NK and NKT cells: The innate–adaptive interface including humoral responses

Publisher Summary Tissue inflammation induced by antigens and pathogens varies considerably, but many diseases have a common pathology including stress and activation of epithelial cells that promotes innate immune responses, providing the host with its first line of defense against infections. Signals generated by subsets of innate lymphocytes, including natural killer (NK) cells, natural killer T (NKT) cells, and antigen presenting cells (APCs) during this early host response determine the nature of downstream adaptive immune responses. NK cells represent a population of specialized lymphocytes capable of recognizing and eliminating a wide range of cancer and virus-infected cells but not normal cells. The function of NK cells is regulated by a fine balance of inhibitory and activating signals, which are mediated by a diverse array of cell-surface receptors. Natural killer (NK) and natural killer T (NKT) cells represent unique lymphoid subsets of the innate immune system that are both critical for some aspects of initiating and directing host adaptive immune responses.These cells share numerous phenotypic markers and functional features, yet are lymphocytes of distinct developmental lineages.As first responders, these innate cells rapidly produce cytokines that modulate various activities of other leukocytes which subsequently influence the ensuing inflammatory response.Moreover, through the expression of effector molecules such as Fas and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) these cells are capable of directly targeting infected or transformed cells for elimination through cytotoxic pathways.Furthermore, both NK and NKT cells polarize T (helper) cell responses and enhance antigen presentation to cytotoxic T-lymphocytes.