Let’s all take a moment and show some gratitude to our immune systems—day in and day out, intricate molecular machines are at work protecting our bodies from the microbial milieu that we inhabit. One of the immune system’s most important constituents are T cells (named after a fascinating organ called the Thymus, where they mature), which are present throughout the body and coordinate immune responses against pathogens. T cells exist in a variety of types—from ‘helper’ CD4+ to ‘killer’ CD8+ cells—but many share a general mechanism of activation: they initially lie dormant before being activated by an immunological stimulus, fight off the threat, then return to the dormant state. This cycle of activation-deactivation is critical for our immune systems to remain nimble in response to infections while avoiding prolonged inflammation and damage to healthy tissue. However, decades of research have revealed another side to this coin: a phenomenon termed T cell exhaustion. Although the exact definition is still under debate, T cell exhaustion generally refers to a progressive loss of functionality in T cells which are persistently stimulated over long periods of time. Exhaustion is theorized to represent a fail-safe mechanism to prevent chronic, deleterious immune overactivation, but exhausted T cells are also implicated in the progression of chronic infections and various cancers, making this an important phenomenon to better understand.
The Lund Lab in the Vaccine and Infectious Disease Division at Fred Hutch studies immune responses to viruses of public health concern. A recent study from the lab led by former lab members Drs. Veronica Davé, Laura Richert-Spuhler, and Tanvi Arkatkar asks an interesting question about T cell exhaustion after infection with herpes simplex virus 2 (HSV-2), a pernicious virus infecting roughly ten percent of people nationwide which poses significant quality of life and health risks. HSV-2 infection—which causes the disease known commonly as genital herpes—is characterized by long periods of viral latency punctuated by brief episodes of recurrence (‘flare-ups’) of painful sores. As Dr. Davé puts it, “When you read about immune activation and T cell exhaustion, it’s usually grouped into two bins: acute activation (which happens when you get a vaccine or the flu) and chronic activation (which happens with a tumor, for example). We quickly realized that HSV-2 infection, with its brief bouts of acute immune activation and deactivation occurring over years-long time periods, doesn’t fit neatly into either of those bins, and presents an interesting opportunity to study how chronic viral reactivation impacts T cell health in the affected tissues.”
To study the effects of chronic HSV-2 reactivation on T cells, the team made two important decisions: first, that they would primarily use samples from HSV-2 positive human patients rather than an animal model, and second, that a longitudinal study design (sampling the same HSV lesions in the same patients over time) would provide the most informative system to relate viral reactivation and subsequent immune involvement. Working with minute quantities of lesion biopsy tissue, Davé and colleagues used cutting-edge multi-parameter flow cytometry to sort and characterize lesion-associated immune cells based on a panel of cell surface markers. Here, she made the first interesting discovery: while viral recurrence leads to a sharp increase in total T cell abundance followed by a gradual return to baseline as lesions healed, the composition of the immune cell compartment (that is, the relative proportions of different T cell and other immune cell types) remained relatively unchanged throughout recurrence and even compared to adjacent, non-lesion tissue.
So, if lesions were rapidly accumulating T cells in response to viral recurrence, where were these T cells coming from? More specifically, Dr. Davé sought to determine whether the increase in T cell numbers was due to proliferation of T cells already at the lesion versus infiltration of T cells into lesion tissue from circulation. The answer—again provided by flow cytometry on tissue samples—was a resounding ‘it depends!’ Lesion tissue from some patients showed a robust increase in T cells positive for the proliferation marker Ki-67 (which peaked and then returned to baseline as the lesion healed), while tissue from other patients showed no such effect. Furthermore, total Ki-67 expression was not correlated with overall T cell abundance at lesions, indicating that this feature of HSV-2 immune response differs between patients and highlighting the utility of the longitudinal, patient-forward study design.
After verifying that lesion-resident T cells express markers of transient activation following flare-ups, Davé and colleagues asked the crucial question: do these tissue-resident T cells become exhausted with subsequent viral recurrence? Probing for the expression of two exhaustion markers—PD-1 and Tim-3—in lesion-resident T cells over time, they discovered that these markers remain stable throughout recurrence and healing. Furthermore, PD-1 and Tim-3 expression levels in tissue-resident T cells and adjacent, non-lesion tissue remained similar throughout the time course of sampling. Since they were limited by the quantities of biopsy tissue they could collect, the team bolstered these results by using a mouse model of HSV-2 challenge, where they showed that isolated T cells retained their ability to secrete inflammatory factors following repeated HSV-2 challenge. Altogether, these results argue that—despite repeated expansion and contraction in response to viral recurrence—tissue-resident T cells don’t become exhausted in response to HSV-2 infection!
“I think the really in-depth, longitudinal characterization we performed on a limited subset of patient samples was both a strength and a challenge,” Dr. Davé notes. “On one hand, it gave us the ability to gather really rich information in a clinically relevant system and characterize the variability in patient immune responses to HSV-2. On the other hand, it also imposed constraints on the amount of tissue we could acquire and the statistical power we had to make far-reaching conclusions.” In all, the study provides a crucial piece of the puzzle towards understanding our response to HSV-2 infection, finding a place in a growing body of diverse work seeking to better understand both fundamental T cell biology and HSV-2 infection for the benefit of public health.
The spotlighted research was funded by the National Institutes of Health and an American Association of Immunologists Career Reentry Fellowship.
Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium member Dr. Martin Prlic contributed to this study.
Davé, V., Richert-Spuhler, L. E., Arkatkar, T., Warrier, L., Pholsena, T., Johnston, C., Schiffer, J. T., Prlic, M., & Lund, J. M. (2023). Recurrent infection transiently expands human tissue T cells while maintaining long-term homeostasis. Journal of Experimental Medicine, 220(9), e20210692.