An analysis of the structural and functional similarities of insect hemocytes and mammalian phagocytes

Abstract

The insect immune response demonstrates a number of structural and functional similarities to the innate immune system of mammals. As a result of these conserved features insects have become popular choices for evaluating the virulence of microbial pathogens or for assessing the efficacy of antimicrobial agents and give results which are comparable to those that can be obtained using mammals. Analysis of the cellular component of the insect and mammalian immune systems demonstrates many similarities. Insect hemocytes recognize pathogens and phagocytose material in a similar manner to neutrophils. The killing of ingested microbes is achieved in both cell types by the production of superoxide and by the release of enzymes in the process of degranulation. Insect hemocytes and mammalian neutrophils are sensitive to the same inhibitors. This review highlights the strong similarities between the phagocytic cells of both groups of animals and demonstrates the potential benefits of using selected insects as in vivo screening systems.

Keywords: Galleria; hemocytes; immunity; insect; invertebrate immunity; mini-model; neutrophil; vertebrate immunity.

Figure 1. Effect of Staphylococcus aureus on viability of G. mellonella larvae. Larvae were inoculated with S. aureus and incubated at 37 °C. A mortality rate of 90% is observed at 72 h. The dark color is due to the process of melanization in response to the growth of the pathogen in the insect. Note that the larvae are supplied with a food source (wood shavings).

Figure 2. Hemocytes of G. mellonella larvae. Hemocytes were recovered from hemolymph of G. mellonella larvae and viewed using an Olympus Microscope. Images show plasmatocytes, spherulocyte, and granular cell.

Figure 3. Mode of action of neutrophil. (A) Unactivated neutrophil: Opsonized bacterium is about to be phagocytosed, NADPH oxidase is not functional and all enzymes are retained on primary (red) and secondary (green) granules within cytoplasm. (B) Activated neutrophil: Opsonized bacterium has been phagocytosed, p40^phox, p47^phox, p67^phox, and rac have translocated from the cytoplasm to join gp91^phox on the cell membrane and as a result superoxide is produced from oxygen with the consequent conversion of NADPH to NADP⁺ and H⁺. Degranulation has commenced releasing enzymes (red, green) from granules into the phagocytic vacuole and outside the cell.