Tom Seijkens

Platelet CD40L mediates thrombotic and inflammatory processes in atherosclerosis

CD40 ligand (CD40L), identified as a costimulatory molecule expressed on T cells, is also expressed and functional on platelets. We investigated the thrombotic and inflammatory contributions of platelet CD40L in atherosclerosis. Although CD40L-deficient (Cd40l(-/-)) platelets exhibited impaired platelet aggregation and thrombus stability, the effects of platelet CD40L on inflammatory processes in atherosclerosis were more remarkable. Repeated injections of activated Cd40l(-/-) platelets into Apoe(-/-) mice strongly decreased both platelet and leukocyte adhesion to the endothelium and decreased plasma CCL2 levels compared with wild-type platelets. Moreover, Cd40l(-/-) platelets failed to form proinflammatory platelet-leukocyte aggregates. Expression of CD40L on platelets was required for platelet-induced atherosclerosis as injection of Cd40l(-/-) platelets in contrast to Cd40l(+/+) platelets did not promote lesion formation. Remarkably, injection of Cd40l(+/+), but not Cd40l(-/-), platelets transiently decreased the amount of regulatory T cells (Tregs) in blood and spleen. Depletion of Tregs in mice injected with activated Cd40l(-/-) platelets abrogated the athero-protective effect, indicating that CD40L on platelets mediates the reduction of Tregs leading to accelerated atherosclerosis. We conclude that platelet CD40L plays a pivotal role in atherosclerosis, not only by affecting platelet-platelet interactions but especially by activating leukocytes, thereby increasing platelet-leukocyte and leukocyte-endothelium interactions.

Deficient CD40-TRAF6 signaling in leukocytes prevents atherosclerosis by skewing the immune response toward an antiinflammatory profile

The CD40–CD40 ligand (CD40L) signaling axis plays an important role in immunological pathways. Consequently, this dyad is involved in chronic inflammatory diseases, including atherosclerosis. Inhibition of CD40L in apolipoprotein E (Apoe)–deficient (Apoe−/−) mice not only reduced atherosclerosis but also conferred a clinically favorable plaque phenotype that was low in inflammation and high in fibrosis. Blockade of CD40L may not be therapeutically feasible, as long-term inhibition will compromise systemic immune responses. Conceivably, more targeted intervention strategies in CD40 signaling will have less deleterious side effects. We report that deficiency in hematopoietic CD40 reduces atherosclerosis and induces features of plaque stability. To elucidate the role of CD40–tumor necrosis factor receptor-associated factor (TRAF) signaling in atherosclerosis, we examined disease progression in mice deficient in CD40 and its associated signaling intermediates. Absence of CD40-TRAF6 but not CD40-TRAF2/3/5 signaling abolishes atherosclerosis and confers plaque fibrosis in Apoe−/− mice. Mice with defective CD40-TRAF6 signaling display a reduced blood count of Ly6Chigh monocytes, an impaired recruitment of Ly6C+ monocytes to the arterial wall, and polarization of macrophages toward an antiinflammatory regulatory M2 signature. These data unveil a role for CD40–TRAF6, but not CD40–TRAF2/3/5, interactions in atherosclerosis and establish that targeting specific components of the CD40–CD40L pathway harbors the potential to achieve therapeutic effects in atherosclerosis.

Blocking CD40-TRAF6 signaling is a therapeutic target in obesity-associated insulin resistance.

Significance Inflammation is a critical contributor to the pathogenesis of metabolic disorders associated with obesity. A group of molecules crucial in regulating the immune system are costimulatory molecules, including CD40. Our current study shows that CD40 acts as a double-edged sword in the metabolic syndrome through the initiation of differential signaling cascades. The CD40-TNF receptor-associated factor (TRAF) 2/3/5 signaling pathway protects against metabolic dysfunction and inflammation associated with obesity; conversely, the CD40-TRAF6 pathway contributes to the detrimental consequences of obesity. In the present study, we therefore designed, validated, and used a small-molecule inhibitor that blocks CD40-TRAF6 interactions. The improvement of insulin resistance by this specific CD40-TRAF6 inhibitor could represent a therapeutic breakthrough in the field of immunometabolism. The immune system plays an instrumental role in obesity and insulin resistance. Here, we unravel the role of the costimulatory molecule CD40 and its signaling intermediates, TNF receptor-associated factors (TRAFs), in diet-induced obesity (DIO). Although not exhibiting increased weight gain, male CD40−/− mice in DIO displayed worsened insulin resistance, compared with wild-type mice. This worsening was associated with excessive inflammation of adipose tissue (AT), characterized by increased accumulation of CD8+ T cells and M1 macrophages, and enhanced hepatosteatosis. Mice with deficient CD40-TRAF2/3/5 signaling in MHCII+ cells exhibited a similar phenotype in DIO as CD40−/− mice. In contrast, mice with deficient CD40-TRAF6 signaling in MHCII+ cells displayed no insulin resistance and showed a reduction in both AT inflammation and hepatosteatosis in DIO. To prove the therapeutic potential of inhibition of CD40-TRAF6 in obesity, DIO mice were treated with a small-molecule inhibitor that we designed to specifically block CD40-TRAF6 interactions; this compound improved insulin sensitivity, reduced AT inflammation, and decreased hepatosteatosis. Our study reveals that the CD40-TRAF2/3/5 signaling pathway in MHCII+ cells protects against AT inflammation and metabolic complications associated with obesity whereas CD40-TRAF6 interactions in MHCII+ cells aggravate these complications. Inhibition of CD40-TRAF6 signaling by our compound may provide a therapeutic option in obesity-associated insulin resistance.

Targeting macrophage Histone deacetylase 3 stabilizes atherosclerotic lesions

Macrophages are key immune cells found in atherosclerotic plaques and critically shape atherosclerotic disease development. Targeting the functional repertoire of macrophages may hold novel approaches for future atherosclerosis management. Here, we describe a previously unrecognized role of the epigenomic enzyme Histone deacetylase 3 (Hdac3) in regulating the atherosclerotic phenotype of macrophages. Using conditional knockout mice, we found that myeloid Hdac3 deficiency promotes collagen deposition in atherosclerotic lesions and thus induces a stable plaque phenotype. Also, macrophages presented a switch to anti‐inflammatory wound healing characteristics and showed improved lipid handling. The pro‐fibrotic phenotype was directly linked to epigenetic regulation of the Tgfb1 locus upon Hdac3 deletion, driving smooth muscle cells to increased collagen production. Moreover, in humans, HDAC3 was the sole Hdac upregulated in ruptured atherosclerotic lesions, Hdac3 associated with inflammatory macrophages, and HDAC3 expression inversely correlated with pro‐fibrotic TGFB1 expression. Collectively, we show that targeting the macrophage epigenome can improve atherosclerosis outcome and we identify Hdac3 as a potential novel therapeutic target in cardiovascular disease.

The immunobiology of CD154-CD40-TRAF interactions in atherosclerosis.

Atherosclerosis is a chronic disease of the large arteries that is responsible for the majority of cardiovascular events. In its pathogenesis, the immune system plays a pivotal role. The effectuation of the immune response through interactions between immune cells that is mediated by co-stimulatory molecules, determine atherosclerosis severity. This review will highlight the role of one of the most powerful co-stimulatory dyads, the CD154 (also known as CD40 ligand, CD40L)-CD40 dyad, in atherosclerosis. Its cell-type specific actions, signal transduction cascades and its therapeutic potentials will be discussed.

Immune Cell Crosstalk in Obesity: A Key Role for Costimulation?

In the past two decades, numerous experimental and clinical studies have established the importance of inflammation and immunity in the development of obesity and its metabolic complications, including insulin resistance and type 2 diabetes mellitus. In this context, T cells orchestrate inflammatory processes in metabolic organs, such as the adipose tissue (AT) and liver, thereby mediating obesity-related metabolic deterioration. Costimulatory molecules, which are present on antigen-presenting cells and naïve T cells in the AT, are known to mediate the crosstalk between the adaptive and innate immune system and to direct T-cell responses in inflammation. In this Perspectives in Diabetes article, we highlight the newest insights in immune cell interactions in obesity and discuss the role of costimulatory dyads in its pathogenesis. Moreover, the potential of therapeutic strategies that target costimulatory molecules in the metabolic syndrome is explored.

Hypercholesterolemia-induced priming of hematopoietic stem and progenitor cells aggravates atherosclerosis

Modulation of hematopoietic stem and progenitor cells (HSPCs) determines immune cell function. In this study, we investigated how hypercholesterolemia affects HSPC biology and atherosclerosis. Hypercholesterolemia induced loss of HSPC quiescence, characterized by increased proliferation and expression of cyclin B1, C1, and D1, and a decreased expression of Rb, resulting in a 3.6‐ fold increase in the number of HSPCs in hypercholesterolemic Ldlr‐/‐ mice. Competitive bone marrow (BM) transplantations showed that a hypercholesterolemic BM microenvironment activates HSPCs and skews their development toward myeloid lineages. Conversely, hypercholesterolemia‐primed HSPCs acquired an enhanced propensity to generate myeloid cells, especially granulocytes and Ly6Chigh monocytes, even in a normocholesterolemic BM microenvironment. In conformity, macrophages differentiated from hypercholesterolemia‐primed HSPCs produced 17.0% more TNF‐α, 21.3% more IL‐6, and 10.5% more MCP1 than did their normocholesterolemic counterparts. Hypercholesterolemia‐induced priming of HSPCs generated leukocytes that more readily migrated into the artery, which resulted in a 2.1‐fold increase in atherosclerotic plaque size. In addition, these plaques had a more advanced phenotype and exhibited a 1.2‐fold increase in macrophages and 1.8‐fold increase in granulocytes. These results identify hypercholesterolemia‐induced activation and priming of HSPCs as a novel pathway in the development of atherosclerosis. Inhibition of this proinflammatory differentiation pathway on the HSPC level has the potential to reduce atherosclerosis.—Seijkens, T., Hoeksema, M. A., Beckers, L., Smeets, E., Meiler, S., Levels, J., Tjwa, M., de Winther, M. P. J., Lutgens, E. Hypercholesterolemia‐induced priming of hematopoietic stem and progenitor cells aggravates atherosclerosis. FASEB J. 28, 2202–2213 (2014). www.fasebj.org

Caveolin-1 deficiency decreases atherosclerosis by hampering leukocyte influx into the arterial wall and generating a regulatory T-cell response

Caveolin‐1 plays a crucial role in atherosclerosis, which is mainly attributed to its effects on low‐density‐lipoprotein (LDL) transcytosis. However, caveolin‐1 has also been implicated in the regulation of inflammation. We investigated the effects of caveolin‐1 deficiency in atherosclerosis with its accompanying changes in plaque‐ and lymphoid‐related immunology and inflammation. Cav1‐/‐ Apoe‐/‐ mice exhibited a 15‐fold reduction in plaque size with plaques containing fewer macrophages, T cells, and neutrophils. Intravital microscopy revealed 83% less leukocyte adhesion to the vessel wall in Cav1‐/‐ Apoe‐/‐ mice, which could be attributed to reduced endothelial chemokine li‐gand‐2 (CCL‐2/MCP‐1) and vascular cell adhesion molecule‐1 (VCAM‐1) expression. Caveolin‐1 deficiency resulted in a 57% increase in regulatory T cells and a 4% decrease in CD4+ effector T cells in lymphoid organs. Bone marrow transplantations revealed that Cav1‐/‐Apoe‐/‐ mice receiving Cav1+/+Apoe‐/‐ or Cav1‐/‐ Apoe‐/‐ bone marrow presented 4‐ to 4.5‐fold smaller plaques with no additional phenotypic changes. In contrast, atherosclerosis was not affected in Cav1+/+ Apoe‐/‐ recipients receiving Cav1‐/‐ Apoe‐/‐ or Cav1+/+ Apoe‐/‐ bone marrow. However, the presence of Cav1‐/‐ Apoe‐/‐ bone marrow was associated with an anti‐inflammatory T‐cell profile. Our study reveals that nonhematopoietic caveolin‐1 determines plaque size, whereas hematopoietic caveolin‐1 regulates lymphoid immune‐modulation. However, both are required for phenotypic modulation of plaques.—Engel, D., Beckers, L., Wijnands, E., Seijkens, T., Lievens, D., Drechsler, M., Gerdes, N., Soehnlein, O., Daemen, M. J. A. P., Stan, R. V., Biessen, E. A. L., Lutgens, E. Caveolin‐1 deficiency decreases atherosclerosis by hampering leukocyte influx into the arterial wall and generating a regulatory T‐cell response. FASEB J. 25, 3838–3848 (2011). www.fasebj.org

Blocking CD40-TRAF6 interactions by small-molecule inhibitor 6860766 ameliorates the complications of diet-induced obesity in mice

Background:Immune processes contribute to the development of obesity and its complications, such as insulin resistance, type 2 diabetes mellitus and cardiovascular disease. Approaches that target the inflammatory response are promising therapeutic strategies for obesity. In this context, we recently demonstrated that the interaction between the costimulatory protein CD40 and its downstream adaptor protein tumor necrosis factor receptor-associated factor 6 (TRAF6) promotes adipose tissue inflammation, insulin resistance and hepatic steatosis in mice in the course of diet-induced obesity (DIO).Methods:Here we evaluated the effects of a small-molecule inhibitor (SMI) of the CD40-TRAF6 interaction, SMI 6860766, on the development of obesity and its complications in mice that were subjected to DIO.Results:Treatment with SMI 6860766 did not result in differences in weight gain, but improved glucose tolerance. Moreover, SMI 6860766 treatment reduced the amount of CD45+ leucocytes in the epididymal adipose tissue by 69%. Especially, the number of adipose tissue CD4+ and CD8+ T cells, as well as macrophages, was significantly decreased.Conclusions:Our results indicate that small-molecule-mediated inhibition of the CD40-TRAF6 interaction is a promising therapeutic strategy for the treatment of metabolic complications of obesity by improving glucose tolerance, by reducing the accumulation of immune cells to the adipose tissue and by skewing of the immune response towards a more anti-inflammatory profile.

Platelet CD40 Exacerbates Atherosclerosis by Transcellular Activation of Endothelial Cells and Leukocytes

Objective— Beyond their eminent role in hemostasis and thrombosis, platelets are recognized as mediators of inflammation. Platelet cluster of differentiation 40 (CD40) ligand (CD40L and CD154) plays a key role in mediating platelet-induced inflammation in atherosclerosis. CD40, the receptor for CD40L, is present on platelets; however, the role of CD40 on this cell type is until now undefined. Approach and Results— We found that in both mice and humans, platelet CD40 mediates the formation of platelet–leukocyte aggregates and the release of chemokine (C-X-C motif) ligand 4. Leukocytes were also less prone to adhere to CD40-deficient thrombi. However, platelet CD40 was not involved in platelet aggregation. Activated platelets isolated from Cd40 −/− Apoe −/− mice adhered less to the endothelium upon injection into Apoe −/− mice when compared with CD40-sufficient platelets. Furthermore, lack of CD40 on injected platelets led to reduced leukocyte recruitment to the carotid artery as assayed by intravital microscopy. This was accompanied by a decrease in endothelial vascular cell adhesion molecule-1, platelet endothelial cell adhesion molecule, VE-cadherin, and P-selectin expression. To investigate the effect of platelet CD40 in atherosclerosis, Apoe −/− mice received thrombin-activated Apoe −/− or Cd40 −/− Apoe −/− platelets every 5 days for 12 weeks, starting at the age of 17 weeks, when atherosclerotic plaques had already formed. When compared with mice that received Apoe −/− platelets, those receiving Cd40 −/− Apoe −/− platelets exhibited a >2-fold reduction in atherosclerosis. Plaques of mice receiving CD40-deficient platelets were less advanced, contained less macrophages, neutrophils, and collagen, and displayed smaller lipid cores. Conclusions— Platelet CD40 plays a crucial role in inflammation by stimulating leukocyte activation and recruitment and activation of endothelial cells, thereby promoting atherosclerosis.