Identification of a new type of invariant Vα14⁺ T cells and responsiveness to a superantigen, Yersinia pseudotuberculosis-derived mitogen

We examined the expression of the H4 T cell activation marker in thymic T cell subpopulations and found that TCR-αβ⁺ CD4⁺ thymic T cells are segregated into three subpopulations based upon H4 levels. Thymic T cells with either no or low H4 expression differentiate via the mainstream differentiation pathway in the thymus. H4(int) thymic T cells, which express a skewed Vβ repertoire of Vβ2, -7, and -8 in their TCRs, show the phenotype of NKT cells: CD44(high), Ly6C(high), NK1.1⁺, and TCR-αβ(low). H4(high) thymic T cells also show a skewed Vβ repertoire, Vβ2, -7, and -8, and predominantly express an invariant Vα14-Jα281⁺ α-chain in their TCRs but constitute a distinct population in that they are CD44(int), Ly6C^-, NK1.1^-, and TCR-αβ(high). Thus, invariant Vα14⁺ thymic T cells consist of ordinary NKT cells and a new type of T cell population. Vβ7⁺ and Vβ8.1⁺ invariant Vα14⁺ thymic T cells are present in DBA/2 mice, which carry mammary tumor virus-7-encoded superantigens, in comparable levels to those in BALB/c mice. Furthermore, Vβ7⁺ invariant Vα14⁺ thymic T cells in DBA/2 mice are in the immunologically responsive state, and Yersinia pseudotuberculosis-derived mitogen-induced Vβ7⁺ invariant Vα14⁺ thymic T cell blasts from DBA/2 and BALB/c mice exhibited equally enhanced responses upon restimulation with Y. pseudotuberculosis-derived mitogen. Thus, invariant Vα14⁺ thymic T cells that escape negative selection in DBA/2 mice contain T cells as functionally mature as those in BALB/c mice.