Human T cell activation: differential response to anti-CD28 as compared to anti-CD3 monoclonal antibodies

CD1 and iNKT cells mediate immune responses against the GBS hemolytic lipid toxin induced by a non-toxic analog

Although hemolytic lipids have been discovered from many human pathogens including Group B Streptococcus (GBS), strategies that neutralize their function are lacking. GBS is a leading cause of pregnancy-associated neonatal infections, and adult GBS infections are on the rise. The GBS hemolytic lipid toxin or granadaene, is cytotoxic to many immune cells including T and B cells. We previously showed that mice immunized with a synthetic nontoxic analog of granadaene known as R-P4 had reduced bacterial dissemination during systemic infection. However, mechanisms important for R-P4 mediated immune protection was not understood. Here, we show that immune serum from R-P4-immunized mice facilitate GBS opsonophagocytic killing and protect naïve mice from GBS infection. Further, CD4+ T cells isolated from R-P4-immunized mice proliferated in response to R-P4 stimulation in a CD1d- and iNKT cell-dependent manner. Consistent with these observations, R-P4 immunized mice lacking CD1d or CD1d-restricted iNKT cells exhibit elevated bacterial burden. Additionally, adoptive transfer of iNKT cells from R-P4 vaccinated mice significantly reduced GBS dissemination compared to adjuvant controls. Finally, maternal R-P4 vaccination provided protection against ascending GBS infection during pregnancy. These findings are relevant in the development of therapeutic strategies targeting lipid cytotoxins.

Lipid analogs reveal features critical for hemolysis and diminish granadaene mediated Group B Streptococcus infection

Although certain microbial lipids are toxins, the structural features important for cytotoxicity remain unknown. Increased functional understanding is essential for developing therapeutics against toxic microbial lipids. Group B Streptococci (GBS) are bacteria associated with preterm births, stillbirths, and severe infections in neonates and adults. GBS produce a pigmented, cytotoxic lipid, known as granadaene. Despite its importance to all manifestations of GBS disease, studies towards understanding granadaene’s toxic activity are hindered by its instability and insolubility in purified form. Here, we report the synthesis and screening of lipid derivatives inspired by granadaene, which reveal features central to toxin function, namely the polyene chain length. Furthermore, we show that vaccination with a non-toxic synthetic analog confers the production of antibodies that inhibit granadaene-mediated hemolysis ex vivo and diminish GBS infection in vivo. This work provides unique structural and functional insight into granadaene and a strategy to mitigate GBS infection, which will be relevant to other toxic lipids encoded by human pathogens.

Granadaene, produced by Group B Streptococcus (GBS), is a long polyene lipid involved in cellular toxicity and hemolytic activity. Here, the authors synthesize and characterize granadaene-like compounds and show that a non-toxic analog diminishes GBS infection in mice when incorporated into a vaccine formulation.

CD30 as a therapeutic target for lymphoma

Hodgkin's lymphoma (HL) and ALK(+) anaplastic large-cell lymphoma (ALCL) have become highly curable due to the success of modern regimens of chemotherapy and radiotherapy. However, up to one-third of the patients experience relapse or do not respond to first-line therapy, and half of them relapse again after secondary therapy with limited options for further treatment. In the last 15 years, monoclonal antibodies (mAbs) directed to surface receptors became a new and valuable therapeutic option in many hematologic malignancies. Due to its restricted expression on normal activated lymphocytes and its high expression on malignant cells, CD30 represents an attractive target molecule for HL and ALCL therapy. However, unconjugated CD30 mAbs have demonstrated a lack of objective clinical responses in patients with recurrent HL. CD30 exhibits complex signaling pathways, and binding of its natural ligand or anti-CD30 mAbs can induce apoptosis but may also promote proliferation and activation depending on the cellular context. Moreover, CD30 rapidly internalizes after crosslinking, which counteracts efficient recruitment of immunologic effectors but also provides the opportunity to transfer cytotoxic payloads coupled to CD30-specific mAbs into the tumor cells. Several tumor targeting approaches have been studied, including radio-immunoconjugates, immunotoxins, immunoRNases, immunokinases, and antibody drug conjugates (ADCs). In 2011, the ADC brentuximab-vedotin, consisting of the CD30-specific chimeric mAb cAC10 and the potent tubulin toxin monomethyl auristatin E, gained regulatory approval as a well tolerated and highly active drug in patients with refractory and relapsed HL and ALCL. SGN-35 is on the way to being incorporated in the standard management of CD30(+) lymphoma with significant therapeutic impact. This review gives a critical overview about anti-CD30 therapies with unconjugated, engineered, and conjugated mAbs and the therapeutic challenges of treatment of CD30(+) lymphoma.

Cgnz1 allele confers kidney resistance to damage preventing progression of immune complex-mediated acute lupus glomerulonephritis

The mechanisms regulating acute and chronic glomerulonephritis are dependent on different genetic mechanisms, where the Cgnz1 allele confers kidney protection in immune complex–mediated proliferative lupus nephritis.

Cgnz1 and Agnz1 on the distal region of mouse chromosome 1 are associated with chronic glomerulonephritis (cGN) and acute GN (aGN). NZM2328.Lc1R27 (R27) was generated by introgressing a C57L/J region where Cgnz1 is located to NZM2328. R27 female mice developed aGN mediated by immune complex (IC) deposition and complement activation without progression to cGN with severe proteinuria. End stage renal disease (ESRD) was not seen in R27 mice as old as 15 mo. Thus, aGN and cGN are under separate genetic control, and IC-mediated proliferative GN need not progress to cGN and ESRD. NZM2328 and R27 female mice have comparable immune and inflammatory parameters. In contrast to NZM2328, R27 mice were resistant to sheep anti–mouse GBM serum-induced nephritis, supporting the hypothesis that aGN is mediated by autoimmunity and resistance to the development of cGN is mediated by end organ resistance to damage. Thus, autoimmunity should be considered distinct from end organ damage. The Cgnz1 region has been mapped to a 1.34 MB region with 45 genes. Nine candidate genes were identified. Clinical relevance of these observations is supported by case studies. Clinical implications and the significance to human lupus and other diseases are presented.

Fine particulate matter (PM₂.₅) air pollution and immune status among women in the Seattle area

Changes in immune status have been suggested as a possible biologic mechanism by which particulate matter (PM) air pollution could lead to adverse health effects. The authors studied associations between ambient PM₂.₅ and immune status among 115 postmenopausal, overweight women in the greater Seattle, Washington, area. The authors evaluated 3-day, 30-day, and 60-day average PM₂.₅ values in relation to inflammation markers (C-reactive protein, serum amyloid A, interleukin-6) and functional assays of cellular immunity (natural killer cell cytotoxicity, T-lymphocyte proliferation) at 3 time points for each woman during 1 year. Three-day averaged PM₂.₅ was inversely associated with anti-CD3-stimulated lymphocyte proliferation. There were no notable associations between the inflammation markers and PM₂.₅. If additional studies confirm our findings, then future health effect assessments for PM₂.₅ should consider changes in cellular immunity as an endpoint that may lead to overt clinical disease.

CD6 synergistic co-stimulation promoting proinflammatory response is modulated without interfering with the activated leucocyte cell adhesion molecule interaction

The CD6 membrane-proximal scavenger receptor cysteine-rich domain (SRCR3) includes the activated leucocyte cell adhesion molecule (ALCAM) binding site. CD6-ALCAM mediates a low-affinity interaction and their long-term engagement contributes to the immunological synapse. Their ligation may play a dual function, facilitating stable adhesion between the antigen-presenting cells and T cells during the early activation phase and later in the proliferative phase of the immune response. This study explored the strength of the CD6 co-stimulatory effect and whether CD6 co-stimulation with its natural ligand ALCAM also contributes to the lymphocyte effector differentiation. It was found that CD6-ALCAM interaction in vitro induced a synergistic co-stimulation of normal human peripheral blood mononuclear cells, defined by Bliss analysis. CD6 co-stimulation enhanced the CD3 proliferative efficacy by 23-34%. Moreover, a fivefold increment in the CD25 molecules number with a distinct gene transcription profile associated with cell activation, differentiation, survival and adhesion molecules was observed over CD3 single activation. Additionally, CD6 co-stimulation in excess interleukin (IL)-2 promotes a preferentially proinflammatory response. Besides, a CD6 membrane-distal domain (SRCR1)-specific non-depleting monoclonal antibody (mAb) inhibited the induced proliferation in the presence of ALCAM, reducing interferon-γ, IL-6 and tumour necrosis factor-α production. These results suggest that CD6 co-stimulation enhances the intrinsic activity of the CD3 activation pathway and contributes to the T helper type 1 subset commitment, enhancing the IL-2 sensitivity of recent activated human lymphocytes. It supports the role of CD6 as a susceptibility gene for pathological autoimmunity leading to tissue inflammation, and its relevance for targeted therapy.

Toxins-useful biochemical tools for leukocyte research

Leukocytes are a heterogeneous group of cells that display differences in anatomic localization, cell surface phenotype, and function. The different subtypes include e.g., granulocytes, monocytes, dendritic cells, T cells, B cells and NK cells. These different cell types represent the cellular component of innate and adaptive immunity. Using certain toxins such as pertussis toxin, cholera toxin or clostridium difficile toxin, the regulatory functions of Gα_i, Gαs and small GTPases of the Rho family in leukocytes have been reported. A summary of these reports is discussed in this review.

Inhibition of T-cell activation in vitro in human peripheral blood mononuclear cells by pimecrolimus and glucocorticosteroids and combinations thereof

Pimecrolimus is an ascomycin macrolactam derivative that has been recently approved for the topical treatment of atopic dermatitis. In this study we report for the first time on a direct comparison of the inhibitory activity of pimecrolimus and the glucocorticosteroids betamethasone 17-valerate, dexamethasone and hydrocortisone at the level of T-cell proliferation and cytokine production. Stimulated human peripheral blood mononuclear cell (PBMC) systems were used that are either sensitive or resistant to calcineurin inhibitors or glucocorticosteroids. Pimecrolimus and the glucocorticosteroids inhibited dose-dependently T-cell proliferation and cytokine production in a sensitive system (anti-CD3 mAb-stimulated PBMC) with the following rank order of potency: pimecrolimus approximately betamethasone 17-valerate approximately dexamethasone > hydrocortisone. In resistant systems (anti-CD3 plus anti-CD28- or Staphylococcal enterotoxin B-stimulated PBMC), pimecrolimus or the glucocorticosteroids alone exerted either no effect, or only a partial inhibitory effect. However, combinations of pimecrolimus with a glucocorticosteroid synergistically and strongly inhibited T-cell proliferation. Taken together, the data indicate that medium potency glucocorticosteroids, such as betamethasone 17-valerate and dexamethasone, are as potent T-cell inhibitors as pimecrolimus. Furthermore, the experimental evidence suggests that combinations of glucocorticosteroids and pimecrolimus could be used clinically to achieve superior therapeutic efficacy, when monotherapy with the individual agents is unsatisfactory.

Molecular construction and characterization of a novel exotoxin fusion protein that selectively blocks the B7:CD28 costimulatory signal system

An important strategy for specifically preventing and treating graft-versus-host and host-versus-graft diseases is to selectively block the B7:CD28/cytotoxic T-lymphocyte A4 costimulatory signal system for induced immune tolerance. In this study, a novel recombinant B7-2-L-PE40KDEL fusion protein was created to target the B7:CD28 system. We used a flexible linker sequence (Gly4Ser)4 and overlapping sequence extension to link the cDNAs encoding a human B7-2 extracellular domain and a mutant truncated form of Pseudomonas exotoxin A (PE), PE40KDEL. This B7-2-L-PE40KDEL fusion gene was then inserted into the pTYB4 expression vector, expressed in Escherichia coli, and purified through Ni-NTA mealty affinity-->MonoQ anion exchange-->Superdex75 gel filtration chromatography 3-step purification protocols. Western blotting demonstrated that the B7-2-L-PE40KDEL fusion protein specifically bound antihuman B7-2 monoclonal antibody and anti-pseudomonas exotoxin A antiserum. We used the Antheprot nucleic acid and protein analyzing software to predict the characteristics of this fusion protein, and showed that the fusion did not confer new antigenicities to the fusion protein. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide tests demonstrated that at doses ranging from 0.2 to 2 microg/mL, this fusion protein specifically killed CD28-overexpressing Jurkat cells but even at doses of 2 microg did not kill CD28-negative Hut28 cells. The results of a one-way mixed lymphocyte reaction demonstrated that the fusion protein has a range of suppressive effects on HLA class I and II matched related donors and recipients, and HLA class I and II mismatched unrelated donors. Taken together, these results demonstrate that we have developed a novel recombinant human B7-2-L-PE40KDEL exotoxin fusion protein that specifically blocks the B7:CD28 costimulatory signal system in a manner that may be of significant importance in preventing and treating graft-versus-host or host-versus-graft diseases.

Cutting Edge: Monovalency of CD28 Maintains the Antigen Dependence of T Cell Costimulatory Responses

CD28 and CTLA-4 are the major costimulatory receptors on naive T cells. But it is not clear why CD28 is monovalent whereas CTLA-4 is bivalent for their shared ligands CD80/86. We generated bivalent CD28 constructs by fusing the extracellular domains of CTLA-4 or CD80 with the intracellular domains of CD28. Bivalent or monovalent CD28 constructs were ligated with recombinant ligands with or without TCR coligation. Monovalent CD28 ligation did not induce responses unless the TCR was coligated. By contrast, bivalent CD28 ligation induced responses in the absence of TCR engagement. To extend these findings to primary cells, we used novel superagonistic and conventional CD28 Abs. Superagonistic Ab D665, but not conventional Ab E18, predominantly ligates CD28 bivalently at low CD28/Ab ratios and induces Ag-independent T cell proliferation. Monovalency of CD28 for its natural ligands is thus essential to provide costimulation without inducing responses in the absence of TCR engagement.

Immuno-suppressive effect of blocking the CD28 signaling pathway in T-cells by an active component of Echinacea found by a novel pharmaceutical screening method

AFTIR (after flowing through immobilized receptor) is a novel method for screening herbal extracts for pharmaceutical properties. Using AFTIR, we identified Cynarin in Echinacea purpurea by its selective binding to chip immobilized CD28, a receptor of T-cells, which is instrumental to immune functioning. The results of surface plasma resonance show that binding between immobilized CD28 and Cynarin is stronger than the binding between CD28 and CD80, a co-stimulated receptor of antigen presenting cells. Cynarin's function was verified by its ability to downregulate CD28-dependent interleukin-2 (IL-2) expression in a T-cell culture line. AFTIR offers promise as an efficient screening method for herbal medicines.

Control of interleukin-2 gene transcription: a paradigm for inducible, tissue-specific gene expression

Interleukin-2 (IL-2) is a key cytokine that controls immune cell function, in particular the adaptive arm of the immune system, through its ability to control the clonal expansion and homeostasis of peripheral T cells. IL-2 is produced almost exclusively by T cells in response to antigenic stimulation and thus provides an excellent example of a cell-specific inducible gene. The mechanisms that control IL-2 gene transcription have been studied in detail for the past 20 years and our current understanding of the nature of the inducible and tissue-specific controls will be discussed.

Simultaneous cross-linking of CD6 and CD28 induces cell proliferation in resting T cells

In the present study, we showed that simultaneous ligation of the monoclonal antibodies (mAb) against CD6 and CD28 induces T-cell proliferation in purified resting T lymphocytes in the absence of T-cell receptor (TCR) occupancy. No cell proliferation was observed when the mAb were cross-linked alone or used simultaneously in the soluble form. T-cell proliferation mediated through CD6/CD28 is accompanied by the up-regulation of interleukin-2 (IL-2) mRNA and expression of IL-2 receptors on the cell surface. In the presence of IL-2-neutralizing mAb the proliferative response of the T cell induced through CD6/CD28 was inhibited dose dependently. Cross-linking mAb to CD6 and CD28 alone or together did not down-regulate the CD3/TCR complex. T-cell proliferation mediated through CD6/CD28 was only partially blocked by the immunosuppressive drug, cyclosporin A (CsA), whereas anti-CD28-induced T-cell proliferation in the presence of the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), was unaffected. In sharp contrast T-cell proliferation mediated by anti-CD6 in the presence of TPA was efficiently blocked by CsA. In addition, two protein kinase C (PKC) inhibitors, GF 109203X and H-7 dose-dependently inhibited T-cell proliferation mediated through CD6/CD28, suggesting that PKC activation may be involved. Furthermore, there was a marked differential dose-dependent inhibitory effect of the PKC inhibitors on T-cell proliferation mediated by the co-ligation of anti-CD6 or anti-CD28 in the presence of anti-CD3, with the former being more sensitive to PKC inhibition. Taken collectively, our results suggest that T-cell activation can occur through an antigen-independent pathway by cross-linking the accessory molecules, CD6 and CD28, and that these two cell surface antigens may have distinct signalling pathways.

Use of the methylxanthine derivative A802715 in transplantation immunology: I. Strong in vitro inhibitory effects on CD28-costimulated T cell activities

BACKGROUND

Recently, methylxanthines such as pentoxifylline (PTX) were shown to be immunosuppressive in vitro. Unfortunately, when used in transplant patients, PTX was poorly active as an immunosuppressant. Here we report that the new methylxanthine derivative A802715 not only is more active than PTX, it also suppresses the cyclosporine (CsA)-resistant "signal two"-dependent pathway of T cell proliferation, making it an interesting drug to associate with CsA.

METHODS

"Signal one"- and "signal two"-dependent T cell activation was investigated with purified human T cells stimulated with immobilized anti-CD3 or anti-CD28 monoclonal antibody (mAb) plus phorbol myristate acetate (PMA) or with a 3T6 mouse fibroblast cell line presenting anti-CD3 mAb on transfected human Fcgamma receptors II (FcgammaRII) in the presence or absence of transfected B7-1 (CD80) molecules.

RESULTS

A802715 was more immunosuppressive in the mixed lymphocyte reaction (MLR) than PTX. A802715 dose-dependently suppressed polyclonal signal one-dependent T cell activation induced by anti-CD3 mAb/PMA. In addition, A802715 also suppressed signal two-dependent T cell proliferation induced by anti-CD28 mAb/PMA. The expression of the interleukin-2 receptor on T cells stimulated by anti-CD3 mAb presented on 3T6/FcgammaRII cells was equally well suppressed by A802715 and PTX. In contrast, interleukin-2 receptor or CD40L (gp39) expression by T cells after stimulation with the same anti-CD3 mAb- 3T6/FcgammaRII cells, but coexpressing transfected B7-1, was only suppressed by A802715. The anticipated synergism between A802715 and CsA was confirmed in MLR assays. Moreover, generation of cytotoxic T lymphocytes during MLR with Epstein-Barr virus-transformed B cells, which strongly express B7-1 and B7-2, was also inhibited by A802715.

CONCLUSIONS

These in vitro data indicate that the A802715 (1) is a stronger immunosuppressant for T cells than PTX, (2) suppresses T cell activation pathways that are resistant to PTX or CsA, and (3) acts synergistically with CsA.

TCR-independent CD28-mediated gene expression in peripheral blood lymphocytes from donors chronically infected with HIV-1

Complete activation of peripheral blood T cells requires both T-cell receptor (TCR) stimulation and CD28 costimulation. Signalling pathways associated specifically with CD28 are not well understood, however, because ligation of CD28 in the absence of TCR stimulation does not give rise to cellular responses in normal cells. In peripheral blood lymphocytes (PBL) from donors chronically infected with human immunodeficiency virus-1 (HIV-1), CD28 can induce viral replication through an alternative pathway that does not require TCR ligation. We have exploited this observation to study CD28-mediated signal transduction using reverse transcriptase-mediated polymerase chain reaction (RT-PCR) to amplify viral RNA. Independent ligation of CD28 on donor PBL induced expression of the HIV-1 tat gene but not the interleukin-2 (IL-2) gene. Viral induction did not occur following pretreatment of cells with actinomycin D, suggesting it was mediated through transcriptional activation of the viral long terminal repeat (LTR). tat was induced in the presence of the protein kinase C inhibitor H-7, but was inhibited by cyclosporin A. Our results demonstrate that CD28 is linked directly to specific signalling pathways leading to de novo induction of genes in PBL.

Analysis of the T-cell activation signaling pathway mediated by tyrosine kinases, protein kinase C, and Ras protein, which is modulated by intracellular cyclic AMP

T-cell receptor (TCR) triggering by an anti-CD3 antibody or phytohemagglutinin (PHA) as well as the treatment with phorbol myristate acetate (PMA), a direct activator of protein kinase C (PKC), induces activation of Ras in T-lymphocytes (Downward, J. et al. (1990)) Nature 364, 719-723). In this paper, we studied the role of Ras in the process of TCR-mediated T-cell activation using a human lymphomic Jurkat cell line. The stimulatory effect of TCR cross-linking on Ras activation was inhibited by herbimycin A, a specific inhibitor of protein tyrosine kinases (PTKs), whereas PMA-induced Ras activation was not affected. On the other hand, calphostin C, a specific inhibitor of PKC, blocked not only PMA-induced, but also TCR-mediated formation of Ras.GTP. Furthermore, down-regulation of PMA-sensitive PKC severely impaired the activation of Ras in response to TCR-stimulation. Tyrosine-phosphorylation and translocation to the particulate fraction of phospholipase C-gamma 1 (PLC-gamma 1) were observed upon T-cell activation. Subcellular localization of PKC was also changed when the cells were stimulated with an anti-CD3 antibody or PMA. While TCR-stimulated translocation of PKC was observed only transiently, PMA-induced translocation of PKC was more sustained. These results suggest that the activation of PLC-gamma 1 by PTK and subsequent activation of PKC are important for TCR-mediated Ras activation in Jurkat cells. An activated form of Ras enhanced the activation of interleukin 2 (IL-2) promoter by TCR stimulation or PMA treatment, although the activated Ras by itself was insufficient for IL-2 promoter activation. On the other hand, a dominant-inhibitory Ras diminished almost completely the activation of IL-2 promoter induced by PMA plus calcium ionophore, indicating that Ras is essential for transduction of T-cell activation signals. Cholera toxin (CTX), which directly activates Gs alpha, is shown to inhibit the activation of IL-2 promoter. TCR-mediated Ras activation, tyrosine phosphorylation and translocation of cellular proteins including ZAP-70, PLC-gamma 1 , and PKC. An activated Gs alpha mutant as well as dibutylyl cAMP (dBcAMP) also showed similar inhibitory effects.

The dendritic cell lineage: a ubiquitous antigen-presenting organization

Dendritic cells are specialized antigen-presenting cells with two unique characteristics: the greatest stimulatory potential and the ability to stimulate naive T-lymphocytes. They originate from the bone marrow and reach their destination via hematogenous or lymphatic migration. Their phenotype is characterized by a high expression of major histocompatibility complex class II molecules and a high expression of adhesion molecules (CD25, CD54, CD58, CD72, and CD80). Pulmonary dendritic cells may be investigated by histologic examination, phenotype analysis, and function studies in a mixed lymphocyte reaction. Their isolation requires enzymatic digestion of lung tissue and subsequent steps of cell separation. The complexity of these manipulations makes it difficult to obtain large numbers of viable cells. A close anatomic relationship with alveolar macrophages underlines a functional interconnection: macrophages down-regulate the antigen-presenting function through release of tumor necrosis factor alpha. Dendritic cells most probably play a major role in lung diseases such as histiocytosis, primary and secondary cancers, and both acute and chronic lung graft rejection. Identification of the precise functional pathways might lead to therapeutic use of modulation of dendritic cell function.

Tumor therapy by immune recruitment with bispecific antibodies

Many strategies for experimental immunotherapy of cancers aim at inducing and expanding tumor-specific cytotoxic T-lymphocytes. One of the most promising approaches uses bispecific monoclonal antibodies (Bi-MAbs) which are able to accumulate and activate human effector cells at the tumor site. Human resting peripheral NK- or T cells targeted by appropriate Bi-MAbs to tumor cells expressing a tumor-associated antigen display multiple signs of activation including proliferation, cytokine secretion, upregulation of cytotoxic peptides and enzymes and induce an efficient tumor cell lysis in vivo. Moreover, tumor-bearing SCID which were treated by effector cell-triggering Bi-MAbs and human peripheral blood lymphocytes had complete regressions of established tumors and most or all animals were cured by human NK-cell or T-cell cytotoxicity, respectively. Local tumor site-specific activation of T-cells was demonstrated, and enhanced granzyme and perforin expression together with the results of inhibition experiments suggest both mechanisms as the major contributors to the cytolytic machinery of Bi-MAb-mediated T-cell cytotoxicity in vivo. The encouraging results of this approach, which is able to cure animals with even advanced disseminated tumors, together with the local site-specific effector cell activation, which suggests minimal side effects, warrant the clinical evaluation of this Bi-MAb approach. As lymphocytes from tumor patients can be adequately activated by the respective Bi-MAbs, the clinical application of Bi-MAbs promises to become a safe, efficient and simple approach which should be readily applicable to the treatment of human malignancies that cannot be cured by standard regimens.

Cyclic AMP sensitive signalling by the CD28 marker requires concomitant stimulation by the T-cell antigen receptor (TCR/CD3) complex

We have previously demonstrated that activation of cAMP-dependent protein kinase (cAK) type I (cAKI, RI alpha 2-C beta 2) mediates the inhibitory effects of cAMP on T-cell replication induced through the TCR/CD3 complex. In the present study we have investigated the effect of cAMP on T-cell DNA synthesis, tyrosine phosphorylation of a 100 kDa protein (pp100) and IL2 mRNA expression, induced through stimulation of the TCR/CD3- and/or the CD28 molecules. Our results demonstrate that tyrosine phosphorylation of pp100 stimulated by anti-CD3 is inhibited by cAMP both in the presence and absence of the phorbol ester PMA, and reflects the changes seen in IL2 mRNA expression and T-cell replication. Combined stimulation with anti-CD3 and anti-CD28, which gives a synergistic response in T-cell replication, gave pp100 phosphorylation and IL2 mRNA expression sensitive to cAMP-dependent inhibition. When PMA was added in addition to anti-CD3 and anti-CD28, the inhibitory effect of cAMP on both T-cell replication and pp100 phosphorylation was completely abolished. The fact that pp100 phosphorylation in response to TCR/CD3-, CD28- and PMA stimulation and cAMP mediated inhibition are identical to the effects of the same stimuli on T-cell proliferation, makes this protein an interesting candidate in downstream signalling from these receptors. In addition, our results are compatible with a model where cAMP, through activation of cAKI, eliminates both the PTK and PKC activating capability of the T-cell receptor at a site(s) proximal to PKC activation. Furthermore, the CD28 molecule which activates PTKs, enters the PTK cascade at a point distal to the target(s) for cAKI action. Therefore, during CD28 signalling PKC activation can be achieved either by TCR/CD3 stimulation (inhibited by cAMP), or directly by PMA (not inhibited by cAMP).

Characterization of a cyclosporin A-sensitive activation pathway in cultured T and natural killer cells

Previously, the authors have described a molecule, identified by the LD6 monoclonal antibody (MoAb), present at the cell surface of long-term cultured T and Natural Killer (NK) cells which is involved in cell triggering. In the study described here the authors used biotin surface labelling and immunoprecipitation to show that LD6 MoAb recognizes a surface protein of approximately 65 kDa. In combination with submitogenic concentrations of phorbol esters (PMA); LD6 MoAb was able to induce accumulation of mRNA specific for GM-CSF, gamma-IFN and TNF-alpha and release of these cytokines by LD6+ T-cell lines. Both lymphokine production and lymphokine-specific mRNA accumulation induced by the LD6 MoAb were blocked totally by Cyclosporin A (CsA). To investigate the mechanism(s) of signal transduction through this activatory pathway, the authors performed Ca++ mobilization experiments. The results of these experiments suggested a role for Ca++ in signal transduction. The Ca++ mobilization induced by LD6 MoAb cross-linking could be inhibited totally by the use of pertussis toxin, indicating a possible role for G proteins in signalling through the LD6 MoAb-reactive molecule. Western blot analysis performed with an anti-phosphotyrosine antibody did not suggest that tyrosine kinase activation has a role.

CD30-antigen-specific targeting and activation of T cells via murine bispecific monoclonal antibodies against CD3 and CD28: potential use for the treatment of Hodgkin's lymphoma

Cross-linking of specific tumor antigens with the T-cell-associated CD3 and CD28 antigens can increase IL-2 secretion, proliferation and antigen-specific cytotoxicity in resting T cells. This cross-linking can be achieved effectively by bispecific monoclonal antibodies (BiMAb) with specificity for both the tumor antigen and CD3 or CD28 antigen, respectively. To take advantage of the enhanced activation of CD3 pre-activated T cells by additional activation via the CD28 antigen, BiMAb OKT3/HRS-3 with reactivity to both CD3 and the Hodgkin's-lymphoma-associated CD30 antigen and the BiMAb 15E8/HRS-3 with reactivity to both CD28 and CD30 antigen were generated by hybridoma fusion. Resting T cells, represented by Jurkat cells (CD3+/CD28+) were specifically activated to produce IL-2 by co-cultivation with an EBV-transformed B-cell line (LAZ509, CD30+/CD19+) only in the presence of the CD30/CD28 cross-linking BiMAb and an additional cross-linking anti-CD3/CD19 BiMAb (OKT3/6A4). Neither the cross-linking BiMAbs alone nor any combination of the monospecific parental MAbs induced a comparable IL-2 production by Jurkat cells in the presence of LAZ509. In addition, using a combination of these BiMAbs, an antigen-dependent cytotoxicity was induced by targeting APC-depleted peripheral blood lymphocytes to CD30+ L540 cells. T cells, previously specifically activated by CD3/CD30 in the presence of CD30 antigen, were cytotoxic to CD30+ cell lines only after incubation with BiMAb anti-CD28/CD30. Neither of the BiMAbs nor any of the parental antibodies induced a comparable effect. Our results indicate that such BiMAbs may offer a new approach for specific immunotherapy of Hodgkin's lymphoma, which takes advantage of cytokine secretion and cytotoxicity of activated T cells.

Co-stimulation with IL-2, but not via CD28, overcomes immunosuppression by breast tumour-derived factors on the in vitro stimulation of human T-cells

In order to investigate the mode of action of tumour-derived immunosuppressive factor from breast cancer (TDS) we examined its function on human T-cells stimulated via CD3 and co-stimulated via CD28 or the IL-2 receptor. When added at the initiation of culture, TDS inhibited anti-CD3 stimulation and co-stimulation by anti-CD28. In contrast, co-stimulation with IL-2 greatly diminished the TDS inhibition of anti-CD3 stimulated cells. Activation by IL-2 alone was also inhibited at the initiation of culture. When PBMC were activated with IL-2 or anti-CD3 for three days and then exposed to TDS for a further 3 days, only the proliferation of the cells pre-activated with IL-2 was inhibited; the cells pre-activated via CD3 were refractory to TDS inhibition. Pre-activation with anti-CD3 for 48 h was required for this to develop. The cytotoxicity of cells pre-activated with anti-CD3 was lower than that of cells exposed to IL-2, but killing obtained from cultures pre-activated with anti-CD3 plus IL-2 was equivalent to that obtained with IL-2 alone and additionally, these pre-activated cells were not subject to inhibition upon subsequent exposure to TDS.

Antibody and B7/BB1-mediated ligation of the CD28 receptor induces tyrosine phosphorylation in human T cells

CD28 is an adhesion receptor expressed as a 44-kD dimer on the surface of a major subset of human T cells. The CD28 receptor regulates the production of multiple lymphokines, including interleukin 2 (IL-2), by activation of a signal transduction pathway that is poorly understood. Here we show that ligation of CD28 by a monoclonal antibody (mAb) or by a natural ligand, B7/BB1, induces protein tyrosine phosphorylation that is distinct from T cell receptor (TCR)-induced tyrosine phosphorylation. CD28-induced protein tyrosine phosphorylation was greatly enhanced in cells that had been preactivated by ligation of the TCR, or by pretreatment with phorbol esters. Rapid and prolonged tyrosine phosphorylation of a single substrate, pp100, was induced in T cells after interaction with B7/BB1 presented on transfected Chinese hamster ovary (CHO) cells. Anti-B7 mAb inhibited B7/BB1 receptor- induced tyrosine phosphorylation, indicating that B7-CD28 interaction was required. CD28-induced tyrosine phosphorylation was independent of the TCR because it occurred in a variant of the Jurkat T cell line that does not express the TCR. Herbimycin A, a protein tyrosine kinase inhibitor, could prevent CD28-induced tyrosine phosphorylation and CD28- induced IL-2 production in normal T cells. The simultaneous crosslinking of CD28 and CD45, a tyrosine phosphatase, could prevent tyrosine phosphorylation of pp100. These results suggest that specific tyrosine phosphorylation, particularly of pp100, occurs directly as a result of CD28 ligand binding and is involved in transducing the signal delivered through CD28 by accessory cells that express the B7/BB1 receptor. Thus, this particular form of signal transduction may be relevant to lymphokine production and, potentially may provide a means to study the induction of self-tolerance, given the putative role of the costimulatory signal in the induction of T cell activation or anergy.

Production of tumor necrosis factor-alpha by naive or memory T lymphocytes activated via CD28

While it is well established that activated T cells can produce tumor necrosis factor alpha (TNF-alpha), it is less clear whether this function is confined to a given subset, e.g., memory cells. To approach this question, we investigated the production of TNF-alpha by human peripheral blood T lymphocytes activated with anti-CD28 mAb since this activation pathway is known to potentiate cytokine production. Under the culture conditions used, the amount of TNF-alpha produced was markedly enhanced compared to that obtained after activation with immobilized anti-CD3 mAb. The enhancement of TNF-alpha production was already apparent after incubation of T cells for 6 hr. Up to 5 ng/ml of TNF-alpha was measured on Day 2 in supernatants of cultures of 10(4) T lymphocytes. To determine the source of the cells producing high amounts of TNF-alpha, T lymphocytes were separated into two subpopulations, namely naive cells (expressing the CD45RA isoform) and memory cells (expressing the CD45RO isoform). While both subpopulations proliferated equally well after stimulation with anti-CD28 mAb, up to 90% of the TNF-alpha produced under these conditions originated from memory T cells. These results thus document that T cell activation via CD28 results in a marked increase in TNF-alpha production without affecting the functional disparity that exists between naive and memory T cells.

A synthetic peptide with sequence identity to the transmembrane protein GP41 of HIV-1 inhibits distinct lymphocyte activation pathways dependent on protein kinase C and intracellular calcium influx

A synthetic peptide containing env amino acid (aa) sequence 581 to 597 of the transmembrane protein gp41 of human immunodeficiency virus type 1 (HIV-1) was tested for its effect on protein kinase C (PKC) and cytoplasmic free Ca2+ [( Ca2+]i) influx-dependent immune functions. We have previously shown that this peptide inhibits PKC-mediated phosphorylation and T-cell receptor-mediated [Ca2+]i influx as well as lymphoproliferation. In this study we demonstrate that the HIV-1 gp41 peptide aa581-597 inhibits lymphoproliferation stimulated via the distinct T-cell-activation molecules CD3, CD2, and CD28, as well as direct stimulation mediated by phorbol ester combined with ionomycin. Further, aa581-597 inhibits both PKC-dependent interleukin 2 (IL 2) production and the [Ca2+]i influx-dependent but PKC-independent induction of IL 2 receptor expression. The HIV-1 gp41 peptide also induces dramatic morphologic changes in lymphocytes, characterized by cytoplasmic ballooning and the acquisition of adherence to plastic, and these changes are dependent on both the length and the temperature of exposure. The results of this study suggest that the HIV-1 gp41 sequence aa581-597 acts at multiple sites to inhibit both PKC activity and [Ca2+]i influx, resulting in the abrogation of several distinct immune functions that are critical for an intact immune response and are defective in HIV-1-infected individuals.

Dissociation between early and late events in T cell activation mediated through CD28 surface molecule

The regulation of early and late events of T cell activation via the CD28 molecule has been investigated, using as an indicator system the differentiated leukemic T cell line Jurkat. Both CD3 and CD28 mAbs induced an increase in (Ca2+)i in Jurkat cells, although with different kinetics, the latter being slower than the former. CD28-mediated (Ca2+)i mobilization was highly sensitive to cholera toxin (ID50 25 ng/ml, vs 300 ng/ml for CD3 stimulation). The inhibitory action of cholera toxin was neither merely due to the increase in intracellular cAMP concentrations, nor to decrease in cell surface expression of the CD28 molecule. To evaluate the effects of cholera toxin on late events of Jurkat cell activation induced by CD28 and CD3 mAbs, the action of cholera toxin and cAMP and CD3- and CD28-mediated IL-2 secretion was analyzed. CD3-induced IL-2 secretion was highly sensitive to cholera toxin (ID less than 5 ng/ml); on the other hand, CD28-induced IL-2 secretion was poorly sensitive to cholera toxin, in sharp contrast to (Ca2+)i mobilization. On the basis of these data, it is hypothesized that the CD28 pathway could be associated with at least two distinct transduction mechanisms, one responsible for the (Ca2+)i rise in Jurkat cells and highly sensitive to cholera toxin, and the other, whose second messenger is unknown, resistant to cholera toxin and responsible for IL-2 secretion.

Immobilized anti-CD5 together with prolonged activation of protein kinase C induce interleukin 2-dependent T cell growth: evidence for signal transduction through CD5

Monoclonal antibodies (mAb) identifying the CD5 antigen were used to stimulate human peripheral blood T lymphocytes. Three out of three anti-CD5 mAb, 10.2, OKT1 and anti-Leu-1 induced vigorous proliferation of purified T cells in the presence of 1.6 nM phorbol 12-myristate 13-acetate (PMA). Immobilization of anti-CD5 mAb on a solid support was necessary for the induction of a proliferative response. Neither 1.6 nM PMA, nor immobilized anti-CD5 mAb were mitogenic as a sole stimulus. mAb identifying CD4, CD7, CD11a, CD18, and major histocompatibility complex class I molecules were not comitogenic with PMA. Anti-CD5/PMA-induced cell proliferation proceeded by an interleukin 2 (IL 2)-dependent mechanism, as was demonstrated by the cell surface expression of the p55 chain of the IL 2 receptor (IL 2R), the production of IL 2 and the inhibition of the proliferative response by anti-IL 2R mAb anti-Tac. There was no strict requirement for detectable numbers of monocytes, although cell proliferation could be enhanced by the monocyte-derived cytokines IL 1 and IL 6. Phorbol 12,13-dibutyrate and mezerein could substitute for PMA in this activation pathway, but synthetic diacylglycerols and phorbol esters that do not activate protein kinase C (PKC) could not, indicating a need for prolonged activation of PKC. T cells activated by anti-CD5/PMA are sensitive to inhibition by cyclosporin A (CsA) and by prostaglandin E2 (PGE2). This contrasts with anti-CD28/PMA-induced T cell proliferation, which is resistant to CsA and PGE2. Cell surface expression of CD5 was strongly up-regulated by PMA, whereas CD3 expression was down-regulated. We conclude that T cell activation can be triggered by engagement of CD5 by immobilized anti-CD5 mAb, combined with prolonged activation of PKC. These data support a role for CD5 as an independent signal transducing molecule.

The effect of the immunosuppressant FK-506 on alternate pathways of T cell activation

Structurally unrelated, FK-506 and cyclosporin (CsA) bind to and inhibit the action of distinct cytoplasmic receptors, FK-506-binding protein (FKBP) and cyclophilin (CyP), respectively. These receptors, termed immunophilins, share no sequence similarity, and yet both have been demonstrated to be capable of catalyzing the cis-trans isomerization of peptidyl-prolyl bonds (rotamase activity). Because FK-506 and CsA bind to different intracellular target structures, we investigated the spectrum of action of FK-506, in comparison to CsA, on T cell activation. We have shown that FK-506, like CsA, is able to inhibit T cell activation mediated not only by the T cell receptor-CD3 complex, but also via another surface molecule, CD2. T cell proliferation, stimulation of interleukin 2 production, and induction of apoptosis were all sensitive to inhibition by both FK-506 and CsA. With each parameter of activation, FK-506 is approximately 10-100-fold more effective than CsA. In contrast, FK-506 did not affect T cell proliferation induced by anti-CD28 monoclonal antibody in the presence of phorbol 12-myristate 13-acetate. This CD28 pathway, however, was inhibited by a structural homology of FK-506, rapamycin, demonstrating that the mechanism of action of FK-506 has specificity. These data suggest that immunophilins or the complex of drug coupled to immunophilin (i.e. FK-506/FKBP, CsA/CyP) are involved in and regulate selective pathways of T cell stimulation.

Activation of peripheral CD8+ T lymphocytes via CD28 plus CD2: evidence for IL-2 gene transcription mediated by CD28 activation

It is well established that peripheral CD8+ and CD4+ T cells display different requirements for in vitro activation by mitogenic mAb. Most CD4+ T cells can be activated by anti-CD3 or mitogenic combinations of anti-CD2. In contrast, CD8+ T cells display minimal responses to CD3 activation, and no proliferation is observed via CD2 activation. Purified peripheral blood CD8+ T cells, stringently depleted of APC, have been studied for their capacity to respond to mAb directed against CD3, CD2 and CD28, used alone or in combination. It is demonstrated that proliferation can be induced by co-stimulation of CD2 and CD28. This does not require autologous APC. CD8+ T cells can also be activated by the combination of anti-CD3 plus anti-CD28 in the presence of APC, but only minimal cell proliferation is obtained in the absence of APC. The response via CD2 plus CD28 is IL-2-dependent, as demonstrated by the ability of mAb against the IL-2 receptor to block proliferation, and is almost completely inhibited by cyclosporine A (CsA). These results suggest that the signal generated by stimulation of CD28 in combination with CD2 differs from that seen with CD28 activation combined with either PMA or CD3. Induction of IL-2 gene activation in CD8+, CD28+ peripheral T cells may therefore require additional "second signals", which are not necessary for activation of CD4+ cells. One such signal might be the interaction between CD28 and its natural ligand.

Tumor necrosis factor beta (TNF-beta) induces binding of the NF-kappa B transcription factor to a high-affinity kappa B element in the TNF-beta promoter

The expression of the gene encoding tumor necrosis factor beta (TNF-beta) (lymphotoxin) is induced in T cells by various extracellular stimuli. We noticed that most such stimuli also activate the NF-kappa B transcription factor. Here we demonstrate binding of purified human NF-kappa B to a sequence within positions -98 to -88 (5'-GGGGCTTCCCC-3') of the TNF-beta promoter, which is conserved between the human and mouse genes. Also the NF-kappa B from the human T-cell line Jurkat, activated upon phytohemagglutinin (PHA)/phorbol 12-myristate 13-acetate (PMA/TPA) treatment in vivo or upon deoxycholate treatment in vitro, binds with high affinity to the sequence in the TNF-beta promoter. Apart from a single mismatch, the site is identical to a cis-activating element that is involved in the inducible expression of the MHC class I gene H-2Kb and which interacts with both the inducible NF-kappa B transcription factor and the constitutive factor KBF1/H2TF1, as we demonstrate here for the site in the TNF-beta promoter. The high homology of the well characterized H-2Kb enhancer sequence with the TNF-beta site with regard to sequence and factor binding strongly supports a physiological role for NF-kappa B in the inducible expression of the TNF-beta gene. Our observation that the TNF-beta protein can rapidly induce the DNA-binding activity of NF-kappa B in Jurkat T cells and transiently increase TNF-beta mRNA levels suggests that NF-kappa B can mediate a positive autoregulation of TNF-beta synthesis.

The endothelial cell as a regulator of T-cell function

These studies have analyzed the antigen-presenting capacities of EC. EC can transcribe, translate and express MHC class II molecules in response to IFN-gamma as well as express class I molecules. The class II dimeric structure is functional, in that allospecific CTL can efficiently kill IFN-gamma-treated EC or fibroblasts, an outcome that can be blocked by antibody to non-polymorphic regions of the class II molecule. Moreover, EC can present antigen in an MHC-restricted manner to resting T cells as well as to antigen-specific cloned T-cell lines. This ability to stimulate primary as well as secondary responses has been further confirmed by experiments using purified populations of naive and memory T cells. In this regard, EC differ from fibroblasts and other non-immune cell types in that they possess costimulator activities necessary for activation of resting T cells. As the local concentration of IL-2 has been shown to be critical in determining the fate of T cells--whether they become activated or anergic--we have investigated the ability of EC to modulate T-cell IL-2 production, believing that this may underlie their ability to act as costimulatory cells. Using PHA-stimulated peripheral blood mononuclear cells or purified CD4+ T cells we have found that EC can augment IL-2 production, typically by 3- to 8-fold. This increased IL-2 production is functional as OKT3-stimulated or sub-optimally PHA-stimulated T cells proliferate more in the presence of EC than in their absence. The major pathway by which EC augment T-cell IL-2 production is cell contact-dependent and involves the CD2:LFA-3 ligand pair. However, use of blocking mAb to CD2 and LFA-3, of PI-LFA-3, and of the immunosuppressive drug CsA has allowed us to reveal the presence of a second signalling pathway. This pathway confers a certain degree of CsA resistance on T cells, but the ligands involved have not yet been identified. We do not find a role for CD28, LFA-1:ICAM-1, VLA-4:VCAM-1 or CD44 in this system. Augmentation is independent of EC metabolism or soluble factors, as fixed cells are almost as efficient as living cells. Similar mechanisms seem also to be involved in more physiological settings, such as alloresponses. Here, proliferation can be blocked by antibodies to CD2 or LFA-3, presumably by blocking of augmented IL-2 production.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of the CD28 receptor in T-cell activation

Antigen-specific T-cell activation is initiated through the T-cell receptor. Recent evidence has shown that a number of additional T-cell surface receptors serve to regulate the responses of antigen-activated T cells. One such molecule, CD28, is a member of a heterophilic cell adhesion complex, and is the receptor for the B-cell-restricted B7/BB-1 antigen. As Carl June, Jeffrey Ledbetter, Peter Linsley and Craig Thompson review here, CD28 serves as the surface component of a novel signal transduction pathway that modulates T-cell lymphokine production and increases the resistance of T-cell responses to various immunosuppressive agents.

T-cell receptor cross-linking transiently stimulates adhesiveness through LFA-1

Effective interaction between T cells and their targets requires that recognition of specific antigen be coordinated with increased cell-cell adhesion. We show that antigen-receptor cross-linking increases the strength of the adhesion mechanism between lymphocyte function-associated molecule-1 (LFA-1) and intercellular adhesion molecules (ICAMs), with intracellular signals transmitted from the T-cell antigen receptor to the LFA-1 adhesion molecule. The increase in avidity is rapid and transient, providing a dynamic mechanism for antigen-specific regulation of lymphocyte adhesion and de-adhesion.