Immuno-Contexture and Immune Checkpoint Molecule Expression in Mismatch Repair Proficient Colorectal Carcinoma

SIMPLE SUMMARY: Consensus Molecular Subtypes have recently been proposed based on molecular and immune landscape of colorectal carcinoma (CRC). Only mismatch repair deficient and hyper-mutated CRC (CRC(dMMR)) can obtain clinical benefits from immune checkpoint blockades; on the other hand, mismatch repair proficient CRCs (CRC(pMMR)) have very limited therapeutic options. This study establishes that CRC(pMMR) displays an immunosuppressive microenvironment containing abundant tumor-associated macrophages (TAMs) and neutrophils, with a reduction in double negative T lymphocytes and B cells and increased exhausted tumor-infiltrating lymphocytes. Poor immunogenicity in CRC(pMMR) is further supported by interferon gamma (IFN-γ) unresponsiveness of both tumor cells and TAMs. ABSTRACT: Colorectal carcinoma (CRC) represents a lethal disease with heterogeneous outcomes. Only patients with mismatch repair (MMR) deficient CRC showing microsatellite instability and hyper-mutated tumors can obtain clinical benefits from current immune checkpoint blockades; on the other hand, immune- or target-based therapeutic strategies are very limited for subjects with mismatch repair proficient CRC (CRC(pMMR)). Here, we report a comprehensive typing of immune infiltrating cells in CRC(pMMR). We also tested the expression and interferon-γ-modulation of PD-L1/CD274. Relevant findings were subsequently validated by immunohistochemistry on fixed materials. CRC(pMMR) contain a significantly increased fraction of CD163(+) macrophages (TAMs) expressing TREM2 and CD66(+) neutrophils (TANs) together with decrease in CD4(−)CD8(−)CD3(+) double negative T lymphocytes (DNTs); no differences were revealed by the analysis of conventional and plasmacytoid dendritic cell populations. A fraction of tumor-infiltrating T-cells displays an exhausted phenotype, co-expressing PD-1 and TIM-3. Remarkably, expression of PD-L1 on fresh tumor cells and TAMs was undetectable even after in vitro stimulation with interferon-γ. These findings confirm the immune suppressive microenvironment of CRC(pMMR) characterized by dense infiltration of TAMs, occurrence of TANs, lack of DNTs, T-cell exhaustion, and interferon-γ unresponsiveness by host and tumor cells. Appropriate bypass strategies should consider these combinations of immune escape mechanisms in CRC(pMMR).