Avian leukocytic cytokines.

Leukocytic cytokines are produced by cells of the immune system and are prominent regulators of the immune response and in some cases various systemic responses. Leukocytic cytokines are released during immune responses and may act in autocrine, paracrine, or endocrine manners. Although over a dozen avian leukocytic cytokines have been described based on functional activities, characterization at the molecular level is not well developed. Two exceptions are 1) myelomonocytic growth factor, a colony-stimulating factor-like cytokine required for the growth and differentiation of hematopoietic precursor cells, particularly myelomonocytic cells; and 2) the avian transforming growth factor-beta (TGF-beta) family of cytokines, which modulate wound healing, bone metabolism, and cellular differentiation. Cytokines with bioactivities similar to mammalian interleukin (IL)-1, IL-2, IL-6, and interferon-gamma have been at least partially purified. Cytokines with bioactivities similar to mammalian IL-8, colony-stimulating factor, and tumor necrosis factor-alpha have been reported but are not well characterized at the molecular level. With a few exceptions, including TGF-beta and thymulin, highly purified leukocytic cytokines of mammalian origin have diminished or no specific activity in avian assay systems. The chicken IL-1 receptor has been cloned and the predicted amino acid sequence shares 60% homology with the human IL-1 receptor. A component of the chicken IL-2 receptor has been partially purified but little is known about other avian leukocytic cytokine receptors. Potential applications of leukocytic cytokines in poultry production originate from their regulation of a variety of functions such as disease resistance, would healing, bone accretion, nutrient partitioning, appetite, growth, and reproduction.