IL-35 is an immunosuppressive cytokine and exerts regulatory effects on T cells, B cells, macrophages and dendritic cells. Neutrophils are important innate immune cells that play key roles in tumor development. The effect of IL-35 on neutrophils remains unknown. Here, we report that IL-35 can induce N2 neutrophil polarization (protumor phenotype) by increasing G-CSF and IL-6 production, and promote neutrophil infiltration into tumor microenvironment. The sustained expression of IL-35 could promote chronic inflammation to augment the proangiogenic and immunosuppressive function of neutrophils. IL-35 stimulated macrophages to secrete proinflammatory cytokines IL-1β and IL-6. IL-1β stimulated γδ T cells to produce IL-17, which in turn increased the production of G-CSF. By increasing the expression of G-CSF and IL-6, IL-35 could up-regulate the expression of MMP-9 and Bv8, and down-regulate TRAIL expression in neutrophils, thus augmenting the proangiogenic function of neutrophils. Moreover, G-CSF/IL-6 induced the enhanced activation of STAT3 and ERK pathways in neutrophils, thus increasing the expression of iNOS to suppress T cell activation. Our findings suggest that IL-35 can promote tumor progression by functioning as an up-stream cytokine to promote cancer-associated inflammation and control neutrophil polarization. Targeting IL-35 might be an important approach for designing new strategy of tumor therapy.

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Circulating tumor cells (CTCs) have been studied well in the prognosis for malignant diseases as liquid biopsy, but their contribution to tumor metastasis is not clearly defined. Here we report that CTCs could promote the metastatic colonization of disseminated carcinoma cells by inducing systemic inflammation and neutrophil recruitment to pre-metastatic organs. Depletion of neutrophils in vivo could effectively abrogate the promoting effect of CTCs on tumor cell metastasis. In the presence of CTCs, the pro-tumor function of neutrophils was augmented, whereas the antitumor function of neutrophils was suppressed. Mechanically, CTC-derived ligands for TLR2 and TLR4 (TLR2/4) induced the systemic inflammation, thus increasing the production of proinflammatory cytokines such as G-CSF and IL-6 that could induce the conversion of neutrophil function from tumor-suppressing to tumor-promoting. Moreover, CTCs induced the production of endogenous TLR2/4 ligands such as S100A8, S100A9, and SAA3, which may amplify the stimulating effect that induces the expression of proinflammatory cytokines. The promoting effect of CTCs on tumor cell metastasis could be abrogated by suppressing inflammatory response with IL-37, an anti-inflammatory cytokine, or blocking CTC-derived ligands for TLR2/4. Identification of the metastatic axis of CTCs/systemic inflammation/neutrophils may provide potential targets for preventing tumor cell metastasis.

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