Optimizing Immunotherapy in Multiple Myeloma: Restoring the Function of Patient’s Monocyte-Derived Dendritic Cells by Inhibiting p38 or Activating MEK/ERK MAPK and Neutralizing Interleukin-6 in the Progenitor Cells.

Previous studies have demonstrated that circulating dendritic cells (DCs) in myeloma patients are functionally abnormal. However, the phenotypic and functional properties of monocyte-derived DCs (MoDCs) from myeloma patients, which are commonly used for immunotherapy in patients, are still poorly defined. This study was undertaken to examine the quality of MoDCs from myeloma patients compared with cells from healthy donors. We analyzed and compared cells from 12 newly diagnosed, stage II or III myeloma patients and 10 healthy donors. MoDCs were generated by culturing adherent cells or purified CD14⁺ monocytes in medium containing GM-CSF and IL-4. A cocktail of IL-1β, IL-6, TNF-α, and PGE2 was used to induce DC maturation. We found that patient-derived MoDCs are phenotypically and functionally defective. Compared with their normal counterpart, patient-derived, immature and mature MoDCs expressed significantly lower levels of CD1a, CD40, CD80, and HLA-DR. The mature cells were also poor at activating alloreactive T cells, presenting recall antigen and activating autologous antigen-specific T cells, and inducing myeloma-specific cytotoxic T-lymphocyte response. These abnormalities may be attributed to elevated production of autocrine cytokines such as IL-6, constitutively activated p38 and STAT3, and inhibited MEK/ERK signaling pathways in the progenitor cells. Treatment with neutralizing IL-6-specific antibody and more importantly, p38 inhibitor, or both, could correct these abnormalities. Treating patient-derived cells with these agents not only significantly increased cell yield, but also produced MoDCs that were as functional as their normal counterpart. We further showed that the p38 inhibitor inhibited the activity of p38 and drastically increased MEK1/2 and ERK1/2 phosphorylation and activation, and a combination of IL-6 neutralizing antibody with the p38 inhibitor further upregulated phosphorylation of MEK and ERK. By examining the role of STAT3 in MoDC differentiation, we found that STAT3 activation did not impair differentiation of patient-derived cells. These results indicate that the p38 inhibitor improved the generation and function of the MoDCs via inhibiting p38 and activating Raf/MEK/ERK signaling pathways, independent of the STAT3 pathway. Thus, this study has delineated the mechanistic defects of MoDCs from myeloma patients, and identified ways for restoring the function of the cells to improve the efficacy of DC-based immunotherapy against multiple myeloma.