Absence of αβ T cells accelerates disuse bone loss in male mice after spinal cord injury

FHL2 regulates microglia M1/M2 polarization after spinal cord injury via PARP14-depended STAT1/6 pathway

Neuronal apoptosis and inflammation exacerbate the secondary injury after spinal cord injury (SCI). Four and a half domains 2 (FHL2) is a multifunctional scaffold protein with tissue- and cell-type specific effects on the regulation of inflammation, but its role in SCI remains unclear. The T10 mouse spinal cord contusion model was established, and the mice were immediately injected with lentiviruses carrying FHL2 shRNA after SCI. The results showed that FHL2 expression was increased following SCI, and then gradually decreased. Moreover, FHL2 depletion aggravated functional impairment, neuronal necrosis, and enlarged lesion cavity areas in the injured spinal cord. FHL2 deficiency facilitated neuronal apoptosis by elevating cleaved caspase 3/9 expression, neuroinflammation by regulating microglia polarization, and bone loss. Indeed, FHL2 deficiency increased the secretion of TNF-α and IL-6, M1 microglia polarization, and the activation of STAT1 pathway but decreased the secretion of IL-10 and IL-4, M2 microglia polarization, and the activation of the STAT6 pathway in the spinal cord. In vitro, FHL2 silencing promoted LPS + IFN-γ-induced microglia M1 polarization through activating the STAT1 pathway and alleviated IL-4-induced microglia M2 polarization via inhibiting the STAT6 pathway. FHL2 positively regulated the expression of poly (ADP-ribose) polymerase family member 14 (PARP14) by promoting its transcription. PARP14 overexpression inhibited FHL2 silencing-induced microglia M1 polarization and relieved the inhibitory effect of FHL2 silencing on microglia M2 polarization. Collectively, the study suggests that FHL2 reduces the microglia M1/M2 polarization-mediated inflammation via PARP14-dependent STAT1/6 pathway and thereby improves functional recovery after SCI.

Contribution of αβ T cells to macrophage polarization and MSC recruitment and proliferation on titanium implants

Physiochemical cues like topography and wettability can impact the inflammatory response and tissue integration after biomaterial implantation. T cells are essential for immunomodulation of innate immune cells and play an important role in the host response to biomaterial implantation. This study aimed to understand how CD4+ and CD8+ T cell subsets, members of the αβ T cell family, polarize in response to smooth, rough, or rough-hydrophilic titanium (Ti) implants and whether their presence modulates immune cell crosstalk and mesenchymal stem cell (MSC) recruitment following biomaterial implantation. Post-implantation in mice, we found that CD4+ and CD8+ T cell subsets polarized differentially in response to modified Ti surfaces. Additionally, mice lacking αβ T cells had significantly more pro-inflammatory macrophages, fewer anti-inflammatory macrophages, and reduced MSC recruitment in response to modified Ti post-implantation than αβ T cell -competent mice. Our results demonstrate that T cell activation plays a significant role during the inflammatory response to implanted biomaterials, contributing to macrophage polarization and MSC recruitment and proliferation, and the absence of αβ T cells compromises new bone formation at the implantation site. STATEMENT OF SIGNIFICANCE: T cells are essential for immunomodulation and play an important role in the host response to biomaterial implantation. Our results demonstrate that T cells actively participate during the inflammatory response to implanted biomaterials, controlling macrophage phenotype and recruitment of MSCs to the implantation site.

Inhibition of TGF-β Signaling Attenuates Disuse-induced Trabecular Bone Loss After Spinal Cord Injury in Male Mice

Bone loss is one of the most common complications of immobilization after spinal cord injury (SCI). Whether transforming growth factor (TGF)-β signaling plays a role in SCI-induced disuse bone loss has not been determined. Thus, 16-week-old male mice underwent sham or spinal cord contusion injury to cause complete hindlimb paralysis. Five days later, 10 mg/kg/day control (IgG) or anti-TGF-β1,2,3 neutralizing antibody (1D11) was administered twice weekly for 4 weeks. Femurs were examined by micro-computed tomography (micro-CT) scanning and histology. Bone marrow (BM) supernatants were analyzed by enzyme-linked immunosorbent assay for levels of procollagen type 1 intact N-terminal propeptide (P1NP), tartrate-resistant acid phosphatase (TRAcP-5b), receptor activator of nuclear factor-kappa B ligand (RANKL), osteoprotegerin (OPG), and prostaglandin E2 (PGE2). Distal femoral micro-CT analysis showed that SCI-1D11 mice had significantly (P < .05) attenuated loss of trabecular fractional bone volume (123% SCI-1D11 vs 69% SCI-IgG), thickness (98% vs 81%), and connectivity (112% vs 69%) and improved the structure model index (2.1 vs 2.7). Histomorphometry analysis revealed that osteoclast numbers were lower in the SCI-IgG mice than in sham-IgG control. Biochemically, SCI-IgG mice had higher levels of P1NP and PGE2 but similar TRAcP-5b and RANKL/OPG ratio to the sham-IgG group. The SCI-1D11 group exhibited higher levels of P1NP but similar TRAcP-5b, RANKL/OPG ratio, and PGE2 to the sham-1D11 group. Furthermore, 1D11 treatment prevented SCI-induced hyperphosphorylation of tau protein in osteocytes, an event that destabilizes the cytoskeleton. Together, inhibition of TGF-β signaling after SCI protects trabecular bone integrity, likely by balancing bone remodeling, inhibiting PGE2 elevation, and preserving the osteocyte cytoskeleton.