HOST-DEFENSE REACTIONS OF INSECTS

Insects have at least three types of barriers to invading foreign matt er at the integument, intestine and hemocoel. The hemocoel has several types of hemocytes and is the final barrier to invaders. The granular cells and plasmatocytes called immunocytes play essential roles in in sect immunity. The foreign or non-self surface is recognized by the im munocytes, and triggers cellular filopodial elongation in which microf ilaments play an indispensable role in phagocytosis. On the other hand , encapsulation and nodule formation involve cooperation between the g ranular cells and plasmatocytcs. In this case, a humoral mediator (cyt okine) is related to cell-cell interactions. Insects also have anti-mi crobial substances, lectin, a prophenoloxidase-activating system (proP O-AS), and a complement-like factor. Cecropin, insect defensin, and at tacin are well-known anti-bacterial substances. In general, they are i nduced on injury or infection as well as at molting or pupation and sh ow a broad spectrum activity. Lectin is also produced by infection or at developmental stages, and plays an important role in agglutinating foreign cells or in mediating the binding of granular phagocytes to th em. In lepidoptera, lectin production is closely regulated by the endo crine system and by phagocytosis of granular cells. Furthermore, proPO -AS is correlated with recognition of foreign materials, particularly with the bacterial cell wall component, peptideglycan or the fungal ce ll wall component, beta-1,3-glucan. Since proPO and the protein for bi nding to peptideglycan or beta-1,3-glucan are synthesized by oenocytoi ds and spherule cells, respectively, these hemocytes can be called imm unocytes. The rapid attachment of granular cells depends on the proPO- AS, suggesting that proPO-AS serves as opsonin in granulocyte adhesion . Complement-like factors are also found in hemolymph, although their defensive role is not confirmed. Like antibodies vertebrates possess, as complement has opsonic or lytic functions, it appears that insect c omplement-like factors could be involved in cytolysis or opsonization of foreign cells.