Glycolipid stimulation of invariant NKT cells mobilizes precursors of mature NK cells and potentiates their participation in immune surveillance against metastatic cancer.

Abstract

Abstract Precursors of mature natural killer (pre-mNK) cells, initially called IFN-producing killer dendritic cells, have been recently characterized as a novel intermediate in NK cell differentiation. Typified by a unique B220+NK1.1+CD11c+MHCII+ phenotype, pre-mNK cells exhibit prolific anti-tumor cytotoxicity while retaining the ability to present antigens thereby activating neighbouring T cells. Invariant NKT (iNKT) cells are another population highlighted for their potential in anti-cancer immunity. They are activated very early and play an important role in transactivating downstream effectors, such as NK cells. The extent to which iNKT cells can activate pre-mNK cells is unknown. We hypothesized that iNKT cells are able to activate pre-mNK cells and potentiate their anti-tumor properties. Wildtype C57BL/6 mice were injected i.p. with the iNKT cell superagonist, α-galactosylceramide (αGC), and various lymphoid tissues were harvested after 5 days for phenotyping or cytotoxicity assays. We have shown, for the first time that iNKT cell activation via αGC results in robust expansion of pre-mNK cells in the spleen, lung and most notably, in the liver. These cells exhibited cytotoxic activity against YAC-1 thymoma and B16 melanoma cancer cells via the granule exocytosis pathway and significantly contributed to overall NK cytotoxic activity in vivo. The anti-cancer responses due to αGC activation have been well documented; however, the contribution of pre-mNK cells in these responses have never been shown. Our findings demonstrate that pre-mNK cells rapidly expand due to iNKT activation while retaining their capacity to kill tumor targets. This may suggest a novel mechanism of targeting pre-mNK cells in anti-cancer therapies.