As observed with human samples, Ag-driven immune responses were notably enhanced in mice immunized with ovalbumin Ag, with increases in cell proliferation, and IFN-γ in cell culture supernatants following blockade in vitro (Fig. 5A, n = 4). Similar enhancements were observed when splenocytes from transgenic OT-II mice, which express the mouse CD4+ T-cell receptor specific for chicken ovalbumin 323–339, were incubated

with ovalbumin Ag in the presence of increasing amounts of anti-sCTLA-4 mAb (Fig. 5B). The examples shown here are typical of several experiments using a range of immunogens, all of which demonstrate that selective AZD1208 mw blockade of sCTLA-4 in vitro, enhances Ag-specific immune responses. We have also found that blockade of sCTLA-4 in vivo, in which mice were immunized under cover of 100 μg/mouse of anti-sCTLA-4 Ab, enhances Ag-specific immune responses (Fig. 5C and Supporting Information Fig. 4). Thus, we were able to address functional blockade of sCTLA-4 using the JMW-3B3 anti-sCTLA-4 mTOR inhibitor mAb in murine models of disease. Finally, given the promise of pan-specific anti-CTLA-4 Ab blockade in the treatment of tumors, including melanoma [30, 31, 34], we investigated whether selective blockade of sCTLA-4 also protected against metastatic melanoma spread in vivo. Mice were infused with

B16F10 melanoma cells and coadministered with anti-sCTLA-4 Ab JMW-3B3, pan-specific anti-CTLA-4 Ab, IgG1 isotype control, or left untreated (Fig. 5D). When mice were sacrificed and examined for metastatic melanoma in the lungs, blockade with either anti-sCTLA-4 or pan-specific anti-CTLA-4 Ab significantly reduced the mean number of metastatic foci

by 44 or 50%, respectively, Phosphoglycerate kinase compared with that with the IgG1 isotype control (p < 0.0001, Mann–Whitney U test). Thus, in this model, inhibition of tumor spread mediated by pan-specific anti-CTLA-4 mAb could be recapitulated by selective blockade of sCTLA-4. This study identifies a potentially important role for the alternatively spliced and secretable CTLA-4 isoform, sCTLA-4, as a contributor to immune regulation. We demonstrate that sCTLA-4 can be produced and has suppressive functions during human T-cell responses in vitro, that the Treg-cell population is a prominent source, and that specific blockade of the isoform can manipulate murine disease in vivo. The general relevance of CTLA-4 to regulatory activity is well recognized from previous work demonstrating both cell intrinsic and extrinsic inhibitory effects on T-cell responses [35, 36]. The sCTLA-4 isoform, in contrast, has received little attention, with interest largely arising because a single nucleotide polymorphism in the 3′ untranslated region of CTLA-4, which reduces sCTLA-4 expression, has been identified as a susceptibility factor for several autoimmune diseases [23, 24].