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

Davis SJ, James WS, Schockmel GA, Simon JH, Somoza C. (1993), Philos Trans R Soc Lond B Biol Sci. 342, 75-81

Abstract

The use of chimeras of rat and human CD4 to probe the HIV-1 gp120 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 gp120, 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 gp120 binding site; and (ii) that chimeras containing residues 33-62 mediate HIV-1 infection. The chimera-binding specificities of gp120 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.

Key figure: The binding of gp120 to soluble forms of the chimeras of rat and human CD4

The various CD4 mutants (mut-2 to mut-6), expressed in soluble form in Chinese hamster ovary cells (Davis et al. 1990), were serially diluted and their binding to gp120 assayed by their ability to inhibit the binding of ¹²⁵I-labelled human sCD4 to gp120 that had been immobilized to plastic with rabbit anti-mouse IgG and an anti-gp120 antibody (mAb 108). The relative affinities of these interactions were compared with those involving rat soluble CD4 (rsCD4) and human soluble CD4 (hsCD4).