Blocking MHC class II on human endothelium mitigates acute rejection.

Acute allograft rejection is mediated by host CD8(+) cytotoxic T lymphocytes (CTL) targeting graft class I major histocompatibility complex (MHC) molecules. In experimental rodent models, rejection requires differentiation of naive CD8(+) T cells into alloreactive CTL within secondary lymphoid organs, whereas in humans, CTL may alternatively develop within the graft from circulating CD8(+) effector memory T cells (T-EM) that recognize class I MHC molecules on graft endothelial cells (EC). This latter pathway is poorly understood. Here, we show that host CD4(+) T-EM, activated by EC class II MHC molecules, provide critical help for this process. First, blocking HLA-DR on EC lining human artery grafts in immunodeficient mice reduces CD8(+) CTL development within and acute rejection of the artery by adoptively transferred allogeneic human lymphocytes. Second, siRNA knockdown or CRISPR/Cas9 ablation of class II MHC molecules on EC prevents CD4(+) T-EM from helping CD8(+) T-EM to develop into CTL in vitro. Finally, implanted synthetic microvessels, formed from CRISPR/Cas9-modified EC lacking class II MHC molecules, are significantly protected from CD8(+) T cell-mediated destruction in vivo. We conclude that human CD8(+) T-EM -mediated rejection targeting graft EC class I MHC molecules requires help from CD4(+) T-EM cells activated by recognition of class II MHC molecules.