Selective expression of major histocompatibility complex (MHC) antigens and modulation of T-cell differentiation in chickens with increased MHC-chromosome dosages

Thymus-Derived Glucocorticoids Set the Thresholds for Thymocyte Selection by Inhibiting TCR-Mediated Thymocyte Activation

Selection processes in the thymus eliminate nonfunctional or harmful T cells and allow the survival of those cells with the potential to recognize Ag in association with self-MHC-encoded molecules (Ag/MHC). We have previously demonstrated that thymus-derived glucocorticoids antagonize TCR-mediated deletion, suggesting a role for endogenous thymic glucocorticoids in promoting survival of thymocytes following TCR engagement. Consistent with this hypothesis, we now show that inhibition of thymus glucocorticoid biosynthesis causes an increase in thymocyte apoptosis and a decrease in recovery that are directly proportional to the number of MHC-encoded molecules present and, therefore, the number of ligands available for TCR recognition. Expression of CD5 on CD4+CD8+ thymocytes, an indicator of TCR-mediated activation, increased in a TCR- and MHC-dependent manner when corticosteroid production or responsiveness was decreased. These results indicate that thymus-derived glucocorticoids determine where the window of thymocyte selection occurs in the TCR avidity spectrum by dampening the biological consequences of TCR occupancy and reveal that glucocorticoids mask the high percentage of self-Ag/MHC-reactive thymocytes that exist in the preselection repertoire.

Selective mitomycin C and cyclophosphamide induction of apoptosis in differentiating B lymphocytes compared to T lymphocytes in vivo

Differentiating B and T lymphocytes differ in sensitivity to a number of environmental toxins and anticancer agents. B lymphocytes are susceptible and T lymphocytes resistant to killing by cyclophosphamide (Cy) metabolites capable of forming DNA interstrand cross-links. However, the mechanisms responsible for the rapid killing and loss of bursal-resident B lymphocytes are unknown. Therefore, we investigated the cellular mechanisms of selective toxicity of two cross-linking drugs, mitomycin C (MMC) and Cy, towards differentiating B and T lymphocyte populations using the chicken embryo model system. Viability of bursal-resident B lymphocytes (bursacytes) decreased starting at 5 h post exposure (PE) to MMC, and was maximally reduced by 71.6% by 10 h PE at the highest dose examined (9.0 micrograms MMC/g). Dose-dependent increases in the percentage of apoptotic bursacytes were observed as early as 5 h PE, and increased to 72% by 10 h PE. This was accompanied by reductions in bursacyte numbers. Cy also induced apoptosis in bursacytes. In contrast, thymus-resident lymphocytes (thymocytes) were much more resistant to the toxic effects of MMC and Cy. Viability of thymocytes was reduced by only 10% in the 9.0 micrograms/g MMC treatment group. In addition, the percentage of thymocytes engaged in apoptosis was much lower than that for bursacytes. MMC induced modest cell cycle inhibition in bursacytes and thymocytes. These data strongly suggest that MMC and Cy-induced diferential toxicity involves primarily early and extensive triggering of apoptosis in differentiating B lymphocytes, leading to rapid reduction of lymphocyte numbers in the embryonic bursa.