The HIV envelope protein, gp120, which appears on the surface of viral particles, mediates entry of the virus into human CD4+ cells and is the sole viral protein that elicits neutralizing antibodies. The gp120 protein is important in both the development of therapeutic agents designed to prevent infection of human cells and as a principal component of a potential AIDS vaccine. Envelope proteins demonstrate a high level of genetic variation. Sequences of gp120 proteins derived from different isolates can vary by as much as 20%, resulting in structural differences that may strongly influence viral replication and immune responses directed against gp120. The purpose of this project is to explore the functional consequences exhibited by gp120 sequence variation. Specifically, we are investigating the interaction between different envelopes derived from genetically distinct viruses and cell surface receptors. During the initial events prior to virus entry, HIV gp120 binds to the CD4 receptor and a coreceptor, either CCR5 or CXCR4, expressed on human T-cells. In addition the HIV envelope binds to a class of receptors termed C-type lectin receptors; a class of receptors that function as antigen-capturing receptors and are not directly involved in the infection of T-cells . We are currently investigating the interaction between gp120 and several C-type lectin receptors. C-type lectin receptors bind a wide range of foreign proteins, including gp120, and in so doing, initiate an immune response against the invading pathogen. HIV is thought to subvert C-type lectin receptors to promote infection and escape the human immune system. Certain C-type lectin receptors, including DC-SIGN and the Mannose receptor, are expressed on the surface of dendritic cells whereby they capture HIV via a high affinity interaction with gp120. Once captured, HIV is taken up by the dendritic cell and targeted for destruction. However, a small amount of virus escapes destruction and effectively hides within the dendritic cell, until it later emerges and infects CD4+ T lymphocytes. The interaction between gp120 and DC-SIGN, similar to that with CD4, is extremely tight. We are using biochemical and immunological techniques to understand in detail how this high affinity interaction is achieved. Thus far, we have made several potentially important observations. The binding of gp120 to DC-SIGN, induces a structural changes in DC-SIGN. We speculate that this change in conformation may be involved in the eventual uptake and internalization of gp120 by dendritic cells. We also have determined that multiple DC-SIGN receptors can simultaneously interact with a single gp120 protein, which effectively increases the strength of the interaction. Finally, we have learned that when DC-SIGN is bound to gp120, subsequent binding of CD4 to gp120 promotes the dissociation of gp120 from DC-SIGN. This suggests ways to inhibit interactions between gp120 and DC-SIGN, and C-type lectin receptors in general.
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