Programmed Cell Death Ligand Expression Drives Immune Tolerogenesis across the Diverse Subtypes of Neuroendocrine Tumours

Introduction: A comprehensive characterization of the tumour microenvironment is lacking in neuroendocrine tumours (NETs), where programmed cell death-1 receptor-ligand (PD-1/PD-L1) inhibitors are undergoing efficacy testing. Objective: We investigated drivers of cancer-related immunosuppression across NETs of various sites and grades using multi-parameter immunohistochemistry and targeted transcriptomic profiling. Methods: Tissue microarrays (n = 102) were stained for PD-L1 and 2 and indoleamine deoxygenase-1 (IDO-1) and evaluated in relationship to functional characteristics of tumour-infiltrating T-lymphocytes (TILs) and biomarkers of hypoxia/angiogenesis. PD-L1 expression was tested in circulating tumour cells (CTCs, n = 12) to evaluate its relationship with metastatic dissemination. Results: PD-L1 expression was highest in lung NETs (n = 30, p = 0.007), whereas PD-L2 was highest in pancreatic NETs (n = 53, p < 0.001) with no correlation with grade or hypoxia/angiogenesis. PD-L1⁺ NETs (n = 26, 25%) had greater CD4⁺/FOXP3⁺ and CD8⁺/PD1⁺ TILs (p < 0.001) and necrosis (p = 0.02). CD4⁺/FOXP3⁺ infiltrate had the highest PD-L1/IDO-1 co-expressing tumours (p = 0.006). Grade 3 well-differentiated NETs had lower CD4⁺/FOXP3⁺ and CD8⁺/PD1⁺ TIL density (p < 0.001), and NanoString immune profiling revealed enrichment of macrophage-related transcripts in cases with poorer prognosis. We identified PD-L1(+) CTC subpopulations in 75% of evaluated patients (n = 12). Conclusions: PD-L1 expression correlates with T-cell exhaustion independent of tumour hypoxia and is enhanced in a subpopulation of CTCs, suggesting its relevance to the progression of NETs. These findings support a potential therapeutic role for PD-L1 inhibitors in a subset of NETs.