AUTOIMMUNE RESPONSES TO ATHEROSCLEROTIC

Atherosclerosis is a chronic, complex arterial disease characterized by intimal lipid accumulation and inflammation. A unique lipid-binding molecule, namely cluster of differentiation 1d (CD1d), may impact atherosclerosis. Structurally, CD1d acts as a nonpolymorphic cell-surface receptor, resembling the major histocompatibility complex-I (MHC-I). While MHC-I restricts peptide antigen presentation to T cells, CD1d presents lipid antigens to T cells named CD1d-restrictedd T cells. Although increased expression of CD1d has been found in human plaques, the exact nature of CD1d-recognized lipids in atherosclerosis remains to be determined. Three groups of lipids may undergo oxidation in atherosclerosis producing atherogenic lipids: phospholipids, fatty acids, and cholesterol. The central hypothesis is that CD1d recognizes and present oxidative lipids to activate CD1d-restricted T cells, and trigger proinflammatory signal transduction In the first part of this study, oxidative phospholipids were identified and characterized as potential autoantigen for CD1d-restricted T cells. Derived from phospholipid 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine by oxidization, 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine (PGPC) is commonly found in atherosclerotic plaques. Upon stimulation with PGPC, spleen-derived CD1d-restricted T cells produced higher levels of cytokines and proliferated at higher rates than those without PGPC stimulation. CD1d deficiency compromised the PGPC-triggered T cell activation, suggesting that PGPC may function as a potentially novel autoantigen for T cells in atherosclerosis. In the second part of this study, CD1d-mediated proinflammatory signaling was evaluated in murine models. Enhanced CD1 expression occurred in spleens of db/db mice with hyperlipidemia. Tumor necrosis factor-alpha (TNF-α) was increased in db/db spleen, while TNF-α receptor expression augmented in the db/db murine heart, in comparison with those in normal mice. The nuclear factor-κ B (NF-κB) expression was enhanced in the db/db heart, whereas CD1d-null mice showed lower NF-κB, implying the involvement of CD1d in inflammation of the spleen and heart tissues in the mice with hyperlipidemia. The current study has identified PGPC as a novel lipid antigen recognized by CD1d-restricted T cells in atherosclerosis. The animal study has also provided evidence that CD1d regulates NF-κB-mediated proinflammatory signaling. Hence, CD1d-restricted T cell responses to autolipid antigen and mediated inflammatory signal may represent a new molecular pathway that triggers cardiovascular tissue injury in atherosclerosis and hyperlipidemia.

The role of CD1d in adipogenesis

Atherosclerosis-associated coronary heart disease is the number one cause of death in the western society, and often triggered by metabolic disorders, such as hyperlipidemia, hypertension, obesity, and diabetes. The CD1 molecules are a group of evolutionarily conservative transmembrane proteins that have recently emerged as novel lipid-binding and transporting molecules. The current study was aimed at illustrating the role of CD1d in regulation of lipid metabolism and adipogenesis. In stably transfected smooth muscle cells where CD1d is overexpressed via a pCMV promoter, high levels of binding of the oxysterol 7-ketocholesterol was found. Adipogenic treatment induces the cells to transdifferentiate into an adipocyte-like morphology. This adipocyte morphology of CD1d transfected SMCs strongly resembles that of the pre-adipocytes 3T3-L1 cells grown in the same adipogenic media. Adipogenic treatment of CD1d transfected 3T3-L1 cells led to an increased accumulation of lipids compared to mock transfected cells. Induction of adipogenic gene expression and activation, such as PPARγ and lipoprotein lipase (LPL), was achieved in adipogenically treated smooth muscle cells as well as 3T3-L1 cells with overexpression of CD1d. For determination of the role of CD1d in regulation of adipogenesis, a CD1d transgenic mouse strain was created using the CD1d-smooth muscle promoter construct. Compared to wild type control mice matched in age and sex, the transgenic mice show an age-dependent increase in abdominal and visceral fat tissue. Histopathological examination demonstrated marked enlargement of adipocytes in the transgenic fat tissue which otherwise remained a normal fat tissue structure. Immunohistochemical analysis of CD1d expression in the fat tissue revealed much stronger membrane CD1d immunostaining in the transgenic tissue than the wild type fat tissue. Under normal chow diets, CD1d-transgenic mice also developed fatty livers. In conclusion, CD1d serves as a regulator of lipid metabolism, which may transducer signals from oxysterols to induce expression of genes important in lipogenesis. These experimental results point to a novel mechanism by which CD1d mediates lipid metabolism in adipose tissue and contributes to the development of obesity. ^