Synthetic peptides corresponding to structural regions of HLA molecules are novel immunosuppressive agents. A peptide corresponding to residues 65–79 of the α-chain of HLA-DQA03011 (DQ65–79) blocks cell cycle progression from early G1 to the G1 restriction point, which inhibits cyclin-dependent kinase-2 activity and phosphorylation of the retinoblastoma protein. A yeast two-hybrid screen identified proliferating cell nuclear Ag (PCNA) as a cellular ligand for this peptide, whose interaction with PCNA was further confirmed by in vitro biochemistry. Electron microscopy demonstrates that the DQ65–79 peptide enters the cell and colocalizes with PCNA in the T cell nucleus in vivo. Binding of the DQ65–79 peptide to PCNA did not block polymerase δ (pol δ)-dependent DNA replication in vitro. These findings support a key role for PCNA as a sensor of cell cycle progression and reveal an unanticipated function for conserved regions of HLA molecules.

We propose that the immunomodulatory effect of the B2702 peptide is caused, in part, by binding to VCAM-1, which then prevents the normal interaction of VCAM-1 with VLA-4.