Bacterial Lipopolysaccharide and Immune Activation in Colorectal Cancer

Abstract

Colorectal cancer (CRC), is the second deadliest cancer in New Zealand. At early stages tumours can be treated with surgery or a combination of surgery and chemotherapy. However, once the cancer has metastasized, treatment options become very limited as surgery and chemotherapy are no longer effective. Immunotherapy was promised to be a treatment for metastatic CRC, however, only a small number of patients respond favourably. Until recently only the tumour itself has been considered in CRC therapy, but the entire tumour microenvironment must be considered. The microenvironment includes the tumour as well as immune cells and bacteria, and each of these components interact with one another. The microbiome is known to affect how colorectal tumours respond to therapy, and lipopolysaccharide (LPS), a component of the Gram-negative bacterial cell wall is a novel link between the microbiome and CRC. The aim of this study is to determine the effect that LPS from three bacterial strains commonly found in the tumour microenvironment has on immune activation in CRC. HT29 and HCT116 CRC cell lines were treated with LPS from Bacteroides fragilis, Fusobacterium periodonticum and Porphyromonas asaccharolytica. Using multiplex assays, secreted checkpoint proteins and cytokine levels were measured and analysed using flow cytometry. RT - qPCR was used to determine LPS induced changes in corresponding gene expression. Flow cytometry was used to determine if LPS caused a change in cell - surface expression of PDL1 on peripheral blood mononuclear cells (PBMCs), HT29 and HCT116 cells lines. Results showed that LPS from F. periodonticum caused a significant increase of secreted IL-8, whilst LPS from B. fragilis and P. asaccharolytica did not induce IL-8 secretion. These results were validated using an ELISA. However, the effect of F. periodonticum -LPS could be suppressed when treated in combination with B. fragilis and P. asaccharolytica. F. periodonticum-LPS also caused a significant increase in both the IL-8 gene (CXCL8) and PDL1 gene (CD247) expression in HT29 cells.

This research has demonstrated how the bacteria in the gut may be interacting with tumour and immune cells in the CRC-microenvironment. In CRC, increased IL-8 is associated with a poorer prognosis and the induction of metastasis. Increased expression of PDL1 on the tumour cell surface is also associated with a poorer prognosis. This research has demonstrated how components of the microbiome may modulate the tumour microenvironment, providing directions for future research in this area.