The prophenoloxidase‐activating system in invertebrates

Abstract

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 beta-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.

Crustacean hematopoiesis

“…This methodological breakthrough enabled detailed studies of various hemocyte characteristics, leading to the identification of several proteins that are primarily important for cellular immunological reactions. For example, the proPO-activating system was shown to reside primarily in the granular hemocytes of Carcinus maenas, Cancer pagurus, Macropipus depurator and Eupagurus bernhardus and, to a minor extent, in semigranular hemocytes ; for reviews of the ProPO-activating system, see Cerenius et al, 2008;Cerenius and Söderhäll, 2004). Previous findings, such as the identification of hemocyanin immunoreactivity in C. maenas hemocytes (Ghiretti-Magaldi et al, 1973), most likely reflected the fact that the purified hemocyanin from the hemolymph of the crab was contaminated with ProPO, which has a similar molecular mass as hemocyanin.…”

Bacterial strategies to overcome insect defences

“…Specialized insect haemocytes can promote melanization, haemocyte aggregation and nodulation, encapsulation or opsonization. Recognition of microbial compounds, such as β-1,3-glucans, lipopolysaccharides and peptidoglycan, ensures that the humoral response is effective 98,99 . Once activated, phenoloxidase catalyses the formation of melanotic nodules that limit the spread of infecting microorganisms or damaged tissues.…”

Innate immune response in insects: recognition of bacterial peptidoglycan and amplification of its recognition signal

“…Innate immunity in invertebrates consists of humoral and cellular components (3,4). The humoral component includes antimicrobial peptides (5), lectins (6), and the prophenoloxidase (proPO) activation cascade (7); the cellular component includes phagocytosis by circulating hemocytes (8). In Drosophila, the Toll signalling pathway is responsible for defense against Gram-positive bacteria and fungi, while the immune deficiency (Imd) pathway is activated primarily to defend against Gram-negative bacteria.…”