Exosomes from Myeloid-Derived Suppressor Cells Carry Biologically Active Proteins

Myeloid-derived suppressor cells (MDSC) are present in most cancer patients where they inhibit natural anti-tumor immunity and are an obstacle to anticancer immunotherapies. They mediate immune suppression through their production of proteins and soluble mediators that prevent the activation of tumor-reactive T lymphyocytes, polarize macrophages toward a tumor-promoting phenotype, and facilitate angiogenesis. The accumulation and suppressive potency of MDSC is regulated by inflammation within the tumor microenvironment. Recently exosomes have been proposed to act as intercellular communicators, carrying active proteins and other molecules between sender cells and receiver cells. In this report we describe the proteome of exosomes shed by MDSC induced in BALB/c mice by the 4T1 mammary carcinoma. Using bottom-up proteomics, we have identified 412 proteins. Spectral counting identified 63 proteins whose abundance was altered >2.-fold in the inflammatory environment. The pro-inflammatory proteins S100A8 and S100A9, previously shown to be secreted by MDSC and to be chemotactic for MDSC, are abundant in MDSC-derived exosomes. Bioassays reveal that MDSC-derived exosomes polarize macrophages toward a tumor-promoting type 2 phenotype, in addition to possessing S100A8/A9 chemotactic activity. These results suggest that some of the tumor-promoting functions of MDSC are implemented by MDSC-shed exosomes.