David J. Pinato a , * , Anu Vallipuram a , Joanne S. Evans a , Clement Wong a , Hua Zhang b , Matthew Brown a , Roberto E. Dina c , Pritesh Trivedi c , Ayse U. Akarca d , Teresa Marafioti d , Francesco A. Mauri a , Rohini Sharma a
25 February 2020
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<sup>+</sup> NETs ( n = 26, 25%) had greater CD4<sup>+</sup>/FOXP3<sup>+</sup> and CD8<sup>+</sup>/PD1<sup>+</sup> TILs ( p < 0.001) and necrosis ( p = 0.02). CD4<sup>+</sup>/FOXP3<sup>+</sup> infiltrate had the highest PD-L1/IDO-1 co-expressing tumours ( p = 0.006). Grade 3 well-differentiated NETs had lower CD4<sup>+</sup>/FOXP3<sup>+</sup> and CD8<sup>+</sup>/PD1<sup>+</sup> 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.