Antigenically Distinct Conformations of CXCR4

Abstract

The major human immunodeficiency virus type 1 (HIV-1) coreceptors are the chemokine receptors CCR5 and CXCR4. The patterns of expression of the major coreceptors and their use by HIV-1 strains largely explain viral tropism at the level of entry. However, while virus infection is dependent upon the presence of CD4 and an appropriate coreceptor, it can be influenced by a number of factors, including receptor concentration, affinity between envelope gp120 and receptors, and potentially receptor conformation. Indeed, seven-transmembrane domain receptors, such as CCR5, can exhibit conformational heterogeneity, although the significance for virus infection is uncertain. Using a panel of monoclonal antibodies (MAbs) to CXCR4, we found that CXCR4 on both primary and transformed T cells as well as on primary B cells exhibited considerable conformational heterogeneity. The conformational heterogeneity of CXCR4 explains the cell-type-dependent ability of CXCR4 antibodies to block chemotaxis to stromal cell-derived factor 1␣ and to inhibit HIV-1 infection. In addition, the MAb most commonly used to study CXCR4 expression, 12G5, recognizes only a subpopulation of CXCR4 molecules on all primary cell types analyzed. As a result, CXCR4 concentrations on these important cell types have been underestimated to date. Finally, while the factors responsible for altering CXCR4 conformation are not known, we found that they do not involve CXCR4 glycosylation, sulfation of the N-terminal domain of CXCR4, or pertussis toxin-sensitive G-protein coupling. The fact that this important HIV-1 coreceptor exists in multiple conformations could have implications for viral entry and for the development of receptor antagonists.The discovery of the receptors used by human immunodeficiency virus type 1 (HIV-1) to infect cells, coupled with a greater understanding of the membrane fusion-inducing conformational changes undergone by the viral envelope protein (Env) upon receptor binding, has identified several promising drug and vaccine targets (reviewed in reference 12). The viral Env protein binds cell surface CD4 with a high affinity, resulting in conformational changes that enable Env to bind a coreceptor (32,54,56). Coreceptor binding is thought to trigger the ability of Env to mediate fusion between the viral and cellular membranes. The major HIV-1 coreceptors are the chemokine receptors CCR5 and CXCR4 (reviewed in reference 11). The R5 virus strains that use CCR5 with CD4 to infect cells are largely responsible for virus transmission and are typically macrophage tropic. The accrual of mutations in Env can lead to X4 virus strains that use CXCR4 in place of CCR5 or R5X4 virus strains that can use both receptors. While X4 virus strains do not always evolve in infected individuals, their emergence is a harbinger of progression to AIDS (51, 52).While virus infection is dependent upon the presence of CD4 and an appropriate coreceptor, it can be influenced by receptor concentration (21,29,30,43,48), affinity between Env and receptors (28), and potenti...

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