A unique population of mucosal Tregs respond to local viral infection

From the Lund and Prlic labs, Vaccine and Infectious Disease Division

Mucosal barrier tissues (mouth, nose, vagina, etc.) represent sites in which the body separates itself from the external environment. While sites like these allow for consumption and absorption of our daily coffee or sugar addictions, mucosal barrier sites are also home to immune cells, primed to respond to pathogens that may try to enter the body. One type of immune cell, CD4+ regulatory T cells (Tregs) are canonically responsible for maintaining balanced immune responses by suppressing other immune cells. However, Tregs also participate in wound healing, allergy response prevention, and tolerance to consumed food nutrients and healthy bacteria at specific tissue sites, suggesting that Treg location impacts non-canonical functions. Tregs have also been found within herpes simplex virus 2 (HSV-2) lesions in the female genitourinary tract, raising questions of how expression profiles of vaginal mucosa Tregs differ from circulating Tregs and how vaginal Tregs respond to viral infection. The Lund and Prlic labs at the Fred Hutchinson Cancer Center characterized previously unstudied vaginal Treg expression profiles as compared to circulating blood or lymphoid Tregs and conducted HSV-2 infection of the mouse vagina to determine if vagina mucosal Treg profiles change in response to infection. Their work was accepted for publication this past month in Mucosal Immunology.

Investigation of Treg profiles from human and mouse samples revealed increased activation profiles and “killer protease” granzyme B expression for Tregs in the vaginal tract mucosal tissues as compared to blood or lymphoid tissue Tregs.
Investigation of Treg profiles from human and mouse samples revealed increased activation profiles and “killer protease” granzyme B expression for Tregs in the vaginal tract mucosal tissues as compared to blood or lymphoid tissue Tregs. Image provided by Dr. Jennifer Lund

To first characterize the Treg populations in the vagina mucosal tissues, the researchers collected samples from healthy women and assayed for Treg abundance and profiled these cells based on which proteins were present on the cell surface. These surface proteins serve as markers that identify Treg subgroups that may differ functionally. The researchers found a sizeable Treg population in the vaginal tissue, comparable to Treg abundances in the blood, suggesting that Tregs localize to the vaginal tract in normal conditions. They also observed an increase in “activation” cell surface markers for Tregs of the vagina as compared to the blood. These activation markers (TIGIT, CD39, CTLA-4, and GITR) suggest that vaginal Tregs may interact with nearby T cells to alter local immune responses. To complement this work, the researchers also observed similar Treg profiles in mice. Specifically, increased activation markers (CTLA-4 and GITR) were present at higher frequencies on Tregs in the mouse vagina as compared to Tregs from the vaginal-draining lymph nodes. Together these data provide two systems in which Tregs located in the vagina mucosal tissues are in an active state as compared to circulating blood or lymphoid Tregs. These findings may indicate that Tregs in the vagina are poised to respond to microbial threats at the mucosal barrier. Interestingly, “we found that while vaginal Tregs are distinct from circulating counterparts, they share properties with other mucosal tissue Tregs, including those that are known to directly aid in tissue repair,” commented first author Dr. Traxinger, a former graduate student in the Lund and Prlic labs. Therefore, it is possible that vaginal Tregs also perform yet-to-be-determined, location-dependent functions.

The researchers next characterized markers of vaginal tissue Tregs during HSV-2 infection in the mouse vagina. Their findings were exciting, and unexpected; Treg activation state in the vagina was enhanced after infection. Based on these Treg activation profiles in uninfected and infected conditions, “Tregs in the vaginal mucosa may have specialized roles in maintaining immunological balance in both steady state and infection. Surprisingly, we also found that following local viral infection or inflammatory conditions, vaginal Tregs express cytotoxic granzyme B (GzmB)— a [“killer protease”] molecule commonly associated with CD8+ cytotoxic lymphocytes and natural killer cells—suggesting that vaginal Tregs acquire GzmB to regulate immune responses during inflammation,” stated Dr. Traxinger. While Tregs have not been extensively studied in the context of mucosal viral infection, “we hypothesize that vaginal Tregs may use GzmB to cull highly activated CD4+ and CD8+ conventional T cell responses during active infection to prevent collateral tissue damage in the mucosa,” stated Dr. Traxinger.

Together, these studies identified active markers on Tregs in the vagina mucosal tissues that increase during HSV-2 infection, implying a role for Tregs in anti-viral immune responses in the vagina. Future studies by these researchers will include “investigating the role of vaginal Tregs in tissue healing after HSV-2-mediated lesions and how vaginal Tregs modulate immune responses during secondary HSV-2 exposure and recurrent lesions,” commented Dr. Traxinger. The researchers propose that their continued study of Tregs in the vagina will provide insight into mucosal tissue vaccine design, as whether Tregs aid or hinder anti-viral T cell responses remains unclear at this mucosal barrier.

The spotlighted research was funded by the National Institutes of Health, the Diseases of Public Health Importance Training Grant, and Viral Pathogenesis Training Grant.

Fred Hutch/University of Washington/Seattle Children's Cancer Consortium member Martin Prlic contributed to this work.

Traxinger B, Vick SC, Woodward-Davis A, Voillet V, Erickson JR, Czartoski J, Teague C, Prlic M, Lund JM. 2022. Mucosal viral infection induces a regulatory T cell activation phenotype distinct from tissue residency in mouse and human tissues. Mucosal Immunol. Doi: 10.1038/s41385-022-00542-7. Epub ahead of print.