Original ArticlesHistopathologic Features of Colitis Due to Immunotherapy With Anti-PD-1 AntibodiesChen, Jonathan H. MD, PhD*; Pezhouh, Maryam K. MD, MSc†; Lauwers, Gregory Y. MD‡; Masia, Ricard MD, PhD* Author Information *Department of Pathology and Laboratory Medicine, Massachusetts General Hospital, Boston †Department of Pathology, Johns Hopkins Medical Institutions, Baltimore, MA ‡Department of Anatomic Pathology, H. Lee Moffitt Cancer Center and Research Institute, Tampa, FL G.Y.L. and R.M. are co-corresponding authors. The authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this article. Correspondence: Ricard Masia, MD, PhD, Department of Pathology and Laboratory Medicine, Massachusetts General Hospital, 55 Fruit St., Warren 219, Boston, MA 02114 (e-mail: [email protected]). The American Journal of Surgical Pathology: May 2017 - Volume 41 - Issue 5 - p 643-654 doi: 10.1097/PAS.0000000000000829 Buy Metrics Abstract Programmed cell death protein 1 (PD-1) blocking agents are novel immunotherapeutics used for treatment of advanced-stage malignancies. They have shown promise in the treatment of several malignancies, with greater efficacy and better tolerability than cytotoxic T-lymphocyte antigen 4 (CTLA-4) blocking agents. However, as with anti-CTLA-4 agents, clinically significant colitis remains an important complication. Although there is growing awareness of the histopathologic features of anti-CTLA-4 therapy, there is little information on the pathologic features of anti-PD-1 colitis. We describe here the histopathologic findings in 8 patients who developed colitis while on anti-PD-1 monotherapy. The most common pattern of injury observed (5/8 cases) was an active colitis with neutrophilic crypt microabscesses and with prominent crypt epithelial cell apoptosis and crypt atrophy/dropout. These latter features are reminiscent of other colitides with prominent apoptosis such as acute graft-versus-host disease or certain drug-induced colitides. The remainder of cases (3/8) showed a lymphocytic colitis-like pattern, characterized by increased intraepithelial lymphocytes and surface epithelial injury. Apoptosis was also often increased in these cases but crypt atrophy/dropout was not present. In patients who experienced recurrence of anti-PD-1 colitis, histologic features were similar to the initial insult but, in addition, features of chronicity developed that mimicked inflammatory bowel disease (basal lymphoplasmacytosis and crypt architectural irregularity, and Paneth cell metaplasia in 1 case). Awareness of the clinical scenario, however, should allow pathologists to suggest anti-PD-1 colitis. Interestingly, recurrent colitis was observed in patients who had been off anti-PD-1 therapy for many months. As anti-PD-1 agents are increasingly used in oncology, we present this series to increase awareness of anti-PD-1 colitis among pathologists, to facilitate its timely diagnosis and treatment. Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved.