Battle over CCL2 for control of the metastatic niche: neutrophils versus monocytes.

Tumor-derived factors, such as proinflammatory cytokines, can increase the hospitality of metastatic sites by recruiting and activating leukocytes to perform supporting roles during metastatic dissemination. These same cytokines, however, are natural danger signals for the immune system and as such can induce anti-tumor immune responses by both adaptive and innate immune cells. The outcome of tumor-derived inflammatory cytokines is probably closely related to the exact repertoire of factors produced by each tumor. Several recent studies have investigated these seemingly contradictory roles of tumor-derived CCL2 with significant clinical implications.