NK and NKT Cells
Natural Killer (NK) cells are cytotoxic lymphocytes that play a primary role in innate immunity, and are specialized to kill tumor cells and virus infected cells. NK cells secrete toxic enzymes, as well as cytokines and chemokines which serve to recruit other immune effector cells in order to remove targeted tumor or virus infected cells. NK cell subsets have been identified that may express CD8a homodimers along with CD56, which are proposed to facilitate their cytotoxic function.
NKT cells are, as their name implies, a unique type of T cell with properties similar to both T cells and NK cells. NKTs do express the alpha beta TCR, as well as certain cell surface proteins shared by NK cells. NKT cells secrete cytokines important to Th1 and Th2 responses, including IL-4 and IFN gamma, the latter of which serves to activate NK cells. More recently, an NKT subset known as invariant NKT (iNKT) cells has been identified to secrete large amounts of IFN gamma and IL-4, and also to express the “Th17” transcription factor and cytokine, ROR gamma and IL-17, respectively. Both NK and NKT cells display the CD161 receptor, which is proposed to regulate the function of each cell type.