Characteristics of immune-infiltrating cells in the tumor microenvironment of appendiceal cancer with peritoneal disease.85

217 Background: Appendiceal cancers (AC) are rare, associated with peritoneal metastases, and respond poorly to current systemic treatments. There are limited studies on the characteristics and prognostic impact of infiltrating immune cells within the tumour microenvironment (TME) of AC with peritoneal metastases. Infiltrating immune cells are associated with survival outcomes and predict the effectiveness of immunotherapies in solid tumours. Different definitions have been developed to describe infiltrating immune cells and associated structures such as tertiary lymphoid aggregates (TLS). The aim of our study was to describe the immune TME in patients with AC peritoneal metastases to understand mechanisms of sensitivity and resistance to chemoimmunotherapies. Methods: Fresh tissue was collected during cytoreductive surgery from consenting patients with AC with peritoneal disease (Jul 2020-Feb 2022). Uncultured Day 0 tissues were fixed in formalin prior to paraffin embedding (FFPE). Haematoxylin and eosin (H&E) staining was performed on sections of FFPE tissue. Sections were evaluated by light microscopy for the presence of immune cells based on morphological appearance. Patterns of infiltrating immune cells were classified based on location (peritumoural, intratumoural, perimucinous, stromal), and aggregation (none, immature, mature). Immune cells forming aggregates were counted and the diameter of aggregates measured using QuPath. Results: We evaluated 25 patients with AC peritoneal disease during the study period. Immune cells were detected in tissues from all except 3 patients; 16 cases (78 immune areas) were low grade (LG) disease of primary appendiceal mucinous neoplasms, and 6 cases (29 immune areas) were high grade (HG) from primary appendiceal adenocarcinomas (Table). Immune cell aggregates had a median diameter of 209 mm (75-491) and median cell count of 892 cells (46-6490). Conclusions: This study represents the first formal description and characterisation of infiltrating immune cells in appendiceal cancer peritoneal metastases. Perimucinous immune cells were not observed in HG disease despite presence of mucin. Intratumoural immune cells were not observed in LG disease, consistent with these tumours being more resistant to chemotherapy. We plan ongoing work correlating the immune features with clinical outcomes alongside immunohistochemical and transcriptomic characterisation of the aggregates. [Table: see text]