The genital epithelium plays a critical role in regulating the sexual transmission of HIV. Expression of intercellular adhesion molecule-1 (ICAM-1) facilitates cell-to-cell virus transmission and is increased by inflammation. Women with Lactobacillus spp.-dominated vaginal microbiota have a decreased risk of HIV acquisition. Lactic acid (LA) is a key metabolite (or bioactive product) of Lactobacillus spp. with antimicrobial and anti-inflammatory properties that is differentially produced by Lactobacillus spp. as L- and D- isomers. LA also acts directly on cervicovaginal epithelial cells to enhance barrier integrity. Here, we investigated whether LA could modulate the expression of ICAM-1 to influence HIV transmission through epithelial cells.
Immortalised ectocervical (Ect), endocervical (End) and vaginal (VK2) epithelial cells were cultured in transwells and treated apically for 1 h with 0.3% L-LA or D-LA (pH 3.9), L- or D-lactate (pH 7.0), or acidified media (pH 3.9, HCl adjusted) simultaneously with toll-like receptor (TLR) agonists poly I:C (PIC, TLR3), FSL-1 (TLR2/6), or PAM3CSK4 (TLR1/2). After 24 h incubation, expression of ICAM-1 mRNA was determined by RNASeq and surface protein expression by flow cytometry.
ICAM-1 mRNA was increased by PIC stimulation of Ect cells, which was reduced 2.2-fold in the presence of L-LA (n=3, FDR< 0.05). Stimulation of Ect cells with PIC significantly increased ICAM-1 protein expression by 1.6-fold (n=4, p< 0.05) compared to unstimulated cells. However, this increase was almost entirely abrogated by treatment with L- or D-LA (both p< 0.01), but not HCl-acidified media, or L- or D-lactate at a neutral pH (all p >0.05), indicating that inhibition of ICAM-1 upregulation is specifically mediated by the uncharged, low pH form of LA. Similar findings were observed for Ect cells treated with FSL-1 and PAM3CSK4, as well as TLR-stimulated End and VK2 cells.
Treatment of cervicovaginal epithelial cells with LA, a microbiome bioactive, specifically inhibits ICAM-1 upregulation associated with TLR stimulation, which may influence mucosal HIV transmission. These findings suggest a potential mechanism by which optimal Lactobacillus-dominated vaginal microbiome may protect against HIV transmission. Future studies will investigate direct cell-to-cell transmission of virus sequestered in epithelial cells to HIV target cells and the effect of LA treatment.