Background/Purpose: Infiltration of monocyte-derived macrophages into the synovial tissue (ST) is a hallmark of rheumatoid arthritis (RA). These infiltrating cells lead to inflammation, local joint effusion, and joint damage via the production of inflammatory cytokines. Macrophages express an array of co-stimulatory receptors, particularly those of the B7 protein family, including the programmed death protein 1 (PD-1) ligands, PD-L1 and PD-L2, and the B7-related protein VSIG4.

In the healthy state, interaction between PD-1 and its ligands results in an inhibitory signalling cascade in the PD-1 expressing T cell, which can in turn inhibit cell proliferation and cytokine release. However, in RA, PD-L1 is conspicuously absent from the ST (Guo, et al., 2018), leaving disease-driving cells expressing PD-1 (such as T peripheral and T follicular helper cells) unchecked. This imbalance in PD-1 signalling may contribute to disease progression and may provide a target for treatment.

Herein, we investigate the potential of ‘reprogramming’ inflammatory synovial fluid macrophages to upregulate inhibitory co-signalling molecules such as PD-L1 and PD-L2, to provide a proof-of-concept study into using these cells as a future cellular-based therapy. Such a therapy may provide a locally administered, anti-T cell based therapeutic approach.

Methods: Synovial fluid mononuclear cells (SFMC) were obtained from patients with active early RA (< 1 year; fulfilling 2010 classification criteria). Cryopreserved SFMCs were cultured for 48 hours in the presence of 10 ng/mL interferon (IFN)-γ or 50 ng/mL dexamethasone, or neither. Following culture, cells were stained for flow cytometry using antibodies against CD14, CD16, CD68, CD163, CD45, PD-L1, PD-L2 and VSIG4 (BD Biosciences), and the viability dye Zombie NIR (BioLegend). Cells were analysed using a Beckman Coulter CytoFLEX flow cytometer and data interpreted using FlowJo software.

Results: SF macrophages were characterised by their expression of CD14, CD16, CD68 and CD163. Flow cytometric analysis revealed that SF macrophages were amenable to phenotypic modulation by exogenous mediators. As expected, Dexamethasone resulted in an increase in a CD163⁺, M2 like phenotype. Comparison of unstimulated control SF macrophages and those cultured with IFNγ showed significantly elevated levels of PD-L1 and PD-L2, and significantly reduced expression of VSIG4. Conversely, SF cells stimulated with dexamethasone exhibited significantly reduced levels of PD-L1, and significantly elevated levels of VSIG4.

Conclusion: Our findings demonstrate, for the first time in RA, the phenotypic plasticity of SF macrophages in vitro. Specifically, we found that culture of RA SF macrophages in the presence of IFNγ increased the expression of PD-L1 and PD-L2, demonstrating the potential of these cells to ‘rebalance’ the PD-1 pathway. Interestingly, these cells exhibited decreased surface protein levels of VSIG4, a B7 family related protein with overlapping function with PD-L1 and PD-L2. Culture with dexamethasone induced the opposite. These findings suggests that while PD-L1, PD-L2 and VSIG4 have similar regulatory characteristics, these proteins may exert differential functions in specific environments.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/manipulation-of-b7-family-member-expression-demonstrates-synovial-macrophage-plasticity-and-possible-future-targets-for-treatment-of-rheumatoid-arthritis/