The NSG-hIL6 Mouse As a Novel Murine Patient Derived Xenograft Model of Plasma Cell Dyscrasias for Study of Disease Biology and Therapeutic Development

Background Plasma cell dyscrasias include pre-malignant and malignant conditions spanning the spectrum from monoclonal gammopathy of undetermined significance (MGUS) to smoldering multiple myeloma (SMM) to active multiple myeloma (aMM) and include the disorders AL amyloidosis and plasma cell leukemia (PCL). Though the last 2 decades has yielded effective therapies against these disorders, there remain no reliably curative treatments, and complications due to multiple myeloma are most often the cause of death. A major reason for the lack of curative therapies is the dearth of relevant research models in which to study these diseases and test new drug candidates. Cell lines that do not reflect the biology of mainstream myeloma are currently the mainstay of disease modeling. Plasma cells do not survive well outside the bone marrow and to this point, little success has been made using primary tumors from patients to model disease. Thus, there is a significant need for more relevant models to build the next, hopefully curative, generation of therapeutics for these diseases.

Research Significance We present a novel immunodeficient murine model able to engraft plasma cell dyscrasias from primary patient tumors and the ability to study basic biology and test efficacy of cellular therapies, representing a significant advance in this field.

Methods The NSG-h(IL6) mouse, developed by Dr. Shultz, builds on the background NSG mouse that lacks adaptive immunity and has limited innate immunity. It expresses a BAC with a piece of human chromosome 7, containing the human IL-6 gene and all associated local promoter and enhancer elements. With the expertise of the stem cell and xenograft core facility (SCXC), mice were preconditioned with busulfan, and 1e6 mononuclear cells (MNCs) isolated from human myeloma patient marrow aspirate were injected into the murine femur. Blood and urine were sampled every 4-5 weeks and analyzed for the presence of human antibodies by ELISA. Deceased mice were sent for histology and their spleens and marrow analyzed for the presence of clonal plasma cells by flow cytometry. Serum protein electrophoresis (SPEP) and complete blood counts (CBC) were noted as hallmarks of disease. Samples from patients with MGUS, SMM, aMM (both newly diagnosed and relapsed/refractory), PCL and AL amyloidosis were engrafted into mice as above. Mice were treated with untraduced T-cells (UTD) or BCMA-directed CARTs 10 weeks after engraftment of active myeloma and blood was assayed for level of immunoglobulins to follow response.

Results Analysis of murine serum and urine by ELISA revealed the presence of rising titers of human antibodies in all engrafted mice and the presence of an M-spike on SPEP. NSG-hIL6 mice conditioned with busulfan showed greater engraftment than non-IL6 containing mice and non-conditioned mice (7 vs 2 vs 0, p<0.005). Light chain specific ELISA showed a clear predominance of one light chain, congruent with the patient sample that was engrafted. Flow cytometric analysis of engrafted murine bone marrow showed the presence of light chain restricted plasma cells. Histology showed the presence of plasma cells in the marrow, spleen and liver of engrafted animals with IHC positivity for specific light chain and CD138. CBC showed pancytopenia (p<0.005) with anemia being most significant in engrafted vs. unengrafted animals. All fresh samples and nearly all frozen samples of MGUS, SMM, aMM, PCL and AL amyloidosis were able to be engrafted at high rates (>50% of mice). Mice injected with column separated CD138+ cells alone showed no engraftment vs. mice injected with total MNCs from the marrow of the same myeloma (3 vs 0, p<0.005 ). BCMA directed CARTs significantly decreased circulating antibody compared to UTD controls (p<0.005).

Conclusion Using the NSG-hIL6 mouse, busulfan conditioning and intrafemur injections, we have been able to engraft all the major plasma cell dyscrasias as denoted by the presence of monoclonal human antibody in these mice and presence of light chain restricted plasma cells congruent with the patient's paraprotein. We have preliminarily shown that the ability to engraft disease is dependent on human supporting cells. Additionally, treatment with BCMA directed CARTs is showing a response. This simple model will allow for further preclinical testing of new agents against multiple myeloma and associated plasma cell neoplasms and allow for study of basic biology of these diseases.

Garfall:Janssen: Other: Independent data monitoring committee; Janssen, Novartis, Tmunity, CRISPR Therapeutics: Research Funding; Janssen, GSK, Amgen, Legend: Consultancy. Carroll:Cartography Bioscences: Membership on an entity's Board of Directors or advisory committees; Janssen Pharmaceuticals: Consultancy. Shultz:Dren Bio, Inc.: Consultancy; Blue Rock Therapeutics: Consultancy; ORNA: Consultancy. Sheppard:Fate Therapeutics: Consultancy, Current equity holder in publicly-traded company; Tmunity Therapeutics: Consultancy, Current equity holder in publicly-traded company; Pfizer: Consultancy, Current equity holder in publicly-traded company; Immunai: Consultancy, Membership on an entity's Board of Directors or advisory committees; Outpace Bio: Consultancy, Membership on an entity's Board of Directors or advisory committees. Stadtmauer:BMS, Celgene, Abbvie, Sorrento: Research Funding.