Specific Vaginal Bacteria Increases HIV Risk in Some Women

South African women with pro-inflammatory bacteria that dominates vaginal microbiomes have more than a 4-fold higher risk of acquiring HIV than those with healthy vaginal bacteria, a recent study indicates.

An estimated 24 million individuals in sub-Saharan Africa are infected with HIV. According to UNAIDS, young African women have up to an 8-fold higher rate of HIV prevalence compared with young men.

Increased inflammation is associated with an increased risk of HIV infection, and cervicovaginal bacteria has been shown to impact baseline inflammation in the female genital tract (FGT).

In a study published in Immunity, investigators sought to determine cervicovaginal bacteria’s role in HIV susceptibility.

Included in the study were participants from the prospective cohort study called FRESH (Females Rising through Education, Support and Health). The participants were black South African women aged 18 to 23 years, who were monitored with high-frequency testing for incident of HIV infection.

The investigators followed 236 HIV-uninfected women with a median follow-up time of 226 days (IQR: 178.5 to 347 days) and a total follow-up time of 198.2 person-years. Despite intensive HIV prevention counseling, 31 women acquired HIV and 205 remained free of HIV. The overall HIV incidence in the FRESH cohort was 8.43 per 100 person-years.

The investigators performed bacterial 16S ribosomal (r)RNA gene and viral shotgun sequencing of cervicovaginal microbiota and assessed their role in HIV acquisition among the study participants. Sequencing of the V4 region of the bacterial 16S rRNA gene in the 236 participants demonstrated the presence of distinct bacterial community types. Based on composition and diversity, they were clustered into 4 basic groups, referred to as cervicotypes (CTs).

The results of the study showed that 10% of women had low diversity communities dominated by Lactobacillus crispatus (CT1, n=23), and 32% were dominated by Lactobacillus iners (CT2, n=74). The findings contrast with white women located in developed countries, of whom 90% have Lactobacillus-dominant cervicovaginal communities.

In the remaining 58% of FRESH participants, the cervicovaginal bacterial microbiome consisted of high-diversity communities with low Lactobacillus abundance, with either Gardnerella vaginalis dominance (CT3, n=68) or a bacterial genus other than Lactobacillus or Gardnerella as the dominant taxon (CT4, n=70).

The most abundant taxa in CT4 were Prevotella, Gardnerella, Shuttleworthia, Sneathia, and Megasphaera, according to the study.

“We think of a healthy microbiome as being Lactobacillus dominant—–that's what we are taught in medical school––but those studies are mostly based on white women in developed countries,” said senior author Douglas Kwon.

“When we did our first study we found that less than 10% of the women in our South African cohort had this classically 'healthy' community," he added. "Seventy percent of our volunteers had diverse bacterial communities with low Lactobacillus abundance. Here we show that not only are those more diverse communities associated with higher levels of genital inflammation but also with significantly increased HIV acquisition.”

Metagenomics sequencing was used to characterize DNA and RNA viral populations in 180 study participants. The investigators identified alphapapillomaviruses and Anelloviridae as the predominant eukaryotic viral taxa present in the FGT in this cohort. Additionally, multiple species of Caudovirales were observed.

The authors noted that no distinct viral community structures were identified within the cohort, nor did they detect a difference in viral composition among different bacterial communities.

The investigators also examined the associations between cervicovaginal bacteria and HIV acquisition. The results of the study showed a greater than 4-fold increase in HIV acquisition among women with high-diversity, low Lactobacillus abundance communities compared with women with L. crispatus dominance (CT4: HR = 4.03, 95% CI: 1.14 to 14.27, p = 0.031, and CT3: HR = 4.22, 95% CI: 1.06 to 16.88, p = 0.042). None of the study participants who acquired HIV had an L. crispatus dominant community.

The relationship between the bacterial microbiome and HIV target cell numbers in the genital tract was examined, revealing a 17-fold increase in HIV target cells in the FGT of women with CT4 versus L. crispatus-dominant communities.

The investigators demonstrated elevated levels of chemokines MIP-1α and MIP-1β in women with high-diversity bacterial communities. Additionally, the findings showed that higher concentrations of cytokines and chemokines was associated with increased HIV acquisition.

Women who became infected with HIV had similar pro-inflammatory cytokine levels as those who remained uninfected within the same CT group, according to the study.

The investigators sought to determine whether specific bacterial taxa was associated with genital inflammation and HIV infection. The results of the study showed that L. crispatus, and to a lesser extent, L. iners were associated with reduced inflammation. Anaerobic taxa was found to be associated with increased inflammation.

In order to more definitively assess a mechanistic role for these bacteria in inducing increased number of FGT HIV target cells, the investigators inoculated germ-free mice intravaginal with L. crispatus or P. bivia.

The experiment showed higher numbers of activated (CD44+) CD4+ T cells in the genital mucosa, but not in the blood of mice inoculated with P. bivia compared with mice that received L. crispatus. Additionally, the mice further treated with P. bivia had increased numbers of mucosal CCR5+ CD4+ T cells. Furthermore, it showed a trend toward elevated genital TNF-α concentrations compared with mice inoculated with L. crispatus. These results indicate specific bacterial taxa may increase HIV infection risk by inducing elevated numbers of HIV target cells in the FGT.

The findings indicated that women with high-diversity genital bacterial communities acquire HIV at 4 times higher rates; activated mucosal CD4+ T cell numbers are elevated in women with high-risk bacteria; specific genital bacteria taxa are linked with reduced or elevated HIV acquisition; and intravaginally administered Prevotella increased activated genital CD4+ T cell numbers in mice.

“We're very excited about these findings,” said first author Christina Gosmann. “We've used modern molecular approaches to characterize the vaginal microbiome and link specific bacteria to HIV acquisition risk in young women living in Sub-Saharan Africa, where HIV is most profound. There is a direct translational application that comes from this work. By identifying bacterial species and communities associated with HIV risk, we provide specific targets that may be leveraged to develop new preventive strategies and to improve the effectiveness of existing preventive measures.”