Background: Type I IFN-based therapies against solid malignancies have yielded only limited success. How IFN affects tumor-associated macrophage (TAM) compartment to impact the therapeutic outcomes are not well understood.
Methods: The effect of an IFN-inducer poly(I:C) on tumor-infiltrating monocytes and TAMs were analyzed using a transplantable mouse tumor model (LLC). In vitro culture systems were utilized to study the direct actions by poly(I:C)-IFN on differentiating monocytes.
Results: We found that poly(I:C)-induced IFN targets Ly6C+ monocytes and impedes their transition into TAMs. Such an effect involves miR-155-mediated suppression of M-CSF receptor expression, contributing to restricting tumor growth. Remarkably, further analyses of gene expression profile of IFN-treated differentiating monocytes reveal a strong induction of Arg1 (encoding arginase-1) in addition to other classical IFN targets. Mechanistically, the unexpected Arg1 arm of IFN action is mediated by a prolonged STAT3 signaling in monocytes, in conjunction with elevated macrophage colony-stimulating factor (M-CSF) signaling. Functionally, induction of ARG1 limited the therapeutic effect of IFN, as inhibition of arginase activity could strongly synergize with poly(I:C) to enhance CD8+ T cell responses to thwart tumor growth in mice.
Conclusions: Taken together, we have uncovered two functionally opposing actions by IFN on the TAM compartment. Our work provides significant new insights on IFN-mediated immunoregulation that may have implications in cancer therapies.
Keywords: Anti-tumor immunity; Arginase; M-CSF; Monocyte maturation; Tumor-associated macrophages; Type I IFN.
Copyright © 2018 The Authors. Published by Elsevier B.V. All rights reserved.