Phenotypic and functional suppression of T cells in human colorectal cancer tissue

Worldwide, more than one million people are diagnosed with colorectal cancer (CRC) every year. Analysis of T cell infiltrate into CRC tumours accurately predicts patient prognosis, but the immune response against CRC is not fully understood. The tumour microenvironment is able to influence T cell infiltrate both functionally and phenotypically and what T cell subsets contribute to the pro or anti-tumour response is not entirely known. Further understanding of the immune microenvironment in CRC may enable identification of novel immune based biomarkers for improving outcome prediction and could potentially lead to the development of new immunotherapies for the treatment of CRC patients. The T cell infiltrate in tumour and matched non-tumour bowel (NTB) tissue from patients with CRC was analysed using a novel 10 colour flow cytometry panel. This analysis revealed a distinct T cell immune environment in CRC tissue, characterised by higher frequencies of CD4+ regulatory (CD25hi FOXp3+) and inflammatory (IL-17+) T cells in combination with lower frequencies of CD4+ activated (CD69+) and CD4+ and CD8+ IL-2 producing (IL-2+) T cells. The frequency of poly-cytokine producing T cells associated with immune protection against tumours (IFN-γ+ IL-2+) were also decreased while polycytokine producing T cells associated with autoimmunity (IFN-γ+ IL-17+) were increased in tumour compared to matched NTB tissue. These findings are consistent with an tumour immune environment permissive of cancer growth. Next analysis was carried out to identify whether the T cell infiltrate in tumour or NTB tissue correlated with traditional prognostic features and disease outcome of CRC patients. The frequency of the majority of T cell subsets did not correlate with prognostic features or disease free survival (DFS) of patients with CRC however mono IL-17 and dual IL-17 IL-2 producing CD4+ T cells were more prevalent in advanced stage tumours. The frequency of IL-2 producing CD4+ and CD8+ T cells in the NTB also correlated with lymph node yield, a positive prognostic factor in patients with CRC. Furthermore, immunohistochemistry analysis of archived specimens revealed that a higher frequency of IL-2 producing T cells at the invasive margin of CRC tumours was associated with shorter disease free survival in patients with CRC, a paradoxical finding to current immunology dogma. These novel findings suggest a significant role for IL-2 and IL-2 producing T cells in the immune response to CRC. T cells recovered from NTB and tumour tissue were then functionally analysed. Responder (CD25low/-) T cells from tumour tissue may be refractory to regulatory T cells mediated IFN-γ suppression but no significant functional difference was observed between regulatory or responder T cells isolated from NTB and tumour tissue. The suppressive environment of CRC was demonstrated by the impaired proliferative ability of T cells from tumour tissue. This may be related to the high level of inhibitory receptors that are expressed on tumour infiltrating T cells. Overall the suppressive immune environment of the CRC tumour tissue was demonstrated both functionally and phenotypically. The discovery of the importance of IL-2 producing T cells and functional impairment of T cells in tumour tissue raises new avenues for study for both potential prognostic biomarkers and immunotherapies