Takamura et al. show that most lung CD8+ TRM cells are not maintained in the inducible bronchus-associated lymphoid tissue (iBALT) but are maintained in specific niches created at the site of tissue regeneration, which are termed as repair-associated memory depots (RAMDs).

Airway resident memory CD8 T (TRM) cells are a distinctive TRM population with a high turnover rate and a unique phenotype influenced by their localization within the airways. Their role in mediating protective immunity to respiratory pathogens, while suggested by many studies, has not been directly proven. This study provides definitive evidence that airway CD8 TRM cells are sufficient to mediate protection against respiratory virus challenge. Despite being poorly cytolytic in vivo and failing to expand after encountering antigen, airway CD8 TRM cells rapidly express effector cytokines, with IFN-γ being produced most robustly. Notably, established airway CD8 TRM cells possess the ability to produce IFN-γ faster than systemic effector memory CD8 T cells. Furthermore, naïve mice receiving intratracheal transfer of airway CD8 TRM cells lacking the ability to produce IFN-γ were less effective at controlling pathogen load upon heterologous challenge. This direct evidence of airway CD8 TRM cell-mediated protection demonstrates the importance of these cells as a first line of defense for optimal immunity against respiratory pathogens and suggests they should be considered in the development of future cell-mediated vaccines.

Resident memory T cells (TRM cells) are an important first-line defense against respiratory pathogens, but the unique contributions of lung TRM cell populations to protective immunity and the factors that govern their localization to different compartments of the lung are not well understood. Here, we show that airway and interstitial TRM cells have distinct effector functions and that CXCR6 controls the partitioning of TRM cells within the lung by recruiting CD8 TRM cells to the airways. The absence of CXCR6 significantly decreases airway CD8 TRM cells due to altered trafficking of CXCR6−/− cells within the lung, and not decreased survival in the airways. CXCL16, the ligand for CXCR6, is localized primarily at the respiratory epithelium, and mice lacking CXCL16 also had decreased CD8 TRM cells in the airways. Finally, blocking CXCL16 inhibited the steady-state maintenance of airway TRM cells. Thus, the CXCR6/CXCL16 signaling axis controls the localization of TRM cells to different compartments of the lung and maintains airway TRM cells.

Resident memory CD8 T (TRM) cells in the lung parenchyma (LP) and airways provide heterologous protection against influenza virus challenge. However, scant knowledge exists regarding factors necessary to establish and maintain lung CD8 TRM. Here we demonstrate that, in contrast to mechanisms described for other tissues, airway and LP CD8 TRM establishment requires cognate antigen recognition in the lung. Systemic effector CD8 T cells could be transiently pulled into the lung in response to localized inflammation, however these effector cells failed to establish tissue residency unless antigen was present in the pulmonary environment. The interaction of effector CD8 T cells with cognate antigen in the lung resulted in increased and prolonged expression of the tissue retention markers CD69 and CD103, and increased expression of the adhesion molecule VLA-1. The inability of localized inflammation alone to establish lung TRM resulted in decreased viral clearance and increased mortality following heterosubtypic influenza challenge, despite equal numbers of circulating memory CD8 T cells. These findings demonstrate that pulmonary antigen encounter is required for the establishment of lung CD8 TRM and may inform future vaccine strategies to generate robust cellular immunity against respiratory pathogens.

The nature and anatomic location of the protective memory CD8+ T cell subset induced by intranasal vaccination remain poorly understood. We developed a vaccination model to assess the anatomic location of protective memory CD8+ T cells and their role in lower airway infections. Memory CD8+ T cells elicited by local intranasal, but not systemic vaccination with an engineered non-replicative CD8+ T cell-targeted antigen confer enhanced protection to a lethal respiratory viral challenge. This protection depends on a distinct CXCR3LO resident memory CD8+ T cell (Trm) population that preferentially localizes to the pulmonary interstitium. Interstitial Trm —by being positioned close to the mucosa where infection occurs—act before inflammation can recruit circulating memory CD8+ T cells into the lung tissue. This results in a local protective immune response as early as one day post-infection. Hence, vaccine strategies that induce lung interstitial Trm may confer better protection against respiratory pathogens

The current influenza vaccine provides narrow protection against the strains included in the vaccine, and needs to be reformulated every few years in response to the constantly evolving new strains. Novel approaches are directed toward developing vaccines that provide broader protection by targeting B and T cell epitopes that are conserved between different strains of the virus. In this paper, we focus on developing mathematical models to explore the CD8 T cell responses to influenza, how they can be boosted, and the conditions under which they contribute to protection. Our models suggest that the interplay between spatial heterogeneity (with the virus infecting the respiratory tract and the immune response being generated in the secondary lymphoid organs) and T cell differentiation (with proliferation occurring in the lymphoid organs giving rise to a subpopulation of resident T cells in the respiratory tract) is the key to understand the dynamics of protection afforded by the CD8 T cell response to influenza. Our results suggest that the time lag for the generation of resident T cells in the respiratory tract and their rate of decay following infection are the key factors that limit the efficacy of CD8 T cell responses. The models predict that an increase in the level of central memory T cells leads to a gradual decrease in the viral load, and, in contrast, there is a sharper protection threshold for the relationship between the size of the population of resident T cells and protection. The models also suggest that repeated natural influenza infections cause the number of central memory CD8 T cells and the peak number of resident memory CD8 T cells to reach their plateaus, and while the former is maintained, the latter decays with time since the most recent infection.

Although influenza virus infection remains a concerning disease for public health, the roles of individual cytokines during the immune response to influenza infection are not fully understood. We have identified IL-36γ as a key mediator of immune protection during both high- and low-pathogenesis influenza infection. mRNA is upregulated in the lung following influenza infection, and mice lacking IL-36γ have greatly increased morbidity and mortality upon infection with either H1N1 or H3N2 influenza. The increased severity of influenza infection in IL-36γ-knockout (KO) mice is associated with increased viral titers, higher levels of proinflammatory cytokines early in infection, and more diffuse pathologic conditions late in the disease course. Interestingly, the increased severity of disease in IL-36γ-KO mice correlates with a rapid loss of alveolar macrophages following infection. We find that the alveolar macrophages from naive IL-36γ-KO mice have higher expression of M2-like surface markers compared with wild-type (WT) mice and show increased apoptosis within 24 h of infection. Finally, transfer of WT alveolar macrophages to IL-36γ-KO mice restores protection against lethal influenza challenge to levels observed in WT mice. Together, these data identify a critical role for IL-36γ in immunity against influenza virus and demonstrate the importance of IL-36γ signaling for alveolar macrophage survival during infection.

Influenza virus is a source of significant health and economic burden from yearly epidemics and sporadic pandemics. Given the potential for the emerging H7N9 influenza virus to cause severe respiratory infections and the lack of exposure to H7 and N9 influenza viruses in the human population, we aimed to quantify the H7N9 cross-reactive memory T cell reservoir in humans and mice previously exposed to common circulating influenza viruses. We identified significant cross-reactive T cell populations in humans and mice; we also found that cross-reactive memory T cells afforded heterosubtypic protection by reducing morbidity and mortality upon lethal H7N9 challenge. In context with our observation that PR8-primed mice have limited humoral cross-reactivity with H7N9, our data suggest protection from H7N9 challenge is indeed mediated by cross-reactive T cell populations established upon previous priming with another influenza virus. Thus, pre-existing cross-reactive memory T cells may limit disease severity in the event of an H7N9 influenza virus pandemic.