NKT cells derive from double-positive thymocytes that are positively selected by CDId

CDId-reactive NKT cells are a separate T cell sublineage. Instructive models propose that NKT cells branch off the mainstream developmental pathway because of their T cell antigen receptor specificity, whereas stochastic models would propose that they develop from precursor cells committed to this sublineage before variable-gene rearrangement. We show here that immature double-positive (DP) thymocytes form the canonical rearranged V-alpha gene of NKT cells at nearly equivalent frequencies in the presence or absence of CDId expression. After interacting with CDId in the thymus, these cells give rise to expanded populations of NKT cells - including both CD4(+) and double-negative lymphocytes in the thymus and periphery - that express this alpha chain. These results confirm the existence of a DP intermediate for CDId-reactive NKT cells. They also show that the early developmental stages of these T cells are not governed by a distinct mechanism, which is consistent with the TCR-instructive model of differentiation.