HIV Transmission and Immunology of the Male Reproductive Tract.

The penile epithelium, encompassing multiple anatomical sites, is the primary location of human immunodeficiency virus (HIV) acquisition in heterosexual men. Although the per-contact risk of penile HIV acquisition is generally low, substantial global discrepancies in HIV prevalence still exist, particularly in low-income regions. In uncircumcised men, the immune milieu of the subpreputial space is a key determinant of HIV risk, with inflammation-mediated epithelial disruption and target cell recruitment facilitating viral infection. Specific bacterial components of the penile microbiome cause local inflammation and enhance susceptibility, while penile circumcision reduces HIV risk by both removing susceptible foreskin tissues and reducing the abundance of these bacteria. The penile urethra is also an important site of HIV acquisition, particularly among circumcised men, but determinants of urethral susceptibility remain poorly understood. Penile-vaginal sex induces transient inflammation and epithelial damage at both the subpreputial space and urethra, likely mediated by mechanical effects and/or the sexual exchange of pro-inflammatory bacteria. This review summarizes knowledge regarding the immunological and microbial determinants of penile HIV acquisition risk, highlights biological factors and sexual practices that shape the penile immune milieu, and discusses current advances in microbiome-targeting interventions as potential HIV prevention strategies.