NKT Cells in Mice Originate from Cytoplasmic CD3-Positive, CD4(−)CD8(−) Double-Negative Thymocytes that Express CD44 and IL-7Rα

Although natural killer T cells (NKT cells) are thought to be generated from CD4(+)CD8(+) (DP) thymocytes, the developmental origin of CD4(−)CD8(−) (DN) NKT cells has remained unclear. In this study, we found the level of NK1.1 expression was highest in DN cells, followed by CD4 and CD8 (SP) and DP cells. The level of NK1.1 expression was highest in CD44(+)CD25(−) (DN(1)) cells, after that CD44(+)CD25(+) (DN(2)), finally, CD44(−)CD25(−) (DN(3)) and CD44(−) CD25(+) (DN(4)) cells. Unexpectedly, cytoplasmic CD3 was not only expressed in SP and DP thymocytes but also in most DN thymocytes at various stages. The mean fluorescence of cytoplasmic and surface CD3 in DN cells was significantly lower than in mature (SP) T and NKT cells in the thymus and spleen. Interestingly, there were more NKT cells in DN-cytoplasmic CD3 expression cells was higher than in DN-surface CD3 expression cells. There were more CD3-NKT cells in DN(1) thymocytes than in TCR-β-NKT cells. NKT cells expressed higher levels of IL-7Rα which was correlated with CD44 expression in the thymus. Our data suggest that T cells and NKT cells follow similar patterns of expression with respect to cytoplasmic and surface CD3. Cytoplasmic CD3 could be used as a marker for early stage T cells. Both cytoplasmic CD3 and surface CD3 were expressed in mature T cells and immature T cells, including the immature cytoplasmic CD3(+) surface CD3(−) and surface CD3(+)TCR-β(−) cells in DN(1)-NKT thymocytes. CD44 could be used as an additional marker of NKT cells which may originate from cytoplasmic CD3-positive DN thymocytes that express CD44 and IL-7Rα in mice.