The immune microenvironment of colorectal cancer - Relationship with survival, sidedness, and pre-diagnostic anthropometry

Colorectal cancer (CRC) is the third most common cancer worldwide. Increasing evidence suggests that CRC should be considered a heterogeneous disease, with multiple differences between proximal and distal tumours. The immune system may, depending on the context, promote or inhibit tumour growth, and different immune cell subsets have been found to be associated with impaired or improved prognosis in CRC. The major aim of this thesis was to investigate the prognostic impact of different immune cell signatures in CRC, with particular reference to primary tumour location, and, furthermore, to perform a characterization of immune cell signatures in relation to anthropometric factors.The study cohort for Papers I-III consists of all 626 cases of CRC in the prospective, population-based cohort Malmö Diet and Cancer Study (MDCS) from 1991 up until December 31, 2008, of which tumours from 557 cases were available for tissue microarray construction, including 201 (36.2%) right-sided and 145 (26.1%) left-sided colon cancers, and 209 (37.7%) rectal cancers. Immunohistochemistry was applied to assess the density of tumour-infiltrating immune cells. For Paper IV, the analyses were restricted to the 584 cases included in the European Prospective Investigation into Cancer (EPIC) cohort, of which the MDCS forms part. Anthropometric measurements were taken at baseline. Cox proportional hazards regression models were applied to study the hazard ratios for survival, and the risk of CRC with particular immune cell compositions.Paper I shows that dense infiltration of B cells is an independent favourable prognostic factor in CRC. Paper II demonstrates that high infiltration of cytotoxic T cells is an independent favourable prognostic factor only in right-sided colon cancer, whereas high infiltration of regulatory T cells is an independent prognostic factor only in rectal cancer. Moreover, re-analysis of the data from paper I revealed that the prognostic impact of B cells is only evident in right-sided tumours.Paper III demonstrates that high expression of programmed cell deaht ligand 1 (PD-L1) on immune cells is an independent favourable prognostic factor only in patients with right-sided and left-sided colon cancer. Paper IV shows that obesity, indicated by several anthropometric factors, is associated with risk of CRC with high infiltration of B cells and cytotoxic T cells but with low infiltration of regulatory T cells in both sexes, albeit with weaker associations in women. Moreover, the results show that obesity is associated with risk of CRC with low PD-L1 expression on immune cells in men, but with high PD-L1 expression on immune cells in women.These results show that the prognostic impact of tumour-infiltrating immune cells in CRC differs according to primary tumour location. It is also demonstrated that obesity might influence the immune microenvironment of CRC. In summary, the findings indicate that primary tumour location, anthropometric factors, and sex are all important factors to include in future studies on the tumour microenvironment of CRC