The role of antigen presentation in tumor-associated macrophages

Macrophages are cells of the myeloid lineage with important roles not only in immune regulation and tissue repair, but also in pathological states such as autoimmune disease and cancer. A plethora of macrophage subtypes exist with distinct phenotypes and functions, not least within the tumor microenvironment (TME) of solid tumors. The abundant macrophages located within the TME are often referred to as tumor-associated macrophages (TAMs). TAMs may be pro-inflammatory with antitumor properties, or may have pro-tumor functions such as angiogenesis. Typically, TAMs are endowed with pro-tumor phenotypes, which has led to strategies to deplete or reprogram TAMs within the TME. Although historically recognized as professional antigen presenting cells (APCs), macrophages are often considered inferior in their abilities to process and present antigens in comparison with dendritic cells (DCs). Notwithstanding, this review gives an overview of the potential accessory role that macrophages might have in antigen processing and presentation to T cells within the TME, with implications for the design of novel immunotherapies.