Functional adaptation of Nef to the immune milieu of HIV-1 infection in vivo.

Nef-mediated down-regulation of MHC class I (MHC-I) molecules on HIV-1-infected cells has been proposed to enhance viral persistence through evasion of host CTLs. This conclusion is based largely on demonstrations that Nef from laboratory HIV-1 strains reduces the susceptibility of infected cells to CTL killing in vitro. However, the function and role of Nef-mediated MHC-I down-regulation in vivo have not been well described. To approach this issue, nef quasispecies from chronically HIV-1-infected individuals were cloned into recombinant reporter viruses and tested for their ability to down-regulate MHC-I molecules from the surface of infected cells. The level of function varied widely between individuals, and although comparison to the immunologic parameters of blood CD4(+) T lymphocyte count and breadth of the HIV-1-specific CTL response showed positive correlations, no significant correlation was found in comparison to plasma viremia. The ability of in vivo-derived Nef to down-regulate MHC-I predicted the resistance of HIV-1 to suppression by CTL. Taken together, these data demonstrate the functionality of Nef to down-regulate MHC-I in vivo during stable chronic infection, and suggest that this function is maintained by the need of HIV-1 to cope with the antiviral CTL response.