Both virus and host components are important for the manifestation of a Nef- phenotype in HIV-1 and HIV-2.

While it has been demonstrated that the Nef protein of simian immunodeficiency virus is obligatory for the establishment of high viral loads and the development of simian AIDS in rhesus macaques, demonstrating a critical role for the human immunodeficiency virus (HIV) Nef protein in tissue culture has been elusive. Data have been contradictory as to whether Nef has a negative or positive influence on in vitro virus replication. In an attempt to define a role for Nef during virus propagation in tissue culture and to obtain virus-host systems that could distinguish between the Nef mutant and wild-type viruses, we have introduced mutations into the nef genes of infectious molecular clones of three HIV-1 strains and two isolates of the HIV-2ROD strain and have investigated the capacity of viruses derived from them to infect a number of CD4-positive T-cell lines and peripheral blood mononuclear cells (PBMC). Mutating the nef gene of all viruses had a modest negative effect on virus production in activated PBMC. In some T-cell lines with some viruses, the effects were severe, and little or no Nef mutant virus could be detected. In other cell lines, the result of mutating the nef gene either had no effect or had a slight negative effect on the replication kinetics. Therefore, whether the consequences of loss of Nef activity can be demonstrated in vitro depends on both the particular virus and the host cell used, suggesting that Nef is exerting its activity on some cellular pathway. In addition, we describe the construction and properties of hitherto unreported infectious molecular clones of the ROD strain of HIV-2.