The Role of Tumour Necrosis Factor in Pulmonary Viral Infections

Abstract

Tumour necrosis factor (TNF) is a key inflammatory cytokine. Excessive production of TNF is detrimental and associated with chronic inflammatory disorders and severe immunopathology observed in acute infections. TNF plays an important antimicrobial role in the host response to infection but there is contradicting evidence regarding its role in defence against viral infections. Nonetheless, many viruses counter the host immune response by targeting the TNF pathway, indicating an important role for this cytokine. Ectromelia virus (ECTV) is an orthopoxvirus that encodes a viral TNF receptor (vTNFR), which is homologue to host TNF receptor (R) II (TNFRII). The precise role of ECTV-encoded vTNFR is not known but it may play a role in the pathogenesis of virus infection. We established that TNF plays a critical role in host survival during respiratory ECTV infection by negatively regulating the degree of lung immunopathology. Similar findings were observed during infection with influenza virus (IAV). In both models of viral infection, TNF deficient mice showed increased susceptibility to infection compared to wild type mice (WT). The increased susceptibility was not due to the inability of these mice to control viral replication. Rather, TNF deficiency leads to exacerbated lung pathology. In the ECTV model, the severe immunopathology was associated with dysregulated production of a number of inflammatory cytokines, many of which utilise the STAT3 signalling pathway. Treating the mice with a STAT3 inhibitor revealed an important role for this signalling pathway in regulating lung pathology during ECTV and IAV infections. Our data revealed that although TNF is associated with inflammation, it regulates inflammation and plays an anti-inflammatory role in the host response to infection. TNF is produced as a transmembrane protein (mTNF) then enzymatically cleaved to release the soluble form (sTNF). Both forms of TNF bind host TNFR and vTNFR. mTNF can also act as a ligand for vTNFR and host TNFR to transmit signals from the outside to inside a cell bearing mTNF through a process termed, reverse signalling. We investigated this phenomenon during ECTV and IAV infection using triple mutant (TM) mice that only express mTNF but not sTNF or TNFRs. We found that treatment of TM and WT mice with exogenous sTNFRII attenuated lung inflammation and pathology but had no effect on viral load, indicating that reverse signalling through mTNF dampened the inflammatory response. Intriguingly, combined treatment with sTNFRII and specific antiviral drug significantly reduced viral load and lung pathology. ECTV-encoded vTNFR (CrmD) contains a TNF-binding domain and a chemokine-binding domain termed SECRET. To determine the function of CrmD during ECTV infection, we used mutant viruses lacking either the TNF-binding domain or both domains to infect mice. The data established that the function of CrmD was to dampen inflammation, likely involving reverse signalling through mTNF. Mice infected with deletion mutants lacking one or both domains of CrmD exhibited significantly exaggerated lung pathology associated with increased production of several inflammatory mediators. Together, these data demonstrate the important roles of TNF and TNF-CrmD signalling pathways in regulating the inflammatory response to ECTV infection.