The rectal mucosal immune environment and HIV susceptibility among young men who have sex with men

The rectal microbiome: understanding its role in HIV transmission

PURPOSE OF REVIEW

Condomless receptive anal intercourse stands out as the sexual practice with highest risk of HIV-1 infection. Recent studies have suggested that the gut microbiome influences susceptibility to HIV transmission. This review explores recent research on host risk factors, the rectal microbiome composition, local inflammation, and bacteria-derived mediators that may affect HIV transmission.

RECENT FINDINGS

Constitutive host factors such as rectal mucosal structure and immune cell populations in the rectum contribute to increased susceptibility. Changes in the composition of the rectal microbiota, influenced by sexual practices and HIV infection modulate immune activation and inflammation, impacting HIV susceptibility. Bacteria-derived mediators may further influence immune responses and HIV replication in the rectal mucosa.

SUMMARY

Understanding the role of the rectal microbiome in HIV transmission has important clinical implications. Targeted interventions that modulate the microbiome may reduce susceptibility to HIV transmission by regulating immune responses and inflammation. Further research into the host-microbiome interactions could lead to novel preventive and therapeutic strategies to mitigate HIV transmission.

Rectal mucosal inflammation, microbiome, and wound healing in men who have sex with men who engage in receptive anal intercourse

Mucosal injury is common during consensual intercourse and induces an inflammatory response that could contribute to pathogen transmission including HIV. Here, we compared mucosal immune and microbiome responses to experimentally induced mucosal injury between men who have sex with men engaging in receptive anal intercourse (MSM-RAI) and men who do not engage in RAI (controls), all without HIV. Rectal mucosal secretions were collected from adult MSM-RAI (n = 19) and controls (n = 6) via anoscopy before and up to eight days after experimentally induced injury. Mucosal healing was evaluated by repeated injury surface area measurements with digital imaging. MSM-RAI demonstrated overall significantly higher concentrations of pro-inflammatory cytokines and a distinct rectal microbiome compared with controls. Wound healing was numerically faster in MSM-RAI but did not meet statistical significance (p = 0.09). Different cytokine injury response patterns were observed between MSM-RAI and controls; however, IL-6 and IP-10 were important mediators in both groups. Microbial guilds, particularly from the Lachnospiraceae and Prevotellaceae families, were associated with rectal mucosal inflammation. This work is the first experimental study of rectal mucosal injury and the immune environment in healthy humans and provides a more nuanced understanding of rectal mucosal inflammation after injury, which can inform our understanding of HIV transmission.

The role of pharmacomicrobiomics in HIV prevention, treatment, and women's health

In the absence of an effective vaccine or curative treatment for HIV, the global HIV/AIDS epidemic continues despite significant advances in treatment and prevention. Antiretroviral therapy (ART) drugs have transformed HIV from a terminal illness to a manageable chronic condition. Likewise, pre-exposure prophylaxis treatment (PrEP) has dramatically reduced transmission in some of the highest risk populations. However, quality of life and life expectancy in people living with HIV (PWH) still lag significantly behind the general population. The mechanisms that reduce the efficacy of PrEP and ART are multifaceted, but one factor that warrants additional attention is the impact of the microbiome on ART and PrEP efficacy, as well as pharmacokinetics more broadly. In this review, we assess the current state of research on the HIV-associated microbiome, how this impacts treatment efficacy, and how microbiome states can alter HIV susceptibility. We also explore how the mechanisms we propose could extend to the efficacy of other drugs and identify promising areas of research that remain understudied. Video Abstract.

Integrated analysis of rectal mucosal microbiome and transcriptome reveals a distinct microenvironment among young MSM

Crosstalk between the microbiome and gut mucosa-resident immune cells plays a pivotal role in modulating immune responses to pathogens, including responses to HIV infection. However, how these interactions may differ between young men who have sex with men (YMSM) disproportionately impacted by HIV, as compared with older adult MSM (AMSM), is not well understood. A broad analysis of associations between the microbiome and rectal transcriptome revealed 10 microbial families/genera correlated with immunologic gene pathways. Specifically, the rectal transcriptome of YMSM was characterized by upregulation of T cell activation/differentiation pathways and signaling from multiple cytokine families compared with AMSM. The microbiome of YMSM was enriched with pathogenic genera, including Peptostreptococcus, shown to be positively correlated with type I IFN pathways important for antiviral immunity. These findings demonstrate that YMSM have a unique immune phenotype and rectal microenvironment and support further evaluation of biological factors that influence rectal HIV transmission.

The impact of sex on HIV immunopathogenesis and therapeutic interventions

Globally, the majority of people living with HIV are women or girls, but they have been a minority of participants in clinical trials and observational studies of HIV. Despite this underrepresentation, differences in the pathogenesis of HIV have been observed between men and women, with contributions from both gender- and sex-based factors. These include differences in the risk of HIV acquisition, in viral load set point and immune activation in responses to viremia, and differences in HIV reservoir maintenance. These differences obligate adequate study in both males and females in order to optimize treatments, but also provide a powerful leverage point for delineating the mechanisms of HIV pathogenesis. The shifts in exposure to sex steroid hormones across a lifespan introduce additional complexity, which again can be used to focus on either genetic or hormonal influences as the driver of an outcome. In this Review, we discuss consistent and reproducible differences by sex across the spectrum of HIV, from acquisition through pathogenesis, treatment, and cure, and explore potential mechanisms and gaps in knowledge.

Sexual behavior is linked to changes in gut microbiome and systemic inflammation that lead to HIV-1 infection in men who have sex with men

Pathogenic changes in gut microbial composition precede the onset of HIV-1 infection in men who have sex with men (MSM). This process is associated with increased levels of systemic inflammatory biomarkers and risk for AIDS development. Using mediation analysis framework, in this report we link the effects of unprotected receptive intercourse among MSM prior to primary HIV-1 infection to higher levels of proinflammatory cytokines sCD14 and sCD163 in plasma and a significant decrease in the abundance of A. muciniphila, B. caccae, B. fragilis, B. uniformis, Bacteroides spp., Butyricimonas spp., and Odoribacter spp., and a potential increase in the abundance of Dehalobacterium spp. and Methanobrevibacter spp. in stools of MSM with the highest number of sexual partners. These differences in microbiota, together with a reduction in the pairwise correlations among commensal and short-chain fatty acid-producing bacteria with a number of sexual partners, support an increase in gut dysbiosis with the number of sexual partners. These results demonstrate the interconnectedness of sexual behavior, immune response, and microbiota composition, notably among MSM participating in high-risk sexual behaviors.

The effect of sexual behavior on HIV-1 seroconversion is mediated by the gut microbiome and proinflammatory cytokines

The association between HIV-1 seroconversion and gut dysbiosis is well documented, and its association with sexual activity is also widely recognized. However, it is not known whether the gut dysbiosis mediates the effects of high-risk sexual behavior on HIV-1 seroconversion. In this report we focused on men who engaged in high-risk sexual behavior where they had receptive anal intercourse with multiple men. We demonstrate that proinflammatory cytokines, sCD14 and sCD163, and gut microbiota mediate the effects of this high-risk sexual behavior on subsequent HIV seroconversion. We discovered changes in the gut microbial ecology, prior to seroconversion, both in terms of the composition as well as inter-relationships among the commensal species. Furthermore, these changes correlate with future HIV seroconversion. Specifically, as the number of sexual partners increased, we discovered in a "dose-response" manner, a decrease in the abundance of commensal and short-chain fatty acid-producing species, A. muciniphila, B. caccae, B. fragilis, B. uniformis, Bacteroides spp., Butyricimonas spp., and Odoribacter spp, and an increase in proinflammatory species Dehalobacterium spp. and Methanobrevibacter spp. These changes were also observed among subsequent HIV seroconverters. Interestingly, we also discovered a reduction in correlations among these commensal and short-chain fatty acid producing bacteria in a "dose-response" manner with the number of sexual partners. Our mediation analysis not only provides a conceptual model for the disease process but also provides clues for future clinical interventions that will manipulate the gut microbiota to treat high-risk subjects to prevent HIV seroconversion.

HIV, asymptomatic STI, and the rectal mucosal immune environment among young men who have sex with men

Young men who have sex with men (YMSM) are disproportionately affected by HIV and bacterial sexually transmitted infections (STI) including gonorrhea, chlamydia, and syphilis; yet research into the immunologic effects of these infections is typically pursued in siloes. Here, we employed a syndemic approach to understand potential interactions of these infections on the rectal mucosal immune environment among YMSM. We enrolled YMSM aged 18-29 years with and without HIV and/or asymptomatic bacterial STI and collected blood, rectal secretions, and rectal tissue biopsies. YMSM with HIV were on suppressive antiretroviral therapy (ART) with preserved blood CD4 cell counts. We defined 7 innate and 19 adaptive immune cell subsets by flow cytometry, the rectal mucosal transcriptome by RNAseq, and the rectal mucosal microbiome by 16S rRNA sequencing and examined the effects of HIV and STI and their interactions. We measured tissue HIV RNA viral loads among YMSM with HIV and HIV replication in rectal explant challenge experiments among YMSM without HIV. HIV, but not asymptomatic STI, was associated with profound alterations in the cellular composition of the rectal mucosa. We did not detect a difference in the microbiome composition associated with HIV, but asymptomatic bacterial STI was associated with a higher probability of presence of potentially pathogenic taxa. When examining the rectal mucosal transcriptome, there was evidence of statistical interaction; asymptomatic bacterial STI was associated with upregulation of numerous inflammatory genes and enrichment for immune response pathways among YMSM with HIV, but not YMSM without HIV. Asymptomatic bacterial STI was not associated with differences in tissue HIV RNA viral loads or in HIV replication in explant challenge experiments. Our results suggest that asymptomatic bacterial STI may contribute to inflammation particularly among YMSM with HIV, and that future research should examine potential harms and interventions to reduce the health impact of these syndemic infections.