Colorectal cancer is the third most common cancer in Australia, making it a major health burden. Up to 25% of patients have disease that spreads to the peritoneum, which leads to a very poor prognosis. Systemic chemotherapy has limited efficacy, offering modest improvement in survival. Aggressive surgery in the form of cytoreductive surgery and hyperthermic intraperitoneal chemotherapy is effective in very selected patients. However, the majority of patients are not suitable surgical candidates. The immune response and the immune landscape in peritoneal metastases has not been assessed previously. Immunotherapy, while effective in other cancers, has an undefined role in peritoneal disease.
Fresh tumour tissue from colorectal peritoneal metastases was used for flow cytometry analysis to evaluate immune cell infiltration. Tissue was further processed to develop robust organoid models. Simultaneous patient matched tumour infiltrative lymphocytes (TILs) were cultured and enriched with interleukin-2 (IL-2). Once organoids were robust and replenishable, they were co-cultured with various concentrations of TILs to assess the cytotoxic ability of the lymphocytes. They were further cultured with TILs with the addition of an anti PD-1 antibody.
Flow cytometry revealed a large CD45 population of mainly T cells. There was an even mix of CD4 and cytotoxic CD8 cells. However, there were unexpectedly high T-regulatory cells, comprising upto 15% of CD4 cells. PD-1 expression on T cells, whilst variable, was upto 75%, suggesting a possible role for anti PD-1 antibody therapy. Co-culture with organoids, TILs and PD-1 antibodies in a novel live cell scanning assay demonstrated organoid killing by TILs increased significantly with the addition of anti PD-1 antibody.
This early pre-clinical data demonstrates a potential role for immunotherapy in the setting of peritoneal disease. These early results need to be expanded with a larger cohort, and validated in an in-vivo setting.