The threshold for autoimmune T cell killing is influenced by B7-1

Autoimmune Diabetes: An Overview of Experimental Models and Novel Therapeutics

Type 1 diabetes (T1D) results from a chronic and selective destruction of insulin-secreting β-cells within the islets of Langerhans of the pancreas by autoreactive CD4(+) and CD8(+) T lymphocytes. The use of animal models of T1D was instrumental for deciphering the steps of the autoimmune process leading to T1D. The non-obese diabetic (NOD) mouse and the bio-breeding (BB) rat spontaneously develop the disease similar to the human pathology in terms of the immune responses triggering autoimmune diabetes and of the genetic and environmental factors influencing disease susceptibility. The generation of genetically modified models allowed refining our understanding of the etiology and the pathogenesis of the disease. In the present review, we provide an overview of the experimental models generated and used to gain knowledge on the molecular and cellular mechanisms underlying the breakdown of self-tolerance in T1D and the progression of the autoimmune response. Immunotherapeutic interventions designed in these animal models and translated into the clinical arena in T1D patients will also be discussed.

A two-pronged mechanism for HIV-1 Nef-mediated endocytosis of immune costimulatory molecules CD80 and CD86

The Nef protein of HIV-1 mediates immune evasion by relocating major histocompatibility complex (MHC) molecules and the immune costimulatory molecules CD80 and CD86 away from the monocytic cell surface. We describe a two-pronged mechanism by which Nef removes CD80 and CD86 from the cell surface. While MHCI, CD80, and CD86 are all internalized via a dynamin-independent pathway, the endocytic mechanism used for costimulatory molecules is distinct from MHCI relocation. Nef expression results in the activation and actin-dependent translocation of Src kinase to the cell periphery. At the cell surface, Src promotes Rac activation via TIAM, a guanine nucleotide exchange factor for Rac. Nef also binds to the cytosolic tails of CD80 and CD86, triggering their endocytosis via Rac-based actin polymerization. These data reveal the use of an unusual molecular mechanism triggered in the host cell by HIV to affect its viral immune evasion strategy and suggest approaches for its subversion.

CD86 has sustained costimulatory effects on CD8 T cells

CD80 and CD86 both costimulate T cell activation. Their individual effects in vivo are difficult to study as they are coordinately up-regulated on APCs. We have studied mice expressing rat insulin promoter (RIP)-CD80 and RIP-CD86 on the NOD and NOD.scid genetic background to generate in vivo models, using diabetes as a readout for cytotoxic T cell activation. Accelerated spontaneous diabetes onset was observed in NOD-RIP-CD80 mice and the transfer of diabetes from 6-wk-old NOD mice to NOD.scid-RIP-CD80 mice was greater compared with NOD-RIP-CD86 and NOD.scid-RIP-CD86 mice, respectively. However, the secondary in vivo response was maintained if T cells were activated through CD86 costimulation compared with CD80. This was demonstrated by greater ability to cause recurrent diabetes in NOD-RIP-CD86 diabetic mice transplanted with 6-wk-old NOD islets and adoptively transferred diabetes from diabetic NOD-RIP-CD86 mice to NOD.scid mice. In vitro, CD80 costimulation enhanced cytotoxicity, proliferation, and cytokine secretion in activated CD8 T cells compared with CD86 costimulation. We demonstrated increased CTLA-4 and programmed death-1 inhibitory molecule expression following costimulation by both CD80 and CD86 (CD80 > CD86). Furthermore, T cells stimulated by CD80 were more susceptible to inhibition by CD4(+)CD25(+) T cells. Overall, while CD86 does not stimulate an initial response as strongly as CD80, there is greater sustained activity that is seen even in the absence of continued costimulation. These functions have implications for the engineered use of costimulatory molecules in altering immune responses in a therapeutic setting.

Cytotoxic T-cells from T-cell receptor transgenic NOD8.3 mice destroy beta-cells via the perforin and Fas pathways

Cytotoxic T-cells are the major mediators of beta-cell destruction in type 1 diabetes, but the molecular mechanisms are not definitively established. We have examined the contribution of perforin and Fas ligand to beta-cell destruction using islet-specific CD8(+) T-cells from T-cell receptor transgenic NOD8.3 mice. NOD8.3 T-cells killed Fas-deficient islets in vitro and in vivo. Perforin-deficient NOD8.3 T-cells were able to destroy wild-type but not Fas-deficient islets in vitro. These results imply that NOD8.3 T-cells use both pathways and that Fas is required for beta-cell killing only when perforin is missing. Consistent with this theory, transgenic NOD8.3 mice with beta-cells that do not respond to Fas ligation were not protected from diabetes. We next investigated the mechanism of protection provided by overexpression of suppressor of cytokine signaling-1 (SOCS-1) in beta-cells of NOD8.3 mice. SOCS-1 islets remained intact when grafted into NOD8.3 mice and were less efficiently killed in vitro. However, addition of exogenous peptide rendered SOCS-1 islets susceptible to 8.3 T-cell-mediated lysis. Therefore, NOD8.3 T-cells use both perforin and Fas pathways to kill beta-cells and the surprising blockade of NOD8.3 T-cell-mediated beta-cell death by SOCS-1 overexpression may be due in part to reduced target cell recognition.

Arming Tumor-Reactive T Cells with Costimulator B7-1 Enhances Therapeutic Efficacy of the T Cells

T cells ectopically expressing costimulators are pathogenic and contribute to autoimmunity against self-antigens. Given that tumor antigens are often self-antigen or mutated self-antigens, we hypothesize that neoexpressing a costimulator on tumor-reactive T cells may likewise enhance their reactivity to tumor. To test this hypothesis, we have expressed B7-1 on OT-1 CD8+ T-cell receptor transgenic T cells via protein transfer (or protein "painting"). Naïve OT-1 T cells, after being painted with B7-1, can self-costimulate themselves, elicit enhanced proliferative and CTL responses to E.G7-ovalbumin tumor cells (expressing a cognate antigen), and become resistant to CD4+CD25+ regulatory T-cell-mediated suppression. Importantly, these T cells, when coimplanted with E.G7-ovalbumin tumor cells into a syngeneic host, are three to nine times more potent than are control T cells (mock painted with human IgG) in inhibiting tumor growth. Further, on transfer into mice bearing established E.G7-ovalbumin tumors, B7-1-painted ex vivo-amplified OT-1 T cells induced complete tumor regression in 65% of treated mice, whereas the control T cells did so in only 28% of treated mice. Finally, on transfer into mice bearing less immunogenic 4T1 breast tumors, B7-1-painted tumor-reactive CD8+ T cells improved the survival of treated mice to a greater extent than did the control T cells. Hence, this study establishes that arming tumor-reactive T cells with a costimulator can enhance their antitumor efficacy.

Use of recombinant lentivirus pseudotyped with vesicular stomatitis virus glycoprotein G for efficient generation of human anti-cancer chimeric T cells by transduction of human peripheral blood lymphocytes in vitro

BACKGROUND

Genetic redirection of lymphocytes that have been genetically engineered to recognize antigens other than those originally programmed in their germlines is a potentially powerful tool for immunotherapy of cancers and potentially also of persistent viral infections. The basis for this procedure is that both cancers and some viruses have developed strikingly similar mechanisms of evading attacks by host immune mechanisms. To redirect human peripheral blood lymphocytes (PBLs) with a chimeric T cell receptor (chTCR) so that they recognize a new target requires a high degree of transfection efficiency, a process that is regarded as technically demanding.

RESULTS

Infection with a retroviral vector carrying a chTCR cassette was shown to transduce 100% of rapidly dividing murine T cells but typically, only approximately 10% of PBLs could be infected with the same vector. In contrast with other retroviruses, lentiviruses integrate their genomes into non-dividing cells. To increase host cell range, vesicular stomatitis virus G protein was pseudotyped with a lentivirus vector, which resulted in approximately 100% PBL transduction efficiency. Signaling of PBLs bearing chimeric receptors was shown by specific proliferation on exposure to cells expressing cognate ligand. Further, T-bodies against CEA showed a startling ability to cause regression of malignant colon tumors in a nude mouse model of human cancer.

CONCLUSION

A lentivirus/VSV pseudotyped virus, which does not require replicating cells for integration of its genome, efficiently transduced a high proportion of human PBLs with chTCRs against CEA. PBLs transduced by infection with a lentivirus/VSV pseudotyped vector were able to proliferate specifically in vitro on exposure to CEA-expressing cells and further they had a startling therapeutic effect in a mouse model of human colon cancer.

A phase I/II trial of oxidized autologous tumor vaccines during the "watch and wait" phase of chronic lymphocytic leukemia

Based on their activity in patients with advanced stage chronic lymphocytic leukemia (CLL), a phase I/II study was designed to evaluate the feasibility, safety, and efficacy of autologous vaccines made from oxidized tumor cells in patients with earlier stage CLL, and to determine an optimal schedule of injections. Eighteen patients (at risk for disease progression and with white blood cell counts between 15 and 100 x 10(6) cells/ml) were injected intramuscularly with 10 ml of oxidized autologous blood (composed mainly of CLL cells) either 12 times over 6 weeks (group 1), 12 times over 16 days (group 2), or 4 times over 6 weeks (group 3). Fourteen out of eighteen patients had Rai stage 0-II disease, while 4/18 had stage III-IV disease but did not require conventional treatment. Partial clinical responses, associated with enhanced anti-tumor T cell activity in vitro, were observed in 5/18 patients of whom three were in group 2. Stable disease was observed in six patients while disease progression appeared not to be affected in the remaining patients. Toxicity was minimal. Vaccination with oxidized autologous tumor cells appears worthy of further investigation and may be a potential alternative to a "watch and wait" strategy for selected CLL patients.

The influence of the major histocompatibility complex on development of autoimmune diabetes in RIP-B7.1 mice

The most important genetic susceptibility factor for type 1 diabetes is encoded in the major histocompatibility complex (MHC). The nonobese diabetic (NOD) mouse, which develops spontaneous diabetes, expresses H-2g7 comprising the MHC class I molecules Kd and Db and the MHC class II molecule I-Ag7. However, neither B6.H-2g7 mice, in which H-2g7 is expressed on the C57BL/6 genetic background, nor the nonobese resistant (NOR) mouse, in which H-2g7 is expressed on a genetic background that is 88% similar to NOD mice, develop diabetes. Immune tolerance can be broken in these diabetes-resistant mice expressing H-2g7 if the costimulatory molecule B7.1 is present on the islet beta cells. This does not occur if only single MHC class I components of the H-2g7 haplotype are present, such as Kd in BALB/c mice or Db in C57BL/6 mice, both of which develop only a low level of diabetes when B7.1 is expressed. The presence of I-Ag7 leads to the development of an autoimmune T-cell repertoire, and local costimulation of CD8 T-cells precipitates aggressive diabetes. This implies that a major role of the MHC class II molecules in diabetes is the development of an autoreactive T-cell repertoire.

Dysregulated B7-1 and B7-2 Expression on Nonobese Diabetic Mouse B Cells Is Associated with Increased T Cell Costimulation and the Development of Insulitis

Little is known about the pathogenic role of B cell dysfunction in T cell-mediated autoimmune disease. We previously reported that B cell hyper-responsiveness, resistance to apoptosis, and accumulation in islets occur during the onset of insulitis, but not in type 1 diabetes (T1D), in NOD mice. In this study we extended these studies to further determine how islet-infiltrated B cells contribute to this inflammatory insulitis. We demonstrate the presence of an increased percentage of B7-1(+) and a decreased percentage of B7-2(+) B cells in the spleen of autoimmune disease-prone NOD and nonobese diabetes-resistant mice compared with the spleen of nonautoimmune disease-prone C57BL/6 and BALB/c mice. An age-dependent differential expression of B7-1 and B7-2 was associated with the development of insulitis and CD4(+)CD25(+) T cell deficiency in autoimmune disease-prone mice. Whereas BCR and LPS stimulation increased B7-2 expression on B cells from autoimmune disease-prone and nonautoimmune disease-prone mice, LPS-induced B7-1 expression was higher on NOD than C57BL/6 B cells. Interestingly, increased expression of B7-1 and B7-2 was found on islet-infiltrated B cells, and this increase was associated with enhanced T cell costimulation. Islet-infiltrated B cells were shown to be a source of TNF-alpha production in islets. B7 blockade of BCR-stimulated NOD B cells by anti-B7-1 and anti-B7-2 mAbs during coadoptive transfer with diabetogenic T cells into NOD.scid mice protected these recipients from T1D. These results suggest that increased B7-1 and B7-2 expression on islet-infiltrated NOD B cells is associated with increased T cell costimulation and the development of inflammatory insulitis in NOD mice.

Molecular cloning and mRNA tissue-expression of two isoforms of the ovine costimulatory molecule CD80 (B7-1)

Using RT-PCR and rapid amplification of cDNA ends (RACE), we cloned two putative alternatively spliced transcripts of the sheep CD80 (B7-1) molecule that encode both transmembrane (TM) and secreted (s) forms of CD80 protein. Comparison of the amino acid sequence of the TM form of ovine CD80 with the sequence of cattle, swine and human CD80 indicated that the deduced protein had a higher degree of similarity to cattle (87% of amino acid identity) than to pig (68%) and human sequence (53% of homology). In tissues, RT-PCR using primers for the TM and the sCD80 transcripts indicated that the expression of both CD80 transcripts was almost exclusively expressed in the hematolymphoid system, with the exception of the uterus. The sCD80 transcript was expressed in peripheral blood mononuclear cells (PBMC), uterus and lymph node, whereas the TM-CD80 transcript was very weakly detected only in PBMC cells. Our result indicates that mRNA transcripts encoding both membrane-bound and secreted CD80 proteins are expressed in sheep like in other animals. However, in contrast with the CD80 molecules from other species, the secreted form of sheep CD80 seems to be the predominant form expressed in the ovine PBMC and other tissues, suggesting that the TM-CD80 represents a rare transcript in this species. Interestingly, the expression of both forms of the CD80 molecule was not affected by treatment of sheep PBMC with Concanavalin A (ConA), as detected by RT-PCR. This is the first report describing the identification of a B7 costimulatory transcript in sheep.

Genetic differentiation of poly I:C from B:9-23 peptide induced experimental autoimmune diabetes

Type 1 diabetes is an immune-mediated disease, in which T cells of the adaptive immune system mediate beta cell destruction. Recently the innate immune system has been linked to etiopathogenesis of several autoimmune diseases including type 1 diabetes, as innate effector cells (e.g. dendritic cells, monocytes/macrophages and NK cells) can prime and promote or regulate (auto)immune responses. We have previously developed an experimental autoimmune diabetes (EAD) model with insulin peptide B:9-23 immunization in transgenic H-2(d)mice expressing the costimulatory molecule B7.1 in their islets (under the Rat Insulin Promotor, RIP). We compared the induction of diabetes with polyinosinic-polycytidylic acid (Poly I:C), a mimic of double stranded viral RNA versus insulin B:9-23 peptide in mice following backcrossing of the B7.1 transgene on to BALB/c mice from original B7.1 C57Bl/6 mice. We find that diabetes induction by Poly I:C is C57Bl/6 associated, whereas B:9-23 peptide induced diabetes and induction of insulin autoantibodies (IAA) are dependent on BALB/c genes. This B:9-23 peptide induced diabetes is consistent with MHC class II H-2(d)being necessary for the response to this peptide. Of note Poly I:C induction of diabetes was lost while B:9-23 induction was retained with backcrossing to BALB/c mice. Interaction of genes and environment (antigenic epitope and viral mimic) can be important in the pathogenesis of immune mediated diabetes and activation of the innate immune system (e.g. Poly I:C) may be one key determinant.

Effect of IL-2Rβ -binding cytokines on costimulatory properties of chronic lymphocytic leukaemia cells: implications for immunotherapy

Weak immunogenicity of chronic lymphocytic leukaemia (CLL) cells may contribute to disease progression and inhibit the effectiveness of immunotherapies, such as vaccines. Agents that can enhance the antigen presenting capabilities of CLL cells might then help to improve the clinical results of immunotherapies. This study investigated the effects of the common gamma chain-binding cytokines, interleukin (IL)-2 and IL-15, on costimulatory properties of primary CLL cells from 51 patients. IL-2 improved the ability of CLL cells to stimulate T cell proliferation and increased the expression of costimulatory molecules (particularly CD80) in a dose-dependent fashion, especially in CLL cells with weak expression of CD38. CD80 and CD86 induction by IL-2 were positively regulated through the mitogen-activated protein kinase pathway, while CD86 expression was negatively regulated through Janus kinase pathways. However, further activation with protein kinase C agonists was required for IL-2 activated CLL cells to stimulate autologous T cells sufficiently to clear bystander CLL cells from mixed lymphocyte responses. IL-15 had similar effects on the costimulatory properties of CLL cells. These results suggest a role for IL-2, or IL-15, in immunotherapeutic strategies for CLL.

B7-CD28 Interaction Promotes Proliferation and Survival but Suppresses Differentiation of CD4−CD8− T Cells in the Thymus

Costimulatory molecules play critical roles in the induction and effector function of T cells. More recent studies reveal that costimulatory molecules enhance clonal deletion of autoreactive T cells as well as generation and homeostasis of the CD25(+)CD4(+) regulatory T cells. However, it is unclear whether the costimulatory molecules play any role in the proliferation and differentiation of T cells before they acquire MHC-restricted TCR. In this study, we report that targeted mutations of B7-1 and B7-2 substantially reduce the proliferation and survival of CD4(-)CD8(-) (double-negative (DN)) T cells in the thymus. Perhaps as a result of reduced proliferation, the accumulation of RAG-2 protein in the DN thymocytes is increased in B7-deficient mice, which may explain the increased expression of TCR gene and accelerated transition of CD25(+)CD44(-) (DN3) to CD25(-)CD44(-) (DN4) stage. Qualitatively similar, but quantitatively less striking effects were observed in mice with a targeted mutation of CD28, but not CTLA4. Taken together, our results demonstrate that the development of DN in the thymus is subject to modulation by the B7-CD28 costimulatory pathway.

Apoptotic cell death in conjunction with CD80 costimulation confers uveal melanoma cells with the ability to induce immune responses

Uveal melanoma is a rare malignancy with a poor prognosis despite current therapeutic intervention. The current investigation focuses on the immunogenicity of uveal melanoma cells genetically modified with recombinant adenovirus encoding CD80 (AdCD80) in contrast to their parental counterpart. We demonstrate that costimulation provided by uveal melanoma cells improved immune responses in vitro as determined by mixed lymphocyte tumour cell cultures and cytotoxic T-cell assays using lymphocytes from healthy donors and uveal melanoma patients. Flow cytometry revealed T-cell stimulation by activated CD4+ and CD8+ T cells. Additionally, autologous lymphocytes proliferated in response to CD80-expressing primary uveal melanomas, indicating that this patient group is suitable for immunotherapy. Moreover, this study utilized AdCD80 modified and parental apoptotic tumour cells, loaded onto immature dendritic cells, as a source of tumour antigen. The ability of live or apoptotic tumour cells to stimulate lymphocyte proliferation and activation was determined. Apoptotic uveal melanoma cells expressing CD80 were efficient at inducing an immune response and served as a potent immunogen. The use of apoptotic uveal melanoma cells in combination with expression of costimulatory molecules could prove a novel adjuvant therapy for the treatment of this disease.

Perinatal blockade of b7-1 and b7-2 inhibits clonal deletion of highly pathogenic autoreactive T cells

A number of in vitro studies have suggested that costimulatory molecules B7-1 and B7-2 and their receptor CD28 can promote clonal deletion, and limited in vivo studies have indicated that CD28 is involved in the clonal deletion of some T cells. However, the significance of B7-mediated clonal deletion in preventing autoimmune diseases has not been studied systematically. Here we report that the perinatal blockade of B7-1 and B7-2 substantially inhibits the clonal deletion of T cells in the thymus and leads to an accumulation of T cells capable of inducing fatal multiorgan inflammation. These results reveal a critical role for costimulatory molecules B7-1 and B7-2 in deleting pathogenic autoreactive T cells in the thymus. The critical role of B7-1 and B7-2 in T cell clonal deletion may explain, at least in part, the paradoxical increase of autoimmune disease in mice deficient for this family of costimulatory molecules, such as cytotoxic T lymphocyte associated molecule 4, CD28, and B7-2. The strong pathogenicity of the self-reactive T cells supports a central hypothesis in immunology, which is that clonal deletion plays an important role in preventing autoimmune diseases.

The level of peptide-MHC complex determines the susceptibility to autoimmune diabetes: studies in HEL transgenic mice

We report a mouse model for the spontaneous development of autoimmune diabetes: the 3A9 T cell receptor (TCR) transgenic mouse, which contains T cells that recognize the 52 - 61 family of hen egg-white lysozyme (HEL) peptides in the context of MHC class II I-A(k) molecules, was bred to the ILK3 mouse, that expresses HEL protein via the rat insulin promoter (RIP). Despite partial tolerance of 3A9 T cells in ILK3 mice, spontaneous diabetes developed in 64 % of 3A9xILK3 mice by 20 weeks of age. We provide evidence that APC from peri-pancreatic nodes have a large content of peptide-MHC complex and stimulate 3A9 T cells. We also report that cross presentation of HEL from beta cells to APC is 26-fold more efficient than presentation of soluble HEL. We previously reported on a biochemical margin of safety, based on the observation that activation of naive 3A9 T cells required 100-fold more peptide-MHC complexes than required for deletion of 3A9 thymocytes. We speculate that the high local density of autologous peptide-MHC complexes can be a determining factor that leads to the activation of autoreactive CD4 T cells and, consequently, to the development of autoimmunity.

B7H costimulates clonal expansion of, and cognate destruction of tumor cells by, CD8(+) T lymphocytes in vivo

B7H/B7RP (hereby called B7H) is a new member of the B7 family of costimulatory molecules and interacts with inducible costimulatory molecule (ICOS). Its function for CD8 T cells has not been reported. We report here that expression of B7H on the tumor cells reduced tumorigenicity and induced immunity to subsequent challenge with parental tumor cells. The immune protection correlates with an enhanced cytotoxic T lymphocyte (CTL) response against P1A, the major tumor antigen expressed in the J558 tumor. To understand the mechanism of immune protection, we adoptively transferred transgenic T cells specific for tumor antigen P1A into mice that bore P1A-expressing tumors. We found that while the transgenic T cells divided faster in mice bearing the B7H(+) tumors, optimal B7H-induced clonal expansion of P1CTL required costimulation by B7-1 and B7-2 on the endogenous host antigen-presenting cells (APCs). Interestingly, when B7H(+) and B7H(-) tumors were coinjected, P1CTL selectively eliminated the B7H(+) tumor cells. Moreover, B7H expressed on the tumor cells made them highly susceptible to destruction by CTL in vivo, even if the CTL was administrated into mice with large tumor burdens. Tumors that recurred in the P1CTL-treated mice lost transfected B7H and/or H-2L(d), the class I molecule that presents the P1A peptide. Taken together, our results reveal that B7H costimulates clonal expansion of, and cognate destruction by CD8(+) T lymphocytes in vivo.

Cancer immunotherapy: insights from transgenic animal models

A wide range of strategies in cancer immunotherapy has been developed in the last decade, some of which are currently being used in clinical settings. The development of these immunotherapeutical strategies has been facilitated by the generation of relevant transgenic animal models. Since the different strategies in experimental immunotherapy of cancer each aim to activate different immune system components, a variety of transgenic animals have been generated either expressing tumor associated, HLA, oncogenic or immune effector cell molecule proteins. This review aims to discuss the existing transgenic mouse models generated to study and develop cancer immunotherapy strategies and the variable results obtained. The potential of the various transgenic animal models regarding the development of anti-cancer immunotherapeutical strategies is evaluated.

Local costimulation reinvigorates tumor-specific cytolytic T lymphocytes for experimental therapy in mice with large tumor burdens

Cytotoxic T cells recognize tumor Ags and destroy cancer cells in vitro. Adoptive transfer studies with transgenic T cells specific for tumor Ags have demonstrated that CTL are effective only in mice with small tumor burdens and thus appear to have limited potential in cancer immunotherapy. Here we used transgenic mice that express the TCR specific for an unmutated tumor Ag P1A and multiple lineages of P1A-expressing tumors to address this critical issue. We found that local costimulation, either by expression of B7-1 on the tumor cells or by local administration of anti-CD28 mAb 37N, reinvigorated the function of CTL specific for the tumor Ag, as it substantially increased the efficacy of CTL therapy for mice with large tumor burdens. Our study suggests that CTL-based immunotherapy can be manipulated to deal with large tumors.

The regulatory role of DR4 in a spontaneous diabetes DQ8 transgenic model

MHC class II molecules are critical determinants of genetic susceptibility to human type 1 diabetes. In patients, the most common haplotype contains the DRA1*0101-DRB1*0401 (DR4) and DQA1*0301-DQB1*0302 (DQ8) loci. To assess directly the relative roles of HLA-DQ8 and DR4 for diabetes development in vivo, we generated C57BL/6 transgenic mice that lack endogenous mouse MHC class II molecules but express HLA-DQ8 and/or DR4. Neither HLA-DQ nor HLA-DR transgenic mice developed insulitis or spontaneous diabetes. However, when they were crossed to transgenic mice (C57BL/6) expressing the B7.1 costimulatory molecules on pancreatic beta cells that do not normally develop diabetes, T cells from these double transgenic mice were no longer tolerant to islet autoantigens. The majority of DQ8/RIP-B7 mice developed spontaneous diabetes, whereas only 25% of DR4/RIP-B7 mice did so. Interestingly, when DQ8 and DR4 were coexpressed (DQ8DR4/RIP-B7), only 23% of these mice developed diabetes, an incidence indistinguishable from the DR4/RIP-B7 mice. T cells from both DR4/RIP-B7 and DQ8DR4/RIP-B7 mice, unlike those from DQ8/RIP-B7 mice, exhibited a Th2-like phenotype. Thus, the expression of DR4 appeared to downregulate DQ8-restricted autoreactive T cells in DQ8DR4/RIP-B7 mice. Our data suggest that although both DQ8 and DR4 can promote spontaneous diabetes in mice with a non-autoimmune-prone genetic background, the diabetogenic effect of the DQ8 allele is much greater, whereas DR4 expression downregulates the diabetogenic effect of DQ8, perhaps by enhancing Th2-like immune responses.

The heat-stable antigen determines pathogenicity of self-reactive T cells in experimental autoimmune encephalomyelitis

Induction of myelin-specific CD4 T cells is a pivotal event in the development of experimental autoimmune encephalomyelitis (EAE). Other checkpoints in EAE pathogenesis have not been clearly defined, although multiple genetic loci are known to influence EAE development. We report here that targeted mutation of the heat-stable antigen (HSA) abrogates development of EAE despite a complete lack of effect on induction of autoimmune T cells. To test whether T-cell expression of HSA is sufficient, we created transgenic mice in which HSA is expressed exclusively in the T-cell lineage. We found that these mice remain resistant to EAE induction. Adoptive transfer studies demonstrate that both T cells and non-T cells must express HSA in order for the pathogenic T cells to execute their effector function. Moreover, HSAIg, a fusion protein consisting of the extracellular domain of the HSA and the Fc portion of immunoglobulin, drastically ameliorates the clinical sign of EAE even when administrated after self-reactive T cells had been expanded. Thus, identification of HSA as a novel checkpoint, even after activation and expansion of self-reactive T cells, provides a novel approach for immunotherapy of autoimmune neurologic diseases, such as multiple sclerosis.

Transgenic overexpression of human Bcl-2 in islet beta cells inhibits apoptosis but does not prevent autoimmune destruction

Insulin-dependent diabetes mellitus results when > 90% of the insulin-producing beta cells in the pancreatic islets are killed as a result of autoimmune attack by T cells. During the progression to diabetes, islet beta cells die as a result of different insults from the immune system. Agents such as perforin and granzymes, CD95 ligand and tumor necrosis factor-alpha, or cytokines and free-radicals have all been shown to cause beta cell apoptosis. The anti-apoptotic protein, Bcl-2, might protect against some of these stimuli. We have therefore generated transgenic mice expressing human Bcl-2 in their islet beta cells. Although Bcl-2 was able to prevent apoptosis induced by cytotoxic agents against beta cells in vitro, Bcl-2 alone could not prevent or ameliorate cytotoxic or autoimmune beta cell damage in vivo.

Differential Coupling of Second Signals for Cytotoxicity and Proliferation in CD8+ T Cell Effectors: Amplification of the Lytic Potential by B7

The role of second signals delivered through B7/CD28 interactions in T cell activation is well documented. However, once CTLs are elicited, TCR-mediated cytotoxicity appears to be uncoupled from the requirement for costimulatory signals. In this study, we show an uncoupling across a broad range of concentrations of peptide, thus demonstrating that cytolysis is a TCR-mediated response that is fully independent of costimulatory signals. However, the same T cell effectors remain fully responsive to B7 engagement, which is able to amplify Ag-mediated proliferation and cytolytic capacity. B7 expression by targets results in an IL-2-mediated proliferative expansion of the effectors concurrent with the elimination of the targets. Thus, costimulation of effectors results in a vast expansion in lytic units over time, which does not occur in the absence of IL-2 or B7. Both TCR-derived and second signals appear to be necessary to achieve this result. These results suggest that B7-expressing APC or a cohort of IL-2-producing helper cells would functionally extend the duration and effectiveness of the cytotoxic response occurring in localized immune responses.

Transgenes and knockout mutations in animal models of type 1 diabetes and multiple sclerosis

In this article, we will examine the roles of transgenic and knockout animals that aid us in understanding two autoimmune diseases-type 1 (insulin-dependent) diabetes and multiple sclerosis. The first sections will focus on studies in type 1 diabetes to show how genetically altered animals have given insight into the role of various immune cell types, autoantigens, co-stimulatory molecules, cytokines and, finally, the role of various effector pathways in the pathogenesis of diabetes. The second section concentrating on the animal model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), will show how animals that express a T-cell receptor derived from a clone able to cause disease have given insight into the pathogenesis of EAE.

Autoaggression and tumor rejection: it takes more than self-specific T-cell activation

Establishment of self-tolerance prevents autoaggression against organ-specific self-antigens. This beneficial effect, however, may in turn be responsible for tumor immune evasion. Thus, dissecting the mechanisms leading to the breakdown of self-tolerance in autoimmune diseases might provide insights for successful antitumor immune therapies. In a variety of animal models, organ- or tumor-specific immunity has been described, focusing on antigen-specific T-cell activation. Here, we discuss two transgenic mouse models which demonstrate that both autoaggression and tumor rejection require more than activated, self-reactive T cells. TCR transgenic mice, which are tolerant to a liver-specific MHC class I antigen, Kb, can be activated to reject Kb-positive grafts, but fail to attack Kb-expressing liver. However, autoaggression occurs when activated T cells are combined with "conditioning" of the target organ by irradiation or infection with a liver-specific pathogen. Similarly, in a mouse model of islet cell carcinoma, neither co-stimulatory tumor cells nor highly activated antitumor lymphocytes provoke an effective immune response against the tumor. Instead, a combination of activated lymphocytes and irradiation is required for lymphocyte infiltration into solid tumors. Both model systems provide evidence that although activated antigen-specific lymphocytes are a prerequisite for autoaggression, effector cell extravasation and appropriate interaction with the target organ/tumor are equally important. Thus, we propose that the organ/tumor microenvironment is a critical parameter in determining the effectiveness of an anti-self immune response.

Cytotoxic T lymphocytes to an unmutated tumor rejection antigen P1A: normal development but restrained effector function in vivo

Unmutated tumor antigens are chosen as primary candidates for tumor vaccine because of their expression on multiple lineages of tumors. A critical issue is whether unmutated tumor antigens are expressed in normal cells, and if so, whether such expression imposes special restrictions on cytotoxic T lymphocyte (CTL) responses. In this study, we use a transgenic approach to study the development and effector function of T cells specific for P1A, a prototypical unmutated tumor antigen. We report here that although P1A is expressed at low levels in normal tissues, including lymphoid tissues, the P1A-specific transgenic T cells develop normally and remain highly responsive to the P1A antigen. The fact that transgenic expression of P1A antigen in the thymus induces T cell clonal deletion demonstrates that normal hematopoietic cells can process and present the P1A antigen and that P1A-specific T cells are susceptible to clonal deletion. By inference, P1A-specific T cells must have escaped clonal deletion due to low expression of P1A in the thymus. Interestingly, despite the fact that an overwhelming majority of T cells in the T cell receptor for antigen (TCR)-transgenic mice are specific for P1A, these mice are no more resistant to a P1A-expressing plasmocytoma than nontransgenic littermates. Moreover, when the same TCR-transgenic mice were challenged simultaneously with B7-1(+) and B7-1(-) tumors, only B7-1(+) tumors were rejected. Therefore, even though P1A can be a tumor rejection antigen, the effector function of P1A-specific CTL is restrained in vivo. These results have important implications for the strategy of tumor immunotherapy.

B7.2 Has Opposing Roles During the Activation Versus Effector Stages of Experimental Autoimmune Thyroiditis

APCs provide costimulatory and down-regulatory signals to Ag-activated T cells through interactions between B7.1 and B7.2 on APCs with either CD28 or CTL Ag-4 expressed on T cells. Recipients of mouse thyroglobulin (MTg)-primed spleen cells activated in the presence of anti-B7.2 had decreased experimental autoimmune thyroiditis (EAT) severity compared with recipients of cells cultured with control rat Ig or anti-B7.1. Blocking B7.2 during in vivo priming also suppressed the ability of MTg-primed spleen cells to transfer EAT, implicating a role for B7.2 for priming and in vitro activation of EAT effector cells. In contrast, administration of anti-B7.2 or anti-B7.2 Fab to recipients of MTg-activated spleen cells increased the severity of EAT compared with recipients receiving control Ig. Thyroids from anti-B7.2-treated recipients had increased expression of IL-4 mRNA compared with thyroids from rat Ig-treated controls. Both B7.1 and B7.2 molecules were expressed in the thyroids of mice with EAT, although B7.2 was more prevalent than B7.1. Administration of both anti-B7.1 and anti-B7.2 to recipient mice suppressed the development of EAT, while anti-B7.1 treatment alone had no effect on EAT severity. The suppression of EAT was not observed when anti-B7.1 and anti-B7.2 treatment was delayed until 7 days after cell transfer, suggesting a requirement for B7 in the initiation of EAT in recipient mice. These results suggest that costimulation is required during the effector phase of EAT and that B7.2 may have opposing roles in the activation versus effector stages of autoreactive T cells.

Genetics of autoimmune diabetes in animal models

Type I diabetes has resisted direct genetic analysis in humans but two excellent models of disease in rodents provide a more readily manipulated alternative for study. These rodent models are being used successfully to localize the genes that are involved in disease pathogenesis in preparation for positional cloning. In addition, mice carrying transgenes and null mutations related to T cell function have been used to demonstrate potential mechanisms for both MHC-dependence and specific effector functions, such as cytokine release and cytotoxicity.