Stem cell transplants are used to put blood stem cells back into the body after the bone marrow has been destroyed by disease, chemotherapy (chemo), or radiation. Depending on where the stem cells come from, the transplant procedure may go by different names:

All of these can also be calledhematopoietic stem cell transplants.

In a typical stem cell transplant for cancer, a person first gets very high doses of chemo, sometimes along with radiation therapy, to try to kill all the cancer cells. This treatment also kills the stem cells in the bone marrow. This is called myeloablation or myeloablative therapy.

Soon after treatment, blood stem cells are given (transplanted) to replace those that were destroyed. The replacement stem cells are given into a vein, much like ablood transfusion. The goal is that over time, the transplanted cells will settle in the bone marrow, where they will begin to grow and make healthy new blood cells. This process is called engraftment.

There are 2 main types of transplants. They are named based on who donates the stem cells.

In this type of transplant, the first step is to remove or harvest your own stem cells. Your stem cells are removed from either your bone marrow or your blood, and then frozen. (You can learn more about this process at Whats It Like to Donate Stem Cells?) After you get high doses of chemo and/or radiation as your myeloablative therapy, the stem cells are thawed and given back to you.

This kind of transplant is mainly used to treat certain leukemias, lymphomas, and multiple myeloma. Its sometimes used for other cancers, like testicular cancer and neuroblastoma, and certain cancers in children. Doctors can use autologous transplants for other diseases, too, like systemic sclerosis, multiple sclerosis (MS), Crohn's disease, and systemic lupus erythematosus (lupus).

An advantage of an autologous stem cell transplantis that youre getting your own cells back. When you get your own stem cells back, you dont have to worry about them (called the engrafted cells or the graft) being rejected by your body. You also dont have to worry about immune cells from the transplant attacking healthy cells in your body (known as graft-versus-host disease), which is a concern with allogeneic transplants.

An autologous transplant graft might still fail, which means the transplanted stem cells dont go into the bone marrow and make blood cells like they should.

Also, autologous transplants cant produce the graft-versus-cancer effect, in which the donor immune cells from the transplant help kill any cancer cells that remain.

Another possible disadvantage of an autologous transplant is that cancer cells might be collected along with the stem cells and then later be put back into your body.

To help prevent any remaining cancer cells from being transplanted along with stem cells, some centers treat the stem cells before theyre given back to the patient. This may be called purging. While this might work for some patients, there haven't been enough studies yet to know if this is really a benefit. A possible downside of purging is that some normal stem cells can be lost during this process. This may cause your body to take longer to start making normal blood cells, and you might have very low and unsafe levels of white blood cells or platelets for a longer time. This could increase the risk of infections or bleeding problems.

Another treatment to help kill cancer cells that might be in the returned stem cells involves giving anti-cancer drugs after the transplant. The stem cells are not treated. After transplant, the patient gets anti-cancer drugs to get rid of any cancer cells that may be in the body. This is called in vivo purging. For instance, lenalidomide (Revlimid) may be used in this way for multiple myeloma. The need to remove cancer cells from transplanted stem cells or transplant patients and the best way to do it continues to be researched.

Doing 2 autologous transplants in a row is known as a tandem transplant or a double autologous transplant. In this type of transplant, the patient gets 2 courses of high-dose chemo as myeloablative therapy, each followed by a transplant of their own stem cells. All of the stem cells needed are collected before the first high-dose chemo treatment, and half of them are used for each transplant. Usually, the 2 courses of chemo are given within 6 months. The second one is given after the patient recovers from the first one.

Tandem transplants have become the standard of care for certain cancers. High-risk types of the childhood cancer neuroblastoma and adult multiple myeloma are cancers where tandem transplants seem to show good results. But doctors dont always agree that these are really better than a single transplant for certain cancers. Because this treatment involves 2 transplants, the risk of serious outcomes is higher than for a single transplant.

Sometimes an autologous transplant followed by an allogeneic transplant might also be called a tandem transplant. (See Mini-transplants below.)

Allogeneic stem cell transplants use donor stem cells. In the most common type of allogeneic transplant, the stem cells come from a donor whose tissue type closely matches yours. (This is discussed in Matching patients and donors.) The best donor is a close family member, usually a brother or sister. If you dont have a good match in your family, a donor might be found in the general public through a national registry. This is sometimes called a MUD (matched unrelated donor) transplant. Transplants with a MUD are usually riskier than those with a relative who is a good match.

An allogeneic transplant works about the same way as an autologous transplant. Stem cells are collected from the donor and stored or frozen. After you get high doses of chemo and/or radiation as your myeloablative therapy, the donor's stem cells are thawed and given to you.

Allogeneic transplants are most often used to treat certain types of leukemia, lymphomas, multiple myeloma, myelodysplastic syndromes, and other bone marrow disorders such as aplastic anemia.

Blood taken from the placenta and umbilical cord after a baby is born can also be used for an allogeneic transplant. This small volume of cord blood has a high number of stem cells in it.

Cord blood transplants can have some advantages. For example, there are already a large number of donated units in cord blood banks, so finding a donor match might be easier. These units have already been donated, so they dont need to be collected once a match is found. A cord blood transplant is also less likely to be rejected by your body than is a transplant from an adult donor.

But cord blood transplants can have some downsides as well. There arent as many stem cells in a cord blood unit as there are in a typical transplant from an adult donor. Because of this, cord blood transplants are used more often for children, who have smaller body sizes. These transplants can be used for adults as well, although sometimes a person might need to get more than one cord blood unit to help ensure there are enough stem cells for the transplant.

Cord blood transplants can also take longer to begin making new blood cells, during which time a person is vulnerable to infections and other problems caused by having low blood cell counts. For a newer cord blood product, known as omidubicel (Omisirge), the cord blood cells are treated in a lab with a special chemical, which helps them get to the bone marrow and start making new blood cells quicker once theyre in the body.

A major benefit of allogeneic transplants is that the donor stem cells make their own immune cells, which could help kill any cancer cells that remain after high-dose treatment. This is called the graft-versus-cancer or graft-versus-tumor effect.

Other advantages are that the donor can often be asked to donate more stem cells or even white blood cells if needed (although this isn't true for a cord blood transplant), and stem cells from healthy donors are free of cancer cells.

As with any type of transplant, there is a risk that the transplant, or graft, might not take that is, the transplanted donor stem cells could die or be destroyed by the patients body before settling in the bone marrow.

Another risk is that the immune cells from the donor could attack healthy cells in the patients body. This is called graft-versus-host disease, and it can range from mild to life-threatening.

There is also a very small risk of certain infections from the donor cells, even though donors are tested before they donate.

Another risk is that some types of infections you had previously and which your immune system has had under control may resurface after an allogeneic transplant. This can happen when your immune system is weakened (suppressed) by medicines called immunosuppressive drugs. Such infections can cause serious problems and can even be life-threatening.

For some people, age or certain health conditions make it more risky to do myeloablative therapy that wipes out all of their bone marrow before a transplant. For those people, doctors can use a type of allogeneic transplant thats sometimes called a mini-transplant. Your doctor might refer to it as a non-myeloablative transplant or mention reduced-intensity conditioning (RIC). Patients getting a mini transplant typically get lower doses of chemo and/or radiation than if they were getting a standard myeloablative transplant. The goal in the mini-transplant is to kill some of the cancer cells (which will also kill some of the bone marrow), and suppress the immune system just enough to allow donor stem cells to settle in the bone marrow.

Unlike the standard allogeneic transplant, cells from both the donor and the patient exist together in the patients body for some time after a mini-transplant. But slowly, over the course of months, the donor cells take over the bone marrow and replace the patients own bone marrow cells. These new cells can then develop an immune response to the cancer and help kill off the patients cancer cells the graft-versus-cancer effect.

One advantage of a mini-transplant is that it uses lower doses of chemo and/or radiation. And because the stem cells arent all killed, blood cell counts dont drop as low while waiting for the new stem cells to start making normal blood cells. This makes it especially useful for older patients and those with other health problems. Rarely, it may be used in patients who have already had a transplant.

Mini-transplants treat some diseases better than others. They may not work well for patients with a lot of cancer in their body or people with fast-growing cancers. Also, although there might be fewer side effects from chemo and radiation than those from a standard allogeneic transplant, the risk of graft-versus-host disease is the same. Some studies have shown that for some cancers and some other blood conditions, both adults and children can have the same kinds of results with a mini-transplant as compared to a standard transplant.

This is a special kind of allogeneic transplant that can only be used when the patient has an identical sibling (twin or triplet) someone who has the exact same tissue type. An advantage of syngeneic stem cell transplant is that graft-versus-host disease will not be a problem. Also, there are no cancer cells in the transplanted stem cells, as there might be in an autologous transplant.

A disadvantage is that because the new immune system is so much like the recipients immune system, theres no graft-versus-cancer effect. Every effort must be made to destroy all the cancer cells before the transplant is done to help keep the cancer from coming back.

Improvements have been made in the use of family members as donors. This kind of transplant is called ahalf-match (haploidentical) transplant for people who dont have fully matching or identical family member. This can be another option to consider, along with cord blood transplant and matched unrelated donor (MUD) transplant.

If possible, it is very important that the donor and recipient are a close tissue match to avoid graft rejection. Graft rejection happens when the recipients immune system recognizes the donor cells as foreign and tries to destroy them as it would a bacteria or virus. Graft rejection can lead to graft failure, but its rare when the donor and recipient are well matched.

A more common problem is that when the donor stem cells make their own immune cells, the new cells may see the patients cells as foreign and attack their new home. This is called graft-versus-host disease. (See Stem Cell Transplant Side Effects for more on this). The new, grafted stem cells attack the body of the person who got the transplant. This is another reason its so important to find the closest match possible.

Many factors play a role in how the immune system knows the difference between self and non-self, but the most important for transplants is the human leukocyte antigen (HLA) system. Human leukocyte antigens are proteins found on the surface of most cells. They make up a persons tissue type, which is different from a persons blood type.

Each person has a number of pairs of HLA antigens. We inherit them from both of our parents and, in turn, pass them on to our children. Doctors try to match these antigens when finding a donor for a person getting a stem cell transplant.

How well the donors and recipients HLA tissue types match plays a large part in whether the transplant will work. A match is best when all 6 of the known major HLA antigens are the same a 6 out of 6 match. People with these matches have a lower chance of graft-versus-host disease, graft rejection, having a weak immune system, and getting serious infections. For bone marrow and peripheral blood stem cell transplants, sometimes a donor with a single mismatched antigen is used a 5 out of 6 match. For cord blood transplants a perfect HLA match doesnt seem to be as important, and even a sample with a couple of mismatched antigens may be OK.

Doctors keep learning more about better ways to match donors. Today, fewer tests may be needed for siblings, since their cells vary less than an unrelated donor. But to reduce the risks of mismatched types between unrelated donors, more than the basic 6 HLA antigens may be tested. For example, sometimes doctors to try and get a 10 out of 10 match. Certain transplant centers now require high-resolution matching, which looks more deeply into tissue types and allow more specific HLA matching.

There are thousands of different combinations of possible HLA tissue types. This can make it hard to find an exact match. HLA antigens are inherited from both parents. If possible, the search for a donor usually starts with the patients brothers and sisters (siblings), who have the same parents as the patient. The chance that any one sibling would be a perfect match (that is, that you both received the same set of HLA antigens from each of your parents) is 1 out of 4.

If a sibling is not a good match, the search could then move on to relatives who are less likely to be a good match parents, half siblings, and extended family, such as aunts, uncles, or cousins. (Spouses are no more likely to be good matches than other people who are not related.) If no relatives are found to be a close match, the transplant team will widen the search to the general public.

As unlikely as it seems, its possible to find a good match with a stranger. To help with this process, the team will use transplant registries, like those listed here. Registries serve as matchmakers between patients and volunteer donors. They can search for and access millions of possible donors and hundreds of thousands of cord blood units.

Be the Match (formerly the National Marrow Donor Program)Toll-free number: 1-800-MARROW-2 (1-800-627-7692)Website: http://www.bethematch.org

Blood & Marrow Transplant Information NetworkToll-free number: 1-888-597-7674Website: http://www.bmtinfonet.org

Depending on a persons tissue typing, several other international registries also are available. Sometimes the best matches are found in people with a similar racial or ethnic background. When compared to other ethnic groups, white people have a better chance of finding a perfect match for stem cell transplant among unrelated donors. This is because ethnic groups have differing HLA types, and in the past there was less diversity in donor registries, or fewer non-White donors. However, the chances of finding an unrelated donor match improve each year, as more volunteers become aware of registries and sign up for them.

Finding an unrelated donor can take months, though cord blood may be a little faster. A single match can require going through millions of records. Also, now that transplant centers are more often using high-resolution tests, matching is becoming more complex. Perfect 10 out of 10 matches at that level are much harder to find. But transplant teams are also getting better at figuring out what kinds of mismatches can be tolerated in which particular situations that is, which mismatched antigens are less likely to affect transplant success and survival.

Keep in mind that there are stages to this process there may be several matches that look promising but dont work out as hoped. The team and registry will keep looking for the best possible match for you. If your team finds an adult donor through a transplant registry, the registry will contact the donor to set up the final testing and donation. If your team finds matching cord blood, the registry will have the cord blood sent to your transplant center.

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Types of Stem Cell and Bone Marrow Transplants

Bone Marrow Stem Cells Comments Off on Types of Stem Cell and Bone Marrow Transplants | December 27th, 2024

Heart disease, whether inherited or acquired, is the leading cause of mortality in both men and women worldwide, accounting for 17.3 million deaths per year.1 The urgent need to improve existing therapies has driven researchers to seek a better understanding of the diverse but inter-related mechanistic origins of heart development and failure, with the ultimate goals of identifying novel pharmacological treatments and/or cell-based engineering approaches to replace damaged heart tissue. Animal models are widely used as surrogates for studying human disease, both in order to recapitulate the complex clinical course of human heart failure and to generate in vitro tools for studying specific aspects of tissue dysfunction.2 Although useful insights have been gained, experimental findings from animal models have not always extrapolated to human disease presentation due to considerable species variation3. Here we describe prominent routes taken towards the goal of cardiac regeneration by focusing on key contributing papers published by Circulation Research in the 60 years since its establishment.

Multipotent adult stem cells have been the focus of most preclinical and clinical studies carried out to date in the field of cardiac regeneration. They represent an attractive source of stem cells since they are relatively abundant, accessible and autologous, and their mechanisms of action for any observed improvement in cardiac function can be potentially delineated. In 1998, Anversa et al. published a field-changing paper challenging the notion that the myocardium is a non-regenerating tissue, by describing the presence of multipotent cardiac stem cells (CSCs) in the adult myocardium that are positive for the hematopoietic progenitor marker c-kit.4 Methods for isolating functionally competent CSCs and mechanisms proving that their activation can reverse cardiac dysfunction were later published by the same group.5, 6 It was this pioneering work and the ability to adequately expand CSCs ex vivo that formed the basis for the first randomized clinical trial of CSC implant in ischemic heart disease patients (SCIPIO trial).7 Phase I of the trial demonstrated a sound safety profile and potential for efficacy in improving ventricular function. In 2004, Messina et al. were able to isolate and expand c-kit+ CSCs from adult murine hearts as self-adherent clusters of progenitor cells, termed cardiospheres.8 This isolation technique later became feasible for human hearts and was used to test the therapeutic efficacy of cardiosphere-derived cells (CDCs) in the CADUCEUS trial.9 The Phase I trial demonstrated a good safety profile and potential for reducing in scar size and regional function compared to controls. More recently, Dey et al. performed detailed characterization of multiple stem cell populations and concluded that c-kit+ CSCs represent the most primitive population of multipotent cardiac progenitors when compared to bone marrow-derived c-kit+ populations, and that CDCs are more closely related to bone marrow stem cells in terms of their gene and protein expression profiles.10 The exact mechanistic and functional outcome implications of such differences are not yet known, but may aid ongoing clinical trials in understanding the biology of these promising cell populations.

Bone marrow-derived mononuclear cells (MNCs) have also garnered considerable interest in regenerative cell therapy as they are easily accessible and autologous, and require minimal expansion. Significantly, evidence of MNC mobilization after myocardial infarction (MI) in mice have supported that bone marrow cells play a role in myocardial healing following injury.11, 12 Randomized human clinical studies of injected MNCs demonstrated a modest increase in left ventricular ejection fraction (LVEF) and a decrease in the New York Heart Association (NYHA) functional classification system.13 Ischemic cardiomyopathy patients receiving MNCs also demonstrated a significant reduction in natriuretic peptide levels.14 Notably, infusion of MNCs with higher colony-forming capacity was associated with lower mortality, raising awareness to the notion that cell viability and quality have a significant impact on therapeutic effect. Mechanistic investigations have suggested that beneficial effects of MNC therapy were a result of neovascularization and paracrine effects rather than cardiomyocyte differentiation.15

Studies of bone marrow-derived mesenchymal stem cells (MSCs) revealed yet another adult stem cell source thought to be suitable for cardiac regeneration. MSCs were reported to readily express phenotypic characteristics of CMs and, when introduced into infarcted animal hearts by intravenous injections, to localize at sites of myocardial injury, prevent tissue remodeling, and improve cardiac recovery.16, 17 Intracoronary infusion of allogeneic mesenchymal precursors (Stro-3+ subpopulation) was also shown to decrease infarct size, improve systolic function, and increase neovascularization in animal MI models.18 These observations led to a pilot human clinical study which confirmed the safety and tolerability of MSCs in humans, and subsequently to a Phase I/II randomized trial.19, 20 More recently, additional evidence has questioned the ability of MSCs to transdifferentiate into cardiomyocytes, instead attributing the mechanism of their therapeutic properties to paracrine effects, neovascularization, and activation of endogenous CSCs.19, 21

Another class of multipotent adult stem cells of particular interest in cardiac cell therapy are CD34+ angiogenic precursors. This interest stems from the relatively impaired angiogenesis seen in ischemic heart disease patients as well as from findings that patients with coronary artery disease have reduced number and migratory activity of angiogenic precursors.22 It has also been observed that CD34+ cell injection ameliorates cardiac recovery in human MI patients by improving perfusion and/or by paracrine effects rather than cardiomyocyte differentiation.23 In one of the largest cell therapy trials to date, Losordo et al. demonstrated that patients with refractory angina who received intramyocardial injections of CD34+ cells experienced significant improvements in angina frequency and exercise tolerance.24 In a subsequent publication, the group identified that CD34+ cells secrete exosomes that might account for some of the improved phenotypes.25 The benefit of CD34+ cells was also shown for non-ischemic cardiomyopathy, when intracoronary injections resulted in a small, but significant improvement in ventricular function and survival.26 More importantly, this study demonstrated that higher intramyocardial homing was associated with better cell therapy response, providing support to prior observations with MNCs that cell delivery method and quality play a significant role in their therapeutic efficacy.

Finally, adipose-derived stem cells (ADSCs) abundantly available from liposuction surgeries have been considered as potential sources of CMs. In 2004, Planart-Blenard et al. reported potential derivation of CMs from human ADSCs by treatment with transferrin, IL-3, IL-6, and VEGF, although at very low event rate (Figure 1).27 Ongoing trials are evaluating the efficacy of this cell population in regeneration of ischemic myocardium, and although complete results have yet to be published, preliminary data are encouraging (Trial identifier: NCT00426868).

Timeline of important discoveries contributing to the field of stem cell cardiac differentiation and characterization (purple and green boxes, above timeline), including the key Top 100 Circulation Research papers discussed in this review (red boxes, below timeline). ESC, embryonic stem cell; iPSC, induced pluripotency stem cell; CMs, cardiomyocytes.

Early attempts at inducing cardiac regeneration involved transplant of skeletal myoblasts or fetal CMs to infarcted canine or rat hearts. Unfortunately, these studies ultimately disappointed the field as myoblasts remained firmly committed to form mature skeletal muscle in the heart28, while extensive cell death coupled with limited proliferation after transplant prevented fetal cardiomyocytes from repairing injury.29 Transplantation of non-contractile committed cells such as fibroblasts and smooth muscle cells into infarcted rat hearts was then briefly thought to enhance heart function, possibly due to aforementioned paracrine effects.30 More recently, several studies have demonstrated in vitro31 and in vivo32 transdifferentiation of mouse fibroblasts into seemingly functional CMs by over-expressing combinations of the cardiac transcription factors Gata4, Mef2c, Tbx5, Hand2, and Nkx2.5. Mouse CMs generated by direct transdifferentiation are positive for CM-specific sarcomeric markers, exhibit electrophysiological and gene expression profiles similar to those of fetal CMs, although this was disputed by other investigators.33In vitro transdifferentiation towards CM-like cells was also reported for human fibroblasts, albeit by more time consuming and less efficient protocols that generated mostly partially reprogrammed CMs.34 Current efforts in this research area focus on optimizing transdifferentiation efficiency and CM maturation, further characterizing derived CMs, and validating that in vitro and in vivo transdifferentiation occur in the absence of experimental artifacts, which can include incomplete silencing of transgene expression from Cre-lox systems, cell fusion events, as well as the possibility of retrovirus transfecting not only dividing fibroblasts but also non-dividing cardiomyocytes in vivo. For this technology to be fully applied in the clinic, a greater understanding of issues that have plagued the field must be reached: (1) the potential consequences of depleting endogenous cardiac fibroblasts to replenish cardiomyocytes; (2) the ability to transfect bystander cells such as smooth muscle and endothelial cells with cardiac transcription factors; and (3) the challenge of triggering immune response against the host cells transfected with viral versus non-viral vectors.

The isolation by Evans and Kaufman of mouse embryonic stem cells (mESCs) in 198135 and the generation of human embryonic stem cells (hESCs) by Thomson in 199836 opened new horizons for in vitro generation of CMs. Many protocols have been developed over the years to maximize the yield and efficiency of pluripotent ESC differentiation to CMs.37 One of the most utilized methods has been the formation of 3D aggregates named embryoid bodies within which cardiac differentiation occurs. In 2002, Xu et al. were amongst the first to optimize cardiac differentiation protocols for hESCs by using DNA demethylating agent 5-azacytidine and enrichment with Percoll separation gradients to obtain up to 70% pure cardiomyocyte populations (Figure 1).38 Later on, rigorous protocol standardization and the use of key signaling factors such as BMP4 and Activin A enabled conversion of hESCs to CMs with over 90% efficiency.39 Consequently, the formation of 3D aggregates, a labor intensive process, has now been largely replaced by differentiation in monolayer cultures, which are more amenable to scale-up and automation.40

The discovery of induced pluripotent stem cell (iPSC) technology41, based partly on principles highlighted by early somatic cell nuclear transfer experiments42, has meant that mature somatic cells such as skin fibroblasts and peripheral blood mononuclear cells (PBMCs) can be reprogrammed with relative ease to acquire an ESC-like phenotype. iPSCs retain the same capacity for high efficiency cardiac differentiation as ESCs, with the added advantages of avoiding ethical debates related to use of human embryos and enabling autologous transplantation of CMs without the need for immunosuppression. These characteristics make iPSCs ideal cellular models to provide a renewable source of CMs for basic research, pharmacological testing, and cell therapy (Figure 2).43

iPSCs are ideal cellular models to provide a renewable source of cardiomyocytes for in vitro disease modeling, pharmacological testing, and therapeutic applications in regenerative medicine.

The use of pluripotent stem cell-derived cardiomyocytes (PSC-CMs), which include both hESC-CMs and iPSC-CMs, for downstream applications requires that their properties be physiologically analogous to human cardiomyocytes in vivo. Assays for CM characterization, such as assessment for cross striations, ultrastructure, and chronotropic drug response, were established decades ago for primary rodent myocytes and published in a highly cited Circulation Research paper by Simpson and Savion in 1982.44 In 1994, Maltsev et al. were able to apply the same assays for extensive characterization of mESC-CMs.45 In addition, rigorous experimental optimization enabled them to identify internal and external solutions for patch-clamp electrophysiological analysis to confirm that CM populations comprised of ventricular, atrial, and nodal sub-types, and exhibited most basic cardiac-specific ionic currents (L-type, ICa, INa, Ito, IK, IK1, IK, ATP, IK, Ach, and If). In 2003, He et al. were among the first to perform similar characterizations of hESC-CMs.46

In vitro derived PSC-CMs have been assessed as potential screening platforms for drug discovery and toxicology studies. Despite their immature fetal phenotype, extensive pharmacological validation confirms their potential utility in drug evaluation.47 Clinically relevant drugs (e.g., adrenergic receptor agonists, anti-arrhythmic agents) have been shown to exert chronotropic and inotropic effects on PSC-CMs. In addition, experimental drugs have been used for in vitro amelioration of diseased phenotypes in human iPSC models of cardiovascular diseases48 and prediction of cytotoxic drug-induced side-effects.49, 50 Accumulated evidence suggests that PSC-CMs can offer the pharmaceutical industry a valuable physiologically relevant tool for validation of novel drug candidates and identification of potential cardiotoxic effects in early drug development stages, thereby easing the huge associated economic and patient care burdens.51, 52

The most successful and widely acknowledged use of PSCs-CMs has so far been in disease modeling. The development of disease models by genome editing of mESCs, a technology that led to award of the Nobel Prize in 2007 for Sir Martin Evans, Mario Capecchi, and Oliver Smithies (Figure 1), has offered new tools for in vivo mechanistic investigation into cardiac illnesses. The discovery of induced pluripotency technologies, which likewise led to the Nobel Prize in 2012 for Sir John Gurdon and Shinya Yamanaka, allowed the generation of patient-specific iPSC-CMs for studying human disease models of familial hypertrophic cardiomyopathy53, familial dilated cardiomyopathy54, long QT syndrome55, Timothy syndrome56, arrhythmogenic right ventricular dysplasia57, and others44 (Figure 2). Beyond the potential ability of these models to reveal insights into pathological disease mechanisms, they also offer unique opportunities to explore promising new genetic therapies58 and to identify loci or pathways related to predisposition towards cardiac disorders, thus enabling refinement of phenotype-to-genotype correlations to improve risk stratification and disease management.

The use of PSC-CMs has also expanded to in vivo applications, with transplantation shown to improve cardiac function in rat and guinea pig models of acute myocardial infarct (MI).59, 60 Effective strategies to deplete potential tumorigenic cells61, 62, induce immunotolerance63, 64, and enhance cell survival65 are being sought. Novel tissue engineering approaches to create engineered heart tissues (EHTs) for aiding cell delivery, survival, alignment and functionality of transplanted PSC-CMs are being developed in parallel.66 Notably, these technologies were pioneered by Thomas Eschenhagens group, who published one of the very first EHT papers in Circulation Research in 2002.67

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Cardiac stem cell biology: a glimpse of the past, present, and future - PMC

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Explained: What is mesenchymal stem cell therapy?  Drug Discovery News

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Scientists Present Research on Novel Cancer Therapies at ASH  City of Hope

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Navigating CAR-T cell therapy long-term complications  Nature.com

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Stem Cell Transplants Offer New Hope for Saving the Worlds Corals  Technology Networks

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Stem Cell Therapy Market Is Expected To Reach Revenue Of  GlobeNewswire

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The Importance of Cellular Therapy in the Clinical Case of a Young Man With a Challenging Precursor B-cell Lymphoblastic Leukemia  Cureus

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High-dose chemotherapy followed by autologous stem cell transplant ineffective for patients with mantle cell lymphoma  News-Medical.Net

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High-dose chemotherapy followed by autologous stem cell transplant ineffective for patients with mantle cell lymphoma - News-Medical.Net

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A search for the perfect match, Apex six year old in need of donor  CBS17.com

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Exclusive: Cell therapy startup Shinobi adds Borges as science chief, Katz as top medical officer  Endpoints News

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Exclusive: Cell therapy startup Shinobi adds Borges as science chief, Katz as top medical officer - Endpoints News

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Sumitomo Chemical and Sumitomo Pharma to Establish Regenerative Medicine and Cell Therapy Joint Venture  

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BrightPath Bio and Cellistic Announces Process Development and Manufacturing Collaboration for Phase 1 Clinical Trial of iPSC-derived BCMA CAR-iNKT cell  Via Ritzau

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BrightPath Bio and Cellistic Announces Process Development and Manufacturing Collaboration for Phase 1 Clinical Trial of iPSC-derived BCMA CAR-iNKT...

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Shinobi Strengthens Leadership to Propel Scalable Immune-Evasive Cell Therapies to the Clinic  The Eastern Progress Online

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Abstract

Induced pluripotent stem cells (iPSCs) are a new type of pluripotent cellsthat can be obtained by reprogramming animal and human differentiated cells. In this review,issues related to the nature of iPSCs are discussed and different methods ofiPSC production are described. We particularly focused on methods of iPSC production withoutthe genetic modification of the cell genome and with means for increasing the iPSC productionefficiency. The possibility and issues related to the safety of iPSC use in cell replacementtherapy of human diseases and a study of new medicines are considered.

Keywords: induced pluripotent stem cells, directed stem cell differentiation, cell replacement therapy

Pluripotent stem cells are a unique model for studying a variety of processes that occur inthe early development of mammals and a promising tool in cell therapy of human diseases. Theunique nature of these cells lies in their capability, when cultured, for unlimitedselfrenewal and reproduction of all adult cell types in the course of theirdifferentiation [1]. Pluripotency is supported by acomplex system of signaling molecules and gene network that is specific for pluripotent cells.The pivotal position in the hierarchy of genes implicated in the maintenance of pluripotency isoccupied by Oct4, Sox2 , and Nanog genes encodingtranscription factors [2, 3]. The mutual effect of outer signaling molecules and inner factors leads tothe formation of a specific expression pattern, as well as to the epigenome statecharacteristic of stem cells. Both spontaneous and directed differentiations are associatedwith changes in the expression pattern and massive epigenetic transformations, leading totranscriptome and epigenome adjustment to a distinct cell type.

Until recently, embryonic stem cells (ESCs) were the only wellstudied source ofpluripotent stem cells. ESCs are obtained from either the inner cell mass or epiblast ofblastocysts [46]. A series of protocols has been developed for the preparation of variouscell derivatives from human ESCs. However, there are constraints for ESC usein cell replacement therapy. The first constraint is the immune incompatibility between thedonor cells and the recipient, which can result in the rejection of transplanted cells. Thesecond constraint is ethical, because the embryo dies during the isolation of ESCs. The firstproblem can be solved by the somatic cell nuclear transfer into the egg cell and then obtainingthe embryo and ESCs. The nuclear transfer leads to genome reprogramming, in which ovariancytoplasmic factors are implicated. This way of preparing pluripotent cells from certainindividuals was called therapeutic cloning. However, this method is technologyintensive,and the reprogramming yield is very low. Moreover, this approach encounters theabovementioned ethic problem that, in this case, is associated with the generation ofmany human ovarian cells [7].

In 2006, the preparation of pluripotent cells by the ectopic expression of four genes Oct4 , Sox2 , Klf4 , and cMyc in both embryonic and adult murine fibroblasts was first reported[8]. The pluripotent cells derived from somatic ones werecalled induced pluripotent stem cells (iPSCs). Using this set of factors(Oct4, Sox2, Klf4, and cMyc), iPSCs were prepared later from variousdifferentiated mouse [914] and human [1517] cell types. Human iPSCs were obtainedwith a somewhat altered gene set: Oct4 , Sox2 , Nanog , and Lin28 [18].Induced PSCs closely resemble ESCs in a broad spectrum of features. They possess similarmorphologies and growth manners and are equally sensitive to growth factors and signalingmolecules. Like ESCs, iPSCs can differentiate in vitro intoderivatives of all three primary germ layers (ectoderm, mesoderm, and endoderm) and formteratomas following their subcutaneous injection into immunodeficient mice. MurineiPSCs injected into blastocysts are normally included in the development toyield animals with a high degree of chimerism. Moreover, murine iPSCs, wheninjected into tetraploid blastocycts, can develop into a whole organism [19, 20]. Thus, an excellent method thatallows the preparation of pluripotent stem cells from various somatic cell types whilebypassing ethical problems has been uncovered by researchers.

In the first works on murine and human iPSC production, either retro or lentiviralvectors were used for the delivery of Oct4 , Sox2 , Klf4 , and cMyc genes into somatic cells. Theefficiency of transduction with retroviruses is high enough, although it is not the same fordifferent cell types. Retroviral integration into the host genome requires a comparatively highdivision rate, which is characteristic of the relatively narrow spectrum of cultured cells.Moreover, the transcription of retroviral construct under the control of a promoter localizedin 5LTR (long terminal repeat) is terminated when the somatic celltransform switches to the pluripotent state [21]. Thisfeature makes retroviruses attractive in iPSC production. Nevertheless, retroviruses possesssome properties that make iPSCs that are produced using them improper for celltherapy of human diseases. First, retroviral DNA is integrated into the host cell genome. Theintegration occurs randomly; i.e., there are no specific sequences or apparent logic forretroviral integration. The copy number of the exogenous retroviral DNA that is integrated intoa genome may vary to a great extent [15]. Retrovirusesbeing integrated into the cell genome can introduce promoter elements and polyadenylationsignals; they can also interpose coding sequences, thus affecting transcription. Second, sincethe transcription level of exogenous Oct4 , Sox2 , Klf4 , and cMyc in the retroviral constructdecreases with cell transition into the pluripotent state, this can result in a decrease in theefficiency of the stable iPSC line production, because the switch from the exogenous expressionof pluripotency genes to their endogenous expression may not occur. Third, some studies showthat the transcription of transgenes can resume in the cells derived fromiPSCs [22]. The high probability thatthe ectopic Oct4 , Sox2 , Klf4 , and cMyc gene expression will resume makes it impossible to applyiPSCs produced with the use of retroviruses in clinical trials; moreover,these iPSCs are hardly applicable even for fundamental studies onreprogramming and pluripotency principles. Lentiviruses used for iPSC production can also beintegrated into the genome and maintain their transcriptional activity in pluripotent cells.One way to avoid this situation is to use promoters controlled by exogenous substances added tothe culture medium, such as tetracycline and doxycycline, which allows the transgenetranscription to be regulated. iPSCs are already being produced using suchsystems [23].

Another serious problem is the gene set itself that is used for the induction of pluripotency[22]. The ectopic transcription of Oct4 , Sox2 , Klf4 , and cMyc can lead to neoplastic development from cells derived from iPSCs,because the expression of Oct4 , Sox2 , Klf4, and cMyc genes is associated with the development ofmultiple tumors known in oncogenetics [22, 24]. In particular, the overexpression of Oct4 causes murine epithelial cell dysplasia [25],the aberrant expression of Sox2 causes the development of serrated polypsand mucinous colon carcinomas [26], breast tumors arecharacterized by elevated expression of Klf4 [27] , and the improper expression of cMyc is observed in 70% of human cancers [28].Tumor development is oberved in ~50% of murine chimeras obtained through the injection ofretroviral iPSCs into blastocysts, which is very likely associated with thereactivation of exogenous cMyc [29, 30].

Several possible strategies exist for resolving the above-mentioned problems:

The search for a less carcinogenic gene set that is necessary and sufficient for reprogramming;

The minimization of the number of genes required for reprogramming and searching for the nongenetic factors facilitating it;

The search for systems allowing the elimination of the exogenous DNA from the host cell genome after the reprogramming;

The development of delivery protocols for nonintegrated genetic constructs;

The search for ways to reprogram somatic cells using recombinant proteins.

The ectopic expression of cMyc and Klf4 genes isthe most dangerous because of the high probability that malignant tumors will develop [22]. Hence the necessity to find other genes that couldsubstitute cMyc and Klf4 in iPSC production. Ithas been reported that these genes can be successfully substituted by Nanog and Lin28 for reprogramming human somatic cells [18;] . iPSCs were prepared from murine embryonic fibroblastsby the overexpression of Oct4 and Sox2 , as well as the Esrrb gene encoding the murine orphan nuclear receptor beta. It has alreadybeen shown that Esrrb , which acts as a transcription activator of Oct4 , Sox2 , and Nanog , is necessary for theselfrenewal and maintenance of the pluripotency of murine ESCs. Moreover, Esrrb can exert a positive control over Klf4 . Thus, the genes causingelevated carcinogenicity of both iPSCs and their derivatives can besuccessfully replaced with less dangerous ones [31].

The Most Effectively Reprogrammed Cell Lines . Murine and humaniPSCs can be obtained from fibroblasts using the factors Oct4, Sox2, and Klf4,but without cMyc . However, in this case, reprogramming deceleratesand an essential shortcoming of stable iPSC clones is observed [32, 33]. The reduction of a number ofnecessary factors without any decrease in efficiency is possible when iPSCsare produced from murine and human neural stem cells (NSCs) [12, 34, 35]. For instance, iPSCs were produced fromNSCs isolated from adult murine brain using two factors, Oct4 and Klf4, aswell as even Oct4 by itself [12, 34]. Later, human iPSCs were produced by the reprogramming offetal NSCs transduced with a retroviral vector only carrying Oct4 [35] . It is most likely that the irrelevanceof Sox2, Klf4, and cMyc is due to the high endogenous expression level of these genes inNSCs.

Successful reprogramming was also achieved in experiments withother cell lines, in particular, melanocytes of neuroectodermal genesis [36]. Both murine and human melanocytes are characterized by a considerableexpression level of the Sox2 gene, especially at early passages.iPSCs from murine and human melanocytes were produced without the use of Sox2or cMyc. However, the yield of iPSC clones produced from murine melanocytes was lower(0.03% without Sox2 and 0.02% without cMyc) in comparison with that achieved when allfour factors were applied to melanocytes (0.19%) and fibroblasts (0.056%). A decreasedefficiency without Sox2 or cMyc was observed in human melanocyte reprogramming (0.05%with all four factors and 0.01% without either Sox2 or cMyc ). All attempts to obtain stable iPSC clones in the absence of both Sox2 andcMyc were unsuccessful [36]. Thus, theminimization of the number of factors required for iPSC preparation can be achieved by choosingthe proper somatic cell type that most effectively undergoes reprogramming under the action offewer factors, for example, due to the endogenous expression of pluripotencygenes. However, if human iPSCs are necessary, these somatic cellsshould be easily accessible and wellcultured and their method of isolation should be asnoninvasive as possible.

One of these cell types can be adipose stem cells (ASCs). This is aheterogeneous group of multipotent cells which can be relatively easily isolated in largeamounts from adipose tissue following liposuction. Human iPSCs weresuccessfully produced from ASCs with a twofold reprogramming rate and20fold efficiency (0.2%), exceeding those of fibroblasts [37].

However, more accessible resources for the effective production of humaniPSCs are keratinocytes. When compared with fibroblasts, human iPSC productionfrom keratinocytes demonstrated a 100fold greater efficiency and a twofold higherreprogramming rate [38].

It has recently been found that the reprogramming of murine papillary dermal fibroblasts(PDFs) into iPSCs can be highly effective with theoverexpression of only two genes, Oct4 and Klf4 ,inserted into retroviral vectors [39;].PDFs are specialized cells of mesodermal genesis surrounding the stem cells ofhair follicles . One characteristic feature of these cells is the endogenous expression of Sox2 , Klf4 , and cMyc genes,as well as the geneencoding alkaline phosphatase, one of the murine and humanESC markers. PDFs can be easily separated from other celltypes by FACS (fluorescenceactivated cell sorting) using life staining with antibodiesagainst the surface antigens characteristic of one or another cell type. The PDF reprogrammingefficiency with the use of four factors (Oct4, Sox2, Klf4, and cMyc) retroviral vectorsis 1.38%, which is 1,000fold higher than the skin fibroblast reprogramming efficiency inthe same system. Reprogramming PDFs with two factors, Oct4 and Klf4 , yields 0.024%, which is comparable to the efficiency of skinfibroblast reprogramming using all four factors. The efficiency of PDF reprogramming iscomparable with that of NSCs, but PDF isolation is steady and far lessinvasive [39]. It seems likely that human PDF lines arealso usable, and this cell type may appear to be one of the most promising for human iPSCproduction in terms of pharmacological studies and cell replacement therapy. The use of suchcell types undergoing more effective reprogramming, together with methods providing thedelivery of pluripotency genes without the integration of foreign DNA into thehost genome and chemical compounds increasing the reprogramming efficiency and substitutingsome factors required for reprogramming, is particularly relevant.

Chemical Compounds Increasing Cell Reprogramming Efficiency. As was noted above,the minimization of the factors used for reprogramming decreases the efficiency of iPSCproduction. Nonetheless, several recent studies have shown that the use of genetic mechanisms,namely, the initiation of ectopic gene expression, can be substituted by chemical compounds,most of them operating at the epigenetic level. For instance, BIX01294 inhibitinghistone methyltransferase G9a allows murine fibroblast reprogramming using only two factors,Oct4 and Klf4, with a fivefold increased yield of iPSC clones in comparison with the controlexperiment without BIX01294 [40]. BIX01294taken in combination with another compound can increase the reprogramming efficiency even more.In particular, BIX01294 plus BayK8644 elevated the yield of iPCSs 15 times, andBIX01294 plus RG108 elevated it 30 times when only two reprogramming factors, Oct4 andKlf4, were used. RG108 is an inhibitor of DNA methyltransferases, and its role in reprogrammingis apparently in initiating the more rapid and effective demethylation of promoters ofpluripotent cellspecific genes, whereas BayK8644 is an antagonist of Ltypecalcium channels, and its role in reprogramming is not understood very well [40]. However, more considerable results were obtained inreprogramming murine NSCs. The use of BIX01294 allowed a 1.5foldincrease in iPSC production efficiency with two factors, Oct4 and Klf4, in comparison withreprogramming with all four factors. Moreover, BIX01294 can even substitute Oct4 in thereprogramming of NSCs, although the yield is very low [41]. Valproic (2propylvaleric) acid inhibiting histone deacetylases canalso substitute cMyc in reprogramming murine and human fibroblasts. Valproic acid (VPA)increases the reprogramming efficiency of murine fibroblasts 50 times, and human fibroblastsincreases it 1020 times when three factors are used [42, 43]. Other deacetylase inhibitors,such as TSA (trichostatin A) and SAHA (suberoylanilide hyroxamic acid), also increase thereprogramming efficiency. TSA increases the murine fibroblast reprogramming efficiency 15times, and SAHA doubles it when all four factors are used [42]. Besides epigenetic regulators, the substances inhibiting the proteincomponents of signaling pathways implicated in the differentiation of pluripotent cells arealso applicable in the substitution of reprogramming factors. In particular, inhibitors of MEKand GSK3 kinases (PD0325901 and CHIR99021, respectively) benefit the establishment of thecomplete and stable pluripotency of iPSCs produced from murineNSCs using two factors, Oct4 and Klf4 [41, 44].

It has recently been shown that antioxidants can considerably increase the efficiency ofsomatic cell reprogramming. Ascorbic acid (vitamin C) can essentially influence the efficiencyof iPSC production from various murine and human somatic cell types [45]. The transduction of murine embryonic fibroblasts (mEFs) with retrovirusescarrying the Oct4 , Sox2 , and Klf4 genes results in a significant increase in the production level of reactive oxygen species(ROS) compared with that of both control and Efs tranduced with Oct4 , Sox2 , cMyc , and Klf4 . Inturn, the increase in the ROS level causes accelerated aging and apoptosis of the cell, whichshould influence the efficiency of cell reprogramming. By testing several substances possessingantioxidant activity such as vitamin B1, sodium selenite, reduced glutathione, and ascorbicacid, the authors have found that combining these substances increases the yield ofGFPpositive cells in EF reprogramming (the Gfp genewas under the control of the Oct4 gene promoter). The use of individualsubstances has shown that only ascorbate possesses a pronounced capability to increase thelevel of GFPpositive cells, although other substances keep theirROSdecreasing ability. In all likelihood, this feature of ascorbates is not directlyassociated with its antioxidant activity [45]. The scoreof GFPpositive iPSC colonies expressing an alkaline phosphatase hasshown that the efficiency of iPSC production from mEFs with three factors (Oct4, Sox2, andKlf4) can reach 3.8% in the presence of ascorbate. When all four factors (Oct4, Sox2, Klf4, andcMyc) are used together with ascorbate, the efficiency of iPSC production may reach8.75%. A similar increase in the iPSC yield was also observed in the reprogramming of murinebreast fibroblasts; i.e., the effect of vitamin C is not limited by one cell type. Moreover,the effect of vitamin C on the reprogramming efficiency is more profound than that of thedeacetylase inhibitor valproic (2propylvaleric) acid. The mutual effect of ascorbate andvalproate is additive; i.e., these substances have different action mechanisms. Moreover,vitamin C facilitates the transition from preiPSCs to stablepluripotent cells. This feature is akin to the effects of PD0325901 and CHIR99021, which areinhibitors of MEK and GSK3 kinases, respectively. This effect of vitamin C expands to humancells as well [45]. Following the transduction of humanfibroblasts with retroviruses carrying Oct4 , Sox2 , Klf4 , and cMyc and treatment with ascorbate, theauthors prepared iPSCs with efficiencies reaching 6.2%. The reprogrammingefficiency of ASCs under the same conditions reached 7.06%. The mechanism ofthe effect that vitamin C has on the reprogramming efficiency is not known in detail.Nevertheless, the acceleration of cell proliferation was observed at the transitional stage ofreprogramming. The levels of the p53 and p21 proteins decreased in cells treated withascorbate, whereas the DNA repair machinery worked properly [45]. It is interesting that an essential decrease in the efficiency of iPSCproduction has been shown under the action of processes initiated by p53 and p21 [4650].

As was mentioned above, for murine and human iPSC production, both retro andlentiviruses were initially used as delivery vectors for the genes required for cellreprogramming. The main drawback of this method is the uncontrolled integration of viral DNAinto the host cells genome. Several research groups have introduced methods fordelivering pluripotency genes into the recipient cell which either do notintegrate allogenic DNA into the host genome or eliminate exogenous genetic constructs from thegenome.

CreloxP Mediated Recombination. To prepareiPSCs from patients with Parkinsons disease, lentiviruses were used,the proviruses of which can be removed from the genome by Cre recombinase. To do this, the loxP site was introduced into thelentiviral 3LTRregions containing separate reprogramming genesunder the control of the doxycyclineinducible promoter. During viral replication, loxP was duplicated in the 5LTR of the vector. As aresult, the provirus integrated into the genome was flanked with two loxP sites. The inserts were eliminated using the temporary transfection ofiPSCs with a vector expressing Cre recombinase[51].

In another study, murine iPSCs were produced using a plasmid carrying the Oct4 , Sox2 , Klf4I, and cMyc genes in the same reading frame in which individual cDNAs were separatedby sequences encoding 2 peptides, and practically the whole construct was flanked with loxP sites [52]. The use ofthis vector allowed a notable decrease in the number of exogenous DNA inserts in the hostcells genome and, hence, the simplification of their following excision [52]. It has been shown using lentiviruses carrying similarpolycistronic constructs that one copy of transgene providing a high expression level of theexogenous factors Oct4, Sox2, Klf4, and cMyc is sufficient for the reprogramming ofdifferentiated cells into the pluripotent state [53,54].

The drawback of the CreloxP system is the incomplete excisionof integrated sequences; at least the loxP site remains in thegenome, so the risk of insertion mutations remains.

Plasmid Vectors . The application of lentiviruses and plasmids carrying the loxP sites required for the elimination of transgene constructsmodifies, although insignificantly, the host cells genome. One way to avoid this is touse vector systems that generally do not provide for the integration of the whole vector orparts of it into the cells genome. One such system providing a temporary transfectionwith polycistronic plasmid vectors was used for iPSC production from mEFs [29]. A polycistronic plasmid carrying the Oct4 , Sox2 , and Klf4 gene cDNAs, as well as aplasmid expressing cMyc , was transfected into mEFs one, three, five,and seven days after their primary seeding. Fibroblasts were passaged on the ninth day, and theiPSC colonies were selected on the 25th day. Seven out of ten experiments succeeded inproducing GFPpositive colonies (the Gfp gene wasunder the control of the Nanog gene promoter). The iPSCsthat were obtained were similar in their features to murine ESCs and did not contain inserts ofthe used DNA constructs in their genomes. Therefore, it was shown that wholesome murineiPSCs that do not carry transgenes can be reproducibly produced, and that thetemporary overexpression of Oct4 , Sox2 , Klf4 , and cMyc is sufficient for reprogramming. The maindrawback of this method is its low yield. In ten experiments the yield varied from 1 to 29 iPSCcolonies per ten million fibroblasts, whereas up to 1,000 colonies per ten millions wereobtained in the same study using retroviral constructs [29].

Episomal Vectors . Human iPSCs were successfully produced fromskin fibroblasts using single transfection with polycistronic episomal constructs carryingvarious combinations of Oct4 , Sox2 , Nanog , Klf4 , cMyc , Lin28 , and SV40LT genes. These constructs were designed on the basis of theoriP/EBNA1 (EpsteinBarr nuclear antigen1) vector [55]. The oriP/EBNA1 vector contains the IRES2 linker sequence allowing theexpression of several individual cDNAs (encoding the genes required for successfulreprogramming in this case) into one polycistronic mRNA from which several proteins aretranslated. The oriP/EBNA1 vector is also characterized by lowcopy representation in thecells of primates and can be replicated once per cell cycle (hence, it is not rapidlyeliminated, the way common plasmids are). Under nonselective conditions, the plasmid iseliminated at a rate of about 5% per cell cycle [56]. Inthis work, the broad spectrum of the reprogramming factor combinations was tested, resulting inthe best reprogramming efficiency with cotransfection with three episomes containing thefollowing gene sets: Oct4 + Sox2 + Nanog + Klf4 , Oct4 + Sox2 + SV40LT + Klf4 , and cMyc + Lin28 . SV40LT ( SV40 large T gene )neutralizes the possible toxic effect of overexpression [57]. The authors have shown thatwholesome iPSCs possessing all features of pluripotent cells can be producedfollowing the temporary expression of a certain gene combination in human somatic cells withoutthe integration of episomal DNA into the genome. However, as in the case when plasmid vectorsare being used, this way of reprogramming is characterized by low efficiency. In separateexperiments the authors obtained from 3 to 6 stable iPSC colonies per 106transfected fibroblasts [55]. Despite the fact that skinfibroblasts are wellcultured and accessible, the search for other cell types which arerelatively better cultured and more effectively subject themselves to reprogramming throughthis method is very likely required. Another drawback of the given system is that this type ofepisome is unequally maintained in different cell types.

PiggyBacTransposition . One promising system used foriPSC production without any modification of the host genome is based on DNA transposons.Socalled PiggyBac transposons containing2linkered reprogramming genes localized between the 5 and3terminal repeats were used for iPSC production from fibroblasts. The integrationof the given constructs into the genome occurs due to mutual transfection with a plasmidencoding transposase. Following reprogramming due to the temporary expression of transposase,the elimination of inserts from the genome took place [58, 59]. One advantage of the PiggyBac system on CreloxP is that the exogenous DNA iscompletely removed [60].

However, despite the relatively high efficiency of exogenous DNA excision from the genome by PiggyBac transposition, the removal of a large number of transposoncopies is hardly achievable.

Nonintegrating Viral Vectors . Murine iPSCs were successfullyproduced from hepatocytes and fibroblasts using four adenoviral vectors nonintegrating into thegenome and carrying the Oct4 , Sox2 , Klf4 , and cMyc genes. An analysis of the obtainediPSCs has shown that they are similar to murine ESCs in their properties(teratoma formation, gene promoter DNA methylation, and the expression of pluripotent markers),but they do not carry insertions of viral DNA in their genomes [61]. Later, human fibroblastderived iPSCs wereproduced using this method [62].

The authors of this paper cited the postulate that the use of adenoviral vectors allows theproduction of iPSCs, which are suitable for use without the risk of viral oroncogenic activity. Its very low yield (0.00010.001%), the deceleration ofreprogramming, and the probability of tetraploid cell formation are the drawbacks of themethod. Not all cell types are equally sensitive to transduction with adenoviruses.

Another method of gene delivery based on viral vectors was recently employed for theproduction of human iPSCs. The sendaivirus (SeV)based vector wasused in this case [63]. SeV is a singlestrandedRNA virus which does not modify the genome of recipient cells; it seems to be a good vector forthe expression of reprogramming factors. Vectors containing either all pluripotencyfactors or three of them (without ) were used for reprogramming the human fibroblast. The construct based on SeV is eliminatedlater in the course of cell proliferation. It is possible to remove cells with the integratedprovirus via negative selection against the surface HN antigen exposed on the infected cells.The authors postulate that reprogramming technology based on SeV will enable the production ofclinically applicable human iPSCs [63].

Cell Transduction with Recombinant Proteins . Although the methods for iPSCproduction without gene modification of the cells genome (adenoviral vectors, plasmidgene transfer, etc.) are elaborated, the theoretical possibility for exogenous DNA integrationinto the host cells genome still exists. The mutagenic potential of the substances usedpresently for enhancing iPSC production efficiency has not been studied in detail. Fullychecking iPSC genomes for exogenous DNA inserts and other mutations is a difficult task, whichbecomes impossible to solve in bulk culturing of multiple lines. The use of protein factorsdelivered into a differentiated cell instead of exogenous DNA may solve this problem. Tworeports have been published to date in which murine and human iPSCs wereproduced using the recombinant Oct4, Sox2, Klf4, and cMyc proteins [64, 65] . T he methodused to deliver the protein into the cell is based on the ability of peptides enriched withbasic residues (such as arginine and lysine) to penetrate the cells membrane. MurineiPSCs were produced using the recombinant Oct4, Sox2, Klf4, and cMycproteins containing eleven Cterminal arginine residues and expressed in E. coli . The authors succeeded in producing murine iPSCs during four roundsof protein transduction into embryonic fibroblasts [65].However, iPSCs were only produced when the cells were additionally treatedwith 2propylvalerate (the deacetylase inhibitor). The same principle was used for theproduction of human iPSCs, but protein expression was carried out in humanHEK293 cells, and the proteins were expressed with a fragment of nine arginins at the proteinCend. Researchers have succeeded in producing human iPSCs after sixtransduction rounds without any additional treatment [64]. The efficiency of producing human iPSC in this way was 0.001%, which isone order lower than the reprogramming efficiency with retroviruses. Despite some drawbacks,this method is very promising for the production of patientspecificiPSCs.

The first lines of human pluripotent ESCs were produced in 1998 [6]. In line with the obvious fundamental importance of embryonic stem cellstudies with regard to the multiple processes taking place in early embryogenesis, much of theinterest of investigators is associated with the possibility of using ESCs and theirderivatives as models for the pathogenesis of human diseases, new drugs testing, and cellreplacement therapy. Substantial progress is being achieved in studies on directed humanESC differentiation and the possibility of using them to correct degenerativedisorders. Functional cell types, such as motor dopaminergic neurons, cardiomyocytes, andhematopoietic cell progenitors, can be produced as a result of ESCdifferentiation. These cell derivatives, judging from their biochemical and physiologicalproperties, are potentially applicable for the therapy of cardiovascular disorders, nervoussystem diseases, and human hematological disorders [66].Moreover, derivatives produced from ESCs have been successfully used for treating diseasesmodeled on animals. Therefore, bloodcell progenitors produced from ESCs weresuccessfully used for correcting immune deficiency in mice. Visual functions were restored inblind mice using photoreceptors produced from human ESCs, and the normal functioning of thenervous system was restored in rats modeling Parkinsons disease using the dopaminergicneurons produced from human ESCs [6770]. Despite obvious success, the fullscale applicationof ESCs in therapy and the modeling of disorders still carry difficulties, because of thenecessity to create ESC banks corresponding to all HLAhaplotypes, whichis practically unrealistic and hindered by technical and ethical problems.

Induced pluripotent stem cells can become an alternative for ESCs in the area of clinicalapplication of cell replacement therapy and screening for new pharmaceuticals.iPSCs closely resemble ESCs and, at the same time, can be produced in almostunlimited amounts from the differentiated cells of each patient. Despite the fact that thefirst iPSCs were produced relatively recently, work on directed iPSCdifferentiation and the production of patientspecific iPSCs isintensive, and progress in this field is obvious.

Dopamine and motor neurons were produced from human iPSCs by directeddifferentiation in vitro [71, 72]. These types of neurons are damaged in many inherited oracquired human diseases, such as spinal cord injury, Parkinsons disease, spinal muscularatrophy, and amyotrophic lateral sclerosis. Some investigators have succeeded in producingvarious retinal cells from murine and human iPSCs [7375]. HumaniPSCs have been shown to be spontaneously differentiated in vitro into the cells of retinal pigment epithelium [76]. Another group of investigators has demonstrated that treating human andmurine iPSCs with Wnt and Nodal antagonists in a suspended culture induces theappearance of markers of cell progenitors and pigment epithelium cells. Further treating thecells with retinoic acid and taurine activates the appearance of cells expressing photoreceptormarkers [75].

Several research groups have produced functional cardiomyocytes (CMs) in vitro from murine and human iPSCs [7781]. Cardiomyocytes producedfrom iPSC are very similar in characteristics (morphology, marker expression,electrophysiological features, and sensitivity to chemicals) to the CMs ofcardiac muscle and to CMs produced from differentiated ESCs. Moreover, murineiPSCs, when injected, can repair muscle and endothelial cardiac tissuesdamaged by cardiac infarction [77].

Hepatocytelike cell derivatives, dendritic cells, macrophages, insulinproducingcell clusters similar to the duodenal islets of Langerhans, and hematopoietic and endothelialcells are currently produced from murine and human iPSCs, in addition to thealreadylisted types of differentiated cells [8285].

In addition to directed differentiation in vitro , investigators apply mucheffort at producing patientspecific iPSCs. The availability ofpluripotent cells from individual patients makes it possible to study pathogenesis and carryout experiments on the therapy of inherited diseases, the development of which is associatedwith distinct cell types that are hard to obtain by biopsy: so the use ofiPSCs provides almost an unlimited resource for these investigations.Recently, the possibility of treating diseases using iPSCs was successfullydemonstrated, and the design of the experiment is presented in the figure. A mutant allele wassubstituted with a normal allele via homologous recombination in murine fibroblastsrepresenting a model of human sickle cell anemia. iPSCs were produced fromrepaired fibroblasts and then differentiated into hematopoietic cell precursors.The hematopoietic precursors were then injected into a mouse from which the skin fibroblastswere initially isolated (Fig. 1). As a result, the initialpathological phenotype was substantially corrected [86].A similar approach was applied to the fibroblasts and keratinocytes of a patient withFanconis anemia. The normal allele of the mutant gene producing anemia was introducedinto a somatic cell genome using a lentivirus, and then iPSCs were obtainedfrom these cells. iPSCs carrying the normal allele were differentiated intohematopoietic cells maintaining a normal phenotype [87].The use of lentiviruses is unambiguously impossible when producing cells to be introduced intothe human body due to their oncogenic potential. However, new relatively safe methods of genomemanipulation are currently being developed; for instance, the use of synthetic nucleasescontaining zinc finger domains allowing the effective correction of genetic defects invitro [88].

Design of an experiment on repairing the mutant phenotype in mice modeling sickle cell anemia development [2]. Fibroblasts isolatedfrom the tail of a mouse (1) carrying a mutant allele of the gene encoding the human hemoglobin -chain (hs) were used for iPSCproduction (2). The mutation was then repaired in iPSCs by means of homological recombination (3) followed by cell differentiationvia the embryoid body formation (4). The directed differentiation of the embryoid body cells led to hematopoietic precursor cells (5)that were subsequently introduced into a mouse exposed to ionizing radiation (6).

The induced pluripotent stem cells are an excellent model for pathogenetic studies at the celllevel and testing compounds possessing a possible therapeutic effect.

The induced pluripotent stem cells were produced from the fibroblasts of a patient with spinalmuscular atrophy (SMA) (SMAiPSCs). SMA is an autosomalrecessive disease caused by a mutation in the SMN1 ( survival motorneuron 1 ) gene, which is manifested as the selective nonviability of lower motor neurons. Patients with this disorder usually die at the age of about two years.Existing experimental models of this disorder based on the use of flatworms, drosophila, andmice are not satisfactory. The available fibroblast lines from patients withSMA cannot provide the necessary data on the pathogenesis of this disordereither. It was shown that motor neurons produced from SMAiPSCs canretain the features of SMA development, selective neuronal death, and the lackof SMN1 transcription. Moreover, the authors succeeded in elevating the SMNprotein level and aggregation (encoded by the SMN2 gene, whose expressioncan compensate for the shortage in the SMN1 protein) in response to the treatment of motorneurons and astrocytes produced from SMAiPSCs with valproate andtorbomycin [89;]. iPSCs and theirderivatives can serve as objects for pharmacological studies, as has been demonstrated oniPSCs from patients with familial dysautonomia (FDA) [90]. FDA is an inherited autosomal recessive disorder manifested as thedegeneration of sensor and autonomous neurons. This is due to a mutation causing thetissuespecific splicing of the IKBKAP gene, resulting in a decreasein the level of the fulllength IKAP protein. iPSCs were produced fromfibroblasts of patients with FDA. They possessed all features of pluripotent cells. Neuralderivatives produced from these cells had signs of FDA pathogenesis and low levels of thefulllength IKBKAP transcript. The authors studied the effect of threesubstances, kinetin, epigallocatechin gallate, and tocotrienol, on the parameters associatedwith FDA pathogenesis. Only kinetin has been shown to induce an increase in the level offulllength IKBKAP transcript. Prolonged treatment with kinetininduces an increase in the level of neuronal differentiation and expression of peripheralneuronal markers.

Currently, a broad spectrum of iPSCs is produced from patients with variousinherited pathologies and multifactorial disorders, such as Parkinsons disease, Downsyndrome, type 1 diabetes, Duchenne muscular dystrophy, talassemia, etc., whichare often lethal and can scarcely be treated with routine therapy [51, 87, 89, 9194]. The data on iPSCs produced by reprogramming somaticcells from patients with various pathologies are given in the Table 1.

Functional categories of M. tuberculosis genes with changed expression level during transition to the NC state

One can confidently state that both iPSCs themselves and their derivativesare potent instruments applicable in biomedicine, cell replacement therapy, pharmacology, andtoxicology. However, the safe application of iPSCbased technologies requires the use ofmethods of iPSCs production and their directed differentiation which minimizeboth the possibility of mutations in cell genomes under in vitro culturingand the probability of malignant transformation of the injected cells. The development ofmethods for human iPSC culturing without the use of animal cells (for instance, the feederlayer of murine fibroblasts) is necessary; they make a viralorigin pathogen transferfrom animals to humans impossible. There is a need for the maximum standardization ofconditions for cell culturing and differentiation.

This study was supported by the Russian Academy of Sciences Presidium ProgramMolecular and Cell Biology.

embryonic stem cells

induced pluripotent stem cells

neural stem cells

adipose stem cells

papillary dermal fibroblasts

cardiomyocytes

spinal muscular atrophy

iPCSs derived from fibroblasts of SMA patients

green fluorescent protein

long terminal repeat

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