SOCS1 negatively regulates the production of Foxp3 + CD4 + T cells in the thymus

SOCS1 profoundly influences the development and peripheral homeostasis of CD8 + T cells but has less impact on CD4 + T cells. Despite the moderate influence of SOCS1 in the development of the total CD4 T‐cell lineage, we show here that SOCS1 deficiency resulted in a 10‐fold increase in Foxp3 + CD4 + T cells in the thymus. Increased numbers of Foxp3 + thymocytes occurred in mice with T‐cell‐specific ablation of SOCS1, suggesting that the effect is T‐cell intrinsic. This increase in Foxp3 + CD4 + cells in SOCS1‐deficient mice also occurred in the absence of IFN‐γ or/and IL‐7 signaling. Increase in CD25 + CD4 + T cells in the absence of SOCS1 could be partly due to enhanced survival by CD25 + CD4 + cells, to a lesser degree CD25 − CD4 + T cells, from SOCS1‐deficient mice with or without T‐cell growth factors.