Phenotypic plasticity of macrophages: a proof-of-concept study for a cell-based targeted therapy for rheumatoid arthritis

Aim: Infiltration of monocyte-derived macrophages into the synovial tissue (ST) is a hallmark of rheumatoid arthritis (RA). These promote inflammation, local joint effusion, and joint damage via the release of cytokines, reactive oxygen species, and tissue-damaging enzymes. However, balancing these are the ‘regulatory’ macrophages with inflammation-resolving properties, characterised by expression of CD206 and MerTK, dominant within the ST of healthy individuals and RA patients in remission. Macrophages exhibit remarkable phenotypic plasticity, and understanding the role of this characteristic in regulating inflammation remains a major challenge. Importantly, whether the infiltrating, inflammatory macrophages of RA ST similarly exhibit phenotypic plasticity, and whether this occurs during remission remains to be studied. Here, we investigated the phenotypic plasticity of synovial macrophages from RA patients and their ability to polarise into ‘regulatory’ CD206+MerTK+ macrophages.

Method: Synovial fluid (SF) mononuclear cells were obtained from patients with active early RA (<1 year; fulfilling 2010 ACR/EULAR classification criteria). Cryopreserved SFMCs were cultured for 48hr in the presence of 10 ng/mL interferon(IFN)γ, 50 ng/mL dexamethasone, 10 μg/mL Infliximab, or diluent. Cultured cells were immunostained and analysed using a Beckman Coulter CytoFLEX flow cytometer.

Results: Prior to culture, the CD45+CD14+CD68+ macrophage populations present in SF were predominantly CD206-MerTK-. After culture without any stimulus, proportions of CD206+MerTK+ macrophages were increased. Treatment with dexamethasone or anti-TNF (Infliximab) resulted in a further increase in proportions of CD206+MerTK+, M2-like macrophages, while culture with IFNγ induced the opposite. The generated CD206+MerTK+ macrophages were phenotypically stable in culture following differentiating agent removal.

Conclusion: Our findings demonstrate that inflammatory SF cells are indeed able to polarise to regulatory, CD206+MerTK+ macrophages. This provides further mechanistic insights into the therapeutic benefits of glucocorticoids and TNF inhibitors, and provides initial proof-of-concept in the use of regulatory macrophages as the target of a cellular-based therapy for patients with RA.