Summary: A major innate defense system in invertebrates is the melanization of pathogens and damaged tissues. This important process is controlled by the enzyme phenoloxidase (PO) that in turn is regulated in a highly elaborate manner for avoiding unnecessary production of highly toxic and reactive compounds. Recent progress, especially in arthropods, in the elucidation of mechanisms controlling the activation of zymogenic proPO into active PO by a cascade of serine proteinases and other factors is reviewed. The proPO-activating system (proPO system) is triggered by the presence of minute amounts of compounds of microbial origins, such as β-1,3-glucans, lipopolysaccharides, and peptidoglycans, which ensures that the system will become active in the presence of potential pathogens. The presence of specific proteinase inhibitors prevents superfluous activation. Concomitant with proPO activation, many other immune reactions will be produced, such as the generation of factors with anti-microbial, cytotoxic, opsonic, or encapsulation-promoting activities.

The prophenoloxidase-activating system in invertebrates.

▪ Abstract Infection in insects stimulates a complex defensive response. Recognition of pathogens may be accomplished by plasma or hemocyte proteins that bind specifically to bacterial or fungal polysaccharides. Several morphologically distinct hemocyte cell types cooperate in the immune response. Hemocytes attach to invading organisms and then isolate them by phagocytosis, by trapping them in hemocyte aggregates called nodules, or by forming an organized multicellular capsule around large parasites. These responses are often accompanied by proteolytic activation of the phenoloxidase zymogen that is present in the hemolymph. A component of insect immune responses to bacteria is the synthesis by fat body and hemocytes of a variety of antibacterial proteins and peptides, which are secreted into the hemolymph. These molecules attack bacteria by several mechanisms. Inducible antifungal proteins have also been recently discovered in insect hemolymph. The promoters for several antibacterial protein genes in ins...

Biological mediators of insect immunity

Role of the prophenoloxidase-activating system in invertebrate immunity.

Cell-mediated immunity in arthropods: hematopoiesis, coagulation, melanization and opsonization.

Invertebrate animals, which lack adaptive immune systems, have developed other systems of biological host defense, so called innate immunity, that respond to common antigens on the cell surfaces of potential pathogens During the past two decades, the molecular structures and functions of various defense components that participated in innate immune systems have been established in Arthropoda, such as, insects, the horseshoe crab, freshwater crayfish, and the protochordata ascidian These defense molecules include phenoloxidases, clotting factors, complement factors, lectins, protease inhibitors, antimicrobial peptides, Toll receptors, and other humoral factors found mainly in hemolymph plasma and hemocytes These components, which together compose the innate immune system, defend invertebrate from invading bacterial, fungal, and viral pathogens This review describes the present status of our knowledge concerning such defensive molecules in invertebrates

/pdf/recent-advances-in-the-innate-immunity-of-invertebrate-41cqts7utq.pdf

Recent advances in the innate immunity of invertebrate animals.

http://www.vliz.be/imisdocs/publications/33406.pdf

Crustacean haemocytes and haematopoiesis

Structure and biological activity of a 1,3-beta-D-glucan-binding protein in crustacean blood.

Host prophenoloxidase expression in freshwater crayfish is linked to increased resistance to the crayfish plague fungus, Aphanomyces astaci.

A cDNA with an open reading frame of 684 base pairs was isolated from a library from blood cells of the crayfish Pacifastacus leniusculus. It codes for a signal sequence and a mature protein of 209 amino acids with a predicted molecular mass of 22.7 kDa. The amino acid sequence consists of four repeated stretches (45-73% identical to each other), indicating that the protein has four domains. The domains have significant sequence similarity to serine proteinase inhibitors of the Kazal family. The three first domains have a leucine residue in the putative reactive site, suggesting that the protein is a chymotrypsin inhibitor. A monomeric 23-kDa proteinase inhibitor, which by amino terminal sequencing of the mature protein was confirmed to be the cloned Kazal inhibitor, was purified from crayfish blood cells. It inhibited chymotrypsin or subtilisin, but not trypsin, elastase or thrombin. The inhibitor seemed to form a 1:1 complex with chymotrypsin or subtilisin. This protein seems to be the first described Kazal inhibitor from blood cells of any animal and the first one with four domains.

https://febs.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1432-1033.1994.tb19005.x

Pia Keyser

Papers

Crustacean haemocytes and haematopoiesis

Structure and biological activity of a 1,3-beta-D-glucan-binding protein in crustacean blood.

Host prophenoloxidase expression in freshwater crayfish is linked to increased resistance to the crayfish plague fungus, Aphanomyces astaci.

Purification and cDNA cloning of a four-domain Kazal proteinase inhibitor from crayfish blood cells