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

Future investigative directions for novel therapeutic targets in head and neck cancer

AREAS COVERED

Here we describe novel agents, their mechanism(s) of action, preclinical results, and ongoing clinical trials in HNSCC.

EXPERT OPINION

Established therapeutic targets in HNSCC include EGFR (cetuximab) and PD-1 (pembrolizumab and nivolumab). Despite the detection of many other possible targets in HNSCC cell lines and patient tumors, no other therapies have successfully advanced to date. Identification of predictive biomarkers may guide the use of targeted agents and combination therapies. Clinical trials supported by strong preclinical data in relevant models are more likely to advance treatment options.

Immunologic Significance of CD80/CD86 or Major Histocompatibility Complex-II Expression in Thymic Epithelial Tumors

Introduction

Unresectable or recurrent thymic epithelial tumors (TETs) have a poor prognosis, and treatment options are limited. This study aimed to investigate the immunologic significance of CD80/CD86 or major histocompatibility complex class II (MHC-II) expression in TETs, as potential predictive biomarkers for immune checkpoint inhibitors (ICIs).

Methods

We analyzed CD80, CD86, MHC class I (MHC-I), and MHC-II expression in TETs using immunohistochemistry and investigated their association with T-cell infiltration or ICI efficacy. In addition, we generated CD80- or MHC-II-expressing mouse tumors, evaluated the effects of ICIs, and analyzed tumor-infiltrating lymphocytes. We also performed tumor-rechallenge experiments in vivo.

Results

We found that approximately 50% and 30% of TETs had high expression of CD80/CD86 and MHC-II in tumor cells, respectively, and that this expression was related to T-cell infiltration in clinical samples. In mouse models, both CD80 and MHC-II increase the effects of ICIs. In addition, senescent T cells and long-lived memory precursor effector T cells were significantly decreased and increased, respectively, in tumor-infiltrating lymphocytes from CD80-expressing tumors, and rechallenged tumors were completely rejected after the initial eradication of CD80-expressing tumors by programmed cell death protein 1 blockade. Indeed, patients with CD80-high thymic carcinoma had longer progression-free survival with anti-programmed cell death protein 1 monoclonal antibody.

Conclusions

Half of the TETs had high expression of CD80/CD86 or MHC-II with high T-cell infiltration. These molecules could potentially increase the effects of ICIs, particularly inducing a durable response. CD80/CD86 and MHC-II can be predictive biomarkers of ICIs in TETs, promoting the development of drugs for such TETs.

Activation of T cell checkpoint pathways during β-cell antigen presentation by engineered dendritic cells promotes protection from type 1 diabetes

Defective immune regulation has been recognized in type 1 diabetes (T1D). Immune regulatory T cell check-point receptors, which are generally upregulated on activated T cells, have been the molecules of attention as therapeutic targets for enhancing immune response in tumor therapy. Here, we show that pancreatic β-cell antigen (BcAg) presentation by engineered tolerogenic dendritic cells (tDCs) that express CTLA4 selective ligand (B7.1wa) or a combination of CTLA4, PD1 and BTLA selective ligands (B7.1wa, PD-L1, and HVEM-CRD1 respectively; multiligand-DCs) causes an increase in regulatory cytokine and T cell (Treg) responses and suppression of the effector T cell function as compared to engineered control-DCs. Non-obese diabetic (NOD) mice treated with BcAg-pulsed CTLA4-ligand-DCs and multiligand-DCs at pre-diabetic and early-hyperglycemic stages showed significantly lower degree of insulitis, higher frequencies of insulin-positive islets, profound delay in, and reversal of, hyperglycemia for a significant duration. Immune cells from the tDC treated mice not only produced lower amounts of IFNγ and higher amounts of IL10 and TGFβ1 upon BcAg challenge, but also failed to induce hyperglycemia upon adoptive transfer. While both CTLA4-ligand-DCs and multiligand-DCs were effective in inducing tolerance, multiligand-DC treatment produced an overall higher suppressive effect on effector T cell function and disease outcome. These studies show that enhanced engagement of T cell checkpoint receptors during BcAg presentation can modulate T cell function and suppress autoimmunity and progression of the disease in T1D.

CTLA-4 (+49A/G) Polymorphism in Type 1 Diabetes Children of Sudanese Population

Background  Type 1 diabetes mellitus (T1DM) is an organ-specific T cell-mediated autoimmune disease, characterized by destruction of pancreatic islets. Cytotoxic lymphocyte antigen-4 ( CTLA-4 ) is a negative regulator of T cell proliferation, thus conferring susceptibility to autoimmunity.

Aims  This study aimed to investigate the association of CTLA-4 +49A/G (rs231775) polymorphism with a risk of T1DM in Sudanese children.

Methods  This a case–control study included 100 children with T1DM, referred to the pediatric clinic at referral pediatric teaching hospital in Gezira State-Sudan. Hundred unrelated healthy controls were recruited from departments in the same hospital. Genomic deoxyribonucleic acid (DNA) was extracted from Ethylenediaminetetraacetic Acid (EDTA)-preserved blood using QIAamp DNA Blood Mini Kit (QIAamp Blood) (QIAGEN; Valencia, CA). The polymerase chain reaction PCR restriction fragment length polymorphism (PCR-RFLP) and sequencing were applied for the CTLA-4 (+49A/G) genotyping. The changes accompanied the polymorphism were evaluated using relevant bioinformatics tools.

Results  The genotype and allele frequencies of the CTLA-4 (+49A/G) polymorphism were significantly different between the patients and controls ( p  = 0.00013 and 0.0002, respectively). In particular, the frequency of the G allele, GG homozygous genotype, and AG heterozygous genotype were significantly increased in patients than in controls ([28% versus 7%, odds ratio (OR) = 5.16, 95% confidence interval [CI] = 2.77–9.65, p  = 0.00] [12% versus 2%, OR = 6.68, CI = 1.46–30.69, p  = 0.01] [32% versus 10%, OR = 4.24, CI = 1.95–9.21, p  = 0.00], respectively). The presence of the G allele (homozygous) showed an influence on the signal peptide polarity, hydrophobicity, and α-helix propensity of the CTLA-protein.

Conclusion  The results further support the association of CTLA-4 (+49A/G) polymorphism and the risk of T1DM in our study population.

Preserving the CTLA-4 Checkpoint for Safer and More Effective Cancer Immunotherapy

A major paradigm in cancer immunotherapy is the use of checkpoint inhibitors to break regulatory mechanisms that usually guard the host against autoimmune diseases. CTLA-4-targeting immunotherapy was the first example that helped establish this paradigm. However, the clinically tested anti-CTLA-4 antibodies exhibit suboptimal efficacy but high toxicity. Recent studies have demonstrated that immunotherapy-related adverse events (irAE) and the cancer immunotherapeutic effect (CITE) represent distinct and therapeutically separable activities of anti-CTLA-4 antibodies. The former is attributable to inactivation of the CTLA-4 checkpoint, while the latter is due to selective depletion of regulatory T cells (Treg) in a tumor microenvironment. Here we argue that for safer and more effective CTLA-4-targeting immune therapy, one should preserve rather than inhibit the CTLA-4 checkpoint while enhancing the efficacy and selectivity of Treg depletion in a tumor microenvironment.

Engineered Dendritic Cell-Directed Concurrent Activation of Multiple T cell Inhibitory Pathways Induces Robust Immune Tolerance

Inhibitory/repressor-receptors are upregulated significantly on activated T cells, and have been the molecules of attention as targets for inducing immune tolerance. Induction of effective antigen specific tolerance depends on concurrent engagement of the TCR and one or more of these inhibitory receptors. Here, we show, for the first time that dendritic cells (DCs) can be efficiently engineered to express multiple T cell inhibitory ligands, and enhanced engagement of T cell inhibitory receptors, upon antigen presentation, by these DCs can induce effective CD4+ T cell tolerance and suppress autoimmunity. Compared to control DCs, antigen presentation by DCs that ectopically express CTLA4, PD1 and BTLA selective ligands (B7.1wa, PD-L1, and HVEM-CRD1 respectively) individually (mono-ligand DCs) or in combination (multi-ligand DCs) causes an inhibition of CD4+ T cell proliferation and pro-inflammatory cytokine response, as well as increase in Foxp3+ Treg frequency and immune regulatory cytokine production. Administration of self-antigen (mouse thyroglobulin; mTg) loaded multi-ligand DCs caused hyporesponsiveness to mTg challenge, suppression of autoantibody production, and amelioration of experimental autoimmune thyroiditis. Overall, this study shows that engineered DC-directed enhanced concurrent activation of multiple T cell coinhibitory pathways is an effective way to induce self-antigen specific T cell tolerance to suppress ongoing autoimmunity.

Glycogen Synthase Kinase 3 Inactivation Compensates for the Lack of CD28 in the Priming of CD8+ Cytotoxic T-Cells: Implications for anti-PD-1 Immunotherapy

The rescue of exhausted CD8⁺ cytolytic T-cells (CTLs) by anti-Programmed Cell Death-1 (anti-PD-1) blockade has been found to require CD28 expression. At the same time, we have shown that the inactivation of the serine/threonine kinase glycogen synthase kinase (GSK)-3α/β with small-interfering RNAs (siRNAs) and small molecule inhibitors (SMIs) specifically down-regulates PD-1 expression for enhanced CD8⁺ CTL function and clearance of tumors and viral infections. Despite this, it has been unclear whether the GSK-3α/β pathway accounts for CD28 costimulation of CD8⁺ CTL function. In this article, we show that inactivation of GSK-3α/β through siRNA or by SMIs during priming can substitute CD28 co-stimulation in the potentiation of cytotoxic CD8⁺ CTL function against the EL-4 lymphoma cells expressing OVA peptide. The effect was seen using several structurally distinct GSK-3 SMIs and was accompanied by an increase in Lamp-1 and GZMB expression. Conversely, CD28 crosslinking obviated the need for GSK-3α/β inhibition in its enhancement of CTL function. Our findings support a model where GSK-3 is the central cosignal for CD28 priming of CD8⁺ CTLs in anti-PD-1 immunotherapy.

CTLA-4 blockade expands infiltrating T cells and inhibits cancer cell repopulation during the intervals of chemotherapy in murine mesothelioma

Cancer immunotherapy has shown promising results when combined with chemotherapy. Blocking CTLA-4 signaling by monoclonal antibody between cycles of chemotherapy may inhibit cancer cell repopulation and enhance the antitumoral immune reaction, thus improve the efficacy of chemotherapy in mesothelioma. The impact of CTLA-4 blockade on the early stage of tumor development was evaluated in a subcutaneous murine mesothelioma model. CTLA-4 blocking antibody was administered following each cycle of chemotherapy, and monotherapy was included as controls. Antitumor effect was evaluated by tumor growth delay and survival of the animals. Tumor cell repopulation was quantified by bromodeoxyuridine incorporation and Ki67 by immunohistochemistry and/or flow cytometry. In vitro cell killing was determined by classic chromium-released assay, and reverse transcription PCR (RT-PCR) was carried out to determine the gene expression of associated cytokines. Anti-CTLA-4 monoclonal antibody was able to inhibit tumor growth at early stage of tumor development. Antitumor effect was achieved by administration of CTLA-4 blockade between cycles of chemotherapy. Tumor cell repopulation during the intervals of cisplatin was inhibited by CTLA-4 blockade. Anti-CTLA-4 therapy gave rise to an increased number of CD4 and CD8 T cells infiltrating the tumor. RT-PCR showed that the gene expression of interleukin IL-2, IFN-γ, granzyme B, and perforin increased in the tumor milieu. Blockade of CTLA-4 signaling showed effective anticancer effect, correlating with inhibiting cancer cell repopulation between cycles of chemotherapy and upregulating tumor-infiltrating T lymphocytes, cytokines, and cytolytic enzymes in a murine mesothelioma model.

Cross-linking of CD80 and CD86 Diminishes Expression of CD54 on EBV-transformed B Cells through Inactivation of RhoA and Ras

Background

Epstein Barr virus (EBV) infected B cells are transformed into lymphoblastoid cell lines. Some researchers suggested some a few similarities between this process and carcinogenesis. We observed the expression of CD80 and CD86, co-stimulatory molecules on EBV-transformed B cells and changes of CD54 expression after stimulation of CD80 and CD86.

Methods

CD80 and CD86 were stimulated using anti-CD80 and anti-CD86 monoclonal antibodies. To assess apoptosis and surface protein expression, flow cytometric analysis was performed. Intracellular signal molecules were evaluated by RT-PCR and immunoblot. Morphology and localization of proteins were examined using inverted or confocal microscope.

Results

Cross-linking of CD80 and CD86 induced apoptosis and interfered with proliferation of EBV-transformed B cells, and dispersion of clumped cells. We also examined that their stimulation induced ROS accumulation and reduced CD54 expression. Interestingly, we observed that CD80 and CD86 diminished the expression of CD54 in different methods. Both CD80 and CD86 down-regulated activation of focal adhesion kinase. CD80 stimulus inhibited CD54 expression through mainly RhoA inactivation, while CD86 down-regulated Ras and JNK phosphorylation.

Conclusion

These results suggest that co-stimulatory CD80 and CD86 molecules, expressed EBV-transformed B cells, may play a role in apoptosis and cell adhesion.

Ovarian cancer vaccine trials and tribulations

Solid tumor vaccine therapy is coming of age. After years of failures, setbacks and negative trials, the first positive trials of antitumor vaccines in humans are being seen. Antitumor vaccine trials have reported an improvement in progression-free survival in breast cancer and an overall survival advantage in prostate cancer. Although, to date no positive Phase III antitumor vaccines trials in ovarian cancer have been reported, recent great strides have been made in improving tumor vaccine target antigens, improving antigen presentation and understanding the mechanisms of immunosuppression associated with tumors. In addition, biological therapies are now being identified that may enhance the efficacy of tumor vaccines. This review summarizes recent trials of ovarian cancer vaccines and addresses future directions to improve vaccine efficacy.

CD80 and CD86 control antiviral CD8+ T-cell function and immune surveillance of murine gammaherpesvirus 68

The interactions between CD80 and CD86 on antigen-presenting cells and CD28 on T cells serve as an important costimulatory signal in the activation of T cells. Although the simplistic two-signal hypothesis has been challenged in recent years by the identification of different costimulators, this classical pathway has been shown to significantly impact antiviral humoral and cellular immune responses. How the CD80/CD86-CD28 pathway affects the control of chronic or latent infections has been less well characterized. In this study, we investigated its role in antiviral immune responses against murine gammaherpesvirus 68 (MHV-68) and immune surveillance using CD80/CD86(-/-) mice. In the absence of CD80/CD86, primary antiviral CD8(+) T-cell responses and the induction of neutralizing antibodies were severely impaired. During long-term immune surveillance, the virus-specific CD8(+) T cells were impaired in IFN-gamma production and secondary expansion and exhibited an altered phenotype. Surprisingly, a low level of viral reactivation in the lung was observed, and this effect was independent of CD28 and CTLA-4. Thus, CD80 and CD86, signaling through CD28 and possibly another unidentified receptor, are required for optimal immune surveillance and antiviral immune responses to murine gammaherpesvirus.

Low surface expression of B7-1 (CD80) is an immunoescape mechanism of colon carcinoma

Artificially enforced expression of CD80 (B7-1) and CD86 (B7-2) on tumor cells renders them more immunogenic by triggering the CD28 receptor on T cells. The enigma is that such B7s interact with much higher affinity with CTLA-4 (CD152), an inhibitory receptor expressed by activated T cells. We show that unmutated CD80 is spontaneously expressed at low levels by mouse colon carcinoma cell lines and other transplantable tumor cell lines of various tissue origins. Silencing of CD80 by interfering RNA led to loss of tumorigenicity of CT26 colon carcinoma in immunocompetent mice, but not in immunodeficient Rag-/- mice. CT26 tumor cells bind CTLA-4Ig, but much more faintly with a similar CD28Ig chimeric protein, thus providing an explanation for the dominant inhibitory effects on tumor immunity displayed by CD80 at that expression level. Interestingly, CD80-negative tumor cell lines such as MC38 colon carcinoma and B16 melanoma express CD80 at dim levels during in vivo growth in syngeneic mice. Therefore, low CD80 surface expression seems to give an advantage to cancer cells against the immune system. Our findings are similar with the inhibitory role described for the dim CD80 expression on immature dendritic cells, providing an explanation for the low levels of CD80 expression described in various human malignancies.

Expression of ICAM-1, HLA-DR, and CD80 on peripheral circulating CD1 α DCs induced in vivo by IFN-α in patients with chronic hepatitis B

AIM: To explore the effects of interferon-α (IFN-α) application on peripheral circulating CD1α dendritic cells (DCs) in patients with chronic hepatitis B, and the expression of HLA-DR, CD80, and ICAM-1 on CD1α DCs in order to explore the mechanism of immune modulation of IFN-α.

METHODS: By flow cytometry technique, changes of CD1α DCs were monitored in 22 patients with chronic hepatitis B treated with IFN-α and in 16 such patients not treated with IFN-α within three months. Meanwhile, the expression of HLA-DR, CD80, and ICAM-1 on CD1α DCs was detected.

RESULTS: In the group of IFN-α treatment, the percentage of CD1α DCs in peripheral blood mononuclear cells was increased after three months of therapy. In patients who became negative for HBV-DNA after IFN-α treatment, the increase of DCs was more prominent, while in control, these changes were not observed. Increased expression of HLA-DR, CD80, and ICAM-1 on CD1α DCs was also observed.

CONCLUSION: CD1α DCs can be induced by IFN-α in vivo, and the immune related molecules such as HLA-DR, CD80, and ICAM-1 are up-regulated to some degree. This might be an important immune related mechanism of IFN-α treatment for chronic hepatitis B.

A mutant B7-1/Ig fusion protein that selectively binds to CTLA-4 ameliorates anti-tumor DNA vaccination and counters regulatory T cell activity

We have shown that a plasmid encoding a B7-1/Ig fusion protein enhanced DNA vaccination against human carcinoembryonic antigen (CEA) more effectively than the plasmid encoding membrane-bound B7-1. However, it was not known if B7-1/Ig acted only by binding CD28 (amplifying a stimulatory signal) or by blocking CTLA-4 on T cells (removing inhibitory signals). Here, we aimed to determine this using a plasmid encoding mutant B7-1/Ig (B7-1wa/Ig), which binds only to CTLA-4 but not to CD28. Our results showed that both the B7-1/Ig and B7-1wa/Ig plasmids, when co-administered with a CEA plasmid, enhanced tumor rejection and the in vitro anti-CEA response. Therefore, B7-1wa/Ig ameliorates DNA vaccination, presumably by binding to CTLA-4. This could result from a number of non-exclusive mechanisms, such as a reduced threshold for T-cell activation, or blockade of CTLA-4/B7-mediated tolerogenic signals in DCs or T cells. We found that, in vitro, a significant fraction of CD3/CD28-activated T cells (in the absence of DCs) expressed CTLA-4 and B7-1. Primed T cells of CTLA-4(+)B7-1(+/-) phenotype acted as regulatory T cells by inhibiting IFNgamma production by re-stimulated CTLA-4(-)B7-1(-) cells, and this was reversed by antibodies against IL-10 or TGF-beta1. Both B7-1wa/Ig and CTLA-4/Ig, which bind to CTLA-4 and B7-1/B7-2 respectively, enhanced IFNgamma production, but not the proliferation or IL-4 release in mixed T-cell populations containing these two cell types. In contrast, CTLA-4(-)B7-1(-) T cells produced IFNgamma which was not affected by B7-1wa/Ig or CTLA-4/Ig. These results suggest that blocking of CTLA-4/B7-1 binding in T cell/T cell interactions blocks negative regulatory signals. This might be the mechanism, at least in part, of the enhancement of anti-tumor immunity by the B7-1wa/Ig and B7-1/Ig plasmids.

Conversion of CTLA-4 from inhibitor to activator of T cells with a bispecific tandem single-chain Fv ligand

Abs or their recombinant fragments against surface receptors of the Ig superfamily can induce or block the receptors' native function depending on whether they induce or prevent the assembly of signalosomes on their cytoplasmic tails. In this study, we introduce a novel paradigm based on the observation that a bispecific tandem single-chain variable region fragment ligand of CTLA-4 by itself converts this inhibitory receptor into an activating receptor for primary human T lymphocytes. This reversal of function results from increased recruitment of the serine/threonine phosphatase 2A to the cytoplasmic tail of CTLA-4, consistent with a role of this phosphatase in the regulation of CTLA-4 function, and assembly of a distinct signalosome that activates an lck-dependent signaling cascade and induces IL-2 production. Our data demonstrate that the cytoplasmic domain of CTLA-4 has an inherent plasticity for signaling that can be exploited therapeutically with recombinant ligands for this receptor.

Exosomes from Plasmacytoma Cells as a Tumor Vaccine

Exosomes are membrane-bound vesicles derived from multivesicular bodies that are externalized by cells through fusion with the plasma membrane. Exosomes have been implicated in cell-to-cell signaling, and those derived from immunologic cells may be involved in both direct and cross-presentation of antigens to T cells. The research presented here evaluated their efficacy as a prophylactic cancer vaccine in a mouse plasmacytoma model. Plasmacytoma cells were shown to release exosomes in vitro, and vaccination with a single dose (5 microg) of exosome protein protected 80% of mice against challenge with wild-type tumors. Protection could be linked to the immune system since vaccinated mice generated specific cytotoxic T lymphocytes, the effects were not seen in SCID mice, and immunity was tumor-specific. Several proteins involved in immunity, including two potential tumor antigens (P1A and intracisternal A particle protein) as well as Hsp70, were demonstrated to be present in exosomes. The authors conclude that exosomes can induce tumor-specific immunity and prevent tumor development and are a potential strategy for future therapeutic tumor vaccination.

Intramuscular gene transfer of soluble B7.1/IgG(1) fusion cDNA induces potent antitumor immunity as an adjuvant for DNA vaccination

Soluble B7.1/IgG Fc fusion protein, which has costimulatory effects, is an effective molecular adjuvant in tumor immune therapy. Here, we describe a nonviral intramuscular (i.m.) gene transfer method to deliver this therapeutic protein. Gene transfer was greatly enhanced by electroporation and highly efficient production of this protein was achieved. Serum levels reached up to 1 microg/ml with considerable length of expression and without apparent systemic adverse effects. Lymphocytes from mice coinjected with soluble B7.1/IgG(1) and carcinoembryonic antigen (CEA)-encoding plasmids showed significantly elevated CEA-stimulated proliferation, cytokine production, and cytotoxic T-lymphocyte (CTL) activity. These mice gained significant protection against a CEA-positive transplanted tumor, in terms of reduced tumor incidence and growth. The effects were superior when soluble B7.1/IgG(1) was expressed as compared to membrane-bound wild-type B7.1. Notably, expression of soluble B7.1/IgG(1) alone did not induce any protection against tumor, confirming its primary role as a costimulatory molecule rather than a direct antitumor agent. The plasmid encoding B7.1/IgG(1) did not have to be injected at the same site as the antigen-encoding plasmid to exert its adjuvant effect, indicating that circulating protein was sufficient. Muscle histopathology revealed minimal damage to DNA-injected muscles. Importantly, we show that, after gene transfer, muscle tissue can produce this protein in large quantity to exert its immune costimulatory effect for cancer therapy and it would be otherwise difficult and expensive to maintain this high a level of recombinant protein.

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.

Differential effect of CD28 versus B7 blockade on direct pathway of allorecognition and self-restricted responses

Immunosuppression with B7 antagonists might have 2 opposite effects: reducing T-cell costimulation through CD28 but also preventing CTLA-4 from transmitting its negative regulatory signal. We therefore hypothesized that a selective blockade of CD28 might be qualitatively different from blocking B7. It was previously reported that CD28 modulation prolongs allograft survival in the rat and reverses induction of experimental autoimmune encephalomyelitis in mice. However, whether CD28 or B7 blockade results in similar immunosuppression on alloimmune and self-restricted responses to soluble antigens has not yet been investigated. Here, we addressed this issue in vitro with antagonist anti-CD28 Fab fragments and in vivo using the modulating anti-rat JJ319 monoclonal antibody. As in the inhibition of B7 with CTLA4 immunoglobulin, anti-CD28 Fab fragments inhibited allogenic T-cell proliferation in mixed cultures. In vivo modulation of CD28 blocked the expansion of alloreactive T cells and promoted their apoptosis. In contrast, selective blockade of CD28 did not modify T-cell proliferative responses and antibody production to soluble antigens, whereas blocking B7 with CTLA4 immunoglobulin did. Our data show that blocking CD28, while leaving CTLA4-B7 interactions undisturbed, inhibits alloreactive CD4+ T-cell expansion but does not modify the response to nominal antigens presented in the context of a self-major histocompatibility complex. That B7 engagement is needed for self-restricted responses whereas engagement of CD28 is not essential adds to the suggestion that another unidentified ligand of B7 might deliver a costimulatory signal in the absence of CD28.

Development and applications of surface-linked single chain antibodies against T-cell antigens

In this report the use of surface-linkage to expand the potential experimental and therapeutic applications of single chain antibody (scFv) constructs is reviewed. A strategy for the generation and functional characterization of surface-linked scFvs that bind selectively to the T-cell proteins CD3epsilon, CD28, and CD152 (CTLA-4) is described in detail. Experimental examples are provided of the use of these constructs to study the positive and negative regulation of T-cell activation and to manipulate the in vivo immunogenicity of tumor cells. In addition, a novel system for Simultaneous T-cell Activation and Retroviral Transduction (START) is described in which retroviral packaging cells are rendered mitogenic for T lymphocytes by combined expression of surface-linked scFvs. Finally, the use of random mutagenesis and yeast surface display to increase the affinity and functional efficacy of scFv constructs is demonstrated.

The T cell activation molecule H4 and the CD28-like molecule ICOS are identical

The recently cloned CD28-like molecule ICOS displays striking similarities with H4, characterized some years ago in the mouse and recently in humans. Both molecules are selectively expressed by activated and germinal center T cells, display similar structure, and display co-stimulatory activities. H4 displays lateral association with the CD3/TCR and is expressed by mature thymocytes. In the mouse, H4 is also expressed at high levels by thymic NKT cells that are resistant to negative selection. The aim of this work was to evaluate whether H4 and ICOS are the same molecule using the C398.4A (binding human and mouse H4) and F44 (binding human ICOS) monoclonal antibody (mAb) in parallel experiments on human T cells. ICOS and H4 displayed the same expression pattern in a panel of T cell lines and the same expression kinetics in phytohemagglutinin-activated T cells. C398.4A completely blocked cell staining by F44, whereas F44 partially blocked C398.4A. H4 and ICOS immunoprecipitates displayed identical SDS-PAGE patterns and H4 immunoprecipitation completely removed ICOS from cell lysates. Finally, the C398.4A mAb specifically stained cells transfected with the human or mouse ICOS. These data prove that H4 and ICOS are the same molecule and that F44 and C398.4A bind partially different epitopes.

Pinpointing when T cell costimulatory receptor CTLA-4 must be engaged to dampen diabetogenic T cells

Engagement of the T cell costimulatory receptor CTLA-4 can potently down-regulate an immune response. For example, in a T cell receptor transgenic mouse model of autoimmune diabetes, CTLA-4 interactions keep pancreatic islet-reactive T cells in check, evidenced by the finding that mAb blockade of CTLA-4 rapidly provokes diabetes in animals that would not normally succumb until many months later. Interestingly, this effect is only observed early in the course of disease, before insulitis is stably entrenched. Here, we have exploited a highly synchronous and easily manipulable transfer system to determine precisely when CTLA-4 must be engaged to check the diabetogenicity of islet-reactive T cells. Our results indicate that CTLA-4 interactions during initial priming of the T cells in the pancreatic lymph nodes are not determinant. Rather, the critical interactions occur when the T cells secondarily reencounter their antigen in the target organ, the pancreatic islets. In addition, we made use of CTLA-4-deficient mice to bolster our interpretation that CTLA-4 engagement has a dampening rather than an enhancing influence on diabetes progression.

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.

Blockade of T cell activation using a surface-linked single-chain antibody to CTLA-4 (CD152)

CTLA-4 (CD152) engagement can down-regulate T cell activation and promote the induction of immune tolerance. However, the strategy of attenuating T cell activation by engaging CTLA-4 has been limited by sharing of its natural ligands with the costimulatory protein CD28. In the present study, a CTLA-4-specific single-chain Ab (scFv) was developed and expressed on the cell surface to promote selective engagement of this regulatory molecule. Transfectants expressing anti-CTLA-4 scFv at their surface bound soluble CTLA-4 but not soluble CD28. Coexpression of anti-CTLA-4 scFv with anti-CD3epsilon and anti-CD28 scFvs on artificial APCs reduced the proliferation and IL-2 production by resting and preactivated bulk T cells as well as CD4+ and CD8+ T cell subsets. Importantly, expression of anti-CTLA-4 scFv on the same cell surface as the TCR ligand was essential for the inhibitory effects of CTLA-4-specific ligation. CTLA-4-mediated inhibition of tyrosine phosphorylation of components of the proximal TCR signaling apparatus was similarly dependent on coexpression of TCR and CTLA-4 ligands on the same surface. These findings support a predominant role for CTLA-4 function in the modification of the proximal TCR signal. Using T cells from DO11.10 and 2C TCR transgenic mice, negative regulatory effects of selective CTLA-4 ligation were also demonstrated during the stimulation of Ag-specific CD4+ and CD8+ T cells by MHC/peptide complexes. Together these studies demonstrate that selective ligation of CTLA-4 using a membrane-bound scFv results in attenuated T cell responses only when coengaged with the TCR during T cell/APC interaction and define an approach to harnessing the immunomodulatory potential of CTLA-4-specific ligation.

Activated and memory CD8+ T cells can be distinguished by their cytokine profiles and phenotypic markers

Dissecting the mechanisms of T cell-mediated immunity requires the identification of functional characteristics and surface markers that distinguish between activated and memory T lymphocytes. In this study, we compared the rates of cytokine production by virus-specific primary and memory CD8+ T cells directly ex vivo. Ag-specific IFN-gamma and TNF-alpha production by both primary and long-term memory T cells was observed in </=60 min after peptide stimulation. Although the on-rate kinetics of cytokine production were nearly identical, activated T cells produced more IFN-gamma, but less TNF-alpha, than memory T cells. Ag-specific cytokine synthesis was not a constitutive process and terminated immediately following disruption of contact with peptide-coated cells, demonstrating that continuous antigenic stimulation was required by both T cell populations to maintain steady-state cytokine production. Upon re-exposure to Ag, activated T cells resumed cytokine production whereas only a subpopulation of memory T cells reinitiated cytokine synthesis. Analysis of cytokine profiles and levels of CD8, LFA-1, and CTLA-4 together revealed a pattern of expression that clearly distinguished in vivo-activated T cells from memory T cells. Surprisingly, CTLA-4 expression was highest at the early stages of the immune response but fell to background levels soon after viral clearance. This study is the first to show that memory T cells have the same Ag-specific on/off regulation of cytokine production as activated T cells and demonstrates that memory T cells can be clearly discriminated from activated T cells directly ex vivo by their cytokine profiles and the differential expression of three well-characterized T cell markers.

Interleukin-4-independent acceleration of cutaneous leishmaniasis in susceptible BALB/c mice following treatment with anti-CTLA4 antibody

BALB/c mice are susceptible to progressive infection with Leishmania major due to the preferential development of CD4(+) T cells that secrete Th2 cytokines. Although Th2 cell development and susceptibility are disrupted by blockade of CD86 function early in infection, CD28-deficient BALB/c mice remain susceptible to leishmaniasis. We therefore examined whether the alternative CD86 ligand, CTLA4, contributes to the expression of susceptibility. BALB/c mice treated for 2 weeks of infection with anti-CTLA4 monoclonal antibody developed more rapidly progressive disease than sham-treated mice, whereas normally resistant C57BL/6 mice were unaffected. The draining lymph node cells of anti-CTLA4-treated BALB/c mice produced up to sixfold more interleukin-4 (IL-4) and IL-13 than control mice in the first 2 weeks of infection, but IFN-gamma synthesis was reciprocally decreased. Anti-CTLA4 treatment of BALB/c mice pretreated with neutralizing anti-IL-4 antibody or genetically deficient in IL-4 also caused significant worsening of leishmaniasis. Exacerbation in IL-4 KO mice was associated with increased IL-13 and decreased gamma interferon (IFN-gamma) and inducible nitric oxide synthase (iNOS) mRNA expression in vivo. These data indicate that anti-CTLA4 antibody induced earlier and more-polarized Th2 responses in susceptible BALB/c mice infected with L. major. The mechanism of disease worsening was partially IL-4 independent, indicating that increased IL-13 and/or decreased IFN-gamma production may have disrupted nitric oxide-based microbicidal responses. We conclude that CTLA4 significantly modulates Th2 development in murine leishmaniasis and that the Th2-polarizing effects of anti-CTLA4 treatment result in IL-4-independent exacerbation of disease.

Complete sequence determination of the mouse and human CTLA4 gene loci: cross-species DNA sequence similarity beyond exon borders

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.

Cutting Edge: Lymphoproliferative Disease in the Absence of CTLA-4 Is Not T Cell Autonomous

Mice deficient for the expression of CTLA-4 develop a lethal lymphoproliferative syndrome and multiorgan inflammation leading to death at about 4 wk of age. Here we show that RAG2-deficient mice reconstituted with CTLA-4-deficient bone marrow do not develop a lymphoproliferative syndrome despite lymphocyte infiltration mainly into pericardium and liver. Moreover, RAG2-deficient mice reconstituted with a mixture of normal and CTLA-4-deficient bone marrow remain healthy and do not develop any disease. Thus, the lethal disease observed in CTLA-4-deficient mice is not T cell autonomous and can be prevented by factors produced by normal T cells.

CD47 Signals T Cell Death

Activation-induced death of T cells regulates immune responses and is considered to involve apoptosis induced by ligation of Fas and TNF receptors. The role of other receptors in signaling T cell death is less clear. In this study we demonstrate that activation of specific epitopes on the Ig variable domain of CD47 rapidly induces apoptosis of T cells. A new mAb, Ad22, to this site induces apoptosis of Jurkat cells and CD3ε-stimulated PBMC, as determined by morphological changes, phosphatidylserine exposure on the cell surface, uptake of propidium iodide, and true counts by flow cytometry. In contrast, apoptosis was not observed following culture with anti-CD47 mAbs 2D3 or B6H12 directed to a distant or closely adjacent region, respectively. CD47-mediated cell death was independent of CD3, CD4, CD45, or p56lck involvement as demonstrated by studies with variant Jurkat cell lines deficient in these signaling pathways. However, coligation of CD3ε and CD47 enhanced phosphatidylserine externalization on Jurkat cells with functional CD3. Furthermore, normal T cells required preactivation to respond with CD47-induced apoptosis. CD47-mediated cell death appeared to proceed independent of Fas or TNF receptor signaling and did not involve characteristic DNA fragmentation or requirement for IL-1β-converting enzyme-like proteases or CPP32. Taken together, our data demonstrate that under appropriate conditions, CD47 activation results in very rapid T cell death, apparently mediated by a novel apoptotic pathway. Thus, CD47 may be critically involved in controlling the fate of activated T cells.

CTLA-4-Mediated Inhibition of Early Events of T Cell Proliferation

CTLA-4 engagement by mAbs inhibits, while CD28 enhances, IL-2 production and proliferation upon T cell activation. Here, we have analyzed the mechanisms involved in CTLA-4-mediated inhibition of T cell activation of naive CD4+ T cells using Ab cross-linking. CTLA-4 ligation inhibited CD3/CD28-induced IL-2 mRNA accumulation by inhibiting IL-2 transcription, which appears to be mediated in part through decreasing NF-AT accumulation in the nuclei. However, CTLA-4 ligation did not appear to affect the CD28-mediated stabilization of IL-2 mRNA. Further, CTLA-4 engagement inhibited progression through the cell cycle by inhibiting the production of cyclin D3, cyclin-dependent kinase (cdk)4, and cdk6 when the T cells were stimulated with anti-CD3/CD28 and with anti-CD3 alone. These results indicate that CTLA-4 signaling inhibits events early in T cell activation both at IL-2 transcription and at the level of IL-2-independent events of the cell cycle, and does not simply oppose CD28-mediated costimulation.

Costimulatory regulation of T cell function

During the past several years, the critical role of costimulatory molecules in regulating T cell responses has been demonstrated. Costimulatory molecule CD28 enhances whereas CTLA-4 downmodulates T cell responses. An understanding of the integration of the signals mediated by costimulatory molecules and the T cell receptor at the cellular and molecular levels is just beginning to be achieved.

CTLA-4 (CD152) Inhibits the Specific Lysis Mediated by Human Cytolytic T Lymphocytes in a Clonally Distributed Fashion

Since the functional outcome of effector T lymphocytes depends on a balance between activatory and inhibitory receptors, we studied the ability of CTLA-4 (CD152) to inhibit the cytolytic function of CTL. In 22 TCRαβ+CD3+8+ CTL clones, activation induced by anti-CD3, anti-CD28, or anti-CD2 mAb was inhibited by anti-CD152 mAb in a redirected killing assay. In eight clones inhibition was &gt;40%, in 10 it ranged between 20–40%, and in four it was &lt;20%. This suggests the existence of a clonal heterogeneity as well as for the ability of CTLA-4 to inhibit CD3/TCR-, CD28-, or CD2-mediated CTL activation. To support further this contention, we used an experimental model based upon Ag-specific CTL. Eight Ag-specific T cell clones that lyse autologous EBV-infected B lymphocytes, but are unable to lyse allogeneic EBV-infected B cell lines, were used in a cytolytic assay in which anti-CD152 mAb or soluble recombinant receptor (i.e., CTLA-4 Ig) were included. In this system, at variance from the redirected killing assay, cross-linking of surface molecules by mAb does not occur. Thus, addition of anti-CD152 mAb or of CTLA-4 Ig and anti-CD80/CD86 mAb to the assay should result in a blockade of receptor/ligand interactions. As a consequence, inhibition of a negative signal, such as that delivered via CD152, should enhance lysis. A &gt;40% increment of target cell lysis was achieved in three of eight clones studied. Since it is not equally shared by all CTL clones, this feature also appears to be clonally distributed.