Apolipoprotein A-I Modulates Regulatory T Cells in Autoimmune LDLr−/−, ApoA-I−/− Mice

Abstract

The immune system is complex, with multiple layers of regulation that serve to prevent the production of self-antigens. One layer of regulation involves regulatory T cells (Tregs) that play an essential role in maintaining peripheral self-tolerance. Patients with autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis have decreased levels of HDL, suggesting that apoA-I concentrations may be important in preventing autoimmunity and the loss of self-tolerance. In published studies, hypercholesterolemic mice lacking HDL apoA-I or LDLr ؊/؊ , apoA-I ؊/؊ (DKO), exhibit characteristics of autoimmunity in response to an atherogenic diet. This phenotype is characterized by enlarged cholesterol-enriched lymph nodes (LNs), as well as increased T cell activation, proliferation, and the production of autoantibodies in plasma. In this study, we investigated whether treatment of mice with lipid-free apoA-I could attenuate the autoimmune phenotype. To do this, DKO mice were first fed an atherogenic diet containing 0.1% cholesterol, 10% fat for 6 weeks, after which treatment with apoA-I was begun. Subcutaneous injections of 500 g of lipid-free apoA-I was administered every 48 h during the treatment phase. These and control mice were maintained for an additional 6 weeks on the diet. At the end of the 12-week study, DKO mice showed decreased numbers of LN immune cells, whereas Tregs were proportionately increased. Accompanying this increase in Tregs was a decrease in the percentage of effector/effector memory T cells. Furthermore, lipid accumulation in LN and skin was reduced. These results suggest that treatment with apoA-I reduces inflammation in DKO mice by augmenting the effectiveness of the LN Treg response.

Apolipoprotein A‐I Modulates Atherosclerosis Through Lymphatic Vessel‐Dependent Mechanisms in Mice

“…We observe longitudinal plaque regression in the thoracic aorta of Ldlr −/− mice following lipid‐free apoA‐I treatment, independently of cholesterol accumulation in lymph or plasma. As reported by Wilhelm et al, this lack of effect in plasma cholesterol could be explained by the potent effect of apoA‐I on immune cell cholesterol balance instead of affecting whole body cholesterol balance 33. ApoA‐I has the ability to relieve excess cholesterol in lipid rafts/microdomains and thus to affect numerous types of signal transduction pathways that rely on these microdomains 30.…”

“…Subcutaneous injections of apoA‐I33 had more potent effects than intradermal injections to reduce the inflammatory cell content in the aortic sinus. This is most likely because of a more direct access to the blood vasculature.…”

“…It has been shown to be a key player in macrophage reverse cholesterol transport regulation through its capacity to package large amounts of cholesterol following its interaction with ABCA1 on cell membranes 29, 56. Interestingly, subcutaneously injected lipid‐free apoA‐I has been reported to reduce accumulation of lipid and immune cells within the aortic root of hypercholesterolemic mice without increasing HDL‐cholesterol concentrations 33, 57. Whereas apoA‐I has been extensively studied in the past decade, the mechanisms by which it mediates its atheroprotective effect are still unclear.…”

“…ApoA‐I was purified from human plasma by sequential ultracentrifugation as previously described 33. The apoA‐I was lyophilized to dryness, dissolved in 6 mol/L guanidine hydrochloride, and then refolded by dialyzing exhaustively against 10 mmol/L ammonium bicarbonate, pH 7.4.…”

“…ApoA‐I is the main protein constituent of plasma high‐density lipoprotein (HDL) and participates in its highly heterogeneous properties 31, 32. Whereas subcutaneous injections of low‐dose lipid‐free apoA‐I treatment do not significantly increase plasma HDL‐cholesterol concentrations, lipid‐free apoA‐I reduces excess cellular cholesterol and reverses the autoimmune‐like phenotype that develops in high cholesterol diet–fed Ldlr −/− apoA‐I −/− double knockout mice 33. Recently, the mechanistic basis explaining this protective effect of small and consistent amounts of apoA‐I in reducing lipid and immune cell accumulation within the aortic root has emerged.…”