Background/Purpose: Rheumatoid arthritis is commonly treated with potent anti-inflammatory glucocorticoids (GC), despite severe side effects such as osteoporosis and insulin resistance associated with chronic dosing of GCs. The prevailing view is that GCs solely rely on repression of cytokines and elimination of immune cells for their anti-inflammatory effects. However, we now show that these direct effects on immune cells are insufficient. GCs modulate transcription by activating the glucocorticoid receptor (GR), a ligand activated transcription factor that binds DNA either as a monomer or dimer. Whereas previously the anti-inflammatory actions of the monomeric GR were considered to be sufficient for immune suppression, we demonstrate for a variety of disease models including arthritis, that GR dimerization is essential to reduce inflammation.

Methods: We induced serum transfer-induces arthritis (STIA) in wild type (wt) mice and in mice with impaired GR dimerization (GR^dim) and treated them with Dexamethasone (Dex) or PBS after the onset of STIA. To test the importance of immune cells, we generated bone marrow chimeric GR^dim mice, reconstituted with wt hematopoietic cells (wt/GR^dim) and wt/wt littermates, induced STIA and treated with Dex or PBS as a control. Disease severity was assessed clinically and histologically and inflamed ankles were analysed by flow cytometry, iTRAQ proteomics and qPCR. Co-culture of wt and GR^dim fibroblast-like synoviocytes (FLS) and bone marrow derived macrophages (BMDM) were used to analyse the interplay of FLS and BMDM in vitro.

Results: GR^dim mice are not able to resolve the infiltration of immune cells in STIA after Dex treatment in contrast to wt control mice. Unexpectedly, we show that wt/GR^dim-chimeras, with impaired GR dimerization in non-immune, radio-resistant cells are resistant to Dex treatment in STIA, too. Flow cytometry analysis of inflamed ankles of wt/wt- and wt/GR^dim-chimeras with STIA reveals a reduction of leukocytes (70%) in wt/wt but not in wt/GR^dim-chimeras treated with Dex. Moreover, Dex induced non-classical, non-activated M2-like macrophages (F4/80+, Ly6C-, MHCII-) are only evident in wt/wt- (+70% compared to wt/wt-chimeras treated with PBS) but not wt/GR^dim-chimeras. Proteomics and gene expression analysis of inflamed ankles confirmed the induction of known M2 polarization markers of macrophages, such as CD163, CD36 and MerTK after Dex treatment of STIA in wt/wt-chimeras, which was absent in wt/GR^dim-chimeras. We corroborated our findings in an in vitro co-culture system of GR^dim FLS with wt primary BMDMs. Concomitant TNF/Dex treatment induced CD163 in wt co-cultures, but not in GR^dimFLS co-cultured BMDMs.  

Conclusion: Taken together, we discovered a novel anti-inflammatory mechanism of corticosteroids that involves GR dimerization dependent gene regulation in non-immune cells to control M2 polarization of macrophages for resolution of arthritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/glucocorticoid-receptor-dimerization-in-stromal-cells-modulates-macrophage-polarization-during-serum-transfer-induced-arthritis/