GM‐CSF deficiency delays neointima formation in a normolipidemic mouse model of endoluminal endothelial damage

The Effects of Exercise on Plaque Volume and Composition in a Mouse Model of Early and Late Life Atherosclerosis

Background

Exercise is associated with a less atherogenic lipid profile; however, there is limited research on the effect of exercise on atherosclerotic plaque composition and markers of plaque stability.

Methods

A total of 110 apolipoprotein (apo)E−/− mice were placed on a chow diet and randomly assigned to control or exercise for a period of 10 weeks, commencing either at 12 weeks of age (the early-stage atherosclerosis, EA group) or at 40 weeks of age (the late-stage atherosclerosis, LA group). At the end of the exercise period, blood was assayed for lipids. Histologic analysis of the aortic sinus was undertaken to assess plaque size and composition that includes macrophage content, monocyte chemoattractant protein (MCP)-1, matrix metalloproteinase-2 (MMP-2), and tissue inhibitors of metalloproteinase 1 and 2 (TIMP-1 and 2).

Results

A total of 103 mice (38 EA, 65 LA) completed the protocol. In the EA group, exercise reduced plasma total cholesterol (TC) (−16%), free cholesterol (−13%), triglyceride (TG) (−35%), and phospholipid (−27%) levels, when compared to sedentary control mice (p < 0.01). In the EA group, exercise also significantly reduced plaque stenosis (−25%, p < 0.01), and there were higher levels of elastin (3-fold increase, p < 0.0001) and collagen (11-fold increase, p < 0.0001) in plaques, compared to control mice. There was an increase in plaque MMP-2 content in the exercise group (13% increase, p < 0.05) but no significant difference in macrophage or MCP-1 content. In the LA group, exercise reduced plaque stenosis (−18%, p < 0.05), but there was no significant difference in plaque composition. There was no difference in macrophage, MCP-1, or MMP-2 content in the LA groups. TIMP-1 was lower with exercise in both the EA and LA groups (−59%, p < 0.01 and −51%, p < 0.01 respectively); however, there was no difference in TIMP-2 levels.

Conclusion

A 10-week exercise period reduces atherosclerotic plaque stenosis when commenced at both early- and late-stage atherosclerosis. Intervening earlier with exercise had a greater beneficial effect on lipids and plaque composition than when starting exercise at a later disease stage.

Cytokine regulation of apoptosis-induced apoptosis and apoptosis-induced cell proliferation in vascular smooth muscle cells

Vascular smooth muscle cells (VSMCs) are the main structural cell of blood vessels, and VSMC apoptosis occurs in vascular disease, after injury, and in vessel remodeling during development. Although VSMC apoptosis is viewed as silent, recent studies show that apoptotic cells can promote apoptosis-induced compensatory proliferation (AICP), apoptosis-induced apoptosis (AIA), and migration of both local somatic and infiltrating inflammatory cells. However, the effects of VSMC apoptosis on adjacent VSMCs, and their underlying signaling and mechanisms are unknown. We examined the consequences of VSMC apoptosis after activating extrinsic and intrinsic death pathways. VSMCs undergoing apoptosis through Fas/CD95 or the protein kinase inhibitor staurosporine transcriptionally activated interleukin 6 (IL-6) and granulocyte-macrophage colony stimulating factor (GM-CSF), leading to their secretion. Apoptosis induced activation of p38MAPK, JNK, and Akt, but neither p38 and JNK activation nor IL-6 or GM-CSF induction required caspase cleavage. IL-6 induction depended upon p38 activity, while Fas-induced GM-CSF expression required p38 and JNK. Conditioned media from apoptotic VSMCs induced VSMC apoptosis in vitro, and IL-6 and GM-CSF acted as pro-survival factors for AIA. VSMC apoptosis was studied in vivo using SM22α-DTR mice that express the diphtheria toxin receptor in VSMCs only. DT administration induced VSMC apoptosis and VSMC proliferation, and also signficantly induced IL-6 and GM-CSF. We conclude that VSMC apoptosis activates multiple caspase-independent intracellular signaling cascades, leading to release of soluble cytokines involved in regulation of both cell proliferation and apoptosis. VSMC AICP may ameliorate while AIA may amplify the effects of pro-apoptotic stimuli in vessel remodeling and disease.

Distinct Differences on Neointima Formation in Immunodeficient and Humanized Mice after Carotid or Femoral Arterial Injury

Percutaneous coronary intervention is widely adopted to treat patients with coronary artery disease. However, restenosis remains an unsolved clinical problem after vascular interventions. The role of the systemic and local immune response in the development of restenosis is not fully understood. Hence, the aim of the current study was to investigate the role of the human immune system on subsequent neointima formation elicited by vascular injury in a humanized mouse model. Immunodeficient NOD.Cg-Prkdc^scidIL2rg^tm1Wjl(NSG) mice were reconstituted with human (h)PBMCs immediately after both carotid wire and femoral cuff injury were induced in order to identify how differences in the severity of injury influenced endothelial regeneration, neointima formation, and homing of human inflammatory and progenitor cells. In contrast to non-reconstituted mice, hPBMC reconstitution reduced neointima formation after femoral cuff injury whereas hPBMCs promoted neointima formation after carotid wire injury 4 weeks after induction of injury. Neointimal endothelium and smooth muscle cells in the injured arteries were of mouse origin. Our results indicate that the immune system may differentially respond to arterial injury depending on the severity of injury, which may also be influenced by the intrinsic properties of the arteries themselves, resulting in either minimal or aggravated neointima formation.

Role of colony-stimulating factors in atherosclerosis

PURPOSE OF REVIEW

The varied effects of colony-stimulating factors (CSFs) on monocytes and macrophages during inflammation and atherosclerosis and its clinical presentation prompt the question whether the differing effects of CSFs dictate macrophage function and disease progression.

RECENT FINDINGS

CSFs can give rise to heterogeneous populations of monocyte-derived macrophages that are characterized by disparate expression of distinct molecules which dictate their ability to process lipid and regulate inflammatory and immune responses. The CSFs have been found within atherosclerotic plaques and in the circulation where their levels may act as predictive biomarkers of disease progression. Accordingly, differing exposure to these factors imparts divergent genomic signatures and functional properties on macrophages and may impact the multifactorial steps involved in atherogenesis, plaque progression and instability.

SUMMARY

Great interest in macrophage heterogeneity in the genesis and progression of atherosclerosis has led to the search for consistent markers of specific subsets in both animal models and humans. A better understanding of the overlap and competition between CSF regulation of macrophage phenotypes is therefore warranted, to allow their characterization in plaques. Subsequent targeted genetic and pharmacological intervention will facilitate the generation of therapeutic approaches to halt the progression and rupture of advanced atherosclerotic plaques.

Immune responses regulating the response to vascular injury

PURPOSE OF REVIEW

Immune modulation of neointimal formation after vascular injury has been investigated for several decades but the complexities involved continue to obscure a clearer understanding of the process. The rapidly changing field of immunology makes this knowledge imperative.

RECENT FINDINGS

The review discusses immune factors involved in the response to vascular injury. Although innate immune responses play a predominantly detrimental role, the adaptive immune response is more complex. Mechanisms of T-cell activation, recruitment, as well as possible regulation are highlighted.

SUMMARY

Progress in understanding the role of the immune system in the response to arterial injury has been impressive. However, recent findings underscore the need to unravel the intricacies involved such as the kinetics and specific pathways of activation, specificity of immune cell involvement, and identification of targets for therapy. This is relevant in light of the increasing reports of immune factors involved in vascular disease and intervention in the clinical setting.