Identification of CD4(−)CD8(−) Double-Negative Natural Killer T Cell Precursors in the Thymus

BACKGROUND: It is well known that CD1d-restricted Vα14 invariant natural killer T (NKT) cells are derived from cells in the CD4(+)CD8(+) double-positive (DP) population in the thymus. However, the developmental progression of NKT cells in the earlier stages remains unclear, and the possible existence of NKT cell presursors in the earlier stages than DP stage remains to be tested. PRINCIPAL FINDINGS: Here, we demonstrate that NKT cell precursors that express invariant Vα14-Jα18 transcripts but devoid of surface expression of the invariant Vα14 receptor are present in the late CD4(−)CD8(−) double-negative (DN)4 stage and have the potential to generate mature NKT cells in both in vivo and in vitro experimental conditions. Moreover, the DN4 population in CD1d knock-out (CD1dKO) mice was similar to those with an NKT cell potential in wild-type (WT) C57BL/6 (B6) mice, but failed to develop into NKT cells in vitro. However, these precursors could develop into NKT cells when co-cultured with normal thymocytes or in an in vivo experimental setting, indicating that functional NKT cell precursors are present in CD1dKO mice. CONCLUSIONS: Together, these results demonstrate that thymic DN4 fraction contains NKT cell precursors. Our findings provide new insights into the early development of NKT cells prior to surface expression of the invariant Vα14 antigen receptor and suggest the possible alternative developmental pathway of NKT cells.