Abstract A33: Impact of Tumor-Derived Exosomes on CD28 Expression in T Cells

Abstract Osteosarcoma is a rare disease with a disproportionate impact, as it is seen mostly in children and adolescents. Despite aggressive treatment, more than 50% of patients succumb to their disease within 10 years of diagnosis, making safe and effective treatments a huge unmet need. Immunotherapy, including immune checkpoint blockade, is drastically changing cancer treatment by increasing the antitumor immune response. Mechanisms of resistance to immune checkpoint blockade are incompletely understood; therefore, there is currently no available method to specifically identify patients who will respond favorably to immunotherapy. Exosomes are extracellular vesicles that can carry proteins, lipids, mRNA, microRNA, and DNA and play a role in extracellular communication. Exosomes can be found in circulation, making them an attractive tool for diagnostic testing. One potential mechanism of immunotherapy resistance could be destabilization of the CD28 coreceptor in T cells by microRNAs secreted in tumor-derived exosomes. Utilizing next-generation RNA-sequencing, we previously identified 2 microRNAs in dogs and 2 microRNAs in humans that were inversely correlated with CD28 expression in 44 human and 39 canine osteosarcoma tissues. Current work entails isolating exosomes from canine and human osteosarcoma tumor cell lines. Using a species mismatched approach, we can identify canine microRNAs in human T cells that were treated with canine exosomes, and vice versa, to verify delivery of microRNAs to T cells via tumor-derived exosomes. We are also using the CRISPR-Cas system to genetically engineer a human osteosarcoma cell line (HOS) that expresses the exosome marker CD63 linked to GFP so we can track exosome uptake by T cells by flow cytometry and determine the effect on CD28 expression levels. Identifying mechanisms of resistance will lead to improved patient selection and identification of therapeutic targets to combat this resistance. Citation Format: Ashley J. Schulte, Lauren J. Mills, Kelly M. Makielski, Taylor A. DePauw, Jaime F. Modiano. Impact of tumor-derived exosomes on CD28 expression in T cells [abstract]. In: Proceedings of the AACR Special Conference on Advances in Liquid Biopsies; Jan 13-16, 2020; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(11_Suppl):Abstract nr A33.