CAF-macrophage crosstalk in tumour microenvironments governs the response to immune checkpoint blockade in gastric cancer peritoneal metastases

Background Peritoneal metastasis is the most common metastasis pattern of gastric cancer. Patients with gastric cancer peritoneal metastasis (GCPM) have a poor prognosis and respond poorly to conventional treatments. Recently, immune checkpoint blockade (ICB) has demonstrated favourable efficacy in the treatment of GCPM. Stratification of best responders and elucidation of resistance mechanisms of ICB therapies are highly important and remain major clinical challenges. Design We performed a phase II trial involving patients with GCPM treated with ICB (sintilimab) combined with chemotherapy. The samples of primary tumours, GCPMs and peripheral blood from patients were collected for single-cell sequencing to comprehensively interpret the tumour microenvironment of GCPM and its impacts on immunotherapy efficacy. Results The GCPM ecosystem coordinates a unique immunosuppressive pattern distinct from that of primary GC, which is dominated by a stroma-myeloid niche composed of SPP1+tumour-associated macrophages (TAMs) and Thrombospondin 2 (THBS2)+matrix cancer-associated fibroblasts (mCAFs). Consequently, this stroma-myeloid crosstalk is the major mediator of ICB resistance in patients with GCPM. Mechanistically, the accumulated THBS2+mCAFs facilitate the recruitment of peritoneum-specific tissue-resident macrophages and their transformation into SPP1+TAMs via the complement C3 and its receptor C3a receptor 1 (C3AR1), thereby forming a protumoral stroma-myeloid niche. Blocking the C3-C3AR1 axis disrupts the stroma-myeloid crosstalk and thereby significantly improves the benefits of ICB in in vivo models. Conclusion Our findings provide a new molecular portrait of cell compositions associated with ICB resistance in patients with GCPM and aid in the prioritisation of therapeutic candidates to potentiate immunotherapy. Data are available in a public, open access repository. Data are available on reasonable request. The sample information is listed in online supplemental table S1. The 10X genomics raw data of this study are deposited in the Genome Sequence Archive for Human database (ID: HRA009064; link: [https://ngdc.cncb.ac.cn/gsa-human/browse/HRA009064][1]). Additionally, the GEXSCOPE single-cell matrix data are deposited in the Mendeley repository (ID: jwkc5t6r55). Previously published scRNA-seq data that were reanalysed and integrated into this study are available in the Gene Expression Omnibus database under accession code GSE183904. All the data that support the findings of this study are available from the corresponding author on reasonable request. [1]: https://ngdc.cncb.ac.cn/gsa-human/browse/HRA009064)