PROJECT DESCRIPTION/ABSTRACT Most HIV-1 infections are acquired through the mucosal surface of either the genital tract or the gastrointestinal tract, two immunologically distinct mucosal sites. Although the rate of HIV-1 transmission across the mucosal surfaces is low, thousands of mucosal transmissions occur each year, relentlessly fueling the global AIDS epidemic. However, despite frequent or repeated mucosal exposure, some individuals appear to be naturally resistant to HIV-1 infection. Natural resistance in highly HIV-Exposed Seronegative (HESN) persons is likely multi-factorial. To address this topic, we hypothesize that (1) HESN cervicovaginal and rectal secretions neutralize HIV-1 or inhibit HIV-1 entry into or dissemination through mucosae and (2) HESN cervicovaginal and rectal secretions promote a non-inflammatory mucosal environment that limits epithelial cell activation, DC uptake and migration, and CD4+ T cell recruitment and activation. Using human vaginal and rectal mucosal explant systems and primary human mucosal cells, our specific aims seek to determine whether human cervicovaginal and rectal secretions from HESN women (1) contain HIV-1 entry-blocking and neutralizing activity for mucosal cells;(2) inhibit HIV-1 mucosal entry into and dissemination through mucosal tissue;and (3) promote a non-inflammatory mucosal environment. Elucidating the contribution of immune factors in mucosal secretions to the natural resistance to HIV-1 in women could promote the development of new prevention strategies.