The peritoneal immune landscape in gastrointestinal peritoneal metastasis patients and the impact of intraperitoneal paclitaxel.

195 Background: Peritoneal metastasis (PM) from gastrointestinal (GI) malignancies respond poorly to traditional systemic therapies. Intraperitoneal (IP) paclitaxel (PTX) is safe and effective in gastric and ovarian PM in Phase III trials and is now being tested for appendiceal-adenocarcinoma PM (AA-PM) (NCT06207305). The peritoneal cavity is immune-cell–rich, yet their diversity across GI-PM subtypes and the impact of IP PTX remain unevaluated. We asked how the peritoneal immune landscape varies across GI-PM subtypes and how, particularly in myeloid cells and T cells, it is altered by IP PTX. Methods: We profiled peritoneal immune components by scRNA-seq and scTCR-seq in patients without PM and in those with PM from AA or colorectal cancer (CRC-PM). We also analyzed AA-PM before and after IP PTX in a clinical trial. In parallel, a mouse PM model with spectrum flow cytometry examined how peritoneal immune cells shape responses to IP PTX. Results: Across all groups, peritoneal fluid contained monocytes, macrophages, T cells, and DC2s, with very few B cells and DC1s. Cell-type frequencies were similar in no-PM and CRC-PM, whereas AA-PM showed marked monocyte enrichment with reduced T cells, consistent with an immune-inhibitory milieu. Within the myeloid compartment, most macrophage subsets—including LYVE1⁺SPP1⁺ tumor-associated macrophages—were comparably prevalent with and without GI PM, indicating baseline anti-inflammatory tone. In contrast, CD8⁺ T cells in GI-PM peritoneal fluid displayed robust activation signatures (TBX21, GZMB, IFNG). In one MSI-high CRC-PM patient on anti-PD-1, the top four TCR clonotypes comprised ~30% of all T cells, indicating strong clonal expansion and potential peritoneal antitumor activity. IP PTX reshaped the peritoneal immune system in AA-PM: TIMD4⁺ resident macrophages decreased post-treatment and macrophages acquired pro-inflammatory programs, including IFN-α/β signaling. In DC2s, GSEA showed upregulated antigen processing/presentation, co-stimulation, cytokine/chemokine, and migratory pathways. Post-treatment peritoneal T cells increased activation and tissue-residency features (CD69, ITGAE, CXCR6, TBX21), suggesting that IP PTX may activate innate cells and augment T-cell responses. In mice, intraperitoneal MC38-luciferase cells established PM, and IP PTX (25 mg/kg twice weekly) markedly reduced tumor bioluminescence. Flow cytometry showed decreased ICAM2⁺ large peritoneal macrophages and increased neutrophils and MHCII⁺ monocytes, recapitulating the patient phenotype. Effector-memory CD4⁺ and CD8⁺ T cells (CD44⁺CD62L⁻) expanded. Anti-tumor activities of these peritoneal macrophages and T cells are under study. Conclusions: Overall, these data demonstrate changes in the immune milieu in GI PM and after IP PTX and suggest cooperative roles for mononuclear phagocytes and T cells in IP PTX efficacy.