Regulatory T cells use programmed death 1 ligands to directly suppress autoreactive B cells in vivo

The mechanisms by which regulatory T cells (T(regs)) suppress autoantibody production are unclear. Here we have addressed this question using transgenic mice expressing model antigens in the kidney. We report that T(regs) were essential and sufficient to suppress autoreactive B cells in an antigen-specific manner and to prevent them from producing autoantibodies. Most of this suppression was mediated through the inhibitory cell-surface-molecule programmed death-1 (PD-1). Suppression required PD-1 expression on autoreactive B cells and expression of the two PD-1 ligands on T(regs). PD-1 ligation inhibited activation of autoreactive B cells, suppressed their proliferation, and induced their apoptosis. Intermediate PD-1(+) cells, such as T helper cells, were dispensable for suppression. These findings demonstrate in vivo that T(regs) use PD-1 ligands to directly suppress autoreactive B cells, and they identify a previously undescribed peripheral B-cell tolerance mechanism against tissue autoantigens.     

Enhanced bone regeneration in rat calvarial defects implanted with surface-modified and BMP-loaded bioactive glass (13-93) scaffolds

The repair of large bone defects, such as segmental defects in the long bones of the limbs, is a challenging clinical problem. Our recent work has shown the ability to create porous scaffolds of silicate 13-93 bioactive glass by robocasting which have compressive strengths comparable to human cortical bone. The objective of this study was to evaluate the capacity of those strong porous scaffolds with a grid-like microstructure (porosity = 50%; filament width = 330 μm; pore width = 300 μm) to regenerate bone in a rat calvarial defect model. Six weeks post-implantation, the amount of new bone formed within the implants was evaluated using histomorphometric analysis. The amount of new bone formed in implants composed of the as-fabricated scaffolds was 32% of the available pore space (area). Pretreating the as-fabricated scaffolds in an aqueous phosphate solution for 1, 3 and 6 days to convert a surface layer to hydroxyapatite prior to implantation enhanced new bone formation to 46%, 57% and 45%, respectively. New bone formation in scaffolds pretreated for 1, 3 and 6 days and loaded with bone morphogenetic protein-2 (BMP-2) (1 μg per defect) was 65%, 61% and 64%, respectively. The results show that converting a surface layer of the glass to hydroxyapatite or loading the surface-treated scaffolds with BMP-2 can significantly improve the capacity of 13-93 bioactive glass scaffolds to regenerate bone in an osseous defect. Based on their mechanical properties evaluated previously and their capacity to regenerate bone found in this study, these 13-93 bioactive glass scaffolds, pretreated or loaded with BMP-2, are promising in structural bone repair.     

Mechanical properties of bioactive glass (13-93) scaffolds fabricated by robotic deposition for structural bone repair

There is a need to develop synthetic scaffolds to repair large defects in load-bearing bones. Bioactive glasses have attractive properties as a scaffold material for bone repair, but data on their mechanical properties are limited. The objective of the present study was to comprehensively evaluate the mechanical properties of strong porous scaffolds of silicate 13-93 bioactive glass fabricated by robocasting. As-fabricated scaffolds with a grid-like microstructure (porosity 47%, filament diameter 330 μm, pore width 300 μm) were tested in compressive and flexural loading to determine their strength, elastic modulus, Weibull modulus, fatigue resistance, and fracture toughness. Scaffolds were also tested in compression after they were immersed in simulated body fluid (SBF) in vitro or implanted in a rat subcutaneous model in vivo. As fabricated, the scaffolds had a strength of 86 ± 9 MPa, elastic modulus of 13 ± 2 GPa, and a Weibull modulus of 12 when tested in compression. In flexural loading the strength, elastic modulus, and Weibull modulus were 11 ± 3 MPa, 13 ± 2 GPa, and 6, respectively. In compression, the as-fabricated scaffolds had a mean fatigue life of ～10⁶ cycles when tested in air at room temperature or in phosphate-buffered saline at 37 °C under cyclic stresses of 1–10 or 2–20 MPa. The compressive strength of the scaffolds decreased markedly during the first 2 weeks of immersion in SBF or implantation in vivo, but more slowly thereafter. The brittle mechanical response of the scaffolds in vitro changed to an elasto-plastic response after implantation for longer than 2–4 weeks in vivo. In addition to providing critically needed data for designing bioactive glass scaffolds, the results are promising for the application of these strong porous scaffolds in loaded bone repair.