Shrimp Immune System – Special Focus on Penaeidin

In this review paper, we summarize the immune system of penaeid shrimp. This review includes the clotting system comprising clot protein and clotting enzyme (transglutaminase II), as well as the prophenoloxidase (proPO) activation system including proPO-activating enzyme (PPAE), proPO and clip domain serine protease homolog (c-SPH). In addition, several shrimp cytokines or cytokine-like molecules found in tiger shrimp Penaeus monodon participating in the defense responses are also discussed. C-SPH could enhance hemocyte adhesion. Shrimp astakine promoted hemocyte proliferation in the hematopoietic tissues and could be down-regulated at the post-transcriptional level by the binding of intracellular molecules such as transglutaminase I and crustin Pm4 to its 3’-untranslatd region. Penaeidin is found to be a dual function molecule as an antimicrobial peptide and an autocrine-acting cytokine. Yet, other antimicrobial substances such as quinone and melanine which are generated during the proPO activation pathway and the reactive oxygen and nitrogen intermediates (ROI and RNI) which are generated during the phagocytic process can also kill and clear the invading microorganisms directly. In addition, Dscam (Down syndrome cell adhesion molecule), a member of the immunoglobulin superfamily (IgSF) which plays an essential role in the alternative adaptive immune system of invertebrates, has also been characterized in shrimp. Penaeid shrimp is one of the most important commercially available aquaculture resources. Although, the culture techniques and field management skills have significantly improved in the recent years, infectious diseases are still a threat to the industry. The study of shrimp immune system could be important in designing the strategies against pathogen infection. For shrimp, the innate immune responses play a major role in combatting invading pathogens and prevent them against diseases. The immune responses involve different factors such as physical barrier, bacteria clearance, encapsulation, clotting reactions, prophenoloxidase system, reactive oxygen intermediates, and antimicrobial activity. The innate immune system uses a variety of pattern recognition receptors (PRR) on the hemocyte membrane to detect the pathogenassociated molecule pattern (PAMP) and transduce signals during pathogen invasion. Hemocytes are then activated and the defense molecules are degranulated against the pathogens. I. THE CLOTTING SYSTEM Coagulation (clot formation) forms a physical barrier to prevent the loss of body fluid and the dissemination of microbes into the haemocoel after an injury or infection. The rapidity and efficiency of coagulation are essential for the survival of invertebrates that rely solely on innate immunity. Transglutaminase (TG) catalyses intermolecular or intramolecular 3-(g-glutamyl) lysine bond formation, resulting in protein polymerization, and plays a role in blood coagulation and post-translational protein remodeling. In shrimp, coagulation is initiated by the activation and lysis of hyaline cells, which release hemocyte components to react with plasma factors [45]. One of the hemocyte components is believed to be a transglutaminase. The plasma factor which has been purified is known as clottable protein (CP) in P. monodon [66]. P. monodon CP resembles the vitellogenin family clotting protein in crayfish [28]. Also, a P. monodon transglutaminase (designated STGI) was cloned from the hemocytes, but no clotting activity was detected. STGI is ubiquitously expressed with a low mRNA level in circulating hemocytes, but a high level in mitotic cells of hematopoietic tissue was shown in an in situ hybridization assay [33]. RT-PCR experiment also showed that STGI transcript decreased when hemocytes matured and were released into the hemolymph. Taken together, these results suggest that transcription and translation of STGI in the hemocytes possibly take place in early developmental stages but are down-regulated in later developmental stages [67]. Chen et al. [15] cloned a TG from shrimp hemocyte cDNA, which has been designated as P. monodon transglutaminase II (STGII). RT-PCR results showed a significant level of Paper submitted 01/31/13; revised 07/31/13; accepted 08/13/13. Author for correspondence: Yen-Ling Song (e-mail: [email protected]). 1 Institute of Zoology, National Taiwan University, Taipei, Taiwan, R.O.C. 2 Department of Life Science, National Taiwan University, Taipei, Taiwan,