Abstract 5817: A Proatherogenic Role for HMGB1 in Atherosclerosis
HMGB1 is a highly conserved nuclear protein that modulates chromatin structure. It is exported from the nucleus into the cytoplasm and secreted by immune cells and is also released by necrotic cells. In vitro, HMGB1 is a chemoattractant for inflammatory and smooth muscle cells and stimulates cytokine secretion by macrophages. Since it is highly expressed in human atherosclerotic lesions we tested the hypothesis that extracellular HMGB1 may be proatherogenic, augmenting development of atherosclerotic lesions in ApoE−/− mice. Initially we confirmed that like human atherosclerotic lesions, lesions in ApoE−/− mice also express HMGB1. To determine its significance for atherosclerosis, ApoE−/− mice were injected i.v. twice weekly with a HMGB1 monoclonal neutralising antibody and fed a high fat diet over 8 weeks. Treatment with the antibody attenuated atherosclerosis by 55%. Macrophage accumulation was reduced by 43% whilst expression of VCAM-1 and MCP-1 were reduced by 48 and 72% respectively. The total (CD11c+) dendritic cell population was reduced by 65% and mature (CD83+) dendritic cells were reduced by nearly 60% suggesting a major effect on the dendritic cell population. Treatment also reduced CD4+ T-cell numbers by 50% and attenuated smooth muscle cell accumulation within developing lesions but did not affect endothelial cells. To determine whether HMGB1 differentially affected the CD4+ T-cell Th1 and Th2 populations we measured Tim3 and Tim1 mRNA levels. Tim 1, a marker for Th2 cells was not detectable in lesions of control ApoE−/− mice or mice treated with the neutralising antibody, but Tim3 mRNA was reduced by 55% with the neutralising antibody. Since in vitro HMGB1 stimulates secretion of proinflammatory cytokines by macrophages we also examined whether proinflammatory cytokine expression was affected in lesions. mRNAs encoding TNF-alpha and IL-1 beta were unchanged but IFN-alpha and IL-6 were reduced by nearly 50% by the neutralising antibody. Also, mRNA encoding ABCA1 was unaffected but mRNA encoding FABP4 was elevated 5-fold. We conclude that HMGB1 contributes to atherosclerosis development by augmenting intima development, stimulating dendritic cell migration, thereby increasing CD4+ Th1T-cell accumulation as well as IFN-alpha and IL-6.