Abstract
The HIV pandemic remains a significant global problem today. Despite major achievements in treatment and control, a cure for HIV/AIDS is still elusive. Part of the difficulty in finding a cure comes from HIV’s ability to evade the host immune system. HIV-1 accomplishes much of this via a protein called Nef. Nef causes downregulation and ultimately degradation of immune receptors/factors. Two of the most important and conserved targets for downregulation by Nef are MHC-I and CD4; their downregulation allows for viral evasion of the adaptive immune responses. A common step in MHC-I and CD4 downregulation is the retention of these immune receptors in the endosomes, which takes place through Nef-mediated hijacking of clathrin adaptor protein complex 1 (AP-1). In one project of this dissertation, we solved a 2.8 Å cryo-electron microscopy (cryo-EM) structure to reveal the mechanistic details of this Nef action. Our results show that Nef primarily interacts with the σ1 subunit of AP-1 for recruiting and downregulating CD4. Furthermore, a single AP-1 protein can be hijacked by two Nef molecules simultaneously for the respective binding of CD4 and MHC-I. In addition to the biological insights, our results also provided new knowledge to facilitate the development of Nef inhibitors, which could potentially become a new class of drugs that may better treat HIV infections. In addition to Nef-mediated mechanisms, HIV also evades host immunity in indirect ways. One such strategy involves the mis-trafficking of the antiviral restriction factor BST-2. This mis-trafficking places BST-2 away from viral assembly sites and into a special location on the cell surface, allowing it to interact with ILT-7 on the surface of plasmacytoid dendritic cells (pDCs). This interaction decreases production of interferons in pDCs, dulling the immune response. In a second project of this dissertation, we aimed to elucidate the interaction between BST-2 and ILT-7 structurally. Although our structural studies did not yield the desired results, we have successfully developed a system that allows for the soluble expression of ILT-7 in E. coli. We also observed a unique interaction between ILT-7 and BST-2. Our progresses here have paved the way for future studies to further elucidate the underlying mechanism.