Induction of alloantigen-specific hyporesponsiveness in human T lymphocytes by blocking interaction of CD28 with its natural ligand B7/BB1

Establishment of long-term tolerance to SRBC in dogs by recombinant canine CTLA4-Ig

BACKGROUND

Blockade of the CD28 costimulatory molecule by recombinant human cytotoxic T lymphocyte (CTL)-associated antigen (CTLA4)-Ig or CD40-CD154 interaction with the monoclonal antibody 5C8 together with donor-specific transfusion led to enhanced engraftment in the canine model of dog leukocyte antigen (DLA)-identical marrow transplantation after 1 Gy total body irradiation. To reduce or eliminate total body irradiation conditioning regimens, we have sought to develop canine specific reagents.

METHODS

We have created a fusion protein of the extracellular domain of canine (c) CTLA-4 linked to the hinge-CH2-CH3 domains of canine IgG1 in a pcDNA3.1+ vector. Chinese hamster ovarian cells were cotransfected with CTLA4-Ig vector and a dihydrofolate reductase-containing vector. Stable, high producing clones were generated.

RESULTS

Cell binding and mixed leukocyte reactions indicated no significant differences in activity between cCTLA4-Ig and human CTLA4-Ig. Mixed leukocyte reaction data indicated that combinations of cCTLA4-Ig and the monoclonal antibody 5C8 were superior in blocking H-thymidine uptake compared to either reagent alone. In dogs, the circulating half-life of cCTLA4-Ig was approximately 7 days with no immune response against the fusion protein. Finally, two injections of cCTLA4-Ig effectively tolerized two dogs against eight consecutive challenges with sheep red blood cells, given over 330 days as indicated by a complete block of IgG antibody production. Tolerance was broken in one of the two dogs when a ninth injection of sheep red blood cell was given subcutaneously in incomplete Freund's adjuvant.

CONCLUSION

cCTLA4-Ig is an effective nonimmunogenic blocking reagent of the CD28 costimulatory pathway in dogs and is a promising reagent for studies of tolerance induction in hematopoietic cell transplantation in the canine model.

The clinical utility of inhibiting CD28-mediated costimulation

SUMMARY

This volume covers many topics in the field of T-cell costimulation. The need for such a volume is testament to the growth of the field. From its beginning as a concept in the 1980s, we have now progressed to the point where many molecules now have functionally defined roles in T-cell costimulation. In addition, the field has progressed 'from bench to bedside'. Abatacept [cytotoxic T-lymphocyte antigen-4 (CTLA-4)-immunoglobulin (Ig) (CTLA-4-Ig)], an inhibitor of CD28-mediated T-cell costimulation, was approved for the treatment of moderate-to-severe rheumatoid arthritis in 2006 by the Food and Drug Administration and in 2007 by the European Medicines Agency. This chapter first presents a personal historical perspective on the early basic studies on the elucidation of the CD28/B7 T-cell costimulatory pathway and the discovery of CTLA-4-Ig. We next present an overview of studies of CTLA-4-Ig in preclinical animal studies. The material discussed in these first two sections is selective rather than exhaustive; their purpose is to provide context for the final section, a summary of human clinical studies performed with abatacept.

Blockade of CD26-mediated T cell costimulation with soluble caveolin-1-Ig fusion protein induces anergy in CD4+T cells

CD26 binds to caveolin-1 in antigen-presenting cells (APC), and that ligation of CD26 by caveolin-1 induces T cell proliferation in a TCR/CD3-dependent manner. We report herein the effects of CD26-caveolin-1 costimulatory blockade by fusion protein caveolin-1-Ig (Cav-Ig). Soluble Cav-Ig inhibits T cell proliferation and cytokine production in response to recall antigen, or allogeneic APC. Our data hence suggest that blocking of CD26-associated signaling by soluble Cav-Ig may be an effective approach as immunosuppressive therapy.

Abatacept, a novel CD80/86-CD28 T cell co-stimulation modulator, in the treatment of rheumatoid arthritis

Rheumatoid arthritis is a chronic systemic inflammatory disease with major articular manifestations. While its aetiology still remains to be resolved, our understanding on the pathogenesis of rheumatoid arthritis has become clearer during two decades enlightening the role of adaptative immunity in the development of the symptoms and signs as well as in the progression of the pathological articular changes taking place in inadequately controlled disease. T lymphocytes are considered to be an important cell type in the pathogenesis of rheumatoid arthritis through production of proinflammatory cytokines, promotion of formation of ectopic lymphoid structures and neovascularization in synovial tissue, promotion autoantibody production by B cells, and activation of synoviocytes and osteoclasts. Abatacept, a CTLA4-Ig fusion protein, represents a new therapeutic approach in rheumatoid arthritis. Abatacept attenuates T cell activation as it regulates the activation of T cells by inhibiting the CD80/86:CD28 co-stimulatory pathway that is required for the proper T cell activation. This MiniReview gives an overview on the mechanism of action of abatacept and summarizes the published clinical data on abatacept in the treatment of rheumatoid arthritis.

Blockade of LIGHT/HVEM and B7/CD28 Signaling Facilitates Long-Term Islet Graft Survival With Development of Allospecific Tolerance

BACKGROUND

Previous studies have shown that blockade of LIGHT, a T-cell costimulatory molecule belonging to the tumor necrosis factor (TNF) superfamily, by soluble lymphotoxin beta receptor-Ig (LTbetaR-Ig) inhibited the development of graft-versus-host disease. The cardiac allografts were significantly prolonged in LIGHT deficient mice. No data are yet available regarding the role of the LIGHT/HVEM pathway in more stringent fully allogeneic models such as skin and islet transplantation models.

METHODS

Streptozotocin-induced chemical diabetic BALB/C mice underwent transplantation with allogeneic C57BL/6 islets and were treated with LTbetaR-Ig, CTLA4-Ig or a combination of both in the early peritransplant period.

RESULTS

Administration of CTLA4-Ig or LTbeta R-Ig alone only increased graft survival to 55 days and 27 days respectively, whereas simultaneous blockade of both pathways significantly prolonged the islet allograft survival for more than 100 days. Long-term survivors were retransplanted with donor-specific (C57BL/6) islets and the grafted islets remained functional for more than 100 days. All of islet allografts were protected against rejection when the mixtures of 1x10(6) CD4+ T cells from tolerant mice and islet allografts were cotransplanted under the renal capsule of the naïve BALB/c recipients.

CONCLUSIONS

These data indicate that: 1) a synergistic effect for prolonged graft survival can be obtained by simultaneously blocking LIGHT and CD28 signaling in the stringent model of islet allotransplantation; 2) development of donor-specific immunological tolerance is associated with the presence of regulatory T-cell activity; and 3) local cotransplantation of the allografts with the regulatory T cells can effectively prevent allograft rejection and induce donor-specific tolerance in lymphocytes-sufficient recipients.

Enhancement of the anti-leukemic activity of cytokine induced killer cells with an anti-CD19 chimeric receptor delivering a 4-1BB-ζ activating signal

OBJECTIVE

There is growing interest in the use of cytokine-induced killer (CIK) cells in cancer therapy. In this study, we sought to maximize the antileukemic activity of anti-CD19 receptor-modified CIK cells against B-lineage acute lymphoblastic leukemia (ALL).

MATERIALS AND METHODS

CIK cells were transduced with retroviral vectors carrying different types of anti-CD19 chimeric receptors: anti-CD19-zeta, anti-CD19-DAP10, anti-CD19-4-1BB-zeta, and anti-CD19-CD28-zeta. A truncated form of the receptor was used as a control. Transduced CIK cells were then analyzed for their cytotoxic activity against ALL cells and for their capability to proliferate and to release cytokines after ALL encounter.

RESULTS

CIK cells were efficiently transduced with all the anti-CD19 retroviral vectors. Anti-CD19 receptor expression conferred powerful killing activity against ALL cells. However, there were clear advantages when receptors containing the co-stimulatory molecules 4-1BB or CD28 were transduced. Such cells had significantly more potent cytotoxicity than cells expressing the anti-CD19-zeta or anti-CD19-DAP10. Moreover, the presence of 4-1BB or CD28 in the receptor increased the production of interleukin (IL)-2, tumor necrosis factor (TNF)-alpha, TNF-beta, IL-5, IL-6, and IL-8 elicited by coculture with ALL cells. Notably, anti-CD19-4-1BB-zeta CIK cells secreted particularly low levels of interleukin-10 and proliferated strongly after contact with ALL cells.

CONCLUSIONS

Anti-CD19 chimeric receptors delivering primary and costimulatory signals render CIK cells powerfully cytotoxic against ALL cells and induce secretion of immunostimulatory cytokines and proliferation. These results support the testing of genetically modified CIK cells in clinical trials.

Effects of cyclosporine on transplant tolerance: the role of IL-2

Allograft(dagger) transplant outcome, rejection or tolerance, depends upon striking a balance between the pertinent cytopathic and regulatory T cells. The drug cyclosporine is a widely used immunosuppressive agent among transplant recipients. Previous studies have demonstrated that cyclosporine blocks apoptosis of activated T cells and the ability of costimulation blockade based regimens to create peripheral transplant tolerance. We now test the hypothesis that the mechanism by which cyclosporine blocks tolerance induction is IL-2 dependent, and linked to a detrimental effect upon T(reg) function. Our study demonstrates that cyclosporine blocks IL-2 gene expression and activation induced cell death (AICD) of alloreactive T effector cells. We also show that cyclosporine abolishes the beneficial effects of a donor specific transfusion (DST) plus anti-CD154 monoclonal antibody (alpha CD154) regimen on enhanced T(regs) function and allograft tolerance induction. Interestingly, provision of IL-2/Fc, a long-lived form of IL-2, completely reverses the detrimental effects of this adjunctive cyclosporine treatment on AICD of alloreactive T effectors, T(regs) function and tolerance induction. Furthermore, in a MHC mismatched islet allograft model, the combination of cyclosporine with IL-2/Fc permitted long-term allograft survival and induced alloantigen specific allograft tolerance. The combination of IL-2/Fc and cyclosporine treatment may provide a new clinical strategy to promote transplant tolerance.

Enhanced cardiac allograft survival by Vav1-Rac signaling blockade in a mouse model

BACKGROUND

Vav1-Rac signaling plays a pivotal role in TCR/antigen and CD28 signals for T cell activation. However, pharmacological interference of this signaling has not been tested in the prevention of alloimmune-mediated allograft rejection. It has been demonstrated that 6-thio-GTP, a metabolite of azathioprine, specifically inhibits Vav1-Rac activity in T lymphocytes. Here we show the immunosuppressive efficacy of 6-thio-GTP in the prevention of cardiac allograft rejection.

METHODS

T cell proliferations were measured by (3)H-thymidine uptake. The immunosuppressive activities of 6-thio-GTP were tested in the cardiac allograft model of C57BL/6 (H-2(b)) to Balb/c (H-2(d)) mice.

RESULTS

6-Thio-GTP inhibited TCR/alloantigen stimulated T cell proliferation and CD28-dependent T cell survival. Administration of 6-thio-GTP (0.5 mg/kg) prolonged graft survival to 13.8+/-2.39 days compared to 8.3+/-0.48 days in PBS controls (p<0.0001). Combination of 6-thio-GTP (0.5 mg/kg) with CsA (15 mg/kg) enhanced graft survival from 15.0+/-1.61 days in CsA treated recipients to 36.8+/-2.17 days in those received 20 days of combination therapy of CsA and 6-thio-GTP (p<0.0001), or to 42.7+/-16.63 days in the group treated with 20 days of CsA and 60 days of 6-thio-GTP (p<0.0001). Lymphocytes from 6-thio-GTP treated recipients with long-term surviving grafts (>60 days) displayed reduced proliferative response to alloantigen and higher frequencies of regulatory T cells (Treg).

CONCLUSION

Vav1-Rac inhibitor 6-thio-GTP prolongs allograft survival alone or in combination with CsA by suppression of alloreactive T cell activation. Our findings suggest the therapeutic potential of pharmacological interference of Vav1-Rac signaling for transplantation.

Uric acid enhances T cell immune responses to hepatitis B surface antigen-pulsed-dendritic cells in mice

AIM: To study the induction of T cellular immune responses in BALB/c mice immunized with uric acid and dendritic cells (DCs) pulsed with hepatitis B virus surface antigen (HBsAg).

METHODS: DCs were generated from bone-marrow cells of BABL/c mice, and then pulsed or unpulsed with HBsAg protein (HBsAg-pulsed-DCs or unpulsed-DCs) in vitro. BABL/c mice were immunized with HBsAg-pulsed-DCs (1 × 10⁶) and uric acid, injected through the tail vein of each mouse. The mice in control groups were immunized with HBsAg-pulsed-DCs alone, unpulsed-DCs alone or 200 μg uric acid alone or PBS alone. The immunization was repeated 7 d later. Cytotoxic T lymphocytes (CTLs) in vivo were determined by the CFSE labeled spleen lysis assay. Spleen cells or spleen T cells were isolated, and re-stimulated in vitro with HBsAg for 120 h or 72 h. Production of IFN-γ and IL-4 secreted by spleen cells were determined by ELISA method; proliferation of spleen T cells were detected by flow cytometry.

RESULTS: The cytotoxicities of HBsAg-specific-CTLs, generated after immunization of HBsAg-pulsed-DCs and uric acid, were 68.63% ± 11.32% and significantly stronger than that in the control groups (P < 0.01). Compared with control groups, in mice treated with uric acid and HBsAg-pulsed-DCs, the spleen T cell proliferation to HBsAg re-stimulation was stronger (1.34 ± 0.093 vs 1.081 ± 0.028, P < 0.01), the level of IFN-γ secreted by splenocytes was higher (266.575 ± 51.323 vs 135.223 ± 32.563, P < 0.01) , and IL-4 level was lower (22.385 ± 2.252 vs 40.598 ± 4.218, P < 0.01).

CONCLUSION: Uric acid can strongly enhance T cell immune responses induced by HBsAg-pulsed-DCs vaccine. Uric acid may serve as an effective adjuvant of DC vaccine against HBV infection.

Molecular construction and characterization of a novel exotoxin fusion protein that selectively blocks the B7:CD28 costimulatory signal system

An important strategy for specifically preventing and treating graft-versus-host and host-versus-graft diseases is to selectively block the B7:CD28/cytotoxic T-lymphocyte A4 costimulatory signal system for induced immune tolerance. In this study, a novel recombinant B7-2-L-PE40KDEL fusion protein was created to target the B7:CD28 system. We used a flexible linker sequence (Gly4Ser)4 and overlapping sequence extension to link the cDNAs encoding a human B7-2 extracellular domain and a mutant truncated form of Pseudomonas exotoxin A (PE), PE40KDEL. This B7-2-L-PE40KDEL fusion gene was then inserted into the pTYB4 expression vector, expressed in Escherichia coli, and purified through Ni-NTA mealty affinity-->MonoQ anion exchange-->Superdex75 gel filtration chromatography 3-step purification protocols. Western blotting demonstrated that the B7-2-L-PE40KDEL fusion protein specifically bound antihuman B7-2 monoclonal antibody and anti-pseudomonas exotoxin A antiserum. We used the Antheprot nucleic acid and protein analyzing software to predict the characteristics of this fusion protein, and showed that the fusion did not confer new antigenicities to the fusion protein. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide tests demonstrated that at doses ranging from 0.2 to 2 microg/mL, this fusion protein specifically killed CD28-overexpressing Jurkat cells but even at doses of 2 microg did not kill CD28-negative Hut28 cells. The results of a one-way mixed lymphocyte reaction demonstrated that the fusion protein has a range of suppressive effects on HLA class I and II matched related donors and recipients, and HLA class I and II mismatched unrelated donors. Taken together, these results demonstrate that we have developed a novel recombinant human B7-2-L-PE40KDEL exotoxin fusion protein that specifically blocks the B7:CD28 costimulatory signal system in a manner that may be of significant importance in preventing and treating graft-versus-host or host-versus-graft diseases.

Efficacy of Modified Levamisole Adjuvant on Inactivated Virus Vaccine

To improve efficacy, especially for the cell-mediated response to inactivated viral vaccines, a modified levamisole (LMS) adjuvant formulation, designated LMS+, was evaluated for its efficacy in mice and chickens, using Newcastle Disease Virus (NDV) as a model pathogen. Compared with oil adjuvant, the killed NDV in LMS+ induced a significantly higher helper T cell type 1 response, as shown by higher levels of interleukin-2, interferon-gamma, T cell proliferation, and delayed-type hypersensitivity responses, without sacrificing the level of IgG production in mice. In addition, vaccine in LMS+ formulation increased the expression of MHC and costimulatory molecules as well as the number of CD11c+ dendritic cells, suggesting that the better response to the LMS+ formulation occurs partly via the maturation of dendritic cells and activation of MHC-antigen presentation and costimulation. Furthermore, this formulation provides 100% protection in chickens after challenge with a lethal dose of virulent NDV strain F48E9 at 1000 ELD50 (50% egg lethal dose). These results demonstrated that modified LMS+ adjuvant could be used to improve both humoral and cell-mediated responses for inactivated viral vaccines and its development as an effective inactivated viral vaccine is warranted.

Antigen-Specific Regulatory T-Cell Subsets in Transplantation Tolerance

Regulatory T cells (Treg) are critical controllers of the immune response. Disturbed Treg function results in autoimmunity, whereas in transplantation Treg are crucial in graft survival and transplant tolerance. Hence therapeutic modalities that influence Treg numbers or function hold great clinical opportunity. Ahead of us are clinical trails studying in vivo Treg induction protocols and immunotherapy with ex vivo expanded Treg. Here we discuss the preferential use and/or induction of antigen-specific Treg subsets with high suppressive power and migratory capacity as a potential therapeutic tool to prevent solid organ transplantation rejection. Accordingly, ex vivoselection procedures to induce and isolate highly suppressive antigen-specific Treg (subsets) are needed. This subject, as well as the Treg-facilitating potential of immunosuppressive agents, is discussed.

Translating the concept of suppressor/regulatory T cells to clinical applications

The in vivo expansion of suppressor/regulatory T cells (Tregs) is a desirable event in autoimmunity and transplantation. Here we summarize the general rules involved in antigen recognition by T cells and describe Tregs and their requirements, discussing different levels of immune intervention.

CTLA-4.FasL inhibits allogeneic responses in vivo

CTLA-4.Fas ligand (CTLA-4.FasL), a paradigmatic 'trans signal converter protein (TSCP)', can attach to APC (via CTLA-4 binding to B7) and direct intercellular inhibitory signals to responding T cells (via FasL binding to Fas receptor), converting an activating APC-to-T cell signal into an inhibitory one. Our previous studies established that CTLA-4.FasL inhibits human primary mixed lymphocyte reactions (MLR) and induces alloantigen-specific hyporesponsiveness ex vivo. The present study extends this to an in vivo context. Using splenocytes from MHC-mismatched C57BL/6 and Balb/c mice, we demonstrated that his(6)CTLA-4.FasL, effectively inhibits murine MLR. Moving in vivo, we demonstrated that subcutaneously administered his(6)CTLA-4.FasL modulates the in vivo response of infused allogeneic splenocytes. his(6)CTLA-4.FasL reduces the number of cells in each cell division, and increases the percentage of dead cells in each division. These findings are consistent with an antigen-induced cell death of the alloreactive cells, and bolsters recombinant TCSP promise as a therapeutic for transplantation diseases.

Novel therapies for rheumatoid arthritis

Recent years have witnessed significant advancements in the therapeutic approach to rheumatoid arthritis. The introduction of biologic agents, in particular inhibitors of tumor necrosis factor (TNF), has ushered a new era in which the goal of therapy has become achieving very low levels of disease activity. Success achieved with the TNF inhibitors has reinvigorated research into targeting other componenets of the dysregulated immune system in RA. A number of approaches, targeting various cytokines, other inflammatory mediators, and populations of immunocompetent cells, seem promising.

CD4+CD25+ Regulatory T Cells in Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (SCT) is a well-established treatment modality for malignant and nonmalignant hematologic diseases. High-dose radio- and/or chemotherapy eradicate the hematopoietic system of the patient and induce sufficient immunosuppression to enable donor stem cell engraftment. The replacement of the recipient's immune system with that of the donor significantly contributes to the success of this treatment, since donor immune cells facilitate stem cell engraftment, provide protection from infections, and eliminate residual malignant or nonmalignant host hematopoiesis, thereby protecting from disease relapse in patients transplanted for leukemia or lymphoma (graft-versus-leukemia effect, GVL). Mediators of these beneficial effects are mature T cells within the stem cell graft. However, donor T cells can also attack host tissues and induce a life-threatening syndrome called graft-versus-host disease (GVHD). The challenge of allogeneic SCT is to find a balance between beneficial and harmful T cell effects, which at present is only insufficiently achieved by the use of immunosuppressive drugs. In the future, it might be possible to replace or support such medications by using the intrinsic regulatory capacity of the transplanted immune system, as represented by T cell subpopulations with suppressive activity, such as CD4+ CD25+ regulatory T (Treg) cells. In various mouse model systems, these cells have been shown to suppress GVHD while preserving the GVL effect. As the characterization of their human counterparts is rapidly progressing, their application in allogeneic SCT might soon be explored in clinical trials.

IL-27 Limits IL-2 Production during Th1 Differentiation

Although the ability of IL-27 to promote T cell responses is well documented, the anti-inflammatory properties of this cytokine remain poorly understood. The current work demonstrates that during infection with Toxoplasma gondii, IL-27R-deficient mice generate aberrant IL-2 responses that are associated with the development of a lethal inflammatory disease. Because in vivo depletion of IL-2 prolongs the survival of infected IL-27R-/- mice, these data suggest that IL-27 curbs the development of immunopathology by limiting parasite-induced IL-2 production. Consistent with this hypothesis, IL-27R-/- CD4+ T cells produce more IL-2 than wild-type counterparts during in vitro differentiation, and when rIL-27 is introduced, it can suppress the expression of IL-2 mRNA and protein by the latter group. Additionally, these studies reveal that, like IL-27, IL-12 can inhibit IL-2 production, and although each employs distinct mechanisms, they can synergize to enhance the effect. In contrast, this property is not shared by closely related cytokines IL-6 and IL-23. Thus, while traditionally viewed as proinflammatory agents, the present findings establish that IL-27 and IL-12 cooperate to limit the availability of IL-2, a potent T cell growth and survival factor. Moreover, because the current studies demonstrate that both can induce expression of suppressor of cytokine signaling 3, a protein that tempers cytokine receptor signaling, they also suggest that IL-27 and IL-12 share additionally inhibitory properties.

The adjuvant effect of levamisole on killed viral vaccines

To explore adjuvants that are capable of promoting Th1-biased immune response, we investigated the usefulness of levamisole (LMS) as one such adjuvant for two different preparations of killed viral vaccines, derived from the foot mouth disease virus (FMDV) or the porcine respiratory reproductive syndrome virus (PRRSV) and tested respectively in BALB/c or C57 BL/6 mice. The results showed that LMS induced different types of immune responses in the host, depending on its dosage. While a high level of serum IgG was induced by using LMS at 2%, the most robust T cell proliferation was induced with LMS at 0.5%. The Th1 and Th2 cytokine profiles, which tracked well with the antibody and T cell responses, were similarly influenced by the dose of LMS. Moreover, the enhanced T cell response correlated with increased expression of MHC and co-stimulatory molecules and decreased expression of the suppressors of cytokine signaling molecules (SOCS1 and SOCS3) in the spleen, suggesting that it is mediated by the antigen presentation, co-stimulator signaling, and cytokine production pathways. These results establish that LMS can be used to induce Th1-biased immune responses when combined with killed-virus-based antiviral vaccines and that such adjuvant effect depends on the optimal LMS dosage.

Effect of Adenovirus-Mediated Transfer of the CTLA4IG Gene in Hamster-to-Rat Xenotransplantation

BACKGROUND

Costimulatory molecules such as CD28 and B7 are essential for T cell activation, as well as playing a role in the T cell receptor and major histocompatibility complex pathway. It is well known that rejection in allotransplantation is diminished by treatment with CTLA4Ig, but whether a similar effect occurs in xenotransplantation remains to be determined.

METHODS

In this study, we investigated whether adenovirus-mediated gene transfer with CTLA4Ig cDNA by intravenous injection to the recipient is effective in suppression in hamster-to-rat cardiac xenotransplantation.

RESULTS

With CTLA4Ig gene transfer, the duration of gene expression was clearly prolonged, based on reduced production of antiadenovirus antibody and shrinkage of the spleen. The survival of cardiac xenografts was significantly prolonged with CTLA4Ig gene transfer compared to the control graft, and survival with combination use of FK506 and CTLA4Ig gene transfer in xenotransplantation was also significantly prolonged compared to that with CTLA4Ig gene transfer alone. Cessation of the cardiac graft in the combination treatment occurred in parallel with the elevation of antihamster IgM antibodies in rat sera.

CONCLUSIONS

These results suggest that adenovirus-mediated CTLA4Ig gene transfer is effective for immunosuppression in hamster-to-rat xenotransplantation.

Cyclosporine Preserves the Anergic State of Human T Cells Induced by Costimulation Blockade In Vitro

BACKGROUND

Costimulation blockade based tolerance-inducing therapies might be disrupted by adjunct conventional immunosuppressive drug use. In the current study, we evaluated the compatibility of various immunosuppressive agents on costimulation blockade-based immunosuppression and T-cell anergy induction of human alloreactive T-cells in vitro. T-cell anergy is crucial in transplantation tolerance.

METHODS

T cell anergy was induced in human mixed lymphocyte cultures in vitro, by monoclonal antibodies directed against the costimulatory ligands CD40 and CD86. The effect of coadministration of conventional immunosuppressive drugs (CsA, rapamycin or FK506) on the inhibitory potential of costimulation blockade and the induction and maintenance of T cell anergy was analyzed.

RESULTS

We found that monoclonal antibodies against CD40 and CD86 and the simultaneous use of conventional immunosuppressive drugs resulted in strong immunosuppression of proliferation and cytokine production. Rapamycin, in contrast to FK506 and CsA, facilitated T-cell apoptosis. However, drug cotreatment prevented costimulation blockade induced T-cell anergy. Induction of human T-cell anergy in vitro required approximately 5 days of culture. Coadministration of drugs at day 5 after the start of mAb treatment, when anergy was established, did not increase the immunosuppressive effect of mAb treatment. But interestingly, in the majority of experiments, in contrast to rapamycin and FK506, CsA did not affect the anergic state when given after T-cell anergy induction. Moreover, the cell death facilitating potential of rapamycin vanished when used later after T-cell activation.

CONCLUSIONS

Timing and choice of conventional drug are crucial in the success of costimulation blockade-based tolerance induction therapies.

Block and tackle: CTLA4Ig takes on lupus

Blockade of antigen nonspecific costimulatory signals is a promising approach for the treatment of autoimmune diseases including systemic lupus erythematosus (SLE). CTLA4Ig, an antagonist of the CD28/B7 costimulatory interaction, effectively prevents SLE onset in several murine models and, when used in combination with cyclophosphamide, can induce remission of active SLE nephritis. In this review we describe the known mechanisms of action of CTLA4Ig both in normal immunity and in autoimmune disease models and address issues about its activity that still need to be resolved. We discuss the preclinical use of CTLA4Ig in murine SLE models and the rationale for a clinical trial in SLE patients.

Selective Costimulation Modulators

T cells have a central role in the orchestration of the immune pathways that contribute to the inflammation and joint destruction characteristic of rheumatoid arthritis (RA). The requirement for a dual signal for T-cell activation and the construction of a fusion protein that prevents engagement of the costimulatory molecules required for this activation has led to a new approach to RA therapy. This approach is mechanistically distinct from other currently used therapies; it targets events early rather than late in the immune cascade, and it results in immunomodulation rather than complete immunosuppression. The fusion protein abatacept is a selective costimulation modulator that avidly binds to the CD80/CD86 ligands on an antigen-presenting cell, resulting in the inability of these ligands to engage the CD28 receptor on the T cell. Abatacept dose-dependently reduces T-cell proliferation, serum concentrations of acute-phase reactants, and other markers of inflammation, including the production of rheumatoid factor by B cells. Recent studies have provided consistent evidence that treatment with abatacept results in a rapid onset of efficacy that is maintained over the course of treatment in patients with inadequate response to methotrexate and anti-tumor necrosis factor therapies. This efficacy includes patient-centered outcomes and radiographic measurement of disease progression. Abatacept has also demonstrated a very favorable safety profile to date. This article reviews the rationale for this therapeutic approach and highlights some of the recent studies that demonstrate the benefits obtained by using abatacept. This clinical experience indicates that abatacept is a significant addition to the therapeutic armamentarium for the management of patients with RA.

Extracellular lysosome-associated membrane protein-1 (LAMP-1) mediates autoimmune disease progression in the NOD model of type 1 diabetes

Treatment (from 5 to 25 weeks of age) with a novel blocking monoclonal antibody, mAb I-10, directed against the plasma membrane (pm) form of LAMP-1, protected against development of autoimmune diabetes in the NOD mouse. A shorter course of treatment, i.e. from 5 to 12 weeks of age, significantly reduced the occurrence of insulitis as well as disease onset. Interfering with pm-LAMP-1 required continuous treatment as tolerance was not observed when treatment was stopped, and no higher proportion of cells with a T regulatory phenotype (e.g. CD4(+)CD25(+)) were induced. The mechanism appears to involve modulating a proinflammatory cytokine, as the proportion of pancreatic-infiltrating IFN-gamma-positive cells was significantly reduced in the mAb I-10-treated group. These results demonstrate an unexpected role for pm-LAMP-1 in autoimmune disease progression, and suggest that further investigation should be performed to understand how this molecule modulates IFN-gamma-driven responses.

Cooperation between CD4 T helper cells is required for the generation of alloantigen-specific, IFN-gamma-producing human CD4 T cells

We have used MLR to assess the cell interactions that enable human PBL stimulated with irradiated, allogeneic PBL to give rise to allospecific, IFN-gamma-producing CD4 T cells. We were able to generate alloantigen-specific, IFN-gamma-producing T cells from peripheral blood. The responder and stimulator cell numbers required for the optimal generation of such cells, enumerated using an enzyme-linked immunospot assay, were separately determined for various combinations of responders and stimulators. The number of antigen-specific, IFN-gamma-producing cells generated per 10(6) responder cells, seeded at the initiation of culture, is critically dependent on the density of the responder cells, with minimal generation of IFN-gamma-producing T cells at low responder densities. These and further observations with fractionated responding PBL show that CD4 T-cell/CD4 T-cell interactions, which are completely independent of CD8() T cells, are necessary for the in vitro generation of alloantigen-specific, IFN-gamma-producing CD4 T cells.

Effects of CD28 blockade on subsets of naive T cells in cats

OBJECTIVE

To determine whether human CTLA4-Ig ([hu]CTLA4-Ig) inhibits costimulation-dependent lymphocyte proliferation in vitro, compare the effects of (hu)CTLA4-Ig with cyclosporine and steroids on CD4+ and CD8+ T-cell lymphocyte proliferation, and determine whether memory T-cell function remains intact in the presence of (hu)CTLA4-Ig.

ANIMALS

29 cats.

PROCEDURE

Peripheral blood mononuclear cells (PBMCs) were stimulated with concanavalin A (costimulation-dependent mitogen) or phorbol 12-myristate 13-acetate and ionomycin (costimulation independent mitogens) alone or in the presence of (hu)CTLA4-Ig, cyclosporine, or dexamethasone; effects of these treatments on lymphocyte proliferation were assessed by incorporation of thymidine labeled with tritium or flow cytometry. Antigen-specific proliferation was determined by stimulating PBMCs from 2 healthy cats seropositive for Toxoplasma gondii with soluble Toxoplasma antigen alone or in the presence of (hu)CTLA4-Ig or cyclosporine.

RESULTS

(hu)CTLA4-Ig inhibited costimulation-dependent lymphocyte proliferation in vitro but had no effect on costimulation-independent lymphocyte proliferation. Compared with mitogen alone, (hu)CTLA4-Ig caused a significant decrease in responder frequency and proliferative capacity of CD4+ T cells; however, the effect on CD8+ T cells was not significant. Cyclosporine alone or with dexamethasone had a significantly greater suppressive effect on responder frequency and proliferative capacity of CD4+ and CD8+ T cells, compared with (hu)CTLA4-Ig. Compared with cyclosporine, (hu)CTLA4-Ig appeared to have a sparing effect on antigen-specific proliferation of memory CD4+ and CD8+ T cells.

CONCLUSIONS AND CLINICAL RELEVANCE

(hu)CTLA4-Ig selectively inhibited costimulation-dependent proliferation of lymphocytes in vitro and had a sparing effect on antigen-specific proliferation of memory cells. The specificity of its mechanism of action suggests that (hu)CTLA4-Ig may prevent allograft rejection but leave memory responses to previously encountered antigens intact.

β2-Adrenergic Receptor Stimulation-Induced Immunosuppressive Effects Possibly through Down-Regulation of Co-Stimulatory Molecules, ICAM-1, CD40 and CD14 on Monocytes

We examined the effects of β2-adrenergic receptor (β2-AR) agonists on the expression of co-stimulatory molecules on lipopoly-saccharide (LPS)-stimulated human peripheral blood mononuclear cells. The study found that β2-AR agonists inhibited the expression of intercellular adhesion molecule-1 (ICAM-1), CD40 and CD14 on monocytes, and that AR agonist activity was antagonized by the selective β2-AR antagonist, butoxamine. The selective β2-AR agonists salbutamol and terbutaline induced a similar co-stimulatory molecule expression pattern. The LPS-induced production of tumour necrosis factor-α was inhibited by AR agonists, and this was also antagonized by butoxamine, and mimicked by salbutamol and terbutaline. The AR agonists also inhibited T-cell proliferation through β2-AR stimulation. This study clearly demonstrated that endogenous catecholamines elicited immunosuppressive effects through β2-AR stimulation, possibly due to down-regulation of the expression of ICAM-1, CD40 and CD14 on monocytes. These results suggested that the sympathetic nervous system might regulate the T-helper cell balance via the peripheral end-effectors of the stress system.

Cytotoxic T-lymphocyte antigen 4-immunoglobulin in rheumatoid arthritis

It is apparent that the potential to significantly affect the immune response exists through artificial modulation of a system of molecules on the surface of T cells that has been designed to specifically provide on-off switches to support or abrogate the activation of T cells. This approach holds considerable promise because it avoids toxicities associated with cell lysis, while theoretically specifically affecting only those T cells, which are being continuously stimulated to become activated. Although there is presently a paucity of studies in humans on the clinical effects of cytotoxic T-lymphocyte antigen 4-immunoglobulin in patients who have rheumatoid arthritis, the existing studies indicate a clear therapeutic value with this approach.

Significant enhancement by anti-ICOS antibody of suboptimal tacrolimus immunosuppression in rat liver transplantation

A member of the costimulatory molecule family, inducible costimulator (ICOS), is expressed on activated T cells and plays a critical role in their primary activation and cytokine production. ICOS is involved in different immune phenomena, such as Th1-mediated autoimmune disease and graft rejection. Although blockade of ICOS costimulation theoretically may protect grafts from rejection, a single dose of anti-ICOS antibody did not result in the prolongation of rat liver allograft survival. However, in this article, we report that anti-rat ICOS antibody markedly enhanced the immunosuppressive activity of a suboptimal dose of tacrolimus (FK506). After fully allogenic DA to LEW liver transplantation, recipients received a single injection of tacrolimus (1 mg/kg, intramuscularly) with or without anti-ICOS antibody (1 mg/kg, intravenously). Recipient survival was significantly prolonged in rats treated with both the antibody and suboptimal tacrolimus (median survival time 44 days vs. 28 days with tacrolimus alone, P <.01). The extent of cell infiltration into the graft was closely associated with prolongation of recipient survival. Our findings thus demonstrate that anti-ICOS antibody immunotherapy combined with suboptimal tacrolimus has a synergistic effect in preventing hepatic allograft rejection and that it may induce long-term graft acceptance intimately associated with a marked reduction of intragraft T lymphocyte infiltration.

CD28 ligation induces transplantation tolerance by IFN-gamma-dependent depletion of T cells that recognize alloantigens

Administration of an agonistic anti-CD28 mAb paradoxically inhibits donor T cell expansion and prevents graft-versus-host disease (GVHD) in mice. Here we examined the mechanism of anti-CD28-mediated immunosuppression and found that anti-CD28 mAb activated, rather than blocked, CD28-mediated signaling in vivo. Anti-CD28 treatment prevented GVHD by selectively depleting alloantigen-activated donor T cells through apoptosis but spared the T cells that did not recognize recipient alloantigens. Overexpression of Bcl-x(L) did not protect T cells from depletion and did not affect GVHD prevention after anti-CD28 treatment. Depletion of activated T cells mediated through CD28 did not depend on the expression of death receptors Fas and TNF receptors type I and II, but both the depletion of activated T cells and the suppressive effect of anti-CD28 mAb on GVHD lethality required donor-derived IFN-gamma production. This study demonstrates that agonistic Ab's specific for the CD28 costimulatory molecule may be used as novel therapeutic agents to abrogate pathogenic T cell responses by selective depletion of activated T cells.

Stem cell–mediated tolerance inducing strategies in organ transplantation

The scope of possible tools to modulate the recipients immune response towards tolerance induction basically includes deletional and non deletional mechanisms, which are currently targeted by various strategies including monoclonal antibodies, cytokine deviation, chimerism induction and the support of regulating T-cells. Here we summarize the main findings in the field derived from experimental animal studies and currently performed clinical studies. This review focuses to give a clinically relevant overview over relevant tolerance inducing concepts, taking into consideration risk profiles and clinical efficacy associated with specific immunosuppressive regiments currently applied in the clinical setting of transplantation.

Monoclonal antibody specific for TIRC7 induces donor-specific anergy and prevents rejection of cardiac allografts in mice

T cell immune response c-DNA (TIRC7) is up-regulated during the early stages of T-cell activation in response to alloantigens. In this study, we analyzed the effects of newly developed monoclonal antibodies (mAb) against TIRC7 in acute cardiac allograft rejection. Fully vascularized heterotopic allogeneic heart transplantation was performed in mice across a full-mismatch barrier (C57Bl/10 into CBA). Recipients received seven injections (day 0-7) of a novel anti-TIRC7 mAb or remained untreated. Graft survival, histology and ex vivo lymphocyte functions were tested. Targeting of TIRC7 with an anti-TIRC7 mAb diminishes lymphocyte infiltration into grafts resulting in delay of morphological graft damage and prolongation of allograft survival. The lymphocytes from anti-TIRC7 mAb-treated animals exhibit hypo-responsiveness without evidence of lymphocyte depletion against the donor allo-antigens. Proliferation and expression of interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) were down-regulated while interleukin-4 (IL-4) and IL-10 expression were spared. Moreover, anti-TIRC7 mAb enhanced up-regulation of CTLA-4 expression but suppressed up-regulation of CD25 on stimulated lymphocytes in vitro and in vivo. Ligation of TIRC7 has important effects on the regulation of co-stimulatory signaling pathways associated with suppressing of T-cell activation. Targeting of TIRC7 may therefore provide a novel therapeutic approach for modulating T cell immune responses during organ transplantation.

Effect of beta2-adrenergic receptor agonists on intercellular adhesion molecule (ICAM)-1, B7, and CD40 expression in mixed lymphocyte reaction

BACKGROUND

The plasma interleukin (IL)-18 level is elevated in acute rejection after organ transplantation. Although beta2-adrenergic receptor (AR) agonists suppress the rejection of organ and tissue transplants, little is known about their action mechanisms. We examined the effects of endogenous catecholamines and beta2-AR agonists on the expression of intercellular adhesion molecule (ICAM)-1, B7.1, B7.2, CD40, and CD40 ligand (CD40L) in human mixed lymphocyte reaction (MLR) and in an in vitro model of acute rejection in the presence or absence of IL-18.

METHODS

ICAM-1, B7.1 B7.2, CD40, and CD40L expression on monocytes was measured by flow cytometry, and the production of interferon (IFN)-gamma and IL-12 was determined by enzyme-linked immunosorbent assay. Lymphocytes proliferation in MLR was measured by [3H]-thymidine uptake. The relevant AR subtypes were characterized using subtype-selective agonists and antagonists.

RESULTS

beta2-AR agonists inhibited the expression of ICAM-1 and CD40 during MLR in the absence of IL-18. Among IL-18-induced expression of ICAM-1, B7.1, B7.2, CD40, and CD40L, beta2-AR agonists inhibited ICAM-1 and CD40 expression. beta2-AR agonists prevented the production of IFN-gamma and IL-12 in the presence of IL-18 but had no effect in the absence of IL-18. beta2-AR agonists inhibited lymphocyte proliferation in IL-18-treated MLR.

CONCLUSIONS

We found that beta2-AR agonists strongly inhibited the expression of ICAM-1 and CD40, irrespective of the presence or absence of IL-18, which is different from that of histamine and prostaglandin E2.

Mechanisms of early peripheral CD4 T-cell tolerance induction by anti-CD154 monoclonal antibody and allogeneic bone marrow transplantation: evidence for anergy and deletion but not regulatory cells

Anti-CD154 (CD40L) monoclonal antibody (mAb) plus bone marrow transplantation (BMT) in mice receiving CD8 cell-depleting mAb leads to long-term mixed hematopoietic chimerism and systemic donor-specific tolerance through peripheral and central deletional mechanisms. However, CD4(+) T-cell tolerance is demonstrable in vitro and in vivo rapidly following BMT, before deletion of donor-reactive CD4 cells is complete, suggesting the involvement of other mechanisms. We examined these mechanisms in more detail. Spot enzyme-linked immunosorbent (ELISPOT) analysis revealed specific tolerization (within 4 to 15 days) of both T helper 1 (Th1) and Th2 cytokine responses to the donor, with no evidence for cytokine deviation. Tolerant lymphocytes did not significantly down-regulate rejection by naive donor-reactive T cells in adoptive transfer experiments. No evidence for linked suppression was obtained when skin expressing donor alloantigens in association with third-party alloantigens was grafted. T-cell receptor (TCR) transgenic mixing studies revealed that specific peripheral deletion of alloreactive CD4 T cells occurs over the first 4 weeks following BMT with anti-CD154. In contrast to models involving anti-CD154 without BMT, BMT with anti-CD154 leads to the rapid induction of anergy, followed by deletion of pre-existing donor-reactive peripheral CD4(+) T cells; the rapid deletion of these cells obviates the need for a regulatory cell population to suppress CD4 cell-mediated alloreactivity.

Cancer vaccine development: protein transfer of membrane-anchored cytokines and immunostimulatory molecules

Many tumor cells escape host-immune recognition by the downregulation or lack of immunostimulatory molecules. Expression of immunostimulatory molecules on tumor cells by gene transfer can be used to induce an antitumor immune response. However, we have previously shown that protein transfer of glycosyl-phosphatidylinositol (GPI)-linked costimulatory molecules is a successful alternative to traditional gene transfer in preparing such a tumor vaccine. Vaccination with membranes modified by protein transfer to express GPI-linked B7.1 (CD80), a costimulatory adhesion molecule, induces protective immunity in mice and allogeneic antitumor T-cell proliferation in humans in vitro. Our goal is to develop an optimal tumor vaccine using tumor membranes modified by protein transfer to target and stimulate antigen-presenting cells (APCs) and T cells. We have investigated the efficacy of expressing GPI-anchored cytokine molecules on the surface of tumor cells. Expression of interleukin-12 (IL-12) on tumor-cell membranes in a GPI-anchored form induces a strong antitumor immune response that is comparable to the effects of secretory IL-12. Because many cytokines act synergistically, we are testing the membrane expression and immunostimulatory effects of cytokines individually as well as in combination to determine potential complementary effects of coexpression on the antitumor immune response. Ultimately, the protein-transfer vaccination may be used in humans alone or in multimodal combination therapies to induce tumor regression and to serve as a protective measure to prevent postsurgical secondary metastases.

Activation of Resting Human Primary T Cells with Chimeric Receptors: Costimulation from CD28, Inducible Costimulator, CD134, and CD137 in Series with Signals from the TCRζ Chain

Chimeric receptors that include CD28 signaling in series with TCRzeta in the same receptor have been demonstrated to activate prestimulated human primary T cells more efficiently than a receptor providing TCRzeta signaling alone. We examined whether this type of receptor can also activate resting human primary T cells, and whether molecules other than CD28 could be included in a single chimeric receptor in series with TCRzeta to mediate the activation of resting human primary T cells. Human CD33-specific chimeric receptors were generated with CD28, inducible costimulator, CD134, or CD137 signaling regions in series with TCRzeta signaling region and transfected by electroporation into resting human primary T cells. Their ability to mediate Ag-specific activation was analyzed in comparison with a receptor providing TCRzeta signaling alone. Inclusion of any of the costimulatory signaling regions in series with TCRzeta enhanced the level of specific Ag-induced IL-2, IFN-gamma, TNF-alpha, and GM-CSF cytokine production and enabled resting primary T cells to survive and proliferate in response to Ag in the absence of any exogenous factors. Inclusion of CD28, inducible costimulator, or CD134 enhanced TCRzeta-mediated, Ag-specific target cell lysis. Chimeric receptors providing B7 and TNFR family costimulatory signals in series with TCRzeta in the same receptor can confer self-sufficient clonal expansion and enhanced effector function to resting human T cells. This type of chimeric receptor may now be used to discover the most potent combination of costimulatory signals that will improve current immunotherapeutic strategies.

Simultaneous administration of a low-dose mixture of donor bone marrow cells and splenocytes plus adenovirus containing the CTLA4Ig gene result in stable mixed chimerism and long-term survival of cardiac allograft in rats

T-cell costimulatory blockade combined with donor bone marrow transfusion may induce mixed chimerism, rendering robust tolerance in transplanted organs and cells. However, most protocols entail high doses of donor bone marrow cells (BMCs) or repeated administration of costly agents that block costimulatory pathways, thus delaying clinical development. To circumvent these shortcomings, we developed a strategy in which the dosage of donor BMCs was reduced but compensated by donor splenocytes (SPLCs). Furthermore, repeated administration of costly agents was replaced with a single injection of adenovirus expressing a gene of interest. In rat cardiac transplantation models, cardiac allografts from DA (RT-1a) rats were transplanted heterotopically into the abdomen of LEW (RT-11) recipient rats. Immediately after cardiac transplantation, an adenovirus vector (AdCTLA4Ig; 5 x 10(9) plaque-forming units) containing the gene for CTLA4Ig was administered to recipients (n = 6) simultaneously with a low dose of donor BMCs (1 x 10(8)/rat) and SPLCs (5 x 10(7)/rat) via the portal vein. The treated LEW recipient rats developed long-lasting mixed chimerism (>10% at >100 days) and exhibited long-term cardiac allografts (mean survival time of > 200 days) compared with control recipients. Moreover, recipients displaying long-lasting mixed chimerism accepted subsequent donor skin allografts while promptly rejecting third-party skin allografts. These results suggest that blockade of the CD28-B7 pathway, using adenovirus-mediated CTLA4Ig gene transfer, in concert with a low dosage of donor BMCs and SPLCs, may represent a feasible strategy to induce stable mixed chimerism and permit long-term survival of cardiac allografts.

Lack of role for CsA-sensitive or Fas pathways in the tolerization of CD4 T cells via BMT and anti-CD40L

Anti-CD40L mAb plus bone marrow transplantation (BMT) and recipient CD8 T-cell depletion permits long-term mixed hematopoietic chimerism and systemic donor-specific tolerance to be achieved across full MHC barriers. Initial tolerance is characterized by peripheral deletion of donor-reactive CD4 cells. In regimens using costimulatory blockade without BMT to achieve allograft survival, cyclosporine inhibited graft survival, suggesting that the combination may not be clinically applicable. We assessed the role of cyclosporine-sensitive mechanisms and the mechanisms of T-cell apoptosis involved in the induction of early peripheral CD4+ T-cell tolerance by BMT with anti-CD40L. Neither a short course of cyclosporine (14 days) nor the absence of FAS-mediated activation-induced cell death (AICD) blocked the induction or maintenance of donor-specific tolerance. IL-2 production was not associated with tolerance induction, consistent with the lack of a role for Fas-mediated AICD. Mice in which passive T-cell death was impaired because of constitutive expression of a Bcl-xL transgene did not develop tolerance with this protocol. These data confirm that deletion of donor-reactive T cells is critical for the induction of mixed chimerism and tolerance. However, the mechanisms involved may differ from those involved in costimulatory blockade regimens that do not include BMT.

CTLA-4. FasL induces alloantigen-specific hyporesponsiveness

The APC:T cell interface can be effectively targeted with immunotherapeutic proteins. We previously described a unique trans signal converter protein, CTLA-4. Fas ligand (FasL), that has the inherent capacities to tether the T cell inhibitor FasL (CD95 ligand) to the surfaces of B7 (CD80 and CD86)-positive APC (via CTLA-4:B7 interaction), and in so doing, to simultaneously interfere with B7-to-CD28 T cell activation signals. Given the continuing need for agents capable of inducing allograft tolerance without generalized immunosuppression, we have explored in depth the functional activity of CTLA-4. FasL in human allogeneic MLR. CTLA-4. FasL inhibits 1 degrees MLR and induces specific hyporesponsiveness in 2 degrees MLR, with both effects only partially reversible with exogenous IL-2. Moreover, the presence of exogenous IL-2 during the 1 degrees MLR does not affect the induction of hyporesponsiveness upon restimulation. Furthermore, CTLA-4. FasL enables partial activation of allostimulated T cells, reduces the fraction of actively dividing cells, and increases the percentage of dead cells among dividing T cells. Taken together, these findings suggest that CTLA-4. FasL-mediated inhibition of secondary alloantigenic responses involves both anergy induction and clonal deletion. Thus, CTLA-4. FasL, a paradigmatic trans signal converter protein, manifests unique functional properties and emerges as a potentially useful immunotherapeutic for modulating alloresponsiveness.

Immunity in the absence of CD28 and CD137 (4-1BB) molecules

We report the generation and immune regulation of mice that are deficient in CD28 and 4-1BB (CD137) genes. These mice were viable, fertile and did not display any overt abnormalities and had a normal T cell phenotype in thymus and spleen. Proliferative responses to anti-CD3 and ConA were enhanced in 4-1BB-/- but not in either CD28-/- or double mutant mice, while levels of interleukin-2 were decreased in all mutant mice. Although the 4-1BB-/- mice displayed increased basal levels of most immunoglobulin isotypes tested, the plateau levels of immunoglobulin G2a, immunoglobulin G2b and immunoglobulin A were particularly high compared to wild type controls. The immunoglobulin class switch to T-dependent antigen was normal in 4-1BB-/- mice but was greatly affected in both CD28-/- and 4-1BB-/- CD28-/- mice. Vesicular stomatitis virus-specific cytotoxic T lymphocyte responses and plaque reduction neutralizing ability was differentially reduced in all mutant mice. Contact sensitivity to allergens showed marginal but not significant change in ear thickness in 4-1BB-/- mice, but an ability to mount contact hypersensitivity to the same antigens was greatly curtailed in CD28-/- and double mutant mice.

High levels of Fas ligand and MHC class II in the absence of CD80 or CD86 expression and a decreased CD4+ T cell Infiltration, enables murine skin tumours to progress

It is still not clear why some tumours will be recognized and destroyed by the immune system, and others will persist, grow, and eventually kill the host. It has been hypothesized that tumour cells might evade immunological destruction by expressing Fas ligand (FasL), a molecule which induces apoptosis in Fas(+) target cells. However, the role of FasL in creating an immune privileged status within a tumour remains controversial. To determine whether FasL is associated with skin tumour progression, we developed a tumour model enabling us to compare two squamous cell carcinomas (SCC). One is a regressor SCC which spontaneously regresses after injection into syngeneic mice. The other is a progressor SCC which evades immunological destruction. Detailed flow cytometric analysis was used to study tumour cell expression of FasL, Fas, CD80, CD86 and MHC class II. We also analysed the percentage of apoptotic tumour cells in vivo using annexin V and correlated skin tumour progression with CD4 and CD8 T cell infiltration. Progressor tumours expressed high levels of FasL in vivo, which was virtually absent from regressor tumours. The percentage of progressor tumours expressing MHC II was significantly greater than regressor tumours, while neither tumour expressed CD80 or CD86 costimulatory molecules. Consistent with a regressor phenotype, the percentage of viable tumour cells was significantly lower for regressor compared to progressor tumours which coincided with a significantly larger CD4(+) T cell infiltrate into the tumour mass. The results suggest that progression of skin tumours occurs if tumour cells express high levels of MHC II but not costimulatory molecules such as CD80 or CD86. This implies that tumours may induce anergy in CD4(+) T cells via MHC II antigen presentation in the absence of costimulation. To ensure escape from the immune system, tumours may then kill these T cells via a FasL-dependent mechanism.

Superior T-cell suppression by rapamycin and FK506 over rapamycin and cyclosporine A because of abrogated cytotoxic T-lymphocyte induction, impaired memory responses, and persistent apoptosis

Immunosuppressive therapy is best achieved with a combination of agents targeting multiple activation steps of T cells. In transplantation, cyclosporine A (CsA) or tacrolimus (FK506) are successfully combined with rapamycin (Rap). Rap and CsA were first considered for combination therapy because FK506 and Rap target the same intracellular protein and thus may act in an antagonistic way. However, in clinical studies, FK506+Rap proved to be effective. To date, there is no in vitro data supporting these in vivo findings, and it is unclear whether the observed effects are T-cell mediated. In a human polyclonal allogeneic in vitro model, we found that although combined drug treatment markedly reduced expansion of naive T cells, T-cell activation occurred irrespective of the drug combination used. The induction of cytotoxic effector T cells was reduced by CsA+Rap but completely abolished by FK506+Rap. Importantly, combined immunosuppression allowed generation of memory CD4+ and CD8+ T cells and hence did not result in T-cell anergy. However, FK506+Rap treatment resulted in a reduced number of allospecific memory T cells showing a decreased cell-cycle turnover and cytokine producing capacity. In contrast, CsA+Rap treatment led to increased memory T-cell numbers responding with elevated kinetics. The ability of Rap to promote apoptosis, which contributes to T-cell suppression, remained unaffected upon combination with FK506 or CsA. These data support the combined use of FK506+Rap over CsA+Rap for immunosuppressive therapy.

The therapeutic potential of costimulatory blockade with CTLA4Ig in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic, systemic, inflammatory disorder characterised by joint inflammation and destruction. Controversy exists regarding the significance and exact role of activated T cells in RA. CTLA4Ig is a soluble fusion protein (cytotoxic T lymphocyte-associated antigen 4 fused to the heavy chain constant region of human IgG1), which represents the first in a new class of agents called costimulation blockers. By blocking the second signal required for optimal T-cell activation, CTLA4Ig has demonstrated efficacy in a variety of spontaneous and induced animal models of autoimmunity. A Phase II clinical study in RA showed CTLA4Ig was efficacious with an acceptable safety profile. These results suggest that activated T cells may be important in RA pathogenesis and that costimulation blockade may be a novel therapeutic approach for this disorder.

The complementary roles of deletion and regulation in transplantation tolerance

Neonatal tolerance of alloantigens was described in mice nearly half a century ago, but unfortunately, the translation of these early findings into the clinical arena proved to be much more challenging than was first anticipated. However, the past decade has seen considerable progress in our understanding of the mechanisms that contribute to transplantation tolerance in experimental models. This review outlines our current understanding of the mechanisms of allograft tolerance, emphasizing the complementary roles of deletion and regulation of alloreactive T cells.