Tumor-cell PD-L1 expression, mismatch repair status, and survival outcomes in colorectal carcinoma: a retrospective Pakistani cohort study

Abstract

Background Tumor-cell programmed death-ligand 1 (PD-L1) expression has been investigated as a biomarker for colorectal carcinoma (CRC), but the reported positivity rates vary by antibody clone, scoring method, and threshold. Tumor-cell PD-L1 is not a validated companion diagnostic tool for CRC. We assessed whether PD-L1 expression scored with the ZR3 clone functions as a clinicopathological or prognostic biomarker in a Pakistani cohort, using microsatellite instability/mismatch-repair (MSI/MMR) status as the reference biomarker. Methods We retrospectively evaluated 60 patients with CRC diagnosed between 2020 and 2023. PD-L1 expression was assessed using the tumor proportion score (TPS) and ZR3 antibody. MSI was determined using polymerase chain reaction-based fragment analysis, and MMR status was determined using four-protein immunohistochemistry. Associations between binary PD-L1 status and 10 clinicopathological variables were analyzed and adjusted using the Benjamini–Hochberg false discovery rate procedure. Overall survival (OS; n=60), disease-free survival (DFS; n=44), and progression-free survival (PFS; n=16) were the exploratory outcomes. Results At a TPS threshold of ≥1%, PD-L1 positivity was identified in 43 of 60 tumors (71.7%), including 31 low-positive and 12 high-positive tumors. This rate exceeded that of most CRC TPS series and likely reflected clone- and threshold-specific assay behavior. No clinicopathological variables were associated with binary PD-L1 status after multiplicity correction. MSI-H/dMMR was present in 12 cases (20.0%) and was associated with the three-level PD-L1 category (Fisher–Freeman–Halton p=0.0017; Cramer’s V = 0.485), but not with binary positivity (p=0.151); the association was confined to the high-positive group. PD-L1 status was not associated with OS (HR 1.43, 95% CI 0.40–5.05; p=0.580), DFS (HR 0.46, 95% CI 0.15–1.45; p=0.185), or PFS (HR 0.67, 95% CI 0.20–2.20; p=0.505). Older age, rather than PD-L1 status, was associated with mortality (adjusted HR 1.68 per 10 years, 95% CI 1.09–2.57; p=0.018). Conclusions ZR3-based tumor-cell PD-L1 TPS was not associated with clinicopathological characteristics or survival in this cohort. The high positivity rate should be interpreted as assay-specific, while the enrichment of MSI-H/dMMR tumors in the high-positive tier was consistent with established biology. ZR3-based TPS should remain a research measure and should not independently guide immunotherapy selection; MSI/MMR testing remains the clinically actionable biomarker for CRC.