Mutational analysis and an alternatively spliced product of B7 defines its CD28/CTLA4-binding site on immunoglobulin C-like domain

Tumor cells ectopically expressing the membrane-bound form of IL-7 develop an antitumor immune response efficiently in a colon carcinoma model

Various approaches employing cytokines and cytokine gene-modified tumor cells have been explored to induce antitumor responses, yet their widespread application has been limited due to efficacy concerns and adverse effects. In this study, interleukin-7 was engineered for expression both as a natural secretory form (sIL-7) and as a membrane-bound form fused with the B7.1 type I transmembrane protein (mbIL-7/B7) on CT26 colon cancer cells. Analysis of the resulting cell clones demonstrated that ectopically expressed sIL-7 and mbIL-7/B7 both retained similar capacities to induce the expansion and activation of CD8+ T cells and to enhance antitumor responses in vitro. While the sIL-7 or mbIL-7/B7 clones showed similar growth in culture, the mbIL-7/B7 clone exhibited lower tumorigenicity in mice compared with the sIL-7 clone or wild-type CT26 cells. Specifically, the mbIL-7/B7 clone failed to form tumors in approximately 60% of the mice injected with it. Moreover, 80% of mice that rejected the mbIL-7/B7 clone developed long-term systemic immunity against CT26 cells. Analysis of immune cells within the tumor masses revealed significant increases in CD4+ T cells, CD8+ T cells, and dendritic cells in tumors formed by the mbIL-7/B7 clone compared to those formed by the sIL-7 clone. These findings suggest that the membrane-bound form of IL-7 with B7.1 is more effective than the secretory form in establishing antitumor immunity within the tumor microenvironment. Our strategy of expressing the mbIL-7/B7 chimera holds promise as a novel approach for tumor therapy, particularly in cases requiring IL-7 supplementation.

Targeted Therapy of B7 Family Checkpoints as an Innovative Approach to Overcome Cancer Therapy Resistance: A Review from Chemotherapy to Immunotherapy

It is estimated that there were 18.1 million cancer cases worldwide in 2018, with about 9 million deaths. Proper diagnosis of cancer is essential for its effective treatment because each type of cancer requires a specific treatment procedure. Cancer therapy includes one or more approaches such as surgery, radiotherapy, chemotherapy, and immunotherapy. In recent years, immunotherapy has received much attention and immune checkpoint molecules have been used to treat several cancers. These molecules are involved in regulating the activity of T lymphocytes. Accumulated evidence shows that targeting immune checkpoint regulators like PD-1/PD-L1 and CTLA-4 are significantly useful in treating cancers. According to studies, these molecules also have pivotal roles in the chemoresistance of cancer cells. Considering these findings, the combination of immunotherapy and chemotherapy can help to treat cancer with a more efficient approach. Among immune checkpoint molecules, the B7 family checkpoints have been studied in various cancer types such as breast cancer, myeloma, and lymphoma. In these cancers, they cause the cells to become resistant to the chemotherapeutic agents. Discovering the exact signaling pathways and selective targeting of these checkpoint molecules may provide a promising avenue to overcome cancer development and therapy resistance. Highlights: (1) The development of resistance to cancer chemotherapy or immunotherapy is the main obstacle to improving the outcome of these anti-cancer therapies. (2) Recent investigations have described the involvement of immune checkpoint molecules in the development of cancer therapy resistance. (3) In the present study, the molecular participation of the B7 immune checkpoint family in anticancer therapies has been highlighted. (4) Targeting these immune checkpoint molecules may be considered an efficient approach to overcoming this obstacle.

Predicting the functional consequences of genetic variants in co-stimulatory ligand B7-1 using in-silico approaches

The purpose of this research is to identify and characterize deleterious genetic variants in the co-stimulatory ligand B7-1, also known as the human cluster of differentiation CD80 marker. The B7-1 ligand and the major histocompatibility complex class II (MHC II) molecules are the main determinants that provide B-cells the required competency to act as antigen presenting cells. For this, participation of both MHC class II molecules and CD80 is required. The interaction of the CD80 ligand with CD28 on the surface 7 of T_H cells plays a key role in the activation of TH cells and progression of B cells through the S phase, hence, leading to their proliferation in mitosis. A set of 2313 genetic variants in the B7-1 ligand have been mapped and retrieved from dbSNP database. Subsequently, 150 non-synonymous single nucleotide polymorphisms (nsSNPs) were mapped and subjected to the sequence and structural homology based predictions, which were further analyzed for protein stability and the disease phenotypes. Finally, we identified 7 potentially damaging nsSNPs in the B7-1 ligand that may affect its interaction with the cognitive receptor CD28, hence, may also interfere with TH cell activation and B cell proliferation. We propose that subsequent experimental analyses (stability, expression and interactions) on these proteins can provide a deep understanding about the effect of these variants on the structure and function of CD80.

Resistance of novel mouse strains different in MHC class I and the NKC domain to the development of experimental tumors

To elucidate the immunological mechanisms critical for tumor progression, we bred novel mouse strains, different in the NKC and H-2D domains. We used inbreeding to generate hybrids of Balb/c and C57BL/6 of stable H-2Db+d-NK1.1neg and H-2Db-d+NK1.1high phenotypes. We analyzed the growth of three established MHC class I-deficient tumor cell lines: TC-1/A9 tumor (HPV-associated) and B16F10 melanoma, both syngeneic to C57BL/6, and the MCB8 (3-methycholanthrene-induced tumor) syngeneic to Balb/c. Furthermore, we induced colorectal carcinoma by azoxymethane-DSS treatment to test the susceptibility to chemically-induced primary cancer. We found that the novel strains spontaneously regressed the tumor transplants syngeneic to both Balb/c (MCB8) and C57BL/6 (B16F10 and TC-1/A9) mice. The H2-Db+d-NK1.1neg, but not the H2-Db-d+NK1.1high strain was also highly resistant to chemically-induced colorectal cancer in comparison to the parental mice. The immune changes during TC-1/A9 cancer development involved an increase of the NK cell distribution in the peripheral blood and spleen along with higher expression of NKG2D activation antigen; this was in correlation with the time-dependent rise of cytotoxic activity in comparison to C57BL/6 mice. The TC-1/A9 cancer regression was accompanied by higher proportion of B cells in the spleen and B220+/CD86+ activated antigen-presenting B cells distributed in the lymphoid organs, as well as in the periphery. The changes in the T-cell population were represented mainly by the prevalence of T helper cells reflected by grown CD4/CD8 ratio, most prominent in the b+d-NK1.1neg strain. The results of the present study imply usefulness of the two novel mouse strains as an experimental model for further studies of tumor resistance mechanisms.

Conservation of structural and functional features in a primordial CD80/86 molecule from rainbow trout (Oncorhynchus mykiss), a primitive teleost fish

In mammals, interaction of CD28 with CD80 or CD86 molecules provides costimulatory signals for T cell activation that leads to increased IL-2 gene and protein expression by activated T cells. Thus far, CD80 and CD86 have been cloned and functionally characterized only in mammals and birds. To shed light into the evolution of CD80 and CD86, we have cloned and functionally characterized a rainbow trout (rt) molecule (rtCD80/86) that shows the highest degree of sequence conservation and phylogenetic relationship with CD80 and CD86 molecules. Moreover, its genomic organization was almost identical to that of human CD86. Rainbow trout possess one membrane-bound and two soluble CD80/86 transcripts, all of which are derived from the same rtCD80/86 gene. The membrane-bound form exhibited its highest degree of expression in lymphoid tissues, particularly on B cells. Incubation of trout leukocytes with LPS and bacteria leads to up-regulation of rtCD80/86 gene expression. Importantly, we show that trout and other teleost fish contain a single CD80/86 gene, thus suggesting that this gene may represent the ancestor from which CD80 and CD86 arose by gene duplication in more evolved species. To gain further insights into the function of rtCD80/86, we have identified and cloned trout IL-2 and have shown that recombinantly produced trout CD80/86 up-regulates the expression of IL-2 in trout blood leukocytes. Significantly, this finding indicates that the capacity to modulate IL-2 expression is a primordial function that has been conserved both in fish and mammalian CD80/CD86 molecules throughout 350 million years of evolution.

Immune responses in patients with metal-on-metal hip articulations: a long-term follow-up

Metal-on-metal (MoM) hip bearings are being inserted into ever-younger patients. The effects on the immune system of chronic exposure are unknown. We investigated the immune response of patients with MoM hip bearings. In patients with MoM implants, the expression of antigen-presenting cell (APC) surface molecules (CD86 and HLA-DR) was seen to be significantly higher (P < .05) than control group. High levels of APC surface molecules suggest an activated state and attempts to propagate an immune response. However, in the same group, the expression of T-cell markers (CD3 and CD28) was low, indicating a small T-cell population. This suggests, despite the activation of APCs, that T cells down-regulate immune responses in MoM articulations. Conversely, in metal-on-polyethylene articulations, expression of T-cell molecules was elevated and expression of APC molecules lowered.

Protective Regulatory T Cell Generation in Autoimmune Diabetes by DNA Covaccination with Islet Antigens and a Selective CTLA-4 Ligand

DNA vaccination of autoimmune diabetes-prone NOD mice with unmodified target islet antigens, i.e., preproinsulin (PPIns) or glutamic acid decarboxylase 65 (GAD65), is poorly protective. However, in this study, we demonstrate protection against disease by covaccination with a mutant B7-1 molecule (B7-1wa) that binds the negative T cell regulator CTLA-4 (CD152), but not CD28. Codelivery of plasmids encoding a PPIns-GAD65 fusion construct and B7-1wa protected against both insulitis and diabetes. In vitro, the T cells of covaccinated mice had negative responses to both insulin and GAD65, and this was restored by adding blocking antibodies to transforming growth factor beta1 (TGF-beta1), suggesting a role for this cytokine. Adoptive transfer experiments revealed that DNA vaccination generated protective CD4(+) regulatory T cells (Tr) of either CD25(+) or CD25(-) phenotype. Furthermore, vaccinated mice had increased numbers of T cells with Tr-associated markers, such as CTLA-4, Foxp3, and membrane-bound TGF-beta1. Tr cells inhibited the responses of diabetogenic T cells to islet antigens, and depletion of T cells expressing membrane-bound TGF-beta1 abolished the suppressive effect. Thus, selective engagement of CTLA-4 during islet-antigen DNA vaccination induces Tr cells that protect against this autoimmune disease.

DNA vaccination with an insulin construct and a chimeric protein binding to both CTLA4 and CD40 ameliorates type 1 diabetes in NOD mice

Type 1 diabetes (T1D), a T-cell-mediated autoimmune disease, could be attributed to many defects in nonobese diabetic (NOD) mice, including deficient expressions of costimulatory molecules that impair antigen presentation. Thus, this deficient antigen presentation may result in a reduced ability to induce a tolerogenic response through negative selection/regulation of autoreactive T cells. Improperly activated T cells seem to be able to induce autoimmune responses causing diabetes. To re-establish tolerance to autoantigens by modulating costimulation, we constructed and tested a new type of DNA vaccine encoding a membrane-bound preproinsulin (mbPPI) and a chimeric gene vector encoding mutant B7.1/CD40L (mB7.1/CD40L) fusion protein. This mutant B7.1 binds CTLA4 but not CD28. We report that young NOD mice immunized with mbPPI along with mB7.1/CD40L DNA vectors significantly reduced diabetes incidence while treatment with CTLA4/IgG1 exacerbated diabetes. In conclusion, the combination of mbPPI and mB7.1/CD40L was able to protect against autoimmunity and diabetes in NOD mice possibly by promoting a more efficient presentation of autoantigen PPI and inducing specific tolerance to PPI by negatively regulating autoreactive T cells.

The B7 family of immune-regulatory ligands

The B7 family consists of structurally related, cell-surface protein ligands, which bind to the CD28 family of receptors on lymphocytes and regulate immune responses via 'costimulatory' or 'coinhibitory' signals.

Evaluation of lymphocyte subpopulations in draining lymph node cells following allergen and irritant

The murine local lymph node assay (LLNA) has been developed as an alternative to guinea pig models for the assessment of the contact sensitization potential. However, there is a need to develop a non-radioisotopic endpoint for the LLNA, because of the radioisotopic method's requiring the use of special facilities. In this study, we investigated to evaluate the lymphocyte subpopulations in the lymph node cells following allergen and irritant treatment. Female Balb/c mice were treated by the topical application on the dorsum of both ears with sensitizers, 2,4-dinitrochlorobenzene (DNCB), toluene diisocyanate (TDI), and α-hexylcinnamaldehyde (HCA), and an irritant, sodium lauryl sulfate (SLS), once daily for three consecutive days. The lymph node (LN) cells were harvested 72h after the final treatment. Phenotypic analysis of lymphocytes subsets was performed with a flow cytometry. The allergens DNCB, TDI, and HCA and an irritant, SLS increased cell number compared to the vehicle. Mice were treated with DNCB, HCA, and TDI showed a preferential increase in the percentage of B220+CD40+ cells compared with vehicle and irritant-treated mice. There was an increase in B220+CD86+ cells of mice treated with DNCB, TDI, and HCA, but no significant increases were observed in mice treated with SLS. Mice were treated with DNCB and TDI showed an increase in the percentage of B220+CD23+ cells compared with vehicle and irritant-treated mice. These results suggest that analysis of B cell activation marker, CD40 on B cells may be useful in differentiating allergen and irritant responses in the draining lymph nodes of chemically treated mice.

Identification of Protein-Protein Interfaces Implicated in CD80-CD28 Costimulatory Signaling

The B7 ligands CD80 and CD86 on APCs deliver either costimulatory or inhibitory signals to the T cell when interacting with their counter-receptors CD28 and CD152 (CTLA-4) on the T cell surface. Although crucial for lymphocyte regulation, the structural basis of these interactions is still not completely understood. Using multivalent presentation and conditions mimicking clustering, believed to be essential for signaling through these receptors, and by applying a combined differential mass spectrometry and structural mapping approach to these conditions, we were able to identify a putative contact area involving hydrophilic regions on both CD28 and CD80 as well as a putative CD28 oligomerization interface induced by B7 ligation. Analysis of the CD80-CD28 interaction site reveals a well-defined interface structurally distinct from that of CD80 and CD152 and thus provides valuable information for therapeutic intervention targeted at this pathway, suggesting a general approach for other receptors.

Costimulatory molecule immune enhancement in a plasmid vaccine model is regulated in part through the Ig constant-like domain of CD80/86

There is great interest in understanding the role of costimulatory molecules in immune activation. In both the influenza and HIV DNA immunization models, several groups have reported that coimmunization of mice with plasmids encoding immunogen and CD86, but not CD80, effectively boosts Ag-specific T cell activation. This difference in immune priming provided an opportunity to examine the functional importance of different regions of the B.7 molecules in immune activation. To examine this issue, we developed a series of chimeric CD80 and CD86 constructs as well as deletion mutants, and examined their immune activating potential in the DNA vaccine model. We demonstrate that the lack of an Ig constant-like region in the CD80 molecule is critically important to the enhanced immune activation observed. CD80 C-domain deletion mutants induce a highly inflammatory Ag-specific cellular response when administered as part of a plasmid vaccine. The data suggest that the constant-like domains, likely through intermolecular interactions, are critically important for immune regulation during costimulation and that engineered CD80/86 molecules represent more potent costimulatory molecules and may improve vaccine adjuvant efficacy.

Impaired germinal center formation and recall T-cell-dependent immune responses in mice lacking the costimulatory ligand B7-H2

B7-H2, which is expressed constitutively on B cells and binds the inducible costimulator (ICOS) on antigen-activated T cells, is a member of the B7 family of costimulatory ligands. We have inactivated B7-H2 in the mouse. B7-H2-/- mice generate normal populations of B and T cells in their various lymphoid organs but have lower basal levels of heavy chain class-switched antibodies in their sera. These mice are able to mount normal immune responses to both type I and type II T-cell-independent antigens. However, their pattern of responses to a T-cell-dependent antigen is altered, with greatly reduced production of antigen-specific heavy chain class-switched antibodies, the levels of which could not be elevated even with repeated immunizations. This suggests a critical role for B7-H2 in the recall phases of the immune response. Germinal center formation is also impaired in the mutant mice. While B cells from the mutant mice could response normally to anti-IgM, anti-CD40, and lipopolysaccharide stimulation, the production of T-helper-type II cytokines such as interleukin-4 (IL-4) and IL-10 by primed CD4+ T cells from mutant mice were reduced. This indicated that the defects in humoral responses and germinal center formation in B7-H2-deficient mice are due to the lack of T-cell-mediated help to the B cells. Hence, B7-H2 on B cells is important for recruiting T-cell help via its interaction with ICOS and plays a critical role in costimulating humoral immune responses.

B7DC/PDL2 promotes tumor immunity by a PD-1-independent mechanism

B7H1 (PDL1) and B7DC (PDL2) are two new members of the B7 family that can interact with PD-1, a putative negative regulator for immune function. Recent studies have provided evidence for inhibitory functions of both members via PD-1. Meanwhile, compelling evidence exists for costimulatory function of both members. Here we demonstrate that expression of B7DC on the tumor cells promotes CD8 T cell-mediated rejection of tumor cells, at both the induction and effector phase of antitumor immunity. Moreover, B7DC binds to PD-1(-/-) cells and enhances T cell killing in a PD-1-independent mechanism. Our results demonstrate a novel pathway for B7DC to promote tumor immunity and may reconcile the apparently contradictory findings on the function of B7DC.

Porcine CD80: cloning, characterization, and evidence for its role in direct human T-cell activation

Previous studies has shown that human anti-pig reactivity in mixed lymphocyte cultures require the indirect presentation of antigens by human antigen presenting cells (APC). Xenoreactivity was inhibited by blockade of human costimulatory molecules. We investigated the role of porcine costimulatory molecules in their ability to activate human T cells directly. Porcine CD80 was cloned from lipopolysaccharide (LPS)-activated porcine lymphocytes. Sequence analysis showed a high degree of conservation in residues involved in CD28/CTLA4. COS cells transfected with porcine CD80 was able to activate human T cells in a cyclosporine independent manner, demonstrating that porcine CD80 can costimulate human T cells. Tumor necrosis factor-alpha (TNF-alpha) activated porcine splenocytes have been shown to up-regulate B7s. In order to test the effect of costimulation blockade in a xeno system, activated splenocytes were cultured with purified CD4+ T cells. The results demonstrated that these cells were capable of activating human T cells and this activation can be blocked by using an antihuman CD80 antibody that demonstrated cross-reactivity to porcine CD80. Non-cross reactive antibodies had no effect, again suggesting direct activation of the human T cells. These data suggest that a reagent that can block both the direct and indirect activation is necessary for a discordant xenotransplant.

B7-CTLA4 interaction promotes cognate destruction of tumor cells by cytotoxic T lymphocytes in vivo

Costimulatory molecules B7-1 and B7-2 (hereby collectively called B7) interact with CD28 and CTLA4 on T cells and promote antitumor immunity. The function of B7-CTLA4 interaction in antitumor CTL response remains controversial. Here we used CD28(-/-) and CD28(+/-) or CD28(+/+) transgenic mice that express the T-cell receptor specific for an unmutated tumor antigen, P1A, and for tumor cells expressing a CTLA4-specific B7 mutant to evaluate the function of CD28-B7 and CTLA4-B7 interactions in induction and effector phases of antitumor immunity. We report that B7-CD28 and B7-CTLA4 interactions promote tumor rejection. However, this is achieved by distinct mechanisms. B7-CD28 interaction enhances T-cell clonal expansion, though a role for this interaction in the effector phase cannot be ruled out. In contrast, B7-CTLA4 interaction enhances the CTL-mediated destruction of tumors, but not T-cell clonal expansion.

Fusion proteins of B7.1 and a carcinoembryonic antigen (CEA)-specific antibody fragment opsonize CEA-expressing tumor cells and coactivate T-cell immunity

Genetic engineering can be used to generate antigen-specific molecules for improved tumor immunotherapy. We have constructed genes coding for fusion proteins consisting of a high-affinity antibody single-chain antibody fragment (scFv) specific for the human carcinoembryonic antigen (CEA) and the costimulation domain of the murine B7.1 molecule (mB7.1) linked to the antibody moiety by an IgG3 peptide linker. The hybrid genes were constructed in 2 orientations, one with the scFv located N-terminal to mB7.1 and one vice versa. Soluble proteins were expressed by CHO cells, purified using anti-idiotype-affinity chromatography and characterized by tumor-cell binding and costimulation activity. When tumor cells expressing CEA on the cell membrane were opsonized with the CEA-specific costimulators, both fusion proteins specifically stimulated murine T-cell preparations to proliferate in a similar manner. Our data suggest that "costimulation coating" of tumor cells may be a suitable approach for activation of a sustained cellular antitumor response. It also provides the opportunity to increase tumor immunogenicity using easily generated soluble fusion proteins that advantageously link biological functions of both the humoral and the cellular arm of the specific immune system.

Selective silencing of full-length CD80 but not IgV-CD80 leads to impaired clonal deletion of self-reactive T cells and altered regulation of immune responses

Co-stimulation provided by the B7 family of proteins underpins the development of protective immunity. There are three identified members of this family: CD80, its splice variant IgV-CD80 and CD86. It has hitherto been difficult to analyze the expression and function of IgV-CD80 since there are no appropriate reagents capable of distinguishing it from CD80. We have generated mice, by gene targeting, the lack CD80 whilst maintaining expression of IgV-CD80. Mutant animals did not delete T cells bearing mammary tumor virus-reactive TCR as efficiently as wild-type animals. We also demonstrate the importance of IgV-CD80 in the responses of recently activated cells and reveal a role for CD80 in sustaining T cell responses. CD86, whilst critical to primary T cell activation, made only a minor contribution to re-activation of normal cells.

Costimulation of T cells

Costimulation is critical to T cell activation. On the antigen-presenting cell the key players are found in the extended family of B7 genes comprising cd80, cd86, B7h/B7RP-1 and B7-H1. cd80 and cd86 encode proteins that bind to CD28 and CTLA4 on T cells. Blocking this pathway with the potent CTLA4-Ig fusion protein shows encouraging potential as a therapeutic agent. While cd80 and cd86 pathways act mainly on naive T cells, B7h/B7RP-1 and B7-H1 seem to exert their effects on antigen-experienced lymphocytes.

T lymphocyte costimulatory molecules in host defense and immunologic diseases

BACKGROUND

Costimulation is an essential component for the optimal induction of T cell-mediated immune responses. Manipulation of the costimulatory pathway with antibodies or genetically-engineered fusion proteins is an important strategy to treat immune-related diseases including allergy, asthma, transplantation and cancer. Recent advances have revealed several new costimulatory molecules, and the functional characteristics of each costimulatory pathway are now becoming clearer.

LEARNING OBJECTIVES

In this review, we summarize basic outlines of the costimulatory systems in terms of molecular structure, expression kinetics and immunological function. We further discuss involvement and therapeutic manipulation of costimulation in several clinical diseases.

DATA SOURCE

The MEDLINE database was used to review the literature related to costimulation.

CONCLUSION

Costimulatory pathways play an essential role in the activation and regulation of T cell immune responses and the induction of T cell tolerance. Therapeutic manipulation of the costimulatory system demonstrates beneficial effects to treat immunological diseases in murine models as well as some clinical situations.

Soluble CD86 is a costimulatory molecule for human T lymphocytes

CD86 is an important costimulatory molecule for the priming and activation of naive and memory T cells, respectively. Here, we show that soluble CD86 is detected in human serum. Soluble CD86 is produced by resting monocytes and results from an alternatively spliced transcript (CD86deltaTM) characterized by deletion of the transmembrane domain. Recombinant CD86deltaTM binds to CD28 and CTLA-4 and induces the activation of T cells after stimulation with anti-CD3 mAb. CD86deltaTM also induces IFNgamma production by virus-specific CD8+ memory human T cells stimulated with the Flu M1 peptide. The concentrations of soluble CD86 found in human serum are sufficient to induce biological activity. Soluble CD86 molecule, therefore, appears to be a functional costimulatory molecule playing a potentially important role in immune surveillance.

Primary structure and functional characterization of a soluble, alternatively spliced form of B7-1

Recent studies have suggested that soluble forms of B7-1 and B7-2 may exist, but transcripts that code for these molecules have not been previously described. In this study, we report the cloning and characterization of an alternatively spliced soluble form of porcine B7-1 (sB7-1) that lacks exons coding for both the transmembrane and cytoplasmic domains. Northern blot analysis of RNA from alveolar macrophages revealed an approximate 3:1 ratio of the transmembrane form of B7-1 mRNA relative to sB7-1 mRNA. Porcine B7-1 was present on the surface of both B and T cells following stimulation with PMA/ionomycin. A histidine-tagged form of porcine sB7-1 (sB7-1-His) interacted with both CD28 and CTLA-4, and effectively blocked IL-2 production from human responder cells stimulated with PHA and either porcine or human stimulator cells. In addition, sB7-1-His inhibited human T cell proliferation in response to porcine or human peripheral blood leukocytes. This study is the first report of an alternatively spliced form of B7 that codes for a soluble protein. Furthermore, these data demonstrate that porcine B7-1 interacts with the human receptors CD28 and CTLA-4, suggesting a potential role for this molecule in pig to human xenotransplantation. Possible physiological functions for the soluble form of B7-1 are discussed.

B7.1 expression by the weakly immunogenic F98 rat glioma does not enhance immunogenicity

Enhanced immunogenicity has been reported following transfection of a variety of immunogenic tumors with the B7.1 co-stimulatory molecule. The purpose of the present study was to determine if transfection of a weakly immunogenic rat brain tumor, the F98 glioma, with the gene encoding B7.1 could enhance its immunogenicity. F98 cells were transfected with a plasmid containing the B7.1 gene, and stable transfectants (F98/B7.1) were obtained. Flow cytometric analysis confirmed the expression of B7.1 and MHC class I antigens on the cell surface. To investigate the effects of B7.1 expression on the tumorigenicity of the F98 glioma, Fischer rats were implanted intracerebrally with either F98 (wild-type) or F98/B7.1 transfected cells. No significant differences in survival times were noted. Mean survival times of 21.8 and 24.0 days were observed for the respective groups at a challenge dose of 103 cells. These differences in survival time were not significant. To determine if expression of B7.1 enhanced the immunogenicity of the F98 glioma, rats were vaccinated weekly for 3 weeks with 107 mitomycin C-treated F98 or F98/B7.1 cells injected subcutaneously and then challenged intracerebrally with F98 cells 1 week later. Unvaccinated animals or those that received wild-type F98 cells as a vaccine had a survival time (mean +/- s.d.) of 22.3 +/- 1.5 days following tumor challenge versus 20.0 +/- 1.7 days for rats that had been vaccinated with F98/B7.1. Although we recognize that it might be possible to design more effective vaccination regimes, nevertheless, our data indicate that transfection of the B7.1 gene into the F98 rat glioma did not enhance its immunogenicity, and that other approaches will be required.

Selective modulation of B-cell activation markers CD86 and I-Ak on murine draining lymph node cells following allergen or irritant treatment

It is well known that T cells are key effector cells in the development of allergic contact dermatitis. However, we and others have shown that mice exposed to contact allergens show a preferential increase in B lymphocytes in the draining lymph nodes (DLN) as seen by an increase in the percentage of B220+ or IgG/IgM+ cells. The purpose of the present investigation was to determine whether chemical allergens, in contrast to irritants, would modulate B-cell activation markers, CD86 and I-Ak, on B cells isolated from DLN of treated mice using the local lymph node assay (LLNA) protocol. Mice were treated on the ears for 3 consecutive days with concentrations of allergens (1-chloro-2,4-dinitrobenzene, alpha-hexylcinnamaldehyde, 4-ethoxymethylene-2-phenyl-2-oxazoline-5-one, and trinitrochlorobenzene), or irritants (benzalkonium chloride and sodium lauryl sulfate), which caused an increase in the number of DLN cells. The DLN were excised 72 h following the final chemical treatment, and the cells were prepared for analysis by flow cytometry. In mice treated with allergens an increase in the median intensity of I-AK and CD86 on B220+ or IgG/IgM+ B cells was observed compared to mice treated with irritants or vehicles. Mice treated with allergens demonstrated an increase in the median intensity of CD86 on B220+ B cells that was dose dependent and peaked at 72 h following the final allergen treatment. The increase in the median intensity of I-AK also was dose dependent but peaked at 96 h. Finally, T and B cells isolated from both allergen- and irritant-treated mice demonstrated an increase in [3H]thymidine incorporation compared to vehicle-treated and naïve mice at 72 h following the final chemical treatment. The results suggest that B cells isolated from DLN of allergen-treated mice are activated and proliferating. Analysis of B-cell activation markers may be useful in differentiating allergen and irritant responses in the draining lymph nodes of chemically treated mice.

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.

Induction of immunologic tolerance for transplantation

In the second half of the 20th century, the transplantation of replacement organs and tissues to cure disease has become a clinical reality. Success has been achieved as a direct result of progress in understanding the cellular and molecular biology of the immune system. This understanding has led to the development of immunosuppressive pharmaceuticals that are part of nearly every transplantation procedure. All such drugs are toxic to some degree, however, and their chronic use, mandatory in transplantation, predisposes the patient to the development of infection and cancer. In addition, many of them may have deleterious long-term effects on the function of grafts. New immunosuppressive agents are constantly under development, but organ transplantation remains a therapy that requires patients to choose between the risks of their primary illness and its treatment on the one hand, and the risks of life-long systemic immunosuppression on the other. Alternatives to immunosuppression include modulation of donor grafts to reduce immunogenicity, removal of passenger leukocytes, transplantation into immunologically privileged sites like the testis or thymus, encapsulation of tissue, and the induction of a state of immunologic tolerance. It is the last of these alternatives that has, perhaps, the most promise and most generic applicability as a future therapy. Recent reports documenting long-term graft survival in the absence of immunosuppression suggest that tolerance-based therapies may soon become a clinical reality. Of particular interest to our laboratory are transplantation strategies that focus on the induction of donor-specific T-cell unresponsiveness. The basic biology, protocols, experimental outcomes, and clinical implications of tolerance-based transplantation are the focus of this review.

T Cell Proliferation-Augmenting Activities of the Gene 3 Protein Derived from a Phage Library Clone with CD80-Binding Activity

We have isolated a phage clone, F2, by panning a phage library with a CTLA4-conformation recognizing mAb (anti-CTLA4 mAb). The unique sequence of 15 amino acids with an internal disulfide bond was inserted in the gene 3 proteins of F2 phage clone (F2-g3p). We show here that 1) F2-g3p was recognized with anti-CTLA4 mAb but not with anti-CD28 mAb, and 2) F2-g3p bound to CD80 but not to CD86. The surface plasmon resonance analysis showed that F2-g3p strongly bound CD80. F2-g3p inhibited the binding of CTLA4 to CD80 but not to CD86. In contrast, F2-g3p weakly inhibited the binding of CD28 with CD80. When hen egg lysozyme (HEL)-primed lymph node cells were stimulated with HEL in the presence of F2-g3p in vitro, cell proliferation was highly potentiated. In the absence of antigenic stimulation, F2-g3p induced no T cell proliferation, indicating the costimulatory nature of F2-g3p. The T cell-augmenting activity of the F2 clone was eliminated when the F2 clone was preincubated with CD80-Ig before the addition to the cultures, indicating the involvement of CD80-binding in the F2-g3p-mediated immunopotentiation. Thus, the F2 motif conferred CD80-binding activity and an immunoregulatory function to the g3p.

Non-glycosylated human B7-1(CD80) retains the capacity to bind its counter-receptors

Though the cell surface-associated costimulator B7-1(CD80) is known to be highly N-glycosylated, the functional significance of this N-glycosylation has not been evaluated. Two experimental approaches were taken to assess the influence of N-glycosylation on human B7-1 function. First, stable K562 transfectants expressing human B7-1 were treated with the N-glycosylation inhibitor tunicamycin. This treatment reduced the levels of B7-1 at the cell surface as judged by both indirect immunofluorescence/flow cytometry and immunoprecipitation analyses. Significantly, the non-glycosylated cell surface-associated B7-1 on tunicamycin-treated cells retained the capacity to bind CTLA-4 x Ig, a soluble derivative of the CTLA-4(CD152) counter-receptor. Second, experiments were performed with bacterially-produced non-glycosylated derivatives of human B7-1, comprising either the complete B7-1 extracellular domain (hB7-1 x ed) or the membrane-proximal IgC-homologue domain of B7-1 in isolation (hB7-1 x IgC). While the hB7-1 x IgC derivative failed to bind to CTLA-4, the larger hB7-1 x ed derivative associated with CTLA-4 x Ig in cell-free binding assays. Futhermore, recombinant hB7-1 x ed effectively blocked B7-1-mediated costimulation in an in vitro T cell proliferation assay, suggesting that this soluble non-glycosylated B7-1 derivative is capable of engaging CD28, the B7 counter-receptor implicated in T cell activation. Taken together, these data indicate that the N-glycosylation of B7-1 is not required for its association with counter-receptors. Moreover, the findings pave the way for the therapeutic use of recombinant bacterial B7-1 derivatives as competitive inhibitors of B7-mediated signals.

B7-CTLA4 interaction enhances both production of antitumor cytotoxic T lymphocytes and resistance to tumor challenge

Expression of B7-family costimulatory molecules CD80 (B7-1) and CD86 (B7-2) on tumor cells enhances host immunity. However, the role of the two B7 receptors, CD28 and CTLA4 (CD152), on T cells in antitumor immune response has not been clearly elucidated. Based on the effects of anti-CD28 and anti-CTLA4 mAbs on T cell response, it was proposed that CD28-B7 interaction promotes antitumor immunity, whereas B7-CTLA4 interaction down-regulates it. A critical test for the hypothesis is whether selective engagement of CTLA4 receptors by their natural ligands CD80 and CD86 enhances or reduces antitumor immunity. Here we used tumors expressing wild-type and mutant CD80, as well as mice with targeted mutation of CD28, to address this issue. We report that in syngeneic wild-type mice, B7W (W88>A), a CD80 mutant that has lost binding to CD28 but retained binding to CTLA4, can enhance the induction of antitumor cytotoxic T lymphocytes (CTL); B7Y (Y201>A), which binds neither CD28 nor CTLA4, fails to do so. Consistent with these observations, B7W-transfected J558 plasmocytoma and EL4 thymoma grow significantly more slowly than those transfected with either vector alone or with B7Y. Optimal tumor rejection requires wild-type CD80. Moreover, expression of a high level of CD80 on thymoma EL4 cells conveys immunity in mice with a targeted mutation of CD28 gene. Taken together, our results demonstrate that B7-CTLA4 interaction enhances production of antitumor CTL and resistance to tumor challenge and that optimal enhancement of antitumor immunity by CD80 requires its engagement of both CD28 and CTLA4.

A B7.1-Antibody Fusion Protein Retains Antibody Specificity and Ability to Activate Via the T Cell Costimulatory Pathway

We describe the construction and characterization of an Ab fusion protein specific for the tumor-associated Ag HER2/neu linked to sequences encoding the extracellular domain of the B7.1 T cell costimulatory ligand. The Ab domain of the fusion molecule will specifically target HER2/neu-expressing tumor cells, while the B7.1 domain is designed to activate a specific immune response. We show that the B7.1 fusion Ab retained ability to selectively bind to the HER2/neu Ag and to the CTLA4/CD28 counter-receptors for B7.1. Specific T cell activation was observed when the B7.1 Ab fusion protein was bound to HER2/neu-expressing cells. The use of the B7.1 Ab fusion protein may overcome limitations of gene transfer and/or standard Ab therapy and represents a novel approach to the eradication of minimal residual disease.

Identification of conserved amino acids in murine B7-1IgV domain critical for CTLA4/CD28:B7 interaction by site-directed mutagenesis: a novel structural model of the binding site

The B7: CD28/CTLA4 interaction plays a major role in T cell responses. Immune intervention targeted at this interaction has demonstrated a vast potential in enhancing tumor immunity and blocking autoimmunity and transplant rejection. However, the structural basis for this interaction is unclear. While we and others have performed site-directed mutagenesis to define amino acids involved in binding CD28 and CTLA4, these residues are localized in different regions, and it is unlikely for all of them to be directly involved. In addition, the effect of the mutations on the overall conformation of B7 has not been systematically evaluated. In this study, we have carried out site-directed mutagenesis to define the amino acids within B7-1 IgV-like domain which participate B7:CD28/CTLA4 interaction. Four anti-B7-1 mAbs that recognize three independent antigenic epitopes on B7-1 were used to monitor the effect of mutations on the overall conformation of B7-1. Of the five mutations in the IgV domain that we have produced, D113 > A appears to interfere with cell surface expression and/or overall conformation of B7-1. while four others do not significantly affect the overall conformation and cell surface expression of B7-1. Among them, G115 > A and Y91 > A eliminated B7-1 binding to both CD28Ig and CTLA4Ig; our previously reported mutants L109 > A and W88 > A selectively affect the B7-1 binding to either CD28Ig or CTLA4Ig. Structural modeling of B7-1 based on the structure of immunoglobulin revealed that these four and other previously identified critical amino acids in both IgV- and IgC-like domains can form a localized structure.

Analysis of the B7 costimulatory pathway in allograft rejection

BACKGROUND

Blockade of the B7/CD28 costimulation pathway with the fusion protein, CTLA4-Ig, has been shown to prolong allograft survival in numerous rodent models, suggesting that this pathway is functionally important in the allograft rejection response. This pathway is complex and consists of at least the B7-1, B7-1a, B7-1cyt II, and B7-2 molecules on the antigen-presenting cell and CD28 and CTLA4 molecules on the T cell.

METHODS

The intragraft transcript expression of the B7 molecules and their counterreceptors was defined using reverse transcriptase-polymerase chain reaction in the vascularized mouse cardiac allograft model. In addition, the functional significance of these molecules was investigated both in vitro in the mixed leukocyte response (MLR) and in vivo in the vascularized mouse cardiac allograft model.

RESULTS

Intragraft expression of B7-1, B7-1a, B7-1cyt II, B7-2, CD28, and CTLA4 transcripts is up-regulated in allografts when compared with both normal untransplanted hearts and syngeneic transplants at between 5 and 12 days after transplant. Both anti-B7-1 and anti-B7-2 monoclonal antibodies alone inhibited T-cell proliferation in the MLR, however, equivalent maximal inhibition was obtained by a combination of these agents or by CTLA4-Ig. Likewise, in the mouse cardiac allograft model, both anti-B7-1 and anti-B7-2 modestly prolonged graft survival. However, an increased survival was obtained with either a combination of anti-B7-1 and anti-B7-2 or CTLA4-Ig. Blockade of the B7/CD28 pathway in the MLR using T cells from CD28 knockout mice had no effect on the proliferative response. Likewise, blockade of the B7/CD28 pathway did not effect the rate of rejection of cardiac allografts by CD28 knockout recipients.

CONCLUSIONS

These data suggest that both B7-1 and B7-2 have an important role in allograft rejection in the mouse vascularized cardiac allograft model.

CTLA-4-B7 interaction is sufficient to costimulate T cell clonal expansion

T cell costimulation, particularly by the B7 family members B7-1 and B7-2, plays a critical role in regulating T cell-mediated immunity. Two molecules on T cells, CD28 and CTLA-4, are known to bind to B7. It has been suggested that CD28-B7 interaction promotes T cell response, whereas B7-CTLA-4 interaction downregulates T cell clonal expansion. However, the proposed responses of individual receptors to B7 have not been verified directly. Here, we report that B7-1 promotes clonal expansion of CD28-deficient T cells, and that the CD28-independent costimulatory activity is mediated by CTLA-4, as it is completely blocked by intact and Fab of anti-CTLA-4 mAb. In addition, a mutant B7-1 molecule, B7W88 >A, which has lost binding to CD28 but retained significant CTLA-4 binding activity, promotes T cell clonal expansion. Furthermore, while presence of CD28 enhances T cell response to B7-1, such response is also completely blocked by anti-CTLA-4 mAb. Taken together, our results demonstrate that B7-CTLA-4 interaction promotes T cell clonal expansion, and that optimal T cell response to B7 is achieved when both CD28 and CTLA-4 interact with B7. These results establish an important function of CTLA-4 in promoting T cell activation, and suggest an alternative interpretation of the function of CTLA-4 in T cell activation.

Induction of hapten-specific tolerance of human CD8+ urushiol (poison ivy)-reactive T lymphocytes

The interaction of CD28 with B7 molecules (CD80 or CD86) is an essential second signal for both the activation of CD4+ T cells through the T-cell receptor and the prevention of anergy. We studied the requirement of hapten-specific human CD8+ cells for CD28 co-stimulation in recognition of hapten, and anergy induction. Urushiol, the immunogenic hapten of poison ivy (Toxicodendron radicans), elicits a predominantly CD8+ T-cell response. Autologous PBMC were pre-incubated with urushiol prior to fixation by paraformaldehyde. Fixed antigen-presenting cells were unable to present urushiol to human CD8+ urushiol-specific T cells. Addition of anti-CD28, however, overcame this antigen-presenting defect, enabling CD8+ cells to proliferate. Fixation of antigen-presenting cells prevents upregulation of B7, and addition of anti-CD28 substitutes for this signal. Proliferation of CD8+ T cells in response to urushiol was blocked by CTLA4Ig, a recombinant fusion protein that blocks CD28/B7 interactions. Preincubation of urushiol-specific CD8+ cells with fixed PBMC + urushiol for 7 d induced anergy. Anergic CD8+ cells were viable and able to proliferate in response to IL-2, but not in response to urushiol. Induction of anergy required the presence of urushiol, and pre-incubation with irradiated PBMC + urushiol did not have this effect. It is proposed that anergy was induced by presentation of urushiol by fixed PBMC, in the absence of adequate co-stimulation signals. Induction of anergy by blocking of co-stimulation could potentially induce clinical hyposensitization to haptens.

Structure/function relationships of axon-associated adhesion receptors of the immunoglobulin superfamily

Evidence is accumulating that axonal members of the Ig superfamily (IgSF) interact in a complex manner with other axonal Ig-like proteins and with proteins of the extracellular matrix. Studies investigating the structure/function relationships of these proteins have highlighted the importance of Ig-like domains near the amino terminus (N-proximal) as both necessary and sufficient for homophilic and heterophilic binding. Although efforts have been made in the past year to correlate the structure and neurite-outgrowth-promoting ability of axonal IgSF members, this work is still at an early stage.

Identification of a costimulatory molecule rapidly induced by CD40L as CD44H

The interaction between CD40 ligand and CD40 is critical for activation of T and B cells in vivo. We have recently demonstrated that this interaction rapidly induces a novel costimulatory activity distinct from B7 and independent of CD28. To study the molecular basis of the costimulatory activity, we have produced a novel monoclonal antibody, TM-1, that binds an 85-kilodalton costimulatory molecule rapidly induced by CD40L. Expression cloning reveals that TM-1 binds CD44H. CD44H expressed on Chinese hamster ovary cells has potent costimulatory activity for clonal expansion of T cells isolated from both wild-type mice and these with a targeted mutation of CD28. Thus, CD44H costimulates T cell proliferation by a CD28-independent mechanism. These results revealed that CD44H is a costimulatory molecule rapidly induced by CD40L.

The amino-terminal immunoglobulin-like domain of activated leukocyte cell adhesion molecule binds specifically to the membrane-proximal scavenger receptor cysteine-rich domain of CD6 with a 1:1 stoichiometry

Activated leukocyte cell adhesion molecule (ALCAM) was recently identified as a ligand for CD6, a signaling receptor expressed on T cells, a subset of B cells, and some cells in the brain. Receptor-ligand binding assays, antibody blocking experiments, and examination of the tissue distribution of these two cell surface proteins suggest that CD6-ALCAM interactions play an important role in mediating the binding of thymocytes to thymic epithelial cells and of T cells to activated leukocytes. Presently, the details of CD6-ALCAM interactions and of signaling through CD6 are unknown. A series of truncated human ALCAM and CD6 immunoglobulin fusion proteins were produced and tested in different binding assays to analyze ALCAM-CD6 interactions in more detail. In this study, we report that the amino-terminal Ig-like domain of human ALCAM specifically binds to the third membrane-proximal scavenger receptor cysteine-rich (SRCR) domain of human CD6. Using thrombin-cleaved Ig fusion proteins containing single or multiple ALCAM or CD6 domains, we were able to determine that the stoichiometry of the interaction between the amino-terminal ALCAM domains and the membrane-proximal CD6 SRCR domain is 1:1. These results provide the first example of an Ig-like domain mediating an interaction with an SRCR domain.

Cytokines and transplantation: Th1/Th2 regulation of the immune response to solid organ transplants in the adult

It has been very tempting to accept the suggestion that the route to rejection or tolerance of organ transplants is determined by T-helper type 1 and type 2 cells, respectively. Much of the data used to support this idea, however, is indirect and therefore cannot be used to imply a causal role for either population as suggested. Recent experiments have been aimed at further expanding knowledge in this area and conclude that the expansion of neither population alone inevitably results in graft damage or tolerance.

Identification of residues in the V domain of CD80 (B7-1) implicated in functional interactions with CD28 and CTLA4

The CD80 (B7-1) molecule is a 45-60-kD member of the immunoglobulin superfamily that is expressed on a variety of cell types of haematopoietic origin. CD80 can provide a critical costimulatory signal to T cells by interacting with the T cell surface molecule CD28. CD80 also binds to the CD28-related molecule CTLA4, which is expressed on activated T cells, Recently, additional ligands of CD28 and CTLA4 have been described in mice and humans. One of them, CD86 (B-70 or B7-2) was characterized at the molecular level. Although similar in predicted structure to CD80, it is distantly related in amino acid sequence. In this study, human CD80 mutants were generated and tested for their ability to maintain the interaction with CD28 leading to adhesion and enhanced IL-2 production. Two hydrophobic residues in the V-like domain of CD80 were identified as critical for binding to CD28 and are also important for the interaction with CTLA4. These residues are adjacent to the epitope of the BB1 antibody, which inhibits CD28-CD80 interactions. One of these residues, Y87, is conserved in all CD80 and CD86 cloned from various species. These results being to unravel the structural requirements for binding to CD28 and CTLA4.