One of the foremost mysteries about iNKT cells is how they are able to mediate such contrasting immunological effects

as find more promoting tumour rejection or clearance of microbial infections, and preventing or ameliorating autoimmune diseases. Previous studies have established that the iNKT cell population contains functionally distinct subsets; for example, CD4− iNKT cells appear to be biased towards production of Th1 cytokines and expression of perforin, whereas CD4+ iNKT cells produce both Th1 and Th2 cytokines and are more notable for up-regulating FAS-ligand after stimulation.37 Thus, it is possible that different iNKT cell subsets become activated in different situations, and mediate distinct effects. This could be a result of differential anatomical localization of iNKT subsets, or of different costimulation requirements. However, as described in the next paragraph, it is not clear that different iNKT cell subsets recognize distinct antigens. Because of their canonical TCR rearrangements, all iNKT cells share the ability to recognize a specific molecular ‘pattern’ in which a galactose or glucose sugar is attached in an α-anomeric conformation to the polar head group of a lipid.38,39 The prototypical synthetic lipid of this type, α-galactosylceramide

(α-GalCer), is a highly potent agonist for iNKT cells.15 Lipids with structural similarity to α-GalCer have been identified from several microbial sources, including a pathogenic Borrelia species.40–43 However, these microbial analogues Selleck SB525334 of α-GalCer generally appear to be substantially weaker TCR agonists than α-GalCer

itself. Importantly, mammalian cells do not seem to produce glycolipids in which the first sugar is attached to the lipid via an α-linkage, and thus the self antigens Integrase inhibitor recognized by iNKT cells apparently do not contain this molecular pattern. The nature of the self antigens recognized by iNKT cells will be discussed at the end of the review; suffice it to note here that there is also as yet no clear evidence that iNKT self-antigen specificities differ according to subset. Another possibility (not mutually exclusive with the subset model) is that the same iNKT cell can mediate distinct functional effects as a result of variations in the activation stimuli in different contexts. We have recently shown that iNKT cells produce cytokines hierarchically in response to increasing TCR signal strength: granulocyte–macrophage colony-stimulating factor (GM-CSF) and IL-13 are activated by exposure to low doses of α-GalCer, higher levels of α-GalCer increase secretion of these cytokines and also induce IFN-γ and IL-4, and production of IL-2 requires the highest amounts of antigen.