Macrophages in multiple myeloma: emerging concepts and therapeutic implications

Abstract

Multiple myeloma, a clonal plasma cell malignancy, has long provided a prototypic model to study regulatory interactions between malignant cells and their microenvironment. Myeloma-associated macrophages have historically received limited scrutiny, but recent work points to central and non-redundant roles in myeloma niche homeostasis. The evidence supports a paradigm of complex, dynamic and often mutable interactions between macrophages and other cellular constituents of the niche. We and others have shown that macrophages support myeloma cell growth, viability and drug resistance through both contact-mediated and non-contact-mediated mechanisms. These tumor-beneficial roles have evolved in opposition to, or in parallel with, intrinsic pro-inflammatory and tumoricidal properties. Thus, simple blockade of protective “don't eat me” signals on the surface of myeloma cells leads to macrophage-mediated myeloma cell killing. Macrophages also enhance the tumor-supportive role of mesenchymal stem/stromal cells (MSCs) in the niche: importantly, this interaction is bidirectional, producing a distinct state of macrophage polarization that we termed “MSC-educated macrophages.” The intriguing pattern of cross-talk between macrophages, MSCs and tumor cells highlights the myeloma niche as a dynamic multi-cellular structure. Targeted reprogramming of these interactions harbors significant untapped therapeutic potential, particularly in the setting of minimal residual disease, the main obstacle toward a cure.

Keywords::

• Monocytes
• monocyte and macrophage biology
• immunotherapy
• myeloma
• immunotherapeutic approaches

Potential conflict of interest:

Disclosure forms provided by the authors are available with the full text of this article at www.informahealthcare.com/lal.

Work in our laboratories is supported by a Leukemia Research Foundation New Investigator Award (F.A.), a Kirschstein National Research Service Award (T32 HL007899, Sheehan) (C.H.), a CTSA TL1 award by the National Center for Advancing Translational Sciences (9U54TR000021, Drezner) (J.L.J.), an American Society of Hematology Trainee Research Award (R.A.D.), the UWCCC Trillium Fund for Multiple Myeloma Research and the UW Carbone Cancer Center Core Grant (P30 CA014520).