Inhibition of an inflammatory response is mediated by a 38-kDa protein of cowpox virus.

The Brighton Red (BR) strain of cowpox virus induces a flat, bright red pock on the chorioallantoic membrane (CAM) of the 12-day-old chick embryo. In contrast, mutants with a deleted 38K gene (which is located 31 to 32 kb from the right-hand end of the virus genome) induced a raised, white, and opaque pock. During the first 24-hr p.i., both CPV-BR and the 38K deletion mutants replicated similarly in the CAM of the chick embryo, as indicated by immunocytochemical detection of similar amounts of virus antigen. By 48 hr p.i., the pocks induced by the mutant and CPV-BR are strikingly different. The pocks induced by the 38K deletion mutants were infiltrated by large numbers of heterophils and macrophages, which correlated with a reduction in the levels of virus antigen and virus infectivity. The CPV-BR pock had an absence of inflammatory cells and increased levels of virus antigen and infectivity. By 72 hr p.i., many of the pocks induced by the mutant were undergoing resolution of the virus infection, as indicated by further decrease of virus antigen and visible signs of healing, whereas CPV-BR pocks continued to be a site of active viral replication. These data are consistent with a model where this 38-kDa protein directly or indirectly inhibits the generation of chemotactic molecules which are elicited during virus replication in the CAM or, alternatively, blocks the interaction of these molecules with cells of the host inflammatory response.