T cells chronically infected with HIV do not contain sufficient Nef to promote CD4 downmodulation in the absence of envelope-mediated effects.
Among HIV viral proteins, envelope glycoproteins and Nef have been both suggested to participate in CD4 downregulation during the course of HIV infection. In a previous study, we provided evidence that a mutant form of CD4 that does not bind gp120 was never downregulated in chronically HIV-1- and HIV-2-infected CEM cells. To further investigate the relative effects of Nef or glycoproteins in CD4 downregulation, recombinant vaccinia virus (VV) vectors were used to express high levels of HIV-1 viral proteins in cells expressing both wild-type and mutant CD4. It was demonstrated that during HIV infection, overexpression of Nef, achieved through the VV expression system, was necessary to induce CD4 downregulation in the mutant CD4-expressing cell model. These results are consistent with the hypothesis that Nef-mediated CD4 downregulation depends on the cellular levels of Nef expression. We concluded that during the late stage of viral replication, CD4 downregulation is mostly due to gp120 and not to Nef because of a low level of Nef expression.