Sustained Complete Response in Refractory Idiopathic Multicentric Castleman Disease Following a Single CD19 CAR T‐Cell Infusion: A Case Report With Mechanistic Insight

ABSTRACT Idiopathic multicentric Castleman disease (iMCD) is rare and life‐threatening; although siltuximab is standard first‐line therapy, a substantial proportion of patients are refractory or experience relapse. We report a 31‐year‐old woman with HHV‐8–negative iMCD with idiopathic plasmacytic lymphadenopathy (IPL; iMCD‐IPL), intractable to five prior lines of therapy, who declined siltuximab and enrolled in ChiCTR1900025419. After fludarabine/cyclophosphamide lymphodepletion, she received 1 × 10 6 CAR + T cells/kg of autologous CD19 CAR T cells on day 0. Grade 1 cytokine release syndrome ( T max 38.2°C) resolved with supportive care; no ICANS occurred. CAR transgene levels expanded (peaking on day +11) and remained detectable through day +58, and flow cytometry showed CD19 + B‐cell depletion from day +7 to day +198. By day +198, she met CDCN criteria for complete biochemical and clinical response with normalization of inflammatory markers and hematologic recovery. Panhypogammaglobulinemia was managed with intravenous immunoglobulin replacement. At > 12 months post‐infusion, she remains in complete, treatment‐free remission. This observation supports a B‐cell–centric model for refractory iMCD‐IPL and motivates prospective evaluation of CD19 CAR T‐cell therapy.