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.

Loss of expression of the recycling receptor, FcRn, promotes tumor cell growth by increasing albumin consumption.

Tumor cells rely on high concentrations of amino acids to support their growth and proliferation. Although increased macropinocytic uptake and lysosomal degradation of the most abundant serum protein, albumin, in Ras-transformed cells can meet these demands, it is not understood how the majority of tumor cells that express wild type Ras achieve this. In the current study we reveal that the neonatal Fc receptor, FcRn, regulates tumor cell proliferation through the ability to recycle its ligand, albumin. By contrast with normal epithelial cells, we show that human FcRn is present at very low or undetectable levels in the majority of tumor cell lines analyzed. Remarkably, shRNA-mediated ablation of FcRn expression in an FcRn-positive tumor cell line results in a substantial growth increase of tumor xenografts, whereas enforced expression of this receptor by lentiviral transduction has the reverse effect. Moreover, intracellular albumin and glutamate levels are increased by the loss of FcRn-mediated recycling of albumin, combined with hypoalbuminemia in tumor-bearing mice. These studies identify a novel role for FcRn as a suppressor of tumor growth and have implications for the use of this receptor as a prognostic indicator and therapeutic target.