Immunological memory refers to the ability of the immune system to “remember” and respond more rapidly and effectively to pathogens previously encountered. Memory T cells generated during previous infections can also be activated in response to subsequent unrelated pathogens. This process occurs via two different mechanisms: T cell receptor (TCR)-dependent activation and TCR-independent activation known as bystander activation. TCR-dependent activation depends on antigen specificity, while bystander activation depends on cytokines.
In a recent study, Dr. Jennifer Lund, a professor in the Vaccine and Infectious Disease Division, and her team demonstrated that bystander-activation of memory CD8 T cells during Herpes Virus 2 (HSV-2) infection reduced disease progression in the female reproductive tract (FRT) of mice. “This is the first study to demonstrate cytokine sensitivity of memory CD8 T cells within the female genital tract,” said Dr. Tanvi Arkatkar a recent PhD graduate from Dr. Lund’s lab and leader of the study. “We show the significance of these cytokine-sensitive, innate-like memory CD8 T cells in limiting early viral burden in a T cell receptor-independent manner in the context of mucosal infection,” she added.
In this study, the authors immunized mice with a recombinant strain of Listeria monocytogenes (LM-OVA), which generated non-specific HSV CD8 T cells, or a strain of LM-OVA expressing HSV glycoprotein B (gB), which generated HSV-specific CD8 T cells. After a month, mice were infected with Herpes simplex virus 2 (HSV-2). Immunization with LM-OVA or LM-OVA-gB resulted in improved clinical outcomes and survival after HSV infection compared with unimmunized mice, suggesting that HSV-specific as well as HSV-nonspecific CD8 T cells provide protection from HSV-2 infection. The observation that HSV-nonspecific CD8 T cells were also protective could mean that bystander-activated T cells are beneficial in this context.
Following up on these findings, the authors examined whether bystander activation of OVA-specific CD8 T cells occurred following HSV-2 infection. There was an increase in the number of OVA-specific CD8 T cells in the vaginal tissue after infection with HSV-2, as well as an increase in the levels of granzyme B and NKG2D, two markers of bystander activation. Additionally, the authors demonstrated that the bystander activation of CD8 T cells was independent of the activation of the TCR. Moreover, the authors found that memory CD8 T cells accumulated in the vaginal tract after immunization with LM-OVA. The authors blocked sphingosine 1-phosphatase (S1P), a metabolite that promotes the exit of the T cells from the circulation and enter the infected tissue to provide site-specific immunity. This block of S1P resulted in a significant decrease in the expression of OVA-specific CD8 T cells in the vaginal tissues, supporting that OVA-specific CD8 T cells are recruited from the circulation into the vaginal tissue where they received bystander activation signals upon HSV-2 infection.
The authors then tested whether cytokine stimulation alone could activate CD8 T cells and affect infection since HSV-2 infection induces cytokines and inflammatory responses. For these studies, CD8 T cells were stimulated with cytokines ex vivo, and then transferred to mice and infected with HSV-2. As expected, the transfer of bystander-activated CD8 T cells delayed the progression of HSV-2 infection as well as reduced the viral load. Additionally, the authors demonstrated that vaginal treatment with type I interferon was sufficient to induce the accumulation and activation of bystander CD8 T cells in the vaginal tract, suggesting that CD8 T cells can become bystander-activated in the absence of cognate antigen after local administration of type I interferon.
Lastly, the authors investigated bystander activation of human CD8 T cells after cytokine treatment of human vaginal tissue. The CD8 T cells were extracted from the tissue and treated with cytokines. The naïve CD8 T cells (which have not yet encountered antigen) did not respond to cytokine treatment. By contrast, memory CD8 T cells expressed granzyme B after cytokine treatment, suggesting bystander activation. Overall, the authors demonstrated bystander activation of human and mouse vaginal CD8 T cells in response to cytokine treatment, and they demonstrated that bystander activation of CD8 T cells delays HSV-2 pathogenesis. “The duration of their activation and the regulatory mechanisms that keep bystander-activated CD8 T cells in check warrant further investigation,” commented Dr. Arkatkar. “We would be particularly interested in exploring the role of regulatory T cells or Tregs in controlling these T cells.”
The spotlighted research was supported by grants from the National institute of Health.
Fred Hutch/University of Washington/Seattle Children's Cancer Consortium member Dr. Martin Prlic contributed to this work.
Arkatkar T, Davé V, Cruz Talavera I, Graham JB, Swarts JL, Hughes SM, Bell TA, Hock P, Farrington J, Shaw GD, Kirby A, Fialkow M, Huang ML, Jerome KR, Ferris MT, Hladik F, Schiffer JT, Prlic M, Lund JM. 2023. Memory T cells possess an innate-like function in local protection from mucosal infection. J Clin Invest. 133(10):e162800.