Triple (GGTA1, CMAH, B2M) modified pigs expressing an SLA class-I^low phenotype - effects on immune status and susceptibility to human immune responses

Porcine xenografts lacking SLA (swine leucocyte antigen) class-I are thought to be protected from human T cell responses. We have previously shown that SLA class-I deficiency can be achieved in pigs by CRISPR/Cas9-mediated deletion of beta2-microglobulin (B2M). Here, we characterized another line of genetically modified pigs where targeting of the B2M locus did not result in complete absence of B2M and SLA class-I, but in significantly reduced expression levels of both molecules. Residual SLA class-I was functionally inert since no proper differentiation of the CD8+ T cell subset was observed in B2Mlow pigs. Cells from B2Mlow pigs were less capable in triggering proliferation of human PBMC in vitro, which was mainly due to non-responsiveness of CD8+ T cells. Nevertheless, cytotoxic effector cells developing from unaffected cell populations (e.g. CD4+ T cells, NK cells) lysed both, targets from SLA class-I+ wildtype and SLA class-Ilow pigs with similar efficiency. These data indicate that absence of SLA class-I is an effective approach to prevent activation of human CD8+ T cells during the induction phase of an anti-xenograft response. However, cytotoxic activity of cells during the effector phase cannot be controlled by this approach.