Abstract
Metastasis remains the primary cause of small cell lung cancer (SCLC)-related deaths. Growing evidence links tumor metastasis with a pre-metastatic microenvironment characterized by an anti-inflammatory response, immunosuppression, and the presence of tumor-derived exosomes. To clarify the relationships among these factors in SCLC, we analyzed SCLC patient samples as well as a mouse model. Among the infiltrating immune cells, our study focused on the tumor-associated macrophages (TAMs), that are well-known to promote tumor progression and metastasis. We found that high expression of the alternatively activated (M2) TAM marker, CD206+ was associated clinically with a poorer prognosis and metastasis state in patients with SCLC. Moreover, infiltrating macrophages (MØ) were found in the metastatic foci of an SCLC mouse model. Additionally, we observed dominant switching to M2 phenotype, accompanied by increased NLRP6 expression. Since tumor-derived exosomes are the key links between the tumor and its immune microenvironment, we further investigated whether SCLC-derived exosomes contributed to the MØ phenotype switch. Our findings showed for the first time that SCLC-derived exosomes induce the M2 switch via the NLRP6/NF-κB pathway, and thus, promote SCLC metastasis in vitro and in vivo. Collectively, these results indicate a novel mechanism by which SCLC-derived exosomes induce immunosuppression of distant MØ to promote systemic metastasis by activating NLRP6. Here, we highlight the close relationship between the tumor-derived exosomes, inflammasomes and immune microenvironment in SCLC metastasis.