We have used a panel of murine mAb against chicken TCR and associated molecules to study the effect of cyclosporin A (CsA) on the ontogeny of the different sublineages of T cells. After injection of CsA (20 mg/kg/day from day 0 to 20) we observed a significant suppression of the normal maturation of the TCR2 (alpha beta TCR) cells in their transition from cortical CD4+CD8+ thymocytes to the mature single positive cells in the thymus medulla. The TCR3 subpopulation, a distinct form of alpha beta-like TCR in chickens, was inhibited from initially developing within the cortex by CsA, indicating that the TCR3 subpopulation is functionally distinct from the TCR2+ cells. In contrast, the maturation and peripheral emigration of TCR1 (gamma delta TCR) cells was unaffected by CsA treatment. Mature splenic T cells sorted for either TCR1+ or TCR2+ subsets were equally sensitive to CsA blockade of Con A-stimulated mitogenesis, indicating that there is no inherent difference in CsA sensitivity between these sublineages. Furthermore, no difference was detected in the expression of class II MHC Ag in thymi of birds treated with olive oil vs CsA. Inasmuch as the mechanism of CsA action appears to involve inhibition of TCR initiated signal transduction for lymphokine synthesis, these data indicate that a similar signaling is involved in thymic repertoire selection for TCR2. The lack of an effect on TCR1 cell maturation suggests that the TCR1 repertoire may not undergo selection in the thymus as do TCR2+ cells.

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