The microenvironment of the lower female reproductive tract is a complex ecosystem composed of epithelial and immune cells and a diverse microbiota. Both host and microbial communities produce metabolites and immune mediators that regulate reproductive health and homeostasis. Disruption of this balance can lead to infection, infertility, miscarriage, preterm birth, and gynaecological disease. A healthy vaginal microbiome is usually dominated by Lactobacillus species, which produce lactic acid (LA) to maintain an acidic environment that protects against pathogens. Whereas, Lactobacillus-depletion, with greater bacterial diversity, is associated with elevated short-chain fatty acids and succinate, reduced acidity, inflammation, loss of epithelial barrier integrity, and greater risk of reproductive disorders. Although LA has been implicated in cytokine release from vaginal epithelial cells (VECs), its mechanisms remain unclear.
This thesis tested the hypothesis that LA is a key regulator of transcription factor activity and innate immune responses in VECs. The research aimed to characterise LA-mediated signalling in response to bacterial and viral agonists, identify potential lactate receptors, and examine the contribution of LA to epithelial barrier function. A proteomics approach was employed to identify novel pathways involved in LA-mediated VEC regulation.
Using a multilayered in vitro VEC model, LA was found to suppress inflammatory signalling, consistent with observations in cervicovaginal fluid samples collected from pregnant women. LA inhibited NFκB, AP-1, IRF3, and upstream kinases, with evidence for HCAR1 receptor involvement. Further support for LA’s role in immune regulation was provided by studies of pregnant women, in whom therapeutic modulation of the vaginal microbiota towards Lactobacillus dominance was associated with increased LA levels and reduced inflammation. Proteomics analysis identified additional LA-responsive pathways related to cytoskeletal organisation, signalling, and metabolism.
Overall, these findings provide new insight into the regulatory role of LA in inflammation of VECs and suggest broader implications for reproductive health and pregnancy outcomes.