Abstract 2907: Exosome secretion is an inheritable property of cancer cells: Single-cell profiling of exosome secretion
Authors: Fathi Mohsen , Joseph Robiya , Martinez-Paniagua Melisa , Adolacion Jay R , An Xingyue , Mahendra Ankit , Gabrusiewicz Konrad , Chatterjee Sujash , Mani Sendurai A. , Varadarajan Navin 
Published: August 2019

Abstract
Decades of research on exosomes, nano-sized vesicles secreted by cells, have revealed novel roles of these vesicles in the formation of pre-metastasis niches that enhances the migration of tumor cells to those sites. Paradoxically, more recent work has suggested that these tumor-derived exosomes can also have an immunostimulatory role, depending on the model studied. The rate of secretion of exosomes by single cells is likely heterogeneous, and a deep profiling of the secretion capacity of individual tumor cells has been largely unexplored.
We have developed a high-throughput single-cell methodology to quantify the dynamic secretion of exosomes from single cells using a modified immunoassay and sought to define: (a) heterogeneity among individual cells within the tumor cell population, and (b) the nature of the cells secreting exosomes within a tumor cell population. In order to investigate these, we chose to study models of triple negative breast cancer: the metastatic tumor lines, 4T1 (mouse) and MDA-MB-231(human); and the non-metastatic lines 67NR (mouse) and MCF7 (human). Our method revealed that MDA-MB-231 single cells secreted exosomes at a rate, ~2 fold higher than MCF7 cells. Surprisingly, the non-metastatic 67NR cells showed a higher secretion rate (~1.5 fold) than metastatic 4T1 cells. Next, we performed single-cell RNA-seq on 67NR and 4T1 single cells. Consistent with our single-cell exosomal profiling results, 67NR cells were significantly (p-value &lt; 0.01) enriched for genes correlated to exosome formation in comparison to the 4T1 cells. Next, in order to study exosomes of single cells, we isolated single cells using an automated micromanipulator which allowed us to establish monoclonal cell lines. Measurement of these monoclonal cells showed that secretor cells of 67NR secrete exosomes with ~2 fold higher rate than non-secretor cells.
Since our in vitro results clearly demonstrated that the secretor clonal cell lines had a higher frequency of exosome secreting single cells, we sought to define the in vivo relevance of these results. Consistent with the hypothesis that the exosomes from 67NR are immune-stimulatory, injection of the secretor clones into Balb/c mice led to lack of primary tumor formation (2/15 mice had tumors) whereas the injection of the non-secretor clones led to tumor formation in 7/15 mice.
In aggregate, our results show that the higher rate of secretion of exosomes from non-metastatic cells can facilitate tumor rejection in vivo. We are currently performing in vitro studies with the 67NR and MDA-MB-231 exosomes to study their impact on immune cells.
Citation Format: Mohsen Fathi, Robiya Joseph, Melisa Martinez-Paniagua, Jay R Adolacion, Xingyue An, Ankit Mahendra, Konrad Gabrusiewicz, Sujash Chatterjee, Sendurai A. Mani, Navin Varadarajan. Exosome secretion is an inheritable property of cancer cells: Single-cell profiling of exosome secretion [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2907.