Regulatory Immune Responses and Repair Mechanisms in Atherosclerosis

Atherosclerosis is a chronic inflammatory disease characterized by the formation of lipid rich plaques in the arterial wall. Rupture of a plaque results in clinical manifestations such myocardial infarction or stroke. Atherosclerosis is a complex disease where both autoimmune responses towards atherosclerosis-related antigens and smooth muscle repair responses play important roles. This thesis contains studies focusing on both regulatory immune responses and tissue repair mechanisms in experimental models of atherosclerosis as well as in patient cohorts. The first part of this thesis investigates the role of regulatory immune responses targeting plaque-related antigens. In paper I, we developed a matrigel-based method to characterize T helper 2 immune responses against human apolipoprotein B100 (ApoB100). We report that matrigel loaded with the antigen of interest can be used to measure antigen-specific immune cell accumulation and cytokine production. In paper II, we report that B cells pulsed with peptide 210 (p210) from ApoB100 coupled to the cholera toxin B subunit (p210-CTB) acquire a regulatory phenotype and induce Tregs in vivo. In the third paper, we unexpectedly found increased frequencies of circulating regulatory T cells in patients with prevalent cardiovascular disease. Our results indicate that the general immune cell activation in patients with prevalent cardiovascular disease can cause a compensatory increase in regulatory T cells to counteract the immune response. In the second part of this thesis, we focused on the role of smooth muscle cells in atherosclerosis. In paper IV, we show that IL-22 is involved in controlling smooth muscle cell phenotype. More specifically, IL-22 deficient atherosclerotic mice develop smaller plaques with increased expression of contractile proteins. Increased numbers of smooth muscle cells remaining in a contractile phenotype in the media and decreased collagen content in the plaques could possibly contribute to the smaller plaque size observed in IL-22 deficient mice. Finally, in paper V, we present data suggesting that high levels of smooth muscle cell growth factors (platelet-derived growth factor, epidermal growth factor, heparin-binding epidermal growth factor) measured in plasma can reflect a fibrous plaque phenotype. In particular, high plasma levels of heparin-binding epidermal growth factor at baseline was associated with a decreased risk for developing a coronary event

Simvastatin therapy attenuates memory deficits that associate with brain monocyte infiltration in chronic hypercholesterolemia

Abstract Evidence associates cardiovascular risk factors with unfavorable systemic and neuro-inflammation and cognitive decline in the elderly. Cardiovascular therapeutics (e.g., statins and anti-hypertensives) possess immune-modulatory functions in parallel to their cholesterol- or blood pressure (BP)-lowering properties. How their ability to modify immune responses affects cognitive function is unknown. Here, we examined the effect of chronic hypercholesterolemia on inflammation and memory function in Apolipoprotein E (ApoE) knockout mice and normocholesterolemic wild-type mice. Chronic hypercholesterolemia that was accompanied by moderate blood pressure elevations associated with apparent immune system activation characterized by increases in circulating pro-inflammatory Ly6Chi monocytes in ApoE-/- mice. The persistent low-grade immune activation that is associated with chronic hypercholesterolemia facilitates the infiltration of pro-inflammatory Ly6Chi monocytes into the brain of aged ApoE-/- but not wild-type mice, and links to memory dysfunction. Therapeutic cholesterol-lowering through simvastatin reduced systemic and neuro-inflammation, and the occurrence of memory deficits in aged ApoE-/- mice with chronic hypercholesterolemia. BP-lowering therapy alone (i.e., hydralazine) attenuated some neuro-inflammatory signatures but not the occurrence of memory deficits. Our study suggests a link between chronic hypercholesterolemia, myeloid cell activation and neuro-inflammation with memory impairment and encourages cholesterol-lowering therapy as safe strategy to control hypercholesterolemia-associated memory decline during ageing

Decreased levels of stem cell factor in subjects with incident coronary events.

It has been proposed that vascular progenitor cells play an important role in vascular repair, but their possible clinical importance in cardiovascular disease has not been fully characterized. Vascular endothelial growth factor A, placental growth factor and stem cell factor (SCF) are three growth factors that are important in recruiting vascular progenitor cells. In this study, we investigated the association between the plasma levels of these growth factors and incident coronary events (CEs)

IL-22 affects smooth muscle cell phenotype and plaque formation in apolipoprotein E knockout mice.

IL-22 is a recently discovered cytokine that belongs to the family of IL-10 related cytokines. It is produced by activated T-cells and innate lymphoid cells and has been suggested to be involved in tissue repair. As both inflammation and repair play important roles in atherosclerosis we investigated if IL-22 deficiency influences the disease process in Apoe(-/-) mice

Liver X receptors are required for thymic resilience and T cell output

The thymus is a primary lymphoid organ necessary for optimal T cell development. Here, we show that liver X receptors (LXRs)-a class of nuclear receptors and transcription factors with diverse functions in metabolism and immunity-critically contribute to thymic integrity and function. LXRαβ-deficient mice develop a fatty, rapidly involuting thymus and acquire a shrunken and prematurely immunoinhibitory peripheral T cell repertoire. LXRαβ's functions are cell specific, and the resulting phenotypes are mutually independent. Although thymic macrophages require LXRαβ for cholesterol efflux, thymic epithelial cells (TECs) use LXRαβ for self-renewal and thymocytes for negative selection. Consequently, TEC-derived LXRαβ protects against homeostatic premature involution and orchestrates thymic regeneration following stress, while thymocyte-derived LXRαβ limits cell disposal during negative selection and confers heightened sensitivity to experimental autoimmune encephalomyelitis. These results identify three distinct but complementary mechanisms by which LXRαβ governs T lymphocyte education and illuminate LXRαβ's indispensable roles in adaptive immunity

On-demand erythrocyte disposal and iron recycling requires transient macrophages in the liver

Iron is an essential component of the erythrocyte protein hemoglobin and is crucial to oxygen transport in vertebrates. In the steady state, erythrocyte production is in equilibrium with erythrocyte removal1. In various pathophysiological conditions, however, erythrocyte life span is severely compromised, which threatens the organism with anemia and iron toxicity2,3. Here we identify an on-demand mechanism that clears erythrocytes and recycles iron. We show that Ly-6Chigh monocytes ingest stressed and senescent erythrocytes, accumulate in the liver via coordinated chemotactic cues, and differentiate to ferroportin 1 (FPN1)-expressing macrophages that can deliver iron to hepatocytes. Monocyte-derived FPN1+ Tim-4neg macrophages are transient, reside alongside embryonically-derived Tim-4high Kupffer cells, and depend on Csf1 and Nrf2. The spleen likewise recruits iron-loaded Ly-6Chigh monocytes, but these do not differentiate into iron-recycling macrophages due to the suppressive action of Csf2. Inhibiting monocyte recruitment to the liver leads to kidney and liver damage. These observations identify the liver as the primary organ supporting rapid erythrocyte removal and iron recycling and uncover a mechanism by which the body adapts to fluctuations in erythrocyte integrity

Induction of T helper 2 responses against human Apolipoprotein B100 does not affect atherosclerosis in ApoE-/- mice.

Immune responses against LDL antigens have been found to play an important modulatory role in atherosclerosis. Immunization with homologous oxidized LDL, as well as human apolipoprotein B100 (ApoB)-derived peptides, inhibit atherosclerosis in hypercholesterolemic animal models of atherosclerosis. However, the role of antigen-specific Th2 responses in atherosclerosis remains to be fully clarified