Promiscuity of the AlloHLA-A2 Restricted T Cell Repertoire Hampers the Generation of Minor Histocompatibility Antigen-specific Cytotoxic T Cells across HLA Barriers

Hematopoietic system-specific miHAs are ideal targets for adoptive immunotherapy after allogeneic HLA (alloHLA)-matched SCT. Adoptive immunotherapy with cytotoxic T cells targeting hematopoietic system-specific miHAs restricted by alloHLA molecules is an attractive strategy to treat relapsed hematologic malignancies after HLA-mismatched SCT. As a proof of principle, we exploited 2 new strategies to generate alloHLA-A2-restricted miHA-specific T cells from HLA-A2^neg donors using a HLA/miHA multimer-guided approach. In one strategy, autologous DCs coated with HLA-A2/miHA complexes were used for in vitro generation of miHA-specific T cells from HLA-A2^neg male donors. In the other strategy, miHA-specific T cells were directly isolated from the peripheral blood of HLA-A2^neg parous females with HLA-A2^pos offspring. Both methods introduced recombinant HLA-A2/miHA complexes as the sole allogeneic target antigen. However, neither method yielded high avidity miHA-specific T cells or prevented the emergence of peptide-dependent promiscuous T cells. The latter T cells resembled miHA-specific T cells so closely with regard to tetramer binding and cytokine production that only extensive testing at a clonal level revealed their nonspecific nature. Therefore, promiscuity of the alloHLA-A2 T cell repertoire of HLA-A2^neg individuals hampers in vitro generation of genuine miHA-specific T cells and limits its use for adoptive immunotherapy after HLA-A2 mismatched SCT.