Inclusion Criteria:  Documented history of consensus histologic, laboratory, and clinical diagnostic criteria of iMCD. Archival and/or baseline incisional/excisional biopsy for retrospective central histologic confirmation of iMCD. CDCNRC-defined disease progression on or after prior treatment with siltuximab at 11 mg/kg q3w without unacceptable toxicity within 12 weeks between the last dose of siltuximab and the date of signed patient informed consent form (ICF). At least 1 measurable abnormal lymph node mass that is ≥1 cm in its longest transverse diameter as assessed by computerized tomography (CT) scan that has not been previously irradiated. Elevated (>10 mg/L) and rising serum CRP in the absence of additional iMCD treatment.  Evidence of at least an additional one of the following laboratory or clinical signs of iMCD per international, evidence-based consensus diagnostic criteria for HIV or HHV 8-negative iMCD:  Anemia, thrombocytopenia, hypoalbuminemia, renal dysfunction, or polyclonal hypergammaglobulinemia. Constitutional symptoms (night sweats, fever (>38°C), weight loss, or fatigue (CTCAE lymphoma B-symptoms score ≥2), large spleen and/or liver, fluid accumulation, eruptive cherry hemangiomatosis/violaceous papules, or lymphocytic interstitial pneumonitis.  Adequate clinical laboratory measurements within 3 weeks prior to study entry in all parameters below:  Absolute neutrophil count ≥1.0 × 109/L, hemoglobin <17 g/dL, and platelets ≥50 × 109/L without transfusion, hematopoietic growth factors, or both for >7 days prior to measurement. AST, ALT, total bilirubin, and alkaline phosphatase ≤5 × ULN. Fasting cholesterol <300 mg/dL and fasting triglyceride <400 mg/dL. Age ≥12 years.  Exclusion Criteria:  Documentation of HIV or HHV-8 infection or presence of other infection-related disorders that resemble clinical or histological features of iMCD Diagnosis of any malignant/benign lymphoproliferative disorders Diagnosis of autoimmune/autoinflammatory disease Treatment with corticosteroids (prednisone dose-equivalent >1 mg/kg/day) within 7 days prior to study entry. History of solid organ transplant, allogeneic bone marrow transplant, or allogeneic peripheral blood stem cell transplant.  Previous malignancy with the following exceptions:  Past malignancy with treatment that was completed at least 2 years before signing informed consent and the patient has no evidence of disease, or Concurrent malignancy that is clinically stable and does not require tumor-directed treatment (eg, nonmelanoma skin cancer and carcinoma in situ)