SUMMARY: We have previously shown that subsp. serotype 3, produces two extracellular inhibitors of the immune system of Saturniid pupae (designated inhibitors A and B; Edlund 1976). Starting from the culture supernatant of a new mutant of with a decreased extracellular proteolytic activity, we have now purified immune inhibitor A (InA). The procedure described consists of three steps: ultrafiltration, precipitation with ammonium sulphate and chromatography on hydroxylapatite. Purified InA gave a single band on polyacrylamide gel electrophoresis using either a gel concentration of 7.5 % (w/v) and reducing and denaturing conditions or a gradient gel and native conditions. In both cases the apparent molecular weight was 78000. A certain amount of proteolytic activity was always co-purified with InA but the two activities could be dissociated by heat or EDTA treatment. Antiserum against purified InA gave only one sharp precipitation band on immunodiffusion against InA with or without EDTA. InA inhibited the killing of by immune haemolymph but did not affect the killing of InA was toxic for Drosophila when injected into the abdomen of adult male flies.


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