Natural killer cell receptor-expressing innate lymphocytes: more than just NK cells

Recently, additional subsets that extend the family of innate lymphocytes have been discovered. Among these newly identified innate lymphoid cells is a subset sharing phenotypic characteristics of natural killer cells and lymphoid tissue inducer cells. These cells co-express the transcription factor ROR gamma t and activating NK cell receptors (NKR), but their lineage and functional qualities remain poorly defined. Here, we discuss recent proposals to place these NKR+ROR gamma t(+) innate lymphocytes on hematopoietic lineage maps. An overview of the transcriptional circuitry determining fate decisions of innate lymphocytes and a summary of current concepts concerning plasticity and stability of innate lymphocyte effector fates are provided. We will conclude by discussing the function of ROR gamma t-expressing innate lymphocytes during inflammatory bowel diseases and in the immune response to tumors.