Identification of indirect CD4+ T cell epitopes associated with transplant rejection provides a target for donor-specific tolerance induction

Antibodies against the donor human leukocyte antigen (HLA) molecules drive late transplant failure, with HLA-DQ donor-specific antibodies (DSAs) posing the highest rejection risk. Here, we investigated the role of indirect CD4⁺ T cell epitopes—donor-derived peptides presented by recipient major histocompatibility complex (MHC) class II—in DSA formation. Antigen mapping of samples from HLA-DQ DSA-positive kidney and heart transplant recipients revealed two polymorphic hotspots in donor HLA-DQ that generated alloreactive peptides. Antigen mapping of indirect CD4⁺ T cell epitopes in a mouse model of fully MHC mismatched skin graft transplantation (BALB/c to C57BL/6) identified a similar epitope (amino acids 287–301) derived from the donor H2-Kd. Tetramer-binding Kd287⁺ CD4⁺ T cells were detected during rejection and their transfer into T cell-deficient mice induced DSA. Systemic delivery of high-dose donor H2-Kd peptides combined with CTLA4-Ig reduced the frequencies of Kd287⁺ CD4⁺ T cells and DSA formation. Thus, targeting a narrow range of donor antigens may prevent DSA formation and improve transplant outcomes.