A New Mouse Model of Multiple Myeloma: Engraftment of Primary Human Multiple Myeloma in SCID/Beige Mice.
Authors: Arditti Fabian D. , Siegel David , Friedman Thea M. , Korngold Robert 
Published: October 2019

Abstract
Multiple Myeloma (MM) is a largely incurable malignancy characterized by the abnormal expansion of clonal plasma cells which primarily compromises the bone marrow (BM) and induces a devastating osteolytic process. There is wide interest in animal models capable of supporting the timely and effective outgrowth of primary human MM. Current Hu/SCID models of MM are useful, but limited by availability of supporting human tissues and the extensive time required for establishment of tumor engraftment. In this study, we achieved the rapid engraftment in SCID/Beige mice of primary human MM from BM aspirates of 6 out of 7 patients and a pleural effusion of another patient, after purging erythrocytes by lysis, rather than ficoll, and injecting the cells orthotopicaly through the knee joint into the femoral BM cavity. Human immunoglobulin (hIg) was detected in the sera of mice by a customized ELISA assay as early as day 6 after the intrafemoral (i.f.) injection, being the first indication of MM engraftment in this model. At 3-4 weeks, the level of hIg was 19 ± 4.8 ug/ml (mean ± SD) in the sera of engrafted mice, ranging from 4.4 to 25.4 ug/ml, compared to uninjected mice (1.8 ± 1 ug/ml). In 7 out of 16 tested mouse urine samples, hIg was also found, although it is not yet clear whether this was in the form of human light chains, and thus similar to the Bence-Jones proteins in patients. In addition to hIg, engraftment was also evidenced at the cellular level by flow cytometric analyses of cell suspensions obtained from different organs of injected mice at end time-points. By 12 weeks after injection, the engraftment of human MM found in the BM of mice, as defined by the co-expression of CD38 and CD138, was 0.60±0.11% (mean ± SD), as compared to uninjected mice (0.01±0.05%). A significantly lower (p=0.028) engraftment was observed in mice injected via the i.v. route (0.34±0.03%). Although MM cells were also found in the spleen of i.f. injected mice (0.99±0.7%), the mouse thymus showed significantly higher levels of engraftment than the mouse BM for all the injected patient samples (2.30 ± 0.90%, p=0.0079). In the long term, (32 weeks post-injection) an increased engraftment was observed in the mouse BM (1.27±0.30, mean % ± SD), but it was still higher in the thymus (4.50±0.80, p=0.012). Furthermore, in an effort to increase tumor engraftment, mice were subjected to mild total body irradiation (2.5 Gy), one day prior to injection of the primary MM cells. This resulted in much higher engraftment in the thymus (19.0±3.4%, p=0.002), but not in the BM or spleen. Altogether, these results suggest the rapid engraftment of primary human MM in SCID/Beige mice without additional transplanted supporting tissue. This model could be suitable for testing prospective new therapies and drugs. Additionally, homing of the malignant plasma cells to the unpopulated thymus of SCID/Beige mice suggests that thymic stromal cells provide a supportive microenvironment for continued growth of the tumor cells. Further studies are warranted to understand the interaction between human MM cells and mouse thymus, and the possible implications into the cross-talk between the same MM cells and human BM stroma.