Effect of triiodothyronine and in vitro growth hormone on avian interleukin-2

Expression and function of chicken bursal growth hormone (GH)

Growth hormone (GH) has several effects on the immune system. Our group has shown that GH is produced in the chicken bursa of Fabricius (BF) where it may act as an autocrine/paracrine modulator that participates in B-cell differentiation and maturation. The time course of GH mRNA and protein expression in the BF suggests that GH may be involved in development and involution of the BF, since GH is known to be present mainly in B lymphocytes and epithelial cells. In addition, as GH is anti-apoptotic in other tissues, we assessed the possibility that GH promotes cell survival in the BF. This work focused on determining the mechanism by which GH can inhibit apoptosis of B cells and if the PI3K/Akt pathway is activated. Bursal cell cultures were treated with a range of GH concentrations (0.1-100nM). The addition of 10nM GH significantly increased viability (16.7±0.6%) compared with the control and decreased caspase-3 activity to 40.6±6.5% of the control. Together, these data indicate that GH is produced locally in the BF and that the presence of exogenous GH in B cell cultures has antiapoptotic effects and increases B cell survival, probably through the PI3k/Akt pathway.

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.

Effects of in vivo thyroid hormone treatment on the expression of interleukin-2 receptors on avian splenocytes

Cornell K strain chickens received a diet supplemented with 0, 0.1 and 1.0 ppm T3 from the day of hatching. At 28 days of age, splenocyte suspensions were prepared and analyzed by flow cytometry for interleukin-2 receptor (IL-2R) and CD3 expression. The low T3 dose increased the percentage of resting small cells expressing IL-2R while the mean fluorescence for this marker was enhanced only after mitogenic activation. This treatment did not alter the number of larger cells positive for IL-2R but did increase their mean fluorescence following mitogenic activation. The high T3 dose depressed the numbers of cells positive for IL-2R and their mean fluorescence amongst all splenocyte preparations. Both levels of T3 enhanced the numbers of CD3-positive cells in all cell preparations. These results suggest that the IL-2R expression can be modulated by in vivo T3 supplementation and that these correlate with the previously demonstrated changes in IL-2-like activity. The regulation of IL-2R expression provides one mechanism through which thyroid status may regulate immune function.