Background/Purpose:   Synovial-lining macrophages play a crucial role in the onset and maintenance of joint inflammation in arthritis. CD68 and CD163 are commonly used markers of synovial macrophages in the RA synovium. CD163 is a cell-surface glycoprotein of the group B cysteine-rich scavenger receptor family that binds to and internalizes hemoglobin-haptoglobin complexes, thereby releasing IL-10 and carbon oxide with strong anti-inflammatory effects. Serum-soluble CD163 levels are significantly elevated in long-standing RA patients. The restrictive expression of CD163 by monocytes-macrophages was confirmed in the affected joint tissues of RA patients; however, the pathogenic roles of CD163-positive macrophages in inflammatory arthritis remain unclear. We investigated the roles of CD163-positive macrophages in arthritis development in mice.

Methods:   A collagen antibody-induced arthritis (CAIA) mouse model was established with a combination of monoclonal anti-type II collagen antibodies and lipopolysaccharide in C57BL/6 (B6)-background CD163 knockout (KO) mice. Arthritis was graded using a 0–16 clinical scale (0–4) per paw. Histological assessment of arthritis severity was performed on paraffin-embedded hematoxylin and eosin-stained sections, and synovial inflammation (inflammatory cell infiltration) and bone erosion were graded in a blinded fashion on an established 0–5 scale. Anti-Iba-1 and anti-NIMP-R14 antibodies were used for immunocytochemical staining to identify synovial tissue-resident macrophages and neutrophils, respectively. Anti-CD163 antibodies were used for histomorphometric quantitation of CD163-positive macrophages in the synovial tissue. Total RNA was isolated from the mouse ankle joints for gene expression analysis of pro-inflammatory cytokines IL-1β and IL-6 and the chemokine CXCL-1 before and 10 days after CAIA induction by quantitative RT-PCR.

Results:   Immunohistochemistry revealed that CD163 antigens were restricted to Iba-1-positive synovial tissue-resident macrophages. The number of CD163-positive cells increased during arthritis. CD163 KO mice exhibited significant exacerbation of clinical scores during arthritis compared with control wild type (WT) B6 mice. Histomorphometric quantification of the arthritic changes in the joint tissues confirmed the clinical assessment, with significant increases in inflammation and bone erosion scores in CD163 KO mice. Recruitment of neutrophils was significantly increased in the synovial tissue of CD163 KO mice compared to that of WT mice. Correspondingly, mRNA expression of CXCL1 was significantly more up-regulated in the inflamed ankle joints obtained from CD163 KO mice than control mice. Additionally, CD163 KO mice showed significantly elevated IL-1b and IL-6 mRNA expression levels in the inflamed synovium.

Conclusion:   CD163 deficiency exacerbates arthritis severity via up-regulation of synovial tissue IL-1b and IL-6. CD163-positive macrophage deficiency induced neutrophil recruitment via up-regulation of CXCL1 in the inflamed synovium. CD163-positive macrophages may play an inhibitory role in the pathogenesis of joint inflammation.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/involvement-of-cd163-positive-macrophages-in-the-pathogenesis-of-arthritis-via-modulation-of-inflammatory-cytokine-and-chemokine-expression-in-the-synovium-of-a-murine-model/