Bacterial vaginosis (BV) is associated with inflammatory responses and an increased transmission rate of HIV, however, the mechanism(s) underlying this are unknown. BV is characterized by a change from a predominantly Lactobacillus spp. environment to a more diverse community, including Gardnerella vaginalis. Inflammatory cells such as neutrophils are critical for innate immune responses, but can also contribute to barrier damage and inflammation. Currently, the role of neutrophils in HIV transmission and BV status is unknown. Here, we hypothesize that BV-associated bacteria will induce neutrophil activation and prolonged life, promoting neutrophil accumulation in the female reproductive tract (FRT), thereby potentially promoting tissue damage.
In order to elucidate the mechanisms for the negative consequences of BV, we collected cervicovaginal cytobrushes from 6 women with BV (Nugent >7), and without BV (Nugent <5). We used flow cytometry to assess phenotype and functionality of neutrophils, and performed in vitro whole blood co-cultures with bacteria associated with BV (G. vaginalis), healthy commensals such as L. iners and L. crispatus, media alone (negative control) and lipopolysaccharide and peptidoglycan (positive controls).
We demonstrated increased neutrophil activation via CD62L downregulation (p=0.0022) and prolonged lifespan via decreased caspase-3 expression (p=0.0022) in the cytobrushes with women with BV. Similarly, in our co-culture experiments we observed more neutrophil activation (p<0.0001) and prolonged lifespan (p=0.079) in cultures with G. vaginalis similar to our positive controls. Of interest, women without BV had reduced inflammatory immune responses similar to our in vitro experiments with cultures containing Lactobacillus spp. that maintained immune cell phenotypes similar to negative controls.
Here, we demonstrate that BV-associated bacteria induce neutrophil activation and delay apoptosis as a potential mechanism for promoting tissue damage. Women without BV, that have healthy vaginal mucosal bacterial communities (Lactobacillus) may support the balance between antimicrobial function of neutrophils and do not induce inflammation, whereas women with BV and dysbiotic vaginal communities (G. vaginalis) potentially lead to neutrophil accumulation and tissue damage. Thus, this study provides potential mechanistic insights into how BV may lead to FRT inflammation and increased HIV transmission.