An Innovative Humanized Mouse Model of Multiple Myeloma (MM) for Studying MM in Its Natural Environment Using Multi-Color Light Sheet Fluorescence Microscopy (LSFM)

Abstract 1809

Various imaging platforms are well established in hematology research. Nevertheless, the three-dimensional architecture of the bone marrow and tumor growth within this microenvironment remain largely uncharacterized. To date the major hindrance to microscopically image tumor engraftment and the immune response in the bone marrow on a single cell level is the compact structure of the bone that is almost impossible to image through.

Therefore, we developed a novel bioluminescent mouse model that recapitulates the clinical characteristics of MM using the new human UMM3 cell line (CD38⁺, CD56⁺, CD138⁺, CD19⁻, CD20⁻), from the pleural effusion of a patient with an IgG lambda myeloma (ISS stage I) as well as the well-characterized RPMI8226 cell line transduced to express eGFP and firefly luciferase (UMM3eGFPluc and RPMIeGFPluc). 1×10⁶ MM cells were injected intravenously into NOD.Cg-Prkdcscid IL2rg (NSG) mice and disease progression and bone marrow (BM) engraftment were monitored twice weekly by in vivo bioluminescence imaging. Both cell lines homed to the BM compartment, reflecting MM pathophysiology. Histological analysis confirmed BM engraftment and showed multiple osteolytic lesions for both UMM3 and RPMI cells.

Since we were interested in imaging the interactions between human MM cells and the bone marrow microenvironment on a single cell level, we employed the multi-color LSFM after decalcification, specific deep-tissue antibody staining and clearing of the bone structures. With this innovative microscopy technique, we were able to establish a novel tool to display tumor cell engraftment in the bone marrow compartment in three dimensions through the intact bone.

We recorded 1500 optical sections for three individual channels each (488, 532, and 647 nm) with an increment of 5μm which allows scanning the whole bone marrow compartment of the sternum within minutes in single-cell resolution. Using higher magnification enabled us to even visualize subcellular components within the bone marrow.

Moreover, tissue autofluorescene, recorded mainly in the 488 nm channel, displayed detailed microanatomical structures which allowed for the localization of individual cells within their anatomical context. We could establish protocols for various fluorophore-coupled antibodies and successfully stained CD138⁺ cells in relation to CD3⁺ cells and to the microenvironment in the bone marrow. The CD138-positive cells infiltrated the bone marrow in a number of small clusters and comprising about 15% of cellular elements in total. Ex vivo bioluminescence imaging of the sternum from UMM3 tumor-bearing mice revealed massive infiltration of luciferase-expressing cells into the bone marrow compartment. This could also be confirmed by flow cytometrical analysis of bone marrow cells which showed eGFP⁺hCD138⁺ cells.

We have successfully introduced a novel technique to study MM cell engraftment and progression in a humanized mouse model. We were able to track the tumor cells both in the living animal by in vivo bioluminescence imaging and on single-cell resolution by multi-color LSFM within the intact bone. Our model may lead to better insights into the pathogenesis of MM and could serve as a model for preclinical testing of new therapeutic approaches for the treatment of MM patients.