Knockout of beta-2 microglobulin reduces stem cell-induced immune rejection and enhances ischaemic hindlimb repair via exosome/miR-24/Bim pathway

Generating universal human umbilical mesenchymal stem cells (UMSCs) without immune rejection is desirable for clinical application. Here we developed an innovative strategy using CRISPR/Cas9 to generate B2M(-)UMSCs in which human leucocyte antigen (HLA) light chain beta 2-microglobulin (B2M) was deleted. The therapeutic potential of B2M(-)UMSCs was examined in a mouse ischaemic hindlimb model. We show that B2M(-)UMSCs facilitated perfusion recovery and enhanced running capability, without inducing immune rejection. The beneficial effect was mediated by exosomes. Mechanistically, microRNA (miR) sequencing identified miR-24 as a major component of the exosomes originating from B2M(-)UMSCs. We identified Bim as a potential target of miR-24 through bioinformatics analysis, which was further confirmed by loss-of-function and gain-of-function approaches. Taken together, our data revealed that knockout of B2M is a convenient and efficient strategy to prevent UMSCs-induced immune rejection, and it provides a universal clinical-scale cell source for tissue repair and regeneration without the need for HLA matching in the future.