Hematopoietic expression of the atypical chemokine receptor 2 inhibits the recruitment of anti-metastatic neutrophils

Purpose: The atypical chemokine receptor 2 (ACKR2) is a scavenger receptor for most inflammatory CC chemokines. It plays a protective role in chronic inflammation, autoimmunity and inflammation-related cancer. The aim of this project is to investigate the role of ACKR2 in primary tumor development and metastatization in different mouse model. Methods: NeuT mice (overexpressing the rat oncogene Her2) crossed with ACKR2-/- mice, orthotopic and intravenous injection of the breast carcinoma cell line 4T1 in Wild Type (WT) and ACKR2-/- Balb/c mice and intravenous injection of the melanoma cell line B16F10 in WT and ACKR2-/- C57BL/6 mice. Tumors and metastasis were studied with in vivo imaging systems, immunohistochemistry and qPCR analysis; blood composition and metastatic organ infiltrate were studied with FACS analysis. Results: Tumor arising in ACKR2-/- NeuT mice showed a more aggressive phenotype when compared to WT NeuT mice. On the contrary, ACKR2-/- NeuT mice were protected from spontaneous lung metastasis. Metastasis protection was registered also using the 4T1 orthotopic model. FACS analysis indicated an increased of Ly6G+ polymorphonuclear cells and Ly6Chigh monocytes in the blood and in the pre-metastatic lungs of ACKR2-/- mice. Depletion experiments demonstrated that neutrophils are implied in metastasis protection. Moreover, ACKR2-/- neutrophils express increased levels of the chemokine receptor CCR2 and have increased CCR2 dependent cytotoxic activity. Discussion: ACKR2 is described to regulate leukocytes recruitment shaping chemokine in tissues but our evidences indicate ACKR2 as a direct regulator of immune cells activity. Conclusion: These results indicate that ACKR2 expression has a dual but opposite role in tumor, inhibiting primary tumor growth but promoting lung metastatization trough the inhibition of anti-metastatic neutrophils.