The gut-associated lymphoid tissue (GALT) is a primary anatomical site for HIV-1 replication, particularly during the early stages of HIV infection, leading to extensive depletion of CD4+ T cells. Accordingly, the principal gut-homing integrin, alpha4beta7 (a4b7), has been identified as an additional cellular receptor for HIV-1 and is emerging as a critical molecule in the pathogenesis of HIV-1 disease. Our previous work demonstrated that IL-7 is a potent inducer of a4b7 expression and activation in both CD4+ and CD8+ T cells. The role of a4b7 in HIV-1 infection is further corroborated by in vivo studies that documented a protective role of anti-a4b7 antibodies in macaques infected with simian immunodeficiency virus (SIV). Intravenous administration of a primatized anti-a4b7 monoclonal antibody (mAb), ACT-1, was found to prevent or delay SIV infection in macaques challenged by repeated low-dose vaginal inoculation. Furthermore, administration of anti-a4b7 mAb in the early post-acute phase of infection was recently found to induce persistent control of SIV replication and preservation of the gut lymphoid tissue after withdrawal of antiretroviral therapy (ART), although subsequent studies did not confirm these results. Since HIV-1 has the capacity to incorporate a range of host-cell proteins into its external envelope, which may affect its cellular tropism and infectivity, we investigated the ability of HIV-1 and SIV to incorporate intern a4b7. We found that a4b7 was one of the host proteins most efficiently incorporated by HIV-1. We investigated the viral components required for such incorporation using viral pseudoparticles generated by expression of either the HIV-1 core protein alone (Gag) or Gag plus the envelope glycoprotein (Env) in the presence or absence of a4b7. Our results show that the presence of Gag alone was sufficient for the viral pseudoparticles to incorporate a4b7, while the addition of Env did not significantly affect the level of incorporation. To determine the functionality of virion-incorporated a4b7, we tested the ability of a4b7-containing HIV-1 virions to bind to the natural integrin ligand, MAdCAM-1, in the presence or absence of various modulators of integrin activation such as the cation Mn++. Binding to MAdCAM-1 was detected even in the absence of activating stimuli, indicating that at least a fraction of the incorporated integrin is present in a functionally active state, but treatment with 1 mM MnCl2, which induces a4b7 to adopt a high-affinity ligand-binding state, dramatically increased capture by MAdCAM-armed beads, indicating that the integrin incorporated by HIV-1 is fully functional and responds to physiological stimulations. To investigate if incorporated a4b7 facilitates virion capture and transfer to susceptible target cells, we transfected MAdCAM-1 into MAdCAM-negative cells and tested their ability to mediate trans-infection of susceptible target cells with either a4b7+ or a4b7- virions. HIV-1 infection was detected only when cells expressing MAdCAM-1 were incubated with a4b7+ virus, while all the other conditions did not yield detectable virus transfer to susceptible cells. Furthermore, we demonstrated that incorporated a4b7 can directly promote infection of susceptible MAdCAM-expressing target cells, such as a unique subset of DC in mesenteric lymph nodes, which were recently shown to express MAdCAM-1 in rhesus macaques. To evaluate the clinical relevance of a4b7 incorporation into HIV-1 virions, we investigated whether and to what extent this phenomenon can occur in vivo in HIV-infected patients and SIV-infected macaques. Virion incorporation of a4b7 was detected in all sera tested, but the levels were significantly higher during the early stage of HIV/SIV infection when the GALT is still populated of CD4+ T cells expressing high levels of the integrin. Finally, to investigate whether virions bearing incorporated a4b7 might specifically home into the intestinal compartment in vivo, where MAdCAM-1 is expressed at high levels, we performed in vivo homing experiments in mice, as murine MAdCAM-1 efficiently binds to human a4b7. HIV-1 virions with incorporated a4b7 were efficiently captured along the lumen of high endothelial venules in intestinal Peyers patches, whereas mice injected with a4b7-negative virus did not show any detectable virus homing to the gut tissue. Virus uptake was specifically inhibited by an anti-a4b7 antibody (ACT-1). These results suggested that a4b7 incorporation may be a critical virulence factor that promotes and sustains HIV-1 infection of the gut compartment, thereby promoting virus spread and pathogenesis, particularly during the early phases of HIV-1 infection. More recently, we have started to investigate whether IL-7 can induce the production of CCR5-binding chemokines, which act as natural antagonists of HIV-1 infection. We found that IL-7, at suprahomeostatic concentrations and in the absence of any concomitant stimulation, is a potent inducer of anti-HIV chemokines, and this effect requires an active cross-talk between T cells and monocytes. These results illustrate a novel potential mechanism of HIV-1 control that does not require the triggering of a full T-cell activation program.
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