Isolation of cowpox virus clones deficient in production of type A inclusions: relationship to the production of diffusible LS antigen.

Cowpox virus clones (A- clones) deficient in production of type A inclusions were isolated from two cowpox strains, Amsterdam and 53. These clones did not differ from their parents in major markers such as pock morphology in chorioallantoic membranes and pathogenicity in the rabbit skin. However, the LS antigens induced by A- clones developed precipitin lines in agar gel diffusion tests, while the antigens from their parents failed to precipitate. Immunofluorescence and agar gel diffusion tests revealed that antigens detectable by antisera against purified type A inclusions and LS antigens were closely related to each other. These findings suggest that the A- clones might be variants of cowpox virus which have lost the ability to assemble LS antigens into type A inclusions.