Controlling Arteriogenesis and Mast Cells Are Central to Bioengineering Solutions for Critical Bone Defect Repair Using Allografts

Reduced angiogenesis and delayed endochondral ossification in CD163-/- mice highlights a role of M2 macrophages during bone fracture repair

While recent studies showed that macrophages are critical for bone fracture healing, and lack of M2 macrophages have been implicated in models of delayed union, functional roles for specific M2 receptors have yet to be defined. Moreover, the M2 scavenger receptor CD163 has been identified as a target to inhibit sepsis following implant-associated osteomyelitis, but potential adverse effects on bone healing during blockage therapy have yet to be explored. Thus, we investigated fracture healing in C57BL/6 versus CD163-/- mice using a well-established closed, stabilized, mid-diaphyseal femur fracture model. While gross fracture healing in CD163-/- mice was similar to that of C57BL/6, plain radiographs revealed persistent fracture gaps in the mutant mice on Day 14, which resolved by Day 21. Consistently, 3D vascular micro-CT demonstrated delayed union on Day 21, with reduced bone volume (74%, 61%, and 49%) and vasculature (40%, 40%, and 18%) compared to C57BL/6 on Days 10, 14, and 21 postfracture, respectively (p < 0.01). Histology confirmed large amounts of persistent cartilage in CD163-/- versus C57BL/6 fracture callus on Days 7 and 10 that resolves over time, and immunohistochemistry demonstrated deficiencies in CD206+ M2 macrophages. Torsion testing of the fractures confirmed the delayed early union in CD163-/- femurs, which display decreased yield torque on Day 21, and a decreased rigidity with a commensurate increase in rotation at yield on Day 28 (p < 0.01). Collectively, these results demonstrate that CD163 is required for normal angiogenesis, callus formation, and bone remodeling during fracture healing, and raise potential concerns about CD163 blockade therapy.

Retention of Human iPSC-Derived or Primary Cells Following Xenotransplantation into Rat Immune-Privileged Sites

In regenerative medicine, experimental animal models are commonly used to study potential effects of human cells as therapeutic candidates. Although some studies describe certain cells, such as mesenchymal stromal cells (MSC) or human primary cells, as hypoimmunogenic and therefore unable to trigger strong inflammatory host responses, other studies report antibody formation and immune rejection following xenotransplantation. Accordingly, the goal of our study was to test the cellular retention and survival of human-induced pluripotent stem cell (iPSCs)-derived MSCs (iMSCs) and primary nucleus pulposus cells (NPCs) following their xenotransplantation into immune-privileged knee joints (14 days) and intervertebral discs (IVD; 7 days) of immunocompromised Nude and immunocompetent Sprague Dawley (SD) rats. At the end of both experiments, we could demonstrate that both rat types revealed comparably low levels of systemic IL-6 and IgM inflammation markers, as assessed via ELISA. Furthermore, the number of recovered cells was with no significant difference between both rat types. Conclusively, our results show that xenogeneic injection of human iMSC and NPC into immunoprivileged knee and IVD sites did not lead to an elevated inflammatory response in immunocompetent rats when compared to immunocompromised rats. Hence, immunocompetent rats represent suitable animals for xenotransplantation studies targeting immunoprivileged sites.

Mast Cells and Vitamin D Status: A Clinical and Biological Link in the Onset of Allergy and Bone Diseases

The immune system is made up by an extremely composite group of cells, whose regulated and harmonious activity is fundamental to maintain health. The mast cells are an essential effector of inflammatory response which is characterized by a massive release of mediators accumulated in cytoplasmic secretory granules. However, beyond the effects on immune response, mast cells can modify bone metabolism and are capable of intervening in the genesis of pathologies such as osteoporosis and osteopenia. Vitamin D is recognized to induce changes in bone metabolism, but it is also able to influence immune response, suppressing mast cell activation and IgE synthesis from B cells and increasing the number of dendritic cells and IL-10-generating regulatory T cells. Vitamin D deficit has been reported to worsen sensitization and allergic manifestations in several different experimental models. However, in clinical situations, contradictory findings have been described concerning the correlation between allergy and vitamin D deficit. The aim of this review was to analyze the close relationships between mast cells and vitamin D, which contribute, through the activation of different molecular or cellular activation pathways, to the determination of bone pathologies and the onset of allergic diseases.

PTH-Induced Bone Regeneration and Vascular Modulation Are Both Dependent on Endothelial Signaling

The use of a bone allograft presents a promising approach for healing nonunion fractures. We have previously reported that parathyroid hormone (PTH) therapy induced allograft integration while modulating angiogenesis at the allograft proximity. Here, we hypothesize that PTH-induced vascular modulation and the osteogenic effect of PTH are both dependent on endothelial PTH receptor-1 (PTHR1) signaling. To evaluate our hypothesis, we used multiple transgenic mouse lines, and their wild-type counterparts as a control. In addition to endothelial-specific PTHR1 knock-out mice, we used mice in which PTHR1 was engineered to be constitutively active in collagen-1α+ osteoblasts, to assess the effect of PTH signaling activation exclusively in osteoprogenitors. To characterize resident cell recruitment and osteogenic activity, mice in which the Luciferase reporter gene is expressed under the Osteocalcin promoter (Oc-Luc) were used. Mice were implanted with calvarial allografts and treated with either PTH or PBS. A micro-computed tomography-based structural analysis indicated that the induction of bone formation by PTH, as observed in wild-type animals, was not maintained when PTHR1 was removed from endothelial cells. Furthermore, the induction of PTH signaling exclusively in osteoblasts resulted in significantly less bone formation compared to systemic PTH treatment, and significantly less osteogenic activity was measured by bioluminescence imaging of the Oc-Luc mice. Deletion of the endothelial PTHR1 significantly decreased the PTH-induced formation of narrow blood vessels, formerly demonstrated in wild-type mice. However, the exclusive activation of PTH signaling in osteoblasts was sufficient to re-establish the observed PTH effect. Collectively, our results show that endothelial PTHR1 signaling plays a key role in PTH-induced osteogenesis and has implications in angiogenesis.

Clinical Application of Bone Marrow Mesenchymal Stem/Stromal Cells to Repair Skeletal Tissue

There has been an escalation in reports over the last decade examining the efficacy of bone marrow derived mesenchymal stem/stromal cells (BMSC) in bone tissue engineering and regenerative medicine-based applications. The multipotent differentiation potential, myelosupportive capacity, anti-inflammatory and immune-modulatory properties of BMSC underpins their versatile nature as therapeutic agents. This review addresses the current limitations and challenges of exogenous autologous and allogeneic BMSC based regenerative skeletal therapies in combination with bioactive molecules, cellular derivatives, genetic manipulation, biocompatible hydrogels, solid and composite scaffolds. The review highlights the current approaches and recent developments in utilizing endogenous BMSC activation or exogenous BMSC for the repair of long bone and vertebrae fractures due to osteoporosis or trauma. Current advances employing BMSC based therapies for bone regeneration of craniofacial defects is also discussed. Moreover, this review discusses the latest developments utilizing BMSC therapies in the preclinical and clinical settings, including the treatment of bone related diseases such as Osteogenesis Imperfecta.

Innovative Techniques for Maximizing Limb Salvage and Function

Concurrent injuries to multiple extremities present unique challenges to the reconstructive surgeon. The primary goal in such scenarios is to optimize functional outcomes. The goal of this article is to present an overview of various techniques necessary to provide sufficient soft tissue and preserve amputation limb lengths and function. The concept of innovative techniques for maximizing limb savage and function is presented using an index patient with multiple extremity third- and fourth-degree burn injuries resulting in nonsalvageable lower extremities and severe left-hand wounds. A review of other potential innovative techniques is discussed. The burn injury resulted in a need for bilateral guillotine below-knee amputations. Above-knee amputation was avoided in the left leg using a parascapular free fasciocutaneous flap, while through-knee amputation was preferred to above-knee amputation in the right leg. The preservation of areas with questionable viability resulted in salvaging the left hand of the patient using digital palmar flaps to resurface the dorsum with creation of a first web-space. Maintenance of maximal viable length of limbs and any residual function in the limbs can be of significant functional benefit to multiple limb amputation patients. Maximizing the limb length in such patients is critical, and typical "rules" that have traditionally been utilized to minimize numbers of operations and optimize prosthetic fit may not apply.

3D printed TCP-based scaffold incorporating VEGF-loaded PLGA microspheres for craniofacial tissue engineering

OBJECTIVE

Vascularization is a critical process during bone regeneration/repair and the lack of tissue vascularization is recognized as a major challenge in applying bone tissue engineering methods for cranial and maxillofacial surgeries. The aim of our study is to fabricate a vascular endothelial growth factor (VEGF)-loaded gelatin/alginate/β-TCP composite scaffold by 3D printing method using a computer-assisted design (CAD) model.

METHODS

The paste, composed of (VEGF-loaded PLGA)-containing gelatin/alginate/β-TCP in water, was loaded into standard Nordson cartridges and promptly employed for printing the scaffolds. Rheological characterization of various gelatin/alginate/β-TCP formulations led to an optimized paste as a printable bioink at room temperature.

RESULTS

The in vitro release kinetics of the loaded VEGF revealed that the designed scaffolds fulfill the bioavailability of VEGF required for vascularization in the early stages of tissue regeneration. The results were confirmed by two times increment of proliferation of human umbilical vein endothelial cells (HUVECs) seeded on the scaffolds after 10 days. The compressive modulus of the scaffolds, 98±11MPa, was found to be in the range of cancellous bone suggesting their potential application for craniofacial tissue engineering. Osteoblast culture on the scaffolds showed that the construct supports cell viability, adhesion and proliferation. It was found that the ALP activity increased over 50% using VEGF-loaded scaffolds after 2 weeks of culture.

SIGNIFICANCE

The 3D printed gelatin/alginate/β-TCP scaffold with slow releasing of VEGF can be considered as a potential candidate for regeneration of craniofacial defects.

Teriparatide Treatment Improves Bone Defect Healing Via Anabolic Effects on New Bone Formation and Non-Anabolic Effects on Inhibition of Mast Cells in a Murine Cranial Window Model

Investigations of teriparatide (recombinant parathyroid hormone [rPTH]) as a potential treatment for critical defects have demonstrated the predicted anabolic effects on bone formation, and significant non-anabolic effects on healing via undefined mechanisms. Specifically, studies in murine models of structural allograft healing demonstrated that rPTH treatment increased angiogenesis (vessels <30 μm), and decreased arteriogenesis (>30 μm) and mast cell numbers, which lead to decreased fibrosis and accelerated healing. To better understand these non-anabolic effects, we interrogated osteogenesis, vasculogenesis, and mast cell accumulation in mice randomized to placebo (saline), rPTH (20 μg/kg/2 days), or the mast cell inhibitor sodium cromolyn (SC) (24 μg/kg/ 2days), via longitudinal micro-computed tomography (μCT) and multiphoton laser scanning microscopy (MPLSM), in a critical calvaria defect model. μCT demonstrated that SC significantly increased defect window closure and new bone volume versus placebo (p < 0.05), although these effects were not as great as rPTH. Interestingly, both rPTH and SC have similar inhibitory effects on arteriogenesis versus placebo (p < 0.05) without affecting total vascular volume. MPLSM time-course studies in untreated mice revealed that large numbers of mast cells were detected 1 day postoperation (43 ± 17), peaked at 6 days (76 ± 6), and were still present in the critical defect at the end of the experiment on day 30 (20 ± 12). In contrast, angiogenesis was not observed until day 4, and functional vessels were first observed on 6 days, demonstrating that mast cell accumulation precedes vasculogenesis. To confirm a direct role of mast cells on osteogenesis and vasculogenesis, we demonstrated that specific diphtheria toxin-α deletion in Mcpt5-Cre-iDTR mice results in similar affects as SC treatment in WT mice. Collectively, these findings demonstrate that mast cells inhibit bone defect healing by stimulating arteriogenesis associated with fibrotic scaring, and that an efficacious non-anabolic effect of rPTH therapy on bone repair is suppression of arteriogenesis and fibrosis secondary to mast cell inhibition. © 2017 American Society for Bone and Mineral Research.

Teriparatide attenuates scarring around murine cranial bone allograft via modulation of angiogenesis

Nearly all bone fractures in humans can deteriorate into a non-union fracture, often due to formation of fibrotic tissue. Cranial allogeneic bone grafts present a striking example: although seemingly attractive for craniofacial reconstructions, they often fail due to fibrosis at the host-graft junction, which physically prevents the desired bridging of bone between the host and graft and revitalization of the latter. In the present study we show that intermittent treatment with recombinant parathyroid hormone-analogue (teriparatide) modulates neovascularization feeding in the graft surroundings, consequently reducing fibrosis and scar tissue formation and facilitates osteogenesis. Longitudinal inspection of the vascular tree feeding the allograft has revealed that teriparatide induces formation of small-diameter vessels in the 1st week after surgery; by the 2nd week, abundant formation of small-diameter blood vessels was detected in untreated control animals, but far less in teriparatide-treated mice, although in total, more blood capillaries were detected in the animals that were given teriparatide. By that time point we observed expression of the profibrogenic mediator TGF-β in untreated animals, but negligible expression in the teriparatide-treated mice. To evaluate the formation of scar tissue, we utilized a magnetization transfer contrast MRI protocol to differentiate osteoid tissue from scar tissue, based on the characterization of collagen fibers. Using this method we found that significantly more bone matrix was formed in animals given teriparatide than in control animals. Altogether, our findings show how teriparatide diminishes scarring, ultimately leading to superior bone graft integration.

Defective bone repair in mast cell-deficient Cpa3Cre/+ mice

In the adult skeleton, cells of the immune system interact with those of the skeleton during all phases of bone repair to influence the outcome. Mast cells are immune cells best known for their pathologic role in allergy, and may be involved in chronic inflammatory and fibrotic disorders. Potential roles for mast cells in tissue homeostasis, vascularization and repair remain enigmatic. Previous studies in combined mast cell- and Kit-deficient Kit^W-sh/W-sh mice (Kit^W-sh) implicated mast cells in bone repair but Kit^W-sh mice suffer from additional Kit-dependent hematopoietic and non- hematopoietic deficiencies that could have confounded the outcome. The goal of the current study was to compare bone repair in normal wild type (WT) and Cpa3^Cre/+ mice, which lack mast cells in the absence of any other hematopoietic or non- hematopoietic deficiencies. Repair of a femoral window defect was characterized using micro CT imaging and histological analyses from the early inflammatory phase, through soft and hard callus formation, and finally the remodeling phase. The data indicate 1) mast cells appear in healing bone of WT mice but not Cpa3^Cre/+ mice, beginning 14 days after surgery; 2) re-vascularization of repair tissue and deposition of mineralized bone was delayed and dis-organised in Cpa3^Cre/+ mice compared with WT mice; 3) the defects in Cpa3^Cre/+ mice were associated with little change in anabolic activity and biphasic alterations in osteoclast and macrophage activity. The outcome at 56 days postoperative was complete bridging of the defect in most WT mice and fibrous mal-union in most Cpa3^Cre/+ mice. The results indicate that mast cells promote bone healing, possibly by recruiting vascular endothelial cells during the inflammatory phase and coordinating anabolic and catabolic activity during tissue remodeling. Taken together the data indicate that mast cells have a positive impact on bone repair.