Cellular immunosuppressive protein in the plasma of parasitized insect larvae.

Evidence that endoparsitoid wasps may suppress the host cellular defense reaction in order to survive within the hemocoel of their host insects was obtained by isolating a cellular immunosuppressive factor from the larval plasma (hemolymph without cells) of the armyworm Pseudaletia separata parasitized with the wasp Cotesia kariyai. The purified proteinaceous factor suppresses hemocyte degranulation and subsequent immune reactions such as nodule formation and encapsulation in the armyworm larvae. The immunosuppressive protein exists not only in the plasma of parasitized larvae but also in the plasma of unparasitized larvae. However, its concentration is much less in unparasitized than in parasitized larvae. Purification procedures involved gel exclusion, ion exchange, and reversed phase high performance liquid chromatography (HPLC). Gel exclusion HPLC and SDS-polyacrylamide gel electrophoresis suggest an approximately 470-kDa hexamer composed of identical subunits of 82 kDa. Aryl amino acids such as tyrosine and phenylalanine comprise about 20% of the total protein amino acid. The sequence of 28 amino acid residues at the amino terminus of the factor was determined to be as follows: Ser-Val-Val-Pro-Thr-Lys-His-Glu-Ile-Lys-Val-Lys-Ala-Val-Glu- Pro-Gln-Tyr-Ile-Xaa-Tyr-Gln-Arg-Lys-Ile-Leu-Ala-Leu-Phe.