Tumour infiltrating lymphocytes and apoptosis are independent features in colorectal cancer stratified according to microsatellite instability status.

BACKGROUND The presence of high level DNA microsatellite instability (MSI-H) in colorectal cancer is associated with an improved prognosis, as is the presence of tumour infiltrating lymphocytes (TILs). It is not clear if TILs contribute directly to the survival advantage associated with MSI-H cancers through activation of an antitumour immune response. AIMS To correlate TIL and apoptosis rates in colorectal cancer stratified by MSI status. METHODS The distribution of TILs was characterised and quantified in a selected series of 102 sporadic colorectal cancers classified according to levels of MSI as 32 MSI-H, 30 MSI-low (MSI-L), and 40 microsatellite stable (MSS). Archival blocks were immunostained using the T cell markers CD3 and CD8, and the B cell marker CD20. Apoptosis of malignant epithelial cells was quantified by immunohistochemistry with the M30 CytoDEATH antibody. RESULTS Positive staining with anti-CD3 and negative staining with anti-CD20 identified virtually all TILs as T cells. The majority of CD3+ TILs (>75%) also stained with anti-CD8. TILs were most abundant in MSI-H colorectal cancers in which 23/32 (72%) scored as TIL positive. Only 5/40 (12.5%) MSS tumours and 9/30 (30%) MSI-L cancers were TIL positive (p<0.0001). MSI-H cancers showed a twofold higher rate of apoptosis (mean (SD) 3.52 (0.34)%) than the MSS cancers (1.53 (0.23)%) while the MSI-L subgroup had an intermediate level (2.52 (0.35)%) (p<0.0001). Overall, there was a small (r=0.347) but significant linear correlation between CD3+ and M30+ random apoptosis counts (p<0.001). However, TILs and apoptosis showed little colocalisation. CONCLUSIONS While TILs might be expected to explain the increased apoptotic rate and improved prognosis of MSI-H cancers, it is likely that TILs and apoptosis are independent characteristics of MSI-H cancers.