Background/Purpose:

Macrophages play an integral role in the progression and persistence of rheumatoid arthritis (RA) through production of degradative enzymes, cytokines, and chemokines and recruitment of additional immune cells which results in progressive joint destruction. We previously demonstrated that naïve mouse joints contain both MHC II⁺ (monocyte-derived) and MHC II^– (tissue-resident) macrophages. Interestingly, we have shown that the monocyte-derived macrophages drive inflammation, while the tissue-resident macrophages are involved in resolution of inflammation. Further, we have noticed that CX3CR1 expression dramatically changes during arthritis in both the MHCII+ and MHCII- macrophage populations. Thus, we optimized a multi-parameter flow cytometry protocol to isolate four synovial macrophage subsets, delimited by MHCII and CX3CR1 expression, to perform subset-specific transcriptomic analysis.

Methods: We modeled arthritis in mice using an acute, inducible, K/BxN serum transfer induced arthritis (STIA), and a chronic, inducible, collagen-induced arthritis (CIA) in 10-12 week old female C57BL/6 mice. Florescence-activated cell sorting was employed to isolate macrophage subsets via expression of MHC II and CX3CR1 throughout the course of arthritis by FACS. RNA was extracted from sorted macrophage populations and processed for RNA sequencing (RNA-seq).

Results:

Analysis of synovial macrophage populations by RNA-seq show that each population has a unique transcriptional profile and identifies a set of genes preferentially expressed in each population that give insight to each population’s role at steady state. We then identified patterns in gene expression between two or more populations or shared between all populations to describe the relationship between macrophages.Analysis of synovial macrophage populations over the course of inflammation reveals that each macrophage subset responds differently across the phases of inflammation. At the initiation of inflammation, the CX3CR1/MHCII double positive and MHCII+CX3CR1- macrophages show the largest transcriptional response with the peak transcriptional change occurring during the propagation stage in all macrophage populations. However, upon resolution of inflammation, CX3CR1/MHCII double positive and CX3CR1+MHCII- macrophages contain a subset of genes that do not return to steady state expression. Macrophage response in our CIA model follows a similar transcriptional pattern to that found in our STIA model, however we do not see a distinct resolution pattern in our macrophage populations.

Conclusion:

We conclude that changes in gene expression from synovial macrophage sub-populations over the course of arthritis coincide with the different phases of joint inflammation. This study provides detailed insight into the heterogeneity of synovial macrophage populations and the relationship between tissue resident and monocyte derived macrophage populations during inflammation. The novel pathways identified here provide new insight into potentially useful targets for therapy.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/dynamics-of-transcriptional-signatures-from-synovial-macrophage-subsets-during-acute-and-chronic-murine-models-of-inflammatory-arthritis/