Human invariant natural killer (iNK) T cells expressing an invariant Valpha24-Jalpha15 T-cell receptor (TCR) are thought to be important regulators of autoimmunity and tumour surveillance. Two major subsets of iNK T cells, CD4+ or CD4- CD8- are known to exist, but the in vivo importance of CD4 expression is unclear. Since interleukin-12 (IL-12) is a key iNK T-cell-activating cytokine, the effect of IL-12 plus or minus the T-cell growth factor IL-2 on a large panel of CD4+ versus CD4- CD8- iNK T-cell clones was examined. Strikingly, IL-12 and IL-2 significantly activated iNK T cells to secrete IL-4, interferon-gamma and granulocyte-macrophage colony-stimulating factor, and up-regulated perforin expression in the absence of TCR stimulation. Furthermore, IL-2 and IL-12 treatment resulted in a preferential increase in apoptosis of CD4- CD8- clones. Thus, independent of TCR activation, IL-2 and IL-12 can directly activate iNK T cells and provide a selective advantage to the CD4+ iNK T-cell population.
