Impact of the Cervicovaginal Microbiome on HIV Susceptibility: An Investigation of Mechanisms and Potential Interventions

Abstract

Sub-Saharan Africa has among the highest HIV infection rates in the world, with an estimated 980,000 new HIV infections in 2017 (Sidebé 2018). Since the majority of HIV transmission occurs through heterosexual sex (UNAIDS, 2014), understanding how HIV infection is established within the female genital tract (FGT) is critical for the development of HIV preventative interventions, and deserves further study. It has been known for decades that bacterial vaginosis (BV), a disease state characterized by increased FGT inflammation, increased pH, and colonization with non-Lactobacillus species, is associated with an increased risk of acquiring HIV (Sewankambo 1997), along with other sexually transmitted infections (STIs), and with health problems during pregnancy such as preterm birth (McGregor 2000). More recently it has also been shown that BV is associated with increased risk of female-to-male HIV transmission (Cohen 2012). In this study, we conducted in vitro experiments on cervical tissue culture models of epithelial cells and fibroblasts. Our results confirmed prior observations regarding inflammation of BV associated bacteria such as Prevotella bivia, Lactobacilus iners, and Gardnerella vaginalis. At set time periods after the introduction of these pro-inflammatory species (with Lactobacillus crispatus for comparison) the supernatants were tested using ELISA assays for various pro-inflammatory cytokines (e.g. IL-6, IL-8), and cell death (via LDH), in order to determine which bacterial species induced pro-inflammatory cytokines. The same experiments were then repeated with the addition of various innate immune inhibitors targeting PAMP (Pathogen-Associated Molecular Pattern) sensing receptors such as TLR-4 (LPS), TLR-2 (peptidoglycan), and TLR-9 (unmethylated CpG) and down-stream inflammatory pathway facilitators (NFκB). The results of these experiments suggested specifically which signaling pathways were responsible for the innate-immune system activation and release of cytokines, and suggested potential interventions to block the process. Strains of bacterial species that have been previously associated with an increased HIV risk were then isolated from the South African FRESH (Females Rising through Education Support and Health) cohort for further testing. Several strains of L. crispatus, G. vaginalis, and P. bivia from FRESH samples were isolated, and their DNA sequenced, to investigate the phylogenetic relationship of these bacteria. Furthermore, three novel FGT species were isolated in culture for the first time, and are now available for further research.