Extracellular Matrix Proteins Mediate HIV-1 gp120 Interactions with α4β7.

Gut-homing alpha 4 beta 7(high) CD4(+) T lymphocytes have been shown to be preferentially targeted by human immunodeficiency virus type 1 (HIV-1) and are implicated in HIV-1 pathogenesis. Previous studies demonstrated that HIV-1 envelope protein gp120 binds and signals through alpha 4 beta 7 and that this likely contributes to the infection of alpha 4 beta 7(high) T cells and promotes cell-to-cell virus transmission. Structures within the second variable loop (V2) of gp120, including the tripeptide motif LDV/I, are thought to mediate gp120-alpha 4 beta 7 binding. However, lack of alpha 4 beta 7 binding has been reported in gp120 proteins containing LDV/I, and the precise determinants of gp120-alpha 4 beta 7 binding are not fully defined. In this work, we report the novel finding that fibronectins mediate indirect gp120-alpha 4 beta 7 interactions. We show that Chinese hamster ovary (CHO) cells used to express recombinant gp120 produced fibronectins and other extracellular matrix proteins that copurified with gp120. CHO cell fibronectins were able to mediate the binding of a diverse panel of gp120 proteins to alpha 4 beta 7 in an in vitro cell binding assay. The V2 loop was not required for fibronectin-mediated binding of gp120 to alpha 4 beta 7, nor did V2-specific antibodies block this interaction. Removal of fibronectin through anion-exchange chromatography abrogated V2-independent gp120-alpha 4 beta 7 binding. Additionally, we showed a recombinant human fibronectin fragment mediated gp120-alpha 4 beta 7 interactions similarly to CHO cell fibronectin. These findings provide an explanation for the apparently contradictory observations regarding the gp120-alpha 4 beta 7 interaction and offer new insights into the potential role of fibronectin and other extracellular matrix proteins in HIV-1 biology. IMPORTANCE Immune tissues within the gut are severely damaged by HIV-1, and this plays an important role in the development of AIDS. Integrin alpha 4 beta 7 plays a major role in the trafficking of lymphocytes, including CD4(+) T cells, into gut lymphoid tissues. Previous reports indicate that some HIV-1 gp120 envelope proteins bind to and signal through alpha 4 beta 7, which may help explain the preferential infection of gut CD4(+) T cells. In this study, we demonstrate that extracellular matrix proteins can mediate interactions between gp120 and alpha 4 beta 7. This suggests that the extracellular matrix may be an important mediator of HIV-1 interaction with alpha 4 beta 7-expressing cells. These findings provide new insight into the nature of HIV-1-alpha 4 beta 7 interactions and how these interactions may represent targets for therapeutic intervention.