Stimulating ideas

Smith V, Brown J,
Hauton C (2003). Immunostimulation in crustaceans: does it really
protect against infection? Fish & Shellfish Immunology. 15, 71–90

This paper gives an extensive insight into the subject of
using immunostimulants to control disease. It focuses on the crustacean immune
system, how it works and how it is affected by immunostimulants to protect
against infection. After 15 years of research and compounds available
commercially as immunostimulants the authors felt that the time was right
for an appraisal of the current state-of-the-art of these substances for
crustaceans to assess how well they live up to their claimed benefits.

There is no evidence that crustaceans, or any other
protostome invertebrate, have clonally derived subsets of cells that permit
specific, adaptive and ‘memory-based’ immunity. Therefore the primary
administration of a non-virulent or attenuated antigen to provoke a protective
pool of recognition and effector cells or molecules in the event of secondary
exposure (vaccine) cannot work in these animals. Therefore immunostimulants are
an interesting way forward.

Immunostimulants receiving most attention comprise of
five main types: live bacteria; killed bacteria (bacterins or bacterial
antigen); glucans; peptidoglycans; and lipopolysaccharides (LPS). Usually these
are administered by immersion, as a dietary supplement or by injection and this
may be an important factor in determining the success of the treatment. Many of
the compounds have direct effects on various aspects of the crustacean immune
system. Glucans, LPS, bacteria and other non-self agents are known to invoke
various in vivo responses, such as change in haemocyte counts and induction of
encapsulation reactions. They also are known to induce prophenoloxidase
activation and melanisation reactions while in vitro glucans have been shown to
initiate cell degranulation and to enhance Phagocytosis. Research has further
demonstrated that phenoloxidase activation by glucans or other non-self
molecules generates a range of immunoactive agents and activities, including
peroxinectin and reactive oxygen species. However administration of immune
stimulants might, at best, be ineffective because of the natural regulatory
molecules within the immune system, or, at worse, be detrimental as has been
reported.

A large number of articles have been published which
report the beneficial effects of immunostimulatory substances. However the
authors of this paper have rigorously inspected these and found that, in a
number of instances, the presented data do not support the conclusions reached.
In several cases poor experimental design and the absence of any statistical
analysis limits the validity of some conclusions. Also, acknowledged by many
authors, is the poor reproducibility of the response for individuals of the
same species within the same treatment group. The inconsistent nature of any
response exhibited by individuals of a single species will severely impede the
efficacy of any marketed stimulant as well as the definition of appropriate
dosing regimes and any ‘booster’ treatments. The problems with the literature
pointed out by the authors did not seem to me that problematic, just that the
authors of the studies had exaggerated results; as in the case of most
scientific literature these days in order to get published.

In conclusion there is a reasonable degree of doubt about the
efficiency of stimulants due to the lack of unequivocal evidence and convincing
reasons why the use of such compounds over prolonged periods might be
detrimental to the host. There is an urgent need for proper large-scale
evaluation of the efficacy of putative immunostimulants through standardized
trials where analyses should include assessment of their ability to up-regulate
the expression of genes encoding defense proteins, as such studies are
necessary to elucidate the mode of action of any promising compounds and
thereby help to refine their use. Findings must be related to functional
end-point assays. Multi-gene analyses, for example, by micro-array technology
are desirable. The optimal dose, route and treatment time for each stimulant at
all appropriate life stages of the target species still need to be found. These
analyses will reveal if treatment with immunostimulants is appropriate for crustaceans,
and for how long protection might reasonably be expected to last.