Macrophages and Therapeutic Resistance in Cancer
Authors: Brian Ruffell, Lisa M Coussens
Journal: Cancer Cell (2015)

How neoplastic cells respond to therapy is not solely dependent on the complexity of the genomic aberrations they harbor but is also regulated by numerous dynamic properties of the tumor microenvironment. Identifying and targeting critical pathways that improve therapeutic efficacy by bolstering anti-tumor immune responses holds great potential for improving outcomes and impacting long-term patient survival. Macrophages are key regulators of homeostatic tissue and tumor microenvironments. Therefore, therapeutics impacting macrophage presence and/or bioactivity have shown promise in preclinical models and are now being evaluated in the clinic. This review discusses the molecular/cellular pathways identified so far whereby macrophages mediate therapeutic responses. How neoplastic cells respond to therapy is not solely dependent on the complexity of the genomic aberrations they harbor but is also regulated by numerous dynamic properties of the tumor microenvironment. Identifying and targeting critical pathways that improve therapeutic efficacy by bolstering anti-tumor immune responses holds great potential for improving outcomes and impacting long-term patient survival. Macrophages are key regulators of homeostatic tissue and tumor microenvironments. Therefore, therapeutics impacting macrophage presence and/or bioactivity have shown promise in preclinical models and are now being evaluated in the clinic. This review discusses the molecular/cellular pathways identified so far whereby macrophages mediate therapeutic responses. Macrophages are represented in all tissues by functionally and phenotypically distinct resident populations that are critical for development and homeostasis (Wynn et al., 2013Wynn T.A. Chawla A. Pollard J.W. Macrophage biology in development, homeostasis and disease.Nature. 2013; 496: 445-455Crossref PubMed Scopus (462) Google Scholar). Under non-pathological conditions, most resident macrophage populations derive from embryonic progenitors and are maintained through local proliferation (Epelman et al., 2014Epelman S. Lavine K.J. Randolph G.J. Origin and functions of tissue macrophages.Immunity. 2014; 41: 21-35Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar). Exceptions to this include intestinal, dermal, and alveolar macrophages at barrier sites (Bain et al., 2014Bain C.C. Bravo-Blas A. Scott C.L. Gomez Perdiguero E. Geissmann F. Henri S. Malissen B. Osborne L.C. Artis D. Mowat A.M. Constant replenishment from circulating monocytes maintains the macrophage pool in the intestine of adult mice.Nat. Immunol. 2014; 15: 929-937Crossref PubMed Scopus (7) Google Scholar, McGovern et al., 2014McGovern N. Schlitzer A. Gunawan M. Jardine L. Shin A. Poyner E. Green K. Dickinson R. Wang X.N. 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Misharin A. et al.Fate mapping reveals origins and dynamics of monocytes and tissue macrophages under homeostasis.Immunity. 2013; 38: 79-91Abstract Full Text Full Text PDF PubMed Scopus (359) Google Scholar) and macrophages in the adult heart that are replaced by circulating bone marrow-derived Ly6C+ inflammatory monocytes over a timescale of several weeks (Molawi et al., 2014Molawi K. Wolf Y. Kandalla P.K. Favret J. Hagemeyer N. Frenzel K. Pinto A.R. Klapproth K. Henri S. Malissen B. et al.Progressive replacement of embryo-derived cardiac macrophages with age.J. Exp. Med. 2014; 211: 2151-2158Crossref PubMed Scopus (37) Google Scholar). Under pathological conditions, there is evidence for both local proliferation and recruitment, with differences observed by tissue location and type of inflammatory insult (Epelman et al., 2014Epelman S. Lavine K.J. Randolph G.J. Origin and functions of tissue macrophages.Immunity. 2014; 41: 21-35Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar). Solid tumors appear to be unique. Preclinical studies indicate minimal macrophage proliferation and shorter half-lives compared with resident macrophages in counterpart homeostatic tissues, measurable in days to weeks (Movahedi et al., 2010Movahedi K. Laoui D. Gysemans C. Baeten M. Stangé G. Van den Bossche J. Mack M. Pipeleers D. In't Veld P. De Baetselier P. Van Ginderachter J.A. Different tumor microenvironments contain functionally distinct subsets of macrophages derived from Ly6C(high) monocytes.Cancer Res. 2010; 70: 5728-5739Crossref PubMed Scopus (268) Google Scholar, Strachan et al., 2013Strachan D.C. Ruffell B. Oei Y. Bissell M.J. Coussens L.M. Pryer N. Daniel D. CSF1R inhibition delays cervical and mammary tumor growth in murine models by attenuating the turnover of tumor-associated macrophages and enhancing infiltration by CD8(+) T cells.OncoImmunology. 2013; 2: e26968Crossref PubMed Scopus (31) Google Scholar). That said, CD68+ cells also positive for proliferating cell nuclear antigen (PCNA) expression have been observed in breast cancers, where they are associated with a poor clinical outcome (Campbell et al., 2011Campbell M.J. Tonlaar N.Y. Garwood E.R. Huo D. Moore D.H. Khramtsov A.I. Au A. Baehner F. Chen Y. Malaka D.O. et al.Proliferating macrophages associated with high grade, hormone receptor negative breast cancer and poor clinical outcome.Breast Cancer Res. Treat. 2011; 128: 703-711Crossref PubMed Scopus (62) Google Scholar). Whether the macrophage lifespan in this context is reflecting diminished tissue integrity and the extent of damage/inflammation or, instead, represents an adaptive process engaged by tumors to support growth is unclear, but production of the C-C chemokine ligand 2 (CCL2) and/or colony-stimulating factor 1 (CSF-1) are necessary to sustain their numbers (Noy and Pollard, 2014Noy R. Pollard J.W. Tumor-associated macrophages: from mechanisms to therapy.Immunity. 2014; 41: 49-61Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar). With the critical role for CCL2 and CSF-1 in recruiting macrophages to neoplastic tissue, there is growing interest in therapeutics targeting these ligands and/or their respective receptors in an effort to ablate the pro-tumorigenic properties of macrophages. This therapeutic approach has led to improved outcomes in a range of pre-clinical models, particularly for agents targeting CSF-1 or the CSF-1 receptor (CSF-1R), the results of which have spurred several clinical trials (Table 1).Table 1Macrophage Therapeutic TargetingPathwayTargetaOnly targets with clinical compounds are listed.Efficacy in Murine ModelsClinical CompoundsClinical Trials in Solid TumorsbData obtained from https://clinicaltrials.gov.RecruitmentCD11bradiation, chemotherapyrovelizumabCSF-1Rsingle agent (GBM, PDAC), chemotherapy, radiation, angiogenesis inhibitorsPLX3397, AMG820NCT01596751 (O); NCT01444404 (C); NCT01349036 (O); NCT01004861 (O); NCT01346358 (O); NCT02265536 (O); NCT01494688 (O); NCT02323191 (O)IMC-CS4/LY3022855,RG7155/RO5509554CCL2single agent (metastasis, PDAC)carlumabNCT00992186 (C); NCT01204996 (C)Neuropilin-1angiogenesis inhibitorsMNRP1685ANCT00747734 (C); NCT00954642 (C)ANG2single agent (mammary), chemotherapy, angiogenesis inhibitorsnesvacumabNCT01271972 (O); NCT01688960 (O)PolarizationIL-4single agent (metastasis), chemotherapy, radiationpascolizumabIL4RαdupilumabIL-13chemotherapylebrikizumab, tralokinumab, GSK679586,FcγRchemotherapyrituximab (CD20), ibrutinib (BTK), R788 (Syk)FunctionIL-6clazakizumab, olokizumab, siltuximab, sirukumabNCT00433446 (C); NCT00385827 (C)NCT00841191 (C)IL-6Rtocilizumab, sarilumabTNF-αmitogen-activated protein kinase inhibitorsadalimumab, certolizumab, etanercept, golimumab, infliximabActivationCD40single agent (PDAC), chemotherapyCP-870,893NCT00711191 (C); NCT01456585 (C)NCT02157831 (C); NCT01008527 (O)NCT02225002 (C); NCT00607048 (C)NCT01103635 (O)O, ongoing; C, completed.a Only targets with clinical compounds are listed.b Data obtained from https://clinicaltrials.gov. Open table in a new tab O, ongoing; C, completed. As monotherapy, CSF-1R inhibition alone impedes the growth of orthotopically implanted pancreatic ductal adenocarcinoma (PDAC) cell lines (Mitchem et al., 2013Mitchem J.B. Brennan D.J. Knolhoff B.L. Belt B.A. Zhu Y. Sanford D.E. Belaygorod L. Carpenter D. Collins L. Piwnica-Worms D. et al.Targeting tumor-infiltrating macrophages decreases tumor-initiating cells, relieves immunosuppression, and improves chemotherapeutic responses.Cancer Res. 2013; 73: 1128-1141Crossref PubMed Scopus (115) Google Scholar), prevents cervical carcinogenesis (Strachan et al., 2013Strachan D.C. Ruffell B. Oei Y. Bissell M.J. Coussens L.M. Pryer N. Daniel D. CSF1R inhibition delays cervical and mammary tumor growth in murine models by attenuating the turnover of tumor-associated macrophages and enhancing infiltration by CD8(+) T cells.OncoImmunology. 2013; 2: e26968Crossref PubMed Scopus (31) Google Scholar), and induces regression of glioblastoma multiforme (GBM) (Pyonteck et al., 2013Pyonteck S.M. Akkari L. Schuhmacher A.J. Bowman R.L. Sevenich L. Quail D.F. Olson O.C. Quick M.L. Huse J.T. Teijeiro V. et al.CSF-1R inhibition alters macrophage polarization and blocks glioma progression.Nat. Med. 2013; 19: 1264-1272Crossref PubMed Scopus (174) Google Scholar). In other tumor models CSF-1R inhibition is without consequence as monotherapy. However, synergism with other modalities, including chemotherapy (DeNardo et al., 2011DeNardo D.G. Brennan D.J. Rexhepaj E. Ruffell B. Shiao S.L. Madden S.F. Gallagher W.M. Wadhwani N. Keil S.D. 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Together, these findings implicate macrophages in regulating therapeutic responses and indicate that durable responses may be obtained by augmenting standard of care or emerging therapies with "macrophage antagonists." This review focuses on the mechanisms underpinning these observations and concludes with a discussion of targeting approaches that extend beyond inhibiting macrophage recruitment. For many solid tumor types, high densities of cells expressing macrophage-associated markers have generally been found to be associated with a poor clinical outcome (Figure 1; Komohara et al., 2014Komohara Y. Jinushi M. Takeya M. Clinical significance of macrophage heterogeneity in human malignant tumors.Cancer Sci. 2014; 105: 1-8Crossref PubMed Scopus (6) Google Scholar, Zhang et al., 2012Zhang Q.W. Liu L. Gong C.Y. Shi H.S. Zeng Y.H. Wang X.Z. Zhao Y.W. Wei Y.Q. 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In humans, however, CD68 expression is widespread and includes granulocytes, dendritic cells, fibroblasts, endothelial cells, and some lymphoid subsets (Gottfried et al., 2008Gottfried E. Kunz-Schughart L.A. Weber A. Rehli M. Peuker A. Müller A. Kastenberger M. Brockhoff G. Andreesen R. Kreutz M. Expression of CD68 in non-myeloid cell types.Scand. J. Immunol. 2008; 67: 453-463Crossref PubMed Scopus (57) Google Scholar, Hameed et al., 1994Hameed A. Hruban R.H. Gage W. Pettis G. Fox 3rd, W.M. Immunohistochemical expression of CD68 antigen in human peripheral blood T cells.Hum. Pathol. 1994; 25: 872-876Abstract Full Text PDF PubMed Scopus (17) Google Scholar, Ruffell et al., 2012bRuffell B. Au A. Rugo H.S. Esserman L.J. Hwang E.S. Coussens L.M. Leukocyte composition of human breast cancer.Proc. Natl. Acad. Sci. USA. 2012; 109: 2796-2801Crossref PubMed Scopus (83) Google Scholar). The use of CD68 for association studies in this context is therefore of variable utility. A clear example of this is non-small-cell lung cancer, where detection of the macrophage scavenger receptors CD163 and CD204, but not CD68, yielded correlations with negative outcome (Chung et al., 2012Chung F.T. Lee K.Y. Wang C.W. Heh C.C. Chan Y.F. Chen H.W. Kuo C.H. Feng P.H. Lin T.Y. Wang C.H. et al.Tumor-associated macrophages correlate with response to epidermal growth factor receptor-tyrosine kinase inhibitors in advanced non-small cell lung cancer.Int. J. Cancer. 2012; 131: E227-E235Crossref PubMed Scopus (10) Google Scholar, Hirayama et al., 2012Hirayama S. Ishii G. Nagai K. Ono S. Kojima M. Yamauchi C. Aokage K. Hishida T. Yoshida J. Suzuki K. Ochiai A. Prognostic impact of CD204-positive macrophages in lung squamous cell carcinoma: possible contribution of Cd204-positive macrophages to the tumor-promoting microenvironment.J. Thorac. Oncol. 2012; 7: 1790-1797Abstract Full Text Full Text PDF PubMed Scopus (9) Google Scholar, Ohri et al., 2011Ohri C.M. Shikotra A. Green R.H. 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These correlations may indicate that macrophage polarization can direct clinical outcome, as also supported by the positive association between the presence of CD68+ cells and survival in colorectal adenocarcinoma (Roxburgh and McMillan, 2012Roxburgh C.S. McMillan D.C. The role of the in situ local inflammatory response in predicting recurrence and survival in patients with primary operable colorectal cancer.Cancer Treat. Rev. 2012; 38: 451-466Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar). Unlike most populations of tumor-associated macrophages that possess pro-tumor and immunosuppressive properties (Biswas and Mantovani, 2010Biswas S.K. Mantovani A. Macrophage plasticity and interaction with lymphocyte subsets: cancer as a paradigm.Nat. Immunol. 2010; 11: 889-896Crossref PubMed Scopus (771) Google Scholar), macrophages in human colorectal cancer have been found to be functionally (and phenotypically) anti-tumor (Edin et al., 2012Edin S. Wikberg M.L. Dahlin A.M. 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Together, these data collectively support the tenet that repolarizing macrophages toward an anti-tumor phenotypic state, either by impeding activities/signals that drive pro-tumor polarization or by delivering exogenous signals that enhance anti-tumor polarization, could act as an alternative and perhaps more efficacious approach to blocking macrophage recruitment, even though these activities and responses are all dynamically regulated in vivo. Indeed, an agonist monoclonal antibody against CD40, a co-stimulatory protein found on professional antigen-presenting cells, has demonstrated efficacy in mouse models of PDAC (Beatty et al., 2011Beatty G.L. Chiorean E.G. Fishman M.P. Saboury B. Teitelbaum U.R. Sun W. Huhn R.D. Song W. Li D. Sharp L.L. et al.CD40 agonists alter tumor stroma and show efficacy against pancreatic carcinoma in mice and humans.Science. 2011; 331: 1612-1616Crossref PubMed Scopus (463) Google Scholar) and patients with PDAC (Beatty et al., 2013Beatty G.L. 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Klaman I. et al.Targeting tumor-associated macrophages with anti-CSF-1R antibody reveals a strategy for cancer therapy.Cancer Cell. 2014; 25: 846-859Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar), these are the first clinical studies to demonstrate the potential efficacy of macrophage-targeted agents. Macrophages produce an array of cytokines, chemokines, polypeptide growth factors, hormones, matrix-remodeling proteases, and metabolites, many of which possess tumor-promoting activities (De Palma and Lewis, 2013De Palma M. Lewis C.E. Macrophage regulation of tumor responses to anticancer therapies.Cancer Cell. 2013; 23: 277-286Abstract Full Text Full Text PDF PubMed Scopus (161) Google Scholar, Noy and Pollard, 2014Noy R. Pollard J.W. Tumor-associated macrophages: from mechanisms to therapy.Immunity. 2014; 41: 49-61Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar, Ruffell et al., 2012aRuffell B. Affara N.I. Coussens L.M. Differential macrophage programming in the tumor microenvironment.Trends Immunol. 2012; 33: 119-126Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar). A caveat to some of these reported activities is that many findings originate from cell culture studies utilizing neoplastic myeloid cell lines or bone marrow-derived macrophages and, therefore, cannot account for the complex milieu of polarization signals to which macrophages would be exposed in vivo (Figure 2). This includes the aforementioned CSF-1 and CCL2, prostaglandin E2 (PGE2), and damage-associated molecular patterns (DAMPs) such as high-mobility group box 1 protein (HMGB1), extracellular ATP, and degraded extracellular matrix components (Ruffell et al., 2012aRuffell B. Affara N.I. Coussens L.M. Differential macrophage programming in the tumor microenvironment.Trends Immunol. 2012; 33: 119-126Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar, Zelenay and Reis e Sousa, 2013Zelenay S. 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Different tumor microenvironments contain functionally distinct subsets of macrophages derived from Ly6C(high) monocytes.Cancer Res. 2010; 70: 5728-5739Crossref PubMed Scopus (268) Google Scholar). The recruitment of macrophages (presumably MHCIILO) into hypoxic regions through Neuropilin-1 also supports an immunosuppressive phenotype (Casazza et al., 2013Casazza A. Laoui D. Wenes M. Rizzolio S. Bassani N. Mambretti M. Deschoemaeker S. Van Ginderachter J.A. Tamagnone L. Mazzone M. Impeding macrophage entry into hypoxic tumor areas by Sema3A/Nrp1 signaling blockade inhibits angiogenesis and restores antitumor immunity.Cancer Cell. 2013; 24: 695-709Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar) that is likely dependent upon HIF-1α (Doedens et al., 2010Doedens A.L. Stockmann C. Rubinstein M.P. Liao D. Zhang N. DeNardo D.G. Coussens L.M. Karin M. Goldrath A.W. Johnson R.S. Macrophage expression of hypoxia-inducible factor-1 alpha suppresses T-cell function and promotes tumor progression.Cancer Res. 2010; 70: 7465-7475Crossref PubMed Scopus (147) Google Scholar). Surprisingly, however, although hypoxia can induce HIF-1α-dependent expression of arginase-1 in macrophages (Doedens et al., 2010Doedens A.L. Stockmann C. Rubinstein M.P. Liao D. Zhang N. DeNardo D.G. Coussens L.M. Karin M. Goldrath A.W. Johnson R.S. Macrophage expression of hypoxia-inducible factor-1 alpha suppresses T-cell function and promotes tumor progression.Cancer Res. 2010; 70: 7465-7475Crossref PubMed Scopus (147) Google Scholar), neither improving tumor oxygenation nor preventing macrophage recruitment into hypoxic areas alters arginase-1 expression (Casazza et al., 2013Casazza A. Laoui D. Wenes M. Rizzolio S. Bassani N. Mambretti M. Deschoemaeker S. Van Ginderachter J.A. Tamagnone L. Mazzone M. 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