SERP1, a serine proteinase inhibitor encoded by myxoma virus, is a secreted glycoprotein that interferes with inflammation.

Myxoma virus is a leporipoxvirus that causes a rapidly lethal, generalized infection known as myxomatosis in the European rabbit (Oryctolagus cuniculus). A characteristic feature of myxomatosis is the specific downregulation of key pathways important for numerous host defenses against the viral infection. The SERP1 gene has significant sequence similarity to the serpin superfamily of serine proteinase inhibitors and is one of many virulence factor genes located within the terminal regions of the myxoma virus genome. Transcriptional analysis of the SERP1 gene in myxoma virus (strain Lausanne) indicates that it is expressed as a late gene and studies using a polyclonal anti-SERP1 antiserum indicate that it encodes a secreted protein with an apparent molecular weight of 55 kDa. Using myxoma virus and recombinant vaccinia virus constructs for experiments with tunicamycin and peptide N-glycosidase F, it is shown that the secreted SERP1 protein is modified by N-linked glycosylation. Mutation of both copies of the SERP1 gene in myxoma virus results in a significant attenuation of the virus, such that more than 50% of infected animals are able to recover from the otherwise lethal infection. Histological analyses of lesions taken from infected animals suggest that in the absence of the SERP1 protein, a more effective inflammatory response occurs, allowing a more rapid resolution of the infection. This suggests that SERP1 contributes to viral pathogenesis by interacting with cellular component(s) involved in the regulation of inflammation.

Transforming growth factor alpha, Shope fibroma growth factor, and vaccinia growth factor can replace myxoma growth factor in the induction of myxomatosis in rabbits.

The epidermal growth factor (EGF) homologues encoded by vaccinia virus, myxoma virus, and malignant rabbit fibroma virus have been shown to contribute to the pathogenicity of virus infection upon inoculation of susceptible hosts. However, since the primary structures of these growth factors and the disease profiles induced by different poxvirus genera vary substantially, the degree to which the various EGF homologues perform similar roles in viral pathogenesis remains unclear. In order to determine whether different EGF-like growth factors can perform qualitatively similar functions in the induction of myxomatosis in rabbits, we created recombinant myxoma virus variants in which the native growth factor, myxoma growth factor (MGF), was disrupted and replaced with either vaccinia virus growth factor, Shope fibroma growth factor, or rat transforming growth factor alpha. Unlike the control virus containing an inactivated MGF gene, which caused marked attenuation of the disease syndrome and substantially less proliferation of the epithelial cell layers in the conjunctiva and respiratory tract, the recombinant myxoma virus strains expressing heterologous growth factors produced infections which were both clinically and histopathologically indistinguishable from wild-type myxomatosis. We conclude that these poxviral and cellular EGF-like growth factors, which are diverse with respect to primary structure and origin, have similar biological functions in the context of myxoma virus pathogenesis and are mitogenic for the same target cells.