The effect of synthetic thymulin on cell surface marker expression by avian T-cell precursors

Developmental immunotoxicity of lead: Impact on thymic function

BACKGROUND

Since the potential effects of early exposure to lead on thymic functions have not been fully characterized, in this study we evaluated the capacity of lead to alter thymic function in juvenile chickens following embryonic exposure.

METHODS

Cornell K strain White Leghorn chicken eggs were administered lead acetate (400 microg/egg) or sodium acetate (control) on embryonic development (E12) with and without thymulin supplementation. Ex vivo production of interferon-gamma (IFN-gamma)-like cytokine by thymocytes and a delayed-type hypersensitivity (DTH) reaction were measured in the juvenile. Additionally, the effects of in vitro exposure to lead on both thymocytes and thymic stromal cells (TSCs) were evaluated.

RESULTS

Following E12 exposure to lead, ex vivo production of IFN-gamma-like cytokine by juvenile-derived thymocytes decreased significantly compared to the control. The same effect was observed when thymocytes were directly exposed to lead in vitro and stimulated with thymic stromal supernatant. In contrast, when TSCs were exposed to lead in vitro, no change was seen in their functional capacity for promoting cytokine production. In ovo supplementation with thymulin partially reversed lead-induced DTH depression without any change in IFN-gamma-like cytokine production. Embryonic exposure to thymulin alone partially depressed the DTH response.

CONCLUSIONS

These results suggest that lead can directly influence thymocyte function in the absence of the thymic microenvironment. Since thymulin levels may influence lead-induced immunotoxicity, embryonic endocrine status may be an important consideration. Lead exposure appears to alter thymic functions directly; however, indirect effects via endocrine factors are not precluded.

The effects of thymulin on macrophage responsiveness to interferon-gamma

The effect of in vivo and in vitro thymulin treatments on macrophage responsiveness to interferon-gamma was evaluated in chickens. Seven-week-old chickens were treated with 0, 1, 10, or 100ng thymulin per 100g body weight. Abdominal exudate cells (AEC), a source of macrophages, were harvested and cultured in the presence of graded levels of recombinant chicken interferon-gamma (ChIFN-gamma). Responsiveness to ChIFN-gamma was determined by measuring the induction of nitric oxide production. One and 2-day thymulin treatment at 10 and 100ng per 100g body weight doses significantly increased responsiveness to ChIFN-gamma while 1ng per 100g body weight had no effect. Other experiments compared the effect of thymulin treatments in Cornell K strain chickens, having normal serum thymulin levels with sex-linked dwarf (SLD) chickens which are deficient in serum thymulin. The dose of thymulin treatment required to significantly increase responsiveness to ChIFN-gamma differed between strains. Finally, the effect of direct in vitro thymulin treatments on macrophage responsiveness to ChIFN-gamma was evaluated. There were no significant increases in responsiveness to ChIFN-gamma between treatment groups within the macrophage cell line, HD-11, when cultured in the presence of 0-200pg thymulin/ml. These data suggest that the effect of thymulin on AEC responsiveness to ChIFN-gamma is indirectly mediated.

The effect of thymulin on avian IL-2 receptor expression

The effect of thymulin on IL-2 receptor (IL-2R) expression by avian splenocytes was examined in the functionally hypothyroid sex-linked dwarf (SLD) and in normal euthyroid K strain chickens. Daily thymulin injections of 0, 0.05 and 5.0 ng/100 g body weight were given from hatching until 4 weeks of age. ConA-treated and non-stimulated splenocytes from these animals were analyzed by flow cytometry for their expression of IL-2Ralpha, CD4 and CD8 cell surface molecules. ConA activation increased the number of IL-2R+ cells within K strain more than in the SLD. Thymulin treatment increased the number of IL-2R+ cells in the SLD but had the opposite effect in K strain chickens. Mitogen activation or thymulin treatment had little effect on the IL-2R density within small cell populations. In contrast, mitogen activation increased the density of IL-2R on larger cell populations in both K and SLD. IL-2R densities on non-stimulated larger cells decreased in the SLD after thymulin exposure. Thymulin treatment produced no effect on the mean IL-2R densities for large activated cells. ConA stimulation increased the number of CD4+ cells in both strains. The density of CD4 expression was modulated by both mitogen activation and thymulin treatment. ConA stimulation produced an increase in the number of CD8+ cells. The SLD had fewer CD8+ cells than did the K strain and thymulin treatment had little effect on this population in either strain. Mitogen stimulation increased the density of CD8 on CD8+ cells but again thymulin treatment had little effect. These results suggest that thymulin can modulate IL-2R expression on splenocytes and that this effect may be dependent upon the thyroidal status of the animal. Further, these data suggest that thymulin has a differential effect on the CD4 and CD8 T-cell subpopulations.

Neuroendocrine control of thymus physiology

The thymus gland is a central lymphoid organ in which bone marrow-derived T cell precursors undergo differentiation, eventually leading to migration of positively selected thymocytes to the peripheral lymphoid organs. This differentiation occurs along with cell migration in the context of the thymic microenvironment, formed of epithelial cells, macrophages, dendritic cells, fibroblasts, and extracellular matrix components. Various interactions occurring between microenvironmental cells and differentiating thymocytes are under neuroendocrine control. In this review, we summarize data showing that thymus physiology is pleiotropically influenced by hormones and neuropeptides. These molecules modulate the expression of major histocompatibility complex gene products by microenvironmental cells and the extracellular matrix-mediated interactions, leading to enhanced thymocyte adhesion to thymic epithelial cells. Cytokine production and thymic endocrine function (herein exemplified by thymulin production) are also hormonally controlled, and, interestingly in this latter case, a bidirectional circuitry seems to exist since thymic-derived peptides also modulate hormonal production. In addition to their role in thymic cell proliferation and apoptosis, hormones and neuropeptides also modulate intrathymic T cell differentiation, influencing the generation of the T cell repertoire. Finally, neuroendocrine control of the thymus appears extremely complex, with possible influence of biological circuitry involving the intrathymic production of a variety of hormones and neuropeptides and the expression of their respective receptors by thymic cells.

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.

Neuroendocrine-immune interactions

The role of the neuroendocrine system in influencing both immune development and function has become an area of active research within many model systems, including the chicken. It is now clear that the neuroendocrine system can exert immediate feedback regulation on the immune system as well as control specific aspects of immune differentiation and development. The primary lymphoid organs of avian species (i.e., the thymus and the bursa of Fabricius) are also known to function as endocrine organs. These produce hormonal products that influence the development of lymphoid cells and that may feed back on the neuroendocrine system. In conjunction with the endocrine activities of the primary lymphoid organs, immune and accessory cells are known to produce a variety of secreted products or cytokines that have the potential not only for the regulation of immune function but also for mediating neuroendocrine activities. Finally, it has been demonstrated in a variety of species that leukocytes are capable of producing endocrine mediators previously believed to be produced only under the direct control of the hypothalamic-pituitary axis. Thus, there are numerous possibilities for bidirectional interactions between the immune and neuroendocrine systems. This discussion focuses primarily on these interactions with an emphasis on the means by which the hormonal mediators, growth hormone and thyroid hormone, may affect the thymus and the thymic microenvironment. The role of the adrenocorticoids and gonadal steroids in regulating immune function and their involvement in immune feedback circuits are also discussed.

The humoral activity of the avian thymic microenvironment

The thymic microenvironment (composed of the lymphoepithelial stroma and the secretory products of the thymic epithelium) provides the required milieu for the development of the thymus-derived lymphocytes (T cells). There is limited information characterizing or identifying the active secretory components of the avian thymus. The work discussed here has focused on examination of the presence, regulation, and activity of one of the thymic hormones (thymulin) in the chicken. A thymulin-like product has been shown to exist in chicken serum as assessed by the mammalian bioassay and an ELISA immunoassay; thymectomy removes this product from the serum. Serum thymulin activity has been shown to be directly related to the thyroid status of the chick with the functionally hypothyroid Cornell sex-linked dwarf strain having lower levels than the euthyroid K strain. Alterations in circulating thymulin concentrations produced by daily thymulin injections resulted in an altered profile of the major peripheral blood T cell subpopulations and produced significant changes in the autoimmune pathology present within the Obese strain chicken. These approaches represent preliminary attempts to study the role of thymulin in avian immune development and in immune-neuroendocrine interactions.