Endothelial cells promote productive HIV infection of resting CD4+ T cells by an integrin-mediated cell adhesion-dependent mechanism

Abstract

<jats:title>Abstract</jats:title><jats:p>Resting CD4+ T cells do not support HIV replication <jats:italic>in vitro</jats:italic>, yet are primary targets of early HIV infection events <jats:italic>in vivo</jats:italic>. There is an established role for factors in the tissue microenvironment, including endothelial cells, in enhancing the susceptibility of resting CD4+ T cells to productive infection, yet the mechanisms behind this are not well understood. Endothelial cells facilitate immune cell trafficking throughout the body. Cell adhesion molecules expressed by endothelial cells engage integrins on activated and memory T cells and mediate transmigration into inflamed tissues. These cell trafficking pathways have overlapping roles in facilitating HIV replication but their relevance to endothelial cell-mediated enhancement of HIV susceptibility in resting CD4+ T cells has not previously been examined. We used flow cytometry to characterize the phenotype resting CD4+ T cells that became productively infected when exposed to HIV in the presence of endothelial cells. Infected CD4+ T cells were primarily central memory cells enriched for high expression of the integrins LFA-1 and VLA-4 and had variable expression of α4β7, CCR6 and CD69. Blocking LFA-1 and VLA-4 on resting CD4+ T cells abrogated infection in the co-culture model, indicating that engagement of these integrins is essential for enhancement of resting CD4+ T cell HIV susceptibility by endothelial cells. Cellular activation of CD4+ T cells did not appear to be the primary mechanism enabling HIV replication since only a small proportion of resting CD4+ T cells became activated over the course of the co-culture and fewer than half of infected cells had an activated phenotype. The demonstration that endothelial cells enhance the cellular HIV susceptibility of resting memory CD4+ T cells through cell trafficking pathways engaged during the transmigration of memory T cells into inflamed tissues highlights the physiological relevance of these findings for HIV acquisition and opportunities for intervention.</jats:p><jats:sec><jats:title>Author Summary</jats:title><jats:p>HIV acquisition risk per coital act is relatively low, but this risk is amplified by various behavioural and biological variables. Genital inflammation is a key biological variable associated with increased risk of HIV acquisition, but the mechanisms driving this are incompletely understood. Inflammation is a complex process, with direct effects on HIV target cells as well as the tissue in which those cells reside and encounter virus. The first HIV target cells <jats:italic>in vivo</jats:italic> are resting memory CD4+ T cells, yet these cells are do not support viral replication when purified and exposed to HIV <jats:italic>in vitro</jats:italic>. Rather, signals from tissue microenvironment are required to support viral replication within resting memory CD4+ T cells. Endothelial cells line tissue vasculature and guide immune cell trafficking to inflamed tissues through engagement of integrins by endothelial-expressed cell adhesion molecules. We show here that these same cell-trafficking pathways enable endothelial cells to promote HIV replication within resting memory CD4+ T cells <jats:italic>in vitro</jats:italic>. Blockade of integrins on resting memory CD4+ T cells prevented endothelial enhancement of HIV infection. These findings further our understanding of the determinants of cellular susceptibility to HIV infection and offer a potential mechanism by which inflammation promotes HIV acquisition.</jats:p></jats:sec>

Phenotypic characterization of single CD4+ T cells harboring genetically intact and inducible HIV genomes

“…The combination of α4 and β1 integrins, also known as the adhesion molecule VLA-4, is involved in trafficking of immune cells to inflammatory sites as well as in T cell co-stimulation 84 . It was recently demonstrated that expression of VLA-4 on resting CD4+ T cells favors their susceptibility to HIV infection in vitro 85 suggesting that VLA-4 may contribute to the reseeding of the reservoir by promoting the migration of resting CD4+ T cells to inflammatory tissues and facilitating their de novo infection during ART 86,87 . Although this model is attractive, results from many studies strongly suggest that reservoir replenishment is limited during ART and that the bulk of latently infected cells is established around the time of ART initiation 88 .…”