The findings of a recent study, which will appear in Cancer Cell this month, provide a clue to the mystery of multiple myeloma cancer cells’ resistance to powerful drugs. A specific type of immune system cell is literally “switching teams” and protecting the cancer cells instead of destroying them.

The study’s authors note that this is the first time that any type of immune system cell has been shown to protect cancer cells.

“The study has identified a defect in a component of the immune system called plasmacytoid dendritic cells (pDCs),” says Dr. Dharminder Chauhan, the study’s co-leader. pDCs are an important component in combating disease, but in the case of multiple myeloma, these cells take on a new identity – they support tumor growth, protect against drugs, and weaken the immune system.

Researchers became interested in studying pDCs when they noticed a high level of these cells in myeloma patients’ bone marrow. While studying laboratory-grown myeloma cells and animals with myeloma, the researchers noticed a release of proteins when pDCs attach to myeloma cells.

What distinguishes this protein release from other immune system responses is the fact that it affects both immune cells and cancer cells. The proteins spur production of myeloma cells, converting the pDCs from immune system cells to guardians of cancer cells.

Researchers do not understand how or why the cells are converted, but they do have a hypothesis. The proteins released during the immune system response may awaken different genes in the pDCs, transforming their protective function to one that promotes cancer. The effect has been likened to bribing a police officer to join a gang of hoodlums.

Fortunately, in discovering pDC conversion, the researchers also found a way to keep the cells’ behavior in line with the immune system.

“Importantly, our study provides a therapeutic remedy for correcting this defect,” Chauhan added. A drug known as cytosine phosphate guanine oligodeoxynucleotides (CpG ODNs) combats this problem by attaching to receptors on the surface of pDC cells. This triggers the pDCs to return to their normal status as immune system cells.

CpG ODNs have been tolerated well in clinical trials for other forms of cancer. Next, Chauhan and his colleagues hope to begin clinical trails with CpG ODNs in myeloma patients.

For more information, please see the press release or the study in Cancer Cell (abstract).