Background/Purpose: Macrophages are critical in maintaining tissue homeostasis, as well as in inflammation and immune response, but their function deteriorates with age increasing susceptibility to arthritis. In the joint synovium, we and others have shown that several distinct populations of macrophage co-exist at steady-state and differ in their ontogeny, localization, and function. Our prior work demonstrated that the epigenomic landscape of macrophages is specified by a combination of transcription factors (TFs) that become active during differentiation or in response to the local environment: however, these processes may be disrupted over time. Here, we investigate how the transcriptional regulation of synovial macrophages is altered in the aging joint.

Methods: We use young (3-6 months), old (20-24 months) mice with a C57BL/6 background. Ankles were dissected, muscle resected, and bone marrow removed. Synovium is exposed and digested into a single-cell suspension for Fluorescence-Activated Cell Sorting (FACS). Macrophages were identified as CD45+CD11B+Ly6G-Ly6C-CD64+ cells and further subdivided into four populations by expression of MHCII and CX3CR1. In addition, monocytes were sorted from the blood and bone marrow. Either RNA-seq or ATAC-seq was performed and sequenced on Illumina Nextseq. Sequencing files were demultiplexed and resulting fastq files were trimmed and aligned to mm10 followed by either mappin to genes with HTseq (RNA-seq) or peak calling and annotation with HOMER (ATAC-seq).

Results: We isolate four macrophage subpopulations which we characterize based on their steady-state identity as CX3CR1+MHCII- tissue-resident synovial lining, CX3CR1+MHCII+ newly infiltrating cells; CX3CR1-MHCII- tissue-resident interstitial, CX3CR1-MHCII+ monocyte-derived interstitial. These populations have distinct epigenomic landscapes in the young joint with the tissue-resident macrophage populations exhibiting more similar transcriptional profiles than the monocyte-derived populations. However, as the mouse ages, the gene expression of all the subpopulations converges on a transcriptional profile similar to young infiltrating macrophages. This is consistent with the observation that there are increased numbers of monocyte-derived macrophages in the joint. Moreover, our results suggest a decrease in the activity of the TF, Mef2C, associated with the expression of tissue-resident genes. By comparing the epigenomic landscape of monocytes in young and aged mice, we observe an decrease in chromatin accessibility at the binding sites of cell-type-specific TFs, such as KLF and PU.1. Thus, the dysfunction of aged synovial macrophages traces back to changes in monocytes, whose contribution increases over time.

Conclusion: These results further our understanding of how the function of different macrophage subpopulations change with age. Future studies will investigate specific targets to reprogram the synovial macrophage compartment in aging.

« Back to ACR Convergence 2021

ACR Meeting Abstracts - https://acrabstracts.org/abstract/transcriptional-regulation-of-synovial-macrophages-in-the-aging-joint/