Stem cell therapy has gained significant attention and made great progress both preclinically and clinically. However, several major challenges must be addressed in clinical stem cell research. These challenges include: (i) viable sources of stem cells, (ii) safety and immunogenicity, (iii) functional heterogeneity, (iv) ensuring post-transplant bioactivity and viability, and (v) targeted delivery and migration capabilities.
The source of stem cells determines the obtainable number of MSCs, their proliferative capacity, and differentiation potential! Currently, the main clinical sources of MSC therapy are bone marrow, adipose tissue, and umbilical cord. Each source has its own advantages and disadvantages.
Bone marrow is the primary source for collecting MSCs but requires invasive surgery, causing pain and infection risk to the donor. Moreover, only about 0.001%-0.01% of mononuclear cells isolated from bone marrow are MSCs, and their proliferative capacity is limited.
Therefore, bone marrow-derived MSCs are gradually being replaced.
Adipose tissue is an alternative MSC source, which can be isolated through minimally invasive procedures and yields approximately 500 times more MSCs than bone marrow. Despite this advantage, adipose-derived MSCs tend to differentiate into adipose tissue, limiting their clinical application.
MSC derived from umbilical cord blood accounts for only 0.2%-1.8% of isolated mononuclear cells but has better proliferation capacity than bone marrow MSCs and can differentiate into bone, cartilage, and adipose tissue. However, the limited volume of cord blood means multiple donors may be required for one treatment.
Age is a major consideration when isolating autologous MSCs, as aging relates to MSC senescence and reduced proliferative and differentiation capacity.
Donor characteristics such as gender, tissue source, and disease status in allogeneic MSCs also affect quality and therapeutic outcomes.
Additionally, different clinical MSC isolation methods vary, making standardization difficult. To address inconsistency from various sources and isolation protocols, the International Society for Cellular Therapy (ISCT) established minimum criteria for human MSCs in 2006.
For hematopoietic stem cells (HSCs), isolation can be from cord blood, bone marrow, and peripheral blood.
Ex vivo expansion of isolated HSCs is critical to generate sufficient therapeutic cells. Challenges include limited growth potential, easy differentiation during culture, and stemness loss over time.
Induced pluripotent stem cells (iPSCs) can originate from various tissues, mainly skin fibroblasts and blood cells. Autologous iPSCs have low immune response, but due to time and cost, allogeneic sources are primarily used. Like other autologous stem cell therapies, iPSC production is time-consuming, creating long wait times for acute indications like heart failure.
Ethically, while iPSCs solve issues related to embryonic stem cells by reprogramming somatic cells into any needed cell type, their unlimited differentiation potential introduces ethical dilemmas, such as reproductive cloning, embryo gene editing, human gamete production, and interspecies chimeras.
Risks related to immunogenicity present a significant challenge to stem cell therapies. MSCs are unique in their low immunogenicity due to low expression of MHC/HLA class I and absence of MHC/HLA class II.
However, allogeneic MSCs are not fully immune privileged. MHC-I mismatch between donor and recipient, especially with increased donor age, can increase immune rejection. Autologous MSCs avoid MHC mismatch but may carry genetic defects from the patient, impairing therapeutic efficacy.
MSC intravenous administration poses a risk of instant blood-mediated inflammatory reaction (IBMIR), an innate immune attack that negatively affects cell engraftment, viability, and overall therapy.
HSCs are more prone to immune rejection, potentially causing graft-versus-host disease (GvHD), where the recipient’s immune system attacks donor cells or donor cells attack recipient organs. Therefore, recipients typically undergo whole-body irradiation or chemotherapy before HSC infusion to reduce GvHD risk and create space in the bone marrow for donor HSC engraftment.
These preconditioning regimens are non-specific and can cause complications affecting various organ functions, require transfusions, secondary tumors, infertility, and infections due to long-term immunosuppression. Less than 25% of patients tolerate these well, and they may be fatal.
Gene therapy offers a promising solution by enabling in situ editing of HSCs, avoiding risks of preconditioning and allogeneic transplantation.
The risk of teratoma formation is the most significant safety concern for clinical iPSC therapy. While unlimited proliferation of iPSCs favors therapeutic applications, differentiation into unwanted cell types carries high tumorigenic potential.
To reduce this risk, robust and efficient directed differentiation methods must be established to minimize undifferentiated cells in final products. Strict purification processes are also necessary to eliminate contaminating cells.
Functional heterogeneity of stem cells arises from various factors, including tissue source, donor characteristics, and intercellular differences within tissues.
MSC characteristics vary greatly depending on source and donor age, gender, disease state, and genetic background.
These differences affect MSC proliferation and differentiation abilities, leading to variable therapeutic outcomes.
Umbilical cord blood-derived MSCs exhibit stronger immunomodulatory capacity, bone marrow MSCs are more effective in promoting regeneration, and adipose-derived MSCs show greater adipogenic differentiation and produce more extracellular matrix components.
MSC cytokine secretion is another critical property that helps regulate tissue repair and overall immune responses, varying by source and donor. For example, adult adipose-derived MSCs secrete higher levels of pro-inflammatory factors than young adipose MSCs, reducing their immunomodulatory capacity.
Donor characteristics play a key role: MSCs from male donors show higher metabolic activity and proliferation, while those from female donors have stronger osteogenic responses. Donor health status also impacts MSC function; MSCs from late-stage osteoarthritis patients show reduced proliferation and diminished chondrogenic and adipogenic differentiation compared to healthy donors.
Age is another key factor, with increasing donor age associated with decreased MSC proliferation and differentiation capacity.
HSC functional heterogeneity may stem from differences in genetic and epigenetic reprogramming during development and maturation, variable positioning within the bone marrow niche, and molecular and cellular stimulus-driven genetic changes.
Aging also plays an important role, with increased myeloid and decreased lymphoid output as cells age.
Post-transplant HSC engraftment efficiency varies by donor age and stem cell source. Studies show donors over 40 are linked to increased acute and chronic GvHD risk and higher transplant-related mortality. Male recipients of female donor HSCs may experience higher non-relapse mortality and GvHD rates.
iPSCs generally show greater heterogeneity than other stem cells because their multi-step reprogramming process can introduce variations.
Additionally, individual genetic differences and reprogramming-associated changes can alter iPSC phenotypes such as differentiation capacity, morphology, and epigenetic profiles.
Maintaining cell viability and function after transplantation is critical, yet presents numerous challenges in stem cell therapies.
Intravenous (IV) injection of MSCs is the most common clinical delivery method, but many studies have shown that MSCs are often trapped in the pulmonary capillaries, surviving for only a few days.
Recent research indicates that intramuscular injection can extend in vivo survival time to over five months, while intraperitoneal or subcutaneous injections result in survival times of 3–4 weeks.
Moreover, many MSCs fail to survive after encountering harsh local microenvironments at disease sites following reinfusion.
The post-transplant viability of MSCs is also affected by cryopreservation and thawing protocols. Studies show that overnight culturing of thawed cells prior to injection can significantly enhance MSC survival.
Successful HSC engraftment and function depend on multiple factors, including conditioning regimens, cell source, donor and recipient age and disease status, and concurrent immunosuppressive medications.
The intensity of the conditioning regimen greatly influences the engraftment rate of HSCs in the bone marrow. Within the bone marrow niche, interactions between HSCs and neighboring non-stem cells are essential to maintaining the balance between quiescence and self-renewal, thus preserving the HSC pool.
For iPSCs, optimizing their differentiation into the desired cell type is key to achieving therapeutic efficacy after delivery. For example, in preclinical studies of iPSC-derived cardiomyocytes, engraftment efficiency depends on the duration of the differentiation process, with 20 days being the optimal time. Therefore, the manufacturing process should be optimized to reduce off-target effects post-delivery and enhance therapeutic outcomes.
The ability of transplanted stem cells to reach the required site in the body is vital for therapeutic effectiveness.
As noted earlier, many MSCs become trapped in lung capillaries, limiting the success of systemic delivery via IV injection, and IBMIR further impairs their homing ability.
Only a small percentage—typically in the low single digits—of intravenously administered MSCs reach their target site. As a result, most late-stage clinical trials for MSC therapies rely on local administration, such as intrathecal, endocardial, or direct lesion injections.
However, local delivery is not always feasible, as many indications require more invasive procedures. To improve MSC homing, several technologies are under investigation, including increased CXCR4 expression, magnetic guidance, cell surface engineering, and ex vivo priming.
HSCs are primarily delivered to the bone marrow to reconstruct its microenvironment. Yet, only about 10% of HSCs successfully engraft in the bone marrow. The source of HSCs also affects engraftment rates—umbilical cord blood HSCs have more delayed engraftment compared to those from bone marrow or peripheral blood.
Intra-bone delivery is being explored as an alternative to standard IV administration to directly place HSCs into the marrow and reduce GvHD risk. However, this method can disrupt the bone marrow niche, limiting its effectiveness.
iPSC-based therapies face significant delivery challenges in solid tissues. Systemic infusion of iPSCs is often insufficient to establish grafts in target organs and may lead to off-target implantation. To address these issues, bioengineered scaffolds are being developed to support the direct implantation of iPSCs into target sites. However, these methods still require invasive surgeries, which limits their broader applicability.
