Long-term acceptance of major histocompatibility complex mismatched cardiac allografts induced by CTLA4Ig plus donor-specific transfusion

Distinct roles for B7-1 (CD-80) and B7-2 (CD-86) in the initiation of experimental allergic encephalomyelitis

The activation and differentiation of T cells require both antigen/MHC recognition and costimulatory signals. The present studies examined the role of B7-1 (CD80) and B7-2 (CD86) costimulation in the prototypic autoimmune disorder, experimental allergic encephalomyelitis (EAE). In adoptively transferred EAE, in vitro activation of myelin basic protein (MBP)-specific lymph node cells was inhibited by the combination of anti-CD80 plus anti-CD86, but not individually. However, in actively induced disease, one injection of anti-CD80 significantly reduced disease, while anti-CD86 exacerbated disease. Interestingly, one injection of CTLA-4Ig suppressed disease, while multiple injections resulted in enhanced disease. Thus, the costimulation provided by B7-1 molecules appears to be important for the development of encephalitogenic T cells. The enhanced disease caused by multiple injections of CTLA-4Ig or a single injection of anti-CD86 suggests an inhibitory function for CD86 interaction with its counterreceptors CD28 and CTLA-4 in EAE. Alternatively, these results are consistent with an essential timing requirement for the coordinated interaction of B7 and CD28 family receptors, and that disruption of this critical timing can have opposing results on the outcome of an immune response.

Survival of mouse pancreatic islet allografts in recipients treated with allogeneic small lymphocytes and antibody to CD40 ligand

Combined treatment with allogeneic small lymphocytes or T-depleted small lymphocytes plus a blocking antibody to CD40 ligand (CD40L) permitted indefinite pancreatic islet allograft survival in 37 of 40 recipients that differed from islet donors at major and minor histocompatibility loci. The effect of the allogeneic small lymphocytes was donor antigen-specific. Neither treatment alone was as effective as combined treatment, although anti-CD40L by itself allowed indefinite islet allograft survival in 40% of recipients. Our interpretation is that small lymphocytes expressing donor antigens in the absence of appropriate costimulatory signals are tolerogenic for alloreactive host cells. Anti-CD40L antibody may prevent host T cells from inducing costimulatory signals in donor lymphocytes or islet grafts.

Long-term hepatic adenovirus-mediated gene expression in mice following CTLA4Ig administration

Recombinant adenovirus vectors are efficient at transferring genes into somatic tissues but are limited for use in clinical gene therapy by immunologic factors that result in the rapid loss of gene expression and inhibit secondary gene transfer. This study demonstrates that systemic coadministration of recombinant adenovirus with soluble CTLA4Ig, which is known to block co-stimulatory signals between T cells and antigen presenting cells, leads to persistent adenoviral gene expression in mice without long-term immunosuppression. This form of immunotherapy greatly enhances the likelihood that recombinant adenovirus vectors will be useful for human gene therapy.

Role of tyrosine kinases in lymphocyte activation: targets for drug intervention

Recent developments in our understanding of lymphocyte receptor-associated signalling events have offered many new potential targets for modifying antigen and cytokine receptor signalling events in immune-related diseases such as allergy, autoimmunity and transplant rejection. As discussed below, these targets are largely tissue-restricted and are functionally confined to a limited set of receptors. Therefore, it is anticipated that selective inhibitors of these signalling events would offer safe and effective therapies for immunologically-based diseases. First, we review T and B cell antigen receptor signalling as targets for inhibiting lymphocyte responses. Second, targets in lymphocyte cytokine receptor signalling pathways are discussed. Finally, we review strategies for inhibition of receptor signalling.

B7-mediated costimulation can either provoke or prevent clinical manifestations of experimental allergic encephalomyelitis

T-cell activation requires signalling provided by ligation of the T-cell receptor for antigen (TCR) and a second antigen (Ag) nonspecific signal, known as costimulation. The B7 receptors, CD80 (B7-1) and CD86 (B7-2), on the Ag-presenting cell (APC), interact with T-cell CD28 or CTLA-4 to deliver a costimulatory signal, which is particularly important for Th1 activation. Experimental allergic encephalomyelitis (EAE) is an autoimmune disorder, induced by Th1 cells directed against myelin antigens that provides an in vivo model for studying the role of B7-mediated costimulation in the induction of a pathological immune response. Using a soluble fusion protein ligand for the B7 receptors, as well as specific monoclonal antibodies specific for either CD80 or CD86, it has been demonstrated that B7 costimulation plays a prominent role in determining clinical disease outcome in EAE. Here we review recent data indicating that a paradoxical exacerbation of disease as well as the expected amelioration of disease can occur with costimulatory receptor blockade.

Collagen-induced arthritis in the BB rat. Prevention of disease by treatment with CTLA-4-Ig

Antigen-specific T cell activation requires two independent signalling events, one mediated through T cell receptor engagement by the antigen-presenting cell-expressed peptide/class II major histocompatibility complex, and the second through the cognate interactions of costimulatory molecules expressed on the T cell and antigen-presenting cell. There is evidence from in vitro and in vivo experimental systems suggesting that the CD28/B7 costimulatory pathway is crucial for induction of maximal T cell proliferation and T helper-B cell collaboration for IgG production. This pathway can be blocked by CTLA-4-Ig, a soluble form of CTLA-4 which binds with high avidity to the CD28 ligands, B7-1 and B7-2. Here, we show that CTLA-4-Ig treatment prevents clinical and histological manifestations of disease in a collagen-induced arthritis model of rheumatoid arthritis in the diabetes resistant BB/Wor rat, when therapy is initiated before immunization with bovine type II collagen (BIIC). Anti-BIIC antibody titers are reduced in CTLA-4-Ig-treated rats compared to diseased control animals. Histologically, joints from CTLA-4-Ig-treated animals show no histological abnormalities, in contrast to control antibody-treated animals, which show complete erosion of the articular cartilage and bone. Despite the efficacy of CTLA-4-Ig in preventing clinical and histological signs of arthritis and reducing antibody responses to BIIC, delayed type hypersensitivity responses to collagen 18 d or more after CTLA-4-Ig treatment ends are similar in CTLA-4-Ig-treated and untreated rats, suggesting that the prolonged disease suppression observed does not result from induction of T cell anergy.

In the absence of a CD40 signal, B cells are tolerogenic

When B cells are deprived of signaling through CD40, they exhibit the ability to induce T cell tolerance. The in vivo administration of anti-gp39 and allogeneic B cells diminished the ability of mice to mount an allogeneic response. Tolerance induction was specific for the haplotype expressed on the allogeneic B cells. Selective allospecific unresponsiveness was induced in the CD8 and CD4 compartments by the administration of anti-gp39 and class II-deficient B cells or class I-deficient B cells, respectively. As predicted by studies with anti-gp39 treatment, diminished allospecific responsiveness was induced by the administration of B cells to mice genetically deficient in gp39. Taken together, these data are consistent with the premise that deprivation of CD40 signaling engenders B cells with enhanced tolerogenicity. These studies provide insights into the tolerogenic capacity of resting B cells and outlines a practical approach to exploit this function.

Long-term inhibition of murine experimental autoimmune encephalomyelitis using CTLA-4-Fc supports a key role for CD28 costimulation

T cell activation involves not only recognition of antigen presented by the MHC, but also nonspecific interactions termed "costimulation." The costimulatory molecules B7-1 and B7-2 are ligands on antigen-presenting cells for the CD28 and CTLA-4 receptors on T cells. Previously, a fusion protein consisting of human CTLA-4 linked to human Fc was shown to bind B7-1 and B7-2 with high avidity and to prevent specific T cell activation. Here we investigated the effects of a recombinant fusion protein consisting of the extracellular domain of human CTLA-4 bound to mouse IgG2a Fc (CTLA-4-Fc) upon experimental autoimmune encephalomyelitis, a T cell-mediated disease that serves as a model for multiple sclerosis. CTLA-4-Fc prevented experimental autoimmune encephalomyelitis in 26 of 28 CTLA-4-Fc-treated mice (median maximum score 0), whereas 28 of 30 mice treated with control mouse IgG2a developed disease (median maximum score 2.75). Less inflammation and virtually no demyelination or axonal loss occurred in CTLA-4-Fc-treated compared with control-treated mice. Activated splenocytes from CTLA-4-Fc-treated mice were able to transfer disease adoptively to naive recipients. These results indicate a key role for the B7/CD28 system in the development of actively induced murine experimental autoimmune encephalomyelitis, suggesting an area of investigation with therapeutic potential for multiple sclerosis.

Cloning and characterization of allograft inflammatory factor-1: a novel macrophage factor identified in rat cardiac allografts with chronic rejection

The development of arteriosclerotic lesions in the Lewis to F344 rat model of chronic cardiac rejection is characterized by macrophage adhesion to the vessel lumen and macrophage infiltration in the neointima prior to smooth muscle cell accumulation. We report the cloning and characterization of allograft inflammatory factor-1 (AIF-1), a novel cDNA that is expressed early and persistently in chronically rejecting cardiac allografts but is absent in cardiac syngrafts and host hearts. The full-length cDNA codes for a hydrophilic polypeptide of 17 kD that contains a 12-amino acid region similar to an EF-hand (calcium-binding) domain. In cardiac allografts AIF-1 transcripts and protein localized to infiltrating mononuclear cells. Analysis of isolated cell populations confirmed that AIF-1 was selectively expressed in macrophages and neutrophils and demonstrated that AIF-1 transcripts could be upregulated by sixfold after stimulation with the T cell-derived cytokine IFN-gamma. Treatment with a diet deficient in essential fatty acids (which attenuates arteriosclerosis) or CTLA-4 Ig (which blocks lymphocyte activation) significantly decreased AIF-1 transcript levels. Upregulation of AIF-1 in the setting of T cell activation suggests that it may play a role in macrophage activation and function.

CD28-B7 blockade after alloantigenic challenge in vivo inhibits Th1 cytokines but spares Th2

Blocking the CD28-B7 T cell costimulatory pathway with the fusion protein CTLA4Ig inhibits alloimmune responses in vitro and in vivo and induces tolerance to cardiac allografts in mice and rats, but the mechanisms mediating the tolerant state in vivo are unknown. Here, we report the effects and potential mechanisms of CTLA4Ig in the rat renal allograft model. LEW rats were nephrectomized and received renal allografts from major histocompatibility complex-incompatible WF rats. While all untreated and control immunoglobulin (Ig)-treated animals acutely rejected their allografts and died, 86% of rats that received a single injection of CTLA4Ig on day 2 after transplantation had prolonged survival (> 60-100 days) with preserved renal function. By contrast, only 29% of animals that received CTLA4Ig on the day of engraftment had prolonged survival. Long-term survivors (> 100 days) exhibited donor-specific tolerance, accepting donor-matched WF but acutely rejecting third-party BN cardiac allografts. Immunohistological analysis of grafts sampled at 1 week after transplantation showed that both control and CTLA4Ig-treated animals had mononuclear cell infiltrates, with a higher percentage of CD4+ cells in the CTLA4Ig-treated group. However, while this was associated with vasculitis and tubulitis in control grafts, there was no evidence of tissue injury in CTLA4Ig-treated animals. The immune response leading to graft rejection in control animals was characterized by expression of the T helper (Th) type 1 cytokines interleukin (IL)-2 and interferon-gamma. In contrast, the persistent CD4+ infiltrate without graft rejection in CTLA4Ig-treated animals was associated with increased staining for the Th2-related cytokines IL-4 and IL-10. Furthermore, grafts from CTLA4Ig-treated animals had marked upregulation of intragraft staining for IgG1, but not IgG2a or IgG2b. Administration of rIL-2 to CTLA4Ig-treated animals restored allograft rejection in 50% of animals tested. These results confirm that blockade of the CD28-B7 pathway after alloantigenic challenge induces donor-specific acceptance of vascularized organ allografts, and indicates in this model that CTLA4Ig inhibits Th1 but spares Th2 cytokines in vivo.

Costimulation in tolerance and autoimmunity

The two signal model of T cell activation predicts that a second costimulatory signal provided by antigen presenting cells (APC) is required in conjunction with the antigenic signal to trigger T cell activation. Considerable evidence indicates that indeed, T cell activation requires such a costimulatory signal which results, at least in part, from the interaction of CD28 with its ligands B7 expressed on all antigen-presenting cells (APC). The second prediction of the two signal model is that T cell receptor engagement in the absence of such a costimulatory signal would lead to specific inactivation of antigen reactive cells. Thus, tissue cells that do not express costimulatory signals would not trigger T cell activation but rather lead to specific inactivation of auto-reactive T cells. By such a model, tolerance to peripheral antigens would be permanently re-established. We review here the evidence suggesting that the CD28-B7 costimulatory pathway might play an important role in T cell tolerance and in the development of autoimmune responses.

Effect of crossmatching on outcome in organ transplantation

The complement-dependent cytotoxicity (CDC) crossmatch and the flow cytometry crossmatch (FCXM) are both used prospectively in renal transplantation, and their use is under evaluation in other types of major organ transplantation. The FCXM is the more sensitive method and better predicts outcome in second and subsequent renal allografts. Improved survival has unmasked the detrimental effect of a positive crossmatch on outcome in liver transplantation. Because of the urgent need of liver transplant candidates, it is unrealistic to defer transplantation until a crossmatch-negative donor is found; however, additional therapeutic measures may be taken to improve outcome for crossmatch-positive liver recipients. Some reports suggest that prospective crossmatching may improve outcome for sensitized heart recipients, and, additionally, recent studies have demonstrated that HLA compatibility between donor and recipient is an independent variable affecting survival after heart transplantation, prompting a reassessment of the current practice of transplanting hearts without consideration of the HLA match.

Prospects for induction of tolerance in renal transplantation

Immunological tolerance is the ultimate goal of transplantation immunobiology. Current therapies involve nonspecific immunosuppression with concomitant risks for infection, malignancy, and drug-specific side effects. By inducing specific immune unresponsiveness to the graft it should be possible to maintain transplants without the need for chronic drug administration and without the risk of nonspecific immunosuppression. This review highlights recent progress in the understanding of immunological tolerance, with special attention to the long-term prospects for successful induction of tolerance in renal transplant patients.

Prevention of T cell anergy by signaling through the gamma c chain of the IL-2 receptor

When stimulated through their antigen receptor without requisite costimulation, T cells enter a state of antigen-specific unresponsiveness termed anergy. In this study, signaling through the common gamma chain of the interleukin-2 (IL-2), IL-4, and IL-7 receptors in the presence of antigen was found to be sufficient to prevent the induction of anergy. After culture with IL-2, IL-4, or IL-7, Jak3 kinase was tyrosine-phosphorylated, which correlated with the prevention of anergy. Therefore, a signal through the common gamma chain may regulate the decision of T cells to either clonally expand or enter a state of anergy.

CD2 is involved in maintenance and reversal of human alloantigen-specific clonal anergy

Induction and maintenance of a state of T cell unresponsiveness to specific alloantigen would have significant implications for human organ transplantation. Using human histocompatibility leukocyte antigen DR7-specific helper T cell clones, we demonstrate that blockade of the B7 family of costimulatory molecules is sufficient to induce alloantigen-specific T cell clonal anergy. Anergized cells do not respond to alloantigen and a variety of costimulatory molecules, including B7-1, B7-2, intercellular adhesion molecule-1 (ICAM-1), and lymphocyte function-associated molecule (LFA)-3. However, after culture in exogenous interleukin (IL)-2 for at least 7 d, anergized cells can respond to alloantigen in the presence of LFA-3. LFA-3 costimulation subsequently restores responsiveness to alloantigen in the presence of previously insufficient costimulatory signals. Expression of CD2R epitope is downregulated on anergic cells and is restored after 7 d of IL-2 culture. The loss of the CD2R is temporally associated with the inability of anergized cells to respond to LFA-3. These results suggest that in addition to blockade of B7 family members, inhibition of CD2 and, potentially, other costimulatory pathways that might reverse anergy will be necessary to maintain prolonged alloantigen-specific tolerance.

Blockade of the CD28 co-stimulatory pathway: a means to induce tolerance

Presentation of antigen to the T-cell receptor (TCR) without co-stimulation results in a state of antigen-specific unresponsiveness on rechallenge, known as anergy in vitro and tolerance in vivo. Mounting evidence suggests that inhibition of the B7-CD28 co-stimulatory pathway is both necessary and sufficient to induce antigen-specific T-cell anergy. Anergy is not static because specific signals are required to maintain this state and prevent its reversal. Attention to these issues will be critical to translate these basic studies to the clinical arenas of transplantation, tumor immunity and autoimmunity.

Systemic release of interleukin-10 during orthotopic liver transplantation

Experimental and clinical observations indicate that the liver allograft is less immunogenic than other organ transplants and can promote immune tolerance. Because interleukin-10 recently emerged as a macrophage and T-cell-derived cytokine with potent immunosuppressive properties, we studied its production in 28 patients undergoing orthotopic liver transplantation. Plasma levels of immunoreactive interleukin-10 dramatically increased within 2 hr after liver allograft reperfusion, with peak levels ranging between 214 and 4998 pg/ml (median = 677 pg/ml). This systemic release of interleukin-10 was transient because it returned to low levels by 48 hr (range = 26 to 51 pg/ml). The higher interleukin-10 levels measured in right atrial blood as compared with portal blood indicated that interleukin-10 was most likely synthesized within the liver graft. To get insight into the cellular origin of interleukin-10, we also measured serum levels of interleukin-4 and interferon-gamma, both produced by T cells, and interleukin-8, a cytokine secreted by macrophages, in eight patients. Interleukin-4 and interferon-gamma levels remained undetectable in most of the patients, whereas interleukin-8 levels paralleled those of interleukin-10. Portal endotoxemia was probably not involved in interleukin-10 production because endotoxin levels remained low (< 20 pg/ml) before and after liver allograft reperfusion. Interleukin-10 plasma levels did not correlate either with cold ischemia time or with the occurrence of rejection episodes. We conclude that orthotopic liver transplantation is associated with a massive release of interleukin-10 and interleukin-8, most likely produced by allograft macrophages.

CTLA-4 can function as a negative regulator of T cell activation

CD28 and CTLA-4 are related glycoproteins found on T cells. Ligation of CD28 following antigen receptor engagement provides a costimulatory signal required for T cell activation. Anti-CTLA-4 antibodies were generated to examine the role of the CTLA-4 receptor on murine T cells. Expression of CTLA-4 as a homodimer is up-regulated 2-3 days following T cell activation. Anti-CTLA-4 antibodies and Fab fragments augmented T cell proliferation in an allogeneic MLR. However, when optimal costimulation and Fc cross-linking were present, anti-CTLA-4 Mabs inhibited T cell proliferation. Together, these results suggest that the MAb may obstruct the interaction of CTLA-4 with its natural ligand and block a negative signal, or directly signal T cells to down-regulate immune function.

The B7 and CD28 receptor families

Current evidence suggests that T-cell receptor (TCR) recognition of antigen bound to the major histocompatibility complex (Ag-MHC) is insufficient to lead to T-cell proliferation or effector function. For a helper T cell to produce sufficient interleukin 2 (IL-2) to allow autocrine-driven clonal expansion, there is a requirement for so-called 'co-stimulatory' or 'accessory' signals in addition to TCR ligation by Ag-MHC. The interaction of the CD28 receptor on T cells with B7 on antigen-presenting cells (APCs) supplies one such co-stimulatory signal. However, the recent discovery that CD28 and B7 are each members of larger gene families suggests that the regulation of co-stimulation is more complex than previously imagined. Here, Carl June and colleagues highlight recent advances in the understanding of the CD28 and B7 receptor families.

Effect of CTLA-4 chimeric protein on rat autoimmune anti-glomerular basement membrane glomerulonephritis

The interaction of the T cell receptor with the antigen/major histocompatibility class II complex is insufficient to induce optimal T cell activation. Co-stimulatory signals, including those provided by CD28/CTLA-4 on T cells and B7 molecules (B7-1, -2, and -3) on antigen-presenting cells, are also required. CD28-B7 interactions can be blocked by a soluble human CTLA-4 chimeric protein (CTLA4Ig). We tested the effect of administration of CTLA4Ig on experimental anti-glomerular basement membrane (GBM) autoimmune glomerulonephritis in Wistar-Kyoto rats induced by immunization with bovine GBM. The disease is characterized by development of antibody to the alpha 3 chain of type IV collagen (Goodpasture's antigen), deposition of rat IgG in GBM, infiltration of the kidney by T cells and macrophages, severe crescent formation and renal failure leading to death in 5-6 weeks. Animals injected with human CTLA4Ig from day 0 to day 14 or to day 35 had reduced disease severity. Beneficial effects were observed even when injections were begun after the onset of glomerulonephritis on day 14. However, the rats developed antibody to the human CTLA4Ig, associated with reduction in levels of circulating CTLA4Ig. The results provide evidence for CD28/CTLA-4 signaling in rat autoimmune glomerulonephritis, and suggest that more effective inhibition of B7-dependent T cell activation, such as might be achieved with homologous CTLA4Ig, could be useful in the treatment of autoimmune diseases.

CD28-B7 interactions in T-cell activation

Previous work has shown that optimal activation of CD4+ T cells requires co-stimulatory signals in addition to the primary signal provided by the antigen receptor. Recent work has demonstrated that CD28 is the primary co-stimulatory signal receptor for T cells, and B7 its natural ligand on antigen presenting cells. In the past year, it has become clear that the importance of CD28-mediated co-stimulatory signals extends to virtually all T-cell subsets. In addition, the existence of multiple ligands that are differentially expressed on antigen-presenting cells has been documented. The picture that is emerging is of a complex and dynamic interplay of co-stimulatory molecules on both the T cell and the antigen-presenting cell that serves to regulate activation. This offers novel approaches to the manipulation of immune responses in vivo.

The role of anergy in peripheral T cell unresponsiveness

When a T cell's encounter with specific antigen results in good signaling through the T cell antigen receptor yet does not lead to a proliferative response, the T cell enters a state of nonresponsiveness, or anergy. Anergy induction can result from a number of different situations, including antigen presentation by costimulation-deficient or "non-professional" antigen presenting cells, pharmacological blocking of T cell proliferation, or chronic stimulation of the T cell receptor by antigen. Anergy is a long-lived but temporary state characterized by a profound inability of the T cell to produce IL-2. Other effector functions may be affected to variable degrees. Anergy has been characterized most carefully under in vitro conditions, but several experimental models have demonstrated that T cells can also become anergic in vivo. This mechanism for tolerance induction may help to ensure that any mature autoreactive T cells which escape thymic deletion are unable to respond to host tissues. Furthermore, an understanding of the mechanism of anergy induction will most certainly lead to beneficial clinical applications, including improving graft acceptance and avoiding such deleterious immune responses as autoimmunity and allergy.

[The regeneration of therapeutic muds from Varzi-Iatchi health resort]

Organ grafts transduced with gene-encoding immunosuppressive molecules are a less toxic approach to preventing graft rejection. Adenovirus vectors have been widely tested with unsatisfactory results, while adeno-associated virus (AAV) is smaller and elicits a low host humoral response. We constructed an AAV vector containing the mouse CTLA4Ig gene. B10 (H2(b)) cardiac grafts were transduced with AAV-CTLA4Ig by coronary infusion. AAV-LacZ vectors were used as reporters and controls, and the expression of beta-gal was determined by X-gal staining. Thirty percent to 40% of myocytes displayed strongly positive X-gal staining after infusion with AAV-LacZ. Additional infusion with vascular dilator reagents did not improve the transduction rate. Survival of B10 heart allografts transduced with AAV-CTLA4-Ig was significantly prolonged in C3H (H2(k)) recipients. These data demonstrate that AAV vectors can efficiently be transduced into the mouse myocardium by coronary infusion. Graft transduction with AAV-CTLA4Ig may be a novel approach to preventing allograft rejection.