Increased Susceptibility to Vaginal Simian/Human Immunodeficiency Virus Transmission in Pig-tailed Macaques Coinfected With Chlamydia trachomatis and Trichomonas vaginalis

Background. Sexually transmitted infections (STIs) are associated with an increased risk of human immunodeficiency virus (HIV) infection, but their biological effect on HIV susceptibility is not fully understood. Methods. Female pig-tailed macaques inoculated with Chlamydia trachomatis and Trichomonas vaginalis (n = 9) or medium (controls; n = 7) were repeatedly challenged intravaginally with SHIVSF162p3. Virus levels were evaluated by real-time polymerase chain reaction, plasma and genital cytokine levels by Luminex assays, and STI clinical signs by colposcopy. Results. Simian/HIV (SHIV) susceptibility was enhanced in STI-positive macaques (P = .04, by the log-rank test; relative risk, 2.5 [95% confidence interval, 1.1-5.6]). All STI-positive macaques were SHIV infected, whereas 3 controls (43%) remained uninfected. Moreover, relative to STI-negative animals, SHIV infections occurred earlier in the menstrual cycle in STI-positive macaques (P = .01, by the Wilcoxon test). Levels of inflammatory cytokines (interferon gamma, interleukin 6, and granulocyte colony-stimulating factor [G-CSF]) were higher in STI-positive macaques during STI inoculation and SHIV exposure periods (P <= .05, by the Wilcoxon test). Conclusions. C. trachomatis and T. vaginalis infection increase the susceptibility to SHIV, likely because of prolonged genital tract inflammation. These novel data demonstrate a biological link between these nonulcerative STIs and the risk of SHIV infection, supporting epidemiological assocations of HIV and STIs. This study establishes a macaque model for studies of high-risk HIV transmission and prevention.