Abstract 2256: Cancer stem cells and exosomal PD-L1 suppress anti-tumor B cells
Authors: Xin Ying , Liu Jiayu , Qin You , Lyu Kexing , Yu Sifei , Lin Ming , Wicha Max , Chang Alfred E. , Li Qiao 
Published: April 2023

Abstract
We have conducted clinical trials for cancer immunotherapy using vaccine-primed lymph node (VPLN) T calls based on our animal studies with tumor-draining lymph node (TDLN) T cells. More recently we have identified antitumor effector TDLN B cells which demonstrated significant tumor reactivity in vitro and therapeutic efficacy in vivo upon adoptive transfer which can be enhanced by anti-PD-L1 administration. It is unknown whether tumor cells, particularly cancer stem cells (CSCs) could suppress the antitumor effector B cells. We found that depletion of B cells with anti-CD20 mAb prior to anti-PD-L1 administration resulted in more aggressive tumor growth, and the anti-tumor efficacy of anti-PD-L1 was significantly reduced in both 4T1 and CT26 murine tumor-bearing hosts, suggesting the involvement of host B cells in the anti-tumor effect of anti-PD-L1. In addition, administration of anti-PD-L1 partially recovered the humoral immune response, confirming the PD-L1/PD-1 pathway involvement in B cell suppression. We detected elevated expression of PD-1 on activated B cells, and higher PD-L1 expression on ALDHhigh CSCs than on ALDHlow non-CSCs. Co-culturing ALDHhigh CSCs with purified B cells significantly reduced IgG secretion, and such CSC-mediated B cell suppression was rescued by adding anti-PD-L1 mAb in a dose-dependent manner to the CSC-B cell co-culture. These experiments thus indicate that CSCs suppress the IgG production by B cells via the PD-L1/PD-1 axis. Tumor cells were recently recognized to suppress T cell function via secretion of exosomes that express PD-L1. To investigate if exosomal PD-L1 contributes to B cell immunosuppression, we generated Rab27ako D5 cells to delete exosomes as well as PD-L1ko D5 cells by CRISPR/Cas9 gene editing. D5-derived exosomes suppressed B cell proliferation and IgG production in vitro, and administration of these exosomes promoted tumor growth and reduced animal survival, revealing the role of tumor-derived exosomes in B cell immunosuppression. Tumor-derived exosomes expressed PD-L1, but PD-L1 was absent in the exosomes isolated from the PD-L1ko tumor cells as evident by western blot and flow cytometry. Deletion of Rab27a or PD-L1 did not affect the proliferation of the tumor cells in vitro. However, Rab27ako or PD-L1ko resulted in significantly enhanced host anti-tumor immunity evident by reduced 4T1 tumor growth in vivo compared with the WT tumor. Importantly, anti-PD-L1 therapy significantly reduced Rab27ako or PD-L1ko 4T1 tumor growth and prolonged animal survival vs. the WT tumor control. We conclude that PD-L1-expresing CSCs and PD-L1-expresing exosomes suppress antitumor effector B cells via the PD-L1/PD-1 axis. To characterize the suppression of CSC-derived exosomes on host B cells as well as T cells, isolation of Rab27ako ALDHhigh CSCs warranties further investigation.
Citation Format: Ying Xin, Jiayu Liu, You Qin, Kexing Lyu, Sifei Yu, Ming Lin, Max Wicha, Alfred E. Chang, Qiao Li. Cancer stem cells and exosomal PD-L1 suppress anti-tumor B cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2256.