Date Published: January 27, 2016
Publisher: Public Library of Science
Author(s): Jonathan Maelfait, Kenny Roose, Lars Vereecke, Conor Mc Guire, Mozes Sze, Martijn J. Schuijs, Monique Willart, Lorena Itati Ibañez, Hamida Hammad, Bart N. Lambrecht, Rudi Beyaert, Xavier Saelens, Geert van Loo, Benjamin R. tenOever.
A20 negatively regulates multiple inflammatory signalling pathways. We here addressed the role of A20 in club cells (also known as Clara cells) of the bronchial epithelium in their response to influenza A virus infection. Club cells provide a niche for influenza virus replication, but little is known about the functions of these cells in antiviral immunity. Using airway epithelial cell-specific A20 knockout (A20AEC-KO) mice, we show that A20 in club cells critically controls innate immune responses upon TNF or double stranded RNA stimulation. Surprisingly, A20AEC-KO mice are better protected against influenza A virus challenge than their wild type littermates. This phenotype is not due to decreased viral replication. Instead host innate and adaptive immune responses and lung damage are reduced in A20AEC-KO mice. These attenuated responses correlate with a dampened cytotoxic T cell (CTL) response at later stages during infection, indicating that A20AEC-KO mice are better equipped to tolerate Influenza A virus infection. Expression of the chemokine CCL2 (also named MCP-1) is particularly suppressed in the lungs of A20AEC-KO mice during later stages of infection. When A20AEC-KO mice were treated with recombinant CCL2 the protective effect was abrogated demonstrating the crucial contribution of this chemokine to the protection of A20AEC-KO mice to Influenza A virus infection. Taken together, we propose a mechanism of action by which A20 expression in club cells controls inflammation and antiviral CTL responses in response to influenza virus infection.
Disease outcome upon exposure to a certain pathogen relies on the capacity of the host to resist and tolerate the infection . Resistance protects the host by suppressing pathogen replication and promoting clearance of the pathogen, a process that is mostly mediated by the innate and adaptive immune system. Tolerance refers to the ability to improve disease outcome without affecting pathogen burden and by limiting tissue damage. An overactive immune response can negatively impact on the disease by causing severe tissue damage . Immunopathology is an important contributor to death during exposure to highly virulent strains of influenza A such as the 1918 H1N1 virus or highly pathogenic avian H5N1 and H7N1 viruses. The mechanisms contributing to immune pathology during flu virus infection have been well documented, and both innate and adaptive immunity seems to be involved [3–6]. However, the exact molecular mechanisms regulating these processes are not well understood.
A20 is an essential negative regulator of NF-κB signaling, and A20 deficient mice die prematurely due to massive multi-organ inflammation triggered by infiltrating intestinal bacteria [46,47]. We showed in this study that specific deletion of A20 in respiratory epithelial cells protects mice from Influenza A virus-induced morbidity and lethality. Viral clearance and the production of the antiviral cytokines IFNα and IFNβ was similar in A20AEC-KO and wild-type mice, in agreement with literature stating that epithelial cells are not the primary producers of type-I IFNs upon respiratory virus infection . Interestingly, although the initial recruitment of innate cells and CTLs into the lungs of A20AEC-KO mice was sufficient to clear the virus by 8 days post infection, monocyte recruitment and the local CTL response in the lung were markedly reduced during later stages of infection. This was rather surprising since A20 is characterized as a negative regulator of the antiviral immune response . Indeed we could confirm such a role for A20 in a surrogate viral infection model using intratracheal instillation of the double stranded RNA mimic poly(I:C) showing hyperactive immune responses in AEC-specific A20 knockout mice.