Constitutive TRIM22 expression within the respiratory tract identifies tissue-specific and cell-type dependent intrinsic immune barriers to influenza A virus infection
Authors: Charman Matthew , McFarlane Steven , Wojtus Joanna K. , Sloan Elizabeth , Dewar Rebecca , Leeming Gail , Al-Saadi Mohammed , Hunter Laura , Carroll Miles , Stewart James P. , Digard Paul , Hutchinson Edward , Boutell Chris 
Published: June 2019

We hypothesized that increased expression of antiviral host factors at portals of viral entry may protect exposed tissues from the constant threat of invading pathogens. Comparative transcriptomic analysis identified the broad-acting restriction factor TRIM22 (TRIpartite Motif 22) to be among the most abundantly expressed antiviral host factors in the lung, a major portal of entry for many respiratory pathogens. This was surprising, as TRIM22 is currently considered to be an interferon stimulated gene (ISG) product that confers protection following the activation of pathogen-induced cytokine-mediated innate immune defences. Using human respiratory cell lines and the airways of rhesus macaques, we experimentally confirmed high levels of constitutive TRIM22 expression in the lung. In contrast, TRIM22 expression in many widely used transformed cell lines could only be observed following immune stimulation. Endogenous levels of TRIM22 in non-transformed cells were sufficient to restrict human and avian influenza A virus (IAV) infection by inhibiting the onset of viral transcription independently of cytokine-mediated innate immune defences. Thus, TRIM22 confers a pre-existing (intrinsic) tissue-specific immune barrier to IAV infection in the respiratory tract. We investigated whether the constitutive expression of TRIM22 was a characteristic shared by other ISGs in human lung tissue. Transcriptomic analysis identified a large group of ISGs and IAV immuno-regulatory host factors that were similarly enriched in the lung relative to other mucosal tissues, but whose expression was downregulated in transformed cell-lines. We identify common networks of immune gene downregulation which correlated with enhanced permissivity of transformed cells to initiate IAV replication. Our data highlight the importance of tissue-specific and cell-type dependent patterns of pre-existing immune gene expression in the intrinsic intracellular restriction of IAV; findings highly relevant to the immune regulation of many clinically important respiratory pathogens.

Author Summary

The respiratory tract is a major portal of virus entry for many clinically important viruses, including seasonal and pandemic influenza A virus (IAV). We reasoned that cells within the respiratory tract might differentially express antiviral host factors to protect against the constant challenge of viral infection. We found the broad-acting antiviral protein TRIM22, conventionally regarded as an interferon stimulated gene (ISG) product upregulated in response to virus infection, to be constitutively expressed to high levels in the lung. We found that constitutive expression of TRIM22 restricted the initiation of human and avian IAV infection independently of cytokine-mediated innate immune defences. We identified pre-existing tissue-specific and cell-type dependent patterns of constitutive immune gene expression that strongly correlated with enhanced resistance to IAV replication from the outset of infection. Importantly, we show that these constitutive patterns of immune gene expression are lost or downregulated in many transformed cell lines widely used for respiratory virus research. Our data highlight the importance of pre-existing tissue-specific and cell-type dependent patterns of constitutive antiviral gene expression in the intracellular restriction of respiratory viral pathogens not captured in conventional cell culture model systems of infection.