Objective: Scavenger receptor class A type I (SR-AI) and SR-AII are expressed by macrophages in particular and bind and internalize a broad range of molecules (including endotoxins, apoptotic bodies, and oxidized low-density lipoprotein). This study was undertaken to investigate the role of SR-AI/II in mediating severe cartilage destruction in antigen-induced arthritis (AIA).
Methods: AIA was induced in the knee joints of SR-AI/II(-/-) mice and wild-type (WT) controls. Joint inflammation and cartilage destruction (chondrocyte death) were measured by examining the histology of total knee joints. Matrix metalloproteinase (MMP)-mediated neoepitopes were measured by immunolocalization using anti-VDIPEN antibodies and chondrocyte activation with anti-S100A8 antibodies. Messenger RNA (mRNA) levels were determined in inflamed synovium using microarray analysis and quantitative reverse transcriptase-polymerase chain reaction. In synovial washouts, cytokines (interleukin-1beta [IL-1beta], IL-10, and tumor necrosis factor alpha) and S100A8/S100A9 were measured using Luminex and enzyme-linked immunosorbent assay.
Results: Levels of SR-AI/II mRNA were strongly elevated in inflamed synovium in AIA. On days 2, 8, and 14 after AIA induction, joint inflammation (exudates/infiltrate) was similar between the 2 groups. In WT mice, severe cartilage destruction was found in multiple cartilage surfaces of the inflamed knee joint on day 14 after AIA induction. MMP-mediated matrix destruction ranged between 40% and 60%, and chondrocyte death was prominent in 40-75% of the cartilage surfaces. In striking contrast, in SR-AI/II(-/-) mice, despite comparable joint inflammation, pronounced cartilage destruction was almost completely absent. Levels of IL-1beta and S100A8/S100A9 were significantly lower on days 7 and 14 after AIA induction, but levels of mRNA for various MMPs (MMP-2, MMP-3, MMP-9, and MMP-13) were comparable.
Conclusion: Our findings indicate that SR-AI and SR-AII are crucial receptors involved in mediating severe cartilage destruction in AIA.