Among the accessory proteins encoded by HIV-1, the 27 kDal membrane associated myristoylated Nef has proved to be enigmatic. The lack of consensus for the results of current in vitro assays measuring Nef effect on virus replication and transcription has required a shift in the focus from the virological to the cellular physiological roles of Nef. There is reasonable agreement in the literature on the potential roles of Nef in modulating certain cellular receptors. In this report, we have tried to address specific properties of Nef in this area by mechanistic studies of Nef effects on CD4 and major histocompatibility complex Class I receptor (MHC-I) and chemokine signalling pathways. We have observed in the past that Nef downregulated CD4 by a bi- modal mechanism. Unquestionably, Nef enhanced CD4 endocytosis was the only demonstrable effect when Nef was expressed acutely in T cell lines that express CD4 constitutively. However, when Nef and CD4 were co-expressed in cells lacking endogenous CD4, we demonstrated a Nef induced defect in the early steps of CD4 synthesis and transport in addition to the accelerated endocytosis of the CD4 receptor. The cytoplasmic CD4 defect may have been due to interference with the intracellular vesicular traffic of certain nascent proteins in Nef expressing cells. The ensuing delay in the recruitment of selected membrane proteins such as CD4 into these vesicles may lead to their premature degradation. MHC-I expression levels and usage status are important in AIDS pathogenesis since perturbation of antigen presentation by infected cells may allow them to escape T killer cell surveillance that is a vital immune mechanism against virus infection. We undertook an exhaustive analysis of potential Nef effects on MHC-I. We found that the Nef effect on MHC-I was quite variable with different MHC-I alleles and different cells. Whereas it is likely that Nef may have enhanced endocytosis and degradation of the susceptible receptor, we could not find consistent evidence for this. Two distinct sets of chemokine co-receptors have been shown to be required for HIV entry in the CD4 positive cells. They are the CCR5 and CXCR4 receptors that are used by the macrophage (M)- tropic and T cell (T)-tropic strains of HIV respectively. Both the CD4 and the chemokine receptor have to be present on the cell to allow HIV entry. Although HIV entry takes place in cells that express mutant coreceptors that have lost the ability to signal through chemokines, it is not clear whether the magnitude of early virus replication events are influenced by the chemokine signaling pathways. We found that Nef proteins of HIV and SIV differentially modulated CCR5 and CXCR4 with potential implications for tropism switching during natural infection.
