The Role of CS1 Fibronectin in HIV-1 Infection of α4β7+ T Lymphocytes

Abstract

A hallmark of Human Immunodeficiency Virus type 1 (HIV-1) pathogenesis is the profound depletion of CD4+ T lymphocytes within gut-associated lymphoid tissues (GALT), and persistent immune dysfunction at mucosal sites. It has been demonstrated that gut-homing CD4+ T cells expressing the α4β7 integrin are preferentially targeted by HIV and play key roles in GALT pathology. Binding between the gp120 subunit of HIV envelope protein and α4β7 has been described and is hypothesized to mediate preferential infection of α4β7+ T cells. However, it has also been reported that many HIV envelopes do not bind to α4β7, and the role of gp120-α4β7 interaction in HIV pathogenesis is controversial. In this dissertation, I investigated the basis for HIV-α4β7 interaction in detail. I discovered that gp120 does not bind to α4β7 as previously reported. Instead, I discovered that mammalian cell lines used to express recombinant gp120 also produced α4β7–reactive isoforms of the extracellular matrix protein fibronectin. These fibronectins co-purified with gp120, and I characterized a method for separating gp120 from fibronectins. Furthermore, I showed that highly purified gp120 bound to CD4 but not to α4β7. Significantly, I also discovered that fibronectins bound to both α4β7 and gp120, and thus mediated indirect gp120-α4β7 binding. These findings resolved the controversy regarding the biochemical basis of gp120-α4β7 interaction, but also suggested that matrix-associated fibronectins may mediate HIV infection of α4β7+ T cells. Based on this, I developed in vitro assays to investigate the role of fibronectins in HIV infection. I discovered that fibronectins captured and delivered HIV preferentially to α4β7+ T cells, and also facilitated T cell activation and efficient cell-to-cell virus transmission. Finally, because α4β7–reactive fibronectins are oncofetal proteins expressed in inflammation but not in healthy tissue, I investigated their expression in the context of infection. Using immunohistochemistry, I provided evidence that α4β7–reactive fibronectins are expressed in vivo in GALT of SIV-infected macaques. Collectively, these studies clarified the nature of HIV interactions with α4β7, identified a novel role for fibronectins in HIV pathogenesis, and provided insight into how these interactions may be targeted therapeutically.