Possible effects of IgG immune complexes on RANKL-mediated osteoclastogenesis

Objectives: The goal of this study was to determine the possible role of IgG-IC on RANKL-mediated osteoclastogenesis.

Methods: To determine if pre-osteoclast cells express Fc receptors, bone marrow cells (BMC) from C57BL/6 mice were treated with M-SCF and/or RANKL. After 3 days of incubation, expression of CD11b, CD14, CD16/32 or CD64 on BMC was analyzed by flow cytometry. BMC isolated from mice deficient in the Fc receptor common γ-chain (FcRγ^-/-) and FcγRIIB (FcγRIIB^-/-), and wild-type mice, were cultured with recombinant M-CSF, and/or sRANKL, in the presence or absence of immobilized or soluble IgG isotypes. TRAP activity, indicative of osteoclastic differentiation, was determined by an enzymatic assay in a 7-day culture.

Results: CD11b+/CD14+ cells, which represent pre-osteoclasts, were found in a higher percentage of BMC treated with M-CSF (11.19%) than non-treated cells (0.34%). Expression of CD16/CD32+ (FcγRIII/FcγRII) or CD 64+ (FcγRI) on CD14+ cells was induced in M-CSF treated cells (1.39% and 7.42%, respectively) compared to non-treated (0.18% and 1.80%, respectively), indicating the induction of FcRγs on osteoclast precursors by M-CSF. TRAP activity was significantly higher in wild-type mouse BMC stimulated with immobilized IgG2a, IgG2b and IgG3 (Kruskall Wallis, p<0.05), as well as soluble IgG2a (p<0.05). Such IgG-isotype-mediated increases of TRAP activity were not seen in BMC from FcRγ-/- or FcγRIIB-/- mice, except immobilized IgG3 in FcγRIIB-/- BMC, which showed increased TRAP activity. Conclusion: Based on expression of Fc receptors on pre-osteoclasts, abundant osteoclastogenic activity after stimulation of wild-type BMC with IgG-ICs and loss of IgG-IC-induced osteoclastogenesis in FcRγ-/- mice, we conclude that IgG-ICs increase osteoclastogenesis triggered by RANKL in periodontal lesions. Support by DE03420, DE18310 (NIH/NIDCR).