Costimulatory blockade by the induction of an endogenous xenospecific antibody response

Monitoring T-Cell Responses in Translational Studies: Optimization of Dye-Based Proliferation Assay for Evaluation of Antigen-Specific Responses

Adoptive therapy with regulatory T cells or tolerance-inducing antigen (Ag)-presenting cells is innovative and promising therapeutic approach to control undesired and harmful activation of the immune system, as observed in autoimmune diseases, solid organ and bone marrow transplantation. One of the critical issues to elucidate the mechanisms responsible for success or failure of these therapies and define the specificity of the therapy is the evaluation of the Ag-specific T-cell responses. Several efforts have been made to develop suitable and reproducible assays. Here, we focus on dye-based proliferation assays. We highlight with practical examples the fundamental issues to take into consideration for implementation of an effective and sensitive dye-based proliferation assay to monitor Ag-specific responses in patients. The most critical points were used to design a road map to set up and analyze the optimal assay to assess Ag-specific T-cell responses in patients undergoing different treatments. This is the first step to optimize monitoring of tolerance induction, allowing comparison of outcomes of different clinical studies. The road map can also be applied to other therapeutic interventions, not limited to tolerance induction therapies, in which Ag-specific T-cell responses are relevant such as vaccination approaches and cancer immunotherapy.

Potential of T-regulatory cells to protect xenografts

PURPOSE OF REVIEW

Immunological barriers still preclude clinical xenotransplantation. The protective role of CD4(+)CD25(+)Foxp3(+) T-regulatory cells (Treg) in allotransplantation is well described and, therefore, could represent a promising therapeutical tool for xenotransplantation. This review addresses the latest findings on Treg in xenotransplantation research.

RECENT FINDINGS

In vivo, costimulation blockade-based strategies including anti-CD154 monoclonal antibodies (mAbs) in combination with rapamycin or anti-LFA-1 mAb prolonged both concordant and discordant islets xenografts survival in a Treg-dependent manner. In vitro, IL-10 secretion was shown to be critical for the suppression of xenogeneic responses mediated by Treg. Moreover, transgenic expression of inducible costimulator-immunoglobulin or PD-L1 on porcine endothelial cells inhibited human T-cell proliferation in vitro and was associated with the induction of Treg and IL-10 secretion. CXCR3 mediated the recruitment of Treg to pig endothelium. Finally, the recruitment of human Treg was enhanced by the immobilization of human CCL17 on pig endothelium.

SUMMARY

There is increasing evidence for the potential of CD4(+)CD25(+)Foxp3(+) Treg to protect xenografts. Induction of Treg in recipients and/or recruitment of human Treg to pig endothelium may represent novel strategies to prevent cell-mediated rejection in pig-to-human xenotransplantation.

Relative efficiency of porcine and human cytotoxic T-lymphocyte antigen 4 immunoglobulin in inhibiting human CD4+ T-cell responses co-stimulated by porcine and human B7 molecules

α1,3-Galactosyltransferase gene-knockout pigs transgenic for porcine cytotoxic T-lymphocyte antigen 4 immunoglobulin (pCTLA4-Ig) have been produced to reduce T-cell-mediated rejection following xenotransplantation. The level of soluble pCTLA4-Ig in their blood was greatly in excess of the therapeutic level in patients, rendering the pigs immune-incompetent. Soluble pCTLA4-Ig produced by these transgenic pigs was evaluated for binding to porcine and human (h) B7 molecules, and for its inhibitory effect on allogeneic and xenogeneic human T-cell responses. Porcine CTLA4-Ig-expressing peripheral blood mononuclear cells (PBMCs) and aortic endothelial cells (AECs) were evaluated for their direct inhibitory effect on hCD4+ T-cell responses. Soluble pCTLA4-Ig and purified hCTLA4-Ig showed similar binding to pB7 molecules, but pCTLA4-Ig showed significantly less binding to hB7 molecules. The pCTLA4-Ig and hCTLA4-Ig inhibited the response of hCD4+ T cells to pAECs equally, but pCTLA4-Ig was less successful in inhibiting the human allogeneic response. The hCD4+ T-cell response to PBMCs from pCTLA4-Ig pigs was significantly lower than that of non-pCTLA4-Ig pigs. Although pCTLA4-Ig was detected in the cytoplasm of pCTLA4-Ig-expressing pAECs, only a minimal level of soluble pCTLA4-Ig was detected in the supernatant during culture, and pCTLA4-Ig-expressing pAECs did not inhibit the xenogeneic direct human T-cell response. High-level tissue-specific production of pCTLA4-Ig may be required for sufficient immunosuppression for organ or cell (e.g., islets) transplantation.

Functional characterization of the ER stress induced X-box-binding protein-1 (Xbp-1) in the porcine system

Background

The unfolded protein response (UPR) is an evolutionary conserved adaptive reaction for increasing cell survival under endoplasmic reticulum (ER) stress conditions. X-box-binding protein-1 (Xbp1) is a key transcription factor of UPR that activates genes involved in protein folding, secretion, and degradation to restore ER function. The UPR induced by ER stress was extensively studied in diseases linked to protein misfolding and aggregations. However, in the porcine system, genes in the UPR pathway were not investigated. In this study, we isolated and characterized the porcine Xbp1 (pXbp1) gene in ER stress using porcine embryonic fibroblast (PEF) cells and porcine organs. ER stress was induced by the treatment of tunicamycin and cell viability was investigated by the MTT assay. For cloning and analyzing the expression pattern of pXbp1, RT-PCR analysis and Western blot were used. Knock-down of pXbp1 was performed by the siRNA-mediated gene silencing.

Results

We found that the pXbp1 mRNA was the subject of the IRE1α-mediated unconventional splicing by ER stress. Knock-down of pXbp1 enhanced ER stress-mediated cell death in PEF cells. In adult organs, pXbp1 mRNA and protein were expressed and the spliced forms were detected.

Conclusions

It was first found that the UPR mechanisms and the function of pXbp1 in the porcine system. These results indicate that pXbp1 plays an important role during the ER stress response like other animal systems and open a new opportunity for examining the UPR pathway in the porcine model system.

Activation requirements and responses to TLR ligands in human CD4+ T cells: comparison of two T cell isolation techniques

Direct regulation of T cell function by microbial ligands through Toll-like receptors (TLR) is an emerging area of T cell biology. Currently either immunomagnetic cell sorting (IMACS) or fluorescence-activated cell sorting (FACS), are utilized to isolate T-cell subsets for such studies. However, it is unknown to what extent differences in T cell purity between these isolation techniques influence T cell functional assays. We compared the purity, response to mitogen, activation requirements, and response to TLR ligands between human CD4(+) T cells isolated either by IMACS (IMACS-CD4(+)) or by IMACS followed by FACS (IMACS/FACS-CD4(+)). As expected, IMACS-CD4(+) were less pure than IMACS/FACS-CD4(+) (92.5%+/-1.4% versus 99.7%+/-0.2%, respectively). Consequently, IMACS-CD4(+) proliferated and produced cytokines in response to mitogen alone and had lower activation requirements compared to IMACS/FACS-CD4(+). In addition IMACS-CD4(+) but not IMACS/FACS-CD4(+) responses were upregulated by the TLR-4 ligand lipopolysaccharide (LPS). On the other hand, TLR-2 and TLR-5 engagement induced costimulation in both IMACS-CD4(+) and highly purified IMACS-/FACS-CD4(+). Altogether these results indicate that small differences in cell purity can significantly alter T cell responses to TLR ligands. This study stresses the importance of a stringent purification method when investigating the role of microbial ligands in T cell function.

Blockade of the 4-1BB (CD137)/4-1BBL and/or CD28/CD80/CD86 costimulatory pathways promotes corneal allograft survival in mice

To explore the roles of 4-1BB (CD137) and CD28 in corneal transplantation, we examined the effect of 4-1BB/4-1BB ligand (4-1BBL) and/or CD28/CD80/CD86 blockade on corneal allograft survival in mice. Allogeneic corneal transplantation was performed between two strains of wild-type (WT) mice, BALB/c and C57BL/6 (B6), and between BALB/c and B6 WT donors and various gene knockout (KO) recipients. Some of the WT graft recipients were treated intraperitoneally with agonistic anti-4-1BB or blocking anti-4-1BBL monoclonal antibody (mAb) on days 0, 2, 4 and 6 after transplantation. Transplanted eyes were observed over a 13-week period. Allogeneic grafts in control WT B6 and BALB/c mice treated with rat immunoglobulin G showed median survival times (MST) of 12 and 14 days, respectively. Allogeneic grafts in B6 WT recipients treated with anti-4-1BB mAb showed accelerated rejection, with an MST of 8 days. In contrast, allogeneic grafts in BALB/c 4-1BB/CD28 KO and B6 CD80/CD86 KO recipients had significantly prolonged graft survival times (MST, 52.5 days and 36 days, respectively). Treatment of WT recipients with anti-4-1BB mAb resulted in enhanced cellular proliferation in the mixed lymphocyte reaction and increased the numbers of CD4(+) CD8(+) T cells, and macrophages in the grafts, which correlated with decreased graft survival time, whereas transplant recipients with costimulatory receptor deletion showed longer graft survival times. These results suggest that the absence of receptors for the 4-1BB/4-1BBL and/or CD28/CD80/CD86 costimulatory pathways promotes corneal allograft survival, whereas triggering 4-1BB with an agonistic mAb enhances the rejection of corneal allografts.

Gal knockout and beyond

Recently, Galalpha1-3Galbeta1-4GlcNAc (Gal) knockout (k/o) pigs have been developed using genetic cloning technologies. This remarkable achievement has generated great enthusiasm in xenotransplantation studies. This review summarizes the current status of nonhuman primate experiments using Gal k/o pig organs. Briefly, when Gal k/o pig organs are transplanted into primates, hyperacute rejection does not occur. Although graft survival has been prolonged up to a few months in some cases, the overall results were not better than those using Gal-positive pig organs with human complement regulatory protein transgenes. Gal k/o pig kidneys rapidly developed rejection which was associated with increased anti-non-Gal antibodies. Although the precise mechanisms of Gal k/o pig organ rejection are not clear, it could result from incomplete deletion of Gal, up-regulation of new antigen (non-Gal antigen) and/or production of non-Gal antibodies. Future work in xenotransplantation should place emphasis on further modification of donors, such as combining human complement regulatory genes with Gal k/o, deleting non-Gal antigens and adding protective/surviving genes or a gene that inhibits coagulation. Induction of donor-specific T- and B-cell tolerance and promotion of accommodation are also warranted.

Prolonged diabetes reversal after intraportal xenotransplantation of wild-type porcine islets in immunosuppressed nonhuman primates

Cell-based diabetes therapy requires an abundant cell source. Here, we report reversal of diabetes for more than 100 d in cynomolgus macaques after intraportal transplantation of cultured islets from genetically unmodified pigs without Gal-specific antibody manipulation. Immunotherapy with CD25-specific and CD154-specific monoclonal antibodies, FTY720 (or tacrolimus), everolimus and leflunomide suppressed indirect activation of T cells, elicitation of non-Gal pig-specific IgG antibody, intragraft expression of proinflammatory cytokines and invasion of infiltrating mononuclear cells into islets.

Distinct effects of CD86-mediated costimulation on resting versus activated human CD4+ T cells

CD80 and CD86 are important in the initiation of T cell immunity. Although their costimulatory function has long been appreciated, it remains unclear whether the biological significance of the two B7 isoforms resides in their different patterns and kinetics of expression or whether differences exist in their function. We have addressed this issue using HLA-DR1 transfectants co-expressing CD80, CD86, or both molecules as stimulators for naïve, memory, and activated human CD4+ T cells. Both CD80 and CD86 efficiently costimulated alloresponses by unseparated peripheral blood CD4+ T cells; however, CD86 was substantially inferior in costimulating alloresponses by separated memory T cells, and completely incompetent in costimulating three human T cell clones. Furthermore, CD80/CD86 double transfectants stimulated lower responses by the clones than cells expressing CD80 alone. That CD86 was actively inhibitory rather than merely neutral was evidenced by the increase in response to the double CD80/CD86 APC when anti-CD86 antibody was added. Furthermore, addition of anti-CTLA-4 Fab to cultures of HLA-DR1 transfectants co-expressing CD86, fully restored the proliferative response. These results indicate that CD80 and CD86 mediate distinct signals in previously activated T cells, and demonstrate that CTLA-4 ligation may dominate the outcome of CD86-mediated costimulation of activated CD4+ T cells.

Acquisition of HLA-DR and costimulatory molecules by T cells from allogeneic antigen presenting cells

There is accumulating evidence that cell surface molecules may be transferred between cells during an encounter. The aim of these experiments was to determine whether transfer of allogeneic material to T cells could influence human alloresponses. CD4(+) cells were cocultured with M1 cell (human fibroblast) transfectants expressing HLA-DR1, CD80 and CD86 alone or in combination. Up to 95% of the allogeneic T cells became positive for HLA-DR and the appropriate costimulatory molecules after only 4 h of coculture. The phenomenon required cell contact and cell membrane fluidity because transfer was abolished by transwell separation of the M1 cells and the T cells or by pre-treatment of the APC with paraformaldehyde. Flow cytometric sorting of T cells after coculture and subsequent mixed lymphocyte assays demonstrated that the T cells that had acquired both HLA-DR and costimulatory molecules could act as potent antigen presenting cells. Finally, matured human dendritic cells were also shown to transfer these molecules to CD4(+) cells, which could then act as antigen presenting cells for unprimed T cells and for a cell line specific for an HLA-peptide complex acquired from the DCs. Taken together, these data suggest a novel pathway for the amplification of human alloresponses.

Achieving permanent survival of islet xenografts by independent manipulation of direct and indirect T-cell responses

Recent success in pancreatic islet allotransplantation has raised expectations but has equally highlighted the acute shortage of donor tissue. The use of xenogeneic tissue would help to address this shortage; however, strong cellular immunity limits the application of this approach. T-cell responses to xenogeneic tissues involve recognition of intact species-mismatched major histocompatibility complex (MHC) molecules, the direct pathway, and xenogeneic proteins presented as peptides by responder-type MHC molecules, the indirect pathway. In this study, we exploited the species difference to selectively and sequentially inhibit direct and indirect xenoresponses after transplantation of porcine islets into mice. Selective inhibition of the direct response was achieved using porcine CTLA4-Ig, which binds preferentially to pig versus mouse B7 molecules. Selective inhibition of the indirect response was achieved using murine CTLA4-Ig, which binds preferentially to mouse B7 molecules. Administration of porcine CTLA4-Ig alone caused modest prolongation of islet survival. Injection of murine CTLA4-Ig alone had a minimal effect. However, the injection of the porcine fusion protein early and the murine homolog late after grafting led to permanent survival of the porcine islets, in the absence of any other immunosuppression. These results suggest that a similar approach could have clinical utility in porcine islet xenotransplantation.

Blockade of indirect recognition mediated by CD4+ T cells leads to prolonged cardiac xenograft survival

The T-cell response to xenografts is induced by direct and indirect recognition of xenoantigens. Although the importance of indirect recognition is well established in vitro, the contribution of this pathway to xenograft rejection in vivo remains to be fully elucidated. We herein investigated the direct contribution of indirect recognition to cardiac xenograft rejection in the rat-to-mouse (PVG.R8-to-C57BL/10) concordant model. Rat xenoantigens invoked a vigorous proliferative response in mouse T cells harvested from naïve or graft recipients at rejection. Indirect recognition predominated the response, as antibodies against mouse class II I-A(b), CD80, or CD86 molecules significantly (45 to 60%) blocked the proliferative response. Importantly, the blockade of indirect recognition in vivo by treating the graft recipients with a monoclonal antibody (mAb) against class II I-A(b) molecule on days 0, 1, and 3 post-transplantation resulted in significant (P < 0.009) prolongation of cardiac xenograft survival (Mean Survival Time (MST) >94 +/- 55 days vs. 7 +/- 0.8 days for controls). In contrast, treatment of recipients with a mAb against mouse class I H-2K(b)/D(b) molecules did not significantly affect graft rejection (MST = 8 +/- 1 days). These results demonstrate that indirect recognition mediated by CD4(+) T cells plays a critical role in the rejection of cardiac grafts in the rat-to-mouse xenogeneic model.

Xenogeneic cells and superantigen induce human T-cell activation in the absence of T-cell recognition of xenoantigen

The extent of interaction between human T-lymphocytes and xenogeneic antigen-presenting cells (APCs) is species-dependent. A successful interaction requires high-affinity receptor-ligand pairing across species and recognition of antigens presented by foreign major histocompatibility complex (MHC). A deficient human T-lymphocyte response to xenogeneic cells is likely the result of a defect in these interactions. However, the requirements for a T-cell response to superantigen (SAg) may differ from those of other T-cell responses. Using irradiated murine splenocytes, which are believed to be incapable of eliciting human T-cell responses, and porcine aortic endothelial cells (PAECs) as the APC populations, we studied the human T-lymphocyte response to antigens presented by these cells. Direct proliferation of human T-lymphocytes to SAg presented by murine APCs was demonstrated; it was blocked by anti-human LFA-1 and anti-murine MHC class II but not by anti-human MHC class II. PAECs also presented SAg to human T-cells, generating a proliferative response greater than the primary response to porcine xenoantigen. Culture of human T-cells with murine splenocytes or PAECs and SAg Staphylococcus enterotoxin A (SEA) for 7 days primed human T-cells to proliferate in a secondary culture in response to autologous APCs. This autologous secondary response was human MHC class II-dependent and was inhibited by anti-human LFA-1, anti-human CD2, and anti-human CD98. Surprisingly, both of these responses were also blocked by anti-SEA, suggesting that despite vigorous washing, a small amount of functionally important SAg was carried over from primary to secondary culture, probably bound to the surface of T-cells. Xenogeneic APCs, even those that fail to stimulate human T-cells directly, can serve as APCs for primary human T-cell responses. After such interactions T-cells can develop secondary responses in autologous interactions based on retention of minute amounts of SAg. Such interactions may have important implications for xenotransplantation.

Xenotransplantation

The continued and growing success of lung allotransplantation has intensified the worldwide shortage of donor organs. Yet, xenotransplantation remains a daunting challenge. Additional molecular incompatibilities and unforeseen complications will continue to be discovered. Progress has been made, notably on the generation of alpha-Gal double knockout pigs. Progressive increases in organ survival times have been seen for most organs after significant investments of time and money. The lung continues to be an organ with the lowest supply of cadaveric donors and the least potential for expanded living donation or mechanical alternatives. As such, the impetus for xenotransplantation is strong. The lung appears to be exquisitely sensitive to xenograft rejection and resistant to strategies that have been moderately successful in other organs. A complex program involving genetically modified donor organs, recipient preparation for antibody removal or tolerance promotion, and multitargeted drug therapy will likely be required for successful clinical application.

Cross-species costimulation: relative contributions of CD80, CD86, and CD40

BACKGROUND

The response of human CD4+ T cells against porcine cells is of comparable magnitude to that induced by human leukocyte antigen-mismatched allogeneic cells. This reflects productive interactions between key costimulatory molecules across the species barrier. Inhibition of these molecular interactions will be crucial in overcoming CD4+ T-cell-mediated rejection of xenografts. We have performed a detailed investigation to determine the expression profiles and relative contributions of the three key costimulatory molecules in the porcine-human xenogeneic response. Whereas only porcine CD86 is constitutively expressed on resting endothelial cells, both CD40 and CD80 are rapidly expressed after activation. All three costimulatory molecules are expressed by professional antigen-presenting cells.

METHODS

We have isolated full-length cDNA clones for human and porcine CD80, CD86, and CD40. Human fibroblast cell lines (M1) coexpressing DR1 were transfected with these cDNAs and used in mixed lymphocyte reactions and flow cytometric studies in vitro.

RESULTS

These data provide the first characterization of the expression profile and functional role of porcine CD80. Functional assays demonstrate that pCD40, pCD80, and pCD86 are independently capable of costimulating human CD4+ T cells, albeit with differing kinetics. Proliferative responses were of comparable magnitude to those obtained when costimulation was provided by human CD40, CD80, and CD86.

CONCLUSIONS

These data have implications for therapy targeting the direct pathway of xenorecognition; costimulatory molecule blockade must be directed against both the B7/CD28 and CD40/CD40L pathways.

Co-stimulatory molecules in islet xenotransplantation: CTLA4Ig treatment in CD40 ligand-deficient mice

Previous work has demonstrated that short-term systemic administration of cytotoxic T lymphocyte antigen-4 (CTLA-4) Ig blocks human pancreatic islet xenograft rejection in mice and induces long-term, donor-specific tolerance, whereas studies on pig pancreatic islet rejection in mice have failed to demonstrate a role for CTLA4Ig in preventing rejection. Treatment with anti-CD40 ligand (L) monoclonal antibodies alone is somewhat effective in prolonging the survival of islet xenografts, but ineffective when applied to skin xenografts. However, simultaneous blockade of the CD28 and CD40 co-stimulatory pathways prolongs the survival of pig skin on recipient mice. To evaluate the role of CD28 and CD40 co-stimulatory pathways in pig islet-like cell cluster (ICC) xenograft rejection in mice, CD40L-deficient mice transplanted with fetal porcine ICCs were given posttransplant treatment with human (h) CTLA4Ig or a human IgG1 chimeric mAb (hL6). Xenografts were evaluated 6 or 12 days after transplantation. Fetal porcine ICC xenografts were protected from rejection in hCTLA4Ig-treated CD40L-deficient mice, whereas xenograft rejection persisted in untreated CD40L-deficient mice. Simultaneous blockade of the CD28 and CD40 co-stimulatory pathways is mandatory to inhibit ICC xenograft rejection in the pig-to-mouse model, because the CD28 and CD40 co-stimulatory pathways seem capable of efficiently substituting for one another.

The functional roles of porcine CD80 molecule and its ability to stimulate and regulate human anti-pig cellular response

BACKGROUND

The pig is currently considered to be the most likely candidate for a xenogenic-organ source. Anti-pig human T-cell response via co-stimulatory molecules has been studied with great interest. The soluble form of porcine CD80 has recently been cloned and characterized, but the sequence of the transmembrane form has not been determined. The purpose of this study was to investigate the functional interaction between porcine CD80 and human T cells using the full-length clone of porcine CD80.

MATERIALS AND METHODS

Specific complementary DNA (cDNA) clones encoding porcine CD80 were isolated and sequenced using rapid amplification of cDNA ends-polymerase chain reaction. Polymerase chain reaction-amplified cDNA coding for the open reading frame of the porcine CD80 transmembrane form was subcloned into an expression vector and then transfected into Chinese hamster ovary (CHO) cells. CHO cells transfected with porcine CD80 (CHO-pCD80) were co-cultured with human CD4+ T cells and then interleukin-2 secretion was measured and transferred pCD80 expression in these human T cells was detected by flow cytometry.

RESULTS

We cloned and determined the complete nucleotide sequence for the transmembrane form of porcine CD80. Results from our T-cell co-stimulatory assay showed significant interleukin-2 production when co-stimulated with CHO-pCD80. Human naïve CD4+ T cells acquired xenogenic pCD80 molecules in the process of T-cell activation.

CONCLUSIONS

Findings from this study seem to suggest that pCD80 has the functional ability to regulate human anti-pig cellular response. In addition, genetic manipulation of porcine co-stimulatory molecules offers a potentially new therapeutic strategy to prevent xenogeneic rejection across species.

Clinical xenotransplantation: pigs might fly?

Xenotransplantation has the potential to deliver an unlimited supply of organs for transplantation. However, this promise has yet to translate into clinical application, despite substantial research efforts in the last decade. Although increasing numbers of studies are being performed in relevant pre-clinical (pig-to-primate) transplantation models, so far these have highlighted the apparent elusiveness of long-term xenograft survival. Humoral rejection remains the main obstacle to success, but control of T cell-mediated rejection will be a problem in the future and there are major concerns about the possible transmission of porcine endogenous retroviruses (PERV) and other infectious agents. This article reviews recent advances in the understanding of acute vascular rejection (AVR), acute T cell-mediated rejection and PERV transmission and highlights some of the strategies that may prove successful in overcoming these problems. Although progress has been slow, the promise of an inexhaustible supply of organs is sufficient reason to continue research in these areas. Assuming the specific problem of AVR can be ameliorated by one of a number of strategies currently under investigation, there are grounds to believe that xenotransplantation will become a clinical reality. Pig xenografts, currently grounded, might eventually fly!

Targeted delivery of anti-CTLA-4 antibody downregulates T cell function in vitro and in vivo

CTLA-4 is a T cell surface molecule that binds to the costimulatory molecules CD80 and CD86 on antigen-presenting cells and downregulates T cell function. Therefore, we wanted to test whether antigen-specific activated T cells could be inhibited through directed CTLA-4 signaling using a bispecific antibody (BiAb) capable of simultaneously binding to CTLA-4 and a tissue-specific antigen. The BiAb was prepared by linking two separate monoclonal antibodies against CTLA-4 and the thyroid-stimulating hormone receptor (TSHR). The mouse B cell lymphoma line M12 (H2(d)) was used to induce alloreactive T cells in CBA/J mice (H2(k)); M12 cells stably transfected with the cDNA encoding murine TSHR (mM12) were used to restimulate the alloresponse in vitro. Results of assays for in vitro T cell proliferation, IL-2 production, and cytotoxicity in the presence of BiAb demonstrated that the BiAb could inhibit the T cell alloresponse when stimulated with mM12 cells but not with M12 cells. This effect was dependent on binding of TSHR-bound BiAb to CTLA-4, since the addition of soluble CTLA-4-Ig blocked the inhibitory effect. Injection of mM12 cells, along with the BiAb, not with antibodies against TSHR or CTLA-4 either separately or together, into CBA/J mice (H2(k)) downregulated alloreactive T cell responses. Our study demonstrated that the presence of CTLA-4 signaling molecules on the surface of target cells can protect those cells from immune attack by antigen-specific T cells and suggested that a similar approach could have potential therapeutic value in transplant rejection and tissue-specific autoimmune diseases.

Complexities of CD28/B7: CTLA-4 costimulatory pathways in autoimmunity and transplantation

Recent advances in the understanding of T cell activation have led to new therapeutic approaches in the treatment of immunological disorders. One attractive target of intervention has been the blockade of T cell costimulatory pathways, which result in more selective effects on only those T cells that have encountered specific antigen. In fact, in some instances, costimulatory pathway antagonists can induce antigen-specific tolerance that prevents the progression of autoimmune diseases and organ graft rejection. In this review, we summarize the current understanding of these complex costimulatory pathways including the individual roles of the CD28, CTLA-4, B7-1 (CD80), and B7-2 (CD86) molecules. We present evidence that suggests that multiple mechanisms contribute to CD28/B7-mediated T cell costimulation in disease settings that include expansion of activated pathogenic T cells, differentiation of Th1/Th2 cells, and the migration of T cells into target tissues. Additionally, the negative regulatory role of CTLA-4 in autoimmune diseases and graft rejection supports a dynamic but complex process of immune regulation that is prominent in the control of self-reactivity. This is most apparent in regulation of the CD4(+)CD25(+)CTLA-4(+) immunoregulatory T cells that control multiple autoimmune diseases. The implications of these complexities and the potential for use of these therapies in clinical immune intervention are discussed.