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The Shrimp Immune System

Thursday, January 19th, 2012

The immune system of crustaceans has become and area of increasing interest for the aquaculture community.  It has been long known that crustaceans, and also arthropods in general, may often harbor pathogenic infections without displaying overt signs of disease.  This fact has been partly responsible for the widespread increase in the adoption of advanced pathogen detection in broodstock in addition to the rapid application of SPF (specific pathogen free) stocks where feasible.  These and related technologies have allowed some level of control of disease in shrimp aquaculture and has undoubtedly resulted in a reduction of catastrophic losses and regional pandemics.  However, the fact remains that even these populations of cultured shrimp are still susceptible to disease.  Facilities near wild populations, that have open circulation systems, or are in close geographic proximity to other shrimp farming facilities have increased risk of disease.

Closed, recirculating, or in-land facilities are often considered the next best method to control the introduction of pathogens; however, when/if an outbreak occurs it is often more catestrophic as the experience and technology to deal with the disease and a lack of natural resistance of the shrimp stocks results in a highly susceptible environment.  Here in, enters the current research into the shrimp immune system.  By studying how the immune system functions in arthropods, or shrimp specifically, molecular interventions may be engineered to block or alter the transmission or virulence of viruses and other pathogens.

A central feature of innate immunity in a wide range of animal species is the Toll-receptor family.  These receptors detect conserved structures found in a wide range of viral and bacterial pathogens.  Once the receptors detect a hint of invading pathogens further immune responses follow.  These receptors were long suspected to be present in shrimp as well and a 2007 report confirmed the existence of Toll-receptors in shrimp, ultimately culminating with the cloning of LvToll1 [1].   More recently two additional receptors, LvToll2 and LvToll3, have been discovered as outlined Developmental & Comparative Immunology [2].  In this paper the authors observed the levels of the various Toll receptors responded distinctly different to different pathogenic agents.  This is not entirely unexpected as other organisms show a similar “combinatorial code” in response to pathogens.

A case in point to help demonstrate the utility of this research is how viruses display tissue specific tropism (simply, which tissues viruses preferentially target).  You need look no further than your own experiences with the common cold and flu viruses.  These viruses target various tissues (mucosal membranes, digestive system, etc), while leaving others (skin, muscles, etc) generally untouched.  Viruses often require specific sets of proteins to be present on a cell to allow for successful invasion.  This selection criteria for tropism is partly what allows viruses to adapt increased virulence properties and efficient transmission strategies.

Just as we see tropism with the cold virus in humans, studies of IMNV in L. vannamei have identified a similar phenomenon.  IMNV is found by quantitative measure to be enriched in the muscle and hemolymph, while being reduced in more peripheral tissues [3].  In light of the Toll receptor research it is clear that some components of the shrimp innate immune system display varying levels of expression in different tissues.  Furthermore, it may be possible that the “natural” pattern of toll gene expression or other innate immune system genes is not sufficient to provide an effective defense against the virus.  In a sense IMNV might have found a “hole” and is exploiting it.  Therefore, in the specific case of IMNV, modulating the expression of some of the toll pathway genes in the muscle may confer additional resistance to IMNV.

By understanding how these receptors respond to infections it could be possible to identify these “holes” that pathogens exploit.  Just like a software patch fixes a vulnerability in a computer operating system, it may be possible to “patch” these organisms to help their immune system more completely fend off viruses and bacteria.  One of TransGenada’s main charges is to identify these exploits and upgrade our shrimp for the betterment of the industry.

For those interested in additional information the following link has more details on the basics of the Toll mediated innate immunity [4].