Distinct Cross Talk of IL-17 & TGF-β with the Immature CD11c TRAF6 -null Myeloid Dendritic Cell-derived Osteoclast Precursor (mDDOCp) May Engage Signaling Toward an Alternative Pathway of Osteoclastogenesis for Arthritic Bone Loss in Vivo

Background: Dendritic cells (DCs), though borne heterogeneous, are the most potent antigen-presenting cells, whose critical functions include triggering antigen-specific naïve T-cell responses and fine-tuning the innate versus adaptive immunity at the osteo-immune and/or mucosal mesenchyme interface. We previously reported that immature myeloid-CD11c DCs/mDCs may act like osteoclast (OC) precursors (OCp/mDDOCp) capable of developing into functional OCs via an alternative pathway of inflammation-induced osteoclastogenesis; however, what are their contribution and signaling interactions with key osteotropic cytokines (i.e., interleukin-17 [IL-17] and transforming growth factor-β [TGF-β]) to bearing such inflammatory bone loss in vivo remain unclear to date.

Methods: Herein, we employed mature adult bone marrow-reconstituted C57BL/6 TRAF6 -null chimeras without the classical monocyte/macrophage (Mo/Mϕ)-derived OCs to address their potential contribution to OCp/mDDOCp-mediated osteoclastogenesis in the chicken type-II-collagen (CC-II)-induced joint inflammation versus arthritic bone loss and parallel associations with the double-positive CD11c TRAP TRAF6-null  DC-like OCs detected in vivo via the quantitative dual-immunohistochemistry and digital histomorphometry for analyses.

Results: The resulting findings revealed the unrecognized novel insight that (i) immature myeloid-CD11c TRAF6 TRAP DC-like OCs were involved, co-localized, and strongly associated with joint inflammation and bone loss, independent of the Mo/Mϕ-derived classical OCs, in CC-II-immunized TRAF6 -null chimeras, and (ii) the osteotropic IL-17 may engage distinct crosstalk with CD11c mDCs/mDDOCp before developing the CD11c TRAP TRAF6 OCs via a TGF-β-dependent interaction toward inflammation-induced arthritic bone loss in vivo.

Conclusion: These results confirm and substantiate the validity of TRAF6 -null chimeras to address the significance of immature mCD11c TRAP DC-like OCs/mDDOCp subset for an alternative pathway of arthritic bone loss in vivo. Such CD11c mDCs/mDDOCp-associated osteoclastogenesis through the step-wise twist-in-turns osteo-immune cross talks are thereby theme highlighted to depict a summative re-visitation proposed.