Structural and functional analysis of the Vaccinia virus O1 virulence protein

Abstract

Poxviruses are double-stranded DNA viruses capable of causing disfiguring and deadly disease in a wide range of hosts, from insects to mammals. Orthopoxviruses (OPXV) encode many proteins that are not essential for viral replication, but are responsible for vast differences in pathogenesis. Of the>200 proteins in the prototypical OPXV vaccinia virus (VACV), many remain functionally cryptic. The objective of these studies was to understand how the VACV O1 protein functions by investigating cell-specific effects that may contribute to virulence. The O1L gene is expressed early as the O1 protein, a 78 kDa protein that lacked N-linked glycosylation. These data are the first to demonstrate the reduced ability of an O1 deletion mutant ([delta]O1) to induce cell migration compared to the parental VACV Western Reserve strain (VACV-WR). [delta]O1-infected cell monolayers also exhibited reduced plaque diameter and clearance in plaque foci. These observations indicated that O1 is a significant contributor to VACV cytopathic effects (CPE) in vitro, in agreement with published reports. The results reported herein are the first to describe an altered immunological response with [delta]O1, as levels of anti-VACV immunoglobulin significantly increased with [delta]O1 infection at a time point (seven days post-infection) when VACV-WR induced VACV-specific antibody levels were comparable to sera from mock-infected mice. [delta]O1 was more immunogenic in an ex vivo antigen presentation assay, although mitogen-induced CD4+ T cell activation during [delta]O1 infection was equivalent to VACV-WR infection. Surprisingly, of all the immune cell types tested, [delta]O1 significantly differed from VACV-WR infection in the metabolic readout of only one cell type - RAW 264.7 macrophages. VACV-WR infected RAW 264.7 macrophages were more metabolically active than [delta]O1-infected cells at higher infectious doses, which may be indicative of a specialized niche for O1 function. Taken together, these data may provide clues into the mechanism of O1 virulence.