T-cell immunity in myocardial inflammation: pathogenic role and therapeutic manipulation.

T-cell-mediated immunity has been linked not only to a variety of heart diseases, including classic inflammatory diseases such as myocarditis and post-myocardial infarction (Dressler's) syndrome, but also to conditions without an obvious inflammatory component such as idiopathic dilated cardiomyopathy and hypertensive cardiomyopathy. It has been recently proposed that in all these conditions, the heart becomes the focus of T-cell-mediated autoimmune inflammation following ischaemic or infectious injury. For example, in acute myocarditis, an inflammatory disease of heart muscle, T-cell responses are thought to arise as a consequence of a viral infection. In a number of patients, persistent T-cell-mediated responses in acute viral myocarditis can lead to autoimmunity and chronic cardiac inflammation resulting in dilated cardiomyopathy. In spite of the major progress made in understanding the mechanisms of pathogenic T-cell responses, effective and safe therapeutic targeting of the immune system in chronic inflammatory diseases of the heart has not yet been developed due to the lack of specific diagnostic and prognostic biomarkers at an early stage. This has also prevented the identification of targets for patient-tailored immunomodulatory therapies that are both disease- and organ-selective. In this review, we discuss current knowledge of the development and functional characteristics of pathogenic T-cell-mediated immune responses in the heart, and, in particular, in myocarditis, as well as recent advances in experimental models which have the potential to translate into heart-selective immunomodulation. LINKED ARTICLES: This article is part of a themed section on Targeting Inflammation to Reduce Cardiovascular Disease Risk. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.22/issuetoc and http://onlinelibrary.wiley.com/doi/10.1111/bcp.v82.4/issuetoc.

Expression of a Chimeric Antigen Receptor Specific for Donor HLA Class I Enhances the Potency of Human Regulatory T Cells in Preventing Human Skin Transplant Rejection.

Regulatory T cell (Treg) therapy using recipient-derived Tregs expanded ex vivo is currently being investigated clinically by us and others as a means of reducing allograft rejection following organ transplantation. Data from animal models has demonstrated that adoptive transfer of allospecific Tregs offers greater protection from graft rejection compared to polyclonal Tregs. Chimeric antigen receptors (CAR) are clinically translatable synthetic fusion proteins that can redirect the specificity of T cells toward designated antigens. We used CAR technology to redirect human polyclonal Tregs toward donor-MHC class I molecules, which are ubiquitously expressed in allografts. Two novel HLA-A2-specific CARs were engineered: one comprising a CD28-CD3ζ signaling domain (CAR) and one lacking an intracellular signaling domain (ΔCAR). CAR Tregs were specifically activated and significantly more suppressive than polyclonal or ΔCAR Tregs in the presence of HLA-A2, without eliciting cytotoxic activity. Furthermore, CAR and ΔCAR Tregs preferentially transmigrated across HLA-A2-expressing endothelial cell monolayers. In a human skin xenograft transplant model, adoptive transfer of CAR Tregs alleviated the alloimmune-mediated skin injury caused by transferring allogeneic peripheral blood mononuclear cells more effectively than polyclonal Tregs. Our results demonstrated that the use of CAR technology is a clinically applicable refinement of Treg therapy for organ transplantation.

T cell trafficking and metabolism: novel mechanisms and targets for immunomodulation.

Coordinated migratory events by naïve and memory T cells are key to effective immunity. Naïve T cells predominantly recirculate through secondary lymphoid tissue until antigen encounter, while primed T cells efficiently localize to antigen-rich lymphoid and non-lymphoid tissue. Tissue-selective targeting by primed T cells is achieved by a combination of inflammatory signals and tissue-selective homing receptors acquired by T cells during activation and differentiation. A large number of molecular mediators and interactions promoting memory T cell migration to non-lymphoid sites of inflammation have been identified. Recently, additional antigen-driven mechanisms have been proposed, which orchestrate the targeted delivery of memory T cells to antigen-rich tissue. Importantly, recent studies have revealed that the T cell metabolic status influences their differentiation and homing patterns. We here summarize these key observations and discuss their relevance for the manipulation of immune anatomy in therapeutic settings.

The highway code of T cell trafficking.

Coordinated migratory events are required for the development of effective and regulated immunity. Naïve T lymphocytes are programmed to recirculate predominantly in secondary lymphoid tissue by non-specific stimuli. In contrast, primed T cells must identify specific sites of antigen location in non-lymphoid tissue to exert targeted effector responses. Following priming, T cells acquire the ability to establish molecular interactions mediated by tissue-selective integrins and chemokine receptors (homing receptors) that allow their access to specific organs, such as the skin and the gut. Recent studies have shown that an additional level of specificity is provided by the induction of specific T cell migration into the tissue following recognition of antigen displayed by the endothelium. In addition, co-stimulatory signals (such as those induced by CD28 and CTLA-4 molecules) have been shown not only to regulate T cell activation and differentiation, but also to orchestrate the anatomy of the ensuing T cell response.

Regulation of T-cell migration by co-stimulatory molecules.

Migration of primed T-cells to the antigenic site is an essential event in the development of effective immunity. This process is tightly regulated in order to ensure efficient and specific responses. Most studies have focused on non-specific mediators of T-cell migration, including integrins and chemokines. However, recent studies have highlighted the key role of the T-cell receptor and co-stimulatory molecules in guiding T-cell access to antigenic tissue. Here, we review the experimental evidence for an essential contribution of co-stimulation-mediated molecular interactions regulating T-cell migration in the development of T-cell immunity and tolerance.

Differential effects of immunosuppressive drugs on T-cell motility.

The best-characterized mechanism of the action of immunosuppressive drugs is to prevent T-cell clonal expansion, thus containing the magnitude of the ensuing immune response. As T-cell recruitment to the inflammatory site is another key step in the development of T-cell-mediated inflammation, we analyzed and compared the effects of two commonly used immunosuppressants, cyclosporin A (CsA) and the rapamycin-related compound SDZ-RAD, on the motility of human CD4+ T cells. We show that CsA, but not SDZ-RAD, inhibits T-cell transendothelial migration in vitro. CsA selectively impaired chemokine-induced T-cell chemotaxis while integrin-mediated migration was unaffected. The inhibition of T-cell chemotaxis correlated with reduced AKT/PKB but not ERK activation following exposure to the chemokine CXCL-12/SDF-1. In addition, CsA, but not SDZ-RAD, prevents some T-cell receptor-mediated effects on T-cell motility. Finally, we show that CsA, but not SDZ-RAD inhibits tissue infiltration by T cells in vivo. Our data suggest a prominent antiinflammatory role for CsA in T-cell-mediated tissue damage, by inhibiting T-cell trafficking into tissues in addition to containing clonal expansion.

A rapid method for labelling CD4+ T cells with ultrasmall paramagnetic iron oxide nanoparticles for magnetic resonance imaging that preserves proliferative, regulatory and migratory behaviour in vitro.

A number of techniques have been developed to track the migration of T cells in vivo, but they all suffer significant shortcomings, including the examination of selected organs rather than the organism as a whole--thus precluding longitudinal studies--or limitations imposed by poor spatial resolution and the application of ionizing radiation. By conjugating the HIV tat peptide to ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles in a reaction yielding a mean valence of 45, a magnetic resonance (MR) contrast agent was synthesised that allowed T cells to be efficiently labelled within just 5 min. The USPIO nanoparticles were incorporated into both the cytoplasm and nucleus of labelled cells, which retained normal in vitro proliferative responses to a polyclonal stimulus; suppressive responses mediated by labelled CD4(+) CD25(+) regulatory T cells; chemotactic responses to the chemokine CXCL-12; and transmigration of an activated endothelial monolayer. We believe that this rapid, efficient and essentially non-toxic approach to labelling both murine and human T cells for MRI holds considerable promise, paving the way for the wider immunological application of this exciting technology.

Isolation of endothelial cells from murine tissue.

The isolation and long-term culture of murine endothelial cells (ECs) has often proven a difficult task. In this paper we describe a quick, efficient protocol for the isolation of microvascular endothelial cells from murine tissues. Murine lung or heart are mechanically minced and enzymatically digested with collagenase and trypsin. The single cell suspension obtained is then incubated with an anti-CD31 antibody, anti-CD105 antibody and with biotinylated isolectin B-4. Pure EC populations are finally obtained by magnetic bead separation using rat anti-mouse Ig- and streptavidin-conjugated microbeads. EC cultures are subsequently expanded and characterised. The surface molecule expression by the primary cultures of murine EC obtained from lung and heart tissue is analysed and compared to that of a murine endothelioma and of primary cultures of murine renal tubular epithelial cells. The phenotype and morphology of these cultures remain stable over 10-15 passages in culture, and no overgrowth of contaminating cells of non-endothelial origin is observed at any stage.

Activated murine endothelial cells have reduced immunogenicity for CD8+ T cells: a mechanism of immunoregulation?

The immunogenic properties of primary cultures of murine lung microvascular endothelial cells (EC) were analyzed. Resting endothelial cells were found to constitutively express low levels of MHC class I and CD80 molecules. IFN-gamma treatment of EC resulted in a marked up-regulation of MHC class I, but no change was observed in the level of CD80 expression. No CD86 molecules were detectable under either condition. The ability of peptide-pulsed EC to induce the proliferation of either the HY-specific, H2-K(k)-restricted CD8(+) T cell clone (C6) or C6 TCR-transgenic naive CD8(+) T cells was analyzed. Resting T cells were stimulated to divide by quiescent peptide-prepulsed EC, while peptide-pulsed, cytokine-activated EC lost the ability to induce T cell division. Furthermore, Ag presentation by cytokine-activated EC induced CD8(+) T cell hyporesponsiveness. The immunogenicity of activated EC could be restored by adding nonsaturating concentrations of anti-H2-K(k) Ab in the presence of an optimal concentration of cognate peptide. This is consistent with the suggestion that the ratio of TCR engagement to costimulation determines the outcome of T cell recognition. In contrast, activated peptide-pulsed EC were killed more efficiently by fully differentiated effector CD8(+) T cells. Finally, evidence is provided that Ag recognition of EC can profoundly affect the transendothelial migration of CD8(+) T cells. Taken together, these results suggest that EC immunogenicity is regulated in a manner that contributes to peripheral tolerance.

Recently activated T cells are costimulation-dependent in vitro.

While the prominent role of B7-mediated signaling in the activation of naive and resting T cells has been exhaustively demonstrated, it is unclear whether costimulation is required in the amplification of an initiated immune response. In this study we have developed a multistep culture system to investigate the costimulation requirements of recently activated alloreactive CD4(+) T cells and the outcome of allorecognition of B7-deficient, MHC class-II-expressing epithelial cells. The results show that following in vitro "priming" with allogeneic costimulation rich antigen presenting cells, T cells can be reactivated to proliferate only if B7-mediated costimulation is provided. Furthermore, recognition of antigen on B7-negative epithelial cells induced allospecific nonresponsiveness in the responder T cells. Finally, the nonresponsive state was not accompanied by IL-4 secretion and appeared to be reversible, since T cell reactivity could be restored by short-term culture in the presence of IL-2. These observations suggest that "primed" T cells remain B7-dependent in vitro and are susceptible to functional inactivation following costimulation-deficient antigen presentation.

Lack of T cell proliferation without induction of nonresponsiveness after antigen presentation by endothelial cells.

BACKGROUND: After priming or reactivation in lymph nodes, T cells recirculate to sites of inflammation, and enter tissues by migrating across activated endothelium. Given that activated endothelial and tissue parenchymal cells express both class I and class II MHC molecules, it is probable that transmigrating T cells encounter cognate antigen on endothelial cells, and on tissue parenchymal cells once they have entered the tissue. METHODS: In this study the consequences of antigen presentation by endothelial and epithelial cells to human CD4+ T cell clones were analyzed and compared by a two-step culture system. RESULTS: T cell clones that required B7-mediated costimulation to be activated were found not to be able to proliferate to antigen presented by either endothelial or epithelial cells, unless trans-costimulation was provided by the addition of B7-transfected cells in the cultures. Furthermore, antigen presentation by epithelial cells induced nonresponsiveness in the T cell clones. In contrast, after cognate recognition on endothelial cells, the ability of the T cell clones to proliferate to a subsequent rechallenge with antigen presented on a specialized APC was unaffected. CONCLUSIONS: These data suggest that endothelial cells have unique properties as antigen-presenting cells, in that they do not influence the subsequent reactivity of cognate T cells.

Antigen recognition influences transendothelial migration of CD4+ T cells.

The functional significance of MHC class II expression by vascular endothelial cells remains obscure. In this study the possibility that Ag presentation by endothelial cells (EC) influences T cell transmigration, facilitating the recruitment of Ag-specific T cells into tissues, was investigated. The frequencies of T cells with specificity for an HLA-DR alloantigen, or for the recall Ag tetanus toxoid (TT), were measured in peripheral blood CD45RO+ (memory) CD4+ T cells before and after transmigration through gamma-IFN-treated EC monolayers. Frequencies of anti-DR17, IL-2-secreting T cells were fourfold higher in the T cells that transmigrated through a monolayer of DR17-expressing EC. Similar increases were seen in TT-specific, DR7-restricted T cells that transmigrated through TT-pulsed, DR7-expressing EC. To examine more directly the effects of cognate recognition of Ag presented by EC, T cell clones were used. For clones that proliferated in a costimulation-independent manner to Ag presented by EC, cognate recognition arrested transmigration. In contrast, Ag presentation by EC to B7-dependent T cell clones, which do not proliferate following cognate recognition of EC, enhanced the rate of transendothelial migration. These data suggest that Ag presentation by EC may serve to augment the recruitment of Ag-specific T cells into tissues and that proliferation and transmigration are mutually exclusive T cell responses.

Antigen presentation by parenchymal cells: a route to peripheral tolerance?

T-cell activation and the development of efficient immune responses requires the delivery, by the antigen-presenting cell, of two distinct signals. The first results from the engagement of the TCR:CD3:CD4 complex, and the second from the interaction of CD28 with the B7 family of co-stimulatory molecules. In this context, the physiological significance and the functional consequences of antigen presentation by B7-deficient parenchymal cells, which express MHC class II molecules as a result of inflammation, remains a matter of debate. In this paper we have attempted to critically review the often conflicting reports on the functional effects of antigen presentation by epithelial and endothelial cells to T cells, both in vitro and in vivo. Our own findings are summarised in a model which is consistent with the suggestion of an important role for antigen presentation by parenchymal cells in the induction and the maintenance of peripheral tolerance.

Mechanisms of peripheral T-cell tolerance.

It is becoming increasingly clear that the control of self-reactivity involves peripheral mechanisms that supplement thymic negative selection. It is now generally accepted that T-cell activation depends upon both T-cell receptor engagement and the delivery of B7-mediated costimulation by specialized antigen presenting cells (APC). In contrast, failure to deliver B7-mediated costimulation can result in the induction of antigen-specific non-responsiveness. In physiological terms, costimulation-deficient antigen presentation is the prerogative of those cells that do not express B7 molecules, even during inflammatory conditions, such as tissue parenchymal cells. The consequences of such costimulation-deficient antigen presentation are illustrated by the allospecific tolerance that is observed in animal models of transplantation following the depletion of bone marrow-derived APC from an allograft. In this paper the possible role of antigen presentation by tissue parenchymal cells in the induction and maintenance of peripheral tolerance is discussed, with particular attention to the important contribution that the liver may make to these events.

Antigen presentation by epithelial cells induces anergic immunoregulatory CD45RO+ T cells and deletion of CD45RA+ T cells.

The immunoregulatory effects of alloantigen presentation by tissue parenchymal cells to resting peripheral blood CD4+ T cells was investigated. Coculture of CD45RO+ (memory) and CD45RA+ (naive) T lymphocytes with primary cultures of MHC class II-expressing epithelial cells rendered both populations of T cells hyporesponsive to a subsequent challenge by the same MHC molecule expressed on EBV-transformed lymphoblastoid B cell lines. However, the mechanisms responsible for the allospecific hyporesponsiveness were distinct. For the CD45RO+ T cells, responsiveness was restored by subsequent culture in the presence of IL-2; the addition of IL-2 had no effect on the reactivity of the CD45RA+ T cells. In contrast, the naive T cells were protected from the induction of nonresponsiveness by the presence of a neutralizing anti-CD95 Ab during the culture with thyroid follicular cells. In addition, the hyporesponsive CD45RO+ T cells effected linked suppression, in that they inhibited proliferation against a third-party DR alloantigen when the third-party alloantigen was coexpressed with the DR Ag against which hyporesponsiveness had been induced. These results suggest that recognition of Ag by T cells on tissue parenchymal cells plays an important role in the maintenance of peripheral T cell tolerance, inducing nonresponsiveness in naive and memory T cells by distinct mechanisms.

Major histocompatibility complex class II-expressing endothelial cells induce allospecific nonresponsiveness in naive T cells.

The role of endothelial cells (EC) in initiating a primary T cell response is of importance in clinical transplantation and autoimmunity since EC are the first allogeneic target encountered by the recipient's immune system and may display tissue-specific autoantigens in the context of an inflammatory response. In this study, we have investigated the antigen-presenting cell function of human umbilical vein-derived EC (HUVEC), depleted of constitutively major histocompatibility complex class II+ cells and induced to express class II molecules by interferon-gamma. The results show that HUVEC do not express B7 but can support proliferation by antigen-specific T cell clones. In contrast, they were unable to initiate a primary alloresponse using three independent HUVEC cultures and MHC class II-mismatched CD4+ T cells from eight donors. The response to HUVEC was reconstituted by trans-costimulation provided by DAP.3 transfectants expressing human B7.1. Coculture of peripheral blood T cells with EC expressing allogeneic DR molecules had markedly different effects on CD45RO+ and RA+ subsets. Subsequent reactivity of the RO+ T cells was unaffected by exposure to EC, indicating a neutral encounter. In contrast, culture with DR+ EC induced allospecific nonresponsiveness in RA+ T cells.