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Influenza A virus (IAV) is a major respiratory pathogen of both humans and animals. The lung is protected from pathogens by alveolar epithelial cells, tissue-resident alveolar macrophages, dendritic cells, and mast cells. The role of alveolar epithelial cells, endothelial cells, and alveolar macrophages during IAV infection has been studied previously. In this study, we address the role of mast cells during IAV infection. Respiratory infection with A/WSN/33 causes significant disease and immunopathology in C57BL/6 mice but not in B6.Cg-KitW-sh mice, which lack mast cells. During in vitro coculture, A/WSN/33 caused mast cells to release histamine, secrete cytokines and chemokines, and produce leukotrienes. Moreover, when mast cells were infected with IAV, the virus did not replicate within mast cells. Importantly, human H1N1, H3N2, and influenza B virus isolates also could activate mast cells in vitro. Mast cell production of cytokines and chemokines occurs in a RIG-I/MAVS–dependent mechanism; in contrast, histamine production occurred through a RIG-I/MAVS–independent mechanism. Our data highlight that, following IAV infection, the response of mast cells is controlled by multiple receptors. In conclusion, we identified a unique inflammatory cascade activated during IAV infection that could potentially be targeted to limit morbidity following IAV infection.

Influenza A virus (IAV) is a widespread infectious agent commonly found in mammalian and avian species. In humans, IAV is a respiratory pathogen that causes seasonal infections associated with significant morbidity in young and elderly populations, and has a large economic impact. Moreover, IAV has the potential to cause both zoonotic spillover infection and global pandemics, which have significantly greater morbidity and mortality across all ages. The pathology associated with these pandemic and spillover infections appear to be the result of an excessive inflammatory response leading to severe lung damage, which likely predisposes the lungs for secondary bacterial infections. The lung is protected from pathogens by alveolar epithelial cells, endothelial cells, tissue resident alveolar macrophages, dendritic cells, and mast cells. The importance of mast cells during bacterial and parasitic infections has been extensively studied; yet, the role of these hematopoietic cells during viral infections is only beginning to emerge. Recently, it has been shown that mast cells can be directly activated in response to IAV, releasing mediators such histamine, proteases, leukotrienes, inflammatory cytokines, and antiviral chemokines, which participate in the excessive inflammatory and pathological response observed during IAV infections. In this review, we will examine the relationship between mast cells and IAV, and discuss the role of mast cells as a potential drug target during highly pathological IAV infections. Finally, we proposed an emerging role for mast cells in other viral infections associated with significant host pathology.

Influenza A virus (IAV), a seasonal respiratory pathogen, results in severe lung pathology. Recent global analysis of lungs from mice infected with highly pathological IAV strains demonstrated enrichment of mast cells, but the role of mast cells during severe pulmonary viral infections has been under studied. We have recently shown that A/WSN/33 causes significant immunopathology in C57BL/6 mice and is mast cell-dependent. A/WSN/33(WSN) was able to directly activate mast cells to produce histamine, leukotrienes, inflammatory cytokines, and anti-viral chemokines. IAV-induced cytokine storm is thought to induce viral immunopathology, therefore we explored the importance of this response from mast cells. Mast cell cytokine production occurred in a RIG-I/MAVS-dependent fashion, but reconstitution of mast cell-deficient mice with RIG-I-/- mast cells generated lung pathology similar to wild type mast cells. Thus, it appears that mast cell degranulation rather than production of cytokines causes WSN-induced lung pathology. Mast cell degranulation occurs early in the viral entry cycle through a RIG-I/MAVS- and MyD88/Trif-independent mechanism. Using recombinant WSN strains, we found an association between the WSN hemaggultinin and neuraminidase proteins in the activation of mast cell degranulation and WSN-induced disease. We have identified a unique inflammatory cascade which could be therapeutically targeted to limit morbidity following infection with influenza virus.

Influenza A virus (IAV), a seasonal respiratory pathogen, has a high capacity to cause pandemic infections resulting in severe lung pathology. Multiple studies using pathological IAV strains have demonstrated the importance of macrophages and neutrophils, but whether the pathological inflammation induced by different IAV isolates is identical has not been directly compared. Here we demonstrate that while both A/WSN/33 (WSN) and A/PR/8/34 (PR8) influenza viruses cause significant immunopathology in mice the inflammatory milieu differs between the two isolates. Specifically, PR8 infection was associated with greater neutrophil recruitment and CCL4 and CXCL2 expression; in contrast, WSN infection was associated mast cell activation, recruitment of a unique Ly6ghigh Ly6chigh monocyte population, and elevated levels of IFNα, G-CSF, and CCL2. One difference between WSN and PR8 is their hemaggultinin molecules preferentially bind to α2,6- and α2,3-linked sialic acids, respectively. Using a recombinant WSN strain whose sialic acid binding preference is switched to α2,3-linkages, we observed minimal mast cell activation and a switch to a more neutrophilic inflammation. Thus, sialic acid binding preference of IAV isolates has clinically important alterations to the inflammatory cascade(s) activated. These findings have important implications for our ability to therapeutically target the host inflammatory response to limit morbidity following influenza virus infection

Influenza A virus (IAV) is a seasonal respiratory pathogen that can result in severe lung pathology. The role of mast cells has been under explored during severe pulmonary viral infections. We found that both A/WSN/33 (WSN) and A/PR/8/34 (PR8) influenza viruses cause significant immunopathology in C57BL/6 mice, but only WSN induced pathology was mast cell dependent. Using in vitro-derived bone marrow cultured mast cells (BMCMC), we found that WSN, but not PR8, directly activated BMCMC to produce histamine, leukotrienes, inflammatory cytokines, and anti-viral chemokines. Moreover, human H1N1, H3N2, and influenza B virus isolates could activate both murine BMCMC and the human mast cell line HMC-1 in vitro. BMCMC activation required infection of mast cells by IAV, which was dependent on the viral hemagglutinin. Cytokine and chemokine production from BMCMC occurs in a RIG-I/MAVS-dependent fashion which required the de novo production of vRNA; conversely, degranulation occurs through a RIG-I/MAVS-independent mechanism. Reconstitution of mast cell deficient mice with RIG-I-/- BMCMC generated lung pathology similar to wild type BMCMC, suggesting that mast cell degranulation, rather than production of cytokines, causes WSN induced lung pathology. Thus, we have identified a unique inflammatory cascade which could be therapeutically targeted to limit morbidity following infection with influenza virus.

The lung is protected from pathogens by alveolar epithelial cells, tissue resident alveolar macrophages, dendritic cells, and mast cells. The role of the mast cell has been under explored during severe pulmonary viral infections. In mice that lack functional mast cells, pathological immune responses are altered depending on the influenza strain. We found that both A/WSN/33 (WSN) and A/PR/8/34 (PR8) influenza viruses generate significant immunopathology in C57BL/6 mice, but only WSN induced pathology was mast cell dependent. To assess the differences in mast cell dependent inflammation response due to influenza strain, we examined the ability of each strain to activate in vitro-derived bone marrow cultured mast cells (BMCMC). We found that WSN, but not PR8, influenza virus activated BMCMC to produce inflammatory cytokines/chemokines, anti-viral chemokines, and degranulate. Importantly, a recombinant PR8 virus that expresses the WSN hemagglutinin (HA) glycoprotein instead of its own could induce cytokine production in BMCMC, demonstrating an important role for the HA glycoprotein for mast cell activation. Mast cell infection and viral RNA synthesis and export were necessary for mast cell activation and occurred in a MAVS-dependent mechanism. Thus, we have identified a unique inflammatory cascade which could be targeted to limit morbidity following infection with influenza virus.

Understanding the innate inflammatory pathways activated during influenza A virus infection is vital to developing effective therapeutics. Our results demonstrate that the inflammatory cascades induced by different influenza A virus strains are not identical. Mice infected with A/WSN/33 show less airway hyper-reactivity than mice infected with A/PR/8/34, likely due to decreased expression of the IL-33/IL-13 axis of inflammation. In addition, we have examined the lipidomic expression profile during flu infections from each of these strains. Lipid extracts were prepared from whole lungs, and specific lipid mediators were both identified and quantified using UHPLC-MS. This approach provides an automated, high-throughput, and high-sensitivity technique to characterize the lipidomic fingerprint of the infection resulting from various influenza A strains. The compounds differentially expressed between strains will ultimately be used to develop targeted therapeutics to lessen both the morbidity and mortality of the seasonal flu.