Macrophages/microglia in glioblastoma: a Zelig-like story of changing phenotypes

In the framework of an increasing number of human cancers being effectively tackled by novel and innovative therapeutic approaches, high-grade glioblastoma (GBM) multiforme stands as a most difficult task for our hopes of effective therapies, or cures. GBM usually localizes at the level of subcortical white matter, in the cerebral hemispheres. Probably because the tumor arises within an immunological sanctuary, immune responses elicited by GBM appear to involve mostly the innate immune system: up to 50% of human GBM tumor mass is represented by tumor-associated macrophages (TAMs), which include both resident microglia and bone marrow-derived macrophages. The source of TAMs, the relative contribution of resident microglia and systemic macrophages, as well as the putative role of TAMs in tumor growth are currently a matter of thorough investigation and dispute.