Ex Vivo Modeling of Multiple Myeloma Provides Basis for Studying Treatment Combinations and Immunotherapy

Background

Multiple myeloma (MM) remains incurable despite treatment advances. While passive immunotherapy such as anti-CD38 antibodies is highly effective, active immunotherapy may provide long-lasting remissions by virtue of triggering memory. A phase 1 nivolumab study, an antibody targeting programmed death-1 (PD1), was unable to yield any responses in multiply relapsed MM patients. Conversely, preliminary trial data of lenalidomide combined with pembrolizumab, a different anti-PD1 antibody, found significantly higher response rates. These two differing outcomes reflect our limited understanding of checkpoint inhibition and immunotherapy in MM. There is a paucity of preclinical models to guide therapeutic studies. Cell lines and xenografted murine models are incapable of exploring active immunotherapy due to a lack of microenvironment and endogenous immune cell signals. Furthermore, malignant cells responsive to drugs in 2-dimensional (2D) cultures are known to display a more resistance in 3D. We have previously demonstrated that B-cell malignancies can be accurately studied using a 3D culture system of patient bone marrow mononuclear cells (BMCs) and can better inform translational trials. Herein we describe an ex vivo, 3D tissue culture model of patient-derived MM samples to more accurately test therapeutics including checkpoint inhibition using ipilimumab, a monoclonal antibody targeting cytotoxic T-lymphocyte antigen 4 (CTLA) which is crucial in co-stimulatory signaling of effector T-cells.

Methods

A 3D extracellular matrix was created using matrigel in 12-well plates. BMCs were isolated from marrow aspirates of 5 MM patients at time of diagnosis and individually cultured. Each patient sample was tested for sensitivity against increasing concentrations of ipilimumab (1X, 3X, and 10X clinical doses) added into supportive medium. Plates were monitored visually by microscopy followed by harvest on day 21 using enzymatic degradation. Unique clonotypic heavy chain immunoglobulin rearrangement (IgH VDJ) from each sample was sequenced, validated and used for semi-quantitative PCR. Semi-QT PCR with clone-specific primers estimated malignant cell survival after harvest. Flow cytometry was used to define cell populations present in culture and to correlate with clonotypic PCR data. T-cell mediated activity was examined by reverse transcription of trizol-extracted, T-cell RNA after harvest.

Results

All samples were successfully cultured, followed for 21 days and harvested. Flow cytometry confirmed presence of T-cell subsets, B-cells, NK cells and dendritic cells before and after culture in 3D. Minimal depletion of clonotypic cells was observed at 3x clinical levels of drug. At 10x simulated clinical therapeutic levels, 3 MM samples demonstrated >90% death of clonotypic MM cells while the other 2 demonstrated 62% and 72% death, respectively, compared to untreated control cultures. The extent to which the drug diffuses into the matrigel is as yet unknown. Flow cytometry of harvested cells suggest that T-cells demonstrate a modest shift toward CD4 and CD8 effector cells. Preliminary mechanistic data from one MM sample using trizol-extracted RNA and reverse transcriptase PCR harvested at 21 days from 3D culture suggests that anti-malignant, cytotoxic T-cell effect may be driven by granzyme B expression. Expanded data from the remaining samples will be presented.

Conclusions

We demonstrate that an ex vivo 3D tissue culture model of MM is both feasible and informative in studying immunotherapy. By culturing unselected BMCs which include stromal cells, immune cells and malignant populations, the 3D culture more closely mimics the tumor microenvironment with both the patient's immune system present as well as stromal supportive signals. In this study, we show that in the presence of active immune effector cells, ipilimumab has activity against patient-derived MM cells. The data suggests the importance of targeted cytotoxic T-cell activation as a primary mechanism of action. We have previously studied standard MM therapeutics such as cytotoxic chemotherapy, immunomodulatory drugs, and proteasome inhibitors in the same way. Consequently, this model is well positioned to study other immunotherapies such as other checkpoint inhibitors, cellular therapy, and combinations. Further testing with therapeutics targeting PD1/PDL1, and adenosine receptors are underway.