GRANULAR CELLS ARE REQUIRED FOR ENCAPSULATION OF FOREIGN TARGETS BY INSECT HEMOCYTES

Haemocytes play an essential role in defending invertebrates against p athogens and parasites that enter their haemocoel, A primary defense r esponse is encapsulation; a process in which haemocytes attach to the foreign organism and kill it. Whether encapsulation requires cooperati on between specific subpopulations of haemocytes is unknown, Using pur ified subpopulations of haemocytes and an in vitro encapsulation assay , we investigated the process of capsule formation in the insect Pseud oplusia includens. Immunocytochemical staining revealed that capsule f ormation involves a three step process, Encapsulation began when granu lar cells attached to the foreign target, This was followed by attachm ent of multiple layers of plasmatocytes. Termination of capsule format ion occurred when a subpopulation of granular cells formed a monolayer around the periphery of the capsule, Neither granular cells nor plasm atocytes were capable of forming a capsule independently, However, pla smatocytes encapsulated targets if granular cells were present or if t argets were preincubated in medium conditioned by granular cells, The effect of granular cell-conditioned medium could be blocked by the add ition of the cell adhesion recognition sequence, RGDS, but not by RGES , These results demonstrate experimentally that granular cells are req uired for encapsulation of foreign targets by plasmatocytes in vitro, and that the role of granular cells in this process involves an RGD-de pendent cell adhesion mechanism.