Macrophages in the Tumor Microenvironment

Solid tumors consist of neoplastic cells, non-malignant stromal cells and migratory haematopoietic cells. Complex interactions between the cell types in this microenvironment regulate tumor growth, progression, metastasis and angiogenesis. There is also strong evidence that this microenvironment is inflammatory and that activation of the innate immune system plays a role in the progression of cancer. One such inflammatory cell that has the potential to promote cancer progression is the macrophage. There is a growing body of pre-clinical and clinical evidence associating abundance of tumor-associated macrophages (TAM) with poor prognosis. According to Condeelis and Pollard, TAM are obligate partners for malignant cell migration, invasion and metastases in many different cancers. These conclusions are based not only on association studies, but also on experiments that show ablation of macrophage function, or their infiltration into experimental tumors, inhibits growth and metastases.