A secreted phospholipase A 2 (BmsPLA 2 ) regulates melanization of immunity through BmDDC in the silkworm Bombyx mori.

Insect immune-associated phospholipase A (PLA ) is an important target of pathogen invasion. Melanization, an effective defense response, has significant correlations with other immune responses to coordinate immune attack against invaders. However, the effect of PLA on melanization has not yet been reported in insects or other arthropods. In this work, we cloned a PLA gene (BmsPLA ), and its protein had characteristic features of secreted PLA (sPLA ). After injection of bacteria, BmsPLA expression and sPLA activity in hemolymph significantly increased. BmsPLA fluorescence was transferred from the cytoplasm to the cell membranes of circulating hemocytes. These results indicated that BmsPLA was related to hemolymph immunity in silkworms. Interestingly, reducing BmsPLA by RNAi decreased melanosis (melanistic hemocytes) levels in vivo and in vitro, while BmsPLA overexpression had the opposite effect. The larval survival and melanization rate in the hemocoel both slowed depending on the PLA inhibitor dosage. These results demonstrated that BmsPLA plays a role in melanization during the immune process of silkworms. Surprisingly, the level of BmDDC matched the degree of melanization in various observations. BmDDC expression showed a significant increase, with the peak occurring later than that of BmsPLA after injection of bacteria, implying that BmsPLA was activated prior to BmDDC. Moreover, the alteration of BmsPLA by RNAi or overexpression led to altered BmDDC levels. These results suggested that BmsPLA regulates the melanization response in silkworms through BmDDC. Our study proposes a new regulatory mechanism of the melanization response and new directions for understanding the complex immune networks of insects. This article is protected by copyright. All rights reserved.