CMS subtypes characterized by high TMB shows immunosuppressive microenvironment that implies resistance to immunotherapy.

Abstract

610 Background: Tumor mutational burden (TMB) is emerging as an important biomarker for immune checkpoint therapy (ICT) response. Yet even in the context of high TMB, ICT are likely ineffective in an immuno-suppressed microenvironment. Here we demonstrate that a well-characterized subtype of CRC, CMS2, associated with Wnt pathway activation, is immunosuppressive despite high TMB. Methods: Tumor/normal-paired DNAseq (WGS or WES) and deep RNAseq (∼200x106reads per tumor) was performed on 464 GI tumors from a commercial database. Samples were classified as high TMB if they had > 200 non-synonymous exonic mutations as previously established (Rizvi et al, 2015). Each sample was assigned to one of the colorectal Consensus Molecular subtypes (CMS) based on RNA classification. A curated panel of 109 genes that discriminate between 22 immune subsets was identified. For each of these immune signatures, a database containing 1880 unselected tumors was used to define a distribution of expression. The study samples were then scored for their deviances within such distributions. Significant enrichment was analyzed between immune subsets, CMS types, TMB status, and somatic mutational status. Results: Compared to other subtypes, CMS1 & CMS2 were significantly high-TMB (adj. p < 3.8E-4 & p < 4.7E-3 respectively). Perplexingly, CMS2 had significantly lower expression of 11 well-established checkpoint and TME markers including LAG3 and PDL1 (adj. p 1.5E-2 & 2.9E-9 respectively), while CMS1 (MSI-enriched) expresses selected TME markers more than other subtypes (PDL1 adj. p < 4.0E-6 & LAG3 adj. p < 1.0E-6). As expected, CMS2 tumors were significantly enriched for likely pathogenic variants in the Wnt-associated gene APC (adj. p < 1.3E-8). Immune-deconvolution indicated substantial exclusion of Tem cells from CMS2 tumors, in line with Wnt/b-catenin blockade of TcmàTem maturation for immunoreactivity. Conclusions: The most common subtype of CRC, CMS2 (~37%), is highly immunosuppressive despite high TMB. ICT is only effective in an immunologically active microenvironment. TMB alone as a biomarker likely is insufficient to indicate the effectiveness of immunotherapy.