Chemotherapy and Immunotherapy as Treatment for MSS Metastatic Colorectal Cancer With High Immune Infiltrate

About 85% of cases of non-metastatic colorectal cancer (CRC) are related to chromosomal instability and have a proficient DNA Mismatch-Repair system (pMMR); which are also called CRC with microsatellite stability (MSS). Other CRC, i.e. 15%, are "microsatellite unstable" (MSI) with deficient DNA Mismatch-Repair system (dMMR). These latter are characterised by generation of many neo-antigens, which result in a high anti-tumour immune response and a high peri- and/or intra-tumour lymphocyte infiltration (TIL). Investigators recently showed, with a prospectively validated immune score, that 14% of localised MSS/pMMR CRC are also highly infiltrated by CD3+ lymphocytes. This same immune score has made it possible to measure high lymphocyte infiltration in hepatic metastases, in particular, in patients treated with XELOX/FOLFOX. Pembrolizumab, an anti-PD1 monoclonal antibody (programmed death-1) is an immune checkpoint inhibitor (ICI) of PD1/PD-L1 axis, recently approved in many cancers. Anti-PD1 antibodies have recently been reported as being very effective in patients with dMMR metastatic CRC (mCRC). In the study by Le DT et al. pMMR mCRC did not seem to benefit from anti-PD1 antibodies. However, it is possible that 20% of pMMR mCRC with a high CD3+ infiltrate in the tumour may be a subgroup of pMMR mCRC sensitive to ICI, as is the case for dMMR mCRC. Lastly, immunogenic cell death induced by chemotherapy, such as oxaliplatin, can increase the efficacy of ICI. The prognostic value of lymphocyte infiltrate has been demonstrated in CRC by several teams. However, no validated test is used in routine clinical practice. Previously, investigators described an automated and reproducible method for analysis of TIL and investigators validated it for clinical use. Automated tests evaluating TIL are performed on virtual slides and have showed that, out of 1,220 tumours tested, 20% were highly infiltrated by CD3+ T cells. Patients presenting with a pMMR CRC with a high immune infi