title: 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.affiliation: Institute for Cancer Research, Department of Medicine I, Comprehensive Cancer Center, Medical University of Vienna, Vienna, Austria.
authors.forename: Barbara
authors.initials: B
authors.lastname: Drobits
authors.forename: Martin
authors.initials: M
authors.lastname: Holcmann
authors.forename: Nicole
authors.initials: N
authors.lastname: Amberg
authors.forename: Melissa
authors.initials: M
authors.lastname: Swiecki
authors.forename: Roland
authors.initials: R
authors.lastname: Grundtner
authors.forename: Martina
authors.initials: M
authors.lastname: Hammer
authors.forename: Marco
authors.initials: M
authors.lastname: Colonna
authors.forename: Maria
authors.initials: M
authors.lastname: Sibilia
mesh_terms: D056704:Adaptive Immunity / Q000276:immunology*; D000276:Adjuvants, Immunologic / Q000627:therapeutic use*; D000634:Aminoquinolines / Q000494:pharmacology / Q000627:therapeutic use*; D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology / Q000627:therapeutic use*; D045744:Cell Line, Tumor; D018932:Chemokine CCL2 / Q000276:immunology; D003713:Dendritic Cells / Q000166:cytology / Q000276:immunology*; D006801:Humans; D000077271:Imiquimod; D008545:Melanoma; D051379:Mice; D008810:Mice, Inbred C57BL; D008811:Mice, Inbred DBA; D018345:Mice, Knockout; D053594:Myeloid Differentiation Factor 88 / Q000235:genetics / Q000276:immunology; D009374:Neoplasms, Experimental / Q000188:drug therapy* / Q000276:immunology; D053633:Receptor, Interferon alpha-beta / Q000276:immunology; D015398:Signal Transduction / Q000276:immunology; D012867:Skin / Q000187:drug effects / Q000276:immunology; D051199:Toll-Like Receptor 7 / Q000235:genetics / Q000276:immunology
chemical_list: D000276:Adjuvants, Immunologic; D000634:Aminoquinolines; D000970:Antineoplastic Agents; D018932:Chemokine CCL2; C508032:Ifnar1 protein, mouse; C507780:Myd88 protein, mouse; D053594:Myeloid Differentiation Factor 88; D051199:Toll-Like Receptor 7; D053633:Receptor, Interferon alpha-beta; D000077271:Imiquimod