Cytoskeletal F-actin polymerization from cytosolic G-actin occurs in the phagocytosing immunocytes of arthropods (Limulus polyphemus and Gromphadorhina portentosa): Does [cAMP](i) play any role?

Phagocytosis is a major defense reaction in arthropods and is accomplished by two blood cells (hemocytes), the granulocyte (GRs) and plasmatocytes (PLs), collectively called immunocytes. Immunocytes (principally the GRs) from two arthropods, Limulus polyphemus (horseshoe crab and Gromphadorhina portentosa (Madagascar hissing cockroach) effectively phagocytose fluorescein isothiocyanate (FITC)-conjugated fluoresbrite microspheres (FITC-FM) and chicken (Gallus domesticus) erythrocytes within 1 hr of incubation, Although actin polymerization and changes in intracellular cAMP ([cAMP](i)) levels occur during the early stages of phagocytosis in vertebrates, these two phenomena have not been studied in arthropod immunocytes, Using the DNase I inhibition assay, we found a decrease in cytosolic G-actin and an increase in the cytoskeletal F-actin in the phagocytosing immunocytes; the total actin in both resting and phago-cytosing immunocytes remained constant, These results showed an 86% increase in F-actin in G. portentosa immunocytes and a 29% increase in those of L. polyphemus after 1 hr of initial incubation with FITC-FM. As in some vertebrates, the role of [cAMP](i) in the early stages of phagocytosis in these two animals-and perhaps in arthropods in general-is variable; although we detected some negligible amounts of [cAMP](i) (0/10-0.80 pmol/cell at different time intervals) in L. polyphemus immunocytes, it was inconclusive whether those in G. portentosa also contained [cAMP](i). Even in L. polyphemus, the difference in the amounts of [cAMP](i) in resting and phagocytosing cells was insignificant (P >0.05). It was also inconclusive whether [Ca2+](i) and/or [Mg2+](i) play any roles in the early stages of phagocytosis in the two arthropods in this study, These results suggest that the two phenomena (F-actin polymerization and levels of [cAMP](i) in arthropods) are basically similar to those in vertebrate neutrophils and macrophages, which suggests that certain immunological mechanisms are conserved in nature. (C) 1996 Academic Press, Inc.