Showing papers in "Fish Physiology in 1996"

2 - The Nonspecific Immune System: Cellular Defenses

Abstract: This chapter describes the morphology and mechanism of cellular defenses in fish. A variety of leukocyte types is involved in nonspecific cellular defenses of fish and includes monocytes/ macrophages, granulocytes, and nonspecific cytotoxic cells (NCCs). Macrophages and granulocytes are mobile phagocytic cells found in the blood and secondary lymphoid tissues and are particularly important in inflammation, which is the cellular response to microbial invasion and/or tissue injury leading to the local accumulation of leukocytes and fluid. Isolated granulocytes are highly mobile and phagocytic and produce reactive oxygen species, but their bactericidal activity is often relatively poor compared with macrophages. A variety of approaches have been used to enrich for NCCs, including dissociation from tumor target cells, density gradient centrifugation, and flow cytometry. The development of inflammatory reactions is controlled by a number of mediators, including cytokines, eicosanoids, complement factors, and other vasoactive compounds released from phagocytes and thrombocytes.

3 – The Nonspecific Immune System: Humoral Defense

Abstract: This chapter examines humoral defense factors by focusing on lysozyme, complement, interferon, C-reactive protein, transferrin, and lectin. Lysozyme is found in a wide range of vertebrates and is one of the defensive factors against an invasion by microorganisms. It splits the β linkages between N -acetylmuramic acid and N -acetylglucosamine in the cell walls of Gram-positive bacteria, thus, preventing them from invading. In fishes, lysozyme is distributed mainly in tissues rich in leucocytes, such as the head kidney, at sites where the risk of bacterial invasion is high, such as the skin, the gills, and the alimentary tract, and in the eggs. It has been demonstrated that Pacific hagfish granulocytes show chemotactic migration in response to the human C5a and lipopolysaccharide (LPS)-activated hagfish plasma. This indicates that specific chemoattractant receptors are present on the surface of hagfish leukocytes and LPS activation of hagfish plasma generates a potent chemotactic product. It has been observed that the catfish alternative complement pathway (ACP) is an efficient defense mechanism against nonpathogenic Gram-negative bacteria that contain no sialic acid, but the catfish ACP is inhibited by the large amount of sialic acid contained in pathogenic Gram-negative bacteria, such as Aeromonas salmonicida and Flavobacterium columnaris , indicating that sialic acid is an important virulence factor for establishing an initial infection.

1 - Cells and Tissues of the Immune System of Fish

Abstract: This chapter focuses on the cell and tissues of the immune system of fish. In teleosts, three types of granulocytes—heterophils, acidophils, and basophils—have been reported. There is enormous variation within the teleosts in both relative abundance and staining reaction of the granulocytes. In the carp, Cyprinus carpio, all three types of granulocytes are found in the blood. Among them, the heterophils and basophils are the least numerous and constitute 1% of the total eukocyte count. The acidophilic granulocytes are rather abundant and constitute 8% of the total leukocyte count. The acidophilic granules are peroxidase positive and contain round or irregular granules with heterogeneous contents. Migrating leukocytes in the basal mucosa and submucosa, as well as in the epithelium but not lymphoid aggregates, have been detected in the hagfish intestine. In the stingray, Dasyatis akajei , meningeal lymphohemopoietic masses appear predominantly in the telencephalon, diencephalon, and mesencephalon. Antigen-binding cells and antibody-producing cells have been found in the lymphohemopoietic tissue of the teleost kidney Recently, a higher number of Ig-positive cells have been found in the head kidney than in the spleen. They occur as scattered cells or forming small pyroninophilic cell clusters, especially after antigenic stimulation. It has been observed that that the cells of the pyroninophilic clusters corresponded to the plasma cell lineage with Ig-producing capacity and to the large lymphoid type plasma cells that gradually differentiate in these cell clusters.

8 - Immunomodulation: Endogenous Factors

Abstract: This chapter examines the different aspects of immunomodulation in the fish. The general health of fish is a function of their environment, the nature of pathogens, and factors intrinsic to the fish themselves. Circulating levels of leukocytes change through time when fish are stressed. Changes following physical types of stressors include depression in the number of lymphocytes in the circulation, or the number of these cells relative to the number of erythrocytes. Negative effects of stress on disease resistance appear to be because of the depression of antibody-synthesizing capability through effects on lymphocytes. Stress effects on the immune system of fish may be mediated by the endocrine system. Catecholamines and corticosteroids are the major stress hormones of vertebrates. Some species of teleosts do not fit the general paradigm for the endocrine stress response, the extremely rapid secretion of catecholamines and rapid secretion of cortisol following the onset of stress. The mechanism of buffering in rainbow trout can be attributed to the elevation in the concentration of corticosteroid-binding globulin as spawning approaches. The globulin reduces the amount of free cortisol that could pass into the ovarian fluid and, subsequently, the egg.

4 - The Specific Immune System: Cellular Defenses

Abstract: This chapter examines the different aspects of the specific immune system in fish. Specific immune responses that are independent of antibody are collectively termed as “cell-mediated immunity.” A number of thymectomy experiments have been performed to determine whether the fish thymus is a source of helper T cells and/or suppressor cells. The outcome of the removal of the thymus in fish appears to depend on the completeness of the operation, the stage of the development of the fish, the interval between thymectomy and immunization, and many other factors, including the type and dose of antigen used. The results are compatible with the premise that the thymus is a source of T cells. The requirement for accessory cells for the activation of T lymphocytes by mitogens has been confirmed for channel catfish peripheral blood lymphocytes. Interleukin-2 (IL-2) was formerly known as “T-cell growth factor” in recognition of its ability to induce the proliferation of T-cells. In mammals, IL-2 is produced primarily by helper (CD4 +ve T) cells, although other subsets of T cells may also be involved. The production of factors with IL-2-like activity in a mixed leukocyte reaction of fish has been demonstrated for carp with peripheral blood and pronephric leukocytes.

6 – Natural Changes in the Immune System of Fish

Abstract: This chapter discusses the natural changes in the immune system of the fish. Natural factors that have an effect on the immune system of the fish include the physiological processes of ontogenetic development and, on the other end of the life cycle, a progressive decline toward senescence and death. A complete histological description of the ontogenetic development of the lymphoid organs—the thymus, kidney, spleen—and gut-associated lymphoid tissue has been performed for relatively few fish species. In trouts, the thymus remains in a superficial position throughout development and throughout life and is separated from the opercular cavity by only a single layer of cells. The ontogenetic development of T cells has been studied in the carp using mouse anticarp thymocyte monoclonal antibodies. It has been found that fry as early as 4 days posthatch had an efficient phagocytic system, the carbon being engulfed by free-wandering macrophages that accumulated under the skin and in the connective tissue, gut, and gills. In the marine teleost, Sebastiscus rnarmoratus , fish as young as 1.5 months were capable of eliciting an allograft response in the same manner as adult fish, as assessed by eye transplantation.

5 - The Specific Immune System: Humoral Defense

Abstract: This chapter discusses the mechanism of humoral defense in the fish. The mechanisms involved in specific humoral defense have been the most thoroughly explored of all the modes of piscine disease resistance. Piscine classes, such as elasmobranchii and osteichthyes, possess extremely wide and varied means of accomplishing physiological goals, including the development of prophylactic immune responses. Initial characterizations of fish antibody structure led to the general supposition that fish Ig was rather comparable to mammalian IgM. C3b as an opsonin can be recognized by a C3b receptor on phagocytic cells. The existence of a possible receptor has been demonstrated in the catfish. This was accomplished by using a yeast cell wall material, zymosan, to stabilize C3b. The first line of humoral defense, where pathogens can be most effectively blocked or neutralized, is the mucosal surface. It is found that when arrayed with an opsonin, such as C3b, phagocytic cells can recognize and digest the pathogen. It is observed that isotype switching and affinity maturation are two other common features of mammalian immunological memory that are primarily associated with the monomeric form of immunoglobulin.

7 – Environmental Factors in Fish Health: Immunological Aspects

Abstract: This chapter describes the immunological aspects of fish health. Fish live closely with their aquatic environment and living epidermal cell membranes have direct contact with all materials carried by water. Wild fish are exposed to many chemicals in their environment that may possess immunomodulatory potential. Some of these substances affecting fish health are from natural sources, such as low pH in some lakes resulting from the accumulation of tannic acid from forest degradation. In vitro assays describing the immunosuppressive effects of copper on the antibody-producing cells in isolated rainbow trout spleen cells shows the direct effect on the humoral immune response. In the assays, Spleen fragments were incubated in tissue culture media for 14 days after being injected with Yersinia ruckeri bacterins. The higher concentrations of copper in the media inhibited the formation of the antibody-producing cells. It is observed that tributyltin, an important antifouling agent used in exterior paints for boats, has immunosuppressive properties on the chemiluminescence response of macrophages in the toadfish.

9 - Infection and Disease

Abstract: This chapter discusses the occurrence of infections and diseases in fishes. In larval fish, infections through the gastrointestinal tract are quite likely to occur with a wide range of fish pathogens because the lethal secretions in the stomach, present in fish at a later stage of development, may still largely be absent. Infections with the kidney disease bacterium, Renibacterium salmoninarum, have been successfully established by the feeding of infected material, and in Pacific salmon farmed in sea water, infections by this route probably help to account for the increasing prevalence of bacterial kidney disease during their rearing. Evidence for the invasion of fish through the intact skin has been convincingly demonstrated with only one bacterial fish pathogen, V. anguillamm . By using routine histopathological techniques, it was been found that in natural and experimental infections of ayu, the skin was the first tissue to be colonized. In Aeromonas salmonicida , serum resistance appears to be related to a protein layer (the A layer) that envelopes the cell and to the long side chains of the A. salmonicida lipopolysaccharide (LPS) that traverse the A layer.