Defence mechanisms in shrimps and disease control by vaccination

Invertebrates lack true antibodies and therefore do not have an immune memory Like vertebrate defence systems, the shrimps also have both cellular and humoral immunity

Cellular components of crustacean immunity Crustacean haemocytes :- Crustacean haemocytes have important roles in the host immune response including recognition, phagocytosis, melanization, cytotoxicity and cell-to-cell communication.

Classification of the haemocyte types hyaline cells are characterised by the absence of any granules in the cell chiefly involved in phagocytosis semi-granular cells contain small granules involved in early non-self recognition and active encapsulation granular cells have large granules Involved in storage and release of the prophenoloxidase (proPO) system and cytotoxicity.

Non-self recognition mechanisms: LPS-binding proteins Beta-1,3 glucan-binding proteins Peptidoglycan binding proteins Lectins /agglutinins

Innate immediate immune reactions Coagulation-clotting Transglutaminase: is involved in clotting process in shrimps. Clotting protein Prophenoloxidase (ProPO) - activating system ProPO in P. monodon has been cloned and different proteins involved in the ProPO system cascade havebeen characterised.

Mechanisms of cellular defence in crustaceans Phagocytosis the process of internalisation of invading pathogens and other small foreign particles by individual blood cells. The phagocytosis is carried out mainly by the hyaline cells. This is the first line of defence in crustaceans against the pathogens, which invades the tissue bypassing the cuticle

Nodule formation Following to the heavy load on phagocytosis, the nodule formation is activated. In nodule formation, the microorganisms are trapped by several layers of haemocytes and subsequent melanization of the nodule. A method of pathogen removal from circulating haemolymph of host by localisation in gills and sinuses in the hepatopancreas.

Encapsulation This process is carried out by the host when the parasite is too large to be engulfed by phagocytosis. Semi-granular cells are primarily involved in the process. Encapsulation is mainly directed against the fungal hyphae or helminth or nematode parasites.

Cytotoxicity The invertebrate blood cells can affect cytotoxic reactions resulting in the destruction of the foreign cells Mechanism and significance remain unknown The cytotoxic reactions are believed to be due to the direct contact of haemocytes with the invading cells

Cell adhesion Cell adhesion is essential in invertebrate immunity in the cellular immune responses of encapsulation and nodule formation. Blood cell adhesion to the infectious organism occurs when they move to and attach at a site of inflammation or infection.

Humoral components of crustacean immunity Lectins (agglutinins: proteins or glycoproteins) ProPO activating system Antimicrobial compounds Serine proteinase inhibitors Clotting reaction

Lectins (agglutinins: proteins or glycoproteins) potential molecules involved in immune recognition and microorganism phagocytosis through opsonization ProPO activating system PO is known to be associated with melanization, sclerotization and wound healing in crustaceans. PO oxidizes phenol into quinones, which will then polymerise non-enzymatically to melanin

Antimicrobial compounds The production of antimicrobial peptides represents a first-line host defence mechanism of innate immunity penaeidins, which were isolated from the shrimp Penaeus vannamei were successful in their role as antimicrobial peptides upon microbial challenge.

Serine proteinase inhibitors Arthropod haemolymph contains proteins with serine proteinase inhibitory activity. These inhibitors may exist in plasma or in haemocyte granules. protecting their hosts from infection by pathogens or parasites. Some may inhibit fungal or bacterial proteases

Clotting reaction Prime importance of innate immune process is 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.