Innate or Non-specific Immunity - The fish immune system

Innate or Non-specific Immunity 

Innate or non-specific immunity refers to various physical and cellular at-tributes that collectively represent the fish first lines of defense against infectious disease. These defense mechanisms come into play immediately or within hours of an antigen’s or an invaders appearance in the body. The non-specific immune resistance includes the following levels:

Physical Barriers

The skin, including the scales in some fishes, and the mucous membranes of the digestive tract are physical barriers that protect the animal from harmful environmental agents and from invasion by microbes. Most of the fluids that are excreted or secreted onto epithelial surfaces, such as mucus and digestive juices, contain chemical components that are anti-microbial. Mucus is secreted by specialized goblet cells in the epidermis of fish and contains immunoglobu-lins (IgM). Mucus also contains precipitins, natural agglutinins, lysins, lysozyme, C-reactive protein and complement. However, the skin and other epithelial surfaces of fishes harbor a variety of microorganisms whose presence and numbers pose no threat to their health.

Soluble (Humoral) Barriers

The internal fluids (humors) of fish contain a number of substances that react with a variety of microbes to lyse, or coat them or to inhibit their growth. Among these substances are inhibitors of microbial growth (transferrin, lactoferrin, ceruloplasmin, metallothionein, cecropins, defensins, magainins), cell lytic enzymes (lysins, lysozyme, proteases), enzyme inhibitors, agglutinins and precipitins and interferons).

1. Acute Phase Proteins and Complement Factors

The concentration of a number of protein types in the blood serum in-creases rapidly during an infection. These proteins are called acute phaseproteins. One representative of the acute phase proteins is the C-reactive protein characterized by its ability to bind to the surface molecules of thecell wall of a wide variety of bacteria and fungi. When the C-reactive pro-tein binds to the surface of bacteria, another group of proteins present in the blood, called complement factors, binds firmly with the immobilized bacteria and the whole complex of bacteria, C-reactive protein, and comple-ment becomes more rapidly engulfed by phagocytes. This process of facili-tating phagocytosis of bacteria by coating them with protein is called opsonization. In fish, C-reactive proteins are naturally present at a level thatis 500 times higher than in mammals, a possible indication of the relative importance of the nonspecific resistance mechanism in fish.

The complement factors of fish blood are also proteins. One of these pro-teins will react spontaneously with surface components of bacteria, notablythe lipopolysaccharide (LPS) found on bacterial cell walls. The same complement factor also reacts with b-1,3 glucans which are structural com-ponents of bacterial and fungal cell walls. This reaction is called an activa-tion because it elicits a sequence of reactions where one complement factor activates the next in a chain reaction that produces protein fragments with different properties. After activation, some of the complement factors are able to cause opsonization of bacteria, whereas others attain the ability to attract phagocytes.

The activation of the complement system by external factors, such as LPS and b1,3 glucans is called the alternative pathway of activation. The com-plexes formed when antibodies react with antigens can also activate the complement system. This is called the classical pathway of complement ac-tivation.

The different proteins in the complement system must act in combination in order to exert an antimicrobial effect. Moreover, lysozyme produced by ph-agocytic cells acts synergistically with the complement factors by its hydro-lytic effect on the bacterial cell wall. Measurement of the level of comple-ment factors and of lysozyme in the serum, and measurement of the phago-cytic and bactericidal activity of phagocytes that operate in concert with these factors, provide ways to quantify the degree of non-specific resistance of an organism.

The C-reactive proteins, complement, and phagocytic cells constitute the most important elements of the non-specific immune system. Because this system exists with its same basic elements in all levels of the evolutionary system from marine invertebrates to warm-blooded animals, it is likely that it has its origin far back in the evolutionary process.

2.  Transferrins and Lactoferrin

Transferrin is a serum protein of 70 to 80 kDa molecular weight belonging to the group of acute phase proteins and present both in mammals and fish. Transferrin plays a possible role in delaying the start of microbial infections by binding the available iron and thereby depriving bacteria of an essential growth factor. The mucus secretions of mammals contain another closely related iron binding and antimicrobial protein called lactoferrin, but this has not yet been found in fish; lactoferrin does, however, enhance the dis-ease resistance of trout when given in the feed.

3.  Caeruloplasmins

This is an acute phase protein present in fish that binds copper and other divalent metal ions. Caeruloplasmin acts as an oxidase that oxidizes diva-lent iron to ferric ions, which then bind to transferrin. By this reaction, ceruloplasmin contributes to depriving bacteria of available iron and diva-lent ions.

4.  Metallothionein

Metallothionein is a peptide rich in cysteine and with a high affinity for metal ions. Metallothionein has been found in several fish species. Its pro-duction is stimulated by metals in the environment and by endotoxins (LPS) from bacteria. It also deprives bacteria of essential metallic ions by sequestering them.

5.  Enzyme Inhibitors

Protease inhibitors that correspond to the mammalian a-2-macroglobulin are present in fish and are believed to play a role in retarding the invasion of pathogens or parasites. The other common groups of protease inhibitors are present in fish serum (inhibitors of serine proteinases, cysteine proteinases, aspastic proteases, metallo proteinases) and some of these belong to the acute phase reactants. Virulent pathogenic microorganisms secrete pro- teolytic enzymes to digest and penetrate the tissues of their host. It is be-lieved that the protease inhibitors of fish serum play a role in defense by neutralizing the proteolytic enzymes produced by these pathogens.

7. Cell Lytic Enzymes

Lysozyme is a bacteriolytic enzyme occurring both in plants and animals. There are large variations in lysozyme activity in different fish species. Lysozyme is present in most tissues and secretions of fish, and the level seems to vary in relation to environmental conditions. Stress due to han-dling of fish and to pollutants reduces lysozyme activity in fish. Because lysozyme is produced in macrophages, the reduced level may be a reflection of a reduction of the macrophage function

Fish serum contains molecules other than lysozyme that cause lysis of bac-teria and probably have a function in the non-specific defense to infections. Fish mucus also contains a trypsin-like protease activity that is able to cause lysis of Gram-negative bacteria. This enzyme is produced in the mu-cus-secreting cells of fish and may function in concert with lysozyme and hemagglutinins in non-specific defense.

8. Agglutinins

Fish serum, skin, and mucus contain factors that resemble immunoglobu-lins or antibodies in their ability to agglutinate and cause lysis of foreign cells and bacteria. These agglutinins are non-specific and they correspond to lectins of invertebrates in their ability to recognize and bind to single sugars on the bacterial surface and mediate phagocytic reactions

Cellular Barriers of the Innate Immune System

1. Inflammation

Inflammation is the observable condition that accompanies damage to the body. It is a localized response to tissue injury and to invading microorgan-isms, characterized by infiltration by granulocytes and macrophages, re-moval of dead cells and foreign cell and debris, followed by tissue repair. Inflammation confers protection by walling off an infected area from the rest of the body. This type of response in fish has been reported against bacterial, viral, fungal, protozoal and parasitic infections. In the higher ver-tebrates especially in mammals, inflammation involves mast cell degranula-tion and the release of vasoactive substances. These cause vasodilation, which increase blood flow and vascular permeability, and adhesiveness of vascular endothelial cells for phagocytic blood cells.

2. Natural Cytotoxic Cells

Some population of cells in fish display a non-induced and non-specific toxicity to foreign cells. The non-specific cytotoxic cells in fish are equiva-lent to the natural killer cells of mammals. They differ from their mamma-lian counterparts by being able to destroy a wider range of foreign cells, and they can even destroy multicellular parasites that attack fish.

3.Phagocytosis in Fish

In fish, macrophages, monocytes and granulocytes are phagocytic and in some species neutrophils are also phagocytic. Intracellular killing by teleost macrophages is similar but slower than found in mammals. These killing activities include lysosomal enzymes, alkaline and acid phosphatases and peroxidases. Activated macrophages produce oxygen metabolites such as super oxide anion in a process known as the respiratory burst. These oxy-gen radicals are bactericidal.