Development of a Novel in Vivo Model for Human Myeloma Via Humanization of the Bone Marrow Niche
Authors: Strowig Till , Hafemann Anja , Hopf Stephanie , Sehgal Kartik , Koduru Srinivas , Kocoglu Mehmet H. , Das Rituparna , Dhodapkar Madhav V. , Flavell Richard A. 
Published: October 2019

Abstract
Abstract 325
Multiple Myeloma (MM) is a human hematopoietic malignancy of the B cell lineage with 5 year survival of only 40%. Most MM patients succumb to progressive disease and therefore novel therapies are urgently needed. MM is characterized by growth of malignant plasma cells in the bone marrow microenvironment (ME) leading to bone erosion and fracture. Interactions between tumor and host ME play a critical role in the biology of MM and are a target of most new therapies against this tumor. The dependence of human MM on ME is further illustrated by the preferential requirement for growth of primary MM cells in implanted human fetal bone, but not the murine bone in immune-deficient mice in the scid-hu model. This model is however suboptimal for preclinical testing of novel therapies and there is a major unmet need for development of newer models to study MM and the interactions between human MM and its microenvironment.
Some of the barriers to growth and xenotransplantation of human cells in immune-deficient Rag2−/−Il2rg−/− mice include macrophage-mediated innate immune rejection, as well as non-cross reactive growth factors. In order to overcome these limitations, Rag2−/−Il2rg−/− mice were genetically engineered to express human versions of the macrophage receptor signal regulatory protein-alpha (SIRPa), as well as several non-cross-reactive growth factors. The combination of these growth factors leads to synergistic enhancement of growth of transplanted human cells in these mice. In order to facilitate the potential ability of these mice to serve as hosts for human MM, these mice were further modified to express a human growth factor that is a critical MM-related cytokine and that is also non-cross reactive between humans and mice.
INA-6 is a human IL6 dependent MM cell line that is unable to grow in standard immunodeficient mice such as NOD/scid/Il2rg−/−, but instead grows only in the implanted fetal bone in the scid-hu model. Injection of genetically humanized mice with INA6 cells led to facile growth of MM cells in the bone, leading to lytic bone disease. Next we tested whether primary tumor cells from MM patients could grow in these genetically engineered mice. Bone marrow mononuclear cells were separated into CD138+ and T cell depleted-CD138- fractions. Injection of either fraction led to growth of MM cells in these mice. These data therefore indicate that the prior inability to reliably grow primary MM cells in mice outside of the scid-hu model was primarily related to the non-cross-reactive cytokines and innate immune barriers mediated by the CD47-SIRPa axis.
These data demonstrate that the genetic humanization of immune-deficient mice to modify the bone marrow niche to express non-cross reactive growth factors leads to facile growth of primary human MM cells in vivo. These mice can therefore serve as a valuable tool to test new patient-specific therapies depending on the genetic makeup of the tumor. Further advancement of this model to include growth of autologous immune cells would lead to humanized immune-competent models much needed for patient-specific preclinical testing of novel targets. Similar approaches can also be extended to develop in vivo models for other hematologic malignancies as well.

Disclosures:
Dhodapkar: Celgene: Research Funding; KHK: Research Funding.