While several clinical studies have shown that HIV-1 infection is associated with increased permeability of the intestinal tract, there is very little understanding of the mechanisms underlying HIV-induced impairment of mucosal barriers. Here we demonstrate that exposure to HIV-1 can directly breach the integrity of mucosal epithelial barrier, allowing translocation of virus and bacteria. Purified primary epithelial cells (EC) isolated from female genital tract and T84 intestinal cell line were grown to form polarized, confluent monolayers and exposed to HIV-1. HIV-1 X4 and R5 tropic laboratory strains and clinical isolates were seen to reduce transepithelial resistance (TER), a measure of monolayer integrity, by 30–60% following exposure for 24 hours, without affecting viability of cells. The decrease in TER correlated with disruption of tight junction proteins (claudin 1, 2, 4, occludin and ZO-1) and increased permeability. Treatment of ECs with HIV envelope protein gp120, but not HIV tat, also resulted in impairment of barrier function. Neutralization of gp120 significantly abrogated the effect of HIV. No changes to the barrier function were observed when ECs were exposed to Env defective mutant of HIV. Significant upregulation of inflammatory cytokines, including TNF-α, were seen in both intestinal and genital epithelial cells following exposure to HIV-1. Neutralization of TNF-α reversed the reduction in TERs. The disruption in barrier functions was associated with viral and bacterial translocation across the epithelial monolayers. Collectively, our data shows that mucosal epithelial cells respond directly to envelope glycoprotein of HIV-1 by upregulating inflammatory cytokines that lead to impairment of barrier functions. The increased permeability could be responsible for small but significant crossing of mucosal epithelium by virus and bacteria present in the lumen of mucosa. This mechanism could be particularly relevant to mucosal transmission of HIV-1 as well as immune activation seen in HIV-1 infected individuals.

The establishment of HIV type 1 (HIV-1) infection is initiated by the stable attachment of the virion to the target cell surface. Although this process relies primarily upon interaction between virus-encoded gp120 and cell surface CD4, a number of distinct interactions influence binding of HIV-1 to host cells. In this study, we report that galectin-1, a dimeric β-galactoside-binding protein, promotes infection with R5, X4, and R5X4 variants. Galectin-1 acts as a soluble adhesion molecule by facilitating attachment of HIV-1 to the cell surface. This postulate is based on experiments where galectin-1 rendered HIV-1 particles more refractory to various agents that block HIV-1 adsorption and coreceptor binding (i.e., a blocking anti-CD4, soluble CD4, human anti-HIV-1 polyclonal Abs; stromal cell-derived factor-1α; RANTES). Experiments performed with the fusion inhibitor T-20 confirmed that galectin-1 is primarily affecting HIV-1 attachment. The relevance of the present findings for the pathogenesis of HIV-1 infection is provided by the fact that galectin-1 is abundantly expressed in the thymus and lymph nodes, organs that represent major reservoirs for HIV-1. Moreover, galectin-1 is secreted by activated CD8+ T lymphocytes, which are found in high numbers in HIV-1-positive patients. Therefore, it is proposed that galectin-1, which is released in an exocrine fashion at HIV-1 replication sites, can cross-link HIV-1 and target cells and promote a firmer adhesion of the virus to the cell surface, thereby augmenting the efficiency of the infection process. Overall, our findings suggest that galectin-1 might affect the pathogenesis of HIV-1 infection.

Sexual transmission of HIV-1 requires virus adsorption to a target cell, typically a CD4؉ T lymphocyte residing in the lamina propria, beneath the epithelium. To escape the mucosal clearance system and reach its target cells, HIV-1 has evolved strategies to circumvent deleterious host factors. Galectin-1, a soluble lectin found in the underlayers of the epithelium, increases HIV-1 infectivity by accelerating its binding to susceptible cells. By comparison, galectin-3, a family member expressed by epithelial cells and part of the mucosal clearance system, does not perform similarly. We show here that galectin-1 directly binds to HIV-1 in a ␤-galactoside-dependent fashion through recognition of clusters of N-linked glycans on the viral envelope gp120. Unexpectedly, this preferential binding of galectin-1 does not rely on the primary sequence of any particular glycans. Instead, glycan clustering arising from the tertiary structure of gp120 hinders its binding by galectin-3. Increased polyvalency of a specific ligand epitope is a common strategy for glycans to increase their avidity for lectins. In this peculiar occurrence, glycan clustering is instead exploited to prevent binding of gp120 by galectin-3, which would lead to a biological dead-end for the virus. Our data also suggest that galectin-1 binds preferentially to CD4, the host receptor for gp120. Together, these results suggest that HIV-1 exploits galectin-1 to enhance gp120-CD4 interactions, thereby promoting virus attachment and infection events. Since viral adhesion is a rate-limiting step for HIV-1 entry, modulation of the gp120 interaction with galectin-1 could thus represent a novel approach for the prevention of HIV-1 transmission.

Although women constitute half of all HIV-1–infected people worldwide (UNAIDS World AIDS Day Report, 2011), the earliest events in the female reproductive tract (FRT) during heterosexual HIV-1 transmission are poorly understood. Recently, we demonstrated that HIV-1 could directly impair the mucosal epithelial barrier in the FRT. This suggested that the HIV-1 envelope glycoprotein gp120 was being recognized by a membrane receptor on genital epithelial cells, leading to innate immune activation. In this study, we report that pattern-recognition receptors TLR2 and -4 bind to HIV-1 gp120 and trigger proinflammatory cytokine production via activation of NF-κB. The gp120–TLR interaction also required the presence of heparan sulfate (HS). Bead-binding assays showed that gp120 can bind to HS, TLR2, and TLR4, and studies in transfected HEK293 cells demonstrated that HS and TLR2 and -4 were necessary to mediate downstream signaling. Exposure to seminal plasma from HIV-1–infected and uninfected men with gp120 added to it induced a significant proinflammatory cytokine response from genital epithelial cells and disruption of tight junctions, indicating a role for gp120 in mucosal barrier disruption during HIV-1 heterosexual transmission. These studies provide, for the first time to our knowledge, a possible mechanism by which HIV-1 gp120 could directly initiate innate immune activation in the FRT during heterosexual transmission.