Fish Diseases and Management

3.2.3.Humoral components of crustacean immunity

1. Lectins (agglutinins: proteins or glycoproteins)

Invertebrate immune system relies on non-self-recognition molecules to ensure efficient defence responses against infectious pathogens that continuously threaten their survival. Lectins from the haemolymph of invertebrates, including crustaceans, have been regarded as potential molecules involved in immune recognition and microorganism phagocytosis through opsonization. Recent reports suggest the involvement of lectins in immune surveillance as non-self-recognition factors, in crustaceans. The precise mechanisms underlying non-self-recognition represent the basis to prevent and control infections as well as to stimulate animal resistance. This is particularly relevant for cultivated aquatic species, especially penaeid shrimps, which are frequently constrained by recurrent diseases that often provoke great economic losses.

2. ProPO activating system

Phenoloxidase (PO), an oxidoreductase, is a multi functional enzyme found widely distributed in microorganisms, plants and animals. This enzyme is part of a complex system of proteinases, pattern recognition proteins and proteinase inhibitors constituting the so-called non-self recognition prophenoloxidase-activating system. PO is known to be associated with melanization, sclerotization and wound healing in crustaceans. In crustaceans, the PO exists in the haemolymph as an inactive pro enzyme, prophenoloxidase (proPO) which is activated by both an endogenous activating system and exogenous reagents. The proPO cascade results in the process of melanization following  its activation to PO by  limited proteolysis by endogenous serine proteases. PO oxidizes phenol into quinones, which will then polymerise non-enzymatically to melanin. The serine proteases can in turn be activated by agents such as lipids, organic solvents and detergents apart from b, 1–3 glucans, peptidoglycan and lipopolysaccharides of Gram-negative and Gram-positive bacteria suggesting that the proPO system might be involved in the non-self recognition in crustaceans. In many crustaceans including shrimps, the proPO activating system is contained in the vesicles of granular and semi-granular cells, being released from the haemocytes through a regulated exocytosis.

3. Antimicrobial compounds

The production of antimicrobial peptides represents a first-line of host defence mechanism of innate immunity that is widespread in nature. Only recently such effectors were isolated in crustacean species, whereas numerous antimicrobial peptides have been characterized from other arthropods, both insects and chelicerates. A family of antimicrobial peptides, named penaeidins, which were isolated from the shrimp Penaeus vannamei were successful in their role as antimicrobial peptides upon microbial challenge. The mature peptides, with molecular masses ranging from 5.4 to 6.6 kDa, are characterised by a proline-rich NH₂-terminal domain and a COOH-terminal domain containing three intramolecular disulfide bridges. The penaeidins display antifungal activity and antibacterial activities, predominantly against Gram-positive bacteria, with a bacterial agglutinating property.

4. Serine proteinase inhibitors

Arthropod haemolymph contains proteins with serine proteinase inhibitory activity. These inhibitors may exist in plasma or in hemocyte granules. The serine proteinase inhibitors in arthropod haemolymph are likely to function in protecting their hosts from infection by pathogens or parasites. Some may inhibit fungal or bacterial proteinases. Others probably have roles in regulating endogenous proteinases involved in coagulation, prophenoloxidase activation, or cytokine activation.

5. Clotting reaction

Crustaceans lack immune memory and therefore have to rely on innate immune reactions. One innate immune process which is of prime importance is the capacity to seal wounds rapidly and efficiently. Clotting process is one such reaction, which is very efficient and rapid, based on a clotting protein present in plasma and a transglutaminase in blood cells. The clotting protein has been cloned and found to belong to the vitellogenin superfamily of proteins. Clotting proteins have also been purified from shrimp and they are very similar in properties to those of crayfish.