Fc Receptor Targeting Reduces Bone Disease in a Pre-Clinical Model of Multiple Myeloma

Background: Multiple Myeloma (MM) is currently incurable, with a median survival of 5-7 years post diagnosis. MM-associated bone disease (MM-OBD), represents a major cause of morbidity and mortality in MM patients. Despite current therapies for MM-OBD exhibiting significant potential (e.g. Zoledronate), their clinical use has been restricted due to severe treatment associated toxicities. Safe novel therapies for MM-OBD are therefore crucially required. Molecular crosstalk between receptor activator of nuclear factor Kappa B ligand (RANKL), present on and secreted by MM plasma cells, and its corresponding receptor (RANK) on osteoclast precursors (OCPs), represents a key mechanism driving osteoclastogenesis and subsequent bone pathology in MM. Our previous studies have demonstrated that Fc receptor (FcR)-mediated signals can inhibit RANKL induced osteoclastogenesis in vitro¹. In addition, findings from preliminary studies show that FcR-mediated signalling in pre-osteoclasts can reduce MM plasma cell driven osteoclastogenesis in vitro. Further interrogation of the underlying molecular mechanisms show that FcR-mediated signals profoundly reduce RANK transcript, and subsequent protein expression, in pre-osteoclasts. However, the effects of FcR engagement on MM-OBD in vivo, and the FcR elicited signalling pathways responsible for inhibition of RANK expression have still to be elucidated.

Aims: This study aimed to determine the in vivo potential of FcR engagement to treat bone disease in a pre-clinical model of MM. Additionally, the mechanisms underlyingFcR-mediated down-regulation of RANK expression in OCPs were interrogated.

Methods: The well-characterised 5TGM1 murine model of MM, together with micro-computed topography (micro-CT), were used to evaluate the effect of FcR engagement on MM-OBD. FcR stimulation was achieved by I.P. injecting mice (every other day, following 5TGM1 I.V. cell injection), with 100 μg/ml of Protein A derived from Staphylococcus aureus (SpA). SpA has been shown to form small immune complexes (SICs) through its affinity for endogenous IgG, which in turn binds to FcγR1 receptors on monocytes and pre-OCs¹. Additionally, the potential involvement of FcR signalling pathways in the down-regulation of RANK in healthy and MM-derived human OCPs was determined via immunoblotting and the use of signalling pathway inhibitors.

Results: Twenty six days post-myeloma cell injection, micro-CT analysis of femurs revealed that mice receiving PBS (vehicle control, n=5) exhibited a significant decrease in bone morphmetric parameters consistent with bone erosion compared to non-myeloma bearing mice (n=3); trabecullar bone volume [BV/TV] = 2.673 vs. 3.449, p=0.034; trabecullar number [Tb.N] = 0.0035 vs. 0.0042, p=0.0041; trabecullar pattern factor [Tb.Pf] = 0.2329 vs. 0.2033, p=0.0393. Importantly, myeloma bearing mice (n=5) receiving SpA, were protected from MM-OBD. In human OCPs (sourced from healthy individuals and MM patients), FcR engagement substantially activated SyK, MEK-ERK1/2, and PI3K signaling cascades. However, inhibition of these pathways failed to restore RANK transcript levels.

Discussion: These findings demonstrate novel mechanisms of RANK gene expression regulation in healthy and MM OCPs, with Fc receptors representing a potential therapeutic strategy for MM-OBD. Further studies will aim to elucidate the molecular mechanisms responsible for FcR-mediated regulation of RANK gene expression.

1. MacLellan, L. M. et al. Co-opting endogenous immunoglobulin for the regulation of inflammation and osteoclastogenesis in humans and mice. Arthritis Rheum.63, 3897-3907 (2011).