The minor histocompatibility antigen HA-3 arises from differential proteasome–mediated cleavage of the lymphoid blast crisis (Lbc) oncoprotein

Abstract

Minor histocompatibility (H) antigens crucially affect the outcome of human leukocyte antigen (HLA)–identical allogeneic stem cell transplantation (SCT). To understand the basis of alloimmune responses against minor H antigens, identification of minor H peptides and their antigenicity-determining mechanisms is essential. Here we report the identification of HA-3 and its encoding gene. The HA-3 peptide, VTEPGTAQY (HA-3^T), is encoded by the lymphoid blast crisis (Lbc) oncogene. We thus show for the first time that a leukemia-associated oncogene can give rise to immunogenic T-cell epitopes that may have participated in antihost and antileukemic alloimmune responses. Genotypic analysis of HA-3^- individuals revealed the allelic counterpart VMEPGTAQY (HA-3^M). Despite the lack of T-cell recognition of HA-3^- cells, the Thr→Met substitution had only a modest effect on peptide binding to HLA-A1 and a minimal impact on recognition by T cells when added exogenously to target cells. This substitution did not influence transporter associated with antigen processing (TAP) transport, but, in contrast to the HA-3^T peptide, HA-3^M is destroyed by proteasome-mediated digestion. Thus, the immunogenicity of minor H antigens can result from proteasome-mediated destruction of the negative allelic peptide.