Hidden reactivation, active immunity: Mapping the immune response to HSV-2

Herpes simplex virus is a master of bad timing—reappearing briefly, often unnoticed, and just long enough to keep the immune system on its toes. In its genital form, herpes simplex virus type 2 (HSV-2) establishes a lifelong infection marked by periods of asymptomatic latency punctuated by abrupt reemergence. During these reactivation events, virus can be released from genital mucosal surfaces—a process known as viral shedding. Importantly, shedding often occurs in the absence of visible lesions or symptoms, enabling ongoing transmission even when infection appears clinically silent. These episodes are typically brief and localized, reflecting a dynamic balance between viral replication and host immune control at the mucosal interface.

While antiviral therapies can reduce the frequency and duration of shedding, they do not eliminate it, underscoring the resilience of HSV-2 reservoirs and the complexity of immune surveillance in genital tissues. Understanding what differentiates periods of containment from episodes of viral escape remains a central challenge in herpes research. Increasingly, attention has turned to the local immune landscape of the genital mucosa, where spatial organization, cell–cell interactions, and activation states may critically shape the outcome of viral reactivation events.

In an article recently published in JCI Insight, scientists in Dr. Jennifer Lund’s lab took a deep dive into the complexities of our immune system and its responses to herpes virus infection. The study drew on samples from 232 participants enrolled in the Kinga Study, a community-based cohort in Kenya focused on understanding how genital immune responses interact with sexually transmitted infections.

Study participants included both an HSV-negative group and a group of participants living with HSV-2 infection. A majority of those in the HSV-2-positive group were symptom-free when their samples were collected, and recent genital sores similarly were rare—highlighting that much of herpes virus activity happens quietly, without obvious symptoms. None of the participants reported taking herpes medications, allowing the researchers to examine immune responses in their natural state.

The participants were generally young, sexually active, and mostly in monogamous relationships. Those with HSV-2 were more likely to have a partner living with HIV, although all participants stayed HIV-negative throughout the study, emphasizing the need to consider how multiple infections shape immune responses. Because many factors—such as age, hormonal contraceptive use, bacterial vaginosis, and recent sexual activity—can influence immune cells in the genital tract, the researchers accounted for these variables when analyzing immune responses.

T cells made up the majority of immune cells found in genital tissues, making them a natural focus for investigation. Yet HSV-2 infection alone was not associated with changes in the overall number or proportion of major T cell subsets in vaginal or cervical tissue, or in the blood. This finding suggests that HSV-2’s impact on immunity is not simply a matter of immune cell accumulation. As co-lead author Rachael Zemek explains, “viruses don’t just affect the infected area—they subtly reorganize and activate immune cells in surrounding tissue.”

This distinction is particularly important in the context of HIV risk. One long-standing hypothesis has been that HSV-2 increases susceptibility to HIV by increasing the number of HIV target cells—CD4+ T cells expressing the CCR5 receptor—near the genital mucosa. While earlier studies suggested this might occur at the tissue surface, this study’s examination of vaginal and cervical tissue did not reveal an overall increase in these target cells. Instead, the findings point toward more nuanced mechanisms involving immune activation and tissue context rather than cell numbers alone.

Upon further investigation, HSV-2 infection was linked to subtle but meaningful changes in the behavior of immune cells in the vaginal tissue. T cells from HSV-2–positive participants showed altered expression of markers involved in immune activation and regulation. For example, CD39—a molecule associated with immune regulation and activation—was elevated on certain T cell subsets, while expression of PD-1 and CXCR3, markers related to immune exhaustion and migration, were reduced. These nuanced changes suggest that the immune system is adapting to the presence of the virus in complex ways that might affect how well it controls infection and inflammation.

Those subtleties became especially clear when the researchers examined immune responses during active viral shedding. Using spatial transcriptomics—a cutting-edge technology that maps gene expression while preserving each cell’s location within intact tissue—the team was able to see how immune cells are arranged and how they behave in their native environment. “Spatial transcriptomics allowed us to capture this organization of immune responses,” Zemek says, adding that the approach revealed patterns that traditional methods would miss.

During shedding episodes, immune cells showed coordinated shifts in both position and function. Cytotoxic T cells moved deeper into the vaginal epithelium, macrophages adopted more inflammatory profiles, and immune cells clustered in ways that suggested increased communication. Dendritic cells also shifted toward more mature, migratory states, while epithelial cells increased expression of inflammatory signals—together painting a picture of an immune system actively responding to viral reactivation.

According to Zemek, this level of spatial and functional detail represents a new direction for the field. “Using spatial transcriptomics, we were able to see exactly where immune cells are located and how they respond in their native tissue environment,” she explains, “providing a level of detail and context that traditional methods can’t capture.” By revealing how immune cells reorganize during viral shedding—rather than simply increasing in number—this work opens new questions about how the immune system self-regulates during HSV-2 reactivation, and how those dynamics may influence both viral control and HIV risk.

Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium Members Drs. Keith Jerome, Nelly Rwamba Mugo, and Evan Newell contributed to this research.

The spotlighted research was funded by the National Institutes of Health.

MacLean F, Zemek RM, Tesfahun Tsegaye A, Graham JB, Swarts JL, Vick SC, Potchen NB, Cruz Talavera I, Warrier L, Dubrulle J, Schroeder LK, Elz A, Sowerby D, Saito A, Thomas KK, Mack M, Schiffer JT, McClelland S, Jerome KR, Chohan BH, Ngure K, Rwamba Mugo N, Newell EW, Lingappa JR, Lund JM, Kinga Study Team. 2025. Genital herpes shedding episodes associate with altered spatial organization and activation of mucosal immune cells. JCI Insight. DOI: 10.1172/jci.insight.197491.