Association of PD-L1 and PD-L2 expression and tumor-infiltrating lymphocytes in BRAF V600E-mutated metastatic colorectal cancer: GI-SCREEN post-hoc analysis

Background: The programmed cell death protein 1 (PD-1)/programmed cell death-ligand 1 (PD-L1)/programmed cell death-ligand 2 (PD-L2) axis is responsible for cancer immune escape, which facilitates disease progression. However, the role of PD-L1 and PD-L2 and tumor-infiltrating lymphocytes (TILs) in metastatic colorectal cancer (mCRC) has not been studied. Materials and methods: We conducted a post-hoc analysis of the Nationwide Cancer Genome Screening Project GI-SCREEN in mCRC. PD-L1 (22C3) and PD-L2 (MEB123.3G2.038) expression in formalin-fixed paraffin-embedded tumor samples was centrally assessed by immunohistochemical assays. TILs were morphologically evaluated using hematoxylin and eosin staining. Clinical information was extracted from the GI-SCREEN database. Inclusion of patients with BRAF V600E mutation was prioritized. Results: Two hundred patients with mCRC (median age 65 years and 116 males) were included in the study. Genomic testing identified RAS mutations in 87 (44%) patients, BRAF V600E mutations in 27 (14%), and microsatellite instability-high status in 8 (4%). Positivity of PD-L1 and PD-L2 was 11% and 47% on tumor cells (TC) and 0% and 64% on immune cells, respectively, and that was associated with the presence of TILs (P = 0.011 for PD-L1, 0.024 for PD-L2). PD-L1+ TC was significantly more frequent in BRAF V600E-mutated tumors (P = 0.03). Even in microsatellite stable tumors, BRAF V600E-mutated tumors were significantly associated with higher expression of PD-L1 on TC than BRAF wild-type (25% versus 8%, P = 0.02). Conclusions: Our study showed a distinct pattern of PD-L1 expression on TC of patients with BRAF V600E-mutated mCRC, which could be a potential therapeutic target for PD-1 blockade.