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Introduction
A major goal of poultry farmers is to minimize the losses, and to attain good flock liveability. Therefore, they strive to improve farm’s environmental conditions and management to maximize bird health. However, commercial poultry environments contain ubiquitous microorganisms that continuously challenge the bird’s immune system. In recent years there has been much research on the role of nutrients that improve bird health through immunomodulation. This paper discusses the importance of such nutrients on the development of immunity in the chicken, in the light of recent work.
The avian immune response
The primary function of the immune response is to provide the bird with the ability to resist invasion and injurious effects of pathogens. Protection is imparted by two types of immune mechanisms. That imparted through the antibodies is called “antibody-mediated or humoral immunity”, and that through cells is known as “cell-mediated or cellular immunity.” In the former, antibodies neutralize, inactivate and weaken the pathogens which are then engulfed by body’s phagocytic cells and killed. However, only those pathogens can be destroyed in this way that are extracellular, such as bacteria. If they are intracellular, such as viruses, then the cell-mediated immune response comes to bird’s rescue. Its basic purpose is to destroy the very cell that harbours the pathogen, and thus protect the bird.
Antibody-mediated immunity
On contact with a pathogen, B-lymphocytes divide and differentiate into plasma cells, which synthesize and secrete large amounts of antibody (immunoglobulins). The antibodies are proteins, made up of amino acids. Each plasma cell can produce up to 300 molecules of immunoglobulins per second. As antibodies are proteins, importance of the adequate supply of proteins, while the bird is producing antibodies cannot be over emphasized. Any deficiency of essential amino acids and sufficient non-essential amino acid nitrogen, particularly at the growing stage, would result in poor antibody response.
Methionine is the first limiting amino acid in most commercial feeds. Requirement for methionine to ensure maximal antibody and cell-mediated immune responses is greater than that for growth (1). Threonine is found in high concentrations in chicken immunoglobulins. Since immunoglobulins depend on amino acid sequences to form the variable regions for antigen binding and provide structural support, threonine deficiency may suppress antibody activity (2). Unlike the mammalian macrophages that synthesize arginine, avian macrophages require exogenous sources of arginine, which are converted to reactive nitrogen intermediate-nitric oxide (3). Nitric oxide can kill ingested bacteria and intracellular parasites, and is also antiviral.
Low dietary vitamin A, in the chicken, causes reduced antibody production in response to infection and following vaccination defective cytotoxic T-lymphocyte responses, reduced antigen-specific T-lymphocyte proliferation, impaired phagocytosis and reduced resistance to infection. This is because retinoids (vitamin A) direct differentiation and development of B-lymphocytes. Recently, a retinoid acid receptor has been identified in the chicken lymphocytes and it has been found that expression of this receptor is closely regulated by dietary vitamin A. It is, therefore, possible that retinoid acid directly amplifies activated genes in bursal lymphocytes, which trigger multiple cellular mechanisms to achieve differentiation (4). Moreover, in the absence of vitamin A, basal cells of bursa of Fabricius undergo squamous metaplasia. The keratinization results in inadequate antibody production and poor differentiation of B-lymphocytes. Responses of T-lymphocytes are also impaired (5).
Cell-mediated immunity
Virus-infected cells and those bacterial diseases where the organisms reside within the cells (eg. avian tuberculosis), the intracellular pathogens are destroyed by cytotoxic T-lymphocytes. They do this through perforins and granzymes, which are proteins.
Dietary zinc deficiency results in thymic atrophy, thymocyte depletion, depressed peripheral T-cell numbers, depressed T-helper cell functions, and poor cell-mediated immune responses. Chickens are unable to produce antibodies against T-cell dependent antigens even though lymphocytes are capable of immunoglobulin production. These effects are due to a decrease in zinc containing hormone, thymulin necessary for lymphocyte development and also of two zinc metalloenzymes, namely, DNA and RNA polymerases, and one zinc-dependent enzyme deoxythymidine kinase (6). The ability of these enzymes to function is destroyed without zinc. Zinc deficiency also impairs interleukin-2 production, which plays an important role in cell-mediated immunity. Therefore, presence of adequate zinc is crucial for normal development, maintenance and function of the immune system. Recently it has been revealed that zinc is also necessary for macrophage function, i.e. phagocytosis (6).
Development of bursa of Fabricius and thymus
During the first several weeks, bursa and thymus develop at a rate much faster than the bird’s body as a whole. Because of the high rate of cell division, a large number of enzyme co-factors are needed to produce an immune response. Deficiencies that are especially damaging to the immune system and which result in greatest incidence of infection include vitamin A, zinc, vitamin E, selenium, vitamin C, copper and manganese.
Nutrients as protectors of tissue damage from the immune system
When the immune system responds against pathogens, it produces a wide variety of harmful substances. The rapidly proliferating cells, of the stimulated immune and phagocytic systems are particularly prone to peroxidative damage by free radicals. Vitamin E is a very effective antioxidant. It is present in cellular membranes and prevents oxidation of unsaturated lipids by free radicals. There is a close working relationship between vitamin E and selenium in their functions within tissues. Selenium is a component of glutathione peroxidase, the enzyme that removes active peroxides from cells before they oxidize unsaturated lipids which are protected by tocopherols (5). Vitamin C and vitamin A also possess antioxidant activity. Vitamin C can inactivate free radicals directly in the cell and can act indirectly by regenerating the antioxidant form of vitamin E. Thus, vitamin E and C act in concert. In addition, superoxide dismutases inactivate free radicals. This group includes both manganese superoxide dismutase, which is located in mitochondria, and copper-zinc-superoxide dismutase, which is found in cytosol (7). The quantity of vitamin E, selenium, copper, zinc and manganese in tissue is directly affected by dietary levels (1).
Acute phase proteins
During the acute phase of an immune response, the greatest nutritional need is for the synthesis and release of various “acute phase proteins” by the liver. Under the influence of interleukin-1, interleukin-6 and tumour necrosis factor-alpha, released from macrophages following infection, liver cells increase protein synthesis and secretion. Their response begins within a few hours of infection and subsides within 24-48 hours. Acute phase proteins give protection and resistance to birds against infection. Synthesis of acute phase proteins requires about 10 times more energy and amino acids than are normally needed by lymphocytes when they are producing an immune response. Moreover, their synthesis is more sensitive to deficiencies of several nutrients, including amino acids (methionine, cysteine) and trace minerals (copper, manganese) than that required for development of specific immunity (1). One of the important acute phase proteins, in the chicken, is copper-containing ceruloplasmin. It protects the bird against the excesses of the immune response by removing oxygen-derived free radicals produced during phagocytosis (8). Thus, the need to synthesize ceruloplasmin during an infection increases the copper requirement.
The ability of poultry to withstand infectious diseases depends upon the integrity of the immune system. Proper functioning of the avian immune system is dependent on the availability of nutrients which mediate cellular functions pertinent to host defense. Proteins, certain vitamins and trace minerals are important in cellular interactions that maintain integrity of the immune system. A unique feature of the avian cellular immune system is that it consists of rapidly dividing cells. Therefore, adequate nutrients are a must for enzyme function that facilitates their proliferation. Certain trace elements and vitamins enhance some cell functions important for disease resistance when added to the diet of poultry.
Conclusion
An adequate supply of good quality protein containing essential amino acids like methionine and also of certain vitamins and trace elements, is a must to ensure protection of birds against infectious diseases through successful operation of the immune mechanisms. Demands for these nutrients are greatly increased during the production of immunity following vaccination and in disease.
References
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- Kidd, M.T. (2000). World’s Poultry Science J., 56: 139.
- Glick, B. (2000). Immunophysiology. In Sturkie’s Avian Physiology. 5th Edn. p. 657. Academic Press. California, U.S.A.
- Friedman, A. and Sklan, D. (1997). World’s Poultry Science J., 53: 185.
- Klasing, K.C. (1998). Comparative Avian Nutrition. pp. 267, 284, 295. CAB International, Wallingford, Oxon, U.K.
- Kidd, M.T. et al. (1996). World’s Poultry Science J., 52: 309.
- Cotran, R.S. et al. (1999). Robbins Pathologic Basis of Disease. 6th Edn. p. 13. Harcourt Asia Pte Ltd., Singapore.
- Chamanza, R. et al. (1999). World’s Poultry Science J., 53: 185.
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