Imiquimod clears tumors in mice independent of adaptive immunity by converting pDCs into tumor-killing effector cells
Barbara Drobits, … , Marco Colonna, Maria Sibilia
Barbara Drobits, … , Marco Colonna, Maria Sibilia
Published February 1, 2012; First published January 17, 2012
Citation Information: J Clin Invest. 2012;122(2):575-585. https://doi.org/10.1172/JCI61034.
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Research Article

Imiquimod clears tumors in mice independent of adaptive immunity by converting pDCs into tumor-killing effector cells

Abstract

Imiquimod is a synthetic compound with antitumor properties; a 5% cream formulation is successfully used to treat skin tumors. The antitumor effect of imiquimod is multifactorial, although its ability to modulate immune responses by triggering TLR7/8 is thought to be key. Among the immune cells suggested to be involved are plasmacytoid DCs (pDCs). However, a direct contribution of pDCs to tumor killing in vivo and the mechanism of their recruitment to imiquimod-treated sites have never been demonstrated. Using a mouse model of melanoma, we have now demonstrated that pDCs can directly clear tumors without the need for the adaptive immune system. Topical imiquimod treatment led to TLR7-dependent and IFN-α/β receptor 1–dependent (IFNAR1-dependent) upregulation of expression of the chemokine CCL2 in mast cells. This was essential to induce skin inflammation and for the recruitment of pDCs to the skin. The recruited pDCs were CD8α+ and induced tumor regression in a TLR7/MyD88- and IFNAR1-dependent manner. Lack of TLR7 and IFNAR1 or depletion of pDCs or CD8α+ cells from tumor-bearing mice completely abolished the effect of imiquimod. TLR7 was essential for imiquimod-stimulated pDCs to produce IFN-α/β, which led to TRAIL and granzyme B secretion by pDCs via IFNAR1 signaling. Blocking these cytolytic molecules impaired pDC-mediated tumor killing. Our results demonstrate that imiquimod treatment leads to CCL2-dependent recruitment of pDCs and their transformation into a subset of killer DCs able to directly eliminate tumor cells.

Authors

Barbara Drobits, Martin Holcmann, Nicole Amberg, Melissa Swiecki, Roland Grundtner, Martina Hammer, Marco Colonna, Maria Sibilia

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Figure 1

TLR7/MyD88-independent effects of Imi in the skin.

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TLR7/MyD88-independent effects of Imi in the skin. Western blot analysis...
Western blot analysis of primary keratinocytes (KCs) isolated from (A) C57BL/6 (WT), (B) Tlr7–/–, and (C) Myd88–/– mice treated with 12 μg/ml Imi or LAL reagent water (control) for the indicated time points. Results are representative of at least 3 independent batches. White lines indicate that samples were run on the same gel but were noncontiguous. (D) Flow cytometric analysis showing annexin V+ cells in primary keratinocyte cultures stimulated with Imi (12 μg/ml) or LAL water for 40 hours (n = 3–4 per group). (E) Apoptosis was measured by active (act.) caspase-3 staining of epidermal sheets from ears of WT, Tlr7–/–, and Myd88–/– mice treated topically with Imi for 7 days or left untreated (Co). Caspase-3+ cells were counted in 10 randomly chosen fields of at least 3 independent samples. *P < 0.05, **P < 0.005.