Quality and quantity: mucosal CD4+ T cells and HIV susceptibility

Purpose of review The HIV sexual transmission probability measured in the context of discordant couples appears too low to fuel the HIV pandemic, but these rates are substantially amplified by specific co-factors. The most consistent predictors of transmission are the HIV levels in the blood and genital tract of an infected individual, each of which increases the transmission probability in a dose-dependent manner. In an analogous fashion, we propose that both the quantity and quality of HIV-susceptible target cells in the exposed genital or rectal mucosa may be key predictors of HIV susceptibility. Recent findings The absolute number of mucosal CD4(+) T cells is increased in several situations that are associated with amplified HIV transmission, particularly during genital infections. In addition, qualitative mucosal T-cell parameters such as immune activation and the expression of the HIV binding molecules CCR5 and/or alpha(4)beta(7) are important determinants of gp120 binding and productive HIV infection. In particular, the Th17 and Th22 cell subsets are enhanced within mucosal compartments and appear to be highly HIV-susceptible. Summary Blockade of specific HIV target cell subsets at the site of exposure, if done in a safe and effective manner, represents an opportunity for new HIV prevention tools.