Apolipophorins and insects immune response

Abstract

Insect lipoproteins, called lipophorins, are non-covalent assemblies of lipids and proteins serving as lipid transport vehicles. The protein moiety of lipophorin comprises two glycosylated
apolipoproteins, apolipophorin I (apoLp-I) and apolipophorin II (apoLp-II), constantly present in a lipophorin particle, and an exchangeable protein, apolipophorin III (apoLp-III). ApoLp-III is an abundant protein occurring in hemolymph in lipid-free and lipid-bound state and playing an important role in lipid transport and insect innate immunity. In immune response apoLp-III serves as a pattern recognition molecule. It binds and detoxifies microbial cell wall components, i.e., lipopolysaccharide, lipoteichoic acid, and β-1,3-glucan. ApoLp-III activates expression of antimicrobial peptides and proteins, stimulates their antimicrobial activity, participates in regulation of the phenoloxidase system and in hemolymph clotting. In addition, the protein is involved in cellular immune response, influencing hemocyte adhesion, phagocytosis and nodule formation, and in gut immunity. Although apoLp-III is the best studied apolipophorin in insect immunity so far, a literature review suggests that all the three apolipoproteins, apoLp-I, apoLp-II and apoLp-III, function together in a coordinated defense against pathogens.