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.

Characterization of the serosal cells surrounding Cotesia kariyai larvae and their role in host immunosuppression.

About half of the serosal cells (Scs) of Cotesia kariyai (Ck) eggs are released as teratocytes into the host body cavity after hatching, and another half remain attached to the larval epidermis until the 1st instar larvae of Ck ecdysis to 2nd instars. To investigate the role of the serosal cells surrounding Ck 1st instar larvae (1st Scs) in immune avoidance, Ck 1st instar larvae with and without Scs removed using dispase were transplanted into Mythimna separata larvae (MsLrv), respectively. As a result, Ck 1st instar larvae surrounded by Scs were less susceptible to the MsL encapsulation than Ck 1st instar larvae without the Scs, suggesting that the Scs are involved in suppressing the encapsulation of the MsL. Furthermore, when the granular cells and plasmatocytes of the MsL involved in the encapsulation were incubated in a medium supplemented with proteins extracted from 1st Scs, the plasmatocytes failed to adhere to glass slides, and did not spread their filopodium and lamellipodium. These findings suggest that 1st Scs express proteins that inhibit filopodium and lamellipodium spreading to prevent the MsL plasmatocytes from adhering to Ck larvae.