Many viral infections, including HIV, are acquired at mucosal surfaces, necessitating the characterization of the immune environment at these barrier tissues. Unfortunately, clinical HIV research often relies on easily collected peripheral blood samples which often do not reflect the immune responses that occur at the site of infection, leaving mucosal immune responses in the context of viral infection poorly understood. Additionally, there is a need to characterize the mucosal immune environment in women infected with genital herpes simplex virus 2 (HSV-2), as the risk of HIV acquisition is increased three-fold in these individuals. This is possibly due to an increase in pro-inflammatory environment and CD4+ T cells, the target cell of HIV, but much remains to be determined about the immune consequences resulting from interaction between HSV-2 and HIV in the genital mucosa. However, it is known that oral tenofovir-based pre-exposure prophylaxis (PrEP) not only prevents HIV infection but also decreases HSV-2 symptoms and viral shedding. Whether PrEP affects the mucosal immune environment of the vaginal tract, especially in conjunction with HSV-2 infection, is unclear. Laura Richert-Spuhler (Vaccine and Infectious Disease Division; formerly of the Lund lab) and colleagues hypothesized that PrEP-mediated HSV-2 suppression could induce changes in the mucosal immune environment, and used genital biopsies and blood samples from HSV-2-positive women on PrEP to profile the immune response during and after cessation of PrEP. Their results were published this month in AIDS.
The authors acquired genital biopsy samples and paired blood from HIV-negative HSV-2-positive women from Uganda and Kenya who engaged in sex with HIV-positive male partners. Samples were collected during PrEP treatment and again two months following cessation of PrEP. Using flow cytometry and immunofluorescent staining of blood, they profiled the cervical and blood T cell compartments during and after PrEP. HIV primarily infects a subset of CD4+ T that express the receptor CCR5, so they focused on this T cell population and found that fewer CD4+ T cells, including the CCR5+ subset, were found in the cervixes of women on PrEP compared to higher numbers once off PrEP. Likewise, CD8+ T cells, which are important in the killing of HIV-infected CD4+ T cells, followed a similar pattern. Interestingly, this trend was reversed in blood, where both CD4+ and CD8+ T cells were increased in frequency during PrEP treatment, and declined once PrEP was discontinued. These results suggest that in HSV2+ women, PrEP differentially alters the immune environment in the genital mucosa compared to blood, highlighting the limitations of immune signatures derived from peripheral blood when studying localized infections and the need for mucosal sampling in genital infection studies.
Finding that PrEP treatment altered the T cell compartment, the authors then investigated the effect of PrEP on other immune subsets. Regulatory T cells (Tregs) are a subset of CD4+ T cells that restrain immune cell activation as well as coordinate an early immune response to HSV-2 and are additionally thought to contribute to protection from HIV infection by limiting proliferation of target CD4+ T cells. As in the conventional T cell subsets, the authors found that Treg numbers were decreased in the mucosa during PrEP, but rebounded after treatment ended, suggesting that Tregs mirror conventional T cell patterns, expanding and contracting at similar rates to appropriately restrain activated T cells. In addition to CD4+ T cells, a subset of tissue macrophages express CCR5 and are susceptible to HIV infection, and this population was likewise decreased in the genital mucosa during PrEP. However, the pro-inflammatory cytokines that govern immune cells were not affected by PrEP treatment in neither the blood nor tissue. These findings suggested that PrEP induces immune quiescence in other mucosal immune cell subsets without affecting the cytokine milieu in the circulation or tissue.
“We found both significant impacts of PrEP usage on the mucosa of HSV-2 seropositive women, and that mucosal immune responses to PrEP usage were distinct from peripheral responses,” Richert-Spuhler said. “Our findings could aid in informing clinical recommendations for continuing PrEP usage in individuals who are at risk for HIV acquisition.” These results complement previous research that demonstrates how mucosa and blood antiviral immune responses can differ greatly within a patient, reinforcing the inadequacy of blood as a proxy for local immune profiles. Additionally, this study suggests that PrEP treatment decreases immune cell populations and activation in the genital mucosa of HSV2-infected patients, including the target cells of HIV. Although these results support the authors’ hypothesis that PrEP-mediated suppression of HSV-2 reduces immune cell responses to active virus, further studies are necessary to directly infer causation, as this study did not collect genital swabs to measure HSV-2 titers in the genital tract. Richert-Spuhler explained: “Our study was partially limited in scope by sample availability and should be confirmed with a rigorous longitudinal timecourse in HSV-2 seropositive and negative women to determine if mucosal alterations during PrEP usage and discontinuation are transient, or stable in nature; and if HSV-2 shedding and clinical symptoms contribute to altered mucosal phenotypes.” However, the observations that both CD4+ and CD8+ T cells, as well as HIV-susceptible macrophages, are decreased during PrEP and rebound after PrEP ceases, are relevant to HIV prevention research. Further research is warranted to determine if this phenomenon is detrimental to or beneficial for HIV prevention, as a lack of CD8+ cytotoxic T cells could lead to decreased HIV clearance while a decrease in CCR5+CD4+ T cells could limit availability of cells susceptible to HIV infection. Their work will continue to interrogate the “exact mechanisms of how HSV-2 seropositivity and PrEP usage may interact to alter susceptibility and mucosal immunity to HIV infection,” which “remains an important topic.”
This work was supported by the National Institute of Allergy and Infectious Diseases of the US National Institutes of Health, the Bill and Melinda Gates Foundation, and the University of Washington STD/AIDS Research Training Fellowship (T32 AI007140).
Richert-Spuhler LE, Pattacini L, Plews M, Irungu E, Muwonge TR, Katabira E, Mugo N, Meyers AFA, Celum C, Baeten JM, Lingappa JR, Lund JM. 2019. Pre-exposure prophylaxis differentially alters circulating and mucosal immune cell activation in HSV-2 seropositive women. AIDS. doi: 10.1097/QAD.0000000000002323.
