Molecular interaction of the CD4 and MHC class II molecules : mapping the contact sites on CD4

T cells expressing CD4 recognize antigens presented by class II following the contact of CD4 with non-polymorphic regions of class II. CD4 enhances T cell activation by acting as an adhesion molecule (co-ligand function), or by bringing the CD4 associated p56$ sp{lck}$ to the vicinity of the TCR (co-receptor function).To dissect the molecular interactions which lead to CD4 function(s), wild-type (WT) and mutant CD4 molecules were expressed in the CD4-dependent 3DT52.5.8 T cell hybridomas. Results showed that multiple sites on CD4 encompassing the CDR1, the CDR3 regions of D1 and the FG loop of D2 are involved in class II interaction. The opposite face containing the CDR2 region also plays a role, either as another class II binding site, or the TCR docking site, or in another function of CD4. Co-receptor function requires a much larger site on CD4, compared to co-ligand function. A stretch of 15 amino acids which links D2 and D3 of CD4 appears to be very important for maintaining CD4 conformation, or to provide CD4 the flexibility required for its interaction with other cell surface molecules, including class II, the TCR, etc.Crystallographic and functional studies have suggested that CD4 may dimerize, although biochemical evidence is lacking. To investigate the CD4 dimerization issue both human and mouse CD4 WT were co-expressed in 3DT52.5.8 cells. Surprisingly this led to a severe disruption of CD4 functions, although it has been shown that both human and mouse CD4 molecules are capable of interacting with human class II efficiently. As expected, co-expression of h-CD4 WT with class II-interaction-deficient CD4 mutants within the CDR1, CDR3 and the FG loop did not rescue CD4 functions. However, co-expression of CD4 WT with mutants from the CDR2 region resulted in an enhanced response. This result suggests that CDR2 mutants do not dimerize with WT molecule, therefore cannot behave as a dominant negative mutant, which is not the case for class II-interaction-deficient mutants from the CDR1, CDR3 and FG loop. Based on these results we suggest a model whereby dimerization involves, at least in part the CDR2 region. Final confirmation of this model awaits further structural data