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

ABSTRACTIL-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 Fox...