Influenza- and MCMV-induced memory CD8 T cells control respiratory vaccinia virus infection despite residence in distinct anatomical niches

Induction of memory CD8 T cells residing in peripheral tissues is of interest for T cell-based vaccines as these cells are located at mucosal and barrier sites and can immediately exert effector functions, thus providing protection in case of local pathogen encounter. Different memory CD8 T cell subsets patrol peripheral tissues, but it is unclear which subset is superior in providing protection upon secondary infections. We used influenza virus to induce predominantly tissue resident memory T cells or cytomegalovirus to elicit a large pool of effector-like memory cells in the lungs and determined their early protective capacity and mechanism of reactivation. Both memory CD8 T cell pools have unique characteristics with respect to their phenotype, localization, and maintenance. However, these distinct features do not translate into different capacities to control a respiratory vaccinia virus challenge in an antigen-specific manner, although differential activation mechanisms are utilized. While influenza-induced memory CD8 T cells respond to antigen by local proliferation, MCMV-induced memory CD8 T cells relocate from the vasculature into the tissue in an antigen-independent and partially chemokine-driven manner. Together these results bear relevance for the development of vaccines aimed at eliciting a protective memory CD8 T cell pool at mucosal sites.

CD85k Contributes to Regulatory T Cell Function in Chronic Viral Infections

Regulatory T cells (Tregs) prevent excessive immune responses and limit immune pathology upon infections. To fulfill this role in different immune environments elicited by different types of pathogens, Tregs undergo functional specialization into distinct subsets. During acute type 1 immune responses, type 1 Tregs are induced and recruited to the site of ongoing Th1 responses to efficiently control Th1 responses. However, whether a similar specialization process also takes place following chronic infections is still unknown. In this study, we investigated Treg specialization in persistent viral infections using lymphocytic choriomeningitis virus (LCMV) and murine cytomegalovirus (MCMV) infection as models for chronic and latent infections, respectively. We identify CD85k as a Th1-specific co-inhibitory receptor with sustained expression in persistent viral infections and show that recombinant CD85k inhibits LCMV-specific effector T cells. Furthermore, expression of the CD85k ligand ALCAM is induced on LCMV-specific and exhausted T cells during chronic LCMV infection. Finally, we demonstrate that type 1 Tregs arising during chronic LCMV infection suppress Th1 effector cells in an ALCAM-dependent manner. These results extend the current knowledge of Treg specialization from acute to persistent viral infections and reveal an important functional role of CD85k in Treg-mediated suppression of type 1 immunity.

Rapid expansion of Treg cells protects from collateral colitis following a viral trigger

Foxp3⁺ regulatory T (Treg) cells are essential for maintaining peripheral tolerance and preventing autoimmunity. While genetic factors may predispose for autoimmunity, additional environmental triggers, such as viral infections, are usually required to initiate the onset of disease. Here, we show that viral infection with LCMV results in type I IFN-dependent Treg cell loss that is rapidly compensated by the conversion and expansion of Vβ5⁺ conventional T cells into iTreg cells. Using Vβ5-deficient mice, we show that these Vβ5⁺ iTreg cells are dispensable for limiting anti-viral immunity. Rather, the delayed replenishment of Treg cells in Vβ5-deficient mice compromises suppression of microbiota-dependent activation of CD8⁺ T cells, resulting in colitis. Importantly, recovery from clinical symptoms in IBD patients is marked by expansion of the corresponding Vβ2⁺ Treg population in humans. Collectively, we provide a link between a viral trigger and an impaired Treg cell compartment resulting in the initiation of immune pathology.