Impact of the Vaginal Microbiome on HIV Transmission

Abstract

HIV is a global epidemic affecting almost 35 million people worldwide. Each year, almost 2 million new infections occur, with over 1 million new infections in women. The majority of these new infections take place in low income countries, primarily Sub-Saharan Africa. HIV transmission is highly understudied within women where each minute, two women are infected with HIV. Increased HIV transmission risk is characterized by three major mechanisms: barrier damage, inflammation, and an altered microbiome. The interplay between these mechanisms and their underlying causes have not been fully elucidated. The broad purpose of this thesis was to address several unanswered questions relating to mechanisms for an altered microbiome state within the female reproductive tract (FRT) and its impact on HIV transmission. We aimed to address three main objectives: 1) determining the impact of vaginal microbial communities on immune cells essential for pathogen protection and epithelial integrity; 2) identifying which specific bacterial species that are associated with HIV transmission and alter pre-exposure prophylaxis (PrEP) drug concentrations; and 3) characterizing the kinetics of PrEP uptake into target cells versus bacteria-mediated drug metabolism. Within the FRT, an altered microbiome consists of the loss of a Lactobacillus dominant community replaced with a diverse, anaerobic community primarily consisting of Gardnerella vaginalis, Atopobium vaginae, Prevotella spp., and other bacterial vaginosis (BV)-associated bacteria. This shift in community is often referred to as vaginal dysbiosis, or bacterial vaginosis (BV). BV is highly prevalent and associated with an 60% increase in HIV acquisition rate. The exact mechanisms by which BV impacts HIV acquisition rates are currently unknown and this thesis aims to address those questions. The role of neutrophils in BV is a major theme of the studies described in chapter II. While neutrophils are important for pathogen containment, evidence has demonstrated their ability to contribute to tissue and barrier damage due to the release of reactive oxygen species and other potentially harmful effector molecules. Here, we demonstrate that BV associated bacteria increase neutrophil activation and increase lifespan, potentially contributing to accumulation in the FRT and reduced epithelial integrity. Healthy vaginal microbial communities, however, do not interfere with homeostatic apoptosis of neutrophils. The study described below provides potential mechanistic insights into how BV may lead to FRT inflammation and increased HIV transmission. The success of pre-exposure prophylaxis (PrEP) drugs such as Truvada, have greatly decreased HIV transmission rates when adhered to. However, the efficacy seen in women highly varies and ranges from 75% to -49% in clinical trials. This variability is far more severe when compared to their male counterparts. While these studies have attributed these varying outcomes to adherence, recent evidence has highlighted the role biological factors, such as the vaginal microbiome, can play in impacting effectiveness of PrEP. In chapter III, through a secondary analysis, we demonstrate that vaginal microbial communities impact HIV incidence rates and that specific taxa, such as G. vaginalis, can directly impact tenofovir drug levels through a bacteria-mediated metabolic process. Indeed, in chapter IV, we show that this metabolism occurs at a faster rate than target cell conversion to the pharmacologically active drug and thus provide evidence linking vaginal bacteria to microbicide efficacy. When evaluating PrEP in women, vaginal microbial communities must be considered and resolving dysbiosis may be critical for enhancing PrEP efficacy and prevention of HIV acquisition in women. Taken together, the data presented in this thesis improves our understanding of the mechanisms contributing to increased HIV transmission risk in women and the impact vaginal microbial communities can have. This thesis alludes to potential therapies that should be further investigated to improve overall health in women, a highly underserved community within HIV prevention studies.