Role of Cysteine-Rich Domains of the vTNFR CrmB in the Immune Evasion Mechanism

After cell infection an immune response is activated in order to eliminate viral expansion. This activation includes the production of inflammatory factors, NK cell activation, apoptosis induction, or activation of cellular antiviral state. Tumor necrosis factor (TNF) is an example of a cytokine that enables the host cell to undergo anti-viral modifications and/or apoptosis. During millions of years of interaction with their hosts large DNA viruses like poxviruses and herpesvirus have designed effective mechanisms to evade cellular immune response. This mechanism results in the production of viral TNF receptors (vTNFRs), soluble proteins secreted after poxvirus infection characterized by mimicking the extracellular domain of TNF superfamily receptors (TNFSFRs). Thus, vTNFRs bind to and inhibit the signalling induced by the host TNF superfamily ligands as a mechanism of immune evasion. Up to four different vTNFRs expressed by poxvirus have been described and named CrmB, CrmC, CrmD and CrmE. It was reported that while CrmC and CrmE only bind to TNFa, CrmD and CrmB were also able to inhibit in vitro the effects of lymphotoxin alpha (LTa). The N-terminal region of these proteins is the TNF-binding site of cellular TNFRs and is characterized by three cysteine-rich domains named CRD1, CRD2, and CRD3. CRD1 contains a conserved preligand assembly domain (PLAD) critical to ligand binding and receptor trimerization, while CRD2 domain has been proved necessary for the TNF-vTNFR interaction. It has been suggested that the responsibility for the specificity of this interaction is provided by the domain CRD3. Nevertheless, the role of CDR3 is still unclear. The purpose of the present work was to study the involvement of CRD3 domain in binding specificity of CrmB. Previous work in our lab suggests that the CRD3 domain of CrmB could be involved in the inhibition of cytotoxicity induced by LTa. In a baculovirus system three mutant proteins of CrmB exchanging the CRD3 domain of CrmC (LM24) or CrmE (LM25) or deleting the CRD3 domain of CrmB (LM26) and analysed the ability of inhibiting the cytotoxic effect of TNFa or LTa. Our results show that while CRD3 domain of CrmB is not responsible for blocking cytotoxic effect induced by murineTNFa, CRD3 of CrmB is necessary to protect L929 cells from hTNFa. The involvement of CRD3-CrmB in the inhibition of cytotoxicity induced by LTa is still unclear. This work contributes to the understanding of strategies used by viruses to evade the immune response which provide us a potent tool that could have therapeutic potential.