Convergent pathways of macrophage polarization: The role of B cells

The construction of an inflammatory microenvironment provides the fuel for cancer development and progression. Hence, solid tumors promote infiltration of leukocyte populations, among which tumor-associated macrophages (TAM) represent a paradigm for cancer-promoting inflammation. TAM orchestrate various aspects of cancer, including diversion and skewing of adaptive responses, cell growth, angiogenesis, matrix deposition and remodelling, the construction of a metastatic niche and actual metastasis, response to hormones and chemotherapeutic agents. Several lines of evidence indicate that TAM show a remarkable degree of plasticity and functional heterogeneity, suggesting that during tumor progression macrophages undergo a phenotypic "switch", eventually exhibiting the alternatively activated, "M2", phenotype that is associated with immunosuppression, promotion of tumor angiogenesis and metastasis. Although recent studies have attempted to address the role of microenvironmental signals on TAM "reprogramming", the interplay between innate and adaptive immunity is emerging as a crucial step of this event. In this issue of the European Journal of Immunology, a study demonstrates that B1 lymphocytes expressing IL-10 play a key role in promoting a pro-tumoral M2-biased phenotype of macrophages. This article defines a new in vivo pathway of macrophage polarization and suggests that targeting B cells is a possible therapeutic intervention to reinstate anti-cancer functions by TAM.