INTRODUCTION: A comprehensive characterisation 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 grade using multi-parameter immunohistochemistry and targeted transcriptomic profiling. METHODS: Tissue microarrays (n=102) were stained for PD-L1 & 2, Indoleamine-deoxygenase-1 (IDO-1) and evaluated in relationship to functional characteristics of tumor-infiltrating T-lymphocytes (TILs) and biomarkers of hypoxia/angiogenesis. PD-L1 expression was tested in circulating tumour cell (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 pNETs (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 was highest PD-L1/IDO-1 co-expressing tumours (p=0.006). Grade 3 well-differentiated NETs had lower CD4+/FOXP3+ and CD8+/PD1+ TILs 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.