FcgammaRIIB inhibits the development of atherosclerosis in low-density lipoprotein receptor-deficient mice.

The immune processes associated with atherogenesis have received considerable attention during recent years. IgG FcRs (FcgammaR) are involved in activating the immune system and in maintaining peripheral tolerance. However, the role of the inhibitory IgG receptor FcgammaRIIB in atherosclerosis has not been defined. Bone marrow cells from FcgammaRIIB-deficient mice and C57BL/6 control mice were transplanted to low-density lipoprotein receptor-deficient mice. Atherosclerosis was induced by feeding the recipient mice a high-fat diet for 8 wk and evaluated using Oil Red O staining of the descending aorta at sacrifice. The molecular mechanisms triggering atherosclerosis was studied by examining splenic B and T cells, as well as Th1 and Th2 immune responses using flow cytometry and ELISA. The atherosclerotic lesion area in the descending aorta was ~5-fold larger in mice lacking FcgammaRIIB than in control mice (2.75 +/- 2.57 versus 0.44 +/- 0.42%; p < 0.01). Moreover, the FcgammaRIIB deficiency resulted in an amplified splenocyte proliferative response to Con A stimulation (proliferation index 30.26 +/- 8.81 versus 2.96 +/- 0.81%, p < 0.0001) and an enhanced expression of MHC class II on the B cells (6.65 +/- 0.64 versus 2.33 +/- 0.25%; p < 0.001). In accordance, an enlarged amount of CD25-positive CD4 T cells was found in the spleen (42.74 +/- 4.05 versus 2.45 +/- 0.31%; p < 0.0001). The plasma Ab and cytokine pattern suggested increased Th1 and Th2 immune responses, respectively. These results show that FcgammaRIIB inhibits the development of atherosclerosis in mice. In addition, they indicate that absence of the inhibiting IgG receptor cause disease, depending on an imbalance of activating and inhibiting immune cells.

Marked upregulation of cholesterol 25-hydroxylase expression by lipopolysaccharide.

During screening of genes upregulated by lipopolysaccharide (LPS; endotoxin) treatment of bone marrow-derived mouse macrophages, it was unexpectedly found that cholesterol 25-hydroxylase (Ch25h) was strongly upregulated. Treatment of macrophages with 10 ng/ml of LPS for 2 h resulted in a 35-fold increase in the expression of Ch25h. In contrast, LPS treatment did not increase the expression of Cyp27a1 or Cyp7b1. The increased Ch25h expression was found to be independent of Myeloid differentiation protein 88 signaling but dependent on Toll-like receptor 4 signaling. LPS treatment of macrophages caused a 6- to 7-fold increase in cellular 25-hydroxycholesterol concentration. When macrophages were treated with increasing concentrations of 25-hydroxycholesterol, a dose-dependent release of CCL5 into the culture medium was observed. Intravenous injection of LPS in eight healthy volunteers resulted in an increase in plasma 25-hydroxycholesterol concentration. The possibility is discussed that 25-hydroxycholesterol may have a role in the inflammatory response, in addition to its more established role in the regulation of cholesterol homeostasis.

Nanomolar concentrations of lysophosphatidylcholine recruit monocytes and induce pro-inflammatory cytokine production in macrophages.

Lysophosphatidylcholine (LPC) has been attributed a pro-inflammatory role in atherosclerosis. Cell culture studies have identified stimulation of cytokine expression and chemotaxis by micromolar (muM) concentrations of LPC. In the present study we have investigated if LPC, in similarity with many other lipid mediators, has pro-inflammatory effects also at nanomolar (nM) concentrations. Cultured mouse bone marrow derived and RAW264.7 macrophages exposed to LPC demonstrated two peaks of increased MIP-2 release and mRNA expression; one at 0.1-10nM and another at muM concentrations. Both concentration ranges of LPC were also found to stimulate THP-1 monocyte chemotaxis. However, stimulation of the cells with muM concentrations of LPC may cause cell injury as increased release of lactate dehydrogenase was observed. Our findings demonstrate two peaks of LPC-induced pro-inflammatory activity, one in the nM and one in the muM range, and indicate that the latter may involve a stress response to lipid cytotoxicity.