The innate-like natural killer T (NKT) cells are essential regulators of immunity. These cells comprise at least two distinct subsets and recognize different lipid antigens presented by the MHC class I like molecules CD1d. The CD1d-dependent recognition pathway of NKT cells is highly conserved from mouse to humans. While most type I NKT cells can recognize αGalCer and express a semi-invariant T cell receptor (TCR), a major population of type II NKT cells reactive to sulfatide utilizes an oligoclonal TCR. Furthermore TCR recognition features of NKT subsets are also distinctive with almost parallel as opposed to perpendicular footprints on the CD1d molecules for the type I and type II NKT cells respectively. Here we present a view based upon the recent studies in different clinical and experimental settings that while type I NKT cells are more often pathogenic, they may also be regulatory. On the other hand, sulfatide-reactive type II NKT cells mostly play an inhibitory role in the control of autoimmune and inflammatory diseases. Since the activity and cytokine secretion profiles of NKT cell subsets can be modulated differently by lipid ligands or their analogs, novel immunotherapeutic strategies are being developed for their differential activation for potential intervention in inflammatory diseases.
KeywordsCD1; cancer; glycolipids; NKT cells; sulfatide; Th1/Th17
NKT CELL SUBSETSThe multifaceted immune system orchestrates interactions between cells to eliminate invading pathogens and maintain homeostatic regulation of self-tolerance. It consists both of cells which respond to antigens never before encountered (the adaptive immune response) and cells which, through evolutionary selection, have been preprogrammed to respond to common environmental antigens (the innate immune response). Among the innate like lymphocyte populations of the immune system are natural killer T (NKT) cells, comprised of at least two subsets, which bridge the innate and adaptive immune responses (see Fig. 1 The pathogenic or protective effects of NKT subsets following stimulation in vivo are determined by their cytokine secretion profile, timing of activation, and the lipid antigens involved. Understanding how these subsets' TCRs bind to their ligands and how they crossregulate each other should lead to the development of novel strategies of immune regulation for intervention in autoimmune diseases in a human leukocyte antigen (HLA)-independent manner (see Fig. 1). Overwhelming evidence of the regulatory function for both type I and type II NKT cells makes them extremely attractive targets for immunotherapeutic manipulation.
NKT CELLS RECOGNIZE BOTH EXOGENOUS AND ENDOGENOUS LIPIDS
INVOLVEMENT AND MODULATION OF TYPE I AND TYPE II NKT CELLS IN
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