Association of Increased F4/80^high Macrophages With Suppression of Serum‐Transfer Arthritis in Mice With Reduced FLIP in Myeloid Cells

Abstract

Objective Macrophages are critical in the pathogenesis of rheumatoid arthritis (RA). We recently demonstrated that FLIP (FLICE-like inhibitory protein) is necessary for the differentiation and/or survival of macrophages. We also identified that FLIP is highly expressed in RA synovial macrophages. This study was performed to determine if the reduction of Flip in macrophages would reduce synovial tissue macrophages and ameliorate serum transfer induced arthritis (STIA). Methods Mice with Flip deleted in myeloid cells (Flipf/fLysMc/+ mice) and littermate controls were employed. STIA was induced by intraperitoneal injection of K/BxN serum. Arthritis was evaluated by clinical score and change in thickness of the ankles. Joints were examined by histology and immunohistochemistry. Cells were isolated from the ankles and bone marrow and examined by flow cytometry, qRT-PCR or Western blot. Results In contrast to expectations, Flipf/fLysMc/+ mice developed more severe arthritis early in the clinical course, however the peak arthritis was attenuated and the resolution phase more complete. Prior to the induction of STIA, tissue resident macrophages were reduced. Further at day 9 post arthritis induction the number of F4/80hi macrophages in the joints of the Flipf/fLysMc/+ mice was not decreased, but increased. Flip was reduced in the F4/80hi macrophages in the ankles of the Flipf/fLysMc/+ mice, while F4/80hi population expressed an anti-inflammatory phenotype in both the Flipf/fLysMc/+ and control mice. Conclusions These observations suggest that reduction of FLIP in macrophages, by increasing the number of anti-inflammatory macrophages, may be an effective therapeutic approach to suppress inflammation, depending upon the disease stage.

Synovial Macrophages in Rheumatoid Arthritis: The Past, Present, and Future

“…However, these methods are all designed to target total SMs without discriminating the different subsets of SMs. As suggested in a previous study [21], F4/80 + resident murine SMs show anti-inflammatory phenotype. Methods that eliminate total SMs could further amplify inflammation by diminishing the number of resident anti-inflammatory SMs in mice.…”

“…At day 9 postarthritis induction, the number of F4/80 hi SMs in the joints of the Flip f/f LysM c/+ mice was increased. Flip was reduced in the F4/80 hi SMs in the ankles of the Flip f/f LysM c/+ mice, while the F4/80 hi population expressed an antiinflammatory phenotype in both the Flip f/f LysM c/+ and control mice [21], suggesting that resident F4/80 + SMs show anti-inflammatory activities. However, we cannot exclude the effects from other myeloid cells because lysozyme M is also expressed in neutrophils and monocytes.…”

Sinomenine Inhibits the Progression of Rheumatoid Arthritis by Regulating the Secretion of Inflammatory Cytokines and Monocyte/Macrophage Subsets

“…PBMCs were washed well with PBS and resuspended in PBS with 1% fetal calf serum (Gibco). Synovial macrophages were identified as CD11b + F4/80 + CD64 + ( 32 , 35 ), spleen or draining lymph node monocytes/macrophages were identified as CD11b + Ly6C + CD43 + ( 36 ), and PBMC monocytes were identified as CD14 + CD16 + ( 17 , 37 ). + denotes an expression level that was ~10-fold above the isotype control or a cell whose subpopulation was separated from the others and could be obviously defined.…”

Synovial tissue macrophages: friend or foe?

“…Tissue-resident macrophages can be self-renewing, controlled by the transcription factor Gata6, and maintain tissue immune homeostasis,28 whereas monocyte-derived macrophages predominate and drive inflammation in chronic disease models 15 29. Successful resolution of inflammation and reinstatement of tissue homeostasis requires differentiation of these proinflammatory macrophages towards an anti-inflammatory phenotype, and the activation/expansion of resident tissue macrophages15 29 30 that synergise for tissue homeostasis by production of anti-inflammatory/immune regulatory mediators (eg, resolvins, IL-10 and transforming growth factor beta (TGFβ)) 31–33. In addition, tissue-resident macrophages contribute to tissue remodelling by removing apoptotic cells (efferocytosis), metabolic products and damaged tissue components34–37 and by reinstating tissue spatial organisation, for example, pigment cell distribution in zebrafish skin 38.…”