Characterisation of the Vaccinia Virus Protein, C16, and its Role in Infection

Abstract

Characterisation and manipulation of Vaccinia virus (VACV) immunomodulators is important for engineering recombinant VACV vaccines with enhanced immunogenicity. To this end, the goal of this study was to characterise the putative immunomodulator, C16, a protein of interest due to proposed similarity to the interleukin 1 receptor antagonist protein. C16 is a highly conserved, early protein of approximately 37 kDa that actively shuttles between the cytoplasm and nucleus in transfected and infected cells. In order to study the function of C16, a VACV mutant lacking the C16L gene (vΔC16) and a revertant virus wherein the gene was reinserted into the deletion mutant (vC16Rev) were constructed. In cell culture, vΔC16 had similar replication kinetics to wild type and revertant control viruses, but formed a smaller plaque. The loss of C16 led to a significantly attenuated phenotype in a murine intranasal model of infection that was characterised by reduced weight-loss and signs of illness. While virus titres between the virus sets were comparable in the initial days of infection, virus titres of vΔC16 dropped more rapidly over the course of the infection. In vΔC16-infected mice there was also an accelerated recruitment and activation of lymphocytes. In contrast, there was no significant difference in the murine intradermal model of infection. Furthermore, the omission of C16 did not decrease the efficacy of protective vaccination. The attenuation of vΔC16 in vivo may be due to the effect of C16 on IL-6 production which has been demonstrated in vitro and in vivo. This is reminiscent of one of the functions of IL-1Ra though the precise mechanism of this downregulation of IL-6 by C16 remains to be determined. C16 also downregulates ISRE-dependent gene expression, via a mechanism independent of STAT1 but which may involve p38 MAPK.