Lipophorin of lower density is formed during immune responses in the lepidopteran insect Galleria mellonella

Injection of heat-killed bacteria into larvae of the greater wax moth Galle ria mellonella is followed by changes in lipoprotein composition in the hem olymph. Density gradient centrifugation experiments revealed that within th e first four hours after injection, a part of larval lipoprotein, high-dens ity lipophorin (HDLp), was converted into a lipoprotein of lower density. S DS-polyacrylamide gel electrophoresis analysis of the gradient fractions an d sequencing of protein fragments, established that the exchangeable apolip oprotein apolipophorin III (apoLp-III), a potent immune-activator, was asso ciated with this newly formed lipophorin. To investigate further the influe nce of lipophorin-associated apoLp-III on immune-related reactions, we perf ormed in vitro studies with isolated hemocytes from G. mellonella and lipop horins from the sphinx moth Manduca sexta, as a natural source of high amou nts of low-density lipophorin (LDLp) and HDLp. The hemocytes were activated to form superoxide radicals upon incubation with LDLp, but not with HDLp. Fluorescence-labeled LDLp was specifically taken up by granular cells. This process was inhibited by adding an excess of unlabeled LDLp, but not by HD Lp. We hypothesize that larval lipophorin formed in vivo is an endogenous s ignal for immune activation, specifically mediated by the binding of lipid- associated apoLp-III to hemocyte membrane receptors.