Pseudorabies virus glycoprotein III derived from virions and infected cells binds to the third component of complement.

Glycoprotein III (gIII) of pseudorabies virus (PRV) was shown to bind to the third component of complement (C3). This was observed only with porcine C3 whereas human C3 showed negligible binding under the conditions tested. PRV virion proteins could be precipitated from supernatants and cell lysates of PRV-infected cells by means of swine-C3 coupled to sepharose. According to their molecular size and their reactivity with anti-gIII monoclonal antibodies, the precipitated PRV proteins represented the fully glycosylated and smaller forms of the gIII protein. Precipitation from PRV virions yielded predominantly the fully glycosylated form of gIII whereas infected cell lysates also contained lower molecular weight gIII proteins. The observed specificity of the virus protein for porcine C3 correlates well with the known host tropism of PRV. Our findings suggest that PRV gIII may exhibit more functions than solely providing attachment to heparin-like moieties on target cell surfaces. As the complement cascade is an important defense mechanism against a variety of pathogens, the interaction with the host C3, the pivotal component of the complement activation, might be a virulence factor of PRV.