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Peptides corresponding to the CD8 and CD4 binding domains of HLA molecules block T lymphocyte immune responses in vitro.

CD4 and CD8 are cell surface glycoproteins that serve as co-receptors for Ag with the TCR. Recent studies have shown that both CD4 and CD8 interact with conserved regions of MHC class II and class I, respectively. To investigate further the roles of CD4 and CD8 in the immune response, we prepared synthetic peptides corresponding to the HLA sequences with which CD4 and CD8 are thought to interact. The peptide corresponding to residues 222 to 235 of the HLA class I heavy chain blocked the differentiation of human CTL precursors into active effect cells but affected neither the ability of PBLs to proliferate in response to mitogen nor the cytotoxic activity of established CTLs. In contrast, the peptide corresponding to residues 134 to 152 of the HLA-DR beta-chain inhibited the differentiation of CTL precursors, the proliferative response of freshly isolated PBL, and the proliferation of an established alloreactive CD4+ T cell clone to Ag. The inhibitory effect of the DR.134-152 peptide on CTL differentiation could be overcome by addition of exogenous IL-2 to the limiting dilution cultures, whereas the effect of the HLA-1.222-235 peptide was unaffected by exogenous IL-2. These results directly demonstrate a functional role for these regions of MHC molecules and underscore the central role of both CD4 and CD8 in the effective initiation of a CTL response.