Cky811 protein expressed by polydnavirus and venom gland of Cotesia kariyai regulates the host Mythimna separata larvae immune response function of C‐type lectin responsible for foreign substance recognition which suppresses its melanization and encapsulation

Cotesia kariyai (Ck) larvae implanted into the body cavity of the Mythimna separata (armyworm) larvae get melanized and encapsulated after adhesion by hemocytes called hyperspread cells (HSCs). The present study showed that HSCs could not adhere to the implanted Ck larvae in armyworm larvae after injection of Ck polydnavirus (CkPDV) + venom (V), thus melanization and encapsulation could not occur. A C‐type lectin called Mys‐IML of the host armyworm larvae was considered to be involved in the recognition of foreign substances which always expressed in hemocytes. The CkPDV DNA encodes a C‐type lectin called Cky811 that has high amino acid homology to Mys‐IML. HSCs did not adhere when CkPDV + V was mixed with the hemolymph of armyworm larvae on glass slides and incubated in vitro, but the addition of anti‐Cky811 antibody enabled HSCs to adhere. The messenger RNA (mRNA) expression of Mys‐IML in armyworm larvae injected with CkPDV + V became undetectable by 6 h. On the contrary, Cky811 mRNA was well expressed in the hemocytes of armyworm larvae injected with CkPDV + V from 0.5 to 6 h. Cky811 protein was also detected in the crude extracts from Ck venom gland + Ck venom reservoir, suggesting that these proteins regulate foreign substance recognition by the armyworm within 0.5 h. These results suggest that CkPDV + V suppresses mRNA expression of Mys‐IML, and that Cky811 protein expressed in hemocytes regulates foreign substance recognition of Mys‐IML, resulting in inhibition of the downstream reaction steps: HSCs adhesion, melanization, and encapsulation. Expression of Cky811, a C‐type lectin encoded by Cotesia kariyai polydnavirus, in hemocytes of the host Mythimna separata inhibit foreign substances recognition by Mys‐IML, a C‐type lectin of the host. This prevents the hyperspread cells of M. separata larvae from attaching to the foreign substance and inhibits their downstream reactions of melanization and encapsulation. This ensures that the eggs and larvae of C. kariyai are not eliminated from the M. separata body cavity. Research Highlights Cky811, encoded by polydnavirus of Cotesia kariyai, inhibits foreign substance recognition by Mys‐IML in Mythimna separata. This prevents the host's hyperspread cells from attaching to the foreign substance and inhibits melanization and encapsulation.