The role of integrin a4B7 binding in HIV-1 subtype C pathogenesis in phenotypically variant CD4+ T cell subsets

• Main Author: Richardson, Simone Irene
• Format: Others
• Language: en
• Published: 2014
• Subjects: Carrier Proteins; T-Lymphocytes; HIV-1
• Online Access: http://hdl.handle.net/10539/15236

The integrin α4β7, which mediates the trafficking of T lymphocytes to the gut associated lymphoid tissue (GALT), a site of rapid HIV replication, has been described as an attachment factor for the HIV envelope protein gp120. While differences in binding affinity of early transmitting and chronic gp120s for α4β7 have previously been noted by others, this has not translated to differences in replication. We aimed to investigate what role this binding interaction has in HIV pathogenesis over time and determine the factors that influence α4β7 reactivity. Understanding the subsets on which α4β7 is expressed may indicate the phenotype of the first host cells infected by HIV. The level of expression of α4β7 on Th17 and Treg CD4+ T cells was of interest as both subsets are important in HIV immunity in the GALT and represented in both the GALT and genital mucosa. All-trans retinoic acid-activated CD4+ T cells isolated from whole blood of healthy donors were incubated with or without HP2/1 (anti-α4 monoclonal antibody) or Act-1 (anti-α4β7 monoclonal antibody) prior to adding virus. Sixty infectious envelope clones were prepared using matched envelope genes from 11 individuals in the CAPRISA 002 Acute Infection cohort representing the T/F virus and variants from 1-39 months post infection (p.i.). Replication was monitored by p24 ELISA over 10 days. ATRA-stimulated CD4+ T cells were subjected to intracellular and surface staining for flow cytometric analysis using a FACSAria to distinguish Th17 and Treg CD4+ T cells and to determine the expression of CD45RA, CCR5, p24, α4β7. The dependence on α4β7 for HIV subtype C replication changes over time and varies across individuals. In three individuals, dependence on α4β7 was higher using T/F viruses and decreased sharply during acute infection (1-3 months post-infection). α4β7 dependence showed an increasing trend in chronic infection over time which was slight in the first year. Factors that influence α4β7 reactivity include glycan distance from α4β7-binding motif, glycan density and length of the V1/V2 region of gp120. Several glycans positioned in conserved regions of gp120 including N234, N332 and N334 were present more or less frequently in viruses with high α4β7 reactivity. There was an association between high dependence on α4β7 for replication at transmission and those T/F viruses that have a S/PDI/V α4β7 binding motif as well T/F viruses found in individuals diagnosed with bacterial vaginosis during acute infection. Several cytokines in the cervicovaginal lavage (CVL) of these individuals during early infection (IL-8, IL-7 and IL-1α) positively correlated with α4β7 dependence for replication. Treg CD4+ T cells expressed slightly higher levels of α4β7 as compared to Th17 CD4+ cells. In addition, Th17 cells in this and other studies were shown to rapidly deplete in vitro following HIV infection while Treg CD4+ T cell were shown to expand. Despite this, Treg CD4+ T cells were significantly more permissive to infection as compared to Th17 CD4+ T cells. Collectively, these data suggest that there is a role for α4β7 in HIV pathogenesis and that the interaction is selected for during transmission by a number of bottlenecks, one of which is the presence of bacterial vaginosis and elevated expression of IL-7, IL-8 and IL-1α. Due to high levels of α4β7 expression and the ability to bind gp120, presence in both the genital tract and the GALT, regulation by ATRA similar to α4β7 upregulation, expansion following HIV infection and elevated permissiveness to acute infection, Treg CD4+ T cells may be a robust vector from the genital mucosa to the GALT shortly after transmission. This population of T cells may be more suited for this function than Th17 CD4+ T cells which are more susceptible to depletion either by HIV or bystander effects. As a result of the α4β7 binding motif being in a highly immunogenic region of gp120 and antibodies directed against this region associated with protection in the only effective vaccine trial to date, further understanding of the interaction between the virus and the integrin provides an opportunity for the development of future vaccine and therapeutic strategies.