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In the recent years, Indian aquaculture has gained tremendous importance because of its contribution to nutritional security, employment opportunity to millions of people, and as a source of foreign exchange. With the gradual decrease in marine catch, the importance of inland aquaculture is increasing with an impressive annual growth rate of over 6%. Out of 2.3mMT total freshwater aquaculture production, contribution from carp alone is 90 %.

To cater the need of growing demand of aquaculture products the practice of fish culture has been changed to intensive-culture system, where fishes are cultured in high density with the introduction of multiple species. In freshwater aquaculture, Indian major carps (rohu-L. rohita; catla-Catla catla; mrigal- C. mrigala) are cultivated into a commercial scale with the increasing trends of culturing catfishes too. Diseases of fish are one of the limiting condition in the success or failure of aquaculture, and in intensive- aquaculture system, diseases like epizootic ulcerative syndroms (EUS), bacterial gill disease, fin rot disease and white spot syndrome, etc. are one of the major threats in terms of economic loss (FAO, NACA-2003).

The health status of aquatic organisms is uniquely related to their environments, which contain very high concentration of microorganisms, many of which are pathogenic. However, under normal conditions fish maintain a healthy status by defending themselves against these potential harmful invaders using a repertoire of innate and specific defense mechanisms (Ellis, 2001). Fish does not have well developed immune system (Magnadottir, B. 2006). Therefore, the innate, natural or nonspecific defense mechanism is extremely important to protect themselves from several pathogens like bacteria, fungus, protozoa and viruses.  Further, the adaptive, acquired or specific immune system characterized by humoral immune response through the production of antibodies is defective in fish because, fish has only IgM.  Till date, only a few monovalent vaccines i.e. against enteric red mouth disease, furunculosis, vibriosis, pasteurellosis and yersiniosis are available (Evelyn, 1997; Larsen and Pedersen, 1997; Vinitnantharat et al., 1999; Gudding and Evesen, 2005). But none of them are efficient enough to protect fish from diseases for a longer period because of the inherent defect in adaptive immune response. In addition to this, molecular heterogeneity and rapid evolution of microbes lead to antigenic variations of pathogens (strain variation) and further enhance the severity of the disease in fish. These data altogether emphasize the role of innate immunity in protecting fish against diseases.

In India, many laboratories (including our institute, CIFA) are working to protect carps and catfishes against diseases. The majority of the objectives of those studies are to develop effective treatment / measures against epizootic ulcerative syndrome (EUS), aeromoniasis caused by A. hydrophila, edwardsiellosis by E. tarda and white spot syndrome in prawn caused by white spot sysndrome virus (WSSV).  However, no effective vaccine is yet available commercially. On the other hand, CIFA has commercially developed chemical reagents (CIFAX, CIFACURE), herbal formulation (AQUAHERBICURE) and immune enhancer (IMMUNOBOOST-C), to protect carps and prawn from diseases, but the mechanism of action of all these compounds remained unknown. It is predicted that these compounds function through the activation of innate/ nonspecific immune responses.

The innate immune system recognizes microorganisms through germline-encoded pattern- recognition receptors (PRRs) such as Toll- like receptors (TLRs) that signal hosts for the presence of infection and induce innate immunity.They are  non-clonal, expressed on all cells of a given type, and independent of immunologic memory. TLRs are expressed constitutively in the host and detect the pathogens regardless of their life-cycle stage. On the other hand, vaccines target only the lymphocytes to induce immunoglobulins (Ig), but failed to protect fish for a longer duration, because only IgM is induced.  Moreover, the success of vaccination depends on immunologic memory, which is lacking in fish. Further, TLRs recognize microbial components that are essential for the survival of the microorganism, and are therefore difficult for the microorganism to alter. These includes lipopolysaccharides (LPS), poly-unmethylated CpG dinucleotides, flagellin, double stranded RNA (dsRNA) and peptidylglycans which are called together as pathogen associated molecular patterns (PAMPs).

Considering these data together, where biological system of fish are deficient in adaptive immune responses, lack immunologic memory, and mostly depend on innate or nonspecific immune responses (Magnadottir, B. 2006),  the roll of TLRs are  expected to be very important. To understand the role of TLRs in innate immunity of fishes, we took three phylogenetically different types of fish, like scaled fish (carps), non scaled fish (catfish) and cartilaginous fish (shark), where no information is available on the presence of TLRs, their expression profiles in different tissues and also their nucleotide sequences. Once we obtain these data, we will be able to understand the evolutionary relationship among these fishes in respect to innate immune profiles contributed by TLR-network.
 
Success of intensive aquaculture with maximum productivity depends on healthy fish, and to maintain healthy stock boosting the innate immune response is immensely important.  In this regard, the importance of toll-like receptor (TLR) is clearly significant because, TLRs signaling activate innate immune responses. At present, there is dearth of information regarding the TLR and its contribution in Indian aquculture and no published report is available indicating the research on toll- like receptors (TLRs) of Indian fishes. Therefore, it’s a prime importance to undertake basic research on TLRs to understand its contribution to induce innate immunity in fish in order to sustain and enhance Indian aquaculture productivity in a predictable time frame