Nef Commandeers Host Cellular Factors to Promote HIV-1 Immune Evasion.

Abstract

Despite mounting an active immune defense, the Human Immunodeficiency Virus (HIV) persists as a low-level infection in the blood in infected individuals. The HIV protein, Nef, is indispensable for disease progression and promotes viral spread by allowing HIV infected cells to evade recognition by the host’s immune system. Cytotoxic T Lymphocytes (CTLs) recognize and lyse cells that present viral antigens in complex with class I-major histocompatibility complex (MHC-I) molecules. Nef directly interacts with, downmodulates, and degrades MHC-I molecules, thus rescuing infected cells from CTL recognition and subsequent death. Nef binds directly to the cytoplasmic tail of specific MHC-I molecules and recruits the clathrin-associated adaptor protein AP-1 to redirect MHC-I from the plasma membrane to the trans-Golgi to endosome trafficking loop. To better understand the mechanism by which Nef promotes HIV immune evasion, we performed a comprehensive screen of domains in the HLA-A2 cytoplasmic tail (Y320SQA323ASSD327), Nef (M20, E62-65, and P72/75/78), and AP-1 (FD172/174) and determined that amino acids in each of these domains are required for HLA-A2 downmodulation and formation of the Nef-MHC-I-AP-1 complex. Importantly, we described key amino acids in the cytoplasmic tail of HLA-A2 that explain why Nef can downmodulate HLA-A and HLA-B, but not the natural killer (NK) cell inhibitory molecules HLA-C or HLA-E. Thus Nef allows infected cells to evade lysis by both CTL and NK cells. Furthermore, we explored the mechanisms of Nef-dependent recruitment of AP-1. We determined that Nef requires functional ARF-1, an AP-1-scaffold protein, to recruit AP-1 to the cytoplasmic tail of MHC-I. Additionally, Nef downmodulates the viral co-receptor CD4 to promote HIV assembly and release by accelerating its endocytosis. The CD4 pathway appears to be distinct from how Nef reroutes MHC-I from the trans-Golgi. However, these disparate pathways ultimately converged in a COP-I coatomer Beta-COP-dependent degradative pathway. Overall, our findings reveal the mechanism by which Nef suppresses recognition of HIV-infected cells and limits clearance by the host’s immune system.