Eubacterium rectale improves the efficacy of anti-PD1 immunotherapy in melanoma via L-serine-mediated NK-cell activation

NK cells, as a type of key immune cell, play essential roles in tumor cell immune escape and immunotherapy. Accumulating evidence has demonstrated that the gut microbiota community affects the efficacy of anti-PD1 immunotherapy and that remodeling the gut microbiota structure is a promising strategy to enhance anti-PD1 immunotherapy responsiveness in advanced melanoma patients; however, the details of the mechanism remain elusive. In this study, we found that Eubacterium rectale (E. rectale) was significantly enriched in melanoma patients who responded to anti-PD1 immunotherapy and a high E. rectale abundance was related to longer survival in melanoma patients. Furthermore, administration of E. rectale remarkably improved the efficacy of anti-PD1 therapy and benefited the overall survival of tumor-bearing mice; moreover, application of E. rectale significantly recruited NK cells into the tumor microenvironment. Interestingly, conditioned medium isolated from an E. rectale culture system dramatically enhanced NK-cell function. Through GC-MS/ UHPLC-MS/MS-based metabolomic analysis, L-serine production was found to be significantly decreased in the E. rectale group; moreover, administration of an L-serine synthesis inhibitor dramatically increased NK-cell activation, which led to enhanced anti-PD1 immunotherapy effects. Mechanistically, supplementation with L-serine or application of the L-serine synthesis inhibitor affected NK-cell activation through Fos/Fosl. In summary, our findings reveal the role of bacteria-modulated serine metabolic signaling in NK-cell activation and provide a novel therapeutic strategy to improve the efficacy of anti-PD1 immunotherapy in melanoma. NK92 cells treated with L-serine(1mM/L) or NCT503 (40μM/L) for 48 hours.NCT503 is an inhibitor of L-serine Synthesis.

自然杀伤细胞（NK cells）作为一类关键免疫细胞，在肿瘤细胞免疫逃逸与免疫治疗中发挥不可或缺的重要作用。越来越多的研究证据表明，肠道菌群群落可影响抗PD-1免疫治疗的疗效，而重塑肠道菌群结构是提升晚期黑色素瘤患者抗PD-1免疫治疗响应性的极具前景的策略；然而其具体机制仍难以阐明。本研究发现，在对抗PD-1免疫治疗产生响应的黑色素瘤患者中，直肠真杆菌（Eubacterium rectale, E. rectale）显著富集，且高丰度的直肠真杆菌与黑色素瘤患者更长的生存期相关。进一步实验显示，给予直肠真杆菌可显著提升抗PD-1治疗的疗效，并延长荷瘤小鼠的总生存期；此外，施加直肠真杆菌可显著将自然杀伤细胞招募至肿瘤微环境中。有趣的是，从直肠真杆菌培养体系中分离得到的条件培养基可显著增强自然杀伤细胞的功能。通过基于气相色谱-质谱联用（GC-MS）/超高效液相色谱-串联质谱（UHPLC-MS/MS）的代谢组学分析，我们发现直肠真杆菌组中的L-丝氨酸产量显著降低；此外，给予L-丝氨酸合成抑制剂可显著增强自然杀伤细胞的活化，进而提升抗PD-1免疫治疗的效果。从机制层面来看，补充L-丝氨酸或施加L-丝氨酸合成抑制剂可通过Fos/Fosl影响自然杀伤细胞的活化。综上，本研究揭示了细菌调控的丝氨酸代谢信号在自然杀伤细胞活化中的作用，并为提升黑色素瘤患者抗PD-1免疫治疗的疗效提供了全新的治疗策略。将NK-92细胞以1mM/L的L-丝氨酸或40μM/L的NCT503处理48小时，其中NCT503为L-丝氨酸合成抑制剂。