Structural mechanisms of costimulation

Site-directed in vitro immunization leads to a complete human monoclonal IgG4 lambda that binds specifically to the CDR2 region of CTLA-4 (CD152) without interfering the engagement of natural ligands

Background

The ability to acquire fully human monoclonal antibodies (mAbs) with pre-defined specificities is critical to the development of molecular tags for the analysis of receptor function in addition to promising immunotherapeutics. Yet most of the arriving affinity maturated and complete human immunoglobulin G (IgG) molecules, which are actually derived from single human B cells, have not widely been used to study the conserved self antigens (Ags) such as CD152 (cytotoxic T lymphocyte antigen-4, CTLA-4) because proper hosts are lacking.

Results

Here we developed an optimized protocol for site-directed in vitro immunizing peripheral blood mononuclear cells (PBMC) by using a selected epitope of human CD152, an essential receptor involved in down-regulation of T cell activation. The resultant stable trioma cell lines constantly produce anti-CD152 mAb (γ4λhuCD152), which contains variable (V) regions of the heavy chain and the light chain derived from the VH3 and Vλ human germline genes, respectively, and yet displays an unusual IgG4 isotype. Interestingly, γ4λhuCD152 has a basic pI not commonly found in myeloid monoclonal IgG4λs as revealed by the isoelectric focusing (IEF) analysis. Furthermore, γ4λhuCD152 binds specifically, with nanomolar affinity, to an extracellular constituency encompassing the putative second complementarity determining region (CDR2) of CD152, whereby it can react to activated CD3⁺ cells.

Conclusion

In a context of specific cell depletion and conditioned medium,in vitro induction of human Abs against a conserved self Ag was successfully acquired and a relatively basic mAb, γ4λhuCD152, with high affinity to CDR2 of CD152 was thus obtained. Application of such a human IgG4λ mAb with designated CDR2 specificity may impact upon and prefer for CD152 labeling both in situ and ex situ, as it does not affect the binding of endogenous B7 ligands and can localize into the confined immunological synapse which may otherwise prevent the access of whole IgG1 molecules.

Suppression of immune responses in collagen-induced arthritis by a rationally designed CD80-binding peptide agent

OBJECTIVE

The CD80/CD86-CD28/CD152 costimulatory pathways transmit signals for CD4+ T cell activation and suppression and are critically involved in the pathogenesis of rheumatoid arthritis (RA). A significant number of CD4+ T cells and macrophages in the rheumatoid synovium express elevated levels of CD80, increasing the potential for costimulation in trans of naive T cells. To determine the effect of blockade of this costimulatory axis in RA, we designed novel CD80-binding peptides and evaluated their therapeutic potential in collagen-induced arthritis (CIA), an animal model of RA.

METHODS

The conserved MYPPPY motif of CD152 adopts a polyproline type II (PPII) helical conformation in the CD80-CD152 complex. The pairing preferences of the critical residues at the CD80-CD152 interface and their propensity to form PPII helices were integrated to design peptides with optimum PPII helical content that selectively block CD80-receptor interactions. The clinical efficacy was tested in DBA/1LacJ mice that were administered the CD80 blocking agents, called CD80-binding competitive antagonist peptides (CD80-CAPs), at the time of immunization with bovine type II collagen or 3 weeks after immunization.

RESULTS

A single administration of select CD80-CAPs significantly reduced the clinical, radiologic, and histologic disease severity in CIA. Importantly, administration of CD80-CAPs during activated immune response significantly suppressed disease development by reducing mononuclear cell infiltration in the joints and mediating peripheral deletion of activated CD4+ T cells.

CONCLUSION

A rationally designed CD80-binding peptide both prevents and suppresses CIA, suggesting a potential application in RA. Apoptosis of activated CD4+ T cells following in vivo blockade suggests that the effects of CD80-CAPs may be long-lasting.

The crystal structure of the costimulatory OX40-OX40L complex

OX40 is a T cell costimulator activated by OX40L. Blockade of the OX40L-OX40 interaction has ameliorative effects in animal models of T cell pathologies. In order to better understand the interaction between OX40 and OX40L, we have determined the crystal structure of murine OX40L and of the human OX40-OX40L complex at 1.45 and 2.4 A, respectively. These structures show that OX40L is an unusually small member of the tumor necrosis factor superfamily (TNFSF). The arrangement of the OX40L protomers forming the functional trimer is atypical and differs from that of other members by a 15 degrees rotation of each protomer with respect to the trimer axis, resulting in an open assembly. Site-directed changes of the interfacial residues of OX40L suggest this interface lacks a single "hot spot" and that instead, binding energy is dispersed over at least two distinct areas. These structures demonstrate the structural plasticity of TNFSF members and their interactions with receptors.

B7.1/NHS76: a new costimulator fusion protein for the immunotherapy of solid tumors

Tumor evasion from immune surveillance is due to the anergic status of tumor-infiltrating lymphocytes, especially T cells. Inappropriate or absent expression of costimulatory molecules such as B7.1 and B7.2 lead to anergy and apoptosis of tumor-infiltrating T cells. To reverse this situation, a tumor-targeted fusion protein, human B7.1/NHS76, was generated by molecular engineering, which retains both the costimulatory activity of B7.1 and the tumor-targeting ability of NHS76 antibody. NHS76 is a human tumor necrosis therapy monoclonal antibody derived from phage display, and is capable of binding intracellular antigens, which are accessible and abundant in necrotic regions of tumors. As human B7.1 can interact functionally with murine B7.1 counter-receptors, the immunotherapeutic potential of this fusion protein was tested in 3 mouse tumor models (Colon 26, RENCA, and MAD109), and animal studies showed a 35% to 55% reduction in tumor volume. To modulate the immune inhibitory microenvironment in tumors, naturally occurring CD4+ CD25+ Treg cells were depleted by cytotoxic CD4 or CD25 antibodies. Combination therapy with anti-Treg and B7.1/NHS76 produced complete regression of established tumors and was associated with increased effector T-cell infiltration in tumors. Rechallenge experiments performed 3 months after mice attained complete remission by combination therapy showed that immunologic memory was established by these treatments. These studies indicate that the targeting of B7.1 to necrotic areas of tumors, where both the release of tumor antigens and infiltrating lymphocytes are prevalent, may be a new approach for the immunotherapy of solid tumors. Our results also suggest that the manifestation of immune-inhibitory factors such as the presence of Treg cells at the tumor site and associated draining lymph nodes may be a major cause for immune system failure to eradicate solid tumors.

The cytoplasmic domain of Fas ligand costimulates TCR signals

Productive T cell activation generally requires costimulation in addition to a signal delivered through the TCR. Although FasL is well-characterized for its capacity to deliver a death signal through Fas, this TNF family member can also transmit a reverse signal to enhance Ag-driven T cell proliferation. In this study, we define this reverse signal through FasL as costimulation by showing it requires TCR coengagement and is CD28 independent. We demonstrate that FasL-mediated costimulation drives FasL recruitment into lipid rafts and association with select Src homology 3 (SH3)-containing proteins. We further show that the proline-rich intracellular domain of FasL is sufficient to costimulate by enhancing the phosphorylation of Akt, ERK1/2, JNK, and FasL itself, by activating the transcription factors NFAT and AP-1, and by enhancing IFN-gamma production. These results elucidate the pathway of costimulation through the death inducer FasL, and comprise the first mechanistic analysis of a newly emerging group of costimulators, the TNF family.

A molecular perspective of CTLA-4 function

Within the paradigm of the two-signal model of lymphocyte activation, the interest in costimulation has witnessed a remarkable emergence in the past few years with the discovery of a large array of molecules that can serve this role, including some with an inhibitory function. Interest has been further enhanced by the realization of these molecules' potential as targets to modulate clinical immune responses. Although the therapeutic translation of mechanistic knowledge in costimulatory molecules has been relatively straightforward, the capacity to target their inhibitory counterparts has remained limited. This limited capacity is particularly apparent in the case of the cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), a major negative regulator of T cell responses. Because there have been several previous comprehensive reviews on the function of this molecule, we focus here on the physiological implications of its structural features. Such an exercise may ultimately help us to design immunotherapeutic agents that target CTLA-4.

CTLA-4 lacking the cytoplasmic domain costimulates IL-2 production in T-cell hybridomas

Optimal T-cell activation depends on the antigen-specific signal mediated by the TCR and engagement of costimulatory receptors such as CD28. CTLA-4, a homologous counterpart of CD28, is considered to be a crucial inhibitory receptor. To test its function separately from CD28 in an antigen-driven and ligand-specific model, we stably transfected the T-cell hybridomas A1.1 and DO11.10, which lack significant endogenous CD28 or CTLA-4 expression, with wild-type CTLA-4 (CTLA-4 WT) and a construct lacking the cytoplasmic tail (tailless [TL]). Functional studies were carried out by co-incubation with APC expressing the B7 ligands for CTLA-4 and appropriate MHC molecules loaded with their cognate antigens. IL-2 production on costimulation of CTLA-4WT and TCR did not differ significantly from untransfected controls. However, coligation of TCR and CTLA-4TL resulted in a vigorous IL-2 response specific for the interaction of CTLA-4 with B7. Thus, lack of the cytoplasmic tail converted CTLA-4 into a costimulatory receptor. This indicates that the CTLA-4 inhibitory function may not be attributable to sequestration of the common B7 ligands when competing with CD28. Rather, ligation of B7 by the CTLA-4 extracellular domain can enhance TCR activation, whereas in the full-length receptor, inhibitory signals mediated by the cytoplasmic domain may override this activation.

Agrin is involved in lymphocytes activation that is mediated by alpha-dystroglycan

It is well established that agrin, an extracellular matrix protein, plays a crucial role in the formation of neuromuscular junctions. Recent evidence indicates that agrin also contributes to immunological synapse formation. However, little is known about how agrin induces the activation of lymphocytes and whose receptors mediate its regulatory effects on these cells. In the present study, agrin was detected in lymphocytes. Up-regulation of agrin expression was involved in lymphocyte activation whereas down-regulation of its expression led to inhibition of both antigen-specific and nonspecific lymphocyte activation, indicating an intrinsic role for agrin in the activation of lymphocytes. Unexpectedly, unlike that found in muscle cells where there is coexpression of muscle-specific kinase (MuSK) and alpha-dystroglycan receptors for agrin, only alpha-dystroglycan could be detected in lymphocytes. Confocal examination showed that alpha-dystroglycan colocalized with agrin in forming the immunological synapse. Down-regulation of alpha-dystroglycan expression inhibited lymphocyte activation even in the presence of agrin. However, agrin involved in down-regulation of alpha-dystroglycan receptors did not increase the inhibitory effect on lymphocytes activation. The anti-alpha-dystroglycan antibody also induced lymphocytes activation. Taken together, these findings strongly indicate that agrin and alpha-dystroglycan mediate lymphocyte activation. Furthermore, agrin-involved lymphocyte activation is mediated by alpha-dystroglycan.

CD137-mediated immunotherapy for allergic asthma

The prevalence of asthma continues to increase. Asthma is caused by a Th2 cell-driven immune response. Its optimal treatment remains a challenge, and a sufficient immunotherapeutic approach to treating asthma has yet to be found. Using a murine asthma model, we show that a single injection of an anti-CD137 (4-1BB) mAb prevents the development of airway hyperreactivity, eosinophilic airway inflammation, excessive mucus production, and elevated IgE during the observation period of 7 weeks. Most importantly, even established disease is completely reversed by anti-CD137 mAb administration. The protection is associated with markedly reduced Th2 cytokine production and increased secretion of the Th1 cytokine IFN-gamma. While B lymphocytes are partly depleted, the number of CD8+ T cells is increased. Blockade of IFN-gamma and depletion of CD8+ T cells during treatment with anti-CD137 mAb reduces in part but does not abrogate the protective effect of CD137 mAb. In contrast, CD137 mAb-mediated CD4+ T cell anergy is critical for the observed effects, since transfer of CD4+ T cells from CD137 mAb-treated mice conveyed protection. These data demonstrate, for the first time to our knowledge, the capacity of anti-CD137 mAb to ameliorate allergic asthma, and they indicate CD137 as a possible target for therapeutic intervention in this disease.

Combination B7-Fc fusion protein treatment and Treg cell depletion therapy

PURPOSE

A B7.1 fusion protein consisting of the extracellular domains of human B7.1 and the Fc portion of human IgG1, called B7.1-Fc, was generated and evaluated for its antitumor potential when used alone or in combination with regulatory T (Treg) cell depletion.

METHODS

A human B7.1-Fc fusion protein was constructed, expressed, purified, and examined for its antitumor activity in experimental mouse tumor models.

RESULTS

Soluble B7.1-Fc showed costimulatory activity of T-cell proliferation in vitro, and when given in vivo, it induced complete regression of Colon 26 tumors after a 5-day treatment regimen. Parallel studies with human B7.2-Fc gave very similar results in the Colon 26 tumor model. Even in mice with established RENCA and Madison 109 tumors, which are poorly immunogenic, B7.1-Fc treatment slowed tumor growth dramatically. In these models, more potent antitumor activity was achieved when B7.1-Fc was used in combination with Treg depletion by i.p. administration of antibody PC61. Rechallenge experiments done with mice that had sustained complete tumor regressions showed that these mice had immunologic memory by their ability to reject subsequent implants. Histologically, B7.1-Fc treatment induced multiple areas of necrosis and infiltration of CD4+ and CD8+ T cells in tumors along with a concomitant dramatic increase in T-cell proliferation in tumor-draining lymph nodes.

CONCLUSIONS

The B7.1-Fc fusion protein seems to be an effective antitumor agent especially in combination with Treg depletion. Its potency in stimulating immune responses and its human origin suggest that clinical studies may be warranted in the future.

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.

ICAM-1 co-stimulates target cells to facilitate antigen presentation

Adhesion molecules are known to mediate cell-cell interactions, particularly those between T cells and antigen-presenting or target cells. Recent studies identified ICAM-1 as a co-stimulatory ligand that binds to lymphocyte function associated antigen-1 (LFA-1), thereby promoting the activation of T cells. As ICAM-1 is expressed on virtually any cell, it becomes a crucial molecule for the activation of CD8(+) T cells in the absence of co-stimulation provided by CD80 and CD86 molecules. In addition, ICAM-1 might function as cell-surface receptor, capable of initiating intracellular signaling. ICAM-1 is associated with other cell molecules, including MHC-I proteins, and our recent data show that productive engagement of ICAM-1 on target cells leads to recruitment of the MHC-I proteins to the contact area and enhances presentation of cognate peptide MHC-I complexes to cytotoxic T cells.

Employing the immunological synapse in AML: Development of leukemic dendritic cells for active specific immunization

Cytotoxic T cells directed against leukemic blasts have been observed in patients with acute myeloid leukemia (AML). However, generation of efficient T-cell responses is hampered due to several factors that enable AML blasts to protect themselves from the patients immune system. Improved immune responses can be established by the differentiation of AML blasts into AML-derived dendritic cells (DC) thereby conserving their intrinsic leukemia specific antigens and obtaining full capacity to present these antigens to naive T cells. This review discusses increased immunogenicity of AML blasts by differentiation into AML-DC and describes ways to augment the AML-DC vaccination approach.

Hierarchical Regulation of CTLA-4 Dimer-Based Lattice Formation and Its Biological Relevance for T Cell Inactivation

CTLA-4 is an activation-induced, homodimeric inhibitory receptor in T cells. Recent crystallographic reports have suggested that it may form lattice-like arrays on the cell surface upon binding B7.1/B7.2 (CD80, CD86) molecules. To test the biological relevance of these CTLA-4-B7 lattices, we introduced a C122A point mutation in human CTLA-4, because this residue was shown to be essential for dimerization in solution. Surprisingly, we found that up to 35% of C122A CTLA-4 dimerized in human T lymphocytes. Moreover, C122A CTLA-4 partitioned within lipid rafts, colocalized with the TCR in the immunological synapse, and inhibited T cell activation. C122-independent dimerization of CTLA-4 involved N-glycosylation, because further mutation of the N78 and N110 glycosylation sites abrogated dimerization. Despite being monomeric, the N78A/N110A/C122A triple mutant CTLA-4 localized in the immunological synapse and inhibited T cell activation. Such functionality correlated with B7-induced dimerization of these mutant molecules. Based on these data, we propose a model of hierarchical regulation of CTLA-4 oligomerization by which B7 binding ultimately determines the formation of dimer-dependent CTLA-4 lattices that may be necessary for triggering B7-dependent T cell inactivation.

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.

A coreceptor interaction between the CD28 and TNF receptor family members B and T lymphocyte attenuator and herpesvirus entry mediator

Immune cell cosignaling receptors are important modulators of immune cell function. For T cells, cosignaling receptors supply necessary secondary signals supporting activation or attenuation after engagement of antigen-presenting cells. The primary cosignaling receptors belong to either the Ig (CD28-like) or TNF receptor (TNFR) superfamilies. The CD28 family is comprised of coinhibitory and costimulatory receptors. The three coinhibitory receptors are cytotoxic T lymphocyte antigen 4, programmed death-1, and B and T lymphocyte attenuator (BTLA). Although cytotoxic T lymphocyte antigen 4 and programmed death-1 interact with B7-Ig family counter receptors, the ligand for BTLA is less clear. From a protein-protein interaction screen, we identified the TNFR family member herpesvirus entry mediator (HVEM) as a counter receptor for BTLA. Here we show that HVEM binds BTLA with high affinity and can form a ternary complex with its known ligands homologous to lymphotoxin, showing inducible expression, and competing with herpes simplex virus glycoprotein D for HVEM, a receptor expressed by T lymphocytes (LIGHT) or lymphotoxin alpha and BTLA. In addition, binding of HVEM to BTLA attenuates T cell activation, identifying HVEM/BTLA as a coinhibitory receptor pair. This study is a demonstration of a direct interaction between the primary T cell cosignaling receptors of the CD28 and TNFR families.

The CD28 family: a T-cell rheostat for therapeutic control of T-cell activation

The CD28 family of receptors (CD28, cytotoxic T-lymphocyte-associated antigen 4 [CTLA-4], inducible costimulator [ICOS], program death-1 [PD-1], and B- and T-lymphocyte attenuator [BTLA]) plays a critical role in controlling the adaptive arm of the immune response. While considerable information is available regarding CD28 and CTLA-4, the function of the more recently discovered members of the CD28 family is less well understood. This review will highlight recent findings regarding the CD28 family with special emphasis on effects the CD28 family has on immunopathology, the discovery of costimulatory antibodies with superagonist function, and the status of clinical trials using various strategies to augment or block T-cell costimulation.

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.

Schistosoma mansoni worms induce anergy of T cells via selective up-regulation of programmed death ligand 1 on macrophages

Infectious pathogens can selectively stimulate activation or suppression of T cells to facilitate their survival within humans. In this study we demonstrate that the trematode parasite Schistosoma mansoni has evolved with two distinct mechanisms to suppress T cell activation. During the initial 4- to 12-wk acute stages of a worm infection both CD4(+) and CD8(+) T cells are anergized. In contrast, infection with male and female worms induced T cell anergy at 4 wk, which was replaced after egg laying by T cell suppression via a known NO-dependent mechanism, that was detected for up to 40 wk after infection. Worm-induced anergy was mediated by splenic F4/80(+) macrophages (Mphi) via an IL-4-, IL-13-, IL-10-, TGF-beta-, and NO-independent, but cell contact-dependent, mechanism. F4/80(+) Mphi isolated from worm-infected mice were shown to induce anergy of naive T cells in vitro. Furthermore, naive Mphi exposed to live worms in vitro also induced anergy in naive T cells. Flow cytometry on in vivo and in vitro worm-modulated Mphi revealed that of the family of B7 costimulatory molecules, only programmed death ligand 1 (PD-L1) was selectively up-regulated. The addition of inhibitory mAb against PD-L1, but not PD-L2, to worm-modulated Mphi completely blocked the ability of these cells to anergize T cells. These data highlight a novel mechanism through which S. mansoni worms have usurped the natural function of PD-L1 to reduce T cell activation during early acute stages of infection before the subsequent emergence of egg-induced T cell suppression in the chronic stages of infection.

The progressive state, in contrast to the stable or regressive state of alopecia areata, is reflected in peripheral blood mononuclear cells

Alopecia areata (AA) is a putative autoimmune disease of the skin with an inflammatory component that can be treated by the local application of contact sensitizers. Here, we explored whether responsiveness toward diphenylcyclopropenone (DPCP) is reflected by the composition and the activation state of peripheral blood mononuclear cells (PBMCs). PBMCs of 43 AA patients, 26 treated and 17 untreated, and of 31 healthy volunteers were tested. AA patients' PBMCs differed from that of healthy donors by a slight increase in CD16- and tumor necrosis factor-alpha (TNF-alpha)-expressing cells. These features were independent of the disease state and treatment. Additional changes in the activation state of PBMCs, upregulation of the costimulatory molecules CD40 and CD80, of the accessory molecule CD154, and of interferon-gamma expression were identified only in AA patients where the disease was advancing, i.e. these changes were independent of the extent of hair loss and were not seen in patients with spontaneous or DPCP treatment-induced, regressing AA. Thus, the progressive state of AA is accompanied by a systemic activation of T cells, and the therapeutic efficacy of treatment can be estimated by restoration of the non-activated state. Furthermore, an increase in CD16(+)- and TNF-alpha-expressing cells may contribute to AA susceptibility.

Involvement of ICOS-B7RP-1 costimulatory pathway in the regulation of immune responses to Leishmania major and Nippostrongylus brasiliensis infections

The ICOS-B7RP-1-mediated T cell costimulatory pathway has been implicated crucial for T cell activation and differentiation. In this study, we investigated the role of this costimulation in the regulation of immune responses to parasitic infections by using blocking antibody against B7RP-1 as well as ICOS-deficient mice. The administration of anti-B7RP-1 monoclonal antibody (mAb) significantly suppressed the footpad swelling in susceptible BALB/c mice upon Leishmania major infection. The observation was consistent not only with the significant suppression of IL-4, IL-5 and IL-10 secretion from lymph node cells, which were derived from L. major-infected mice, but also with the significant reduction of total serum IgE and IgG(1) in anti-B7RP-1 mAb-treated BALB/c mice. Infection of ICOS-deficient mice with L. major also suggested the impaired Th2 immune responses in the absence of this costimulation. The immunological function of ICOS-B7RP-1 costimulatory pathway in infection was further confirmed by infecting anti-B7RP-1 mAb-treated wild type or ICOS-deficient mice with Nippostrongylus brasiliensis. The characteristic elevation of total serum IgE and eosinophilia upon N. brasiliensis infection was suppressed by blocking this costimulation. Moreover, the protection to N. brasiliensis adult worms was suppressed in anti-B7RP-1 mAb-treated wild type or ICOS-deficient mice. These results suggest the crucial role of this costimulatory pathway in the regulation of Th2-biased T cell differentiation and in host immune responses against L. major and N. brasiliensis infections.

Expression of costimulatory molecules in peripheral blood mononuclear cells of atopic asthmatic children during virus-induced asthma exacerbations

BACKGROUND

Respiratory viruses are the most frequent triggers of acute asthma exacerbations. Herein we investigate costimulatory molecule expression on peripheral blood mononuclear cells (PBMC) during such exacerbations.

METHODS

Eleven children with atopic asthma were followed prospectively and respiratory symptoms were recorded on diary cards. A blood sample and nasopharyngeal wash (NPW) were obtained at baseline and subsequently during an exacerbation. PBMC were immunophenotyped using flow cytometry. NPW samples were examined for the presence of respiratory viruses by RT-PCR.

RESULTS

A virus was detected in 73% of exacerbations and none at baseline. A drop of NK cells and a marginal increase of monocytes were the only changes of cell count during the exacerbation. A significant downregulation of B7-2 on NK cells and of B7-1 on monocytes was also observed during exacerbations.

CONCLUSIONS

The above observations are in contrast to in vitro findings showing an upregulation of costimulatory molecules after exposure of blood cells to viruses or allergens. It is possible that activated immune cells leave the blood stream to migrate to the inflammation site during acute asthma exacerbations.

Emerging paradigms of T-cell co-stimulation

The analysis of recent data reveals that T-cell co-stimulation is a hierarchical process with elements of mutual interdependence between individual co-stimulators. The expression and function of co-stimulatory molecules is biased on various T-cell subsets and is dependent on the T-cell differentiation state. The classical paradigm of T-cell co-stimulation by professional antigen-presenting cells has to incorporate the newly recognized concept of T-cell co-stimulation in the interaction with peripheral tissues, such as endothelial or epithelial cells. The two signal paradigm of T-cell co-stimulation is being replaced by a multisignal integration concept of central and peripheral co-stimulation.

Single-nucleotide polymorphism g.1548G > A (E469K) in human ICAM-1 gene affects mRNA splicing pattern and TPA-induced apoptosis

The single-nucleotide polymorphism (SNP) g.1548G > A (E469K) in the human intercellular adhesion molecule-1 (ICAM-1) gene has been suggested to have an association with several types of inflammatory diseases. The polymorphism is located at the three-base position upstream of the splice donor site that produces an alternatively spliced short isoform (ICAM-1-S). To clarify its functional relevance, we studied RNA splicing patterns by comparing cells with different genotype (G/G cells and A/A cells). G/G cells expressed a lower amount of ICAM-1-S mRNA than A/A cells. Since ICAM-1-S has no transmembrane or intracellular domain, ICAM-1 signal transduction and cell-cell contact including Fas-FasL interaction may be influenced. In addition, we studied the effect of this change on FLIP-L mRNA and apoptosis. FLIP-L mRNA tended to decrease, while cell death induced by phorbol 12-myristate 13-acetate was increased. These results suggest that the g.1548 polymorphism modifies inflammatory immune responses by changing cell-cell interaction and then regulating apoptosis.

Structural and functional analysis of the costimulatory receptor programmed death-1

PD-1, a member of the CD28/CTLA-4/ICOS costimulatory receptor family, delivers negative signals that have profound effects on T and B cell immunity. The 2.0 A crystal structure of the extracellular domain of murine PD-1 reveals an Ig V-type topology with overall similarity to the CTLA-4 monomer; however, there are notable differences in regions relevant to function. Our structural and biophysical data show that PD-1 is monomeric both in solution as well as on cell surface, in contrast to CTLA-4 and other family members that are all disulfide-linked homodimers. Furthermore, our structure-based mutagenesis studies identify the ligand binding surface of PD-1, which displays significant differences compared to those present in the other members of the family.

TCR subunit specificity of CTLA-4-mediated signaling

Cytotoxic T-lymphocyte-associated antigen (CTLA)-4 is an activation-induced receptor that down-regulates T cell responses by antagonizing B7-dependent costimulation and/or by transducing a negative signal. The mechanism of CTLA-4-mediated negative signaling is unknown. Recently, it has been postulated that CTLA-4 inhibits T cell activation by causing specific dephosphorylation of the T cell receptor (TCR)-zeta chain of the antigen-receptor complex through an lck-dependent recruitment of the Src homology-2-containing tyrosine phosphatase-2. To test this hypothesis, we generated stably transfected T cell clones expressing doxycycline-inducible CTLA-4 with CD25:TCR-zeta (CD25-zeta) or CD25:CD3-epsilon (CD25-epsilon) fusion proteins. In these clones, ligation of CD25-zeta or of CD25-epsilon with antibodies against CD25 induced full T cell activation, as illustrated by extracellular signal-regulated kinase (ERK) activation and interleukin (IL)-2 production. More importantly, coligation of CTLA-4 with CD25-zeta or of CTLA-4 with CD25-epsilon in the respectively transfected clones inhibited ERK activation and IL-2 production, demonstrating that CTLA-4 does not specifically inhibit signals from TCR-zeta but can also inhibit signals from CD3-epsilon. Our results suggest that the target specificity of CTLA-4 is determined by its coligation with any given transmembrane receptor rather than by its intracellular mediators.

Molecular interactions mediating T cell antigen recognition

Over the past decade, key protein interactions contributing to T cell antigen recognition have been characterized in molecular detail. These have included interactions involving the T cell antigen receptor (TCR) itself, its coreceptors CD4 and CD8, the accessory molecule CD2, and the costimulatory receptors CD28 and CTLA-4. A clear view is emerging of how these molecules interact with their ligands at the cell-cell interface. Structural and binding studies have confirmed that the proteins span small but comparable distances and that, overall, they interact very weakly. However, there have been important surprises as well: that TCR interactions with peptide-MHC are topologically constrained and characterized by considerable conformational flexibility at the binding interface; that coreceptors engage peptide-MHC with extraordinarily fast kinetics and at angles apparently precluding direct interactions with the TCR bound to the same peptide-MHC; that the structural mechanisms allowing recognition by costimulatory and accessory molecules to be weak and yet specific are very heterogeneous; and that because of differences in both binding affinity and stoichiometry, there is enormous variation in the stability of the various costimulatory receptor/ligand complexes. These studies provide the necessary framework for exploring how these molecular interactions initiate T cell activation.

Viewpoint: therapeutic implications of CTLA-4 compartmentalization

Understanding the regulatory events involved in the activation and inactivation of T cells is crucial to develop therapeutic approaches for autoimmune diseases and for organ transplantation. Co-stimulatory signals delivered through the CD28 receptor and inhibitory signals through CTLA-4 are required for the proper modulation of T cell responses and the induction and maintenance of peripheral tolerance. Manipulation of these signals is emerging as a potential strategy to prevent allograft rejection in different animal models. Recent data on the compartmentalization and the structural features of CTLA-4 within T cells provides critical information not only on the molecular basis of T cell inactivation by CTLA-4, but also on the key requirements for the successful development of therapeutic strategies targeting this molecule.

Critical contribution of CD28-CD80/CD86 costimulatory pathway to protection from Trypanosoma cruzi infection

The CD28-CD80/CD86-mediated T-cell costimulatory pathway has been variably implicated in infectious immunity. In this study, we investigated the role of this costimulatory pathway in resistance to Trypanosoma cruzi infection by using CD28-deficient mice and blocking antibodies against CD80 and CD86. CD28-deficient mice exhibited markedly exacerbated T. cruzi infection, as evidenced by unrelenting parasitemia and 100% mortality after infection with doses that are nonlethal in wild-type mice. The blockade of both CD80 and CD86 by administering specific monoclonal antibodies also exacerbated T. cruzi infection in wild-type mice. Splenocytes from T. cruzi-infected, CD28-deficient mice exhibited greatly impaired gamma interferon production in response to T. cruzi antigen stimulation in vitro compared to those from infected wild-type mice. The induction of T. cruzi antigen-specific CD8(+) T cells was also impaired in T. cruzi-infected, CD28-deficient mice. In addition to these defects in natural protection against T. cruzi infection, CD28-deficient mice were also defective in the induction of CD8(+)-T-cell-mediated protective immunity against T. cruzi infection by DNA vaccination. These results demonstrate, for the first time, a critical contribution of the CD28-CD80/CD86 costimulatory pathway not only to natural protection against primary T. cruzi infection but also to DNA vaccine-induced protective immunity to Chagas' disease.

Assembling atomic resolution views of the immunological synapse

Antigen-dependent T-cell activation is initiated by the organized interactions of a host of cell-surface receptors in the interface between the T cell and an antigen-presenting cell. Recent structural studies of many of the receptors that comprise this 'immunological synapse', including integrins, cadherins and co-stimulatory molecules, reveal in detail the molecular interactions that bridge the synapse. Additionally, structural analyses of the cytoplasmic tails of integrins and co-stimulatory receptors in complex with intracellular signaling proteins are providing insight into the mechanisms that couple receptor clustering on the cell surface with the initiation of cytoplasmic signaling.

Molecular modeling and functional mapping of B7-H1 and B7-DC uncouple costimulatory function from PD-1 interaction

B7-H1 and B7-DC are ligands for PD-1, a receptor implicated in negative regulation of T and B cell functions. These ligands, however, also costimulate T cell responses. It remains elusive whether or not costimulation is mediated through PD-1. By comparative molecular modeling and site-directed mutagenesis, we found that nonconserved residues between these ligands on the A'GFCC'C" face mediate interaction with PD-1. This indicates significant structural heterogeneity of the interactions between PD-1 and its ligands. Importantly, ligand mutants with abolished PD-1 binding capacity could still costimulate proliferation and cytokine production of T cells from normal and PD-1-deficient mice. Our results reveal unique binding characteristics of B7-H1 and B7-DC and provide direct evidence for an independent costimulatory receptor other than PD-1.

The nature of molecular recognition by T cells

Considerable progress has been made in characterizing four key sets of interactions controlling antigen responsiveness in T cells, involving the following: the T cell antigen receptor, its coreceptors CD4 and CD8, the costimulatory receptors CD28 and CTLA-4, and the accessory molecule CD2. Complementary work has defined the general biophysical properties of interactions between cell surface molecules. Among the major conclusions are that these interactions are structurally heterogeneous, often reflecting clear-cut functional constraints, and that, although they all interact relatively weakly, hierarchical differences in the stabilities of the signaling complexes formed by these molecules may influence the sequence of steps leading to T cell activation. Here we review these developments and highlight the major challenges remaining as the field moves toward formulating quantitative models of T cell recognition.

Negative regulation of T cell receptor-lipid raft interaction by cytotoxic T lymphocyte-associated antigen 4

Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) is an essential negative regulator of T cell activation. Recent evidence suggests that CTLA-4 association with the immunological synapse during contact with antigen-presenting cells is important for its inhibitory function. In the present study, we observed a direct interaction of CTLA-4 with the phosphorylated form of T cell receptor (TCR)zeta within the glycolipid-enriched microdomains associated with the T cell signaling complex. In this setting, CTLA-4 regulated the accumulation/retention of TCRzeta in the signaling complex, as the lipid raft fractions from CTLA-4KO T cells contained significantly higher amounts of the TCR components when compared with wild-type littermates. In contrast, coligation of CTLA-4 with the TCR during T cell activation selectively decreased the amount of TCRzeta that accumulated in the rafts. These results suggest that CTLA-4 functions to regulate T cell signaling by controlling TCR accumulation and/or retention within this a critical component of the immunological synapse.

Chimeric co-stimulatory molecules that selectively act through CD28 or CTLA-4 on human T cells

CD28 and CTLA-4 (CD152) play a pivotal role in the regulation of T cell activation. Upon ligation by CD80 (B7-1) or CD86 (B7-2), CD28 induces T cell proliferation, cytokine production, and effector functions, whereas CTLA-4 signaling inhibits expansion of activated T cells and induces tolerance. Therefore, we hypothesized that co-stimulatory molecules that preferentially bind CD28 or CTLA-4 would have dramatically altered biological properties. We describe directed molecular evolution of CD80 genes derived from human, orangutan, rhesus monkey, baboon, cat, cow, and rabbit by DNA shuffling and screening. In contrast to wild-type CD80, the evolved co-stimulatory molecules, termed CD28-binding protein (CD28BP) and CTLA-4-binding protein (CTLA-4BP), selectively bind to CD28 or CTLA-4, respectively. Furthermore, CD28BP has improved capacity to induce human T cell proliferation and interferon-gamma production compared with wild-type CD80. In contrast, CTLA-4BP inhibited human mixed leukocyte reaction (MLR) and enhanced interleukin 10 production in MLR, supporting a role for CTLA-4BP in inducing T cell anergy and tolerance. In addition, co-stimulation of purified human T cells was significantly suppressed when CTLA-4BP was cotransfected with either CD80 or CD28BP. The amino acid sequences of CD28BP and CTLA-4BP were 61 and 96% identical with that of human CD80 and provide insight into the residues that are critical in the ligand binding. These molecules provide a new approach to characterization of CD28 and CTLA-4 signals and to manipulation of the T cell response.

CTLA-4: new insights into its biological function and use in tumor immunotherapy

The discovery of multiple costimulatory cell surface molecules that influence the course of T cell activation has increased our appreciation of the complexity of the T cell response. It remains clear, however, that CD28 and cytotoxic T lymphocyte antigen 4 (CTLA-4) are the critical costimulatory receptors that determine the early outcome of stimulation through the T cell antigen receptor (TCR). Details of how the T cell integrates TCR stimulation with the costimulatory signals of CD28 and the inhibitory signals of CTLA-4 remain to be established, but unique features of the cell biology of CTLA-4 provide important insights into its function. We summarize here recent findings that suggest a previously unrecognized role for CTLA-4 in the regulation of T cell responses. We also describe preclinical and clinical results that indicate manipulation of CTLA-4 has considerable promise as a strategy for the immunotherapy of cancer.