A human IL-6–Expressing PDX model of multiple myeloma recapitulates clinical features and supports patient-derived tumor engraftment

Abstract Background: Multiple myeloma (MM) is a malignancy of plasma cells characterized by bone destruction, anemia, and renal failure. Although therapeutic advances have improved outcomes, patients frequently relapse, and a definitive cure remains elusive. A major barrier to curative drug development is the lack of widely-available patient-derived xenograft (PDX) models that accurately replicate MM biology and clinical presentation. Existing models, particularly those using cell lines, often fail to reflect the slower growth kinetics and therapeutic resistance seen in primary disease. While humanized cytokine mouse models have shown promise, their complexity and limited accessibility have hindered widespread adoption. Here, we demonstrate a simplified and commercially-available human IL-6–expressing NOG mouse model enables consistent engraftment of primary MM cells and recapitulates hallmark clinical disease features. Methods Six super-immunodeficient NOG mice expressing human IL-6 (NOD.Cg-Prkdc^scid Il2rg^tm1Sug Tg(CMV-IL6)1-1Jic/JicTac, Taconic Biosciences®) were used. The hIL-6 transgene, driven by the CMV promoter, was introduced via microinjection into fertilized NOD oocytes, followed by backcrossing with NOG mice. Serum IL-6 was quantified using an immunoassay (IDEXX BioAnalytics), based on Luminex xMAP technology. Mice were conditioned with 30 mg/kg busulfan (IP) 24 hours prior to tail vein injection of 1×10⁶ CD138⁺ frozen MACS-enriched primary MM cells harboring a t(4;14) translocation. T cells were inactivated using OKT3 (1 μg/million cells) on ice for 30 minutes prior to injection. Tumor engraftment was monitored biweekly from week 12 using serum protein electrophoresis (SPEP; Helena® QuickGel). At endpoint, bone marrow (BM) was collected for flow cytometry, imaging, and complete blood counts. Results ELISA confirmed stable hiL-6 expression in plasma, with a mean of 58.03 pg/mL (range: 49.16–68.75 pg/mL). Four of six mice developed a monoclonal gamma spike (M-spike) by SPEP, with a median detection time of day 141 (range: 117–159), and progressive increase over time. Flow cytometry on BM revealed ~45% CD138⁺ plasma cells with immunophenotype CD45-/CD19-/CD138+/CD38+/CD56+, consistent with MM (mean 45.2%, range 32.2%-60.1%). Therapeutic targets including BCMA and CD70 were expressed at levels typically observed in primary samples. Minimal amount of plasma cells was detected in spleen. Complete blood counts confirmed anemia in engrafted mice compared to NSG controls: Hemoglobin: 10.7 vs. 14.3 g/dL (p < 0.05), Hematocrit: 29.2 vs. 38.8% (p < 0.001), Red blood cell count: 6.0 vs. 8.24 × 10⁶/μL (p < 0.0001). BM processing revealed brittle femurs, suggestive of underlying bone disease. This was further supported by micro-CT imaging, which demonstrated medullary bone loss in the vertebrae. Conclusion This human IL-6–expressing NOG PDX model enables robust engraftment of primary MM cells and recapitulates key clinical features, including anemia, bone marrow plasma cell infiltration, and bone disease. The model represents a valuable and accessible tool for studying MM biology and testing emerging therapies. Ongoing work includes renal function assessment, cytospin-based morphological validation, and genomic profiling (e.g., exome sequencing) pre- and post-engraftment. Future studies will evaluate CAR-T cell therapies, including strategies targeting CD70 in high-risk MM.