CD1d-Independent NKT Cells in β2-Microglobulin-Deficient Mice Have Hybrid Phenotype and Function of NK and T Cells

Unlike CD1d-restricted NK1.1+TCRαβ+ (NKT) cells, which have been extensively studied, little is known about CD1d-independent NKT cells. To characterize their functions, we analyzed NKT cells in β2-microglobulin (β2m)-deficient B6 mice. They are similar to NK cells and expressed NK cell receptors, including Ly49, CD94/NKG2, NKG2D, and 2B4. NKT cells were found in normal numbers in mice that are deficient in β2m, MHC class II, or both. They were also found in the male HY Ag-specific TCR-transgenic mice independent of positive or negative selection in the thymus. For functional analysis of CD1d-independent NKT cells, we developed a culture system in which CD1d-independent NKT cells, but not NK, T, or most CD1d-restricted NKT cells, grew in the presence of an intermediate dose of IL-2. IL-2-activated CD1d-indpendent NKT cells were similar to IL-2-activated NK cells and efficiently killed the TAP-mutant murine T lymphoma line RMA-S, but not the parental RMA cells. They also killed β2m-deficient Con A blasts, but not normal B6 Con A blasts, indicating that the cytotoxicity is inhibited by MHC class I on target cells. IL-2-activated NKT cells expressing transgenic TCR specific for the HY peptide presented by Db killed RMA-S, but not RMA, cells. They also killed RMA (H-2b) cells that were preincubated with the HY peptide. NKT cells from β2m-deficient mice, upon CD3 cross-linking, secreted IFN-γ and IL-2, but very little IL-4. Thus, CD1d-independent NKT cells are significantly different from CD1d-restricted NKT cells. They have hybrid phenotypes and functions of NK cells and T cells.