(M-488) Identifying Vaginal Microbes and Metabolites with Pro- and Anti-Histamine Potential

In recent years, the vaginal microbiome, a complex ecosystem of microorganisms, has emerged as a critical player in women's health and disease, potentially influencing many conditions including bacterial vaginosis, preterm birth, endometriosis, and the risk of transmission of sexually transmitted infections. However, there remains a considerable knowledge gap concerning the interaction between the vaginal microbiome and host biology. Histamine is a bioactive amine that plays a key role in the mechanism of allergic response. Signaling through the histamine pathway by binding of histamine to one of its receptors (H1, H2, H3, and H4) results in the production of clinical allergic symptoms, which has significant implications for both inflammation and immune responses. In the female reproductive tract, histamine receptors H1 and H2 are dominant, indicating that histamine signaling may play a role in many inflammatory conditions relevant to women’s health. However, the link between the vaginal microbiome and changes in histamine signaling remains poorly understood.

This project aims to bridge this gap, intending to identify microbes and metabolites within the vaginal microbiome that mimic the function of histamines and antihistamines. By exploring these newly identified microbes and metabolites, their impact on host histamine signaling, and the relevance of these insights to inflammatory pathways in women’s reproductive health, our goal is to highlight new paths for developing more effective treatments and preventative approaches targeting vaginal microbiome-histamine signaling interactions.

Data from the HIV pre-exposure prophylaxis (PrEP) study was used in this project, which gathered vaginal swabs and biopsies from 405 East African women. These women were categorized into two cohorts - the first group included 315 women, aged 18-51, with diverse exposures to HIV-risk factors. The second group consisted of 90 women aged between 25-51 who had consented to provide multiple genital samples. Vaginal biopsies were collected from these women for transcriptomics data, and their swabs were processed for proteomics and metabolomics data. The relationships between pairs of factors, including vaginal microbiome composition-host proteomics, vaginal microbiome composition-host transcriptomics, vaginal metabolomics-host proteomics, and vaginal metabolomics-host transcriptomics were determined using Spearman correlation analysis. 

Gene signatures of antihistamine drugs homochlorcyclizine, antazoline, and hydroxyzine were gathered from the Library of Integrated Network-Based Cellular Signatures (LINCS) database. These signatures were derived from the MCF7 breast cancer cell line exposed to these drugs at a 10 μM concentration for a 24-hour period. Using a Fisher's Exact test, these antihistamine-induced gene signatures were compared to the signatures of microbes and metabolites obtained from the Spearman correlation analysis. This comparison was conducted separately for upregulated and downregulated gene signatures. For each drug, scatter plots were created comparing gene signatures of microbes/metabolites and antihistamine drugs, as well as histamine alone. From this analysis, microbes or metabolites that significantly overlap with both upregulated and downregulated gene signatures for the drugs were identified. These findings give insights into potential mimicry of drug activity by certain microbes or metabolites.

Upon comparison of gene signatures, many microbes and metabolites exhibited similar upregulated and downregulated gene signatures to those of the antihistamine drugs homochlorcyclizine, antazoline, and hydroxyzine, and to the histamine ligand alone. Of note, universal anti-histamine factors common to all antihistamine drugs include xanthine, citrulline, hydroxyisocaproate, imidazole propionic acid, and phenyllactate. Xanthine is a purine base that is used as a bronchodilator. Citrulline is an amino acid that promotes vasodilation. Hydroxyisocaproate is a product of leucine metabolism and is considered an anti-catabolic substance, increases muscle mass, and has anti-inflammatory properties. Imidazole propionic acid is an intermediate in the metabolism of histidine and has been shown to affect insulin signaling. Phenyllactate is an organic acid with antimicrobial activity and immune-modulatory effects. These metabolites all exhibit anti-allergic properties, which may be pertinent to investigate further in their application to the vaginal microbiome.

While most antihistamine-mimicking metabolites do not share a similar gene signature to histamine, there appears to be some similarity in gene signatures of microbes when compared to both. Lactobacillus strains and Gardnerella vaginalis were found to have similar gene signatures to both antihistamine drugs and histamine. Major vaginal Lactobacillus species, Prevotella species and Gardnerella vaginalis are commonly present in vaginal microbiomes that are both negative and positive for bacterial vaginosis (BV), acting as both a vaginal symbiont and an opportunistic pathogen. More work is needed to identify the relationship between these microbes and histamine.

This analysis reveals several metabolites and microbes that exhibit gene signatures mimicking those of antihistamine drugs. Universal anti-histamine factors such as xanthine, citrulline, hydroxyisocaproate, imidazole propionic acid, and phenyllactate have been identified, which may hold significant relevance in anti-inflammatory treatments. Certain Lactobacillus strains, Prevotella strains and Gardnerella vaginalis have gene signatures similar to both antihistamine drugs and histamine, indicating their dual roles in the vaginal environment as beneficial symbionts and potential pathogens. We presents an avenue for further investigation into how these identified microbes and metabolites might influence the pathogenesis of women’s health conditions through the regulation of inflammatory and immune pathways, including histamine signaling, and aim to leverage this knowledge to identify novel therapeutic strategies.