Abstract 5622: scRNA-seq reveals restructuring of the immune microenvironment of multiple myeloma following treatment with an IAP antagonist
Authors: Stein Caleb K. , Meermeier Erin , Welsh Seth , Garbitt Victoria , Sharik Meaghen , Bergsagel P. Leif , Banovich Nicholas , Keats Jonathan , Chesi Marta 
Published: September 2020

Abstract
Previously, the IAP antagonist LCL161 was shown to have robust anti-myeloma activity in a transgenic mouse model where an acute inflammatory response accompanied a reduction in tumor burden. In order to fully examine alterations to the immune microenvironment, murine splenocytes from twenty mice transplanted with Vk12598 myeloma and administered single-agent treatment regimens of LCL161, cyclophosphamide, or bortezomib were harvested after 24 hours and subjected to single cell RNA-seq sequencing.
After initial removal of erythrocytic and mitochondrial expressing cells, count matrices of over 50,000 total cells span twenty murine whole spleen samples. In an examination of the myeloid component, we identified 13 clusters of immune cells with unique compositions across samples due to differing treatment regimens and initial tumor burden. While minimally present in all seven non-transplanted mice, macrophages with elevated expression of complement activation system genes (C1q) were the most abundant myeloid cell population in each of our untreated, Vk12598 transplanted mice. Alternatively, populations of cDC2, pDC, moDC, and monocytes lacking expression of MHC class II markers were fixtures for healthy controls while markedly less abundant in tumor-bearing mice.
Tumor burdens in three samples subjected to LCL161 therapy were significantly reduced 24-hours post treatment and accompanied by an expansion of late-stage neutrophils and migratory DCs with expression features mirroring those of DCs from mesenteric lymph nodes. This composition of immune cells was distinct from all other sample including those treated with cyclophosphamide or bortezomib. In addition, relative proportions of B, T, and NK cells were nearly fully reconstituted, matching levels observed in healthy controls, in two of these samples. In a direct functional assessment, expression levels of Fscn1 and Ccr7 (key over-expressed genes in these migratory DCs) markedly increased following treatment of bone marrow derived DCs with LCL161. Overall, the transcriptional profile of these migratory DCs indicates their pivotal role in maturation of DCs (Fscn1, Tmem123) as well as the recruitment of T and NKT cells (Cxcl16, Ccr7) and increase in antigen presentation (Cd63)–all of which may serve to ‘warm' tumors, increasing T cell infiltration necessary for immunotherapy approaches.
With the increased resolution of scRNA-seq, we observe unique alterations in the composition of the immune microenvironment of MM induced by treatment with LCL161 that further detail its unique mechanism of anti-MM activity.
Citation Format: Caleb K. Stein, Erin Meermeier, Seth Welsh, Victoria Garbitt, Meaghen Sharik, P. Leif Bergsagel, Nicholas Banovich, Jonathan Keats, Marta Chesi. scRNA-seq reveals restructuring of the immune microenvironment of multiple myeloma following treatment with an IAP antagonist [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5622.