Phagocytic Mesothelial Cells in Pericardial Effusion Following Allogeneic Peripheral Blood Stem Cell Transplantation for Chronic Active Epstein–Barr Virus Infection: A Distinctive Cytomorphological and Immunocytochemical Case Study

Phagocytic activity of mesothelial cells is a rare phenomenon requiring careful differentiation from histiocytic hemophagocytosis to ensure appropriate patient management, as these phenomena could exhibit distinct clinical implications. In this study, we report a case of phagocytic mesothelial cells found in pericardial effusion that developed 18 months after allogeneic peripheral blood stem cell transplantation for chronic active Epstein-Barr virus (EBV) infection. The patient presented with fever and dyspnea, alongside mild chronic graft-versus-host disease, affecting the shoulders and skin. Cytological examination revealed mesothelial cells containing intracytoplasmic lymphocyte-like structures and hemosiderin granules, confirmed by Berlin blue staining. Immunocytochemical analysis indicated pan-cytokeratin (AE1/AE3), calretinin, and desmin positivity as well as CD163 negativity, confirming their mesothelial origin. The ferritin level of the patient remained at 370.0 ng/mL. EBV DNA was undetectable in peripheral blood. The clinical course was favorable, without systemic complications or recurrent effusion. This case highlights the importance of accurate cytomorphological and immunocytochemical characterization of phagocytic mesothelial cells in post-transplantation settings.