Download THE IMMUNE SYSTEM OF SHRIMP Introduction Penaeid

THE IMMUNE SYSTEM OF SHRIMP
By : Franklin S. Martínez ([email protected])
Nicovita-ALICORP SAA Technical Service
Introduction
Penaeid shrimp aquaculture is an important economic activity worldwide. Nevertheless,
shrimp production has been seriously affected by diseases, mostly those caused by viruses
(Flegel, 2006) and Vibrio bacteria (Bachère, 2000). Shrimp resistance to invading
organisms is strongly influenced by its immune status. The defense mechanisms of
crustaceans is less developed than that in finfish and other vertebrates (Figure 1). More
specifically, crustaceans have no adaptive memory. In other words, they do not have the
ability of producing immunoglobulins, so that they apparently depend only on innate
defense systems (Roch, 1999). Understanding shrimp defense mechanisms in combination
with different strategies can contribute to improve disease management. In fact, several
different methods exist to learn about the health/disease status of these animals, including
the evaluation of immune/physiological parameters such as hemolymph protein
concentration, total hemocyte counts, phenoloxidase activity, free radical production,
phagocytic activity, and field variables including stress tests, survival, and growth rates,
which can be used as shrimp health status indicators (Rodríguez y Le Moullac, 2000). The
purpose of this bulletin is to review some major defense mechanisms of shrimp.
A
B
Vertebrates
Adaptive
Memory
Invertebrates
Non specific
Non specific
Figure 1. The immune system of (A) vertebrates, and (B) invertebrates
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Shrimp defense mechanisms
The innate defense system – also known as natural or non-specific defense system –
includes both cellular and humoral components (Figure 2) which work in jointly
coordination for the detection/elimination of all foreign organisms potentially hazardous for
the host (Jiravanichpaisal et al., 2006). Cellular defense components include all those
reactions performed directly by hemocytes (phagocytosis, encapsulation, nodule
formation). On the other hand, humoral components include the activation and release of
molecules stored within hemocytes, such as anticoagulant proteins, agglutinins,
phenoloxidase enzyme, antimicrobial peptides, protease inhibitors, etc. (Jiravanichpaisal et
al., 2006; Holmblad and Söderhäll, 1999.)
Function of crustacean immune system
The cuticle works as the first physical barrier, and it contains antimicrobial substances
(Söderhäll and Cerenius, 1992). Once the pathogen has crossed the outer defense barriers,
hemocytes play an important role in the crustacean immune response. In addition to
participating in the inactivation of invading organisms, hemocytes are also involved in the
regulation of different physiological functions i.e., exoskeleton hardening, cuticle damage
healing, coagulation, carbohydrate metabolism, and protein/amino acid transportation and
storage (Jiravanichpaisal et al., 2006.)
Innate Immunity
(Natural or Non-specific)
Cellular
Components
- Phagocytosis
- Encapsulation
- Formation of nodules
Humoral
Components
- Anticoagulant proteins
- Agglutinins
- Phenoloxidase enzyme
- Antimicrobial peptides
- Free radicals
Figure 2. Cellular and humoral components of crustacean immune system
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Hemocyte classification is based on the presence and size of 3 types of cytoplasmic
granules: hyaline, semi-granular, and granular hemocytes (Figure 3). Even though the
proportion and function of hemocytes can vary among species, it is generally considered
that granular and semi-granular hemocytes have the ability of producing melanin by the
pro-phenoloxidase system (Johansson y Söderhäll, 1989). On the other hand, hyaline
hemocytes and – in a lesser extent – semi-granular hemocytes are responsible for the
phagocytosis process (Giulianini et al., 2007.)
Pathogen recognition
The first step in the immune process is the recognition of microorganisms. This process is
carried out by hemocytes through molecules that have the ability of recognizing structures
in the cell walls of invading organisms, such as attachment proteins, and by the recognition
of β-1,3-glucans, lipopolysaccharides, and peptidoglycans (Lin et al., 2006; Vargas-Albores
and Yepiz-Plascencia, 2000). Once invading organisms are detected, hemocytes get
activated then a whole series of mechanisms is triggered to control or remove the intruders.
Phenoloxidase activity
The phenoloxidase system has been recognized as an efficient defense mechanism against
the non-self. This system is stored and produced by semi-granular and granular hemocytes,
and it can be activated by a minimum presence of microbes. Activation of the prophenoloxidase system results in the production of melanin, a dark-brown pigment
responsible – among other processes – for inactivating foreign particles, and preventing
their spread throughout the host body, as well as for healing cuticle damages
(Sritunyalucksana and Söderhäll, 2000).
A
B
12.4 × 7.8
C
14.8 × 8.3
13.6 × 9.5
Figure 3. Classification of hemocytes: (A) Hyaline, (B) Semi-granular, (C) Granular
(Modified photos from Giulianini et al., 2007)
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Free radicals and antioxidant mechanisms
Destroying the phagocytized materials involves the intracellular production of free radicals.
During contact with and recognition of the pathogen, host enzymes like NADPH-oxidase
are activated, which in turn increase oxygen consumption, resulting in the production of
free radicals such as superoxide anions (O2-) and hydrogen peroxide (H2O2), among others
(Muñoz et al., 2000; Rodríguez and Le Moullac, 2000). These free radicals can directly kill
the invading organism or work in combination with nitrogen compounds (nitric oxide), or
exert a synergistic effect with lysozymes (Roch, 1999).
Nevertheless, free radicals do not discriminate host-self cells from microbes, so that they
can be deleterious in the event of acting in the extra-cellular spaces. Under normal
conditions, the potential damage caused by free radicals is regulated by mechanisms such
as antioxidant molecules i.e., ascorbic acid, poly-unsaturated fatty acids, and antioxidant
enzymes (superoxide dismutase, various peroxidases) (Dandapat et al., 2003; CampaCórdova et al., 2002.)
Phagocytosis, encapsulation and nodule formation
Phagocytosis is the most common reaction of defense cell mechanisms. By this process,
cells (hemocytes) ingest and destroy invading pathogens, foreign particles or modified
(aged) cells of the body itself (Secombes, 1996.)
Encapsulation and nodule formation (Figure 4) are processes by which several hemocytes
cooperate with each other aiming to stop the action of invading organisms, when the host is
attacked by either extremely-large particles or numerous tiny particles, to be ingested then
destroyed by individual cells (Söderhäll y Cerenius, 1992).
A
B
C
Figure 4. Defense cell processes include: (A) Phagocytosis, (B) Encapsulation, and (C)
Nodule formation. Hemocytes are represented in green, while the invading
organisms appear in red
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References
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Generation of superoxide anion and SOD activity in haemocytes and muscle of
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Dandapat, J., G. B. N. Chainy and K. J. Rao. 2003. Lipid peroxidation and antioxidant
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Flegel, T. W. 2006. Detection of major penaeid shrimp viruses in Asia, a historical
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Tumpis Edition
Editors: Dagoberto Sánchez [email protected] Carlos Ching [email protected]
Máximo Quispe [email protected]
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