Suspension of thymic emigration promotes the maintenance of antigen-specific memory T cells and the recall responses

Recirculating Th2 cells induce severe thymic dysfunction via IL-4/STAT6 signaling pathway

Thymic involution happened early in life, but a certain ratio of activated CD4⁺ T cells will persistently recirculate into the thymus from the periphery and it have been suggested to be able to inhibit the development of embryonic thymocytes. Our present study was aimed to elucidate the specific mechanism how activated CD4⁺ T cells could influence upon developing thymocytes by using fetal thymic organ culture (FTOC) and kidney capsule transplantation. Our results demonstrated that Th2 cells were found to play a fundamental role in the inhibition of embryonic thymocyte development since a very low concentration of Th2 cells could obviously reduce the total number of thymocytes. And this effect was not tenable in other Th cell type. Notably, IL-4, the major cytokine secreted by Th2 cells, was suggested the key factor playing the inhibition role. In addition to reduced cell population, the proportion of double positive (DP) T cells was also heavily decreased. Furthermore, we demonstrated that it was the downstream effector signal transducer and activator of transcription 6 (STAT6) of IL-4 partially manipulate this inhibition. Together, these findings reveal a novel influence of Th2 cells re-entering the thymus on thymic involution.

Thymic homing of activated CD4+ T cells induces degeneration of the thymic epithelium through excessive RANK signaling

Activated T cells have been shown to be able to recirculate into the thymus from the periphery. The present study was aimed to elucidate the functional consequences of thymic homing of activated T cells upon developing thymocytes and thymic epithelial cells (TEC). In the presence of activated T cells, especially CD4⁺ T cells, T cell development was found to be inhibited in thymic organ cultures with markedly reduced cellularity. Thymic transplantation demonstrated that the inhibitory effect was most likely due to a defective microenvironment. As the major component of the thymic stroma, the TEC compartment was severely disturbed after prolonged exposure to the activated T cells. In addition to reduced cell proliferation, TEC differentiation was heavily skewed to the mTEC lineage. Furthermore, we demonstrated that RANKL highly expressed by activated CD4⁺ T cells was primarily responsible for the detrimental effects. Presumably, excessive RANK signaling drove overproduction of mTECs and possibly exhaustion of epithelial progenitors, thereby facilitating the deterioration of the epithelial structures. These findings not only reveal a novel activity of activated T cells re-entering the thymus, but also provide a new perspective for understanding the mechanism underlying thymic involution.