The morbidity and mortality associated with respiratory virus infection is felt most keenly among the elderly. T cells are necessary for viral clearance, and many age-dependent intrinsic T cell defects have been documented. However, the development of robust T cell responses in the lung also requires respiratory DCs (rDCs), which must process antigen and migrate to draining LNs (DLNs), and little is known about age-related defects in these T cell–extrinsic functions. Here, we show that increases in prostaglandin D₂ (PGD₂) expression in mouse lungs upon aging correlate with a progressive impairment in rDC migration to DLNs. Decreased rDC migration resulted in diminished T cell responses and more severe clinical disease in older mice infected with respiratory viruses. Diminished rDC migration associated with virus-specific defects in T cell responses and was not a result of cell-intrinsic defect, rather it reflected the observed age-dependent increases in PGD₂ expression. Blocking PGD₂ function with small-molecule antagonists enhanced rDC migration, T cell responses, and survival. This effect correlated with upregulation on rDCs of CCR7, a chemokine receptor involved in DC chemotaxis. Our results suggest that inhibiting PGD₂ function may be a useful approach to enhance T cell responses against respiratory viruses in older humans.

Influenza infections induce a rapid, but transient, dendritic cell (DC) migration from the lungs to the lymph nodes (LNs) that is followed by substantial recruitment of DCs into the lungs without subsequent migration to the LNs. Given that peripheral DCs are primarily thought to be involved in the initiation of adaptive immunity after migration into lymphoid tissues, what role these newly lung-recruited DCs play in influenza virus immunity is unclear. In this study, we demonstrate that loss of non-LN migratory pulmonary DC subsets increases mortality, sustains higher viral titers, and impairs pulmonary CD8 T cell responses. Reconstitution of the lungs with pulmonary plasmacytoid DCs, CD8α+ DCs, or interstitial DCs restores CD8 T cell responses in a cell contact–, major histocompatability complex I–, and influenza peptide–dependent manner. Thus, after their initial activation in the LN, protective influenza-specific CD8 T cell responses require additional antigen-dependent interactions, specifically with DCs in the lungs.

The autoreactive T cells that escape central tolerance and form the peripheral self-reactive repertoire determine both susceptibility to autoimmune disease and the epitope dominance of a specific autoantigen. SJL (H-2^s) mice are highly susceptible to the induction of experimental autoimmune encephalomyelitis (EAE) with myelin proteolipid protein (PLP). The two major encephalitogenic epitopes of PLP (PLP 139–151 and PLP 178–191) bind to IA^s with similar affinity; however, the immune response to the PLP 139–151 epitope is always dominant. The immunodominance of the PLP 139–151 epitope in SJL mice appears to be due to the presence of expanded numbers of T cells (frequency of 1/20,000 CD4⁺ cells) reactive to PLP 139–151 in the peripheral repertoire of naive mice. Neither the PLP autoantigen nor infectious environmental agents appear to be responsible for this expanded repertoire, as endogenous PLP 139–151 reactivity is found in both PLP-deficient and germ-free mice. The high frequency of PLP 139–151-reactive T cells in SJL mice is partly due to lack of thymic deletion to PLP 139–151, as the DM20 isoform of PLP (which lacks residues 116–150) is more abundantly expressed in the thymus than full-length PLP. Reexpression of PLP 139–151 in the embryonic thymus results in a significant reduction of PLP 139–151-reactive precursors in naive mice. Thus, escape from central tolerance, combined with peripheral expansion by cross-reactive antigen(s), appears to be responsible for the high frequency of PLP 139–151-reactive T cells.

Elimination of influenza virus-infected cells during primary influenza virus infections is thought to be mediated by CD8⁺ T cells though perforin- and FasL-mediated mechanisms. However, recent studies suggest that CD8⁺ T cells can also utilize TRAIL to kill virally infected cells. Therefore, we herein examined the importance of TRAIL to influenza-specific CD8⁺ T cell immunity and to the control of influenza virus infections. Our results show that TRAIL deficiency increases influenza-associated morbidity and influenza virus titers, and that these changes in disease severity are coupled to decreased influenza-specific CD8⁺ T cell cytotoxicity in TRAIL^−/− mice, a decrease that occurs despite equivalent numbers of pulmonary influenza-specific CD8⁺ T cells. Furthermore, TRAIL expression occurs selectively on influenza-specific CD8⁺ T cells, and high TRAIL receptor (DR5) expression occurs selectively on influenza virus-infected pulmonary epithelial cells. Finally, we show that adoptive transfer of TRAIL^+/+ but not TRAIL^−/− CD8⁺ effector T cells alters the mortality associated with lethal dose influenza virus infections. Collectively, our results suggest that TRAIL is an important component of immunity to influenza infections and that TRAIL deficiency decreases CD8⁺ T cell-mediated cytotoxicity, leading to more severe influenza infections.

This manuscript reviews the current literature on the importance of innate immune cells in the control of influenza virus infections.Adaptive immune responses are critical for the control and clearance of influenza A virus (IAV) infection. However, in recent years, it has become increasingly apparent that innate immune cells, including natural killer cells, alveolar macrophages (aMϕ), and dendritic cells (DC) are essential following IAV infection in the direct control of viral replication or in the induction and regulation of virus-specific adaptive immune responses. This review will discuss the role of these innate immune cells following IAV infection, with a particular focus on DC and their ability to induce and regulate the adaptive IAV-specific immune response.