The Role of Lectins in Finfish: A Review

The immune system of vertebrates involves both innate and acquired immune responses. The innate immunity is more generalized with robust response whereas the other has a highly specific response to infectious pathogens. Because of the lack of specialized lymphatic organs, innate immunity is an important mode of defense in fishes. The less specific innate immune system acts mainly through complement pathway which depends on pattern-based recognition of self and non-self targets by host lectins and associated proteins. This ultimately results in the clearance of target cells. Lectins are glycoproteins which possess at least one carbohydrate recognition domain (CRD) that specifically and reversibly binds to a carbohydrate which is widely distributed in bacteria, fungi and viruses, and thus activates the innate immune system. Lectins are involved in many biological functions including cell adhesion, phagocytosis, complement activation and innate immunity. Fish lectins have also an important role in fertilization, embryogenesis and morphogenesis. Based on the structure, binding specificities and calcium dependency, lectins are classified into different families like C-type lectins, F-type lectins, galectins, intelectins, rhamnose binding lectins, I-type lectins, Lily-type lectins etc. Lectins such as ficolins, galectins, calnexin, pentraxin, F-type lectins, intelectins, mannose-binding proteins (MBPs) are known to play an important role in innate immunity and disease resistance. The skin forms an important immune structure in fish and the skin mucus is reported to have lectins, as well as stomach, intestine, liver, gills, eggs, skin, serum, and plasma of different families of fish. In addition to their important role in cellular recognition as pattern recognition receptors, the interaction of lectins with carbohydrates has been explored in various fields of research where saccharide specificity is essential. These proteins are invaluable tools for the study of simple or complex carbohydrates, in solution or on cell surface, as well as for cell characterization. The action of the innate immune system in recognition of a pathogen is initiated by pathogen recognition receptors (PRRs). This can detect and respond to the pathogen-associated molecular patterns (PAMPs). PRRs also detect death-associated molecular patterns (DAMPs) which are endogenous molecules released by damaged or stressed cells. Lectins belong to the humoral component of innate immune system and are involved in recognition of PAMPs effecting in agglutination and neutralization of potential microbial pathogens or activation of complement components. Taken together, various study reports suggest that fin fish lectins plays a major role in immune recognition of microbial pathogens and facilitate their clearance by phagocytosis. The present review discusses the occurrence and biological role of lectins in finfish.