Parasitology Today, vot. 5, no. 6, 1989 171 i i ii Cellular Immunity in Crustaceans and the proPO System M,W. Johansson and K. S6derh ill The molecular mechanism of cellular immunity in arthropods has until recently been largely unknown, but with the d~.velopment of a technique to isolate and handle the different blood cell types of crustaceans and with the purification of several proteins associated with the so-called proPO system of freshwater crayfish the processes have now begun to be better understood. In this article Mats ff ohansson and Kenneth Srderh~ill discuss the function of the proPO system in cellular immune reactions in crustaceans and in particular the role of a protein with a molecular mass of 76 kDa, which has been shown to be involved in the communication between the different blood cell types of crayfish. REWEWS Department of Physiological Botany University of Uppsala PO Box 540 S-751 21 Uppsala Sweden There is growing research interest in arthropod defence mechanisms, partly because many haematophagous insects are vectors for several important parasites of man and livestock, and also because crus- taceans are becoming of increasing econ- omic importance in aquaculture. It is commonly agreed that arthropods and invertebrates in general, do not pos- sess immunoglobulins J. Nevertheless, with an open circulatory system, they must have immediate, non-inducible defence and coagulation mechanisms to entrap parasites and prevent blood loss upon wounding ~. These reactions are car- ried out primarily by the blood cells or haemocytes 1. Their vital role in such pro- cesses can be shown in experimental depletion of the number of free haemo- cytes, after which the animal becomes more susceptible to infection2. Smaller foreign particles are phagocytosed by cir- cula.ting haemocytes as well as by immobilized phagocytes, and if present in large numbers, foreign particles are also immobilized in nodular aggregates of haemocytes 1. Larger entities are encap- sulated by haemocytes that attach and spread over them, and that eventually build up into several layers of cells~. Wounding induces very :rapid clotting in arthropods, and the haemocytes also take part in this process by aggregating within the wound and by providing factors required for plasma gelationl. Apart from these immediate defence reactions, arthropods have long-term inducible humoral immunity, mediated by antibacterial proteins 3'4 that are newly synthesized upon infection or wounding. It has long been recognized that the cellular defence reactions in invertebrates 1989, Elsevier Science Publishers t.td, (UK) 0165~ 147/89/$02.00 are most often accompanied by melan- ization 1'2,s. The key enzyme in the syn- thesis of the pigment melanin is phenoloxidase, which is present in arthro- pod blood (haemolymph) as an inactive pro-enzyme, prophenoloxidase (proPO). ProPO is activated in a stepwise process by microbial cell wall components such as the 13-1,3-glucans of fungi, or the lipo- polysaccharides (LPS) and peptidogly- cans of Gram-negative and Gram-positive bacteria, respectively 2'5-7 (Fig. 1). Apart from its role in melanization, the results from several experiments have implied that components of the putative proPO- activating system of arthropods z'5'6 (Fig. 1) stimulate several cellular defence reac- tions, including (1) phagocytosis2's'9, (2) nodule formation2'1°, (3) encapsulation 2 and (4) haemocyte locomotion 1°. The importance of the proPO system in defence is also shown by the fact that certain parasites have developed ways to evade the system or to inhibit its acti- vation. For example, some entomo- pathogenic fungi produce protoplasts without a glucan-containing cell wall in the host haemolymph5, and toxins from other fungi inhibit both the glucan- triggered activation of the system and the glucan-stimulated locomotion of insect haemocytes ~°. During oviposition in some parasitic species of wasp, viruses are also injected into the host that inhibit proPO activation 11. The fact that the peptide, melittin, of bee venom blocks the acti- vation of proPO, but not phenoloxidase activity, may also be relevant herez. The biochemistry of activation of the proPO system and its roles in arthropod immunity are becoming clearer, since progress has now been made not only in