PEG-rIL-10 treatment decreases FoxP3(+) Tregs despite upregulation of intratumoral IDO.

IL-10 has been classically defined as a broad-spectrum immunosuppressant and is thought to facilitate the development of regulatory CD4(+) T cells. IL-10 is believed to represent one of the major suppressive factors secreted by IDO(+)FoxP3(+)CD4(+) Tregs. Contrary to this view, we have previously reported that PEGylated recombinant IL-10 (PEG-rIL-10) treatment of mice induces potent IFNγ and CD8(+) T-cell-dependent antitumor immunity. This hypothesis is currently being tested in clinical trials and we have reported that treatment of cancer patients with PEG-rHuIL-10 results in inhibition and regression of tumor growth as well as increased serum IFNγ. We have continued to assess PEG-rIL-10's pleiotropic effects and report that treatment of tumor-bearing mice and humans with PEG-rIL-10 increases intratumoral indoleamine 2, 3-dioxygenase (IDO) in an IFNγ-dependent manner. This should result in an increase in Tregs, but paradoxically our data illustrate that PEG-rIL-10 treatment of mice reduces intratumoral FoxP3(+)CD4(+) T cells in an IDO-independent manner. Additional investigation indicates that PEG-rIL-10 inhibits TGFβ/IL-2-dependent in vitro polarization of FoxP3(+)CD4(+) Tregs and potentiates IFNγ(+)T-bet(+)CD4(+) T cells. These data suggest that rather than acting as an immunosuppressant, PEG-rIL-10 may counteract the FoxP3(+)CD4(+) Treg suppressive milieu in tumor-bearing mice and humans, thereby further facilitating PEG-rIL-10's potent antitumor immunity.