Mechanism and Function of Monoclonal Antibodies Targeting Siglec-15 for Therapeutic Inhibition of Osteoclastic Bone Resorption

Background: Bone-resorbing osteoclasts express the Siglec-15 receptor on their plasma membrane. Results: Siglec-15 antibodies inhibit osteoclast differentiation and induce receptor endocytosis and degradation. Conclusion: These results establish that Siglec-15 antibodies target osteoclasts in vivo and reveal a likely mechanism of action. Significance: Siglec-15 could serve as a target of novel therapeutics for osteoporosis and cancer-associated bone loss. The use of monoclonal antibodies to target functionally important cell-surface proteins on bone-resorbing osteoclasts represents a promising approach for treatment of cancer-associated bone loss and other skeletal pathologies. Previously, we identified Siglec-15, a little studied sialic acid-binding receptor, as a candidate target that is highly up-regulated during osteoclast differentiation induced by the cytokine receptor activator of NF-B ligand (RANKL). In this report, we confirm that Siglec-15 is localized to the plasma membrane where it can be targeted by monoclonal antibodies to inhibit differentiation of functional osteoclasts in vitro. Furthermore, we found that treatment of mice with these antibodies led to a marked increase in bone mineral density, consistent with inhibition of osteoclast activity. Interestingly, osteoblast numbers were maintained despite the anti-resorptive activity. At the molecular level, Siglec-15 interacts with the adapter protein DAP12 and can induce Akt activation when clustered on the osteoclast cell surface, which likely represents its normal signaling function. Importantly, we discovered that monoclonal antibodies induce rapid internalization, lysosomal targeting, and degradation of Siglec-15 by inducing receptor dimerization. This study defines a key regulatory node that controls osteoclast differentiation and activity downstream of RANKL and supports further development of Siglec-15 antibodies as a novel class of bone loss therapeutics.