Coreceptor function of mutant human CD4 molecules without affinity to gp120 of human immunodeficiency virus

Despite extensive mutational studies on the human CD4 molecule and its affi nity to human immunodeficiency virus (HIV) envelope glycoprotein gp120, cor eceptor functions of such mutant molecules have only been examined by indir ect measurement of their affinity to class II major histocompatibility comp lex (MHC) molecules, In this report, coreceptor functions of mutant human C D4 molecules, which have no or reduced affinity to an HIV envelope protein, gp120, were assessed in a murine T cell receptor/class II MHC recognition system. The substitution of human C" beta strand with the murine homologous segment resulted in the loss of the coreceptor function as well as in the complete loss of gp120 binding capacity, corroborating the consensus that P he-43 in C" beta strand plays crucial roles in both situations. However, si multaneous replacement of the C'-C" loop along with the C" beta strand by h omologous murine segments rescued the coreceptor function, whereas gp120 bi nding capacity remained negative. Further analysis indicated that insertion of lysine be tween Gly-41 and Ser-42 can partially compensate for the core ceptor function lost by the Phe-43 --> Val mutation. Although the corecepto r function of these mutant CD4 molecules in a human T cell recognition syst em is yet to be determined, these observations necessitate a re-evaluation of the role played by Phe-43 in coreceptor function. Examination of the sen sitivities of the mutant CD4 molecules expressed on HeLa cells to infection by a T cell-tropic HIV-1 strain indicated that only those mutants that had completely lost gp120 binding capacity were resistant to the infection, Al l mutants having whole C" substitution, irrespective of additional substitu tions or their coreceptor functions, mere resistant to the infection.