Programmed death-ligand 1 (PD-L1) and its receptor programmed death 1 (PD-1) act as immune suppressors to shield tumor cells from killing of T cells, which are essential immune checkpoint in cancer immunotherapy. Targeting PD-1/PD-L1 axis has applicated for first-line or combination therapy in various cancer types and gained a powerful clinical effect. However, poor clinical response rate of PD-1/PD-L1 blockade have become increasingly prominent. Immunosuppressive tumor microenvironment and variable tumor immune escape mechanisms are important reasons for the poor response of PD-1/PD-L1 blockade. Targeting myeloid-derived suppressor cells (MDSCs) aim to unlock the immunosuppressive tumor microenvironment and promote the infiltration and activation of T cells, which is essential to enhance the response rate and clinical efficacy of PD-1/PD-L1 blockade. CCL2 is a chemokine that mainly attracts MDSCs to the tumor sites leading to form immunosuppressive tumor microenvironment. We firstly observed that high level expression of CCL2 correlated with poor response to PD-1/PD-L1 blockade as well as worse overall survival of patients. Meanwhile, CCL2 positively associated with human MDSCs makers and PD-L1 indicated that CCL2 is a novel potential target of tumor microenvironment. Thus, we performed drug screening from Chinese medicines and lycorine was identified to inhibit CCL2 mRNA level of tumor cells. Consistently, 10 uM of lycorine certainly retarded CCL2 secretion of tumor cells with ELISA assay. Mechanistically, lycorine controlled the expression of CCL2 through potential STAT3 regulation in tumor cells. Furthermore, lycorine enhanced the anti-tumor effect of PD-L1 blockade in MC38 tumor-bearing mice. We observed that lycorine improved tumor microenvironment through increasing the infiltration of CD8*T cells and reducing the accumulation of MDSCs. Consistently, in vitro co-culture assay confirmed that lycorine significantly reduced MDSCs accumulation including M-MDSC and PMN-MDSC. Collectively, Lycorine controls CCL2 expression to reduce MDSCs accumulation in the tumor microenvironment and unravel its inhibitory effect on T cells, eventually leading to enhance the response to PD-LI blockade. Our study provides a novel potential combination strategy and application for cancer immunotherapy.

程序性死亡配體 1 (Programmed death-ligand 1, PD-L1) 及其受體程序性死亡蛋白(rogrammed cell death prorein 1, PD-1)作為免疫抑制因子來保護腫瘤細胞免受工細胞的殺傷，是腫瘤免疫治療中重要的免疫檢查點。靶向 PD-1/PD-L1軸巳應用於多種癌症類型的一線或聯合治療並獲得了強大的臨床效果。然而，PD-1/PD-L1 免疫療法臨床反應率差的問題日益突出。免疫抑制性腫瘤微環境和多變的腫瘤免疫逃逸機制是 PD-1/PD-L1免疫療法響應不佳的重要原因。靶向髓系 來源的抑制性細胞 (Myeloid-derived suppressor cell, MDsCs) 旨在解鎖免疫抑制性腫瘤微環境，促進 工細胞的浸潤和激活，這對於提高 PD-1/PD-L1 免疫療法的臨床反應率至關重要。CCL2 是一種超化因子，主要將 MDSCs 或腫瘤相關巨噬細胞 (Tumor associated macrophages, TAMs) 趨化到腫瘤部位，從而形成免疫抑制性腫瘤微環境。我們首先觀察到 CCL2 的高水平表達與對 PD-1/PD-LI 免疫療法的不響應患者以及患者的較差的生存相關。同時 CCL2 與 MDSCs 的生物標誌物、PD-L1 密切相關，表明 CCL2是腫瘤微環境的新的潛在靶點。我們從中藥來源化合物中進行篩選，發現石蒜鹼可抑制腫瘤細胞的 CCL2 mRNA 水平。在ELISA 實驗中，10 1M 的石蒜鹼可明顯抑制腫瘤細胞的 CCL2 分泌。機制上，石蒜鹼通過 STAT3 抑制腫瘤細胞中 CCL2 的表達。 此外，石蒜鹼增強了 PD-L1 抗體在 MC38 荷瘤小鼠中的抗腫瘤作用。石蒜鹼通過增加 CD8+T 細胞的浸潤和減少MDSCs 的招募來改善腫瘤微環境。同時體外 共培養實驗證實，石蒜鹼顯著降低了 MDSCs 的招募，包括 M-MDSC 和 PMN- MDSC. 綜上，石蒜鹼通過靶向趨化因子 CCL2，減少腫瘤免疫微環境中 MDSCs 的浸潤，解除其對 工細胞的抑製作用從而增強 PD-L1 抗體的療效。該聯合治療方案為腫瘤免疫治療提供一種新的聯合策略及應用。