Molecular mechanisms of MHC-I downmodulation by HIV-1 Nef.

HIV-1 Nef is an essential pathogenic determinant required for the timely development of AIDS. To avoid both cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells Nef downmodulates the MHC-I allotypes most important for CTL recognition (HLA-A and B) but not the MHC-I necessary to avoid NK cells (HLA-C and E). This allotype specific downmodulation has been attributed to particular amino acids within the cytoplasmic tail of MHC-I, although the function of these residues was unknown. We have demonstrated that Nef disrupts HLA-A2 trafficking through a direct interaction. Furthermore, amino acids in the cytoplasmic tail of MHC-I that are required for downmodulation are also required for Nef binding. Therefore, the differential downmodulation of MHC-I by Nef is conferred by Nef's ability to bind MHC-I. In addition to downmodulatiog MHC-I, Nef also downmodulates the viral receptor CD4. Mutational analyses have revealed that separate domains of Nef are required for the downmodulation of these two molecules. To further elucidate the mechanism of MHC-I downmodulation, we examined the role of the Nef domains required for MHC-I downmodulation. We found that each of these regions was required for co-precipitation of Nef with MHC-I, and their ability to downmodulate MHC-I could be rescued by fusing them to MHC-I. Further analysis of these fusion proteins revealed that Nef activated a trafficking signal in the MHC-I cytoplasmic tail. Nef decreases the cell surface expression of HLA-A and B by altering their regular trafficking. While HLA-C is not affected by Nef it is normally expressed poorly on the cell surface. Studies with chimeric molecules revealed that both the extracellular and cytoplasmic tail domains of HLA-CW4 contributed to this low surface expression. While the extracellular domain of HLA-CW4 affected early trafficking events in the endoplasmic reticulum, the cytoplasmic tail domain affected later trafficking events after the medial-Golgi. The regulation of MHC-I trafficking is important for the immune system. As such, viruses have evolved multiple strategies to avoid viral antigen presentation by MHC-I. Studies aimed at deciphering these mechanisms will allow for the development of agents to disrupt these strategies, and will provide valuable information about the normal MHC-I antigen presentation process.Ph.D.Biological SciencesCellular biologyMicrobiologyMolecular biologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124337/2/3137963.pd