Poster Presentation Australasian Society for Immunology Annual Scientific Meeting 2014

The thymus achieves two tasks essential for the maintaining immune tolerance: negative selection that deletes potentially self-reactive T cells via apoptosis and the generation FoxP3⁺CD4⁺CD25⁺ regulatory T (T_reg) cells that act in the periphery to suppress self-reactive T cells that escape thymic negative selection. The relationship between these two processes within the thymus is poorly understood. Here, we tested the effect of specifically ablating thymic deletion on the development of T_reg cells in the thymus.

To address this issue, we generated mice with conditional deficiency in two key mediators of apoptosis, Bak and Bax, before negative selection (in Bax^ΔVavBak^-/- and Bax^ΔCd4Bak^-/- mice) or after negative selection (Bax^ΔFoxp3 Bak^-/-). We found that blocking apoptosis before negative selection resulted in a striking 5-10 fold accumulation of FoxP3⁺CD4⁺ T_reg cells with low expression of CD25 (IL2-receptor alpha). This accrual of FoxP3⁺CD4⁺CD25^low cells was apparent in three-day old neonatal Bax^ΔCd4Bak^-/-mice and continued in adulthood. The cells were refractory to IL2, IL7 and anti-CD3 stimulation suggesting that the accumulation was not due to cytokine deprivation or the lack of T cell stimulation. Using intrathymic FITC injections to track thymic emigration, we found that the FoxP3⁺CD4⁺CD25^low cells were not exported into the peripheral circulation. Yet, EdU labeling experiments revealed continual turnover within this population, indicating they are not the product of a “dead-end” pathway. Therefore, this subset may comprise an alternative precursor of mature CD25⁺Foxp3⁺ cells that arises from thymocytes escaping deletion.

Overall, the data suggest that failure of apoptosis before negative selection rescues thymocytes that that go on to adopt a FoxP3⁺CD4⁺CD25^low phenotype in the thymus. This study advances our understanding about T_reg cell differentiation and raises further mechanistic questions about how this accumulation affects the T_eff /T_reg cell repertoire in the periphery and how this may change the course of immune responses to self and foreign antigens.