The costimulatory molecules B7-1 and B7-2 regulate T lymphocyte activation by delivering activating signals through CD28 and inhibitory signals through cytotoxic T lymphocyte-associated antigen 4 (CTLA-4). The importance of CTLA-4-mediated inhibition was demonstrated by the uncontrolled T cell activation and lymphoproliferative disease that develops in CTLA-4-deficient (-/-) mice. To examine the role of B7 signaling in the activation of CTLA-4-deficient T cells, we bred CTLA-4(-/-) mice with mice lacking B7-1, B7-2, or both B7 molecules. The CTLA-4/B7-1(-/-) and the CTLA-4/B7-2(-/-) mice develop lymphoproliferation and enhanced T cell activation. Mice lacking CTLA-4, B7-1, and B7-2 have a normal life-span, and do not have lymphocytic infiltrates in any organs, or increased T cell activation. Therefore, the two B7 molecules have overlapping functions, since either B7-1 or B7-2 alone can cause the CTLA-4(-/-) phenotype. Elimination of both B7-1 and B7-2 from the CTLA-4- deficient mouse abrogates the lymphocyte activation and disease, and does not reveal evidence for additional stimulatory CD28 ligands. The CTLA-4(-/-) phenotype can be reproduced with anti-CD28 antibody in mice lacking CTLA-4, B7-1, and B7-2, but wild-type mice are unaffected by the same treatment. This suggests that the inhibitory function of CTLA-4 can overcome strong CD28-mediated signaling in vivo.

Costimulatory molecules, termed B7.1 and B7.2, are present on the surfaces of APC and are important for the activation of T lymphocytes specific for both foreign Ags and autoantigens. We have examined the role of B7 costimulation in the MRL-lpr/lpr murine model of human systemic lupus erythematosus. MRL-lpr/lpr mice receiving both anti-B7.1 and anti-B7.2 Abs expressed significantly lower anti-small nuclear ribonucleoprotein particles (snRNP) and anti-dsDNA autoantibodies than did untreated mice. Anti-B7.2 Ab treatment alone inhibited anti-dsDNA autoantibody expression while having no effect on anti-snRNP autoantibody expression. Anti-B7.1 Ab treatment alone did not change the expression of either anti-snRNP or anti-dsDNA autoantibodies. Parallel studies performed in MRL-lpr/lpr mice genetically deficient in either B7.1 or B7.2 expressed autoantibody profiles comparable to those found in wild-type MRL-lpr/lpr mice. However, B7.1-deficient MRL-lpr/lpr mice exhibited distinct and more severe glomerulonephritis while B7.2-deficient MRL-lpr/lpr mice had significantly milder or absent kidney pathology as compared with age-matched wild-type mice. These studies indicate that each B7 costimulatory signal may control unique pathological events in murine systemic lupus erythematosus that may not always be apparent in autoantibody titers alone.

T cells require CD28/CTLA-4 costimulatory molecule interactions in addition to Ag-specific signals through the TCR for in vivo effector Th cell function. Some studies have suggested that the ligands for these costimulatory molecules may differentially influence effector T cell function with B7-2 favoring a type 2 response and B7-1 favoring a type 1 response, while other studies have suggested that these molecules may be redundant. The recent development of B7-2-deficient mice permits the direct analysis of the requirement of B7-2 during a type 2 immune response to an infectious pathogen. We have examined, in B7-2-deficient mice, effector Th cell function and the associated type 2 immune response following infection with Heligmosomoides polygyrus, a natural murine parasitic nematode. Elevations in cytokine gene expression and protein secretion were pronounced and comparable in inoculated B7-2-/- and B7-2+/+ mice at day 8 after H. polygyrus inoculation. However, by day 14 after infection, increases in T cell cytokine expression were markedly inhibited in H. polygyrus-inoculated B7-2-/- mice. Furthermore, elevations in serum IgE and germinal center formation were inhibited at later stages of the immune response, while elevations in serum IgG1 persisted. These findings suggest that certain T-dependent components vary in their B7-2-dependency during the type 2 immune response. They further demonstrate that B7-2 interactions are not necessary for the initiation of the type 2 immune response, but are instead required for its progression after the development of effector T cells.

We have generated mice deficient in the expression of the lymphocyte cell surface antigen CD48 (Blast-1, BCM1, sgp-60) by gene targeting in embryonic stem cells. Mice homozygous for the CD48 mutation (CD48(-/-) mice) are severely impaired in CD4(+) T cell activation. Proliferative responses to mitogens, anti-CD3 mAb, and alloantigen are all reduced. Experiments in which T cells and antigen-presenting cells from either wild-type or CD48(-/-) mice were cocultured reveal that CD48 is important on both T cells and antigen-presenting cells. The most dramatic impairment was observed in experiments in which highly purified T cells were stimulated through the T cell receptor in the presence of the phorbol ester, phorbol 12-myristate 13-acetate. The results of these experiments raise the possibility that CD48 plays a role in signaling through the T cell receptor.

The past year has seen significant advances in our understanding of the role of cytotoxic T lymphocyte antigen 4 (CTLA-4) in regulating T cell activation and tolerance. Recent studies indicate that CTLA-4 not only counterbalances CD28 signals but also can inhibit T cell responses independently of CD28. Recent work has also revealed a role for CTLA-4 in regulating Th1/Th2 differentiation. Manipulation of CTLA-4 in animal models of autoimmunity has shown that CTLA-4 regulates both the initiation and the progression of autoimmune diseases.

The granulomatous inflammation in infection with the helminth Schistosoma mansoni represents a cellular hypersensitivity reaction mediated by, and dependent upon, MHC class II-restricted CD4+ Th cells sensitized to parasite egg Ags. The current work examines the role and significance of the B7:CD28/CTLA-4 pathway in providing the costimulation necessary for the activation of these pathogenic T cells. In vitro T cell responses in B7-1-/- mice, 7-8 wk postinfection, were no different from wild-type controls, but the absence of B7-2 molecules resulted in a decrease in egg Ag-induced proliferation with increased IFN-gamma production. Both B7-1-/- and B7-2-/- mice exhibited intact granuloma formation. In contrast, CD4+ Th cells from B7-1/2 double-deficient mice displayed a dramatic loss of proliferative capacity upon stimulation with egg Ag. Most strikingly, these T cells secreted only IFN-gamma, but not IL-4 and IL-10, a pattern entirely opposite to that displayed by wild-type controls. Despite these major differences in T cell reactivity, B7-1/2-/- mice had only a limited reduction of granuloma size and fibrosis, without appreciable difference in cellular composition. These results show that substantial granuloma formation can occur under conditions of limited T cell expansion and restricted Th1-type cytokine production. They also support the notion that the combined effect of B7 signaling is not as critical for Th1 cell activation as it is for the development of the Th2 dominant environment characteristic of the evolving schistosome infection in H-2b mice.

B7-1 (CD80)-transfected P815 tumor cells were previously shown to elicit tumor-eradicating immunity that leads to the regression of B7-1+ P815 tumors after transient growth in normal syngeneic (DBA/2) mice. Here, we show that not only the B7-1 molecule but also the B7-2 (CD86) molecule contributed to the eradication of B7-1+ P815 tumors. The B7-1 molecule that contributed to the eradication of B7-1+ P815 tumors was expressed not only on the tumor cells but also on host APCs, including MAC-1+ cells. The B7-2 molecule that contributed to the eradication of B7-1+ P815 tumors was expressed only on host APCs, such as B220+ cells, and not on the tumor cells. In spite of the fact that B7-expressing host APCs contributed to the eradication of B7-1+ P815 tumors, only CD8+ T cells without help from CD4+ T cells were important for tumor eradication. Taken together, these findings indicate that in addition to the ability of B7-1-transfected tumor cells to stimulate CD8+ T cell-mediated tumor-eradicating immunity directly, such tumor cells can also stimulate CD8+ T cell-mediated tumor-eradicating immunity indirectly as a result of cross-priming through B7-expressing host APCs.

Mice lacking CTLA-4 die at an age of 2-3 wk due to massive lymphoproliferation, leading to lymphocytic infiltration and destruction of major organs. The onset of the lymphoproliferative disease can be delayed by treatment with murine CTLA4Ig (mCTLA4Ig), starting day 12 after birth. In this study, we have characterized the T cells present in CTLA-4-deficient mice before and after mCTLA4Ig treatment. The T cells present in CTLA-4-deficient mice express the activation markers, CD69 and IL-2R; down-regulate the lymphoid homing receptor, CD62L; proliferate spontaneously in vitro and cannot be costimulated with anti-CD28 mAb consistent with a hyperactivated state. The T cells from CTLA-4-deficient mice survive longer in culture correlating with higher expression of the survival factor, Bcl-xL, in these cells. Most significantly, the CD4+ T cell subset present in CTLA-4-deficient mice secretes high levels of IL-4 and IL-5 upon TCR activation. Treatment of CTLA-4-deficient mice treated with mCTLA4Ig reverses the activation and hyperproliferative phenotype of the CTLA-4-deficient T cells and restores the costimulatory activity of anti-CD28 mAb. Furthermore, T cells from mCTLA4Ig-treated mice are not skewed toward a Th2 cytokine phenotype. Thus, CTLA-4 regulates CD28-dependent peripheral activation of CD4+ T cells. This process results in apoptosis-resistant, CD4+ T cells with a predominantly Th2 phenotype that may be involved in the lethal phenotype in these animals.

The induction of spongiform myeloencephalopathy by murine leukemia viruses is mediated primarily by infection of central nervous system (CNS) microglia. In this regard, we have previously shown that CasBrE-induced disease requires late, rather than early, virus replication events in microglial cells (W. P. Lynch et al., J. Virol. 70:8896-8907, 1996). Furthermore, neurodegeneration requires the presence of unique sequences within the viral env gene. Thus, the neurodegeneration-inducing events could result from microglial expression of retroviral envelope protein alone or from the interaction of envelope protein with other viral structural proteins in the virus assembly and maturation process. To distinguish between these possible mechanisms of disease induction, we engineered the engraftable neural stem cell line C17-2 into packaging/producer cells in order to deliver the neurovirulent CasBrE env gene to endogenous CNS cells. This strategy resulted in significant CasBrE env expression within CNS microglia without the appearance of replication competent virus. CasBrE envelope expression within microglia was accompanied by increased expression of activation markers F4/80 and Mac-1 (CD11b) but failed to induce spongiform neurodegenerative changes. These results suggest that envelope expression alone within microglia is not sufficient to induce neurodegeneration. Rather, microglia-mediated disease appears to require neurovirulent Env protein interaction with other viral proteins during assembly or maturation. More broadly, the results presented here prove the efficacy of a novel method by which neural stem cell biology may be harnessed for genetically manipulating the CNS, not only for studying neurodegeneration but also as a paradigm for the disseminated distribution of retroviral vector-transduced genes.

Blockade with B7 antagonists interferes with CD28:B7 and CTLA-4:B7 interactions, which may have opposing effects. We have examined the roles of CD28:B7 and CTLA-4:B7 on in vivo alloresponses. A critical role of B7:CD28 was demonstrated by markedly compromised expansion of CD28-deficient T cells and diminished graft-versus-host disease lethality of limited numbers of purified CD4+ or CD8+ T cells. When high numbers of T cells were infused, the requirement for CD28:B7 interaction was lessened. In lethally irradiated recipients, anti-CTLA-4 mAb enhanced in vivo donor T cell expansion, but did not affect, on a per cell basis, anti-host proliferative or CTL responses of donor T cells. Graft-versus-host lethality was accelerated by anti-CTLA-4 mAb infusion given early post-bone marrow transplantation (BMT), mostly in a CD28-dependent fashion. Donor T cells obtained from anti-CTLA-4 mAb-treated recipients were skewed toward a Th2 phenotype. Enhanced T cell expansion in mAb-treated recipients was strikingly advantageous in the graft-versus-leukemia effects of delayed donor lymphocyte infusion. In two different systems, anti-CTLA-4 mAb enhanced the rejection of allogeneic T cell-depleted marrow infused into sublethally irradiated recipients. We conclude that blockade of the selective CD28-B7 interactions early post-BMT, which preserve CTLA-4:B7 interactions, would be preferable to blocking both pathways. For later post-BMT, the selective blockade of CTLA-4:B7 interactions provides a potent and previously unidentified means for augmenting the GVL effect of delayed donor lymphocyte infusion.

Blocking the interaction of the CD28 costimulatory receptor with its ligands, CD80 and CD86, inhibits in vivo immune responses, such as allograft rejection, and in some instances induces tolerance. Previously, we found that CTLA4Ig, which blocks the CD28/CTLA-4 (CD152) ligands CD80 and CD86, can be used to induce transplantation tolerance to vascularized allografts. Recent data suggest that an intact CD152-negative signaling pathway is essential for induction of tolerance to nominal Ags. Here, we show that blockade of CD152 using an anti-CD152 mAb at the time of transplantation prevents the induction of long-term allograft survival by agents that target CD80 and CD86. In contrast, CD152 signals are not required for the maintenance of established graft survival. We also report for the first time that blockade of CD86 alone can induce long-term graft survival. This requires that anti-CD86 mAb is given on the day of transplantation and also depends upon an intact CD152 pathway. This result, plus experiments using CD80-deficient mice, suggests a dominant role for CD80 molecules on donor cells as the relevant ligand for CD152. We additionally find that blockade of CD152 at the time of transplantation does not interfere with the effectiveness of anti-CD154 mAbs, suggesting distinct mechanisms for inhibition of graft rejection by blocking the CD28 vs CD154 pathways.

Covalent linkage of a bacterial polysaccharide to an immunogenic protein greatly enhances the carbohydrate's immunogenicity and induces polysaccharide-specific B-cell memory in vivo. These findings have spurred the development of glycoconjugate vaccines for serious bacterial infections. The specific B-cell-T-cell interactions responsible for recruitment of T-cell help by glycoconjugate vaccines are not well defined. We used mice deficient in molecules critical for stimulatory, cognate B-cell-T-cell interactions to study how T cells improve the immunogenicity of a glycoconjugate vaccine against group B streptococcal disease. Isotype switching to immunoglobulin G (IgG) was abrogated in mice deficient in major histocompatibility complex (MHC) class II antigen (Ag)-T-cell receptor (TCR), B7-CD28, or CD40-CD40L interactions. However, expression of either the B7-1 or the B7-2 molecule on antigen-presenting cells was sufficient for optimal T-cell costimulation. T cells activated by the vaccine also played a pivotal role in determining the magnitude of the IgM response to the polysaccharide. Comparable results were obtained with pathway antagonists. These data suggest that MHC class II Ag-TCR, B7-CD28, and CD40-CD40L interactions are critical for immune responses to glycoconjugate vaccines in vivo.

CTLA4 (CD152), a receptor for the B7 costimulatory molecules (CD80 and CD86), is considered a fundamental regulator of T-cell activation. In this paper, we present the complete primary structure of the mouse and human CTLA4 gene loci. Sequence comparison between the mouse and the human CTLA4 gene loci revealed a high degree of sequence conservation both for homologous noncoding regions (65-78% identity) and for coding regions (72-98% identity), with an overall score of 71% over the entire length of the two genes. Of the CTLA4 genomic regions aligned, five simple repetitive elements were found in the mouse locus, whereas two simple repetitive sequences were localized on the human locus. RNA blot analysis of mouse and human primary tissues indicated that both CTLA4 and T-cell receptor transcripts were found in most organs with generally higher levels in lymphoid tissues. The conservation of CTLA4 gene patterning raises the possibility that constrained gene evolution of CTLA4 may be linked to conserved transcriptional control of this locus.

Blockade of the CD28/CTLA4/B7 costimulatory pathway using CTLA4-Ig has great therapeutic potential, and has been shown to prolong allograft survival in a variety of animal models. To gain further insight into the mechanism by which costimulatory blockade prevents allograft rejection, we studied cardiac allograft survival in the complete absence of B7 costimulation using mice lacking B7-1 and B7-2 (B7-1/B7-2-/- mice). To determine the role of B7 on donor vs recipient cells, we used B7-1/B7-2-/- mice as either donors or recipients of allografts. Wild-type (WT) recipients acutely reject fully allogeneic hearts from both WT and B7-1/B7-2-/- mice. In contrast, B7-1/B7-2-/- recipients allow long-term survival of grafts from both WT and B7-1/B7-2-/- mice, with minimal histologic evidence of either acute or chronic rejection in grafts harvested after 90 days. The B7-1/B7-2-/- mice acutely reject B7-1/B7-2-/- allografts if CD28 stimulation is restored by the administration of Ab to CD28 and can mount an alloresponse in mixed lymphocyte reactions. Therefore, B7-1/B7-2-/- mice are capable of generating alloresponses both in vivo and in vitro. Our results demonstrate that in the alloresponse to mouse heterotopic cardiac transplantation, B7 molecules on recipient cells rather than donor cells provide the critical costimulatory signals. The indefinite survival of allografts into B7-1/B7-2-/- recipients further shows that the absence of B7 costimulation alone is sufficient to prevent rejection.

All mammals produce heparin, a negatively charged glycosaminoglycan that is a major constituent of the secretory granules of mast cells which are found in the peritoneal cavity and most connective tissues. Although heparin is one of the most studied molecules in the body, its physiological function has yet to be determined. Here we describe transgenic mice, generated by disrupting the N-deacetylase/N-sulphotransferase-2 gene, that cannot express fully sulphated heparin. The mast cells in the skeletal muscle that normally contain heparin lacked metachromatic granules and failed to store appreciable amounts of mouse mast-cell protease (mMCP)-4, mMCP-5 and carboxypeptidase A (mMC-CPA), even though they contained substantial amounts of mMCP-7. We developed mast cells from the bone marrow of the transgenic mice. Although these cultured cells contained high levels of various protease transcripts and had substantial amounts of mMCP-6 protein in their granules, they also failed to express mMCP-5 and mMC-CPA. Our data show that heparin controls, through a post-translational mechanism, the levels of specific cassettes of positively charged proteases inside mast cells.

We have used T cells from B7-1-deficient TCR transgenic DO11.10 mice to demonstrate a functional role for B7-1 on T cells. B7-1-deficient DO11.10 T cells produce more IL-4 than wild-type DO11.10 T cells, suggesting that B7-1 expressed by T cells regulates the differentiation of IL-4-producing cells. In addition, we found that IL-4 inhibits B7-1 expression by wild-type DO11.10 T cells. Our results suggest that there is a reciprocal relationship between B7-1 expressed on T cells and IL-4 production, which results in a modulatory feedback loop. When high levels of IL-4 are produced by T cells, B7-1 expression by T cells is inhibited, which allows amplification of IL-4 production by these T cells. When low levels of IL-4 are produced by T cells, B7-1 expression by these T cells is increased, and a further reduction in IL-4 production follows. However, in addition to being influenced by IL-4, B7-1 expression by T cells is affected by peptide concentration and by B7 costimulation from APCs. The studies presented here demonstrate that B7-1 on T cells as well as on APCs regulates IL-4 production. However, whereas B7-1 expression on APCs can promote IL-4 production, IL-4 production is inhibited by B7-1 on T cells.

The tumor necrosis factor family molecule Ox40-ligand (Ox40L) has been identified as a potential costimulatory molecule and also has been implicated in T cell homing and B cell activation. To ascertain the essential functions of Ox40L, we generated and characterized Ox40L-deficient mice. Mice lacking Ox40L exhibit an impaired contact hypersensitivity response, a dendritic cell-dependent T cell-mediated response, due to defects in T cell priming and cytokine production. In contrast, Ox40L-deficient mice do not have defects in T cell homing or humoral immune responses. In vitro, Ox40L-deficient dendritic cells are defective in costimulating T cell cytokine production. Thus, Ox40L has a critical costimulatory function in vitro and in vivo for dendritic cell:T cell interactions.

To examine the role of CTLA-4 in Th cell differentiation, we used two newly generated CTLA-4-deficient (CTLA-4-/-) mouse strains: DO11. 10 CTLA-4-/- mice carrying a class II restricted transgenic TCR specific for OVA, and mice lacking CTLA-4, B7.1 and B7.2 (CTLA-4-/- B7.1/B7.2-/- ). When purified naive CD4+ DO11.10 T cells from CTLA-4-/- and wild-type mice were primed and restimulated in vitro with peptide Ag, CTLA-4-/- DO11.10 T cells developed into Th2 cells, whereas wild-type DO11.10 T cells developed into Th1 cells. Similarly, when CTLA-4-/- CD4+ T cells from mice lacking CTLA-4, B7. 1, and B7.2 were stimulated in vitro with anti-CD3 Ab and wild-type APC, these CTLA-4-/- CD4+ T cells produced IL-4 even during the primary stimulation, whereas CD4+ cells from B7.1/B7.2-/- mice did not produce IL-4. Upon secondary stimulation, CD4+ T cells from CTLA-4-/- B7.1/B7.2-/- mice secreted high levels of IL-4, whereas CD4+ T cells from B7.1/B7.2-/- mice produced IFN-gamma. In contrast to the effects on CD4+ Th differentiation, the absence of CTLA-4 resulted in only a modest effect on T cell proliferation, and increased proliferation of CTLA-4-/- CD4+ T cells was seen only during secondary stimulation in vitro. Administration of a stimulatory anti-CD28 Ab in vivo induced IL-4 production in CTLA-4-/- B7.1/B7.2-/- but not wild-type mice. These studies demonstrate that CTLA-4 is a critical and potent inhibitor of Th2 differentiation. Thus, the B7-CD28/CTLA-4 pathway plays a critical role in regulating Th2 differentiation in two ways: CD28 promotes Th2 differentiation while CTLA-4 limits Th2 differentiation.

The importance of B7 costimulation in regulating T cell expansion and peripheral tolerance suggests that it may also play a significant regulatory role in the development of autoimmune disease. It is unclear whether B7 costimulation is involved only in the expansion of autoreactive T cells in the periphery, or if it is also required for effector activation of autoreactive T cells in the target organ for mediating tissue injury and propagating autoimmune disease. In this study, the role of B7-CD28 costimulation and the relative importance of B7 costimulators for the induction and effector phases of experimental autoimmune encephalomyelitis (EAE) induced by myelin oligodendrocyte glycoprotein (MOG) peptide were examined. Wild-type, B7-1/B7-2-deficient mice, or CD28-deficient C57BL/6 mice were immunized with MOG 35-55 peptide. Mice lacking both B7-1 and B7-2 or CD28 showed no or minimal clinical signs of EAE and markedly reduced inflammatory infiltrates in the brain and spinal cord. However, mice lacking either B7-1 or B7-2 alone developed clinical and pathologic EAE that was comparable to EAE in wild-type mice, indicating overlapping functions for B7-1 and B7-2. Resistance to EAE was not due to a lack of induction of T helper type 1 (Th1) cytokines, since T cells from B7-1/B7-2(-/-) mice show reduced proliferative responses, but greater interferon gamma production compared with T cells from wild-type mice. To study the role of B7 molecules in the effector phase of the disease, MOG 35-55-specific T lines were adoptively transferred into the B7-1/B7-2(-/-) and wild-type mice. Clinical and histologic EAE were markedly reduced in B7-1/B7-2(-/-) compared with wild-type recipient mice. These results demonstrate that B7 costimulation has critical roles not only in the initial activation and expansion of MOG-reactive T cells, but also in the effector phase of encephalitogenic T cell activation within the central nervous system.