Eosinophils: The unsung heroes in cancer?

Figures & data

Figure 1. Life-cycle models of eosinophils. Eosinophils are produced in the bone marrow through a series of progenitors from the most immature stem cells (Hematopoietic Stem Cells: HSC), Multipotent Progenitor Cell: MPP) to committed eosinophil progenitors (Semper, Muehlberg et al.) that finally generate mature eosinophils. This process of proliferation and differentiation is driven by transcription factors (e.g. GATA-1). IL-5, together with IL-3 and GM-CSF, controls eosinophil development in the bone marrow. Recent evidence indicates that IL-33 precedes IL-5 in regulating eosinophilopoiesis via the activation of the IL-33R receptor, ST2. Circulating human eosinophils selectively express IL-5Rα, CCR3, EMR1, CRTH2, and Siglec-8. Eosinophil can leave the bloodstream and target the bone marrow, liver, and spleen for elimination. In some tissues, eosinophils can be phagocytosed by macrophages. Eosinophils express a wide spectrum of integrins (α and β) and can roll and adhere to VCAM-1 and ICAM-1 on activated endothelial cells. This interaction favors the chemotactic activity of several chemokines (eotaxin-1/CCL11, eotaxin-2/CCL24, eotaxin-3/CCL26, and RANTES) that activate the CCR3 receptor highly expressed on eosinophils. This explains the propensity of eosinophils to leave the bloodstream and migrate into inflamed tissues and certain tumors. Several cells (fibroblasts, epithelial and endothelial cells, smooth muscle cells, T cells, macrophages, and eosinophils itself) are a major source of these chemokines. IL-5, IL-33 and presumably other cytokines locally produced by both immune cells (eosinophils, ILC2, T cells, and mast cells) in tumor microenvironment and by tumor cells can prolong the life span of eosinophils at site of tumor growth.

Table 1. Anti-tumorigenic activity of eosinophils in experimental tumors.

Type of Cancer	Model	Eosinophil detection	Observed effects
Colon	Human Colo-205 cell line	May-Grünwald-GiemsaAnti-EPX, Anti-MBP	Eosinophils induce direct tumor cell killing through adherence and release of cytotoxic granules in vitro (Gatault, Delbeke et al. 2015; Legrand, Driss et al. 2010)
Fibrosarcoma	Mice implanted with MCA-induced fibrosarcoma cells	Carbol's chromotrope-hematoxylin	Eosinophils protect from tumor growth in vivo and induce direct tumor cell killing in vitro (Simson, Ellyard et al. 2007)
Hepatocellular	Mice implanted with MH134 hepatocellular carcinoma cells	Anti-EPX	Eosinophils inhibit tumor growth in vivo and exert tumor citotoxicity in vitro (Kataoka, Konishi et al. 2004)
Hodgkin's lymphoma	Human T-cell and B-cell lymphoma cell lines	N.I.	ECP is cytotoxic for Hodgkin's lymphoma tumor cells in vitro (Glimelius, Rubin et al. 2011)
Melanoma	Mice implanted with B16.F10 melanoma cells	Anti-Siglec-F	Eosinophils inhibit tumor growth and lung metastasis in vivo and induce tumor cell killing in vitro (Carretero, Sektioglu et al. 2015) (Lucarini, Ziccheddu et al. 2017)
	Mouse model of experimental lung melanoma B16.F10 metastasis	Anti-ECP	Eosinophil infiltration into the lung prevents tumor metastasis in vivo (Ikutani, Yanagibashi et al. 2012)
	Mouse model of experimental lung melanoma B16.F10 metastasis	Carbol's chromotrope-hematoxylin; Anti-MBP	Influx of degranulating eosinophils into the lung prevents tumor metastasis in vivo and eosinophil lysates are cytotoxic for tumor cells in vitro (Mattes, Hulett et al. 2003)
Prostate	Human DU 145 and PC-3 cell lines	N.I.	Activated eosinophils from allergic and asthmatic individuals inhibit tumor cell growth in vitro (Furbert-Harris, Parish-Gause et al. 2003)

EPX: eosinophil peroxide; MBP: major basic protein; ECP: eosinophil cationic protein; N.I.: not indicated.

Table 2. Role of eosinophils in clinical cancers.

Effect on cancer	Type of Cancer	Eosinophil staining	Tissue of detection	Clinical outcome
Anti-tumoral	Colorectal	H&E EPX	Tumor (stroma)	Eosinophil score associated with decreased risk for all-cause and colorectal cancer death (Prizment, Vierkant et al. 2016)
		H&E	Tumor (peritumoral)	Eosinophils have a significant favorable impact on prognosis of colorectal cancer patients (Harbaum, Pollheimer et al. 2015)
		H&E	Tumor	High eosinophil counts are associated with a significantly better prognosis following surgery (Fernandez-Acenero, Galindo-Gallego et al. 2000)
		H&E	Tumor	High eosinophil counts correlate with prolonged OS (Nielsen, Hansen et al. 1999)
		Giemsa	Tumor	Eosinophils significantly correlate with absence of metastasis and prolonged OS (Pretlow, Keith et al. 1983)
	Gastric	N.I.	Tumor Blood	High pre-operative eosinophil counts correlate with prolonged OS after surgery (Iwasaki, Torisu et al. 1986)
	Larynx	H&E	Tumor	Presence of eosinophils correlate with prolonged OS after surgery (Thompson, Bradley et al. 1994)
	Nasopharyngeal	Luna	Tumor	Eosinophils in EGFR-positive tumors correlate with better prognosis (Fujii, Yamashita et al. 2002)
	Oral	Congo red	Tumor	High eosinophil score negatively correlates with metastatic grade (Jain, Kasetty et al. 2014)
		Morphometric	Tumor	Eosinophil score correlate with prolonged OS (Dorta, Landman et al. 2002)
	Breast	N.I.	Blood	Low pre-operative eosinophils counts correlates risk of recurrent disease after surgery (Ownby HE, Roi LD et al. Cancer 1983)
	Melanoma	Smear test	Blood	Eosinophil counts correlate with OS at any disease stage independently of therapy (Moreira, Leisgang et al. 2017)
Pro-tumoral	Cervical	H&E	Tumor	Eosinophil infiltration increases with cancer progression (Xie, Liu et al. 2015)
		H&E	Tumor	Eosinophil infiltration predicts a worse OS (van Driel, Hogendoorn et al. 1996)
	Hodgkin's lymphoma	H&E	Tumor	Eosinophils are unfavorable prognostic factors for OS and freedom from treatment failure (Enblad, Sundstrom et al. 1993) (von Wasielewski, Seth et al. 2000)
	Leukemic/ lymphoma	Morphometric	Blood	Eosinophils are unfavorable prognostic factors for OS (Utsunomiya, Ishida et al. 2007)

H&E: haematoxilyn and eosin; EPX: eosinophil peroxide; N.I.: not indicated.

Figure 2. Role of eosinophils in survival rate of cancer patients. Kaplan-Meier plots depict the overall survival rate of all the indicated cancer types in function of the expression of indicated genes in cancer tissues. Survival events (vertical black lines) of patients were stratified based on the low (red color) or high expression (green color) of Siglec8 (Sialic Acid Binding Immunoglobulin-like Lectin 8, GenBank/Entrez ID: 27181), EPX (Eosinophil Peroxidase, GenBank/Entrez ID: 8288), and CLC (Charcot-Leyden crystal protein, GenBank/Entrez ID: 1178), three specific markers for human eosinophils. The survival rate data were extracted from patients in publicly available expression array datasets by the web-based tool SurvExpress (http://bioinformatica.mty.itesm.mx:8080/Biomatec/SurvivaX.jsp). Each graph depicts a distinct patient's cohort. LS, Liposarcoma; DLBCL, Diffuse Large B cell lymphoma; AC, Adenocarcinoma; UC: Urothelial carcinoma; GB, Glioblastoma. P_Cox, Log-Rank (Mentel-Cox) P value calculated by SurvExpress. P values below 0.05 were considered statistically significant.

Figure 3. Roles of eosinophils in human and experimental tumors. In green boxes are indicated those tumors in which eosinophils play a protective role. In red boxes those tumors in which eosinophils play a pro-tumorigenic role. In mixed red/green boxes are presented tumors in which eosinophils play both a pro- and anti-tumorigenic role in different studies.

Figure 4. Possible mechanisms by which eosinophils and their mediators play a protumorigenic or an antitumorigenic role. Eosinophils in TME can promote tumor initiation and progression through the release of ROS, angiogenic factors, metalloproteinase-9, and the induction of epithelial-to-mesenchymal transition. Eosinophils can exhibit antitumor activity through direct tumor cell cytotoxicity mediated by ROS, granzyme, TNF-α, eosinophil cationic proteins (ECP, EPX, MBP), and IL-18. IFN-γ produced by eosinophils favors M1 polarization of TAMs. Eosinophils can also exert antitumor activity indirectly through the attraction of CD8⁺ T cells via the production of CCL5, CXCL9, and CXCL10.