SummaryHuman leukocytes were stimulated in vitro with peptides corresponding in sequence to the highly variable helix of the a1 domain of various HLA-B and -C molecules. A CD4'CD8 -cytotoxic T cell line, CTLAV, that is specific for the HLA-B7 peptide presented by HLA-DR11 .1 was obtained. The HLA-DR11.2 molecule, which only differs at three residues from HLA-DR11 .1, did not present the HLA-B7 peptide to CTLAV Peptides from the a1 domain helix of other HLA-A and HLA-B molecules, but not HLA-C molecules, competed with the HLA-B7 peptide for binding to HLA-DR11.1. A cell line (WT50) that coexpresses HLA-B7 and HLA-DR11 .1 was killed by CTLAV in the absence ofany added HLAB7 peptide. The processing and presentation ofHLAB7 in these cells appears to be through the endogenous, and not the exogenous, pathway of antigen presentation. Thus, Brefeldin A inhibits presentation and chloroquine does not . Furthermore, introduction of purified HLA-B7 molecules into HLA-DR11 .1', HLA-B7 -cells by cytoplasmic loading via osmotic lysis of pinosomes, but not by simple incubation, rendered them susceptible to CTLAV killing. These results provide an example of class II major histocompatibility complex (MHC) presentation of a constitutively synthesized self protein that uses the endogenous pathway ofantigen presentation . They also emphasize the capacity for presentation of MHC peptides by MHC molecules.

HLA-loss variants of an EBV-transformed B lymphoblastoid cell line (EBV-LCL) 721 were used to investigate whether human MHC molecules other than known class I or class II were involved in autologous T cell responses. Bulk lymphocyte cultures of purified T cells primed to an autologous variant EBV-LCL that fails to express HLA-class II and has reduced cell surface HLA-class I expression, and oligoclonal TCR-gamma delta-bearing lines derived from them, could lyse both this EBV-LCL and an independently derived, class II expressing autologous variant EBV-LCL that bears no HLA-A, -B, or -C, suggesting the presence of additional HLA-like restriction elements. Cold target inhibition of cytolysis mediated by these lines indicated that a shared or cross-reactive MHC controlled restriction element other than the known MHC determinants was retained by the EBV-LCL variants. Single-cell derived clones from these T cell lines which expressed only the TCR-gamma delta showed this same target cell specificity pattern, proving recognition of MHC-controlled determinants by autologous gamma delta T cells. Anti-gamma delta antibody could inhibit cytolysis by the gamma delta-expressing lines, suggesting that the TCR-gamma delta was involved in recognition of the EBV-LCL targets. Flow cytometric analysis with separate HLA-reactive antibodies indicated that the restriction element for these cytolytic responses is a molecule serologically cross-reactive with HLA-B and -C Ag, yet is a determinant that cannot be HLA-A, -B, -C, -DR, -DQ or -DP.