Immune rejection of allogeneic transplants has been a major issue for developing new stem cell therapies, as patients and cell donors must be immunologically matched. Various studies have trialled potential solutions to this incompatibility, such as modifying stem cells to express engineered HLA molecules capable of evading both T and NK cells. However, these cells may not work with human transplants, or could pose unanticipated safety issues, and the challenge to develop a safe and completely “universal” transplant donor cell has not yet been solved.
In this study, two strategies for the creation of universal donor cells will be explored and optimised with mouse melanoma cells; 1) eliminating essential MHC class I component B2M while upregulating immune suppressor PD-L1, and 2) a whole genome CRISPR activation screen to find genes conferring immune resistance, both tested in vivo. B2M knockout + PD- L1 overexpression cells demonstrated a minor increase in tumour growth and survival after allogeneic transplant, showing reduced immune infiltration compared to the wild type, but also revealed a loss of cell viability within the tumour mass. This suggests an increased capability of the modified cells to evade immune attack, but with a loss of fitness in the cell population potentially through reduced cell diversity. The whole genome CRISPR activation screen revealed a cohort of promising candidate transplant acceptance genes, including factors associated with the immune system, cell cycle, olfactory system and calcium ion pathways, however, further validation of these hits is necessary before drawing firm conclusions. This unbiased approach may allow for the discovery of novel genes in the pathways of transplant rejection, potentially uncovering new solutions to the urgent and ongoing problem that is transplant rejection.