Bone repair and regeneration: The Immune Component Is A Healing Partner

The Inflammatory Phase:

The initial inflammatory phase is crucial, with immune cells flooding the fracture site. Macrophages, neutrophils, and lymphocytes do more than just clear the debris; they release cytokines and growth factors that coordinate the healing process.

1. Neutrophils: These first responders initiate an acute inflammatory response, attack microbes, and regulate bone homeostasis.

2. Macrophages: Key players in inflammation regulation and tissue regeneration, macrophages exist in two functional states: pro-inflammatory (M1) and anti-inflammatory (M2). Initially, M1 macrophages dominate, clearing necrotic tissue and secreting cytokines like TNF-α, IL-1β, and IL-6, which recruit cells needed for repair such as osteoblasts and mesenchymal stem cells (MSC). They also stimulate angiogenesis by releasing VEGF.

3. T Cells: CD4+ T helper cells release interferon-γ (INFγ) and TNF-α which recruit MSC and immune cells to the fracture site and promote MSC differentiation into osteoblasts. (Maruyama et al., 2020; Schlundt et al., 2015; Yang & Liu, 2021)

    

Callus Formation:

As the bone repair progresses, a transition from M1 to M2 macrophages occurs. The timing of this transition is critical for the resolution of inflammation and the progression from damage control to tissue regeneration. M2 macrophages release anti-inflammatory cytokines like IL-19 and TGF-β, which suppress inflammation and promote callus formation.

1. M2 macrophages: These cells play a critical role in promoting MSC differentiation into chondrocytes, which form the soft callus. They also support osteoblast activity, transforming the callus into hard bone, and secrete factors like BMP-2, BMP-4, and TGF-β1, which drive osteoblast proliferation. (Clark et al., 2017; Schlundt et al., 2015; Yang & Liu, 2021)

2. Angiogenesis: M1 macrophages initiate blood vessel formation through VEGF, while M2 macrophages promote angiogenesis via MMP9 and VEGF. (Schlundt et al., 2015) 

3. Regulatory T cells (Tregs):  Tregs help resolve inflammation by releasing IL-10 and TGF-β and regulate osteoclast activity through cytotoxic T-lymphocyte associated protein 4 (CTLA4), ensuring proper bone regeneration. They also inhibit CD4+ T cells to block excessive secretion of IFN-γ and TNF-α, facilitating MSC-based bone healing.

4. CD8+ T cells: These cells release RANKL to facilitate osteoclast recruitment and activity, which aids in the removal of the fibrin thrombus during callus formation.

5. B cells: B cells produce osteoprotegerin, which inhibits RANKL, regulating osteoclast differentiation and promoting bone formation.

Bone Remodeling Phase:

In this final phase, immune cells continue to regulate the balance between bone resorption and formation, ensuring the repaired bone gains strength.

1. M1 and M2 Macrophages: The interplay between M1 and M2 macrophages is crucial in this phase. M1 macrophages promote osteoclastogenesis and bone resorption via RANKL and TNF-α, while M2 macrophages support bone formation by secreting TGF-β and VEGF. 

2. Osteoblasts: These cells, along with immune cells, secrete osteoprotegerin which inhibits RANKL, thereby preventing excessive osteoclast activation and ensuring proper balance between bone resorption and formation. (Baht et al., 2018; Schlundt et al., 2015)