Tumour-associated macrophage polarisation and re-education with immunotherapy

Monocytes/macrophages constitute important contributors of cancer-associated inflammation. Through their plasticity and capacity to become polarised by tumours towards less activatory and more immunosuppressive (M2) phenotypes, tumour-associated macrophages (TAM) are thought to support tumour progression. Orchestrated by T helper 2 (Th2)-biased stimuli, macrophage recruitment, activation and polarisation in tumour microenvironments is associated with poorer clinical outcomes. Their key roles in supporting tumour progression and their capacity for plasticity have focused targeted and immunotherapeutic strategies to counteract macrophage pro-tumourigenic activities and to re-ignite their tumour-cytotoxic power. Therapeutic approaches include blockade of macrophage recruitment into tumours, suppression of TAM survival, re-polarisation towards an M1-like phenotype and antibody therapies to enhance TAM anti-tumoural activities. Future immunotherapeutic directions may include monoclonal antibodies with enhanced effector functions. Antibodies of different classes, including those of the IgE class, shown to restrict tumour growth by harnessing monocyte/macrophage cytotoxic properties in pre-clinical cancer models, may synergise or re-educate these potent immune sentinels to destroy rather than support tumours. Opportunities for monitoring monocyte/macrophage polarisation or activatory signatures in patients may inform clinical management.