Lepidoptera constitute a major group of agricultural and forestry pests. Therefore, investigating the immune mechanisms of the model species Bombyx mori may provide valuable insights for the development of improved pest management strategies. In insects, phenoloxidase (PO) and dopa decarboxylase (DDC) in immune melanization have been widely studied individually, yet their relative contributions have rarely been investigated. Here, we demonstrate that pharmacological inhibition of either PO or DDC in Escherichia coli-infected larvae significantly suppresses hemolymph melanization, with PO inhibition causing a more pronounced reduction than DDC inhibition. Consistently, RNA interference-mediated knockdown of BmPPO1 or BmPPO2 markedly decreased hemolymph melanization following infection. This results in both PO and DDC contributing to immune-induced hemolymph melanization, with PO playing a dominant role in this process. In contrast, compared to PO inhibition, DDC inhibition leads to significant damage to hemolymph antibacterial activity and cellular immune responses, including hemocyte aggregation, encapsulation, and phagocytosis. In addition, compared with the knockdown of BmPPO1 or BmPPO2, the knockdown of BmDDC leads to a more severe decrease in antibacterial activity and cellular immune function. Exogenous addition of dopamine can partially rescue cell damage, indicating that both DDC and PO play a role in cellular immunity, but DDC has a slightly stronger effect. Overall, this study provides important insights into the immunity of hemolymph in insects and other arthropods.
Hu et al. (Thu,) studied this question.