A new study in the journal Blood investigates why myeloma cells often fail to respond to the chemotherapy intended to kill them. The study reports that macrophages found in bone marrow may be keeping multiple myeloma cells from dying. Macrophages are white blood cells that typically destroy invading cells, including tumor cells.

In this study, myeloma cells were isolated from the bone marrow of myeloma patients and macrophages from the blood of healthy donors. The two cell types were grown together, both in and out of direct contact with one another. Cultures were then treated with dexamethasone (Decadron) or melphalan (Alkeran), drugs intended to induce myeloma cell death.

Results showed that macrophages, when cultured in direct contact with myeloma cells, prevented the myeloma cells from dying. When cultured with myeloma cells but not in direct contact with them, macrophages no longer protected the myeloma cells.

To further examine this cell-to-cell contact effect, the researchers added an adhesion antibody that binds to myeloma cells in place of macrophages. Protection given to the myeloma cells by macrophages was significantly reduced because the antibody prevented macrophages from binding to the myeloma cells.

The researchers performed another experiment that confirmed the decrease in number of dying myeloma cells was not due to their engulfment by macrophages but to greater protection conferred by the macrophages.

Finally, biopsies performed by the researchers show that macrophages were scarcely found in the bone marrow of healthy controls but did heavily infiltrate the bone marrow of myeloma patients.

The group’s findings may have significant clinical implications, as they suggest that macrophages may play an important role in protecting myeloma cells from chemotherapy-induced death.

For more information, please see the article in Blood (abstract).