Influenza viral infection is well-known to predispose to subsequent bacterial superinfection in the lung but the mechanisms have remained poorly defined. We have established a murine model of heterologous infections by an H1N1 influenza virus and Staphylococcus aureus. We found that indeed prior influenza infection markedly increased the susceptibility of mice to secondary S. aureus superinfection. Severe sickness and heightened bacterial infection in flu and S. aureus dual-infected animals were associated with severe immunopathology in the lung. We further found that flu-experienced lungs had an impaired NK cell response in the airway to subsequent S. aureus bacterial infection. Thus, adoptive transfer of naive NK cells to the airway of prior flu-infected mice restored flu-impaired antibacterial host defense. We identified that TNF-α production of NK cells played an important role in NK cell-mediated antibacterial host defense as NK cells in flu-experienced lungs had reduced TNF-α expression and adoptive transfer of TNF-α–deficient NK cells to the airway of flu-infected mice failed to restore flu-impaired antibacterial host defense. Defected NK cell function was found to be an upstream mechanism of depressed antibacterial activities by alveolar macrophages as contrast to naive wild-type NK cells, the NK cells from flu-infected or TNF-α–deficient mice failed to enhance S. aureus phagocytosis by alveolar macrophages. Together, our study identifies the weakened NK cell response in the lung to be a novel critical mechanism for flu-mediated susceptibility to bacterial superinfection.

Crohn’s disease is a chronic inflammatory condition of the gastrointestinal tract in which alterations to the bacterial community contribute to disease. Adherent-invasive E. coli (AIEC) are associated with human Crohn’s disease, however their role in intestinal immunopathology is unclear due to the lack of an animal model compatible with chronic timescales. Here we establish chronic AIEC infection in streptomycin-treated conventional mice (CD-1, DBA/2, C3HeN, 129e, C57BL/6), enabling the study of host response and immunopathology. AIEC induces an active Th17 response, heightened levels of proinflammatory cytokines and fibrotic growth factors, with transmural inflammation and fibrosis. Depletion of CD8+ T cells increases cecal bacterial load, pathology and intestinal fibrosis in C57BL/6 mice suggesting a protective role. Our findings provide evidence that chronic AIEC infections result in immunopathology similar to that seen in Crohn’s disease. With this model, research into the host and bacterial genetics associated with AIEC-induced disease becomes more widely accessible.

No abstract

Staphylococcus aureus remains a common cause of nosocomial bacterial infections and are often antibiotic resistant. The role of NK cells and IL-15 and their relationship in host defense against extracellular bacterial pathogens including S. aureus remain unclear. We have undertaken several approaches to address this issue using wild type (WT), IL-15 gene knock-out (KO), and NK cell-depleted mouse models. Upon pulmonary staphylococcal infection WT mice had markedly increased activated NK cells, but not NKT or γδ T cells, in the airway lumen that correlated with IL-15 production in the airway and with alveolar macrophages. In vitro exposure to staphylococcal products and/or coculture with lung macrophages directly activated NK cells. In contrast, lung macrophages better phagocytosed S. aureus in the presence of NK cells. In sharp contrast to WT controls, IL-15 KO mice deficient in NK cells were found to be highly susceptible to pulmonary staphylococcal infection despite markedly increased neutrophils and macrophages in the lung. In further support of these findings, WT mice depleted of NK cells were similarly susceptible to staphylococcal infection while they remained fully capable of IL-15 production in the lung at levels similar to those of NK-competent WT hosts. Our study thus identifies a critical role for NK cells in host defense against pulmonary extracellular bacterial infection and suggests that IL-15 is involved in this process via its indispensable effect on NK cells, but not other innate cells. These findings hold implication for the development of therapeutics in treating antibiotic-resistant S. aureus infection.

Lung immunopathology is the main cause of influenzamediated morbidity and death, and much of its molecular mechanisms remain unclear. Whereas tumor necrosis factor-␣ (TNF-␣) is traditionally considered a proinflammatory cytokine, its role in influenza immunopathology is unresolved. We have investigated this issue by using a model of acute H1N1 influenza infection established in wild-type and TNF-␣-deficient mice and evaluated lung viral clearance, inflammatory responses, and immunopathology. Whereas TNF-␣ was up-regulated in the lung after influenza infection, it was not required for normal influenza viral clearance. However, TNF-␣ deficiency led not only to a greater extent of illness but also to heightened lung immunopathology and tissue remodeling. The severe lung immunopathology was associated with increased inflammatory cell infiltration, anti-influenza adaptive immune responses, and expression of cytokines such as monocyte chemoattractant protein-1 (MCP-1) and fibrotic growth factor, TGF-␤1. Thus, in vivo neutralization of MCP-1 markedly attenuated lung immunopathology and blunted TGF-␤1 production following influenza infection in these hosts. On the other hand, in vivo transgenic expression of MCP-1 worsened lung immunopathology following influenza infection in wild-type hosts. Thus, TNF-␣ is dispensable for influenza clearance; however, different from the traditional belief, this cytokine is critically required for negatively regulating the extent of lung immunopathology during acute influenza infection.