In vitro cellular immunity to unrelated pathogens in chickens infected with fowlpox virus

Peritoneal macrophages recovered from chickens 15 to 20 days after inoculation with fowlpox virus and showing a delayed hypersensitivity reaction against fowlpox antigens demonstrated an enhanced antimicrobial effect against fowlpox virus as well as unrelated viruses and bacteria. Inoculation of normal chicken macrophage cultures with fowlpox virus resulted in approximately a 200-fold increase in virus titer by 96 h, whereas the virus showed less than a fourfold increase in macrophage cultures from fowlpox-immune chickens. Similarly, the titer of Newcastle disease virus increased by more than 1 log in cultures of normal macrophages, whereas the titer decreased by approximately 1 log after infection of fowlpox-immune macrophages. Vaccinia, vesicular stomatitis, and herpes simplex viruses, which are not natural pathogens of chickens, failed to replicate in cultures of either normal or fowlpox-immune macrophages. By using Salmonella gallinarum, it was further demonstrated that the antimicrobial activity of fowlpox-immune macrophages encompassed not only nonspecific viruses but also bacteria. Infection of normal macrophages with Salmonella resulted in intracellular replication of the organism between 0 and 24 h as determined by microscope examination and viable bacteria counts. In contrast, cultures of fowlpox-immune macrophages failed to show an increase in intracellular organisms and showed a marked decrease in viable bacteria. In conclusion, these studies clearly showed that cellular immunity, previously demonstrated in mammalian species, develops in chickens after infection with fowlpox virus.