1). Excessive Treg activity is observed in persistent

infections such as murine models of Leishmaniasis, malaria and tuberculosis [39–41] and in human diseases such as upper GI persistence of Helicobacter pylori, human immunodeficiency virus (HIV) and hepatitis C virus (HCV) infections [42–45], suggesting the possibility of a link between pathogen persistence and Treg-mediated suppression. Subversion of Treg function for the generation of appropriate immune responses to effect efficient pathogen clearance may therefore be an advantage or, indeed, a necessity. Indeed, accumulating evidence supports the MK0683 supplier assertion that interactions between Tregs and an infective/inflammatory environment leads to the subversion of their suppressive function. The salient experiments demonstrate a direct effect of Toll-like receptor (TLR) ligation on Tregs to block their suppression [46,47] and modulation of dendritic cell (DC) activity by lipopolysaccharide (LPS) to induce restricted Treg activity [48] in a manner that is

independent of direct ligation of the TLR on Tregs[49,50]. Indeed, appropriately activated DC can break the ‘anergic’ state of Tregs and promote proliferation in this usually hypoproliferative population [51]. Our own (unpublished) observations and those of others suggest that proinflammatory cytokines, Ku-0059436 cost in particular IL-1β, IL-6 and tumour necrosis factor (TNF)-α, released by DC following interaction with pathogens, can subvert the suppressive effects of Tregs. Both IL-1β and IL-6 can block Treg-mediated suppression of effector cell proliferation [48,52], although IL-6 may require the presence of IL-1 to overcome regulation [49]. There are some data from humans to suggest that TNF-α

can inhibit Treg function [53] with some supporting, but circumstantial, evidence showing a numerical increase in forkhead box P3 (FoxP3)+ Casein kinase 1 T cells and restoration of defective regulatory function in patients with rheumatoid arthritis treated with anti-TNF-α therapy [54]. The inevitable question is whether subverted Tregs remain ‘dormant’ Tregs or undergo a stable change of phenotype to an alternative lineage. IL-17 is a proinflammatory cytokine with non-redundant functions in the clearance of extracellular pathogens (see also [55] for further detail). This is seen readily in both IL-17R-deficient mice, which demonstrate great susceptibility to lethal bacterial infections [56,57], and in IL-17-deficient humans as part of the hyper-immunoglobulin E (IgE) syndrome (HIES), where recurrent infections are a feature [58,59]. The significant proinflammatory features of IL-17 have been reviewed previously, as has the compelling evidence for the role of IL-17 in inflammatory/autoimmune conditions of mice and the considerable body of evidence suggesting an important role for IL-17 in the aetiopathogenesis of inflammatory and autoimmune diseases in humans [60,61].