Fish Immunology (1+1)

Unit 13 - Defence mechanisms in crustaceans

13.3. Cellular immune components
Encapsulation
When the pathogen like parasites which are too big to engulf by phagocytes invade the crustaceans, the haemocytes present in haemolymph collaborate and deposit on the surface of the parasite and block off the parasite from circulation. Normally the cells involved in the encapsulation is semigranular cells.
Cytotoxicity
Haemocytes are capable of destroying tumor and non-tumor cells present in the host by secreting cytotoxic molecules. Cells that are mainly responsible for cytotoxicity is Granular and semigranular cells which are similar to mammalian natural killer cells (NK cells). Once these cytotoxic cells recognize the foreign particle or tissue, they attach to the foreign particle or tissue and release the cytotoxic chemicals, that kills the target.
Clotting reaction
Clotting reaction is the mechanism of formation of extracellular and intravascular clots for rapid sealing of wounds. Clotting prevents the loss of haemolymph and invading pathogens. Intravascular clots produce stasis by adhering to the walls of blood vessels and extracellular clots
Inhibit movement of pathogens and make them vulnerable to phagocytosis.
Two different coagulation mechanisms

• Haemocyte derived clotting cascade
• Transglutaminase dependant clotting reaction

Haemocyte derived clotting cascade
Haemocyte derived clotting cascade can be seen in horseshoe crab. This mechanism is activated by lipopolysaccharide (LPS) of bacteria leading to the cascade of reaction where the activated clotting enzyme catalyzes the change of a soluble protein (coagulogen) into an insoluble clot (coagulin). That seals the wound and entraps parasites or pathogens that are trying to invade the host.
Transglutaminase dependant clotting reaction
Transglutaminase dependant clotting reaction can be seen in crustaceans like shrimp, lobster, crab, cray fish etc. Here clottable protein (CP) play a major role. TGase is an enzyme released from the haemocyte during the invasion of pathogen or injury. TGase after release activates by calcium ions (Ca2+) then catalyze the linking of CP into large insoluble aggregates. These insoluble aggregates are responsible for clotting process.
Prophenoloxidase (proPO) system
The prophenoloxidase (proPO) system is an efficient innate immune response against foreign materials in the haemolymph of invertebrates. This system is involved in proPO mediated melanization process, in response to infection, invasion or wound healing.
It is initiated by the recognition of lipopolysaccharides and peptidoglycans from bacterial cell wall and β-1, 3 glucans from fungal cell wall, even when they are in minute amounts. This suggests that the system will be activated in the presence of pathogens. proPO system contains pattern-recognition proteins (PRPs), several zymogenic proteinases and prophenoloxidase (proPO) enzyme.
proPO is synthesized in the hemocytes and are localized in the granules of the hemocytes. When β-1, 3 binding protein (β-1, 3 BP) binds to β-1, 3 glucans, it becomes activated and binds specifically to a cell surface associated protein, a superoxide dismutase (SOD). The recognition of non-self materials triggers degranulation of semigranular and granular cells. Prophenoloxidase activating enzymes (proppA) is released during the process which is further activated to ppA by the pathogen associated molecular patterns (PAMPs). Active ppA converts the proPO to phenoloxidase. PO is a bifunctional copper containing enzyme, which is known as a tyrosinase and catalyzes the early steps in the pathway to melanin formation. Melanin is a pigment which is black in color and helps the host to neutralize the pathogen.