Tumor microenvironment (TME) formation is a major cause of immunosuppression. The TME consists of a considerable number of macrophages and stromal cells that have been identified in multiple tumor types. CCL2 is the strongest chemoattractant involved in macrophage recruitment and a powerful initiator of inflammation. Evidence indicates that CCL2 can attract other host cells in the TME and direct their differentiation in cooperation with other cytokines. Overall, CCL2 has an unfavorable effect on prognosis in tumor patients because of the accumulation of immunosuppressive cell subtypes. However, there is also evidence demonstrating that CCL2 enhances the anti-tumor capability of specific cell types such as inflammatory monocytes and neutrophils. The inflammation state of the tumor seems to have a bi-lateral role in tumor progression. Here, we review works focusing on the interactions between cancer cells and host cells, and on the biological role of CCL2 in these processes.

Pyroptosis has been reported to improve the immunosuppressive tumor microenvironment and may be a strategy to enhance osteosarcoma treatment. The extent to which modulation of mitochondria could induce tumor pyroptosis to enhance immunotherapy has not been characterized. We synthesized a mitochondria-targeting polymer micelle (OPDEA-PDCA), in which poly[2-(N-oxide-N,N-diethylamino)ethyl methacrylate] (OPDEA) was used to target mitochondria and the conjugated dichloroacetate (DCA) was used to inhibit pyruvate dehydrogenase kinase 1 (PDHK1). This conjugate induced pyroptosis through initiation of mitochondrial oxidative stress. We found that OPDEA-PDCA targeted mitochondria and induced mitochondrial oxidative stress through the inhibition of PDHK1, resulting in immunogenic pyroptosis in osteosarcoma cell lines. Moreover, we showed that OPDEA-PDCA could induce secretion of soluble programmed cell death-ligand 1 (PD-L1) molecule. Therefore, combined therapy with OPDEA-PDCA and an anti-PD-L1 monoclonal antibody significantly suppressed proliferation of osteosarcoma with prolonged T cell activation. This study provided a strategy to initiate pyroptosis through targeted modulation of mitochondria, which may promote enhanced antitumor efficacy in combination with immunotherapy.