Canarypox and fowlpox viruses as recombinant vaccine vectors: an ultrastructural comparative analysis.

Due to their natural host-range restriction to avian species, canarypox virus (CP) and fowlpox virus (FP) represent efficient and safe vaccine vectors, as they correctly express transgenes in human cells, elicit complete immune responses, and show protective efficacy in preclinical animal models. At present, no information is available on the differences in the abortive replication of these two avipox viruses in mammalian cells. In the present study, the replicative cycles of CP and FP, wild-type and recombinants, are compared in permissive and non-permissive cells, using transmission electron microscopy. We demonstrate that in non-permissive cells, the replicative cycle is more advanced in FP than in CP, that human cells, whether immune or not, are less permissive to avipox replication than monkey cells, and that the presence of virus-like particles only occurs after FP infection. Overall, these data suggest that the use of FP recombinants is more appropriate than the use of CP for eliciting an immune response.