FOXP3+ cells in both PB and LN yielded positive staining with Akt inhibitor the newly developed anti-murine/human Helios antibody clone 22F6, consistent with the notion that they were naturally occurring Treg cells. Stimulation of mononuclear cells of LN origin with concanavalin A (Con A) in vitro yielded increased proportions and median fluorescence intensity of FOXP3 expression by both CD4+ and CD8+ T cells. Removal of the Con A and continued culture disclosed a CD4+ FOXP3high population, distinct from the CD4+ FOXP3intermediate T cells; very few CD8+ FOXP3high T cells were observed, though CD8+ FOXP3intermediate cells were present in

equal abundance to CD4+ FOXP3intermediate cells. The CD4+ FOXP3high T cells were thought to represent activated Treg cells, in contrast to the FOXP3intermediate cells, which were thought to be a more heterogeneous population comprising predominantly activated conventional T cells. Co-staining with interferon-γ (IFN-γ) supported this notion, because the FOXP3high T cells were almost exclusively IFN-γ−, whereas

the FOXP3intermediate cells expressed a more heterogeneous IFN-γ phenotype. Following activation of mononuclear cells with Con A and interleukin-2, the 5% of CD4+ T cells showing the highest CD25 expression (CD4+ CD25high) were enriched in cells Talazoparib in vivo expressing FOXP3. These cells were anergic in vitro, in contrast to the 20% of CD4+ T cells with the lowest CD25 expression (CD4+ CD25−), which proliferated readily. The CD4+ CD25high FOXP3high T cells were able to suppress the proliferation of

responder CD4+ T cells in vitro, in contrast to the CD4+ CD25− cells, which showed no regulatory properties. Regulatory T (Treg) cells play a crucial role in the maintenance of peripheral tolerance.1,2 Abnormalities of Treg-cell number or function have been implicated in several autoimmune3–5 and allergic6–8 diseases, and Treg cells play a pivotal role in the maintenance Sitaxentan of allograft tolerance.9–11 Despite limiting collateral damage in the immune response against certain microbes, Treg cells have also been implicated in the pathogenesis of a number of infectious diseases – either by promoting persistence of the pathogen by inhibiting anti-microbial effector responses or by acting as a cellular reservoir of the pathogen.12–15 Such pathomechanisms have been demonstrated in both rodents and higher mammals, including veterinary species: for example, Treg cells are known to be a reservoir of productive feline immunodeficiency virus infection16–19 and are induced in the periphery by porcine reproductive and respiratory syndrome virus.20,21 The manipulation of Treg-cell number or function therefore holds promise as a novel therapy for infectious disease or as a component of more effective vaccination strategies.