A small population of vasculitogenic T cells expands and has skewed T cell receptor usage after culture with syngeneic smooth muscle cells

Inflammatory and immune responses in the arterial media

Inflammatory arterial diseases differentially affect the compartments of the vessel wall. The intima and adventitia are commonly involved by the disease process, with luminal and microvascular endothelial cells playing a critical role in the recruitment and activation of leukocytes. In contrast, the avascular media is often spared by immune-mediated disorders. Surprisingly, vascular smooth muscle cells (VSMCs), the predominant and often exclusive cell type of the media, are capable of robust proinflammatory responses to diverse stressors. The multiple cytokines and chemokines produced within the media can profoundly affect macrophage and T cell function, thus amplifying and shaping innate and adaptive immune responses. On the other hand, VSMCs and the extracellular matrix that they produce also display significant anti-inflammatory properties. The balance between the pro- and anti-inflammatory effects of VSMCs and their extracellular matrix versus the strength of the inciting immunologic events determines the pattern of medial pathology. Limitations on the extent of medial infiltration and injury, defined as medial immunoprivilege, are typically seen in arteriosclerotic diseases, such as atherosclerosis and transplant vasculopathy. Conversely, breakdown of medial immunoprivilege that manifests as more intense leukocytic infiltrates, loss of VSMCs, and destruction of the extracellular matrix architecture is a general feature of certain aneurysmal diseases and vasculitides. In this review, we consider the inflammatory and immune functions of VSMCs and how they may lead to medial immunoprivilege or medial inflammation in arterial diseases.

Murine aortic smooth muscle cells acquire, though fail to present exogenous protein antigens on major histocompatibility complex class II molecules

In the present study aortic murine smooth muscle cell (SMC) antigen presentation capacity was evaluated using the Eα-GFP/Y-Ae system to visualize antigen uptake through a GFP tag and tracking of Eα peptide/MHCII presentation using the Y-Ae Ab. Stimulation with IFN-γ (100 ng/mL) for 72 h caused a significant (P < 0.01) increase in the percentage of MHC class II positive SMCs, compared with unstimulated cells. Treatment with Eα-GFP (100 μg/mL) for 48 h induced a significant (P < 0.05) increase in the percentage of GFP positive SMCs while it did not affect the percentage of Y-Ae positive cells, being indicative of antigen uptake without its presentation in the context of MHC class II. After IFN-γ-stimulation, ovalbumin- (OVA, 1 mg/mL) or OVA_323–339 peptide-(0.5 μg/mL) treated SMCs failed to induce OT-II CD4⁺ T cell activation/proliferation; this was also accompanied by a lack of expression of key costimulatory molecules (OX40L, CD40, CD70, and CD86) on SMCs. Finally, OVA-treated SMCs failed to induce DO11.10-GFP hybridoma activation, a process independent of costimulation. Our results demonstrate that while murine primary aortic SMCs express MHC class II and can acquire exogenous antigens, they fail to activate T cells through a failure in antigen presentation and a lack of costimulatory molecule expression.

Participation of blood vessel cells in human adaptive immune responses

Circulating T cells contact blood vessels either when they extravasate across the walls of microvessels into inflamed tissues or when they enter into the walls of larger vessels in inflammatory diseases such as atherosclerosis. The blood vessel wall is largely composed of three cell types: endothelial cells lining the entire vascular tree; pericytes supporting the endothelium of microvessels; and smooth muscle cells forming the bulk of large vessel walls. Each of these cell types interacts with and alters the behavior of infiltrating T cells in different ways, making these cells active participants in the processes of immune-mediated inflammation. In this review, we compare and contrast what is known about the nature of these interactions in humans.

CD4+ T cells sensitized by vascular smooth muscle induce vasculitis, and interferon gamma is critical for the initiation of vascular pathology

Primary vasculitis is the result of idiopathic inflammation in blood vessel walls. T cells are believed to play a critical role, but the nature of the pathological T-cell response remains obscure. In this study, we provide evidence that CD4(+) T lymphocytes, activated in the presence of syngeneic vascular smooth muscle cells, were sufficient to induce vasculitic lesions after adoptive transfer to recipient mice. Additionally, the disease is triggered in the absence of antibodies in experiments in which both the donors of stimulated lymphocytes and the transfer recipients were mice that were deficient in B cells. Tracking and proliferation of the transferred cells and their cytokine profiles were assessed by fluorescence tagging and flow cytometry. Proliferating CD4(+) T cells were evident 3 days after transfer, corresponding to the occurrence of vasculitic lesions in mouse lungs. The transferred T lymphocytes exhibited Th1 and Th17 cytokine profiles and minimal Th2. However, 1 week after vasculitis induction, effector functions could be successfully recalled in Th1 cells, but not in Th17 cells. Additionally, in the absence of constitutive interferon-γ expression, T cells sensitized by vascular smooth muscle cells failed to induce vasculitis. In conclusion, our results show that Th1 cells play a key role in eliciting vasculitis in this murine model and that induction of the disease is possible in the absence of pathogenic antibodies.

Human vascular smooth muscle cells lack essential costimulatory molecules to activate allogeneic memory T cells

OBJECTIVE

The arterial media, populated by vascular smooth muscle cells (VSMC), is an immunoprivileged compartment and, in contrast to the intima or adventitia containing endothelial cells, is generally spared by inflammatory processes, such as arteriosclerosis. To determine mechanisms of medial immunoprivilege, we investigated the ability of human VSMC versus endothelial cells to activate allogeneic T cells in vitro.

METHODS AND RESULTS

Unlike cultured endothelial cells, cultured VSMC do not activate allogeneic memory CD4 or CD8 T cells and fail to effectively support T-cell proliferation to the polyclonal activator, phytohemagglutinin, consistent with a defect in costimulation function. Although many costimulators are comparably expressed on both cell types, endothelial cells but not VSMC basally express OX40 ligand and upregulate inducible costimulator ligand in response to proinflammatory cytokines. OX40 ligand-transduced, but not control- or inducible costimulator ligand-transduced, VSMC acquire the capacity to stimulate allogeneic memory CD4 T cells to produce cytokines and to proliferate in the presence of supplemental l-tryptophan. OX40 ligand overexpression, although not essential, also enhances allogeneic memory CD8 T-cell responses to VSMC after l-tryptophan supplementation.

CONCLUSIONS

The inability of cultured VSMC to activate memory T cells results from a lack of essential costimulators, particularly OX40 ligand, in addition to indoleamine 2,3-dioxygenase-mediated tryptophan depletion.

Induction of Indoleamine 2,3-Dioxygenase in Vascular Smooth Muscle Cells by Interferon-γ Contributes to Medial Immunoprivilege

Atherosclerosis and graft arteriosclerosis are characterized by leukocytic infiltration of the vessel wall that spares the media. The mechanism(s) for medial immunoprivilege is unknown. In a chimeric humanized mouse model of allograft rejection, medial immunoprivilege was associated with expression of IDO by vascular smooth muscle cells (VSMCs) of rejecting human coronary artery grafts. Inhibition of IDO by 1-methyl-tryptophan (1-MT) increased medial infiltration by allogeneic T cells and increased VSMC loss. IFN-gamma-induced IDO expression and activity in cultured human VSMCs was considerably greater than in endothelial cells (ECs) or T cells. IFN-gamma-treated VSMCs, but not untreated VSMCs nor ECs with or without IFN-gamma pretreatment, inhibited memory Th cell alloresponses across a semipermeable membrane in vitro. This effect was reversed by 1-MT treatment or tryptophan supplementation and replicated by the absence of tryptophan, but not by addition of tryptophan metabolites. However, IFN-gamma-treated VSMCs did not activate allogeneic memory Th cells, even after addition of 1-MT or tryptophan. Our work extends the concept of medial immunoprivilege to include immune regulation, establishes the compartmentalization of immune responses within the vessel wall due to distinct microenvironments, and demonstrates a duality of stimulatory EC signals versus inhibitory VSMC signals to artery-infiltrating T cells that may contribute to the chronicity of arteriosclerotic diseases.

Interferon-γ Axis in Graft Arteriosclerosis

Cardiac transplantation is the most effective treatment for advanced heart failure. Despite improvements in immunosuppression therapy that prevent acute rejection, cardiac allografts fail at rates of 3% to 5% per posttransplant year. The hallmark morphological lesion of chronically failing cardiac allografts, also seen in chronic renal and liver graft failure, is luminal stenosis of blood vessels, especially of conduit arteries. Late graft failure results from widespread secondary ischemic injury to the graft parenchyma rather than direct immune-mediated damage. Although this process affects the entire graft vasculature, graft arteriosclerosis is a suitable term to describe the problem because it applies to different types of failing organs and because it emphasizes the central feature, namely an accelerated form of arterial injury and remodeling. The precise pathogenesis of graft arteriosclerosis is unknown. In this review, we make the case that the signature T-helper type 1 cytokine, interferon (IFN)-γ, is a key effector in graft arteriosclerosis, which, together with the IFN-γ–inducing cytokine interleukin-12 and IFN-γ–inducible chemokines such as CXCR3 ligands, constitute a positive feedback loop for T-cell activation, differentiation, and recruitment that we refer to as the IFN-γ axis. We evaluate the evidence to support this hypothesis in clinical observational and experimental animal studies. Additionally, we examine the regulation of IFN-γ production within the artery wall, the effects of IFN-γ on vessel wall cells, and the outcome of therapeutic agents on IFN-γ production and signaling. These observations lead us to suggest that new therapies for graft arteriosclerosis should be optimized which focus on reducing IFN-γ synthesis or actions.

Dendritic cell transmigration through brain microvessel endothelium is regulated by MIP-1alpha chemokine and matrix metalloproteinases

Dendritic cells (DCs) accumulate in the CNS during inflammatory diseases, but the exact mechanism regulating their traffic into the CNS remains to be defined. We now report that MIP-1alpha increases the transmigration of bone marrow-derived, GFP-labeled DCs across brain microvessel endothelial cell monolayers. Furthermore, occludin, an important element of endothelial tight junctions, is reorganized when DCs migrate across brain capillary endothelial cell monolayers without causing significant changes in the barrier integrity as measured by transendothelial electrical resistance. We show that DCs produce matrix metalloproteinases (MMP) -2 and -9 and GM6001, an MMP inhibitor, decreases both baseline and MIP-1alpha-induced DC transmigration. These observations suggest that DC transmigration across brain endothelial cell monolayers is partly MMP dependent. The migrated DCs express higher levels of CD40, CD80, and CD86 costimulatory molecules and induce T cell proliferation, indicating that the transmigration of DCs across brain endothelial cell monolayers contributes to the maintenance of DC Ag-presenting function. The MMP dependence of DC migration across brain endothelial cell monolayers raises the possibility that MMP blockers may decrease the initiation of T cell recruitment and neuroinflammation in the CNS.

Autoantibodies to vascular smooth muscle are pathogenic for vasculitis

We have previously shown that microvascular smooth muscle activates CD4+ T lymphocytes in sterile co-culture, presents antigen, and produces inflammatory cytokines. Adoptive transfer of lymphocytes co-cultured with syngeneic smooth muscle cells to healthy recipient mice results in vasculitic lesions predominantly in postcapillary venules. The present study assessed the pathogenic role of immunoglobulin and B cells in a murine model of vasculitis. Here, we show that transferred B cells, including plasmablast cells, accumulated, persisted, and proliferated in lung and secondary lymphoid organs of recipient mice. The induction of vasculitis was accompanied by production of IgM and IgG2a autoantibodies specific for vascular smooth muscle intracellular antigens. Circulating immunoglobulin had a pathogenic role in this vasculitis model, because the disease could be induced by transfer of serum from vasculitic mice to untreated animals but not by transfer of serum depleted of anti-smooth muscle autoantibodies. Additionally, the pathogenic mechanisms triggered by the transfer of vasculitogenic serum were dependent on T lymphocytes because both wild-type and B cell-deficient mice developed the disease after serum transfer, whereas RAG2-deficient mice did not. Thus, immunoglobulin and cell-mediated pathways work in concert to produce vasculitis in this model.