Characterization of the early stages of thymic NKT cell development

Upon reaching the mature heat stable antigen (HSA)^low thymic developmental stage, CD1d-restricted Vα14-Jα18 thymocytes undergo a well-characterized sequence of expansion and differentiation steps that lead to the peripheral interleukin-4/interferon-γ–producing NKT phenotype. However, their more immature HSA^high precursors have remained elusive, and it has been difficult to determine unambiguously whether NKT cells originate from a CD4⁺CD8⁺ double-positive (DP) stage, and when the CD4⁺ and CD4⁻CD8⁻ double-negative (DN) NKT subsets are formed. Here, we have used a CD1d tetramer-based enrichment strategy to physically identify HSA^high precursors in thymuses of newborn mice, including an elusive DP^low stage and a CD4⁺ stage, which were present at a frequency of ∼10⁻⁶. These HSA^high DP and CD4⁺ stages appeared to be nondividing, and already exhibited the same Vβ8 bias that characterizes mature NKT cells. This implied that the massive expansion of NKT cells is separated temporally from positive selection, but faithfully amplifies the selected TCR repertoire. Furthermore, we found that, unlike the DN γδ T cells, the DN NKT cells did not originate from a pTα-independent pathway bypassing the DP stage, but instead were produced during a short window of time from the conversion of a fraction of HSA^low NK1.1^neg CD4 cells. These findings identify the HSA^high CD4⁺ stage as a potential branchpoint between NKT and conventional T lineages and between the CD4 and DN NKT sublineages.