Abstract
NK1.1 and its human homolog CD161 are expressed on NK cells, subsets of CD4(+) and CD8(+) T cells, and NKT cells. While the expression of NK1.1 is thought to be inhibitory to NK cell function, it is reported to play both costimulatory and coinhibitory roles in T-cells. CD161 has been extensively studied and characterized on subsets of T-cells that are MR1-restricted, IL-17 producing CD4(+) (T(H)17 MAIT cells) and CD8(+) T cells (Tc17 cells). Non-MAIT, MR1-independent CD161-expressing T-cells also exist and are characterized as generally effector memory cells with a stem cell like phenotype. Gene expression analysis of this enigmatic subset indicates a significant enhancement in the expression of cytotoxic granzyme molecules and innate like stress receptors in CD8(+)NK1.1(+)/CD8(+)CD161(+) cells in comparison to CD8(+) cells that do not express NK1.1 or CD161. First identified and studied in the context of viral infection, the role of CD8(+)CD161(+) T-cells, especially in the context of tumor immunology, is still poorly understood. In this review, the functional characteristics of the CD161-expressing CD8(+) T cell subset with respect to gene expression profile, cytotoxicity, and tissue homing properties are discussed, and application of this subset to immune responses against infectious disease and cancer is considered.