The impact of the immune microenvironment in patients with GEP-NETs.

267Background: The immune microenvironment in neuroendocrine tumors (NETs) remains largely unexplored with very limited data evaluating its impact on survival. We aimed to characterize the immune microenvironment and lymphocytic infiltrate in patients with gastroenteropancreatic (GEP) NET. Methods: We performed comprehensive immune profiling for CD3, CD8, PD-1, IDO, and PD-L1 expression in two cohorts of patients with primary GEP-NET: patients who had short progression free survival (PFS) ≤ 4 years (n = 12) or long PFS > 4 years (n = 14) following surgical resection. Tumor associated immune infiltrates in the tumor, inner and outer invasive front were recorded and quantified. The percentage of PD-L1 membranous expression was manually counted in tumor, stroma and invasive front. To account for the correlation among multiple samples within the same patient, generalized estimating equations (GEE) were used for model estimation and hypothesis testing. Results: Patients with shorter PFS had larger primary tumo...