Costimulatory molecules in human atherosclerotic plaques: an indication of antigen specific T lymphocyte activation

The Size and Phenotype of Virus-Specific T Cell Populations Is Determined by Repetitive Antigenic Stimulation and Environmental Cytokines

Based on the expression of the TNFR SFP CD27, two Ag-primed CD8(+) T cell subsets can be discerned in the circulation of healthy individuals: CD27(+) T cells that produce a variety of cytokines but do not display immediate cytolytic activity; and cytotoxic CD27(-) T cells, which secrete only IFN-gamma and TNF-alpha. The mechanism that controls the generation of these different phenotypes is unknown. We show that CMV reactivation not only increases the number of virus-specific T cells but also induces their transition from a CD27(+) to a CD27(-) phenotype. In support of a relation between pool size and phenotype in a cohort of latently infected individuals, the number of Ag-specific CD27(-) CD8(+) T cells was found to be linearly related to the total number of CMV-specific CD8(+) T cells. In vitro studies revealed that the acquisition of the CD27(-) phenotype on CMV-specific T cells depended on the interaction of CD27 with its cellular ligand, CD70. Expression of CD70 was proportional to the amount of antigenic stimulation and blocked by the CD4(+) T cell-derived cytokine IL-21. Thus, induction of CD70, which may vary in distinct viral infections, appears to be a key factor in determining the size and phenotype of the CMV-specific T cell population in latently infected individuals.

CD40 ligand is selectively expressed on CD4+T cells and platelets: implications for CD40-CD40L signalling in atherosclerosis

Atherosclerosis is a degenerative inflammatory disease of the vascular system. Endothelial cells (ECs), smooth muscle cells, and macrophages, key elements in atherosclerosis, all have the potential to express the CD40 receptor and are thus susceptible to potent pro-inflammatory signals by CD40 ligand (CD40L)-bearing cells. CD40L is a TNF-alpha-related membrane protein originally identified on activated T cells. The recent recognition of platelets as an abundant source of CD40L led to a reassessment of the involvement of CD40L in atherosclerosis. In the present report, CD40L(+) T cells were identified in the intima of atherosclerotic tissues within macrophage infiltrates and in areas of neovascularization. These CD40L(+) T cells were CD4(+), CD69(+), but negative for CD8, CD25, CD28, and ICOS. In some specimens, CD40L(+) platelets were identified in the intima and in plaque ruptures. Contrary to previous reports, CD40L was not observed on ECs, smooth muscle cells, and macrophages in atherosclerotic tissues or in vitro at the protein and mRNA levels. Functionally, flow chamber experiments demonstrated that stimulation of ECs via CD40 is sufficient to recruit neutrophils and T cells from whole blood to ECs and suggested that CD40L(+) platelets contribute significantly to the recruitment of inflammatory cells to damaged endothelium in vivo. However, due to the short half-life of platelet CD40L, the chronic CD40L-driven inflammatory component can only be sustained by activated CD4(+) T cells. Contrary to current understanding, the contribution of CD40L to chronic inflammation in atherosclerosis is thus antigen-driven and MHC-dependent. This conclusion has significant therapeutic implications.

Increased expression of T cell activation markers (CD25, CD26, CD40L and CD69) in atherectomy specimens of patients with unstable angina and acute myocardial infarction

Atherosclerotic plaques contain a chronic immune mediated inflammation in which T cells play an important role. A previous study revealed that the numbers of interleukin-2 receptor-positive T cells is increased in culprit lesions of patients with acute coronary syndromes; a finding of considerable interest since it indicates a recent change in the intraplaque T cell mediated immune response. Confirmation of this observation is important, because it could provide insight into the onset of the acute event. We have, therefore, expanded our earlier work by using a panel of different T cell activation markers (CD25, CD26, CD40L, CD69). The study is based on 58 culprit lesions from patients who underwent coronary atherectomy. There were four groups of patients: chronic stable angina (n=13), stabilized unstable angina (n=16), refractory unstable angina (n=15), and acute myocardial infarction (AMI; n=14). Activated T cells were expressed as a percentage of the total of CD3-positive cells. CD25, CD26, CD40L, and CD69/CD3 percentages increased with the severity of the coronary syndrome. In patients with AMI all percentages were significantly higher than in patients with chronic stable angina. CD25, CD26, CD40L, and CD69/CD3 percentages in patients with an unstable condition (refractory unstable angina and AMI) were significantly higher than those in patients with a stable condition (chronic stable or stabilized unstable angina) The finding that the percentage of T cells with recent onset activation is significantly increased in the culprit lesions of patients with acute coronary syndromes suggests strongly that a recent change in pathogenic stimulation has occurred leading to local T cell activation.

NKT cell subsets in infection and inflammation

We recently identified two stably expressed cell surface markers, IL-18R and ST2L, which are selectively expressed on T1/NK1 and T2/NK2 cells, respectively. Here we use these molecules in direct ex vivo analysis of PBMCs from patients with AIDS, psoriasis (PS) atherosclerosis and to show the importance of these markers as determinants of the functional dichotomy of lymphocyte subsets, in particular NKT. In a cohort of 22 HIV patients made up of a mixture of long term non-progressors, seroconvertors, progressors and asymptomatics, we found a clear NKT1 to NKT2 shift (P=0.001) in the HIV-infected individuals. We also show a predominance of NKT2 cells over NKT1 cells in the PBMCs of patients with mild to moderate PS (N=13, P=0.005) but not in atopic dermatitis or healthy controls. However, in patients (N=6) requiring surgery for aneurysm, a predominance of Type 1 (IL-18R(+)) NKT lymphocytes over NKT2 was detected among infiltrating lymphocytes isolated from atherosclerotic plaques. Our data therefore demonstrate that ST2L and IL-18R could serve as important determinants of the immune status of human diseases.

Human atherosclerotic plaques express DC-SIGN, a novel protein found on dendritic cells and macrophages

The association of autoimmune phenomena with atherosclerosis suggests that plaques may contain specialized antigen-presenting cells, dendritic cells (DCs). DC-SIGN is a C-type lectin expressed by DCs. This study assessed whether human atherosclerotic plaques expressed DC-SIGN and several other macrophage/DC markers. Plaques from human coronary and carotid arteries and aorta contained DC-SIGN-immunoreactive cells. Double-labelling showed co-expression of DC-SIGN and macrophage/DC lineage markers CD14, CD68, HLA-DR, and S100. There was no immunoreactivity for the DC activation markers CD83 or CMRF-44. Since DC-SIGN mediates adhesion to T-lymphocytes and endocytosis, its expression in atherosclerotic plaques may have functional implications. Activated DCs migrate quickly from areas of inflammation to regional lymph nodes, possibly explaining the paucity of activated DCs in atherosclerotic plaques. In conclusion, this study has shown that DC-SIGN is expressed in atherosclerosis.

Endothelial cell and cAMP regulation of T-cell CD40 ligand: relevance of calcium/calmodulin-dependent kinase IV signalling

CD40 ligand (CD40L) expression is now recognized to contribute in the pathogenesis of vascular disease. Because increased CD40L has been associated with myocardial infarction, effects of endothelial cells and cAMP with respect to CD40L regulation may be of clinical relevance. In the present study, endothelial cells are shown to markedly increase CD40L on naïve CD4+ T cells with a more modest effect on memory T cells. Furthermore, the addition of dibutyryl cyclic AMP (dbcAMP) synergistically increased naïve cell CD40L but inhibited memory cell CD40L. Although it has previously been recognized that human endothelial cells can increase T-cell CD40L, this is the first description of the difference in responses of naïve and memory cells and the first demonstration of synergistic effects of endothelial cells and cAMP on CD40L regulation. Consistent with previous reports that CD40L regulation is distinctive, another marker of early activation (CD69) was not similarly regulated. The mechanisms of CD40L regulation were related to calcineurin and calcium/calmodulin dependent kinase IV (CaMKIV) signalling pathways. Endothelial cell costimulation of CD40L was found to be dependent upon calcineurin activity while cAMP actions to increase CD40L were dependent upon CaMKIV. Expression of a dominant negative CaMKIV construct further indicated an important role for CaMKIV in regulation of CD40L and cAMP responses. These data indicate that endothelial cell costimulation can interact with cAMP through calcium signalling pathways to synergistically enhance CD40L expression. Because increased CD40L is associated with atherosclerotic plaque and instability, results are relevant to the pathogenesis of atherosclerosis and myocardial infarction.

Increased CD8(+) T cells associated with Chlamydia pneumoniae in symptomatic carotid plaque

BACKGROUND AND PURPOSE

The presence of Chlamydia pneumoniae has been reported in carotid atheroma, but its causative effect in the activation of an atherosclerotic plaque to a prothrombotic state remains unproved. Antigen- mediated activation of T lymphocytes within plaque may represent a mechanism by which infection can result in plaque conversion. The goal of the present study was to characterize the T-cell subtype profile related to the presence of C pneumoniae in patients with symptomatic versus asymptomatic carotid atherosclerosis.

METHODS

We studied 14 plaques (5 symptomatic and 9 asymptomatic) positive for C pneumoniae confirmed by polymerase chain reaction and 14 plaques (6 symptomatic and 8 asymptomatic) from age- and stenosis-matched patients negative for C pneumoniae by polymerase chain reaction. T-cell subpopulations of T-helper, T-cytotoxic, and T-memory lymphocytes were identified through indirect enzyme immunohistochemistry with anti-CD3+, anti-CD4+, anti-CD8+, and anti-CD45RO+ monoclonal antibodies, respectively. Results are expressed as the number of positive cells per millimeter squared.

RESULTS

In the absence of C pneumoniae, symptomatic plaques had a modest but significant increase of CD3+ (89.6 versus 55.3, P=0.013), CD4+ (57.3 versus 32.7, P=0.01), and CD45RO+ (82.8 versus 43.7, P=0.007), but not CD8+ T cells (28.5 versus 25.5, P=0.245) compared with asymptomatic. However, in the presence of C pneumoniae, there was significant increase of all T-lymphocyte subtypes in symptomatic plaques, including CD8+ (76.8 versus 30.3, P=0.03), CD3+ (192.1 versus 80.4, P=0.004), CD4+ (111.9 versus 37.9, P=0.003), and CD45RO+ (120.2 versus 72.9, P=0.003) cells compared with asymptomatic plaques. With use of 2-way ANOVA, both the presence of chlamydia and symptoms were associated with significantly higher T-cell counts (P<0.005 for all subtypes).

CONCLUSIONS

Although all patients with symptomatic disease show a modest elevation in the concentration of intraplaque lymphocytes, a preferential increase in CD8+ class I-restricted T cells is observed in symptomatic carotid plaque positive for C pneumoniae. These data provide incentive to further explore the role of Chlamydia in the modification of immune-mediated mechanisms in active atherosclerotic plaque.

Monocytes influence the fate of T cells challenged with oxidised low density lipoproteins towards apoptosis or MHC-restricted proliferation

Atherosclerosis has been implicated in myocardial infarction, stroke and a host of cardiovascular diseases. The presence of activated T lymphocytes and macrophages, and the increased expression of HLA-DR antigen are consistent with the notion of immune activity in the atherosclerotic plaque. The nature of the causative antigen has not been established although oxidised low density lipoproteins (oxLDL) that accumulate in atherosclerotic plaques could fulfil this role. Here, we report that monocytes play a key role in influencing the fate of purified peripheral human T lymphocytes from healthy donors when the cells are exposed to LDL oxidised under the controlled conditions of water radiolysis. Our data showed that oxLDL generated under these conditions were chemoattractants for T cells. However, they induced a state of apoptosis in T lymphocytes cultured in the absence of monocytes. The extent of apoptosis was related to the degree of oxidation of LDL and the time of T cell exposure to oxLDL. OxLDL-dependent apoptosis did not involve a scavenger-like receptor. CD4(+) cells were more sensitive to the apoptotic effect of oxLDL than CD8(+) cells. OxLDL-primed (12 h) autologous monocytes triggered a robust proliferation of T lymphocytes cultured in the absence of oxLDL. The strength of T cell stimulation was related to the degree of oxidation of the LDL used in priming. Heterologous monocytes exposed to oxLDL under similar conditions induced a response that was not different than monocytes exposed to untreated LDL (natLDL) which did not induce T cell proliferation. Fucoidan did not modify the oxLDL-, monocyte-dependent T cell response to proliferation, suggesting that a scavenger-like receptor was not involved. The expression of the HLA-DR marker and the B7.2 protein were up-regulated in monocytes exposed to oxLDL but not to natLDL. The levels of B7.1 were unchanged. Our data are consistent with the notion that monocytes are critical for T cell survival in the presence of oxLDL and MHC-restricted T cell proliferative response to oxLDL.

Interleukin-15 expression in atherosclerotic plaques: an alternative pathway for T-cell activation in atherosclerosis?

T-cell activation in atherosclerotic plaques is thought to be initiated by plaque-derived antigens, such as oxidized LDL (oxLDL). An alternative pathway of T-cell activation independent of antigen stimulation, mediated by the cytokine interleukin (IL)-15, was recently described. We investigated IL-15 expression in atherosclerotic plaques in relation to plaque morphology, inflammatory cells, T-cell activation, and oxidation-specific epitopes by use of immunohistochemistry. In situ hybridization was used to evaluate IL-15 mRNA expression. We also studied the proliferative response of plaque-derived T-cell lines to IL-15 in vitro using [(3)H]thymidine incorporation. Fresh-frozen specimens were classified as fibrous (n=9), fibrolipid (n=8), and lipid-rich (n=14) plaques; normal vessels (n=4) served as reference. Expression of IL-15 mRNA and protein was found almost solely in fibrolipid and lipid-rich plaques, associated with oxLDL-positive macrophages. Sequential immunostains revealed colocalization between IL-15- and CD40L-positive T cells. Moreover, plaque-derived T-cell lines were highly responsive to IL-15. Hence, IL-15 could provide a pathway for antigen-independent T-cell activation.

Detection of Chlamydia pneumoniae-reactive T lymphocytes in human atherosclerotic plaques of carotid artery

Linkage between Chlamydia pneumoniae infection and atherosclerosis has been confirmed in several studies, but the precise role of this organism in the disease process is not known. We investigated the relation and reactivity of T lymphocytes of human carotid plaques to C pneumoniae antigens. Tissue specimens were obtained from 17 patients who underwent carotid endarterectomy. Immunohistological staining and/or in situ hybridization revealed the presence of C pneumoniae in 11 (64%) of the 17 of the cases. Inflammatory infiltration seen in the vessel walls consisted primarily of CD45RO+ T-memory lymphocytes (median 80%, range 50% to 90%), whereas CD20+ B cells and monocytes were in minor proportion. In vivo activated T lymphocytes were propagated from the specimens with interleukin-2, and the antigen specificity of the established T-cell lines (TLLs) was analyzed against C pneumoniae elementary body antigen. TLLs were established from all 17 carotid tissues but none from the control specimens of ascending aorta. C pneumoniae was recognized as a specific T-cell-stimulating antigen in 7 (41%) of 17 cases. Further analyses of the C pneumoniae-reactive TLLs showed that chlamydial 60-kDa heat-shock protein induced specific proliferation in 5 (71%) of 7 cases and revealed 2 haplotype (DRB1*1502 and DQB1*06) binding motifs in human 60-kDa heat-shock protein. C pneumoniae was identified as a specific microbial antigen recognized by 41% of TLLs propagated from in vivo activated plaque T cells. Our results suggests that cell-mediated immunity to C pneumoniae plays a role in the atherosclerotic process and that this response may involve autoimmunity.

Atherosclerosis, inflammation, and infection

In recent years, it has been shown that inflammation plays an important role in the pathogenesis of atherosclerosis. Activated macrophages, T lymphocytes, and mast cells are present in atherosclerotic plaques, which has led to the notion that the inflammatory response is an immune-mediated process. Complicated lesions, moreover, appear to be associated with an increase in the amount of the inflammatory response and in these patients, increased levels of acute phase proteins are present. The appreciation that atherosclerosis is an immune-mediated inflammatory disease has also led to renewed interest in the potential role of infectious agents in initiating or modulating atherosclerosis. Seroepidemiological studies have shown raised antibody titres against several micro-organisms. However, as yet, there are hardly any data available that provide a sound scientific basis for an infectious origin. Of all potential candidate organisms, Chlamydia pneumoniae appears as the one most likely involved in atherogenesis. C. pneumoniae has been retrieved from atherosclerotic tissues; the level of raised plasma titres correlates with the severity of symptomatic atherosclerotic disease; and the incidence of C. pneumoniae-responsive T cells in peripheral blood is increased in patients with coronary heart disease. It also appears that in some patients T cells generated from atherosclerotic plaques respond to C. pneumoniae. At the present state of knowledge, however, it is fair to state that the relationship between infection, intraplaque inflammation, and atherosclerosis still remains hypothetical, despite the increasing evidence that such a relationship could exist.

Chemokines and atherosclerosis

Chemokines or chemotactic cytokines represent an expanding family of structurally related small molecular weight proteins, recognised as being responsible for leukocyte trafficking and activation. Soon after the discovery of this class of cytokines, about a decade ago, monocyte chemoattractant protein-1 (MCP-1) was found to be highly expressed in human atherosclerotic lesions and postulated to be central in monocyte recruitment into the arterial wall and developing lesions. In this review, we will discuss our present knowledge about MCP-1 and its receptor CCR2 and their role in atherogenesis. Although less well established, other chemokines such as RANTES, MIP-1alpha and MIP-1beta have also been implicated in atherosclerotic lesion formation as are a number of more recently discovered chemokines like MCP-4, ELC and PARC. The role of these chemokines in the progression of atherosclerosis will be discussed as well as the emerging role of IL-8, mostly know for its effects on neutrophils. Particular attention will be given not only to the involvement of chemokines in the inflammatory recruitment of monocytes/macrophages, but also to their role in the related local immune responses and vascular remodelling which occur during the formation of unstable atherosclerotic plaques.