Noncellular nonspecific defence mechanisms of fish

Abstract

Fish tissues and body fluids contain naturally occurring proteins or glycoproteins of non-immunoglobulin (Ig) nature that react with a diverse array of environmental antigens and may confer an undefined degree of natural immunity to fish. They consist of microbial growth inhibitory compounds that include “acute phase” proteins such as transferrins, caeruloplasmin, and metallothionein. Their action is simply to chelate metal ions and deprive bacteria and other parasites of essential inorganic ion sources. Both serum and cellular interferons are found in fish, and this anti-virus protein has been demonstrated mainly in salmonids during viral disease studies. Enzyme-inhibitors (α2-macroglobulin and other α-globulins) thus far detected in fish appear to be antibacterial proteinases, and are involved in the inhibition of extracellular proteases secreted by fish pathogens. Fish also possess a variety of relatively specific lytic molecules that cause cell lysis, and some of these materials are hydrolase enzymes (lysozyme, chitinase, chitobiase) whose main actions are against bacteria and fungi. In addition, mucus contains trypsin-like proteinases which destroy gram-negative bacteria. Nonspecific lysins and agglutinins against erythrocytes and other cellular antigens are found in serum, eggs, and skin mucus. The lysins, including toxins, some of which are bacteriolytic in activity, are, in their mode of action, natural or spontaneous, antibody-independent and noncomplement-mediated. In contrast, specific hemolytic antibodies (Ig), which complex with antigens, bind complement, and cause complement-mediated immune lysis, are reported to exist. The agglutinins are generally reactive toward certain sugar residues on erythrocyte or bacterial cell walls, and in most cases act as lectins or lectin-like molecules. Natural lysins and agglutinins behave in a similar way as antigen-induced antibodies or Igs, but exhibit a high degree of cross-reactions, due to the occurrence of similar carbohydrate determinants on many types of microbial cell surface. As with mammals, both C-type (calcium-dependent) and S-type (thiol-dependent) lectins are present in fish. They more resemble invertebrate lectins than those of higher animals. Fish lectins appear to play antibacterial or antifungal roles and in some instances seem to be involved in egg-sperm fusion, polyspermy prevention, and embryo development. Natural, non-Ig precipitins (e.g. α-precipitin and C-reactive proteins) are found largely, but not exclusively, in fish serum and precipitate with simple monosaccharides or long chain polysaccharides of certain stereochemistry and glycosidic linkages. Their functions remain unknown, but C-reactive protein is induced following stress-induction and exposure to inflammatory agents. Many of the above mentioned “defence” substances are present in skin mucus and possess the capacity to react with potentially infective microorganisms including parasites. Mucus thus acts as an immediate defence barrier to invasion and/or colonisation of pathogens.