Effects Of ABC-transporter A1 Deficiency On T Cell Cholesterol Homeostasis And Function

ATP binding cassette transporter Al?ABCA1?,a membrane-associated protein,mediates the cholesterol efflux to lipid-free/poor apoAI.It has been thought that the regulation of ABC Al on cholesterol homeostasis and functions of macrophages is crucial for protection against atherosclerosis.However,compared with control mice,macrophage-specific ABCA1-deficient mice develop the comparable atherosclerotic lesions,indicating that macrophages are not target cells responsible for the atheroprotection of ABCAl.Recent studies have shown that T cell-mediated immunity plays an important role in the development of atherosclerosis?AS?.Nevertheless,the role of ABC Al in cholesterol homeostasis and functions of T lymphocytes remains unclear.Aims:To determine the effects of ABCAl deletion on cholesterol homeostasis and functions of T lymphocytes.Methods:The Cre/Loxp system was used to generate T cell-specific ABCA1 knockout mice(ABCA1^CD4-/CD4-).The deficiency of ABCA1 in T cells was validated by PCR and Q-PCR.The specific deletion of ABCA1 was further clarified by cholesterol efflux experiments.Two animal models were selected to investigate effects of ABCAl deficiency on T cell functions:?1?An experimental autoimmune encephalomyelitis?EAE?model was established by immunizing mice with the antigen polypeptide MOG35-55 of myelin oligodendrocyte glycoprotein?MOG?.?2?T cell-specific ABC A1-deficient mice on LDLr^-/-background(ABCA1^CD4-/CD4-LDLr^-/-)were generated,and atherosclerotic lesions were induced by Western type diet?WTD?.In EAE model,we scored the disease severity and determine the degree of inflammatory infiltration and demyelination in spinal cords by histological examinations.The development of AS was evaluated at the aortic roots of animals by Oil-red O staining.Immunohistochemistry?IHC?was employed to determine the lesion compositions and T cell accumulation.T-cell activation,proliferation,differentiation and apoptosis were detected by flow cytometry.Results:ABCAl expressions on splenic CD4⁺T cells of ABCA1^CD4-/CD4-mice was less than 10%of that of control mice?P<0.001?,while T cell-depleted splencytes of two groups have comparable ABCA1 expression,indicating that ABCA1 is specificly deleted in T cells of ABCA1 ^CD4-/CD4-mice.ABCAl deletion significantly inhibited cholesterol efflux from T cells to apoAI?0.28 fold,P<0.001?,but had little effects on HDL-mediated cholesterol efflux.Interestingly,impaired cholesterol efflux was associated with a slight decrease rather than increase of membrane lipid rafts?CD4⁺T:0.91-fold,CD8+T:0.93-fold,P<0.01?on T cells of ABCAl^CD4-/CD4-mice as compared to ABCA1+/+mice.Despite no effects on the development of T cells in thymus,ABCAl deficiency did affect the production and phenotypes of peripheral T cells.As compared to ABCA1+/+/mice,ABCAl^CD4-/CD4-mice showed reduced number of memory effector CD4⁺T cells in blood?0.79 fold,P<0.05?and spleens?0.86 fold,P<0.05?.This might be resulted from reduced cell activation?CD25+%:-0.85 fold,P<0.05?and subsequent inhibition of proliferation??0.85 fold,P<0.05?as well as induction of apoptosis??1.15 fold,P<0.05?.Moreover,ABCAl deificiency reduced the production of nature-occuring Tregs in thymus?0.85 fold,P<0.05?,but enhanced the generation of inducible Tregs in peripheral organs and in vitro??1.20 fold,P<0.05?.After induction of EAE by immunization with MOG35-55,ABCA 1^CD4-/CD4-mice showed less clinical scores?0.54 fold,P<0.001?as compared to ABCA1+/+ mice.Absence of ABCA1 in T cells inhibited the in-vivo proliferation?-0.51 fold,P<0.01?and the MOG-specific in-vitro proliferation?0.65 fold,P<0.05?of splenic CD4⁺T cells.In addition,ABCA 1^CD4-/CD4-mice also have more Tregs and reduced number of Th17 effector cells in both spleens?Tregs:1.23 fold;Th17:0.67 fold,P<0.05?and spinal cords?Tregs:1.70 fold;Th17:0.60 fold,P<0.05?.Male ABCA1^CD4-/CD4-LDLr^-/-mice and ABCAl+/+ LDLr^-/-littermates were given a Western type diet for 9 weeks to induce formation of atherosclerotic plaques.Despite no difference in plasma levels of total cholesterol,ABCA1^CD4-/CD4-LDLr^-/-mice showed less lesion development?0.79 fold,P<0.05?as compared to control littermates,associated with reduced T cell accumulation in plaques and surrounding adventitias?plaque:0.58 fold,P<0.05;adventitia:0.68 fold,P<0.05?.Specific deletion of ABCA1 in T cells did not affect the development of T cells in thymus and the production of peripheral T cells under hypercholesterolemia.However,both memory CD4⁺and CD8+T cells decreased significantly in the spleen,which may be the result of decreased activation and proliferation of T cells in vivo??0.75 fold,P<0.05?.Interestingly,in aorta lymph nodes,ABCAl deletion reduced percents of not only memory effector cells?-0.80 fold,P<0.05?but also CD4⁺Thl?0.57 fold,P<0.01?and CD8+TC1?0.72 fold,P<0.05?,despite no effects on induction of Tregs.Conclusion:1.ABCA1 mediates cholesterol efflux from T cells to apoAI and plays important roles in T cell cholesterol homeostasis;2.ABCA1 deficiency on T cells suppresses activaiton and proliferation of T cells and induces the apoptosis after activaition,thereby leading to reduced Thl 7 effector cells and increased Tregs for alleviation of EAE;3.ABCA1 deficiency on T cells inhibits the development of atherosclerosis via reduction of memory effector T cells and Thl and TC1 effectors.Therefore,downregulation of ABCA1 on T cells maybe a potential therapeutic strategy for immune disorders.