Aim: It has been reported that oxidized low-density lipoprotein (oxLDL) forms a stable and non-dissociable complex with β₂-glycoprotein I (β₂GPI) and that IgG anti-β₂GPI autoantibodies are able to recognize this complex, thus facilitating macrophage-derived foam cell formation in patients with antiphospholipid syndrome (APS). However, the immunopathological mechanisms of oxLDL/β₂GPI complexes in promoting foam cell formation are not fully understood. In this study, we examined the role of toll-like receptor 4 (TLR4) in the oxLDL/β₂GPI/anti-β₂GPI complex-induced transformation of mouse peritoneal macrophages to foam cells.
Methods: Oil red O staining and optical density (OD) measurements of intracellular stained oil red O solution were used to monitor the transformation of peritoneal macrophages to foam cells in TLR4-competent C3H/HeN and TLR4-mutant C3H/HeJ mice. During foam cell formation induced by the oxLDL/β₂GPI/anti-β₂GPI complex, the expression of TLR4 and activation of nuclear factor kappa B (NF-κB) were confirmed by analyzing the protein and mRNA levels of these compounds. Furthermore, the related active molecule expression during foam cell formation induced by the oxLDL/β₂GPI/anti-β₂GPI complex was examined in the presence or absence of TLR4.
Results: The data showed that treatment with the oxLDL/β₂GPI/anti-β₂GPI complex markedly increased foam cell formation, the TLR4 expression, NF-κB activation, the tissue factor (TF) expression and tumor necrosis factor-α (TNF-α) and monocyte chemotactic protein-1 (MCP-1) secretion in the C3H/HeN mice. However, the transformation of macrophages to foam cells and the expression levels of phosphorylated NF-κB, TF, TNF-α and MCP-1 were significantly reduced in the C3H/HeJ mice treated with the oxLDL/β₂GPI/anti-β₂GPI complex. In addition, compared with that achieved by oxLDL alone, the oxLDL/β₂GPI complex decreased foam cell formation and the related signaling molecule expression in the C3H/HeN mice.
Conclusions: Our results indicate that TLR4 plays an important role in the process of oxLDL/β₂GPI/anti-β₂GPI complex-induced transformation of macrophages to foam cells, which may accelerate the development of atherosclerosis in the setting of APS. However, β₂GPI alone functions as an antiatherogenic protein by preventing the foam cell formation induced by oxLDL.