Autoimmune Pancreatitis Type 2 Diagnostic Utility of PD-L1 Immunohistochemistry

Background: Autoimmune pancreatitis (AIP) encompasses a heterogenous disease group that includes IgG4-related type 1 AIP and non-IgG4-related type 2 AIP. Clinically and on imaging, type 2 AIP mimics type 1 AIP, other forms of chronic pancreatitis and pancreatic ductal adenocarcinoma (PDAC); therefore, discriminatory markers may aid proper diagnosis. Herein, we examine the expression of PD-L1 and indoleamine 2,3-dioxygenase (IDO1) as a diagnostic tool to distinguish type 2 AIP from other forms of pancreatitis and PDAC. Design: We evaluated 35 pancreatectomy specimens diagnosed with type 2 AIP and potential mimics of this disease including type 1 AIP (n=14), chronic pancreatitis-not otherwise specified (n=10), groove pancreatitis (n=14), and PDAC (n=278). We scored inflammatory infiltrates, fibrosis and atrophy and performed immunohistochemical staining for PD-L1 and IDO1. We validated our findings on a series of endoscopic ultrasound-guided biopsies from patients with suspected type 2 AIP and inflammatory and neoplastic mimics of this disease (n=37). Results: The mean age of patients with type 2 AIP was 50 years with a F:M ratio of 1.2:1. Patients with type 2 AIP showed pancreatic ductal staining for PD-L1 and IDO1 in 69% (24/35) and 60% (15/25) of cases, respectively. PD-L1 reactivity was noted in 3% of patients with other forms of chronic pancreatitis and 3% of PDACs; notably, peritumoral ducts and acini were negative. Eight of 9 endoscopic ultrasound-guided biopsies with pancreatic ductal epithelium from patients with type 2 AIP were positive for PD-L1, while the inflammatory and neoplastic mimics were negative. Collectively, the sensitivity and specificity of PD-L1 as a marker of type 2 AIP was 70% and 99%, respectively. Conclusions: Ductal PD-L1 reactivity has the potential to distinguish type 2 AIP from other forms of pancreatitis and PDAC.