Introduction Vaginal lactobacilli are associated with favourable sexual health outcomes and acidify the vagina to pH <4.0 by producing 0.3–1% D and L isomers of lactic acid (LA). Epithelial cells that line the vagina and cervix have barrier and immune functions in the lower female reproductive tract (FRT). Here we investigate the immune modulatory effects of L-LA on lower FRT epithelial cells that might influence HIV susceptibility.

Methods The effect of apically applied L-LA (0.3% w/w, pH3.9) was assessed on vaginal (VK2), endocervical (End), ectocervical (Ect) epithelial cell lines and primary ectocervical cells grown in transwells. Elicited immune mediators were quantified following apical stimulation with toll-like receptor (TLR) agonists ± L-LA by flow cytometry and luminex-based assays.

Results L-LA had little impact on FRT epithelial cell viability. Stimulation of FRT epithelial cell lines with the TLR3-agonist poly (I:C) (PIC) induced high-levels of pro-inflammatory cytokines (IL-6/IL-8), and their variable induction with TLR agonists Pam (3) CSK(4) (TLR1/2) and lipopolysaccharide (TLR4). Conversely, L-LA treatment significantly reduced PIC-induced IL-6 (~30-fold) and IL-8 (3–4.5–fold, p£0.03) secretion, compared to PIC-only treated FRT epithelial cell lines. Irrespective of TLR stimulation, L-LA elicited a 4–11-fold (p < 0.01) increase in the anti-inflammatory cytokine IL-1RA in FRT epithelial cell lines. Neither 0.3% L-LA at neutral pH nor acidity alone (HCl, pH 3.9) elicited the abovementioned effects indicating that immune modulation is mediated by the protonated form of L-LA and is not due to low pH. L-LA also reduced PIC-induced secretion of RANTES and MIP3a in all cells, associated with recruitment of HIV target cells to the mucosa. Similar anti-inflammatory effects of L-LA were observed in primary ectocervical cells.

Conclusion L-LA found in lactobacillus-dominated vaginal microbiota elicits an anti-inflammatory effect on lower FRT epithelial cells and dampens inflammation induced by microbial TLR agonists suggesting a role in mitigating inflammation-induced HIV susceptibility at the vaginal mucosa.

Disclosure of interest statement This study was funded by NHMRC Project APP1088564. No pharmaceutical grants were received in the development of this study.