Role of NKT cells in the digestive system. IV. The role of canonical natural killer T cells in mucosal immunity and inflammation

Lymphocytes that combine features of T cells and natural killer (NK) cells are named natural killer T (NKT) cells. The majority of NKT cells in mice bear highly conserved invariant V alpha chains, and to date two populations of such canonical NKT cells are known in mice: those that express V alpha 14 and those that express V alpha 7.2. Both populations are selected by nonpolymorphic major histocompatibility complex class I-like antigen-presenting molecules expressed by hematopoietic cells in the thymus: CD1d for V alpha 14- expressing NKT cells and MR1 for those cells expressing V alpha 7.2. The more intensely studied V alpha 14 NKT cells have been implicated in diverse immune reactions, including immune regulation and inflammation in the intestine; the V alpha 7.2 expressing cells are most frequently found in the lamina propria. In humans, populations of canonical NKT cells are found to be highly similar in terms of the expression of homologous, invariant T cell antigen-receptor alpha-chains, specificity, and function, although their frequency differs from those in the mouse. In this review, we will focus on the role of both of these canonical NKT cell populations in the mucosal tissues of the intestine.