Mast Cells Support Multiple Myeloma Survival and Engraftment through Secreted Factors

Background

Multiple Myeloma remains an incurable and fatal cancer with high morbidity. Despite the advent of highly efficacious therapies, all patients eventually relapse. The reasons why all therapies eventually fail are unclear. An understudied area of myeloma persistence is the bone marrow microenvironment in which cancer cells reside. When myeloma cells are removed from the bone marrow, they rapidly die. Little is known about which cells and structures in the bone marrow support myeloma cells, and even less is known about how they achieve that support. During the development of the NSG-hIL6 patient derived xenograft (PDX) model of myeloma (Hasanali et al. JCI Insight. 2024), we noted a significant infiltration of human mast cells and asked if this these cells had an effect on myeloma survival and persistence directly.

Research Significance

We present a study highlighting the supportive role of human mast cells in myeloma engraftment and persistence, exhibiting both the potential for the NSG-hIL6 model to study the myeloma microenvironment and new targets for further research.

Methods

Vk-myc mouse myeloma cells were injected into normal B6 mice or mast cell deficient Sash mice. Mouse sera were assessed for an M-spike by serum protein electrophoresis. Bone marrow mononuclear cells (BMMNCs) from a new diagnosis myeloma patient and in vitro derived human mast cells (hMast) from human stem cell collections were acquired and then injected into busulfan conditioned NSG-hIL6 mice as BMMNCs alone, mast cells alone or both. Levels of human immunoglobulin were assessed from mouse sera (a surrogate for myeloma engraftment) at 4-8 week intervals by ELISA. BMMNCs from myeloma patients were co-cultured with hMast cells in vitro and assessed for live myeloma cells after 72 hours by flow cytometry for intracellular light chains. We performed similar experiments using a tissue culture insert to test the need for mast cell-myeloma cell contact. Using supernatants from in vitro co-culture experiments and the Proteome Profiler Array from R&D Biosciences, we identified 10 cytokines secreted by mast cells that were not secreted by myeloma BMMNCs. We then treated myeloma BMMNCs with each cytokine alone or in combination.

Results

Normal B6 mice engrafted Vk-myc cells by 3 weeks post injection compared to mast cell deficient Sash mice which engrafted at 4 weeks (p=0.015). Myeloma BMMNCs co-injected with mast cells into the NSG-hIL6 model showed engraftment of patient myeloma at 20 weeks post injection compared to BMMNCs injected alone at 28 weeks (p=0.033). In vitro culture of myeloma BMMNCs with mast cells showed a 20.9% increase in viability compared to myeloma BMMNCs alone (p=0.01), and this increase was maintained even if BMMNCs were separated from mast cells by a tissue culture insert, suggesting a secreted factor or factors was responsible for the survival difference. Proteome Profiler Array of supernatants showed sCD14, Dkk-1, DPPIV, CXCL5, FGD-15, IGFBP2, IL6, MiP-3b, Osteopontin and uPAR increased in mast cell containing conditions. We treated myeloma BMMNCs with each cytokine individually or all in combination. There was no survival advantage to any cytokine alone, but when all were combined, there was a 10% increase in surviving myeloma cells (p=0.02).

Conclusion

These data highlight mast cells’ ability to directly support the engraftment and survival of human myeloma cells in vitro and in vivo through a milieu of secreted cytokines, opening the door to further study of novel myeloma survival factors. These results also support the NSG-hIL6 model as a tool to study the human myeloma microenvironment.