Most importantly, engagement of the GITR resulted in potent anti-tumor responses including eradication of established Meth-A sarcomas [8], poorly immunogenic B16 melanoma [9], and CT26 buy JQ1 colon tumors [10]. Conversely, inhibition of GITR/GITR-L interactions by administration of soluble GITR-Fc resulted in prolongation of allograft survival potentially by preventing GITR-L-mediated reversal of Treg-cell-mediated suppression [11]. GITR knockout mice and mice treated with a blocking GITR-Fc had reduced inflammation,

tissue damage, and reduced mortality in a model multiple organ failure [12]. While the costimulatory effects of GITR engagement on Teff cells are clear, controversial results have been reported on the effects of GITR engagement on Treg cells in vivo [11]. Some studies have demonstrated enhancement of Treg-cell numbers following treatment of mice with recombinant Fc-GITR-L [13] and mice expressing a GITR-L transgene in B cells had an increase in the ratio of Treg cells/Tconv cells and a delay in the onset of experimental autoimmune encephalitis [14].

Conversely, several studies in tumor models have described a decrease in the percentage of Foxp3+ T cells in the tumor, as well as a redistribution of the intracellular localization of Foxp3 [15]. However, interpretation of some of these studies that used anti-GITR mAbs is complicated as administration of anti-GITR in vivo can result in depletion of Treg cells [16]. In the present study, we have used Resminostat a nondepleting, recombinant Fc-GITR-L and combinations of GITR WT and GITR KO Treg cells and Teff cells to reexamine the effects of GITR PI3K inhibitor stimulation on each subpopulation in both unmanipulated mice and in a well-characterized model of inflammatory bowel disease (IBD). We demonstrate that the effects of that Fc-GITR-L-induced GITR signaling are complex and depend on the physiologic environment in the host as

well as the activation state of the Treg cells and Teff cells. The implications of these results regarding the therapeutic manipulation of the immune response by members of the TNFRSF are discussed. Previous studies have demonstrated that engagement of the GITR provides a costimulatory signal for activation of the proliferation of both CD4+ and CD8+ Foxp3− T cells in vitro [2, 3], while engagement of the GITR on Foxp3+ Treg cells in vitro stimulated their proliferation in the presence of IL-2, but in the absence of TCR stimulation [1]. To assess the effect of GITR engagement in vivo, we administered Fc-GITR-L, a nondepleting soluble recombinant protein dimer that has been shown to enhance tumor immunity [17] or human IgG1 as a control to unmanipulated mice. Fc-GITR-L administration in the absence of any other exogenous stimulation significantly increased Foxp3+ T-cell frequency and absolute numbers on day 3 after treatment (Fig. 1A–C).