Retinoic acid–inducible gene I–like receptors (RLRs) are cytosolic RNA sensors critical for initiation of antiviral immunity. Activation of RLRs following RNA recognition leads to production of antiviral genes and IFNs for induction of broad antiviral immunity. Although the RLRs are ubiquitously expressed, much of our understanding of these molecules comes from their study in epithelial cells and fibroblasts. However, RLR activation is critical for induction of immune function and long-term protective immunity. Recent work has focused on the roles of RLRs in immune cells and their contribution to programming of effective immune responses. This new understanding of RLR function in immune cells and immune programming has led to the development of vaccines and therapeutics targeting the RLRs. This review covers recent advances in our understanding of the contribution of RLRs to immune cell function during infection and the emerging RLR-targeting strategies for induction of immunity against cancer and viral infection.

Influenza A virus activates cellular inflammasome pathways, though it remains unknown whether many of the effector proteins downstream of inflammasome activation promote virus clearance or instead promote pathological inflammation. We investigated the role of gasdermin D (GSDMD), a pore-forming inflammasome effector protein that allows cellular release of inflammatory molecules and eventual pyroptotic lysis of cells. GSDMD knockout (KO) mice infected with influenza virus exhibited reduced weight loss and mortality compared to infected wild type (WT) mice. Lung viral titers were similar between genotypes, indicating that GSDMD does not directly affect virus replication. Instead, we observed that GSDMD KO mice had less severe lung inflammation, histopathology, and immune cell infiltration, suggesting that GSDMD promotes tissue-damaging immune responses following infection. Global transcriptomic analysis revealed significant decreases in specific inflammatory gene programs in GSDMD KO lungs, including decreased neutrophil chemotaxis and activation gene signatures, which were confirmed by decreased neutrophil elastase measurements and decreased neutrophil numbers in the lung. Indeed, exogenous depletion of neutrophils starting at day 3 post infection in WT mice recapitulated the protective phenotype observed in GSDMD KO mice, implicating neutrophils as central players in the GSDMD-dependent pathological response to influenza virus. Overall, these findings reveal an important role for GSDMD during influenza virus-induced lung inflammation, pathogenesis, and neutrophil accumulation. Therapeutic interventions targeting the GSDMD/neutrophil axis may provide an effective means to treat severe influenza virus infection.

Interferon-lambda (IFN-λ) is currently being investigated in a phase III clinical trial as a therapeutic against COVID-19. Exogenous IFN-λ restricts SARS-CoV-2 in vitro and in Balb/c and C57Bl/6 (WT) murine models of infection. However, roles of endogenously produced IFN-λ in SARS-CoV-2 pathogenesis are not currently known, and the overall mechanisms by which IFN-λ modulates the induction of protective immune responses in SARS-CoV-2 infections remains to be elucidated. We find that IFN-λ receptor deficient mice (Ifnlr1−/−) infected with mouse-adapted SARS-CoV-2 lose significantly more weight and have increased SARS-CoV-2 viral replication compared to WT through day 5 post infection. Intriguingly, Ifit3 and Ifitm3 are increased in the lungs of Ifnlr1−/− mice compared to WT following infection, despite similar IFN-α/β and IFN-λ mRNA levels, suggesting compensatory increases in type I IFN signaling are not driving the increased weight loss or ISG induction observed in Ifnlr1−/− mice. Global transcriptomics revealed induction of a suppressive immunoregulatory signature with increased IL-10 as a hallmark in Ifnlr1−/− lungs, suggesting IFN-λ is critically involved in regulating appropriate immune activation to limit SARS-CoV-2 pathogenesis. Histological analysis revealed a significant increase in CD45+ cells (but not neutrophils) in the lungs of Ifnlr1−/− mice compared to WT on day 5 post infection, and we identified increases in pathways associated with myeloid cell function and T cell activation in Ifnlr1−/− by comparative transcriptomics. Overall, broadening the understanding of how IFN-λ regulates SARS-CoV-2 infection, pathogenesis, and immunity will inform the utilization of IFN-λ as an immunotherapy and adjuvant. Supported by NIH Award K22 AI146141 to EAH.