Modulation of the Inflammatory Response and Bone Healing

Abstract

The optimal treatment for complex fractures and large bone defects is an important unsolved issue in orthopedics and related specialties. Approximately 5-10% of fractures fail to heal and develop non-unions. Bone healing can be characterized by three partially overlapping phases: the inflammatory phase, the repair phase, and the remodeling phase. Eventual healing is highly dependent on the initial inflammatory phase, which is affected by both the local and systemic responses to the injurious stimulus. Furthermore, immune cells and mesenchymal stromal cells (MSCs) participate in critical inter-cellular communication or crosstalk to modulate bone healing. Deficiencies in this inter-cellular exchange, inhibition of the natural processes of acute inflammation, and its resolution, or chronic inflammation due to a persistent adverse stimulus can lead to impaired fracture healing. Thus, an initial and optimal transient stage of acute inflammation is one of the key factors for successful, robust bone healing. Recent studies demonstrated the therapeutic potential of immunomodulation for bone healing by the preconditioning of MSCs to empower their immunosuppressive properties. Preconditioned MSCs (also known as "primed/ licensed/ activated" MSCs) are cultured first with pro-inflammatory cytokines (e.g., TNFα and IL17A) or exposed to hypoxic conditions to mimic the inflammatory environment prior to their intended application. Another approach of immunomodulation for bone healing is the resolution of inflammation with anti-inflammatory cytokines such as IL4, IL10, and IL13. In this review, we summarize the principles of inflammation and bone healing and provide an update on cellular interactions and immunomodulation for optimal bone healing.

Keywords: anti-inflammatory cytokines; bone healing; immunomodulation; inflammation; mesenchymal stromal cell; preconditioning; pro-inflammatory cytokines.

Figure 1

Schematic summary of the stages of bone healing and the temporal pattern of the relative immune cells and cytokines/growth factors expression. Bone healing can be viewed as a three-stage biological phase (inflammation, repair, and remodeling) which can be further divided into six main sub-steps: hematoma, inflammation, soft callus formation, hard callus formation, remodeling, bone healing. After fracture, immune cells including PMNs, NK cells, mast cells, and platelets (platelets are not truly cells as they have no nuclei) are activated in the early stage of the inflammation and the secreted cytokines/chemokines subsequently recruit and activate monocytes/macrophages to further play important roles throughout this process. The pro-inflammatory cytokines including IL1, IL6, TNFα are essential signals during the early stages of bone fracture. In addition, TNFα increases again in the late repair phase, and several pro-inflammatory cytokines (e.g., IL1, IL6, TNFα) are highly expressed in the remodeling phase. The control switch of expression patterns from a pro-inflammatory to an anti-inflammatory response (IL4, IL10, IL13) in the late stages of inflammation is critical to fracture repair.

Figure 2

Schematic summary of the cellular and molecular effects after preconditioning of MSCs. The upper small rectangle boxes depict the different stimuli used to precondition MSCs. The middle level boxes indicate the upregulation of cytokines and chemokines by MSCs after stimulation. The effects of preconditioned MSCs on a cellular level are represented on the bottom of the figure. The stimuli factors and their respectively triggered outputs are linked by matched color arrows and boxes. IL17A-preconditioned MSCs increase IL6 secretion and regulatory T cell generation and inhibit Th1 cytokine secretion such as TNFα, IFNγ, IL2, and IL10. IFNγ-preconditioned MSCs promote the secretion of immunomodulatory molecules such as IDO, PGE2, HGF, TGFβ, and CCL2, suppress T cell and NK cell proliferation and polarize macrophages to an M2 phenotype. TNFα-preconditioned MSCs promote the secretion of immunoregulatory mediators such as PGE2, IDO, and HGF, suppress T cell proliferation. Hypoxia-preconditioned MSCs promote the secretion of PGE2, IDO, VEGF, and bFGF, suppress T cell proliferation, and polarize macrophages to an M2 phenotype.

Figure 3

Potential strategies of immunomodulation for bone healing in acute (A) and chronic (B) inflammation. The optimal inflammatory process for bone healing is mediated by an initial and optimal transient stage of acute inflammation, followed by the resolution of inflammation (green line). Excess (solid red line) or deficiency/inhibition of acute inflammation (dashed red line), and chronic inflammation (dotted red line) interfere with the healing of bone defects.