Role of intraperitoneal paclitaxel in eosinophil activation and recruitment in the peritoneal cavity and anti-tumor effects against peritoneal metastasis from gastric cancer.

471 Background: Recent studies indicate that the efficacy of chemotherapy is significantly influenced by the tumor immune microenvironment. Peritoneal cavity contains many immune cells, however, the relationship between immune response within the peritoneal cavity and tumor response remains poorly understood. Methods: Single cell suspensions were obtained from ascites or peritoneal lavages from 41 patients with PM from GC. In 28 patients, cells were collected both before and after 1-3 courses of IP chemotherapy. These samples were immunestained with mAbs targeting to specific lymphoid or myeloid subsets, and their proportions in CD45(+) leukocytes were analyzed using flowcytometry. Eosinophils were purified by magnetic cell sorting for subsequent RNA-Seq analysis. Results: Among 41 patients with PM, tumor leukocyte ratio (TLR) calculated as CD326(+) tumor cells divided by CD45(+) leukocytes varied from 0.002% to 58.1%. TLR was not correlated with any ratios of immune cells, however, the ratios of CD8(+) T cells, NK cells and CD16(-) CD193(+) eosinophils tended to decrease with the increase in TLR. Conversely, the ratios of CD19(+) B cell, CD14(+) macrophages and CD16(+) neutrophils increased with elevated TLR. Mean survival time of these patients was 17.7 months. None of the leukocyte subsets showed significant association with patient outcome. In 28 patients, the ratios of lymphocytes mostly decreased with significant difference in CD4(+) T cells and CD19(+) B cells after IP treatment. In contrast, the ratio of CD11b(+) myeloid cells increased in peritoneal cavity after IP treatment. Among the myeloid cells, the ratio of CD16(-) CD193(+) eosinophils significantly increased. The change was particularly prominent in in post-treatment negative peritoneal lavage cytology (CY0) patients (M=0.47% vs M=10.0%, p<0.0001), while the difference was not significant in post-treatment positive peritoneal lavage cytology (CY1) patients (M=0.93% vs M=0.97%, p=0.84). Patients with eosinophil ratio of ≥2% after IP chemotherapy in peritoneal cavity had significantly longer overall survival compared to those with lower eosinophil ratios (17.3 mo vs 26.7 mo, p=0.034; HR =0.23[0.06-0.89]). The peritoneal eosinophils after IP treatment exhibited elevated levels of CD11b and CD63 expression compared to circulating eosinophils (p<0.01), while SSC-A levels tended to be lower. Gene ontology pathways revealed enrichment of cytokine-mediated signaling pathway, extracellular matrix organization and response to interferon-gamma in peritoneal eosinophils (p<0.0001). Conclusions: IP-PTX induces eosinophil activation and recruitment in the peritoneal cavity, while also affecting other types of immune cells. These effects may potentially contribute to the anti-tumor response against peritoneal metastases.