Inflammatory responses and the generation of chemoattractant activity in cowpox virus-infected tissues

Histological examination of the lesions produced on the chick chorioallantois infected with cowpox virus shows extensive haemorrhage but there are few inflammatory cells. However, infection with a deletion mutant, white cowpox virus, results in little haemorrhage but there is massive polymophonuclear cell infiltration. Extracts from tissues infected with the parental, red cowpox virus contained little or no chemoattractant activity detectable in micropore filter assays. However, white cowpox virus-infected tissue extracts, including cellular extracts from infected tissue cultures, had a marked effect in vitro on the migration of both neutrophils and monocytes. The chemoattractant activity induced in ovo by white cowpox virus infection has sedimentation characteristics in sucrose density gradients that are similar to leukotactic factors shown previously to be present in the allantoic fluid of chick embryos infected with paramyxoviruses. Induction of chemoattractant activity did not occur after infection of chick chorioallantois with a recombinant white cowpox virus constructed to express a protein that is responsible for the haemorrhagic character of red cowpox virus. This gene product has been shown previously to have homology with various serine protease inhibitors. The significance of these studies to the immunogenicity and pathogenicity of vaccinia recombinant viruses is discussed.