Phenotypic characterization of CD8+NKT cells

We describe a novel CD8+NKT cell population expressing TCRα /β or TCRγ /δ . These CD8+NKT cells were prominent in the liver, and except for the thymus, virtually absent in other lymphoid organs. CD8+NKT cells expressed activation markers and comprised a high proportion of Ly49+ cells. The development of the majority of CD8+NKT cells expressing TCRα /β, but not TCRγ /δ, depended on classical MHC class I. No CD8+NKT cells were detectable in young athymic mice, whereas the cells expressing TCRγ /δ, but not TCRα /β, appeared randomly in aged athymic mice. CD8+NK1+ TCRα /β cells showed polyclonal TCRVβ usage and were virtually devoid of TCRVα14. CD8+NK1+ TCRγ /δ cells predominantly expressed TCRVγ1, 2 and 4, and Vδ4, 5, 6 and 7. CD8+NKT cells, in particular those expressing TCRγ /δ, were a major population in early life. IFN‐γ, but not IL‐4, was induced in CD8+NKT cells by in vitro stimulation, independent of the TCRα /β or TCRγ /δ lineage. Hence, these cells represent a unique, though heterogeneous T cell population that shares markers with, but is distinct from, both conventional NKT cells and conventional CD8+ T cells, and that may play a role in immune regulation.