Antigen-independent adhesion of CD4+ T lymphocytes to Epstein-Barr virus (EBV)-transformed B cells is mediated by CD2Aymphocyte function-associated antigen (LFA)-3 and LFA-lhntracellular adhesion molecule (1CAM)-1. Although some anti-CD4 antibodies block the antigen-independent adhesion of CD4+ T lymphocytes, the CD4-HLA class I1 interaction does not appear to significantly contribute to the forces of cell adhesion since CD4+ Tcells equally bind HLA class 11+ and HLA class II-mutant B cells. In addition, conjugates formed between CD4+ Tcells and HLA class II-B cells remain stable for at least 1 h while CD4+T/HLA class 11+ B cell conjugate percentages promptly drop off. Down-regulation of CD4 or spontaneous low expression of CD4 also results in a persistance of conjugates formed with B cells.The role of the CD4-HLA class I1 interaction has been further studied by investigating the inhibitory effect of synthetic 12-mer peptides analogous to HLA class I1 and containing the Arg-Phe-Asp-Ser sequence conserved in the p1 domain. These peptides were previously found to inhibit HLA class II-restricted Tcell responses, this sequence being thought to be involved in CD4-HLA class 11 interaction. These peptides block conjugate formation of CD4+ resting Tcells or clones but not of CD8+ T cells, by interacting with the T cells as shown by preincubation experiments. Down-regulation of CD4 or spontaneous low expression results in the loss of the inhibitory activity. The peptide-mediated inhibition is neutralized by a soluble dimeric CD4 molecule. Alteration within the Arg-Phe-Asp-Ser sequence results in a significant loss of inhibition. It is thus proposed that the CD4-HLA class I1 interaction negatively regulates antigen-independent adhesion of T cells, this interaction involving the highly conserved Arg-Phe-Asp-Ser sequence in the HLA class I1 p1 sequence as a CD4-binding site.