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 accompli shed by two blood cells (hemocytes), the granulocyte (GRs) and plasmat ocytes (PLs), collectively called immunocytes. Immunocytes (principall y 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 wi thin 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 studie d in arthropod immunocytes, Using the DNase I inhibition assay, we fou nd a decrease in cytosolic G-actin and an increase in the cytoskeletal F-actin in the phagocytosing immunocytes; the total actin in both res ting and phago-cytosing immunocytes remained constant, These results s howed 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 ear ly stages of phagocytosis in these two animals-and perhaps in arthropo ds in general-is variable; although we detected some negligible amount s of [cAMP](i) (0/10-0.80 pmol/cell at different time intervals) in L. polyphemus immunocytes, it was inconclusive whether those in G. porte ntosa also contained [cAMP](i). Even in L. polyphemus, the difference in the amounts of [cAMP](i) in resting and phagocytosing cells was ins ignificant (P >0.05). It was also inconclusive whether [Ca2+](i) and/o r [Mg2+](i) play any roles in the early stages of phagocytosis in the two arthropods in this study, These results suggest that the two pheno mena (F-actin polymerization and levels of [cAMP](i) in arthropods) ar e basically similar to those in vertebrate neutrophils and macrophages , which suggests that certain immunological mechanisms are conserved i n nature. (C) 1996 Academic Press, Inc.