Macrophage Paracrine Signalling Differentially Affects Fibroblast-Induced Collagenous Tissue Remodelling

Upon injury, tissue repair often leads to a loss in functionality, organisation, and structure. The immune system, particularly macrophages, is crucial during tissue healing. Macrophages polarise into pro-inflammatory M1 and anti-inflammatory M2 subsets, regulating various stages of tissue healing. Macrophages steer fibroblasts in the process of extracellular matrix degradation, synthesis, and rearrangement. However, the direct role of paracrine signalling by different macrophage phenotypes on fibroblast-induced structural tissue remodelling remains elusive. Therefore, this study aimed to explore how paracrine factors from M1, M2a, and M2c macrophages affect fibroblast remodelling abilities in an in vitro model system.

Macrophages were polarised in vitro, and their conditioned medium or cytokine-enriched medium containing specific macrophage-secreted factors was added to fibroblast-populated reconstituted collagen tissues.

Macrophage-conditioned media led to changes in fibroblast-induced tissue compaction for all macrophage subsets. The presence of macrophage polarising factors in the conditioned medium, particularly LPS/IFNγ, and high serum levels directly affected tissue compaction and matrix remodelling gene expression. Without these confounding factors, M1 cytokine-enriched medium led to reduced tissue compaction when compared to M2a/M2c cytokine-enriched media. MMP activity analysis showed that matrix degradation likely contributed to tissue compaction.

Factors secreted by M1 macrophages resulted in reduced tissue compaction compared to M2a/M2c macrophages in an in vitro model of tissue remodelling, suggesting a diminished capacity for fibroblasts to remodel the extracellular matrix. Importantly, factors to polarize macrophages and serum are regarded as confounding factors in studying the effect of paracrine signalling by macrophages on tissue remodelling.