Effect of triiodothyronine on the expression of T cell markers and immune function in thyroidectomized White Leghorn chickens.

Hypothyroid K-strain chickens were produced by neonatal thyroidectomy and treatment with 6-propyl-2-thiouracil. Thyroidectomized birds were given 0, 1.5, 4.5, 15, or 45 micrograms/kg body wt of triiodothyronine (T3) by daily injection. At 5 weeks of age, thymocytes were prepared for flow cytometric analysis of CT-1a, CD3, CD4, and CD8 expression. Sham-operated birds had the smallest proportion of CT-1a+ cells and the brightest CT-1a+ cells. Unsupplemented thyroidectomized birds presented an inverse picture, while T3-treated thyroidectomized birds were intermediate. Fewer and less brightly labeled CD3+, CD4+, and CD8+ cells were associated with sham-operated birds or with higher levels of T3 replacement. Low levels (1.5 microgram/kg body wt) or no T3 treatment produced a greater proportion of positive, highly fluorescent cells. The ratios of CD4+ to CD8+ thymocytes were increased (P less than or equal to 0.05) by T3 supplementation. Functionally, thyroidectomy produced a decrease in mitogen-stimulated proliferation of peripheral blood lymphocytes. This effect was ameliorated by T3 supplementation. Further, thyroidectomy produced an elevation of plasma growth hormone concentrations. These results suggest that thyroid factors and alterations of thymic status significantly affect the generation of specific thymus-derived lymphocyte populations and their functional capabilities, perhaps due to changes in the thymic microenvironment. These alterations may have important consequences for the development of immunocompetence and disease resistance in chickens.