TCR stimulation drives cleavage and shedding of the ITIM receptor CD31.

CD31 is a transmembrane molecule endowed with T cell regulatory functions owing to the presence of 2 immunotyrosine-based inhibitory motifs. For reasons not understood, CD31 is lost by a portion of circulating T lymphocytes, which appear prone to uncontrolled activation. In this study, we show that extracellular T cell CD31 comprising Ig-like domains 1 to 5 is cleaved and shed from the surface of human T cells upon activation via their TCR. The shed CD31 can be specifically detected as a soluble, truncated protein in human plasma. CD31 shedding results in the loss of its inhibitory function because the necessary cis-homo-oligomerization of the molecule, triggered by the trans-homophilic engagement of the distal Ig-like domain 1, cannot be established by CD31(shed) cells. However, we show that a juxta-membrane extracellular sequence, comprising part of the domain 6, remains expressed at the surface of CD31(shed) T cells. We also show that the immunosuppressive CD31 peptide aa 551-574 is highly homophilic and possibly acts by homo-oligomerizing with the truncated CD31 remaining after its cleavage and shedding. This peptide is able to sustain phosphorylation of the CD31 ITIM(686) and of SHP2 and to inhibit TCR-induced T cell activation. Finally, systemic administration of the peptide in BALB/c mice efficiently suppresses Ag-induced T cell-mediated immune responses in vivo. We conclude that the loss of T cell regulation caused by CD31 shedding driven by TCR stimulation can be rescued by molecular tools able to engage the truncated juxta-membrane extracellular molecule that remains exposed at the surface of CD31(shed) cells.

Antiangiogenic treatment prevents adventitial constrictive remodeling in graft arteriosclerosis.

BACKGROUND: Lumen loss in graft arteriosclerosis is the consequence of the development of a thick neointima and constrictive arterial remodeling. The latter is due to adventitial chronic inflammation and excessive perivascular collagen deposition. We reasoned that blockade of the portal of entry of inflammatory effectors may constitute a strategy to prevent constrictive arterial remodeling. METHODS AND RESULTS: We found that an anti-angiogenic therapy (ABT-510 nonapeptide), devoid of direct immunomodulatory properties, dramatically reduced adventitial angiogenesis by 66% (P<0.0001) in the rat aortic interposition model of graft arteriosclerosis. The associated decreased entry of inflammatory cells (44%; P<0.00001) resulted in drastic reduction of collagen deposition (57%; P<0.0001) thereby preventing subsequent adventitial constrictive remodeling and reduction of lumen surface area (5.26+/-0.74 vs. 8.58+/-2.48 microm2; Control vs. ABT-510-treated rats; P<0.0001). ABT-510 had no effect on the development of the neointima. CONCLUSION: This work supports the idea that targeting angiogenesis may act synergistically with conventional immunosuppressive therapy in preventing graft arteriosclerosis, a crucial feature of chronic graft rejection.

Direct and indirect effects of alloantibodies link neointimal and medial remodeling in graft arteriosclerosis.

OBJECTIVE: Chronic vascular rejection, the main cause of allograft failure, is characterized by the destruction of smooth muscle cells (SMCs) in the media concomitantly with the proliferation of SMCs in the adjacent neointima. We hypothesized that alloantibodies might be responsible for these 2 opposite but coordinated events. METHODS AND RESULTS: We used the rat aortic interposition model of chronic vascular rejection. During the rejection process, a neointima composed of proliferating SMCs from the recipient developed, whereas the SMCs in the media, all of donor origin, underwent apoptosis. Alloantibody deposition was detected only in the media. Using in vitro cultures experiments, we observed that alloantibody binding to donor SMCs exerts (1) a rapid upregulation of the transcription of growth factors genes, followed by (2) the induction of apoptosis after 24 hours. The transient production of growth factors by donor SMCs in response to the binding of alloantibodies induced the proliferation of recipient SMCs in culture supernatant transfer experiments. Additional data suggest that among the repertoire of alloantibodies, those directed against major histocompatibility complex I might carry the remodeling effect. CONCLUSIONS: Our data suggest that during chronic vascular rejection, alloantibody binding to donor medial SMCs is a crucial event that links neointimal and medial remodeling.

Reduced immunoregulatory CD31+ T cells in patients with atherosclerotic abdominal aortic aneurysm.

BACKGROUND: Cell-mediated immunity is considered to contribute to the pathogenesis of abdominal aortic aneurysms (AAA). In particular, infiltrating macrophages and CD8+ T lymphocytes participate in the destruction of the aortic wall extracellular matrix and smooth muscle cells. We surmise that these pathological events are controlled by circulating regulatory lymphocytes. METHODS AND RESULTS: Circulating CD4+/CD31+ cells were reduced in AAA patients (n=80, 8.9+/-0.6%) as compared with controls (n=69, 13.7+/-0.8%; P<0.001) and inversely proportional to AAA size. Exclusion of the aneurysm by an endoprothesis did not affect CD31+ T cell values. Reduction of blood CD4+/CD31+ cells was not attributable to their enrichment in AAA tissue. In contrast, CD8+/CD31+ cells were slightly reduced in the blood while increased in the aneurysmal tissue (29.2+/-0.5 versus 20.2+/-4.7% in blood, n=6; P<0.05). Remarkably, high percentages of CD4+/CD31+ cells were able to regulate proliferation and cytokine production of CD8+ lymphocytes, as well as CD8+ cell-mediated cytotoxicity of aortic smooth muscle cells (P<0.01). Finally, CD4+/CD31+ cells reduced the production and activity of metalloproteinase-9 by lipopolysaccharide-stimulated macrophages. CONCLUSIONS: Circulating CD4+/CD31+ T cells regulate macrophage and CD8+ T cell activation and effector function in the arterial wall. Their reduction might promote the development of AAA.

The proatherogenic role of T cells requires cell division and is dependent on the stage of the disease.

OBJECTIVE: The mechanism by which T cells exert a proatherogenic potential is unclear. In order to determine whether this potential requires their replication, we crossed atherosclerosis-prone apolipoprotein E knockout mice (ApoE degrees) with transgenic mice in which exclusive and conditional ablation of dividing T cells relies on their specific expression of the herpes simplex type 1 thymidine kinase (TK) suicide gene. METHODS AND RESULTS: We first showed that conalbumin-immunized ApoE degrees TK mice mounted a significant immune response to the antigen that was fully and specifically blocked by an in vivo ganciclovir (GCV) treatment. Next, ApoE degrees TK mice and ApoE degrees mice were treated or not with GCV either during the first 4 weeks (GCV 1 to 4w), the last 4 weeks (GCV 5 to 8w), or during 8 weeks (GCV 1 to 8w). Strikingly, ApoE degrees TK mice displayed a dramatic decrease in lesion development in the GCV 1 to 8w and GCV 5 to 8w groups, whereas the GCV had no effect when administered during the first 4 weeks. In protected mice, the inflammatory parameters in lesions, the percentage of CD69+ CD3+ splenocytes, and the circulating natural killer T cells were reduced. CONCLUSIONS: The present study, therefore, shows that the proatherogenic potential of T cells is crucial in the progression of fatty streaks to mature plaques and requires cell division.

Reduced immunoregulatory CD31+ T cells in the blood of atherosclerotic mice with plaque thrombosis.

OBJECTIVE: Lymphocyte activation is thought to play a major role in the pathogenesis of atherosclerotic complications such as plaque thrombosis. Circulating CD31+ T cells have been shown to regulate human T cell activation. Aim of this study was to evaluate whether the proportion of circulating immunoregulatory CD31+ T cells is correlated to the occurrence of plaque thrombosis in aged apolipoprotein (apo) E knockout (KO) mice. METHODS AND RESULTS: CD31+ T cell depletion of spleen T cells enhanced proliferation (P<0.05) and interferon-gamma production (P<0.01) while reducing interleukin (IL)-4 (P<0.001) and IL-10 (P=0.001) secretion in response to minimally modified low-density lipoprotein. CD31+ T cells were counted in 65 apoE KO mice (46-week-old) by flow cytometry. Organizing thrombi could be documented in 28 of 195 (14%) lesions and in at least one of the aorta root lesions in 23 of 65 mice (35%). CD31+ T cell count was significantly reduced in mice showing plaque thrombosis (72.3+/-1.5% versus 84.1+/-1.2%; P<0.0001), but such reduction did not follow induced plaque rupture or experimentally controlled thrombosis. CONCLUSIONS: Reduced CD31+ T cells in circulating blood is a hallmark of atherosclerotic plaque thrombosis. Our data suggest that CD31+ T cells may play an important regulatory role in the development of plaque thrombosis.

[Immunological aspects of atherosclerosis]. / Aspects immunologiques de l'athérome.

The pathogenesis of atherosclerosis remains incompletely understood. Accumulation of oxidized lipoproteins (oxLDL) within the vascular wall drives a related immune response very early during the disease course. Such an immune response is self-amplified and eventually escapes from physiologic control mechanisms. Certain lymphocytes may become pathogenic. B cells play a protective role by producing antibodies able to neutralize oxLDL. Elucidation of the immune control mechanisms in atherosclerosis will open the way to new therapeutic perspectives.

A CD31-derived peptide prevents angiotensin II-induced atherosclerosis progression and aneurysm formation.

AIMS: The loss of the inhibitory receptor CD31 on peripheral T lymphocytes is associated with the incidence of atherosclerotic complications such as abdominal aortic aneurysms (AAA) in patients and plaque thrombosis in mice. However, we have recently discovered that a small fragment of extracellular CD31 remains expressed on the surface of the apparently 'CD31-negative' T-cells and that it is possible to restore the CD31-mediated T-cell inhibition in vivo by using a synthetic CD31-derived peptide. Here, we wanted to evaluate the therapeutic potential of the peptide in an experimental model of accelerated atherosclerosis and AAA formation. METHODS AND RESULTS: The effect of the murine CD31-derived peptide (aa 551-574, 1.5 mg/kg/day, sc) was evaluated on the extent of atherosclerotic plaques and the incidence of AAA in 28-week-old apolipoprotein E knockout mice (male, n ≥ 8/group) submitted to chronic angiotensin II infusion. The therapeutic mechanisms of the peptide were assessed by evaluating its effect on immune cell functions in vivo and in vitro. The prevalence of angiotensin II-induced AAA correlated with the loss of extracellular CD31 on T-cells. CD31 peptide treatment reduced both aneurysm formation and plaque size (P < 0.05 vs. control). Protection was associated with reduced perivascular leucocyte infiltration and T-cell activation in vivo. Functional in vitro studies showed that the peptide is able to suppress both T-cell and macrophage activation. CONCLUSION: CD31 peptides could represent a new class of drugs intended to prevent the inflammatory cell processes, such as those underlying progression of atherosclerosis and development of AAA.

Atheroprotective effect of CD31 receptor globulin through enrichment of circulating regulatory T-cells.

OBJECTIVES: This study was designed to evaluate whether replacing CD31 (PECAM-1) signaling can restore the regulation of lymphocyte activation and improve experimental atherosclerosis. BACKGROUND: Atherosclerosis, the principal cause of myocardial infarction and stroke, is due to the development of a pathogenic immune response within the vascular wall and is aggravated by the reduction of regulatory T-cells. CD31, a transmembrane adhesion molecule with inhibitory signaling functions, is physiologically expressed on blood and vascular resting cells but is lost in pathologic conditions associated with atherosclerosis. METHODS: Replacement therapy with a CD31 receptor globulin (Rg) was delivered by in vivo gene transfer in 6-week-old apolipoprotein E knockout mice (n = 14 per group) every 5 weeks for 6 months. Control groups were treated with a truncated CD31Rg or with vehicle alone. At the end of the study, plaque size and morphology and blood T-cell compartment were analyzed in all mice. RESULTS: Atherosclerotic lesions of CD31Rg-treated mice were smaller (p < 0.01) and showed less neovascularization and intraplaque hemorrhage (p < 0.05) compared with control subjects. Furthermore, circulating regulatory T-cells were increased in vivo (p < 0.01) and showed normal suppressive function on proliferation of conventional T-cells in vitro. Indeed, CD31Rg treatment led to blunted blood T-cell activation (p < 0.05) and reduced T-cell infiltration within plaques (p < 0.01). CONCLUSIONS: Our data suggest that CD31 plays a key role in the regulation of the immune response linked to atherosclerosis. CD31-targeting therapeutic approaches may therefore be envisaged for preventing and treating atherosclerotic diseases.