A Unique Pre-Clinical Model Providing Access to a Drug Resistant Population within the Multiple Myeloma Stem Cell Niche in a Novel 3-D Culture System for Bone Marrow.
Authors: Kirshner Julia , Thulien Kyle J. , Martin Lorri D. , Marun Carina Debes , Reiman Tony , Belch Andrew R. , Pilarski Linda M. 
Published: October 2019

Abstract
Bone marrow (BM), a site of hematopoiesis, is a multicellular tissue with a complex architecture. Multiple myeloma (MM) is an incurable plasma cell malignancy where even patients in remission succumb to an inevitable relapse. While considerable progress has been made towards understanding and treating MM, to date, there is no culture system which can recapitulate the complex interactions within the BM microenvironment. Current failure to grow the MM clone within the context of human microenvironment hampers progress into the understanding of the biology of MM and design of biologically relevant therapies. Here we present an in vitro three-dimensional (3-D) tissue culture model which recapitulates the human BM microenvironment allowing for the growth and expansion of the MM clone. Cells from the BM aspirates are grown in a fibronectin, laminin and collagen rich ECM designed to reconstruct in vitro endosteum and central marrow, mimicking the in vivo microenvironment of the BM. Proliferation and redistribution of cells within reconstructed ECM results in stratification of the culture, mimicking the in vivo condition where cells occupy individual niches. Cellular composition of the culture is maintained in accordance with the proliferation properties of the BM where osteoblasts, osteoclasts, adipocytes and stromal cells differentiate along with the full complement of the hematopoietic cells. BM cultures from normal donors are well-organized with osteoclasts and hematopoietic cells occupying distinct positions in the ECM. In contrast, reconstructed BM from MM patients is disorganized in 3-D where osteoclasts intermingle with the hematopoietic compartment. The MM malignant clone is expanded in 3-D cultures as measured by real-time quantitative PCR (rqPCR) for genomic clonotypic VDJ sequences. Malignant B and plasma cells proliferate in these cultures and FISH analysis reveals that their progeny harbor chromosomal abnormalities identical to those that mark the malignant clone prior to culture. Preclinical testing of emerging therapeutics targeted for multiple myeloma is hindered by the failure of the current models to sustain growth of the myeloma clone. In the 3-D culture, myeloma clone expands within its native environment providing an ideal preclinical model where conventional (Melphalan) and novel (Velcade) therapeutics efficiently and selectively kill their target cells. In the 3-D BM culture model, non-proliferating, label retaining cells (LRC) concentrate at a putative endosteum-marrow junction, where hematopoietic stem cells have been shown to localize in vivo, suggesting that the drug-resistant myeloma stem cells localize to the endosteal niche. In a colony-forming assay, drug-resistant LRC purified from the 3-D cultures form clonal colonies composed of malignant cells with patient specific clonotypic VDJ sequences. Recapitulation of the BM architecture in vitro is a first step towards the identification and therapeutic targeting of the elusive myeloma stem cell.