Induced pluripotent stem cells (iPSCs) are made from mature, adult cells by a combination of genetic engineering and cell culture techniques. Master genes are transfected into mature cells, which are then cultured as they grow and revert to more immature states. Eventually, a population of cells grow in culture that have some, though not all of the characteristics, of embryonic stem cells. Because these cells are pluripotent, they should, theoretically have the ability to differentiate into any adult cell type. Also, since they are derived from a patient’s own cells, they should be tolerated by the patient’s immune system and should not experience tissue rejection.I
Or should they? Experiments with cells derived from iPSCs have generated mixed results. If C57BL/6 (B6) mice are transplanted with iPSC-derived cells, such cells show some levels of recognition by the immune system. However, another study has concluded that various lineages of B6 iPSC-derived cells are not recognized by the immune system when transplanted under the kidney capsule of B6 mice. Why the contradiction?
Yang Xu and his colleagues at the University of California, San Diego have attempted to resolve this controversy by utilizing a mouse model system. Xu and his colleagues used the same B6 transplantation model and transplanted a variety of different cells derived from iPSCs that were made from cells that came from the same laboratory mice.
Xu and others showed that iPSC-derived and embryonic stem cell (ESC)-derived cells are either tolerated or rejected, depending upon WHERE they are transplanted. You see the immune system depends upon a network of cells called “dendritic cells” to sample the fluids that circulate throughout the body and identify foreign substances. Some locations in our bodies are chock-full of dendritic cells, while other locations have a paucity of dendritic cells. When iPSC or ESC-derived cells are transplanted under the kidney capsule, they survive and thrive. The kidney capsule has a distinct lack of dendritic cells. However, if these same cells, which were so nicely tolerated under the kidney capsule, are transplanted under the skin or injected into muscles, they were rejected by the immune system. Why? These two sites are loaded with dendritic cells.
Therefore, the rejection of iPSC-derived cells by the patient’s body is more of a function of where the cells are transplanted than the cells themselves. Mind you, poor quality iPSCs can produce derivatives that are rejected by the immune system, but high-quality iPSCs can differentiate into cells that are accepted by the immune system, but it is wholly dependent on where they are transplanted.
Perhaps, transplanted IPSC derivatives will need the immune system suppressed for a short period of time and after they become integrated into the patient’s body, the immune suppression can be lifted. Alternatively it might be possible to induce tolerance to the transplanted cells with immunological tricks. Either way, understanding why iPSCs-derived cells are rejected or accepted by the patient’s immune system is the next step to using these amazing cells for regenerative medicine.
Xu’s paper appeared in the journal Stem Cells – DOI: 10.1002/stem.2227.