Transmembrane form of tumor necrosis factor : immunobiology and role in infection and inflammation

Abstract

Tumor necrosis factor (TNF) is a key cytokine secreted by leukocytes. Although it plays an antimicrobial role in response to infection, many of its effects are detrimental to the host and overproduction is known to lead to pathology. TNF is produced as a transmembrane protein, mTNF, which is proteolytically cleaved to release a secreted form of the protein (sTNF). Of the strategies used by viruses to counter the host, many are directed toward TNF, underscoring the importance of this cytokine in the host response. Ectromelia virus (ECTV) is an orthopoxvirus and a natural mouse pathogen, which causes a systemic infection and disease similar to smallpox in humans. Like other orthopoxviruses, ECTV encodes a viral TNF receptor (vTNFR) that has significant homology to its mammalian counterpart. In this thesis, we investigated the function of TNF in the host response during an ECTV infection. We have found that ECTV-infected TNF-deficient (TNF{u207B}/{u207B}) animals show increased susceptibility and succumb to infection (Chapter 3). This is not due to their inability to control viral replication, but may instead be due to their inability to appropriately modulate the immune response. In comparison to Wild type animals, lungs from infected TNF{u207B}/{u207B} mice show severe pathology with diffuse congestion, edema, infiltration of inflammatory cells, and necrosis of bronchial epithelium. Our data suggest, paradoxically, that TNF plays an anti-inflammatory role in the host response to ECTV infection. The data from ETCV-infected mTNF{u0394}/{u0394} .TNFRI{u207B}/{u207B}.II{u207B}/{u207B} mice (expressing only mTNF, but not sTNF, TNFRI and TNFRII) indicate that much of the anti-inflammatory function of TNF is afforded by the mTNF form of the cytokine (Chapter 4). We have demonstrated that mTNF participates in reverse signalling, a process whereby interaction with TNFR leads to the transmission of signal in the direction of the cell expressing mTNF. ECTV-infected mTNF{u0394}/{u0394}l.TNFRI{u207B}/{u207B}.II{u207B}/{u207B} mice exhibit similar lung pathology to TNF{u207B}/{u207B} mice and are also highly susceptible to infection. Remarkably, treatment of these animals with soluble TNFR (sTNFR), results in a marked reduction in pathology, without affecting virus titers. We extended our investigation on the evaluation of combined sTNFR and antiviral therapy on the course of ECTV infection in mice. The combination treatment markedly reduces clinical manifestations of respiratory mousepox which is associated with minimal lung pathology (Chapter 4). These show that reverse signalling through mTNF plays an important role in the pathogenesis of poxviral disease by dampening inflammation and resulting immunopathology. We delved into the details of reverse signalling in vitro by using primary bone marrow-derived macrophages (BMDM) from mTNF{u0394}/{u0394}.TNFRI{u207B}/{u207B}.II{u207B}/{u207B} mice induced by mammalian or viral TNFRs. Our findings show that reverse signalling results in the inhibition of several genes that are downstream of the NF-{u03BA}B signalling pathway. Further investigations showed that reverse signalling through mTNF results in the inhibition of NF-{u03BA}B activation (Chapter 5). In Chapter 6, we investigated the role of crmD, an ECTV-encoded vTNFR, in vivo during respiratory mousepox and our data suggest that crmD in the virus also participates in reverse signalling to dampen immunopathology. This may be an important pathway that the virus utilize to allow survival of the host long enough for it to be transmitted to another host. These findings present a possibility of utilizing reverse signalling through mTNF in developing new therapeutic strategies to limit immunopathology associated with inflammatory and infectious diseases. Show more