The HIV envelope protein, gp120, mediates entry of viral particles into CD4+ cells. gp120 binds to the CD4 receptor and a co-receptor, either CCR5 or CXCR4. These receptors are expressed on a subset of human lymphocytes and macrophages, and thus it is these cells that are productively infected by HIV. Because gp120 is the only viral protein against which neutralizing antibodies are elicited, it is a primary target of therapeutic agents designed to prevent infection of human cells by HIV, and a key component of a potential AIDS vaccine. gp120 is also recognized by C-type lectin receptors, and other yet unidentified receptors. C-type lectin receptors including DC-SIGN, the Mannose receptor, and Langerin are expressed on the surface of dendritic cells. These receptors capture HIV via a high affinity interaction with gp120. Unlike CD4 and chemokine receptors, these ancillary receptors are likely not to mediate productive infection. However, interactions between gp120 and these receptors may alter the biological function of the cells on which they are expressed. The purpose of this project is twofold: 1) biochemically characterize interactions between gp120 and these ancillary receptors 2) identify as yet unidentified receptors to which gp120 binds. C-type lectin receptors typically function as antigen-capturing receptors. They bind a wide range of foreign proteins, including gp120, and in so doing, initiate an immune response against the invading pathogen. Like the interaction between gp120 and CD4, the interaction between gp120 and these C-type lectin receptors exhibits a high affinity. However, unlike CD4, these receptors capture HIV via a high affinity interaction with carbohydrate residues that are post-translationally attached to gp120. Once captured, HIV particles are taken inside the dendritic cell where the majority of virus is degraded. We are initially focusing on interactions between gp120 and DC-SIGN, a lectin receptor that is widely expressed on dendritic cells. We are employing biochemical and immunological techniques to understand in detail the mechanism by which DC-SIGN efficiently captures gp120 and have made several noteworthy observations. When gp120 binds DC-SIGN, it changes the structure of DC-SIGN. We speculate that this change in conformation may be involved in the eventual uptake and internalization of gp120 into dendritic cells. In addition the structure of gp120 is also altered. In its unbound state, gp120 is highly flexible, however once attached to DC-SIGN, gp120 conformation is more constrained. Understanding this constrained conformation may provide useful information for the design gp120-based subunit vaccines. ? ? Envelope proteins derived from different isolates reflect a high level of genetic variation, differing in sequence by as much as 20%. Such variation can substantially influence interactions between gp120 and DC-SIGN, as envelope proteins derived from different viruses bind DC-SIGN with varying strength and some gp120s can be recognized by more than one DC-SIGN simultaneously. Understanding these differences, which may impact the transmissibility of particular HIV isolates, may provide insight as to how one might inhibit interactions between gp120 and DC-SIGN, and C-type lectin receptors in general.
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