The Vigorous Immune Microenvironment of Microsatellite Instable Colon Cancer Is Balanced by Multiple Counter-Inhibitory Checkpoints

Abstract

We examined the immune microenvironment of primary colorectal cancer (CRC) using immunohistochemistry, laser capture microdissection/qRT-PCR, flow cytometry and functional analysis of tumor infiltrating lymphocytes. A subset of CRC displayed high infiltration with activated CD8+ CTL as well as activated Th1 cells characterized by IFN-γ production and the Th1 transcription factor Tbet. Parallel analysis of tumor genotypes revealed that virtually all of the tumors with this active Th1/CTL microenvironment had defects in mismatch repair, as evidenced by microsatellite instability (MSI). Counterbalancing this active Th1/CTL microenvironment, MSI tumors selectively demonstrated highly up-regulated expression of multiple immune checkpoints, including five – PD-1, PD-L1, CTLA-4, LAG-3 and IDO – currently being targeted clinically with inhibitors. These findings link tumor genotype with the immune microenvironment, and explain why MSI tumors are not naturally eliminated despite a hostile Th1/CTL microenvironment. They further suggest that blockade of specific checkpoints may be selectively efficacious in the MSI subset of CRC.

Blockade of LAG3 enhances responses of tumor-infiltrating T cells in mismatch repair-proficient liver metastases of colorectal cancer

“…The observed differences between MMR-proficient LM-CRC and MMR-proficient primary CRC are to some extent reminiscent of those found between MMR-deficient primary CRC and MMR-proficient primary CRC by Llosa et al. 31 They demonstrated that MMR-deficient primary CRC displayed higher infiltration with CD8 + CTL as well as upregulated expression of PD-1, PD-L1, CTLA4 and LAG3 compared to MMR-proficient primary CRC, and suggested that these differences contributed to the enhanced clinical responsiveness to anti-PD-1 therapy of microsatellite instable CRC compared with microsatellite stable CRC. 31 In addition, we found that LM-CRC contained increased frequencies of professional APC subsets (mDC and monocytes) (Fig.…”

“…31 They demonstrated that MMR-deficient primary CRC displayed higher infiltration with CD8 + CTL as well as upregulated expression of PD-1, PD-L1, CTLA4 and LAG3 compared to MMR-proficient primary CRC, and suggested that these differences contributed to the enhanced clinical responsiveness to anti-PD-1 therapy of microsatellite instable CRC compared with microsatellite stable CRC. 31 In addition, we found that LM-CRC contained increased frequencies of professional APC subsets (mDC and monocytes) (Fig. 1C), which may result in improved presentation of tumor antigens to intra-tumoral T cells as well as more PD-L1 and MHC-II expression in LM-CRC compared with primary CRC, because in LM-CRC mDC and monocytes expressed more PD-L1 and MHC-II than B cells (Fig.…”

“…30 They also have higher expression levels of inhibitory checkpoint molecules, probably to resist immune-mediated tumor elimination. 31 Together, enhanced immune cell infiltration and upregulation of inhibitory immune checkpoints may render MMR-deficient CRC more sensitive to PD-1/PD-L1 blockade than MMR-proficient CRC.…”

Immunoprofiles of colorectal cancer from Lynch syndrome

“…Immune evasive mechanisms include loss of the MHC class I subunit B2M and up-regulation of the T-cell inhibitor PD-L1. 13,14 Differences in mechanisms, e.g. related to deregulation of MHC class I expression, apply between sporadic and hereditary MSI tumors, and motivate mechanistic explanation related to the variable response rates observed in sporadic and Lynch syndrome-associated MSI tumors treated with anti-PD-1 inhibitors.…”

“…2,5 This anti-tumorigenic effect may be counter-balanced by immune-evasive mechanisms, such as loss of the MHC class I subunit, beta-2-microglobulin (B2M) and up-regulation of the T-cell inhibitor programmed death 1 ligand 1 (PD-L1). 13,14 B2M loss has been associated with loss of MHC class I receptors suggesting that this subset may not elicit neoepitope-induced T cell responses. 4,15 In contrast, high levels of PD-L1 have been associated with T cell exhaustion that can be reversed by inhibition of the programmed death 1 (PD-1)/PD-L1 binding.…”