The recognition of chimeras of rat and human CD4 by HIV-1 gp120 and by monoclonal antibodies.

The use of chimeras of rat and human CD4 to probe the HIV-1 gpl20 and antibody binding properties of CD4 is reviewed. Short segments of human CD4 sequence were substituted for the equivalent regions of rat CD4 which does not bind gpl20, and analysis of the properties of these chimeras established: (i) that residues 33-58 of the NH2-terminal domain of human CD4 encompass the high-affinity gpl20 binding site; and (ii) that chimeras containing residues 33-62 mediate HIV-1 infection. The chimerabinding specificities of gpl20 and a large panel of anti-CD4 antibodies were also determined. This allowed a critical test of the popular notion that receptor mimics appear at high frequency among antibodies elicited by immunization with receptor ligands and that anti-idiotypic antibodies can be used to identify novel receptors. The data suggest that such mimics appear infrequently, if at all, a result which is consistent with the failure of the anti-idiotype approach to identify new genes encoding receptors with prescribed functions.