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. Unlike CD4 and chemokine receptors, these ancillary receptors are not likely to mediate productive infection. However, interactions between gp120 and these receptors may alter the biological function of the cells on which they are expressed. We have focused our recent efforts on the interaction between gp120 and an unidentified receptor expressed on the surface of natural killer cells. Natural killer cells play a central role in innate immune responses against foreign pathogens. Natural killer cells obtained from HIV infected individuals exhibit profound functional defects in vitro. They fail to lyse target cells efficiently, and have a reduced capacity to secrete interferon gamma. ? We have undertaken two complementary approaches to study the interaction between gp120 and natural killer cells. The first approach is to characterize the effect of gp120 on the function of natural killer cells. We have determined that gp120 treatment reduces the capacity of natural killer cells to lyse target cells and to secrete interferon gamma. Because these two activities are central to the proper function of natural killer cells, we conclude that gp120-treatment exerts a profound negative effect on natural killer cell function. We note that the deficiencies in natural killer cell function mediated by gp120 overlap the functional defects exhibited by natural killer cells obtained from HIV infected individuals. This correlation supports the concept that gp120 contributes to the global immune dysfunction that is a hallmark of HIV disease. ? The second approach is to understand mechanistically how gp120 disrupts the function of natural killer cells. Unlike CD4+ T-cells, natural killer cells do not express detectable levels of CD4 on their outer membrane. Nor do they express any of the known C-type lectin receptors that bind gp120. Thus the receptor to which gp120 binds on the surface of natural killer cells is currently unknown. To identify this receptor we developed a highly sensitive binding assay that employs fluorescently labeled recombinant gp120 proteins. Binding of these labeled proteins to the surface of natural killer cells can be detected using standard flow-cytometry techniques. Using this system we have made two observations that provide important information that will hopefully allow us to identify this unknown receptor. First, we determined that gp120 binding to natural killer cells requires the presence of divalent cations, e.g. calcium. This requirement is associated with a limited number of receptors, and represents a significant clue to the identity of the gp120-binding receptor on natural killer cells. Second, we have determined that gp120s derived from genetically distinct viral isolates exhibit very large differences in their capacity to bind to natural killer cells. For example we find that a gp120 derived from subtype C, the dominant HIV subtype in Africa and Asia binds much more strongly to natural killer cells than does a gp120 derived from subtype B, the dominant HIV subtype found in North America. Although the precise significance of these differences is unclear, this information is extremely useful in helping us to ultimately identify the unknown receptor. To that end we are currently employing cutting-edge proteomics technology to identify the gp120-binding receptor on natural killer cells.
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