Recent research has brought about a clear understanding that successful fracture healing is based on carefully coordinated cross-talk between inflammatory and bone forming cells. In particular, the key role that macrophages play in the recruitment and regulation of the differentiation of mesenchymal stem cells (MSCs) during bone regeneration has been brought to focus. Indeed, animal studies have comprehensively demonstrated that fractures do not heal without the direct involvement of macrophages. Yet the exact mechanisms by which macrophages contribute to bone regeneration remain to be elucidated. Macrophage-derived paracrine signaling molecules such as Oncostatin M, Prostaglandin E2 (PGE2), and Bone Morphogenetic Protein-2 (BMP2) have been shown to play critical roles; however the relative importance of inflammatory (M1) and tissue regenerative (M2) macrophages in guiding MSC differentiation along the osteogenic pathway remains poorly understood. In this review, we summarize the current understanding of the interaction of macrophages and MSCs during bone regeneration, with the emphasis on the role of macrophages in regulating bone formation. The potential implications of aging to this cellular cross-talk are reviewed. Emerging treatment options to improve facture healing by utilizing or targeting MSC-macrophage crosstalk are also discussed.

ReviewCite this article: Goodman SB et al. 2014 Novel biological strategies for treatment of wear particle-induced periprosthetic osteolysis of orthopaedic implants for joint replacement. J. R. Soc. Wear particles and by-products from joint replacements and other orthopaedic implants may result in a local chronic inflammatory and foreign body reaction. This may lead to persistent synovitis resulting in joint pain and swelling, periprosthetic osteolysis, implant loosening and pathologic fracture. Strategies to modulate the adverse effects of wear debris may improve the function and longevity of joint replacements and other orthopaedic implants, potentially delaying or avoiding complex revision surgical procedures. Three novel biological strategies to mitigate the chronic inflammatory reaction to orthopaedic wear particles are reported. These include (i) interference with systemic macrophage trafficking to the local implant site, (ii) modulation of macrophages from an M1 ( pro-inflammatory) to an M2 (anti-inflammatory, pro-tissue healing) phenotype in the periprosthetic tissues, and (iii) local inhibition of the transcription factor nuclear factor kappa B (NF-kB) by delivery of an NF-kB decoy oligodeoxynucleotide, thereby interfering with the production of pro-inflammatory mediators. These three approaches have been shown to be viable strategies for mitigating the undesirable effects of wear particles in preclinical studies. Targeted local delivery of specific biologics may potentially extend the lifetime of orthopaedic implants.

BackgroundMesenchymal stem cells (MSCs) are capable of immunomodulation and tissue regeneration, highlighting their potential translational application for treating inflammatory bone disorders. MSC-mediated immunomodulation is regulated by proinflammatory cytokines and pathogen-associated molecular patterns such as lipopolysaccharide (LPS). Previous studies showed that MSCs exposed to interferon gamma (IFN-γ) and the proinflammatory cytokine tumor necrosis factor alpha (TNF-α) synergistically suppressed T-cell activation.MethodsIn the current study, we developed a novel preconditioning strategy for MSCs using LPS plus TNF-α to optimize the immunomodulating ability of MSCs on macrophage polarization.ResultsPreconditioned MSCs enhanced anti-inflammatory M2 macrophage marker expression (Arginase 1 and CD206) and decreased inflammatory M1 macrophage marker (TNF-α/IL-1Ra) expression using an in-vitro coculture model. Immunomodulation of MSCs on macrophages was significantly increased compared to the combination of IFN-γ plus TNF-α or single treatment controls. Increased osteogenic differentiation including alkaline phosphate activity and matrix mineralization was only observed in the LPS plus TNF-α preconditioned MSCs. Mechanistic studies showed that increased prostaglandin E2 (PGE2) production was associated with enhanced Arginase 1 expression. Selective cyclooxygenase-2 inhibition by Celecoxib decreased PGE2 production and Arginase 1 expression in cocultured macrophages.ConclusionsThe novel preconditioned MSCs have increased immunomodulation and bone regeneration potential and could be applied to the treatment of inflammatory bone disorders including periprosthetic osteolysis, fracture healing/nonunions, and osteonecrosis.Electronic supplementary materialThe online version of this article (doi:10.1186/s13287-017-0730-z) contains supplementary material, which is available to authorized users.

Chronic inflammation is associated with upregulation of the transcription factor NF-κB and excessive inflammatory cytokine secretion by M1 macrophages. The anti-inflammatory cytokine IL-4 converts pro-inflammatory M1 macrophages into an anti-inflammatory and tissue regenerative M2 phenotype, thus reducing inflammation and enhancing tissue regeneration. We have generated NF-κB responsive, or constitutively active IL4-expression lentiviral vectors transduced into murine bone marrow-derived mesenchymal stromal cells (MSCs). MSCs with a constitutively active IL-4 expression vector produced large quantities of IL-4 continuously whereas IL-4 secretion was significantly induced by lipopolysaccharide (LPS) in the NF-κB sensing MSCs. In contrast, LPS had no effect on MSCs with IL-4 secretion driven by a constitutively active promoter. We also found that intermittent and continuous LPS treatment displayed distinct NF-κB activation profiles, and this regulation was independent of IL-4 signaling. The supernatant containing IL-4 from the LPS treated MSCs suppressed M1 marker (iNOS and TNFα) expression and enhanced M2 marker (Arginase 1, CD206, and IL1Ra) expression in primary murine macrophages. The IL-4 secretion at the basal, non-LPS induced level was sufficient to suppress TNFα and enhance Arginase 1 at a lower level, but had no significant effects on iNOS, CD206, and IL1Ra expression. Finally, IL-4 secretion at basal or LPS-induced levels significantly suppressed osteogenic differentiation of MSCs. Our findings suggest that the IL-4 secreting MSCs driven by NF-κB sensing or constitutive active promoter have great potential for mitigating the effects of chronic inflammation and promoting earlier tissue regeneration.

Inflammation is a defensive mechanism for pathogen clearance and maintaining tissue homeostasis. In the skeletal system, inflammation is closely associated with many bone disorders including fractures, nonunions, periprosthetic osteolysis (bone loss around orthopedic implants), and osteoporosis. Acute inflammation is a critical step for proper bone-healing and bone-remodeling processes. On the other hand, chronic inflammation with excessive proinflammatory cytokines disrupts the balance of skeletal homeostasis involving osteoblastic (bone formation) and osteoclastic (bone resorption) activities. NF-κB is a transcriptional factor that regulates the inflammatory response and bone-remodeling processes in both bone-forming and bone-resorption cells. In vitro and in vivo evidences suggest that NF-κB is an important potential therapeutic target for inflammation-associated bone disorders by modulating inflammation and bone-remodeling process simultaneously. The challenges of NF-κB-targeting therapy in bone disorders include: (1) the complexity of canonical and noncanonical NF-κB pathways; (2) the fundamental roles of NF-κB-mediated signaling for bone regeneration at earlier phases of tissue damage and acute inflammation; and (3) the potential toxic effects on nontargeted cells such as lymphocytes. Recent developments of novel inhibitors with differential approaches to modulate NF-κB activity, and the controlled release (local) or bone-targeting drug delivery (systemic) strategies, have largely increased the translational application of NF-κB therapy in bone disorders. Taken together, temporal modulation of NF-κB pathways with the combination of recent advanced bone-targeting drug delivery techniques is a highly translational strategy to reestablish homeostasis in the skeletal system.

Novel evidence-based prosthetic designs and biomaterials facilitate the performance of highly successful joint replacement (JR) procedures. To achieve this goal, constructs must be durable, biomechanically sound, and avoid adverse local tissue reactions. Different biomaterials such as metals and their alloys, polymers, ceramics, and composites are currently used for JR implants. This review focuses on (1) the biological response to the different biomaterials used for TJR and (2) the chronic inflammatory and foreign-body response induced by byproducts of these biomaterials. A homeostatic state of bone and surrounding soft tissue with current biomaterials for JR can be achieved with mechanically stable, infection free and intact (as opposed to the release of particulate or ionic byproducts) implants. Adverse local tissue reactions (an acute/chronic inflammatory reaction, periprosthetic osteolysis, loosening and subsequent mechanical failure) may evolve when the latter conditions are not met. This article (Part 2 of 2) summarizes the biological response to the non-metallic materials commonly used for joint replacement including polyethylene, ceramics, and polymethylmethacrylate (PMMA), as well as the foreign body reaction to byproducts of these materials.

Mesenchymal stem cell (MSC)‐based therapy is a promising strategy for bone repair. Furthermore, the innate immune system, and specifically macrophages, plays a crucial role in the differentiation and activation of MSCs. The anti‐inflammatory cytokine Interleukin‐4 (IL‐4) converts pro‐inflammatory M1 macrophages into a tissue regenerative M2 phenotype, which enhances MSC differentiation and function. We developed lentivirus‐transduced IL‐4 overexpressing MSCs (IL‐4 MSCs) that continuously produce IL‐4 and polarize macrophages toward an M2 phenotype. In the current study, we investigated the potential of IL‐4 MSCs delivered using a macroporous gelatin‐based microribbon (μRB) scaffold for healing of critical‐size long bone defects in Mice. IL‐4 MSCs within μRBs enhanced M2 marker expression without inhibiting M1 marker expression in the early phase, and increased macrophage migration into the scaffold. Six weeks after establishing the bone defect, IL‐4 MSCs within μRBs enhanced bone formation and helped bridge the long bone defect. IL‐4 MSCs delivered using macroporous μRB scaffold is potentially a valuable strategy for the treatment of critical‐size long bone defects.

Total joint replacement (TJR) has revolutionized the treatment of end-stage arthritic disorders. This success is due, in large part, to a clear understanding of the important interaction between the artificial implant and the biology of the host. All surgical procedures in which implants are placed in the body evoke an initial inflammatory reaction, which generally subsides over several weeks. Thereafter, a series of homeostatic events occur leading to progressive integration of the implant within bone and the surrounding musculoskeletal tissues. The eventual outcome of the operation is dependent on the characteristics of the implant, the precision of the surgical technique and operative environment, and the biological milieu of the host. If these factors and events are not optimal, adverse events can occur such as the development of chronic inflammation, progressive bone loss due to increased production of degradation products from the implant (periprosthetic osteolysis), implant loosening or infection. These complications can lead to chronic pain and poor function of the joint reconstruction, and may necessitate revision surgery or removal of the prosthesis entirely. Recent advances in engineering, materials science, and the immunological aspects associated with orthopaedic implants have fostered intense research with the hope that joint replacements will last a lifetime, and facilitate pain-free, normal function.