Scientists trial implant to patch up the heart  BBC.com

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Revolutionary Stem-Cell Patches Offer Hope For Heart Repair  Evrim Aac

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The cure for a broken heart could be stem cell patches  The Times

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The cure for a broken heart could be stem cell patches - The Times

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The science around terminal inactivation and deletion of genetic codes of heredity in somatic cells was postulated by the Weismann barrier theory [1]. The somatic cell nuclear transfer (SCNT) demonstration asserted the fact that the genetic code in somatic cells is not discarded, and that reactivation of the same is a possibility through careful manipulations [2]. Developmental biology entered a new dimension of achievement when the discovery of embryonic stem cells (ESCs) and their pluripotency was exhibited, and further research identified that on fusion of somatic cells like fibroblasts, and T-lymphocytes with ESCs, reprogramming of the former through expression of genes associated with pluripotency becomes a possibility [3,4]. The findings around SCNT and ESC fusion identified the possibility of reversion in somatic cells indicating the presence of reprogramming factors that bear the potential to act as epigenetic memory erasing factors [5]. The earliest study around generation of pluripotent stem cells from fibroblasts was linked to introduction of four crucial transcription factors including octamer binding transcription factor 3/4 (Oct3/4), sex determining region Ybox 2 (SRY-Sox2), Krppel-like factor 4 (Klf4), and cellular-Myelocytomatosis (c-Myc) (OSKM) [6]. The allogenic trait of ESCs, risk of immune rejection in the recipient along with need for lifetime immunosuppression, and the ethicality around using the same, makes human induced pluripotent stem cells (iPSCs) an established candidate for regenerative therapies as they were found to not impact the host immune system [7]. The introduction of the iPSCs technology happened in the year 2006, and since then multiple observational studies have recounted its impact on cardiac diseases, ophthalmic conditions, as well as neurological disorders [8,9,10]. Figure 1 highlights the process of generating iPS cells.

Showing the process of progression and generating iPSC cells. Detailed description of creating iPSCs with reprogramming factors and differentiating them into a variety of cell types.

The nuclear reprogramming strategies, without compromising on safety and quality for therapeutic applications, include the integrative or nonintegrative transfer systems using viral or nonviral vectors. The first iPSCs were generated by integrating viral vectors, more popularly the retrovirus wherein the resultant iPSCs exhibited failure in complete expression of endogenous genes of pluripotency [11]. The more efficient viral vector has been documented to be the lentiviral vector (LV), which has recorded a reprogramming efficiency of between 0.11% [12,13,14]. To ensure increased safety for therapeutics, nonviral integrative systems have also been worked upon involving use of two plasmids; once encoding for c-Myc, and the other for the four reprogramming factors [15]. However, this system was also shown to have risk of integration, and low reprogramming efficiency. In case of nonintegrative nonviral systems for reprogramming, delivery of pluripotency marker genes has been done using self-replicating vectors, and cytoplasmic RNA. Though easy to work with, the reprogramming efficiency has been found to be lower than LV [16]. Today, research has identified possibility of successful reprogramming using microRNAs (miRNAs) which exhibit improved efficiency, wherein use of c-Myc has been replaced with miR-291-3p, miR-294, and miR-295 to generate homogenous colonies of human iPSCs [17]. The reprogramming methods have been highlighted in Table 1.

Reprogramming strategies for iPSCs in human species. Various programming strategies with ensuring safety and quality for therapeutic applications include the integrative or nonintegrative transfer systems using viral or nonviral.

There are many assays, including molecular and functional, to evaluate the developmental efficiency of iPSCs. These include alkaline phosphatase staining of pluripotency markers, DNA demethylation, retroviral silencing, and factor independence involving assessment of self-renewal in the absence of dox-inducible trans genes. The functional assays include teratoma formation, chimera development, tetraploid complementation, germline transmission, and in vitro differentiation [14]. Considering the low reprogramming efficiency in iPSCs, many studies have identified blocks in lineage conversion. Reprogramming pathway studies in fibroblasts have identified the repel factor to be involved in mesenchymal-to-epithelial transition (MET) and BMP receptor signaling [27,28]. Further studies on the refractory fibroblasts indicate negative iPSC generation in spite of prolonged culturing and presence of homogeneous factor expression indicating loss of somatic program, and activation of endogenous pluripotency genes to be the main roadblocks in formation of iPSCs [14]. The other limiting factor has been linked to expression levels of Nanog locus which are activated late in the reprogramming process and thus limit efficiency of conversion [29]. Gene silencing by DNA methylation, involving the pluripotency genes nanog and Oct4 which causes blockage in binding of transcription factors, has also been linked to causing interference in reprogramming [30]. Though the four most popular reprogramming factors have been Oct4, Sox2, Klf4, and c-Myc, human iPSCs have also been derived using expression of Oct4, Sox2, Nanog, and Lin28, indicating that pluripotent ground state becomes achievable through activation of different transcription factors [21]. The detailed derivation of iPSC along with the assay has been highlighted in Figure 2.

Schematic representation on derivation and assay for human iPSCs. Detailed schematic representation of derivation of iPSC with the various assays to evaluate the developmental efficiency.

The therapeutic potential of iPSC towards personalized cell therapy and disease modelling, has extended the functionality beyond laboratory tables as a research tool in murine and human models. Animal studies have identified promising potential of iPSC around treatment of genetic disorders, including sickle cell anemia; disease modelling of complex degenerative conditions like diabetes, Alzheimers disease, and the feasibility to be used in organ transplantation without risk of rejection and need of immunosuppression [14,31]. Few highlights on the therapeutic potential of iPSCs have been summarized in Table 2. The focus of the current review is to highlight and discuss the therapeutic roles of human iPSCs in different conditions and the future.

Few highlights of iPSC-disease models and the investigated therapy. The example of therapeutic potential of iPSC towards personalized cell therapy and disease modelling, has extended the functionality of the pluripotency beyond laboratory tables as a research tool in murine and human models.

Pluripotency and self-renewal are unique characteristics of iPSC that make them ideal for disease modelling and regenerative medicine. Their ability to indefinitely differentiate into cells of all the three germ layers makes them an important source for treating injuries as well as diseases. The availability of generating patient-specific iPSC with high efficiency and safety through protocols involving biochemical and epigenetic aspects expands the therapeutic potential of this tool. This can be assessed from the fact that a clinical trial involving iPSC-derived dopaminergic neurons have been initiated for Parkinsons disease after successful in vivo studies involving immunodeficient mice highlighted no risk of tumorigenicity [43]. Further, tissue resident macrophages, which are critical for immunity and derived from human-iPSCs, have been found to be immunologically different and better than the traditional monocyte-derived macrophages. Studies have shown human iPSC macrophages to restrict Mycobacterium tuberculosis growth in vitro by >75%, and were found to be capable of mounting antibacterial response when challenged with pathogens [44]. The greatest niche for iPSCs is the ability to generate the same from different donor categories including the diseased, and healthy making its application in the clinical setting at any stage a feasibility without the ethical issues around the ESCs.

The fundamental use of iPSC in regenerative medicine remains undisputed, but the tumorigenic potential of residual undifferentiated stem cells necessitates the need to devise strategies to remove the same from differentiated cells. Different study reports multiple treatment methodologies for eliminating undifferentiated iPSCs and one such recent publication identified undifferentiated hiPSCs to be sensitive to treatment involving medium supplemented with high concentration of L-alanine [45]. Another study assessed the efficacy of plasma-activated medium (PAM) in eliminating undifferentiated hiSPCs through inducing oxidative stress. This study found PAM to selectively eliminate undifferentiated hiPSCs cocultured with normal human dermal fibroblasts, which were the differentiated cells. Lower expression of oxidative-stress related genes in the undifferentiated hiPSCs were found to be the underlying cause for PAM-selective cell death [46]. A recent study report describes the use of salicylic diamines to remove residual undifferentiated cells from iPSC-derived cardiomyocytes. Salicylic diamines were found to exert their specific cytotoxic activity in the pluripotent stem cells by inhibiting the oxygen consumption rate. Teratoma formation was also found to be abolished in comparison to untreated cells [47].

Non-communicable diseases, including cardiovascular conditions, have emerged to be one of the leading causes for mortality in developed as well as developing nations. The trigger for myriad heart conditions exists both in genetics and the environment, which makes studying disease etiology in animal models complicated and inefficient. Animal model studies indicate up to 90% failure in new drug clinical trials, highlighting the limitation around prediction of safety and efficacy among humans. The iPSCs-based disease models have been studied for cardiac channelopathies including hereditary long QT syndrome (LQTS), dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), and arrhythmogenic right ventricular cardiomyopathy (ARVC); the endothelial cell disease including familial pulmonary arterial hypertension (FPAH); the smooth muscle cell condition including Williams-Beuren syndrome (WBS), and Marfan syndrome (MFS) [8].

LQTS is an inherited fatal arrhythmia syndrome and around 17 genes have been associated with congenital LQTS, including the three main genes; KCNQ1 (LQT1), KCNH2 (LQT2), and SCN5A (LQT3), together which account for ~75% of clinically definite cases. The current therapeutic intervention includes -blockers and a surgical procedure named left cardiac sympathetic denervation. Though genetic markers have been defined, the occurrence of variance of unknown significance (VUS) in 1 of 3 patients adds to the dilemma of inconclusive diagnosis. The need for better diagnostic platforms to assess outcome of genetic variants as well as different therapeutics led to the introduction of iPSCs. Many studies have worked to improve the differentiation efficiency, cellular maturation, and lineage specificity, develop new high-throughput assays for cellular phenotyping, and promote clinical implementation of patient-specific genetic models. A study by Wu J.C. et al. [48], utilized patient iPSC-derived cardiomyocytes (iPSC-CMs) and devised various strategies to reduce heterogeneity. These include derivation of chamber-specific cardiomyocytes, cultivation for extended period, 3-dimensional and mechanical conditioning, rapid electric stimulation, and hormonal stimulation; use of multicellular preparations to reduce intercellular variability; and development of high-throughput cellular phenotyping using optogenetic sensors including genetically coded voltage and calcium indicators. Further, this study also established the utility of iPSC-CMs to distinguish between pathogenic and benign variants to improve diagnosis and management of LQTS using CRISPR genome editing. This study, using iPSC-CMs, also identified factors causative for prolonged QT including upregulation of genes; DLG2, KCNE4, PTRF, and HTR2C and downregulation of CAMKV gene. Thus iPSC-based model platforms aid in developing a better understanding around intractable clinical problems associated with diseases like LQTS.

In case of DCM, characterized by ventricular chamber enlargement, and dilation as well as systolic dysfunction, human derived iPSCs have been used to investigate the excitation-contraction-coupling machinery, response to positive inotropic interventions, and study the proteome profile. This study utilized DCM patient specific-iPSC derived from skin fibroblasts and identified defects in assembly and maintenance of sarcomeric structure in the mutated iPSC-CM, as well as lower response to -adrenergic stimulation with isoproterenol, and increased [Ca2+] out and angiotensin-II. This indicates mutated CM from DCM patients to express blunted inotropic response [49]. In case of HCM which is the most common cause of sudden death among the young, iPSC models have been used to identify pathogenesis of the condition. Once such study involving iPSC-CM derived from patients in a maternally inherited HCM family positive for the mitochondrial 16s rRNA gene (MT-RNR2) mutation m.2336T > C identified mitochondrial dysfunction, and ultrastructure defects among the carriers. Further, reduction in levels of mitochondrial proteins, the ATP/ADP ratio, and mitochondrial potential was also found. These lead to increase in intracellular Ca2+ levels, that becomes causative for HCM-specific electrophysiological abnormalities [50]. Recent studies have also generated peripheral blood mononuclear cells-derived iPSC from HCM patient positive for the myosin binding protein C (MYBPC3) pathogenic mutation c.33693370 insC by the episomal method, which underwent successful differentiation to triblast cells with normal male karyotype, and expression of pluripotent markers indicating its usefulness as a tool to study HCM [51].

The iPSC models around FPAH have identified modification of BMPR2 signaling causing reduced endothelial cell adhesion, migration, survival, as well as angiogenesis. The autosomal dominant BMPR2 disease causing mutation has been found to be only 20% penetrant and the use of iPSC identified increased BIRC3 to be related to improved survival, indicating the potential to use protective modifiers of FPAH for developing treatment strategies in the future [52]. The iPSC model around WBS with haploinsufficiency found deficiency of elastin and the patient-derived smooth muscle cell to be immature and highly proliferative with defects in function and contractile properties. The rescue was done by upregulating elastin signaling and use of anti-proliferative drug rapamycin [53]. In case of MFS, disease pathogenesis investigation using iPSCs identified defects in fibrillin-1 accumulation, degradation of extracellular matrix, abnormal activation of transforming growth factor-, and cellular apoptosis [54].

The iPSC technology is also largely viewed to promote pre-clinical drug trials and screening over animal models to overcome differences in electrophysiological properties between human and animal cardiomyocytes. Studies have shown patient-derived iPSCs to exhibit higher sensitivity towards cardiotoxic drugs that could be the cause for change in action potential and arrhythmia [55]. Studies which have analyzed the beat characteristics of 3D engineered cardiac tissues have proven the occurrence of physiologically relevant changes in cardiac contraction in response to increasing concentrations of drugs like verapamil (multi-ion channel blocker) and metoprolol (-adrenergic antagonist) [56].

Thus, iPSC has been successfully used to model and understand pathogenesis of different cardiac diseases, providing insights on pathways around progression as well as for assessment of drug toxicity. These highlight the potential to use iPSC-based models for precision medicine in clinical use.

Theoretically iPSC has the potential to be programmed to form any cell in the human body, and coupled with improvements in reprogramming techniques, this technology has advanced our knowledge on disease pathology, developing precise therapeutics, as well as fuel advances in regenerative medicine [57]. In case of neurodegenerative conditions, and psychiatric disorders, the genetic predisposition and its relation to the disease pathophysiology is complex, and often there is alteration at structural as well as functional levels. In case of schizophrenia, which is aptly termed the disease of the synapses, studies have generated iPSC from family members positive for a frameshift mutation in schizophrenia 1 (DISC1) and used gene editing to generate isogenic iPS cell lines. This study found depletion of DISC1 protein among the mutation carriers, along with dysregulation of genes associated with synapses and psychiatric disorders in the forebrain. This mutation causes deficit of synaptic vesicles among the iPS-cell derived forebrain neurons. This identification of transcriptional dysregulation in human neurons, highlights a new facet involving synaptic dysregulation in mental disorders [58]. The technology of stem cell therapy has also been used to restore the functionality in many degenerative conditions including that of the retina that leads to loss of vision. Studies have evaluated the use iPSC to overcome challenges posed by use of stem cell therapy. The proposed strategy revolves around transplantation of photoreceptors with or without the retinal pigment epithelium cells for treating retinal degradation, with minimal risk using iPSC [59].

Degenerative disease generally progresses through multiple differentiation stages, and using iPSC models, these pathways of transition can be easily identified to assess cause as well as etiopathology better. Amyotrophic lateral sclerosis (ALS) involves loss of neurons from the spinal cord and motor cortex causing paralysis and death. The research around advancement of therapeutics, requires supply of human motor neurons positive for the causative genetic mutations that will also aid in understanding the root cause of motor neuron death. One study documented the production of iPS from ALS patient specific-skin fibroblasts from two sisters. Both were identified to be positive for the L144F (Leu144 Phe) mutation of the superoxide dismutase (SOD1) gene that is associated with a slowly progressing form of ALS. This study found successful reprogramming to be possible with only four factors; KLF4, SOX2, OCT4, and c-MYC. Further, the severe disability state of the patients used for harvesting in this case did not seem to block the transformation process or efficiency [60]. Fanconi anemia (FA) is an inherited bone marrow failure syndrome and is a chromosomal instability disorder needing transplantation of hematopoietic grafts from HLA-identical sibling donors. The reduced quality of the hematopoietic stem cells from the bone marrow of the affected limits the benefit of gene therapy trials. Studies have worked upon formation of genetically corrected FA-specific iPSCs through non-hematopoietic somatic cells reprogramming to generate large number of genetically-stable autologous hematopoietic stem cells for treating bone marrow failure in FA. The reprogramming was done on dermal fibroblasts involving two rounds of infection with mouse-stem-cell-virus-based retrovirus encoding amino-terminal flag-tagged version of the four transcription factors; OCT4, SOX2, KLF4, c-MYC. A batch of genetically corrected somatic cells using lentiviral vectors encoding FANCA or FANCD2 was also used for reprogramming to overcome the predisposition to apoptosis found in FA cells. The FANCA involved fibroblasts also underwent successful transformation to generate iPSCs. This study also found restoration of the FA pathway as a necessity to generate iPS from somatic cells of FA patients. The persistent FANCA expression in the FA-iPS cells indicated successful generation of genetically corrected FA-iPSCs with functional FA pathway, and disease-free status [61].

Parkinsons disease (PD) is a common chronic progressive disorder due to loss of nigrostriatal dopaminergic neurons. The pathophysiology of the disease is complex and research till date lacks complete understanding. Further, sporadic cases are not linked to any genetic variation. Development of patient-specific invitro iPSC models have been attempted to understand disease etiology better. Studies have worked upon generating iPSCs from sporadic cases of PD, which have been successfully reprogrammed to form dopaminergic neurons free of the reprogramming factors. This study utilized doxycycline-inducible lentiviral vectors that were excised with Cre-/lox-recombinase, resulting in generation of iPSC free of programming factors, and which retained all the pluripotent characteristics after removal of transgenes. This removal of promoter and transgene sequences from the vector reduced risk of oncogenic transformation and re-expression of the transduced transcription factors. This study highlighted the possibility of generating stable iPS-cell line in PD for better disease modelling [62]. Another study worked on improving the safety of human and non-human primate iPSC derived dopaminergic neurons for cell transplantation treatment in PD. This study found the protocol of NCAM(+)/CD29(low) sorting to result in enriching ventral midbrain dopaminergic neurons from the pluripotent stem cell-derived neural cell populations. Further, these neurons also exhibited increased expression of FOXA2, LMX1A, TH, GIRK2, PITX3, EN1, and NURR1 mRNA. These neurons were also found to bear the potential to restore motor function among the 6-hydroxydopamine lesioned rats, 16 weeks after transplantation. Further, the primate iPSC-derived neural cell was found to have survived without any immunosuppression after one year of autologous transplant, highlighting the proof-of-concept around feasibility and safety of iPSC-derived transplantation for PD [10].

Type 1 diabetes is an autoimmune condition involving destruction of the -cells of the pancreas wherein transplantation with -cells as islet tissues or the entire pancreas is suggested as an alternative over the traditional exogenous insulin supplementation. However, these come with risk of rejection, need of immunosuppression, apart from difficulty in the physiological control on blood glucose levels. To circumvent this block, generation of -cells or islet tissues from human pluripotent stem cells like iPSCs has been attempted. Many studies have generated pancreatic -like cells which secrete insulin in response to stimuli like potassium chloride [63]. However, co-excretion of glucagon, and somatostatin, apart from releasing unsuitable amounts of insulin; make these clinically inferior. iPSC-derived pancreatic endoderm cells have been shown to retain the potential to differentiate and are functionally comparable with adult -cells. Further, the shortage of donor islet has been overcome using iPSCs, as pancreatic cells generated from these have been evaluated in clinical trials as a new source for transplantation therapy. The differentiation of iPSCs through mimicking the natural in vivo process was facilitated using a combination of growth factors including Nodal-activin, Wnt, retinoic acid, hedgehog, epidermal and fibroblast growth factor, bone morphogenetic protein, and Notch to activate as well as inhibit the key signaling pathway. This study thus highlighted the possibility of generating patient-specific fully functional pancreatic tissue for transplantation over donor islet for diabetes treatment [64].

These studies highlight the development around iPSCs and transplantation technology for treatment of degenerative diseases as well as use them as disease models. The ability to generate patient-specific iPSC from skin biopsies, increases safety of autologous transplants without risk of immunorejection.

The treatment for blood disorders involves need for mature red blood cells/erythrocytes from the bone marrow or umbilical cord blood, for blood transfusion, and is limited due to incompatibility in blood group and Rh antigens, and risk of infections [65]. Erythropoiesis is a complex process for generation of mature erythrocytes from the precursor erythroblasts that are difficult to culture in vitro, as the entire process occurs in the bone marrow mediated by complex interaction between cellular and extracellular environment involving hormones, cytokines, and growth factors [66]. Further, the fully differentiated red blood cells (RBCs) are not proliferative, and setting up a system for erythropoiesis-like maturation in precursor cells is a challenge. Further, recruitment of donors, need for rare blood group types, as well as safety in sensitive population groups, add to the roadblock [67]. Studies have investigated human pluripotent stem cells, including iPSCs as an alternative source for unlimited supply of functional erythrocytes. Studies have discussed different methods devised for RBC production, including using PSCs by repeating the developmental haematopoiesis; reprogramming somatic cells through transcription factors including OCT4, SOX2, c-MYC, KLF4, NANOG, LIN28; and stimulating the maturation of hematopoietic stem cells isolated from peripheral or umbilical cord blood [67,68]. The advantage of using iPSCs is their ability to differentiate into any cell type, and can be maintained indefinitely, thus becoming a potential source for cell replacement therapies. The potential of iPSc becomes highlighted by the fact that the French National Registry of People with a Rare Blood Phenotype/Genotype claims a single iPSc clone from their database could meet 73% of the needs of sickle cell disease patients [69]. This highlights that a limited number or RBC clones have the potential to supply to the majority needs of alloimmunized patients with rare blood groups.

Studies have also worked on developing iPSC models for blood malignancies including myelodysplastic syndromes (MDS), acute myeloid leukemia (AML), and myeloproliferative neoplasms (MPN). A study worked on generating iPSC clones from bone marrow and blood of patients by integrating mutational analysis with cell programming to generate different iPSC clones which represent different disease stage as well as spectrum of the diseases including predisposition, low- and high-risk conditions. Additionally, the researchers also utilized the CRISPR/Cas9 system to introduce as well as correct mutations in the iPSCs. This study found iPSC from AML patients upon differentiation exhibited the leukemic phenotype, and the derived hematopoietic stem cells contained two immunophenotypically distinct cell populations; an adherent and non-adherent fraction, wherein the adherent fraction cells continuously renewed and generated the non-adherent cells. The AML-iPSC thus generated was found to exhibit characteristics of the leukemia stem cell model thus becoming an efficient model for molecular analysis and studying key functional aspects to be utilized for developing better therapeutics [70]. In case of chronic myeloid leukemia (CML), the BCR-ABL gene fusion is the major disease driver, and treatment involves use of tyrosine kinase inhibitor (TKI), causing remission in the vast majority of the cases. Studies have shown the CML-iPSCs to not be affected by TKI even in presence of BCR-ABL expression, indicating absence of dependency in this state of differentiation. The CML-iPSCs factors essential for maintenance of BCR-ABL positive and iPSCs including phosphorylation of AKT, JNK, ERK1/2 remained unchanged while the expression of STAT5 and CRKL was decreased. Further, the hematopoietic cells derived from CML-iPSC regained TKI sensitivity thus facilitating understanding on the disease pathogenesis better [71,72]. In case of MDS, reprogramming to generate iPSCs has been done from patients with del7q mutation, which is the signature for the disease. The iPSCs with the mutation upon hematopoietic differentiation were found to generate low quantities of CD34+/CD45+ myeloid progenitor cells. Further, studying genetically engineered clones as well as the MDS-iPSC-del7q clone from the patient, the researchers functionally mapped MDS phenotype to regions 7q32.37q36.1, which is linked to loss of hematopoietic differentiation potential [73]. To highlight the efficiency of iPSC-technology in precision oncology, studies have also created isogenic iPSCs with del7q and mutation SRSF2 P95L, each of these connected to a specific phenotype and drug response [74].

Human iPSC preclinical models also exist for monogenic blood disorders including thalassemia, and hemoglobinopathies for gene and cell therapy. Pilot trial investigations have explored the safety and effectiveness of mobilizing CD34+ hematopoietic progenitor cells in beta-thalassemia major adults. Further, the CD34+ were transduced with globin lentiviral vector, wherein the vector-encoded beta-chain was found to be expressed at normal hemizygous protein output levels in NSG mice. This trial thus validated an effective protocol for beta-globin gene transfer among thalassemia major CD34+ hematopoietic progenitor cells [75]. The risk of insertional mutagenesis using hematopoietic stem cells can be overcome through iPSCs which can be cloned and the clones with vector integration in the safe harbor sites become possible. The genomic safe harbors (GSHs) ensure that the inserted new genetic material functions as predicted, and do not cause any alterations to the host genome [76]. Studies have shown the use of gene editing tools in case of beta-thalassemia to not be successful in expression of beta-globin in the corrected locus, because of the developmental immaturity of the iPSCs. In such cases, insertion of globin gene copy in the GSH site like AAVS1 has been recommended as an alternative approach [77]. Human iPSC models for gene therapy have also been developed and studied for primary immunodeficiency syndromes, including chronic granulomatous disease (CGD) caused by mutations in genes which code for the phagocyte NADPH oxidase that produces reactive oxygen species (ROS) that kill bacteria. Studies have shown genetically corrected CGD-iPSCs from macrophages and neutrophils using CRISPR/Cas9 system in the single intronic mutation of the CYBB gene to exhibit antimicrobial activity through generation of ROS and phagocytosis [78].

Thus, the potential of iPSCs to study etiology of complex diseases which manifest late in life, as well as to identify markers for precision therapeutics, is worth exploring in the arena of clinical biomedical research. Human iPSC-based models are a true success in our understanding of disease pathogenesis away from the animal models.

Organ donations are a key clinical need to treat end-stage organ failure conditions, and in often cases, patients are left to fight the acute shortage for the same. This apart, from identifying HLA-matched donors, handling risk of infections and rejection, as well as life-long immunosuppression, to a great extent damages quality of life for the affected as well as leads to loss of crucial time. Human iPSCs are being evaluated as a potential source for generating organs that can overcome roadblocks of shortage as well as risk of rejection. Studies have explored the possibility of generating a three-dimensional vascularized and functional liver organ from human iPSCs [79,80,81]. Generation of hepatocyte-like cells using iPSC technology has been reviewed to be fundamentally beneficial for treatment of severe liver disease, screening for drug toxicities, in liver transplantation, as well as to facilitate basic research [21]. Liver organogenesis involves delamination of specific hepatic cells from the foregut endodermal sheet to form a liver bud, which is then vascularized. One study prepared hepatic endoderm cells from human iPSCs through direct differentiation, wherein 80% of the treated cells were found to be positive for the cell fate determining hepatic marker; HNF4A. Further, to stimulate early organogenesis, the iPSCs were cocultured with stromal cells, human umbilical vein endothelial cells, and human mesenchymal stem cells, and after 48h of seeding, the human iPSCs were found to be self-organized into three-dimensional cell clusters visible macroscopically. This iPSC-derived liver bud, when further assessed by quantitative polymerase chain reaction (PCR) and microarray assay for expression analysis, highlighted the pattern to be similar to human fetal liver cell-derived liver buds. Hemodynamic stimulation to form organ was done by cranial window model, and the iPSC-derived tissue was found to perform liver-specific functions including protein synthesis and human-drug specific metabolism actions. This proof-of-concept study highlights the potential to use organ-bud transplantation for organ regeneration [82]. Figure 3 highlights the process of liver development and hepatic differentiation from hiPSCs.

Process of liver development and hepatic differentiation from hiPSCs. The process of isolated cells from patients can be cultured and reprogrammed into patient-specific hiPSCs and quick comparison from natural liver development.

Hepatocytes represent 80% of the liver mass and are the specialized epithelial cells crucial for maintaining homeostasis. The hepatic differentiation involves induction of endoderm differentiation by activin A, fibroblast growth factor 2 (FGF2), and bone morphogenetic protein 4 (BMP4), and such generated hepatocytes have been found to retain features of human liver including lipid and glycogen storage, urea synthesis, etc. Cholangiocytes in the inner space of the bile duct tree have also been generated from the common progenitor hepatoblast, through downregulation of signaling factors including epidermal growth factor (EGF), interleukin 6 (IL-6), Jagged 1, sodium taurocholate, and the generated cholangiocytes have been detected to express mature markers including SOX9 (SRY-Box Transcription Factor 9), OPN (Osteopontin), CK7 (Cytokeratin 7), CK19 (Cytokeratin 19), etc. The kupffer cells are the largest population of resident macrophages in the human body and also facilitate liver regeneration after an ischemic injury. Studies have demonstrated generation of iPSC-derived kupffer cells from macrophage precursors by adding a hepatic stimulus [83,84].

Another study evaluated lung regeneration by endogenous and exogenous stem cell mediated therapeutic approaches. Physiologically the tissue turnover rate in lung is slow and any insult to the regeneration process can lead to development of chronic obstructive pulmonary disease (COPD) as well as idiopathic pulmonary fibrosis. Bone marrow stem cells, embryonic stem cells, as well as iPSCs have shown excellent regenerative capacity to repair injured lung by generating whole lung in the lab using de-cellularized tissue scaffold and stem cells [85]. Lung organogenesis involves proximodistal patterning, branching morphogenesis, alveolarization, and cellular differentiation [86]. A study by Mou et al. [87], described generation of multipotent lung and airway progenitors from mouse ESCs and patient-specific cystic fibrosis (CF) iPSCs. The definitive endoderm from mouse ESCs were converted to foregut endoderm and then into replicating lung endoderm+Nkx2.1 (earliest marker of lung endoderm), which further transformed to a multipotent embryonic lung progenitor and airway progenitor cells. This study further highlighted that precise timing of the BMP, WNT, FGF signaling pathways are crucial for induction of NKX2.1. This study also utilized the same strategy to develop disease-specific lung progenitor cells from CF-iPSCs to make a model platform to study lung diseases. Further, the disease-specific lung progenitors were also engrafted in immunodeficient mice. One study derived lung progenitor cells with ~80% efficiency from iPSCs which differentiated onto alveolar epithelium both in vitro and in vivo. This study used Activin/BMP-4/bFGF treatment to obtain definitive endoderm from iPSC, which was further exposed to a series of pathway inhibitors (BMP, TGF-, WNT), followed by longer exposure to FGF-19, KGF, BMP-4 and a small molecule CHIR99021 to mimic Wnt pathway to generate anterior foregut endoderm. The generated lung progenitors were further differentiated to many pulmonary progenitor cells including basal cells, goblet cells, ciliated cells, in vitro as well as in immunodeficient mice [88].

Studies have also utilized iPSC-derived organ models to study pathogenesis of the coronavirus disease-2019 (COVID-19). One study established a screening strategy to identify drugs that reduce angiotensin converting enzyme 2 (ACE2) using human ESCs-derived cardiac cells and lung organoids, as the infection occurs due to binding of the virus to ACE2 on the cell membrane. Target analysis revealed treatment with antiandrogenic drugs to reduce ACE2 expression, thus protecting the lung organoids from the SARS-CoV-2 infection. Clinical studies on COVID-19 identified patients with prostate disease, with elevated levels of circulating androgen to pose increased risk for high disease severity [89]. Another study utilized human lung stem-cell based alveolospheres to generate insights on SARS-CoV-2 mediated interferon response and pneumocyte dysfunction. This study described a chemically defined modular alveolosphere culture system for propagation and differentiation of the human alveolar type 2 (AT2) derived from primary lung tissue. The cultured cells were found to express ACE2 and transcriptome analysis of the infected alveolospheres were found to mirror features of the COVID-19 infected human lung, together with the interferon-mediated inflammatory response, loss of surfactant proteins, and apoptosis. Further, infected alveolospheres when treated with low dose interferons, a reduction in viral replication was noted. Thus, human stem-cell based models have also added insight to COVID-19 pathogenesis [90]. In case of use of iPSC three-dimensional model, a study by Huang et al. [91] found the derived AT2 to be susceptible to SARS-CoV-2 with decreased expression of surfactant proteins, and cell death, exhibiting delayed type I interferon response with multiplicities of infection of 5 and interferon-stimulated genes. Another study assessed inhibitor of SARS-CoV-2 infection using lung and colonic organoids from the gut. The derived iPSCs in three-dimensional, were positive for SARS-CoV-2 infection. In case of immune response, the tumor necrosis factor (TNF) and interleukin-17 (IL-17) signatures were noted after 24 h with multiplicities of infection of 0.1. This study also screened US Food and Drug Administration (USFDA) approved entry inhibitors including imatinib, mycophenolic acid, and quinacrine dihydrochloride; wherein treatment at physiologically relevant levels highlighted inhibition of SARS-CoV-2 infection both in iPSC-lung organoids and colonoids, indicating that iPSC models also prove to be a valuable source for safe drug screening [92].

Development of organ-specific progenitor cells which progress into the complete three-dimensional organ in a lab highlights the potential of iPSCs in regenerative medicine. Further, the impact of organ-system models to study infection pathology, highlights the wide clinical arena in which iPSC-technology can be used.

The iPSCs have been generated for modelling pathogenesis of many diseases, and one of the most notable additions to the same is cancer, including models for familial cancer syndromes. One such study reports on the successful establishment of Li-Fraumeni Syndrome (LFS) patient-derived iPSC to study role of p53 in development of osteosarcoma. LFS being a heterogenous cancer condition, osteosarcoma is one of the types wherein relevance of germline p53 mutations have been highly reported. The pre-existing murine LFS models have been insufficient in charting the entire tumor landscape and patient-derived iPSCs in this regard have demonstrated the feasibility to effectively study human cancer syndromes. Studies have found the LFS-derived mesenchymal stem cells to exhibit low expression of targets of p53 including p21 and MDM2; highlighting their ability to retain the defective p53 function from the parental fibroblasts. Further, p53 knockdown was found to cause upregulation of osteogenic markers in LFS osteoblasts, and the possibility to attain osteosarcoma-related phenotypes in LFS iPSC-derived osteoblasts was found. Further, gene expression analysis in LFS-derived osteoblasts was found to correlate with poor patient survival, and decreased time for recurrence. The impaired H19 restoration was also found to repress tumorigenic potential [36]. Another study involving modelling of osteosarcoma from LFS derived-iPSC identified the LFS osteoblasts to recapitulate oncogenic properties of osteosarcoma proving to be an excellent model to study disease pathogenesis [93]. In case of Noonan syndrome (NS) characterized by germline PTPN11 mutations, studies which have derived hiPSCs from hematopoietic cells and which harbor the PTPN11 mutations were found to successfully recapitulate features of NS. The iPSC-derived NS myeloid cells were found to exhibit increased STAT5 signaling and enhanced expression of micro-RNAs viz. miR-223 and miR-15a. Further, reducing miR-223 function was found to normalize myelogenesis, highlighting the role of micro-RNA dysregulation in early oncogenesis [94]. Human iPSC-derived hereditary cancer models have also aided in identifying BRCA1-deleted tumor niche to be the cause for disease progression [95].

The iPSC models around cancer aid in overcoming the hurdles posed by traditional cancer cell line systems, which may lose the characteristics of the original tumor with time, and further harnessing primary cancer cells at different stages of carcinogenesis is not feasible. The established iPSC reprogramming strategies can aid in differentiation of cancer cells to target cell lineages which can aid in studying each of the different stages in cancer progression [96]. The iPSCs developed from primary tumors, as well as cancer cell lines are invaluable tools to study genetic alterations early-on in familial cancer syndromes which is crucial in understand disease pathogenesis. Apart from cancer cell lines, patient-derived xenograft models have also been proven to be efficacious for understanding tumor heterogeneity, genetic alterations, and testing efficacy of cytotoxic drugs. However, the need for successful engraftment, technical challenges, and variable growth rates, are the key limitations. Even in case of animal models, high rate of mortality, and absence of metastasis are the limitations [97,98,99]. Advancements in iPSC models have also led researchers to be able to design autologous iPSC-based vaccine which presents a broad spectrum of tumor antigens to the immune system of the mice, and also found success in eliciting a prophylactic reaction against multiple cancer types. These studies highlight the great promise iPSC-based autologous vaccines present towards cancer prevention as well as therapy [100].

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Induced Pluripotent Stem Cells (iPSCs)Roles in Regenerative Therapies ...

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An important step in developing a therapy for a given disease is understanding exactly how the disease works: what exactly goes wrong in the body? To do this, researchers need to study the cells or tissues affected by the disease, but this is not always as simple as it sounds. For example, its almost impossible to obtain genuine brain cells from patients with Parkinsons disease, especially in the early stages of the disease before the patient is aware of any symptoms. Reprogramming means scientists can now get access to large numbers of the particular type of neurons (brain cells) that are affected by Parkinsons disease. Researchers first make iPS cells from, for example, skin biopsies from Parkinsons patients. They then use these iPS cells to produce neurons in the laboratory. The neurons have the same genetic background (the same basic genetic make-up) as the patients own cells. Thus scientist can directly work with neurons affected by Parkinsons disease in a dish. They can use these cells to learn more about what goes wrong inside the cells and why. Cellular disease models like these can also be used to search for and test new drugs to treat or protect patients against the disease.

iPS cells - derivation and applications:Certain genes can be introduced into adult cells to reprogramme them. The resulting iPS cells resemble embryonic stem cells and can be differentiated into any type of cell to study disease, test drugs or-after gene correction-develop future cell therapies

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iPS cells and reprogramming: turn any cell of the body into a stem cell

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Induced pluripotent stem cells are derived from skin or blood cells that have been reprogrammed back into an embryonic-like pluripotent state that enables the development of an unlimited source of any type of human cell needed for therapeutic purposes. For example, iPSC can be prodded into becoming beta islet cells to treat diabetes, blood cells to create new blood free of cancer cells for a leukemia patient or neurons to treat neurological disorders.

Using iPSC technology, center researchers have reprogrammed skin cells into active motor neurons, egg and sperm precursors, liver cells, bone precursors, and blood cells. In addition, patients with untreatable diseases such as, ALS, Rett syndrome, Lesch-Nyhan syndrome, and Duchenne muscular dystrophy donate skin cells to our center for iPSC reprogramming In stem cell research, scientists can reprogram cells that have undergone differentiation, such as skin or blood cells, to revert back into an embryonic-like state. The resulting cells are called induced pluripotent stem cells. reprogramming In stem cell research, scientists can reprogram cells that have undergone differentiation, such as skin or blood cells, to revert back into an embryonic-like state. The resulting cells are called induced pluripotent stem cells. research. The generous participation of patients and their families in this research enables our scientists to study these diseases in the laboratory in the hope of developing new treatment technologies.

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Nanoparticle that cuts middlemen could improve stem cell therapy  Futurity: Research News

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Coordinated immune networks in leukemia bone marrow microenvironments distinguish response to cellular therapy  Science

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GATA2 mutated allele specific expression is associated with a hyporesponsive state of HSC in GATA2 deficiency syndrome  Nature.com

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How the bone marrow microbiome responds to immunotherapy  Chemical & Engineering News

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My Experience With Stem Cell Therapy: Snake Oil or Silver Bullet?  GearJunkie

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Continuing Education Activity

Hematopoietic stem cell transplant (HPSCT), sometimes referred to as bone marrow transplant, involves administering healthy hematopoietic stem cells to patients with dysfunctional or depleted bone marrow. There are several types of HPSCT in clinical use, and transplanted cells may be obtained from several sources. This procedure has several benefits and may be used to treat malignant and non-malignant conditions. It helps to augment bone marrow function. In addition, depending on the disease being treated, it may allow for the destruction of malignant tumor cells. It can also generate functional cells that replace dysfunctional ones in cases like immune deficiency syndromes, hemoglobinopathies, and other diseases. Survival rates after HPSCT are increasing, but morbidity due to complications of the procedure continues. This activity reviews the indications for HPSCT, the different options by which to obtain donor cells, including the advantages and disadvantages of each, and the acute and chronic complications of the procedure. Additionally, it highlights the role of the interprofessional team in managing patients who undergo HPSCT to improve patient outcomes and decrease procedure-associated morbidities.

Objectives:

Describe the malignant and non-malignant indications for hematopoietic stem cell transplants.

Contrast the advantages and disadvantages of different types of hematopoietic stem cells.

Outline the potential complications of hematopoietic stem cell transplants and apply strategies to ameliorate these risks.

Describe the need for a well-integrated, interprofessional team approach to improve care for patients undergoing hematopoietic stem cell transplants.

Hematopoietic stem cell transplant (HPSCT), sometimes referred to as bone marrow transplant, involves administering healthy hematopoietic stem cells to patients with dysfunctional or depleted bone marrow. This procedure has several benefits. It helps to augment bone marrow function. In addition, depending on the disease being treated, it may allow for the destruction of malignant tumor cells. It can also generate functional cells that replace dysfunctional ones in cases like immune deficiency syndromes, hemoglobinopathies, and other diseases.

History and Evolution

Hematopoietic stem cell transplantation (HPSCT) was first explored for use in humans in the 1950s. It was based on observational studies in mice models, which showed that infusion of healthy bone marrow components into a myelosuppressed bone marrow could induce recovery of its function in the recipient.[1]These animal-based studies soon found their clinical application in humans when the first successful bone marrow transplant was performedbetween monozygotic twins in New York in 1957 to treatacute leukemia.[2]The performing physician, E. Donnell Thomas, continued his research on the development of bone marrow transplantationand later received the Nobel Prize for Physiology and Medicine for his work. The first successfulallogeneicbone marrow transplant was reported in Minnesota in 1968 for a pediatric patient with severe combined immunodeficiency syndrome.[3]

Since then, allogeneic and autologous stem cell transplants have increased in the United States (US) and worldwide. The Center for International Blood and Marrow Transplant Research (CIBMTR) reported over 8000 allogeneic transplants performed in the US in 2016, with an evengreaternumber of autologous transplants; autologous transplants have steadily outpaced allogeneic transplants over time.[4][5]

Definitions

Major Histocompatibility Complex (MHC)

The human MHC genes on the short arm of chromosome 6 (6p) encode for human leukocyte antigens (HLA) and are highly polymorphic. These polymorphisms lead to significant differences in the resultant expressed human cell-surface proteins. They are divided into MHC class I and MHC class II.

Human Leukocyte Antigens (HLA)

The HLA proteins are expressed on the cellular surface and play an essential role in alloimmunity. HLA class I molecules, encoded by MHC class I, can be divided into HLA-A, HLA-B, and HLA-C. These proteins are expressed on all cell types and present peptides derived from the cytoplasm and recognized by CD8+ T cells. HLA class II molecules are classified as HLA- DP, HLA-DQ, and HLA-DR, are encoded by MHC class II, can be found on antigen-presenting cells (APCs), andare recognized by CD4+ T cells.

Syngeneic Bone Marrow Transplantation

The donor and the recipient are identical twins. The advantages of this type of transplant include no risk of graft versus host disease (GVHD) or graft failure. Unfortunately, however, only a very fewtransplant patients will have an identical twin available for transplantation.

Autologous Bone Marrow Transplantation

The bone marrow products are collected from the patient and are reinfused after purification methods. The advantage of this type of transplantis no risk of GVHD. The disadvantage is that the reinfused bone marrow products may contain abnormal cells that can cause relapse in the case of malignancy; hence, theoretically, this method cannot be used in all cases of abnormal bone marrow diseases.

Allogeneic Transplantation

The donor is an HLA-matched family member, an unrelated HLA-matched donor, or a mismatched family donor (haploidentical).

Engraftment

The process by which infused transplanted hematopoietic stem cells produce mature progeny in the peripheral circulation.

Preparative Regimen

This regimen comprises high-dose chemotherapy or total body irradiation (TBI) or both, which are administered to the recipient before stem cell infusion to eliminate the largest number of malignant cells and induce immunosuppression in the recipient so that engraftment can occur.

Malignant Disease

Multiple Myeloma

Studies have shown increased overall survival and progression-free survival in patients younger than 65 years when consolidation therapy with melphalan is initiated, followed by autologous stem cell transplantation and lenalidomide maintenance therapy.[6]The study showed a favorable outcome of high-dose melphalan plusHPSCT compared to consolidation therapy with melphalan, prednisone, and lenalidomide. It also showed better outcomes in patients who received maintenance therapy with lenalidomide.

Hodgkin and Non-Hodgkin Lymphoma

Studies have shown that in cases of recurrent Hodgkin and Non-Hodgkin lymphomas that do not respond to initial conventional chemotherapy, chemotherapy followed by autologous stem cell transplantation leads to better outcomes. A randomized controlled trial by Schmitz showed a better outcome at three years of high-dose chemotherapy with autologous stem cell transplant compared to aggressive conventional chemotherapy in relapsed chemosensitive Hodgkin lymphoma. However, the overall survival was not significantlydifferent between the two groups.[7]CIBMTR reports that his group of malignancies accounts for the second highest number of HPSCTs in the US, after multiple myeloma.

Acute Myeloid Leukemia (AML)

Allogeneic stem cell transplant has been shown to improve outcomes. It may prolong overall survival in patients with AML who fail primary induction therapy and do not achieve a complete response.[8]The study recommended that early HLA typing for patients with AML is beneficial if they fail induction therapy and are considered for HPSCT.

Acute Lymphocytic Leukemia (ALL)

Allogeneic stem cell transplant is indicated in refractory and resistant cases of ALL when induction therapy fails for a second time to induce remission. Some studies suggest an increased benefit of allogeneicHPSCT in patients with high-risk ALL, including patients with the Philadelphia chromosome and those with t(4;11).[9]

Myelodysplastic Syndrome (MDS)

Allogeneic stem cell transplant is considered curative in cases of disease progression and is only indicated in intermediate- or high-risk patients with MDS.

Chronic Myeloid Leukemia (CML) and Chronic Lymphocytic Leukemia (CLL)

Patients with CML and CLL received the fewest number of allogeneic transplants in 2020.HPSCT has high cure rates for CML, but because tyrosine kinase inhibitors pair high success rates with a low adverse risk profile, HPSCTis reserved for patients with refractory disease.

Myelofibrosis, Essential Thrombocytosis, and Polycythemia Vera

Allogeneic stem cell transplant has been shown to improve outcomes in patients with myelofibrosis and those diagnosed with myelofibrosis preceded by essential thrombocytosis or polycythemia vera.[10]

Solid Tumors

Autologous stem cell transplant is consideredthestandard of care in patients with testicular germ cell tumors that are refractory to chemotherapy; in this case, refractory is defined as the third recurrence with chemotherapy.[11]HPSCT has also been studied in medulloblastoma, metastatic breast cancer, and other solid tumors.

Non-Malignant Diseases

Aplastic Anemia

Systematic and retrospective studies have suggested an improved outcome with HPSCT in acquired aplastic anemia compared to conventional immunosuppressive therapy.[12]In a study of 1886 patients with acquired aplastic anemia, transplanted cells collected from the bone marrow produced superior outcomes compared to those collected from the peripheral blood.[13]Patients with aplastic anemia need a preparative regimen, as they still can develop immune rejection to the graft.

Severe Combined Immune Deficiency Syndrome (SCID)

Large retrospective studies have shown increased overall survival in infants with SCID when they received the transplant early after birth before the onset of infections.[14]

Thalassemia

Allogeneic stem cell transplant from a matched sibling donor is an option to treat certain types of thalassemia and has shown 15-year survival rates reaching near 80%. However, recent retrospective data showed similar overall survival compared to conventional treatments withmultiple blood transfusions.[15]

Sickle CellDisease

An allogeneic stem cell transplant is recommended to treat sickle cell disease.[16]

Other Non-malignant Diseases

HPSCT has been used to treat chronic granulomatous disease, leukocyte adhesion deficiency, Chediak-Higashi syndrome, Kostman syndrome, Fanconi anemia, Blackfan-Diamond anemia, and enzymatic disorders.Moreover, the role ofHPSCT is expanding in non-malignant autoimmune diseases, including systemic sclerosis and systemic lupus erythematosus, and has already shown promising results in cases like neuromyelitis optica.[17][18][19][20][21][22][23][24][25] It is also considered best practice for relapsing-remitting multiple sclerosis.[26][27]

There are no absolute contraindications for hematopoietic stem cell transplant.

Special equipment exists for collecting, preserving, and administering stem cell products.

An interprofessional team approach is amainstay of ensuring the high-quality collection and infusion of stem cell products.

Preparation includes:

Preparativeregimen:high-dose chemotherapy ortotal body irradiation (TBI) or both

Collection of hematopoietic stem cells

Instant infusion or cryopreservation followed by infusion

Mechanism of Action

The mechanism of action of HPSCT in leukemia is based on the effect of the graft and donor immunity against malignant cells in recipients. These findings were demonstrated in a study that involved over 2000 patients with different leukemias treated with HPSCT. The study showed the lowest relapse rates were in patients who received non-T-cell-depleted bone marrow cells and those who developed GVHD compared to patients who received T-cell-depleted stem cells, those who did not develop GVHD, and patients who received syngeneic grafts. These findings support the notion that donor cellular immunity is central to engraftment efficacy against tumor cells.[28]

The mechanism of action of HPSCT in autoimmune diseases is believed to be secondary to the increase in T-cell regulatory function, which promotes immune tolerance. However, more studies are needed to determine the exact physiology.

In hemoglobinopathies, the transplanted stem cells produce functional cells after engraftment that replace the diseased cells.

Administration

HLA Typing

HLA typing is essential to determine the most suitable donor for stem cell collection. In theory, matched, related donors are the best candidates, followed by matched unrelated donors, cord blood, and haploidentical donors. HLA typing is analyzed at either an intermediate-resolution level, which entails detecting a small number of matched alleles between the donor serum and the recipient, or at a high-resolution level to determine the specific number of polymorphic alleles at a higher level. Polymerase chain reaction and next-generation sequencing are used for HLA typing, and the results are reported as a score correlating with a match of two alleles for a specific HLA type. Different institutions use a different number of HLA subtypes for the eligibility of donors. However, studies that showed high-resolution matching for HLA-A, HLA-B, HLA-C, and HLA-DRB1 were associated with improved survival and outcomes.[29]The Blood and Marrow Transplant Clinical Trials Network (BM CTN) has proposed donor HLA assessment and matching recommendations.[30]

The process may vary depending on the source of the stem cell site collection, whether it is bone marrow, peripheral blood, or cord blood. Moreover, there is a slight difference based on whether it is autologous, allogeneic, or syngeneic HPSCT. For example, the procedure consists of the initial mobilization of stem cells, in which peripheral blood stem cells are collected, given the low number and the need for high levels of progeny cells. This is thenfollowed by a preparative regimen and, finally, infusion.

Mobilization and Collection

Mobilization and collection procedures involve using medication to increase the number of stem cells in the peripheral blood, given that there are insufficient stem cells in the peripheral blood. Medications include granulocyte colony-stimulating factors (G-CSF) or chemokine receptor 4 (CXCR4) blockers like plerixafor. G-CSF is believed to enhance neutrophils to release serine proteases, which break vascular adhesion molecules and promote the release of hematopoietic stem cells from the bone marrow. Plerixafor blocks the binding of stromal cell-derived factor-1-alpha (SDF-1) to CXCR, leading to stem cell mobilization to the peripheral blood.[31]CD34+ is considered the marker for progenitor hematopoietic stem cells in the peripheral blood, and usually, a dose of 2 to 10 x 10/kg CD34+ cells/kg is needed for proper engraftment. Chemotherapy can sometimes be used to mobilize hematopoietic stem cells; this process is termed chemoembolization.

The usual site of bone marrow collection is the anterior or posterior iliac crest. The aspiration procedure can be performed under local or general anesthesia. Common complications include pain and fever; serious iatrogenic complications occur in less than 1% of cases. Each aspiration contains 15 mL, and multiple aspirations are done. The goal is to collect 1 to 1.5 L of bone marrow product from the aspirations. The dose of nucleated cells from bone marrow should range between 2 to 4 x 10 cells/kg; overall survival and long-term engraftment are strongly influenced by cell dose in allogeneic HPSCT.[32]

Preparative Regimen

The preparative regimen consists of the administration of chemotherapy with or without total body irradiation for the eradication of malignant cells and induction of immune tolerance for the transfused cells to engraft properly. This process is not limited to patients with malignancies. It extends to cases like aplastic anemia and hemoglobinopathies, given that these patients have intact immune systems that could cause graft failure if there is no conditioning.

The administration of the preparative regimen should immediately precede the HPSCT. As a general rule, the effect of the regimen should produce bone marrow suppression within 1 to 3 weeks of administration. The preparative regimen is divided into myeloablative conditioning and reduced-intensity conditioning. Different combination regimens are used in the preparative period, depending on the disease being treated, existing comorbidities, previous radiation exposure, and the source of the harvested hematopoietic stem cells.

Reduced-intensity conditioning is preferred in patients who are older, have had prior radiotherapy, have comorbidities, and have a history of extensive chemotherapy before HPSCT.[33]The advantages of using reduced-intensity conditioning include less need for transfusion due to transient post-transplant pancytopenia, less chemotherapy-induced liver damage, and less radiation-induced lung damage.[34]However, the relapse rates after reduced-intensity conditioning are higher. Nevertheless, these regimens are better tolerated and have a better safety profile in specific patient populations.

Most chemotherapies used in preparative regimens consist of potent immunosuppressive agents like high doses of cyclophosphamide, alkylating agents like busulfan, nucleoside analogs like fludarabine, and many other agents like melphalan, anti-thymocyte globulin, rituximab, and gemcitabine. Totalbodyirradiation is performed using fractionated doses; there is less pulmonary toxicity than with a one-dose regimen.[35]

Reinfusion of either fresh or cryopreserved stem cells can occur in an ambulatory setting and takes up to two hours. Before the infusion begins, quality measures are performed to ensure the number of CD34+ cells is sufficient.

In the particular case of SCID, there is no need for a preparative regimen in patients receiving cells from HLA-matched siblings. This is because no abnormal cells need to be eliminated, and the immunosuppression caused by SCID can prevent graft rejection.

Advantages and Disadvantages of Different Hematopoietic Stem Cells

One advantage of peripheral blood stem cell transplant (PBSCT) is a more rapid engraftment rate than the bone marrow-derived stem cells; recovery in the former is two weeks and is delayed for five days more in the latter. Using a post-transplant immunosuppressive regimen to prevent GVHD can prolong the increase in bone marrow products.[36] Moreover, the rate of acute GVHD between PBSCT and bone marrow transplantation appears to be similar in HLA-identical matched related donors.[36]However, chronic GVHD is a more common occurrence after PBSCT, which could lead to more complications. Two-year overall survival rates seem to be similar regardless of stem cell origin.[37]Other studies comparing bone marrow-derived transplant andPSCT concluded that the psychological burden due to chronic GVHD and the 5-year ability to restore normal activities, including returning to work, was better in the bone marrow-derived transplant group.[38]

The advantages of cord blood transplant include the rapid collection and administration times, which facilitate treating urgent conditions, less frequent infections, lower rates of GVHD with the same rate of GVT, and less need for a stringent identical HLA. The disadvantages include delayed engraftment, a higher possibility of graft rejection, and higher rates of disease relapses. The cord blood transplant is most commonly used in patients without matched-related or unrelated donors. One major study demonstrated the utility of cord blood transplants in patients with thalassemia-major and sickle cell disease,indicating similar 6-year overall survival rates compared to the bone marrow-derived transplants.[39]

The most important factors affecting the success of cord blood transplant are the total nucleated cell dose and HLA matching; the recommended minimum dose of total nucleated cells for successful engraftment is 2 x 10^7 cells/kg. Theoretically, strict HLA matching is not required in the case of cord blood transplant as cord blood is devoid of mature T cells, but studies have shown better outcomes when matching recipients at HLA-A, HLA-B, HLA-C, and HLA-DRB1.[40]Given that a single cord blood unit might not contain the required amount of nucleated cells, a double cord transplant is used. However, only one cord blood transplant product will dominate within three months of infusion. Further, randomized controlled trials failed to show a significant difference in outcome, benefits, or risks between double cord blood and a single cord blood transplant.[41][42]

Haploidentical stem cell transplantation involves administering bone marrow products from a first-degree related haplotype-mismatched donor.[43]This helps underserved patients without broad access to resources as they have fewer chances of having a matched unrelated donor.[44]The advantages of this method include lower cost and rapid availability of hematopoietic cell products. However, the disadvantages include hyperacute GVHD, which increases mortality and graft rejection.[45]This has been overcome by the depletion of T cells responsible for the reaction mentioned above, but this also leads to delayed immune recovery and decreased graft versus tumor effect. Recently strategies including selective depletion of subsets of T cells, including alpha-beta, have shown improved outcomes compared to conventional ex vivo depletion of large T-cell populations.[46]

Complications after bone marrow transplant may be acute or chronic. Many factors can affect these adverse events, including age, baseline performance status, the source of stem cell transplant, and the type and intensity of the preparative regimen. Acute complications occur in the first 90 days, including myelosuppression with neutropenia, anemia, or thrombocytopenia; sinusoidal obstruction syndrome; mucositis; acute graft versus host disease; bacterial infections with gram-positive and gram-negative organisms; Herpesviridaeinfections; and fungal infection withCandidaand Aspergillus. Chronic complications include chronic GVHD, infection with encapsulated bacteria, and reactivation of the varicella-zoster virus.

Antimicrobial Prophylaxis

Levofloxacin is usually given orally or intravenously and initiated on the first day post-transplant. It is continued until the absolute neutrophil count is more than 1000 cells/microL or until the discontinuation of prednisonein cases of GVHD.[47]

Prophylaxis against Pneumocystis jirovecii (PCP)is warranted, given the immunosuppression following a hematopoietic stem cell transplant.[48]Trimethoprim-sulfamethoxazole (TMP-SMX) is usually used, and several dosing regimens have been proposed. TMP-SMX may be given twice weekly until the patient is off immunosuppression.[49]Antifungal infection prophylaxis with fluconazole is recommended for one month following the transplant as it has been shown to decrease the incidence of fungal infections. No difference was seen when fluconazole was compared to voriconazole.[50][51]However, voriconazole is used in patients with an elevated risk of developing severe antifungal infections.Anti-viral prophylaxis is achieved with acyclovir, continued for one month to prevent herpes-simplex virus and one year to prevent varicella-zoster virus.[52]Prophylaxis against cytomegalovirus is only recommended in patients who test positive by PCR, and the treatment of choice is ganciclovir.

One unique syndrome encountered with cord stem cell transplant is cord colitis which involves diarrhea in recipients of cord blood and is believed to be secondary to Bradyrhizobium enterica,which usually responds to a course of metronidazole or levofloxacin.[53]

Sinusoidal Obstruction Syndrome (SOS)

Sinusoidal obstruction syndrome (SOS), or veno-occlusive disease (VOD), results from chemotherapy during a preparative regimen and occurs within six weeks of HPSCT. This syndrome consists of tender hepatomegaly, jaundice due to hyperbilirubinemia, ascites, and weight gain due to fluid retention. The incidence is reported to be 13.6% in an analysis study assessing the existing literature on the incidence of the disease.[54]The pathophysiology consists of endothelial damage to the hepatic sinusoids leading to obstruction and necrosis of the centrilobular liver.[55]The destruction of the sinusoids leads to hepatic failure and hepatorenal syndrome, which areresponsible for the related mortality. The agents most commonly implicated in causing this syndrome are oral busulfan and cyclophosphamide. Using intravenous busulfan has been shown to decrease the occurrence of SOS.[56]

The diagnosis of SOS is clinical and is based on hyperbilirubinemia greater than 2 mg/dL in the presence of the aforementioned clinical findings. Treatment consists of ursodeoxycholic acid, which has been shown to significantly decrease the occurrence of SOS when given pre- and post-transplant.[57]Another medication, defibrotide, has shown efficacy in treating SOS when it occurs.[58][59]

Idiopathic Pneumonia Syndrome (IPS)

Idiopathic pneumonia syndrome usually occurs in the first 90 days post-transplant. The incidence is low and is related to the direct chemotoxicity of the preparative regimen. Treatment with steroids is standard, although no randomized controlled clinical trials have been done to support their efficacy. Recently, etanercept has been studied; adding soluble TNF-inhibitors to steroids has not shown added efficacy.[60]

Graft Rejection or Failure

A loss of bone marrow function after reconstitution following infusion of hematopoietic stem cells or no gain of function after infusion is termed graft rejection or failure. The incidence of failure is highest when there is a high HLA disparity; this disparity is highestin cases of cord blood and haploidentical donors and lowest with autologous and matched donor siblings. Factors responsible for graft failure include but are not limited to functional residual host immune response to the donor cells, a low number of infused cells, in vitro damage during collection and cryopreservation, inadequate preparative regimen, and infections.

Chimerism refers to the presence ofa cell population from a person in the blood of a different person. Evaluating for chimerism is an important step in ensuring engraftment and success of the transplantation. This evaluation is done by checking the expression of CD33, which indicates the presence ofgranulocytes, and CD3, which indicates the presence ofT cells, and confirming that most of thecells present are from the donor. The importance of effective chimerism has beendemonstrated in many studies that showed decreased relapse rates and increased survival in allogeneic transplantation.[61]

Graft Versus Host Disease (GVHD)

Graft versus host disease (GVHD) is a reaction between T cells from the donor in an allogeneic transplant and the recipient's HLA polymorphic epitopes, leading to a constellation of symptoms and manifestations. GVHD may be acute or chronic; each is sub-categorized into classic and late-onset, classic, and chronic overlap.[62]

Acute GVHD usually develops within three months. However, it can develop after three months and is then termed delayed acute GVHD. Prophylaxis is generally achieved with calcineurin inhibitors, methotrexate, and anti-thymocyte globulins. The severity of GVHD is estimatedusingthe Glucksberg scale, which classifies acute GVHD from grade I to VI. Treatment with either high-dose prednisone or methylprednisolone isindicated in higher-grade disease.[63]

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YolTech Therapeutics to Initiate a Clinical Trial for YOLT-204, a First-in-Class Bone Marrow-Targeted In Vivo Gene Editing Therapy for -Thalassemia  The Manila Times

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Doctors retrieve stem cells from 20-month-old to treat thalassaemic sister  The Times of India

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Exploring the Regenerative Mechanisms Behind Spinal Cord Injury Repair  Genetic Engineering & Biotechnology News

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Electrically Active Transplantable Material Could Treat Brain and Spinal Cord Injuries  HospiMedica

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Unlocking Spinal Cord Regeneration: Astrocytes Lead the Way  Neuroscience News

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School of Medicine professor receives grant to study improved cancer treatments  Mercer University

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Can The Heart Heal Itself? Expert Explains Breakthroughs In Cardiac Regeneration  Onlymyhealth

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Skip the Botox and Try One of These Growth Factor Serums Instead  ELLE

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