However, here we concentrate on evidence for differential sensitivity as measured by T cell effector functions. Thornton and Shevach described

a co-culture system to measure Treg-mediated suppression that not only provided important mechanistic data on the requirements for suppression, but also laid down a template for demonstrating the functional activity of Tregs. The classical suppression assay involves the co-culture of CD25+ Tregs and CD25– responder T cells over a range of suppressor : responder ratios and measurement of the extent to which Tregs restrain the proliferation of CD25– T cells [40]. There is almost no area of Treg selleck screening library biology which has not been assessed by some modification of this basic technique. This assay has been used to compare the regulatory function of different subsets of Tregs[64], of in vitro-activated versus freshly explanted Tregs[65–68], of Tregs from sites of inflammation [69], of nTregs and iTregs[26] and of Tregs in infected versus healthy mice [70] and humans [71]. The findings of

many of these studies informed further in-vivo experiments and they have greatly enhanced our knowledge of Treg function. However, the specificity and activation status of regulatory and effector T cell populations as well as the cytokines present in the microenvironment and the activation status of antigen-presenting cells (APCs) will influence the capacity of Tregs to suppress in vivo. These conditions are often not well modelled in vitro and this caveat represents the greatest limitation of this type of assay. Particularly in mice, Torin 1 concentration most often the responder population used for in vitro suppression assays are CD4+CD25– T cells from naive mice, and such cells are highly susceptible to Treg-mediated suppression.

Indeed, it has been suggested that the window of susceptibility to Treg-induced suppression in vitro is regulated tightly and restricted to the first 12 h of stimulation [72]. Limiting the Carbohydrate proliferation or cytokine production of highly activated polarized T cells is a much more demanding task, and this may be why a clear comparison of the capacity of Tregs to limit the activity of polarized Th1, Th2 and Th17 cells is missing from the literature. It has been shown, however, that while Tregs can suppress the priming of Th2 responses, they are unable to suppress the proliferation or cytokine production of established Th2 effectors unless they themselves are pre-activated in vitro[73]. The importance of the comparative activation status of effectors and Tregs has been well illustrated. Tregs at sites of inflammation, for example, are typically more highly activated than peripheral Tregs[74,75], and this draws into question the extrapolation of functional assays carried out using mismatched responder : suppressor co-cultures and argues in favour of sampling both Tregs and effector T cells from the tissue of interest wherever possible [44,69,76].