Innate immune responses of a lepidopteran insect, Manduca sexta

Many innate immune mechanisms are conserved throughout the animal kingdom. Manduca sexta, a widely used model for insect biochemical research, employs these mechanisms to defend against invading pathogens and parasites. We have isolated from M. sexta hemolymph a group of proteins (hemolin, peptidoglycan recognition proteins, beta-1,3-glucan recognition proteins, and C-type lectins), which serve as a surveillance mechanism by binding to microbial surface molecules (e.g. peptidoglycan, lipopolysaccharide, lipoteichoic acid, and beta-1,3-glucan). The binding triggers diverse responses such as phagocytosis, nodule formation, encapsulation, melanization, and synthesis of anti-microbial peptides/proteins. Some of these responses are mediated and coordinated by serine proteinase cascades, analogous to the complement system in mammals. Our current research is focused on the proteolytic activation of prophenoloxidase (proPO) - a reaction implicated in melanotic encapsulation, wound healing, and protein cross-linking. We have isolated three proPO-activating proteinases, each of which requires serine proteinase homologs as a cofactor for generating active phenoloxidase. The proteinases and proteinase-like molecules, containing one to two clip domains at their amino-terminus, are acute-phase proteins induced upon an immune challenge. Inhibitory regulation of the proteinases by serpins and association of the proteinase homologs with a bacteria-binding lectin are important for ensuring a localized defense response. Additional serine proteinases expressed in M. sexta hemocytes and fat body have been discovered. Future research efforts will be aimed at elucidating the proteinase cascade for proPO activation and investigating the roles of proteinases in other immune responses such as processing of plasmatocyte-spreading peptide.