CD117(+)CD44(+) Stem T Cells Develop in the Thymus and Potently Suppress T-cell Proliferation by Modulating the CTLA-4 Pathway

BACKGROUND: CD117 is expressed on double-negative (DN; CD4(–)CD8(–)) cells (Nat Rev Immunol 14:529–545; 2014), but whether it is expressed in other stages and its subsequent functions are unclear. We used an improved method of flow cytometry to analyze different populations of thymocytes (Sci Rep 4:5781; 2014). The expression of CD117 and CTLA-4 were directly assayed in the early stage of thymocytes. METHODS: Flow cytometry was used to analyze different populations of thymocytes, and T-cell proliferation assays, RT-PCR, and real-time RT-PCR were used to characterize the stem cells and examine the function of CD44(+)CD117(+) cells. RESULTS: In DN cells, CD117 expression was greatest on CD44(+)CD25(+) cells (DN(2)), followed by CD44(+)CD25(–) (DN(1)), CD44(–)CD25(+) (DN(3)), and CD44(–)CD25(–) (DN(4)) cells. In thymocytes, CD117 expression was highest in DN cells, followed by single-positive (SP; CD4 or CD8) and double-positive (DP; CD4(+)CD8(+)) cells. Especially, CD117 expression was positively associated with CD44 and CTLA-4 expression. CTLA-4 expression was highest in DN cells, followed by SP and DP cells. CTLA-4 expression was positively associated with CD25, CD44, and Foxp3 expression. CD44(+)CD117(+) T cells expressed more CTLA-4, which suppressed T-cell proliferation and blocked CTLA-4 to cause antibody-induced T-cell proliferation. CONCLUSION: These results suggest that CD44(+)CD117(+) T cells are stem cells and a specific T-cell phenotype that initially develops in the thymus, but they do not progress through DN(3) and DN(4) stages, lack a DP stage, and potently suppress T-cell proliferation and modulate the CTLA-4 pathway.