The very properties that make pluripotent stem cells revolutionary also hide significant risks that researchers are working to overcome.
Imagine a future where failing hearts are rebuilt with new muscle, paralyzed nerves are reconnected, and damaged organs are regenerated. This is the promise of human pluripotent stem cells (hPSCs)—master cells that can become any cell type in the body. Yet, behind this extraordinary potential lie two formidable challenges: immunogenicity (the ability to provoke immune attacks) and tumorigenicity (the potential to form tumors). This article explores how scientists are confronting these hurdles to unlock the full therapeutic potential of stem cells.
Human pluripotent stem cells come in two main forms with distinct characteristics and challenges.
Derived from early embryos, ESCs were the first type of human pluripotent stem cells discovered. They offer tremendous differentiation potential but face ethical concerns regarding their source.
Created by reprogramming adult cells back to an embryonic-like state, iPSCs were discovered in 2006 and offered a potentially unlimited source of patient-matched cells 1 9 .
Immunogenicity refers to how likely a transplanted cell is to be recognized as "foreign" and attacked by the recipient's immune system. The primary triggers are Major Histocompatibility Complex (MHC) molecules—proteins on cell surfaces that help the immune system distinguish between self and non-self 1 .
MHC expression in undifferentiated hPSCs
MHC expression during differentiation
Immunogenicity of autologous iPSCs 6
Undifferentiated hPSCs have a unique immune profile: they express low levels of MHC class I molecules and no MHC class II molecules or co-stimulatory proteins, potentially giving them immune-privileged status 4 . However, this changes dramatically once they differentiate:
During differentiation, making cells more visible to immune cells 4
Controls these changes through DNA methylation and histone modifications 4
Can create immunogenic proteins not typically present in adult tissues 1
In a landmark 2011 study published in Nature, Zhao et al. made a surprising discovery that challenged conventional thinking 6 . When they transplanted iPSCs derived from inbred C57BL/6 mice back into the same strain of mice, the cells were frequently rejected—despite being genetically matched.
| Cell Type | MHC Class I Expression | MHC Class II Expression | Risk of Immune Rejection |
|---|---|---|---|
| Undifferentiated hPSCs | Low | Absent |
Lower (but high tumor risk)
|
| Differentiated hPSC-derivatives | Variable (often increased) | Absent (unless professional APCs) |
Variable
|
| Autologous iPSC-derivatives | Patient-matched | Absent |
Low to Moderate (due to abnormal proteins)
|
| Allogeneic hPSC-derivatives | Donor-specific | Absent |
High
|
The ability to form tumors—particularly teratomas—represents one of the most significant safety concerns in hPSC-based therapies. These benign tumors contain a chaotic mix of tissues representing all three embryonic germ layers and can form when even a few undifferentiated pluripotent cells remain in a therapeutic product 2 .
hPSCs share several disturbing similarities with cancer cells 9 :
The reprogramming factors used to create iPSCs further complicate this picture. The original Yamanaka factors include c-Myc, a well-known oncogene that significantly increases tumor risk in iPSC-derived tissues 9 .
Prolonged culturing of hPSCs introduces additional risks. Over time, cells with genetic advantages—particularly those with mutations in cancer-related genes like TP53—can dominate the culture 2 . Common chromosomal abnormalities occur in chromosomes 1, 12, 17, 20, and X, mirroring aberrations found in human germ cell tumors 2 .
| Genetic Abnormality | Frequency | Associated Genes | Potential Risk |
|---|---|---|---|
| Chromosome 12p gains | Common | NANOG | Increased pluripotency, tumorigenic aggressiveness |
| Chromosome 17q gains | Common | BIRC5 (SURVIVIN) | Anti-apoptotic, enhanced survival |
| Chromosome 20 gains | Common | BCL2L1 | Anti-apoptotic, enhanced survival |
| TP53 mutations | 30% of later passage lines | TP53 | Loss of tumor suppressor function |
| Copy Number Variations (CNVs) | Frequent | Multiple | Genomic instability |
To understand how scientists discovered the immunogenicity of iPSCs, let's examine the groundbreaking 2011 study that first challenged the assumption of immune tolerance for autologous cells.
Zhao and colleagues designed an elegant series of experiments using mouse models 6 :
They generated iPSCs from mouse embryonic fibroblasts using both retroviral (ViPSCs) and episomal (EiPSCs) approaches, alongside embryonic stem cells from two mouse strains (C57BL/6 and 129/SvJ)
These cells were transplanted into syngeneic C57BL/6 mice to observe teratoma formation
Researchers tracked teratoma growth, regression, and immune cell infiltration
They performed global gene expression profiling of teratomas to identify differentially expressed genes
The results were striking 6 :
As expected, syngeneic ESCs efficiently formed teratomas without rejection, while allogeneic ESCs were rapidly rejected
In contrast, both retroviral and episomal iPSCs showed significant immunogenicity in syngeneic recipients, with T-cell infiltration and teratoma regression
The researchers identified multiple abnormally expressed genes in iPSC-derived teratomas, and demonstrated that several of these gene products directly contributed to immunogenicity
This study provided crucial evidence that the reprogramming process itself could introduce immunogenic elements, meaning that patient-specific iPSC therapies might not be automatically immune-privileged 6 .
| Transplant Scenario | Cell Type | Teratoma Formation | Immune Response |
|---|---|---|---|
| Syngeneic (B6→B6) | B6 ESCs | Efficient, no rejection | None detected |
| Allogeneic (129/SvJ→B6) | 129/SvJ ESCs | Failed, rapid rejection | Severe T-cell response |
| Syngeneic (B6→B6) | B6 ViPSCs | Mostly rejected | T-cell infiltration |
| Syngeneic (B6→B6) | B6 EiPSCs | Mostly immunogenic | T-cell infiltration, tissue damage |
Advancing hPSC research while managing immunogenicity and tumorigenicity risks requires specialized tools and reagents.
While immunogenicity and tumorigenicity present significant hurdles, researchers have developed multiple innovative approaches to address them.
Using CRISPR/Cas9 to delete MHC molecules and express immunomodulatory proteins like PD-L1 to create "universal donor" stem cells 3
Choosing inherently less immunogenic cell types like retinal pigment epithelial cells, which have demonstrated success in clinical trials 1
Using localized rather than systemic immunosuppression to minimize side effects 4
Methods like fluorescence-activated cell sorting using surface markers to remove undifferentiated cells from therapeutic products 9
Engineering hPSCs with inducible "suicide genes" like herpes simplex virus thymidine kinase or inducible caspase-9 that can eliminate transplanted cells if tumors form 3
Using compounds that selectively target undifferentiated cells, such as BIRC5 inhibitors that specifically induce apoptosis in hPSCs 2
As research advances, the landscape of hPSC-based therapies continues to evolve. Recent clinical trials show promising results—patients with Parkinson's disease and macular degeneration have shown improvement after receiving autologous iPSC-derived cells without significant immune rejection or tumor formation 9 . Ongoing trials are exploring hPSC-derived cardiomyocytes for heart failure and pancreatic cells for diabetes 1 3 .
iPSC-derived retinal pigment epithelial cells showing promise in clinical trials
Dopaminergic neurons derived from iPSCs demonstrating therapeutic potential
hPSC-derived cardiomyocytes being tested for myocardial regeneration
Pancreatic beta cells from hPSCs in development for insulin production
The future likely lies in combination approaches that address both immunogenicity and tumorigenicity simultaneously. This might include using hypoimmunogenic engineered cells with built-in suicide switches, along with improved purification methods to eliminate undifferentiated cells 3 .
CRISPR-based modification to create universal donor cells with reduced immunogenicity
Novel methods to ensure complete removal of undifferentiated cells before transplantation
Inducible suicide genes as fail-safe mechanisms against tumor formation
Patient-specific therapies with optimized immune matching and safety profiles
The journey from laboratory discovery to widespread clinical application remains challenging, but with continued scientific innovation and careful attention to safety, the remarkable potential of pluripotent stem cells may yet revolutionize medicine as we know it.