Uncovering of functional alternative CTLA-4 counter-receptor in B7-deficient mice

The role of superantigens in the immunobiology of retroviruses

Murine mammary tumour viruses (MMTVs) are retroviruses that encode superantigens capable of stimulating T cells via superantigen-reactive T cell receptor V beta chains. MMTVs are transmitted to the suckling offspring via the milk. We have established that class II and B cell-deficient mice that were foster nursed by virus-secreting mice do not transfer infectious MMTVs to their offspring. No MMTV proviruses could be detected in the spleen and mammary tissue of these mice and there was no deletion of MMTV superantigen-reactive T cells. These results confirm that superantigen expression in the context of MHC class II molecules is required for MMTV transmission. We conclude that B cells are essential for the completion of the viral life cycle in vivo. This indicates that B cells are infected first and that viral amplification takes place only if infected B cells present the MMTV superantigen on their surface which, in turn, results in activation of T cells expressing the appropriate T cell receptor V beta chains. These activated T cells stimulate B cells which enables viral replication. Human T cells carry all the structural features required for an efficient response to murine retrovirally encoded superantigens. Superantigen-like stimulation of human T cells has been demonstrated in both infectious and autoimmune diseases. Human immunodeficiency virus may encode a superantigen but this has not been proven.

Induction of a Th1 response from Th2-polarized T cells by activated dendritic cells: dependence on TCR:peptide-MHC interaction, ICAM-1, IL-12, and IFN-gamma

Dendritic cells (DC) are important regulators of T cell immunity. The degree of stimulation, the pattern of costimulatory molecules expressed, and the cytokines secreted by DC dictate the nature of the effector and memory cells generated, particularly with respect to their Th1 or Th2 phenotypes. In this study, we demonstrate that the addition of activated DC to spleen cultures containing established Th2-polarized CD4(+) T cells was sufficient to suppress Th2 and induce Th1 cytokines in a recall response, a phenomenon referred to as phenotype reversal. The ability of activated DC to induce phenotype reversal displayed exquisite Ag specificity. The DC activator B7-DC cross-linking Ab (XAb) was >10,000-fold more efficient at inducing phenotype reversal than the TLR agonists CpG-oligodeoxynucleotide and Gardiquimod. Characterization of the mechanisms governing phenotype reversal revealed the requirement for cognate interaction between the TCR:peptide-MHC complex, the expression of the costimulation/adhesion molecule ICAM-1, and secretion of IL-12 and IFN-gamma by the activated DC. The requirement for the costimulation/adhesion molecule SLAM (signaling lymphocytic activation molecule) was found to be quantitative. Thus, activation of DC, particularly by crosslinking B7-DC, can modulate well-established Th2 T cell responses in an Ag-specific manner. Because the regulation of mouse and human DC by B7-DC XAb overlaps in several significant ways, immune modulation with B7-DC XAb is a potential strategy for treating Th2-mediated diseases.

Viral interference with B7-1 costimulation: a new role for murine cytomegalovirus fc receptor-1

Murine CMV (MCMV), a beta-herpesvirus, infects dendritic cells (DC) and impairs their function. The underlying events are poorly described. In this study, we identify MCMV m138 as the viral gene responsible for promoting the rapid disappearance of the costimulatory molecule B7-1 (CD80) from the cell surface of DC. This was unexpected, as m138 was previously identified as fcr-1, a putative virus-encoded FcR. m138 impaired the ability of DC to activate CD8+ T cells. Biochemical analysis and immunocytochemistry showed that m138 targets B7-1 in the secretory pathway and reroutes it to lysosomal associated membrane glycoprotein-1+ compartments. These results show a novel function for m138 in MCMV infection and identify the first viral protein to target B7-1.

A mechanistic study of immune system activation by fusion of antigens with the ligand-binding domain of CTLA4

Fusion proteins consisting of the ligand-binding domain of CTLA4 covalently attached to an antigen (Ag) are potent immunogens. This fusion strategy effectively induces Ag-specific immunity both when introduced as a DNA-based vaccine and as a recombinant protein. CTLA4 is a ligand for B7 molecules expressed on the surface of antigen-presenting cells (APCs), and this interaction is critical for the fusion protein to stimulate Ag-specific immunity. We show that interaction of the fusion protein with either B7-1 or B7-2 is sufficient to stimulate immune activity, and that T cells are essential for the development of IgG responses. In addition, we demonstrate that human dendritic cells (DCs) pulsed with CTLA4-Ag fusion proteins can efficiently present Ag to T cells and induce an Ag-specific immune response in vitro. These studies provide further mechanistic understanding of the process by which CTLA4-Ag fusion proteins stimulate the immune system, and represent an efficient means of generating Ag-specific T cells for immunotherapy.

Vaccinia virus infection of mature dendritic cells results in activation of virus-specific naïve CD8+ T cells: a potential mechanism for direct presentation

Understanding how vaccinia virus (VV) generates immunity necessitates an appreciation for how this virus interacts with dendritic cells (DC), which are the most potent activators of naïve CD8(+) T cells. In order to optimally activate naïve CD8(+) T cells, DC must undergo maturation, during which costimulatory molecules are upregulated and cytokines are produced. In this report, we show that VV infection of immature murine bone marrow-derived DC (BMDC) failed to induce maturation. Similar results were obtained when CD8(+) DC were analyzed, a subset shown previously to be important in vivo in the generation of a vaccinia-specific response. The finding that VV infection of DC resulted in APC that were incapable of initiating T-cell activation was surprising given the previously reported role for direct presentation in the generation of anti-VV CD8(+) T-cell responses in mice. To address the potential mechanism responsible for direct presentation, we tested the hypothesis that previously matured DC were susceptible to vaccinia virus infection and could present newly synthesized VV-derived epitopes for CD8(+) T-cell activation. Our results show, that during VV infection of mature DC, threshold levels of viral protein are produced that promote T-cell activation. These results suggest that, even though VV cannot mature DC, previously matured DC exposed to VV can generate a VV-specific CD8(+) T-cell response providing a potential mechanism by which direct infection results in T-cell activation in vivo.

New therapeutics that treat rheumatoid arthritis by blocking T-cell activation

Despite the recent introduction of several new biological products, there remains a significant unmet medical need in rheumatoid arthritis. A focus on the aberrant activation of autoimmune T cells, which is integral to pathogenesis, is a promising approach involved in several of these new therapies. In choosing a molecular target for the modification of T-cell function, it is argued in this article, that within co-stimulatory pathways, CD80 could have a more compelling rationale than CD86. Data are presented showing that CD80-mediated T-cell activation can be inhibited using a small-molecule antagonist, which offers the potential to prevent the inflammatory process leading to joint destruction.

T-cell activation via CD26 and caveolin-1 in rheumatoid synovium

CD26 is a T-cell costimulatory molecule with dipeptidyl peptidase IV (DPPIV) activity in its extracellular region. We previously reported that recombinant soluble CD26 enhances peripheral blood T-cell proliferation induced by the recall antigen tetanus toxoid (TT). Recently, we demonstrated that CD26 binds caveolin-1 on antigen-presenting cell (APC), and that residues 201–211 of CD26 along with the serine catalytic site at residue 630, which constitute a pocket structure of CD26/DPPIV, contribute to binding to caveolin-1 scaffolding domain. In addition, following CD26–caveolin-1 interaction on TT-loaded monocytes, caveolin-1 is phosphorylated, with linkage to NF-κB activation, followed by upregulation of CD86. Finally, reduced caveolin-1 expression on APC inhibits CD26-mediated CD86 upregulation and abrogates CD26 effect on TT-induced T-cell proliferation, and immunohistochemical studies revealed an infiltration of CD26+ T cells in the sublining region of rheumatoid synovium and high expression of caveolin-1 in the increased vasculature and synoviocytes of the rheumatoid synovium. Taken together, these results strongly suggest that CD26–cavolin-1 interaction plays a role in the upregulation of CD86 on TT-loaded APC and subsequent engagement with CD28 on T cells, leading to antigen-specific T-cell activation such as the T-cell-mediated antigen-specific response in rheumatoid arthritis.

CTLA-4 interacts with STAT5 and inhibits STAT5-mediated transcription

Cytotoxic T-lymphocyte antigen-4 (CTLA-4; CD152) is a member of the immunoglobulin gene superfamily with strong homology to the receptor CD28 with which it shares the ligands CD80 and CD86. Unlike CD28, a potent costimulator of T-cell responses, CTLA-4 is transiently expressed on the cell surface of activated T cells and appears to operate predominantly as a negative regulator of T-cell proliferation. Signal transduction mechanisms utilized by CTLA-4 remain unclear although several mechanisms have been implicated. In this study, we show that the cytoplasmic domain of CTLA-4, but not of CD28, binds to STAT5 in yeast two-hybrid assay and in coimmunoprecipitation assays. Mutations of Tyr165 and Tyr182 in CTLA-4 did not abrogate the interaction of STAT5 with CTLA-4. Finally, the overexpression of CTLA-4 in Jurkat T cells inhibits STAT-mediated activation of STAT5 responsive elements. These results suggest that CTLA-4 and STAT5 interact in T cells and that this interaction is important for CTLA-4 signalling.

Molecular cloning, expression and functional characterization of miniature swine CD86

CD86 is one of the key molecules involved in the co-stimulation of T cells. The complete cDNA encoding CD86 molecule of miniature swine was cloned and analyzed. A comparison of two CD86 amino acid sequences of miniature swine and domestic swine showed only three amino acid differences suggesting that it is unlikely to affect the major structural features of the miniature swine CD86 (msCD86). In the expression study, constitutive expression of CD86 mRNA was detected in various tissues, and the aberrant expression of the transcriptional variant (putative soluble form) was noted. The cDNA and amino acid sequences for this variant were determined and compared with those for the human soluble CD86, which was previously reported to co-stimulate the T cells. Interestingly, an alignment of the two sequences revealed that 51 amino acids corresponding to the sequence for the boundary of the extracellular and intracellular domains including the transmembrane domain are deleted at almost an identical location within the full form of CD86 from both species. This suggests the possibility of a co-stimulatory function of the putative soluble msCD86. In order to determine if the cloned msCD86 molecules has co-stimulatory activity, the proliferative responses of the human CD4(+) T cells to the msCD86-transfected COS cells were measured in the presence of Con A. The results revealed that CD86/COS, but not the mock/COS, efficiently co-stimulated the proliferation of the Con A-stimulated CD4(+) T cells and this co-stimulatory effect was blocked by CTLA4-Ig. The structural and functional information on the miniature swine CD86 from this study will enable a further genetic manipulation of CD86 as a therapeutic strategy for controlling the xenogeneic T cell immune responses mediated by the CD86-CD28 signal pathway.

Dietary eicosapentaenoic acid modulates CTLA-4 expression in murine CD4+ T-cells

We have demonstrated that downregulation of proliferation by CD4(+) T-cells in mice fed n-3 PUFA diets is dependent on the involvement of CD28. Therefore, we hypothesized that the balance of co-stimulatory and downregulatory properties of CD28 and CTLA-4, respectively, would be altered by diet. Mice were fed a control corn oil (CO)-enriched diet devoid of n-3 PUFA, or diets enriched with either docosahexaenoic acid (DHA) or eicosapentaenoic acid (EPA) for 14d. The proliferation of splenic CD4(+) T-cells was suppressed by DHA and EPA following stimulation with anti-CD3 and anti-CD28. Surprisingly, the number of surface CD28 molecules was not reduced in activated CD4(+) T-cells from either group of n-3 PUFA-fed mice. However, in mice fed EPA, CTLA-4 protein levels were enhanced significantly 72 h post-activation (P<0.01). Therefore, we conclude that dietary EPA may suppress CD4(+) T-cell activation by enhancing the downregulatory co-receptor CTLA-4, while not altering the levels of CD28.

Human CTLA4 knock-in mice unravel the quantitative link between tumor immunity and autoimmunity induced by anti-CTLA-4 antibodies

Although results from preclinical studies in animal models have proven the concept for use of anti-cytotoxic T-lymphocyte antigen 4 (CTLA-4) antibodies in cancer immunotherapy, 2 major obstacles have hindered their successful application for human cancer therapy. First, the lack of in vitro correlates of the antitumor effect of the antibodies makes it difficult to screen for the most efficacious antibody by in vitro analysis. Second, significant autoimmune side effects have been observed in a recent clinical trial. In order to address these 2 issues, we have generated human CTLA4 gene knock-in mice and used them to compare a panel of anti-human CTLA-4 antibodies for their ability to induce tumor rejection and autoimmunity. Surprisingly, while all antibodies induced protection against cancer and demonstrated some autoimmune side effects, the antibody that induced the strongest protection also induced the least autoimmune side effects. These results demonstrate that autoimmune disease does not quantitatively correlate with cancer immunity. Our approach may be generally applicable to the development of human therapeutic antibodies.

Organ transplantation—how much of the promise has been realized?

Since the introduction of organ transplantation into medical practice, progress and optimism have been abundant. Improvements in immunosuppressive drugs and ancillary care have led to outstanding short-term (1--3-year) patient and graft survival rates. This success is mitigated by several problems, including poor long-term (>5-year) graft survival rates, the need for continual immunosuppressive medication and the discrepancy between the demand for organs and the supply. Developing methods to induce transplant tolerance, as a means to improve graft outcomes and eliminate the requirement for immunosuppression, and expanding the pool of organs for transplantation are the major challenges of the field.

Porin of Shigella dysenteriae enhances Toll-like receptors 2 and 6 of mouse peritoneal B-2 cells and induces the expression of immunoglobulin M, immunoglobulin G2a and immunoglobulin A

Porin of Shigella dysenteriae type 1 increased the mRNA levels for Toll-like receptors TLR2 and TLR6, by 1.8-fold and twofold, respectively, in peritoneal cavity B-2 cells from C57BL/6 mice, implicating that the co-expression of TLR2 and TLR6 occurs as a combinatorial repertoire in response to porin. Among the two key TLRs, TLR2 and TLR4, which are primarily responsible for recognizing the majority of bacterial products, TLR2 alone participates in porin recognition. TLR2 expression was increased on B-2 cells, whereas the expression of TLR4 remained unaffected. Besides TLRs, mRNA for myeloid differentiation factor 88 (MyD88), an effector molecule associated with the TLR-mediated response, was enhanced by twofold, suggesting its involvement in the activity of porin. The B-2 cells showed a 1.8-fold increase in mRNA expression of the signalling molecule, nuclear factor-kappa B (NF-kappaB), in the presence of porin. Porin treatment of B-2 cells selectively up-regulated the expression of the costimulatory molecule, CD86, by 4.4-fold. Porin induced the cell-surface expression of immunoglobulin (Ig)M, of IgG2a preferentially among the IgG subclasses, and of IgA, on B-2 cells. The porin-mediated inductions of IgG2a and IgA were augmented by interleukin-6 on B-2 cells, by 2.7- and 1.6-fold, respectively.

Up-regulation of CD80-CD86 and IgA on mouse peritoneal B-1 cells by porin of Shigella dysenteriae is Toll-like receptors 2 and 6 dependent

Porin of Shigella dysenteriae type 1 increased the mRNA levels for Toll-like receptor (TLR) 2 and TLR6 by 1.5- and 2.9-fold respectively, of peritoneal cavity B-1a and B-1b cells, implicating that coexpression of TLR2 and TLR6 is essential as a combinatorial repertoire for recognition of porin by the B-1 cells. Among the two key TLRs, TLR2 and TLR4, which are primarily responsible for recognizing majority of the bacterial products, TLR2 and not TLR4, participates in porin recognition. TLR2 got increased on both the B-1 cell populations whereas the TLR4 expression remained unaffected. Besides TLRs, mRNA for MyD88, an effector molecule associated with TLR-mediated response was enhanced by 1.8-fold that suggests of its involvement in the activity of porin. Both of the B-1 cell populations expressed strongly the mRNA for NF-kappaB in the presence of porin, that was 2.4-fold more than untreated control, conforming to the earlier finding that coexpression of TLR2 and TLR6, resulted in robust NF-kappaB activation for signaling. Porin treatment of B-1 cell populations of C57BL/6 mice, and C3H/HeJ mice in particular, selectively up-regulated the expression of the costimulatory molecules. CD80 expression got enhanced on the B-1a cells whereas CD86 got solely expressed on B-1b cells. Porin-induced cell surface expression of IgM and IgA on B-1 cell populations from C57BL/6 mice. The IgA-generating capacity, hallmark of mucosal immune response, was confirmed with B-1 cells of C3H/HeJ, the lipopolysaccharide non-responder mouse, in response to the protein. The porin-mediated induction of IgA was augmented by interleukin-6 on B-1a and B-1b cells, by 2.4- and 2.6-fold, respectively. The IgA expressed on both B-1a and B-1b cell surfaces after 72 h of culture was found to bind to the 38 kDa monomer of porin confirming it to be anti-porin IgA antibody.

B7-1 and B7-2 selectively recruit CTLA-4 and CD28 to the immunological synapse

The reported affinity differences between CD28 and CTLA-4 binding to B7-1 and B7-2 may serve to selectively regulate CD28 and CTLA-4 function by differentially recruiting and/or stabilizing these molecules at the immunological synapse. Here we show that ligand binding is important for the accumulation of both CD28 and CTLA-4 at the synapse. While CD28 is recruited to the synapse in the absence of B7-1 and B7-2 binding, it is not effectively stabilized there, as its localization can be disrupted by CTLA-4. In the case of CTLA-4, ligand binding is critical for its concentration at the synapse. We also demonstrate that the affinity and avidity differences in ligand binding translate into selective recruitment of CD28 or CTLA-4 to the immunological synapse--B7-1 is the major ligand mediating CTLA-4 localization, while B7-2 is the main ligand for CD28 concentration at the synapse.

Cytoplasmic entry of Listeria monocytogenes enhances dendritic cell maturation and T cell differentiation and function

Protective immunity to the intracellular bacterial pathogen, Listeria monocytogenes, is mediated by a vigorous T cell response. In particular, CD8(+) cytolytic T cells provide essential effector function in the clearance of bacterial infection. The cytoplasmic entry of Listeria facilitated by listeriolysin O is an essential feature not only of the bacteria's virulence, but of the ability of the bacteria to elicit protective immunity in the host. To determine how cytoplasmic entry of Listeria regulates the development of protective immunity, we examined the effects of this process on the maturation of murine dendritic cells (DC) and on their ability to prime naive CD8(+) T cell responses. Costimulatory molecules (CD40, CD80, and CD86) were induced by listerial infection only when the bacteria invaded the cytoplasm. In addition, the production of IL-12, IL-10, IL-6, and TNF-alpha was most efficiently triggered by cytosolic Listeria. Naive T cells primed by peptide-loaded DC infected with either wild-type or nonhemolytic mutant Listeria proliferated equivalently, but a much larger proportion of those primed by wild-type Listeria monocytogenes produced IFN-gamma. Costimulatory molecules induced by cytosolic entry regulated T cell proliferation and, as a result, the number of functional T cells generated. DC-produced cytokines (specifically IL-12 and IL-10) were the major factors determining the proportion of T cells producing IFN-gamma. These data highlight the requirement for listerial cytoplasmic invasion for the optimal priming of T cell cytokine production and attest to the importance of this event to the development of protective CTL responses to this pathogen.

Spontaneous T cell apoptosis in feline immunodeficiency virus (FIV)-infected cats is inhibited by IL2 and anti-B7.1 antibodies

Lymph node (LN) T cells from feline immunodeficiency virus (FIV)-infected cats have an increased expression of B7 co-stimulatory molecules as well as their ligand CTLA4, resembling an activation phenotype shown to induce anergy and apoptosis in activated T cells. In addition, LN T cells from FIV-infected cats also show increased spontaneous apoptosis compared to uninfected animals. The apoptosis observed in these animals occurs primarily in T cells expressing B7 and CTLA4, suggesting a role for B7 and CTLA4 interactions in the induction of anergy/apoptosis. In order to investigate the role of B7 and CTLA4 interactions on T cell apoptosis in LN T cells from FIV-infected cats, we performed blocking experiments by measuring T cell apoptosis in LN T cell cultures treated with anti-feline B7.1, B7.2, and CTLA4 specific antibodies, as well as interleukin (IL)-2. The addition of IL2, the primary cytokine produced by B7/CD28 interactions, resulted in a significant decrease of T cell apoptosis in cultured LN cells as assessed by two-color flow cytometry and TUNEL assay. The addition of anti-B7.1 antibodies significantly inhibited T cell apoptosis in FIV-infected cats with low-level plasma viremia, while addition of anti-B7.2 and anti-CTLA4 antibodies had no affect. These results suggest a role of B7 signaling in the increased spontaneous apoptosis observed in LN T cells from FIV-infected animals.

Porin of Shigella dysenteriae enhances mRNA levels for Toll-like receptor 2 and MyD88, up-regulates CD80 of murine macrophage, and induces the release of interleukin-12

Sera of patients convalescing from shigellosis reacted strongly and specifically with the 38,000 Da monomer of porin of Shigella dysenteriae type 1. Since human, the only natural host of S. dysenteriae type 1, recognized the protein through humoral immune response, it is of great significance to study the surface-exposed outer membrane antigen as an adjuvant. Porin treatment of CD11b+ peritoneal cavity (PerC) MPhi of BALB/c mouse was found to up-regulate CD80 on cell surface and had no effect on CD86 expression. The surface expression of CD80 got increased by 1.6-fold in the presence of gamma interferon (IFN-gamma) supporting selective regulation of the B7-1 (CD80) member of the B7 family. MPhi released 7.25 pg of interleukin-12 (IL-12) in the presence of porin. The protein in combination with IFN-gamma augmented profoundly the release of IL-12 by 2.6-fold. Porin-mediated induction of IL-12 release would therefore influence Th1-type response, known to be preferentially triggered due to up-regulation of CD80 expression. Treatment of PerC MPhi by the protein showed an increase of mRNA for both Toll-like receptor (TLR)2 and myeloid differentiation factor 88 (MyD88) by 2- and 2.3-fold respectively, emphasizing that TLR2 is essential for recognition of S. dysenteriae type 1 porin. Understanding the mechanism of adjuvanticity of porin of S. dysenteriae type 1 is a necessary step towards the development of a better adjuvant against shigellosis.

Ebastine inhibits T cell migration, production of Th2-type cytokines and proinflammatory cytokines

BACKGROUND

Cytokine imbalance and cellular migration to inflammatory sites are critical components of allergic diseases. Redirecting cytokine imbalance and inhibiting cell migration therefore represent important therapeutic strategies for the treatment of these disorders.

OBJECTIVES

To study the in vitro effect of ebastine, a novel non-sedating H1 receptor antagonist, on cytokine secretion and migration of activated T cells, as well as production of pro-inflammatory cytokines by macrophages.

METHODS

Peripheral T cells obtained from healthy volunteers were cultured in wells coated with the combination of anti-CD3 monoclonal antibody (mAb) and anti-CD26 mAb, anti-CD3 mAb and anti-CD28 mAb, or anti-CD3 mAb with PMA, in the presence or absence of ebastine. T cell proliferation and the production of cytokines were measured by [3H]thymidine incorporation assay and ELISA, respectively. In addition, transendothelial migration of T cells and production of pro-inflammatory cytokines by macrophages were examined.

RESULTS

Ebastine inhibited T cell proliferation and the production of IL-4, IL-5, IL-6, and TNF-alpha by T cells under each co-stimulatory condition tested, whereas it exhibited no effect on the production of IL-2 or IFN-gamma. In addition, T cell migration and the production of such pro-inflammatory cytokines as TNF-alpha and IL-6 by macrophages were inhibited by ebastine.

CONCLUSIONS

These results indicate that ebastine has a specific inhibitory effect on Th2-type cytokine production. Moreover, ebastine inhibited T cell migration and pro-inflammatory cytokine production by T cells and macrophages, suggesting that ebastine might be useful for the treatment of T cell-mediated allergic inflammatory disorders, including asthma, atopic dermatitis, and Th2-type autoimmune diseases.

The role of ICOS and other costimulatory molecules in allergy and asthma

Activation and differentiation of T cells play a critical role in the pathogenesis of allergies and asthma. Upon encounter with specific antigen, naïve T helper precursor (ThP) cells become activated, an event that is regulated not only by engagement of the T cell receptor (TCR) with peptide presented in the context of MHC class II molecules, but also by a number of costimulatory signals. CD28 engagement by B7-1 and B7-2 on resting ThP cells provides a critical signal for initial cell cycle progression, interleukin-2 production and clonal expansion. However, in recent years, other related members of the immunoglobulin (Ig) family, such as inducible costimulatory molecules (ICOS) and the TNF receptor family members which include OX40, have also been demonstrated to play an important role in providing unique and complementary signals that regulate the outcome of immune responses. These positive costimulatory signals are counterbalanced by signals that dampen down immune responses and include CTLA-4, PD-1 and the recently described Ig superfamily members BTLA and TIM-3. This review discusses the role of these costimulatory signals and their potential involvement in the pathogenesis of asthma and allergic responses.

Role of costimulatory pathways in the pathogenesis of multiple sclerosis and experimental autoimmune encephalomyelitis

Multiple sclerosis is an immune-mediated disorder of the central nervous system. T lymphocytes are thought to play a central role in the initiation and potentially in the propagation of this disease. Two signals are required for T-cell activation. The first signal consists of the interaction of the T-cell receptor with antigen presented by the MHC molecule on antigen-presenting cells. The second signal requires engagement of costimulatory receptors on T cells with their ligands on antigen-presenting cells. Several costimulatory pathways have been shown to play an important role in T-lymphocyte activation. Here we will review the current literature on the contribution of the B7-1/2-CD28/CTLA-4, inducible costimulatory molecule-B7h, programmed death pathway 1-programmed death pathway ligand 1/ligand 2, CD40-CD154, OX40-OX40 ligand, and CD137-CD137 ligand pathways to the pathogenesis of multiple sclerosis and their potential roles as therapeutic targets.

Distinct costimulation dependent and independent autoreactive T-cell clones in primary biliary cirrhosis

BACKGROUND & AIMS

Previous work has suggested that CD4+ CD28- or costimulation-independent T cells are increased in autoimmune diseases. In this study, we compared frequency and qualitative characteristics of autoreactive costimulation-independent or CD4+ CD28- T cells in primary biliary cirrhosis (PBC) by taking advantage of the well-defined immunodominant autoepitope of the E2 component of pyruvate dehydrogenase (PDC-E2).

METHODS

We determined the frequency of costimulation-independent autoreactive T cells that respond to PDC-E2 163-176 and the frequency of CD4+ CD28- T cells. Finally, we determined the role of biliary epithelial cells (BEC) as both an antigen-presenting cell or, alternatively, as a target cell for T-cell-mediated cytotoxicity.

RESULTS

The precursor frequency of costimulation-independent CD4+ T cells that respond to PDC-E2 163-176 and the frequency of CD4+ CD28- T cells were dramatically elevated in PBC. Furthermore, 2 types of T-cell clones that respond to PDC-E2 163-176 emerged from this study. One type was costimulation dependent and the other costimulation independent. Both types of clones lyse BEC in a similar effector target (E/T) ratio distribution. However, BEC did not help the proliferation of any T-cell clones. Furthermore, costimulation-independent T-cell clones do not become anergic by BEC.

CONCLUSIONS

In PBC, costimulation-independent autoreactive T cells, which do not become anergic, increase and maintain the autoimmune response. In controls, although autoantigens are expressed on BEC and autoantigen-reactive T cells exist around BEC, autoantigen-reactive T cells are costimulation dependent and will become anergic and maintain peripheral tolerance.

Macrophage distinguishes Vibrio cholerae hemolysin from its protease insensitive oligomer by time dependent and selective expression of CD80–CD86

The monomeric and oligomeric forms of Vibrio cholerae hemolysin (HlyA), a membrane damaging toxin that forms transmembrane pentameric diffusion channels in target eukaryotic membrane, show a pronounced difference in protease susceptibility, presumably due to masking of sensitive peptide bonds during oligomerization. In this work, we examined if resistance of a protein to proteolytic processing affects the expression of costimulatory molecules, CD80 and CD86, on macrophage exposed to the same antigen. The murine peritoneal cavity macrophages expressed both CD80 and CD86 after 24 h of incubation with HlyA monomer but failed to express the costimulatory molecules when treated with the HlyA oligomer. The expression of CD80 molecule on macrophage after 48 h by the HlyA oligomer that failed to express the costimulatory molecules after 24 h indicates that proteolytic processing plays a decisive role in expression of CD80 and CD86 on cell surface.

Molecular mechanisms regulating Th1 immune responses

The T helper lymphocyte is responsible for orchestrating the appropriate immune response to a wide variety of pathogens. The recognition of the polarized T helper cell subsets Th1 and Th2 has led to an understanding of the role of these cells in coordinating a variety of immune responses, both in responses to pathogens and in autoimmune and allergic disease. Here, we discuss the mechanisms that control lineage commitment to the Th1 phenotype. What has recently emerged is a rich understanding of the cytokines, receptors, signal transduction pathways, and transcription factors involved in Th1 differentiation. Although the picture is still incomplete, the basic pathways leading to Th1 differentiation can now be understood in in vitro and a number of infection and disease models.

Informatics and the immune system: the expanding IL-1 and B7 protein families

Remarkable advances in our knowledge of the genome sequence have led to the creation of databases that afford the opportunity to discover genes based on the presence of specific sequence motifs. The application of this strategy to the identification of proteins in families of immunological interest has had a visible impact on the sizes of the interleukin 1/18 and the CD28/B7 costimulatory molecule families, among many others. This accelerated pace of discovery presents a new challenge to match the rate of discovery with biological understanding of the functions of these extended protein families.

Intestinal graft-versus-host disease: mechanisms and management

Allogeneic haematopoietic stem cell transplantation remains the treatment of choice for a number of malignancies. However, graft-versus-host disease (GVHD) has long been regarded as a serious complication of this procedure. Although GVHD may affect any organ, intestinal GVHD is particularly important because of its frequency, severity and impact on the general condition of the patient. Recent studies have led to progressive elucidation of the mechanism of GVHD. Donor T cells are critical for the induction of GVHD, because depletion of T cells from bone marrow grafts effectively prevents GVHD but also results in an increase of leukaemia relapse. It has been shown that the gastrointestinal tract plays a major role in the amplification of systemic disease because gastrointestinal damage increases the translocation of endotoxins, which promotes further inflammation and additional gastrointestinal damage. Consequently, the management of intestinal GVHD (and the intestine itself) is a subject that should be highlighted. In this article, approaches to the prevention of intestinal GVHD are discussed after being classified into three categories: regimens in common clinical use, regimens under investigation and original regimens used at our hospital. The standard regimen that is used most widely for prevention of GVHD is cyclosporin plus short-term methotrexate. Corticosteroids can be added to this regimen but careful consideration of the adverse effects of these hormones should be considered. Tacrolimus is a newer, more potent alternative to cyclosporin. T-cell depletion (TCD) after transplantation has been shown to prevent acute GVHD, however, the survival benefit of TCD has not been as great as expected. Mycophenolate mofetil can be useful for the treatment of acute GVHD as part of combination therapy. Regimens currently under investigation in animal experiments include suppression of inflammatory cytokines and inhibition of T-cell activation, and, specifically at our institution, hepatocyte growth factor gene therapy. The evidence-based therapy used at our institution includes systemic antibacterial therapy (including eradication of intestinal bacteria) to prevent the intestinal translocation of lipopolysaccharide and avoid the subsequent increase of various inflammatory cytokines. In addition, because of the similarities between intestinal GVHD and ulcerative colitis, sulfasalazine, betamethasone enemas and eicosapentaenoic acid have been used to treat intestinal GVHD in some patients.

B7-1 costimulatory molecule is critical for the development of experimental autoimmune myasthenia gravis

Following immunization with acetylcholine receptor (AChR), MHC class II-restricted, AChR-specific CD4 cell activation is critical for the development of experimental autoimmune myasthenia gravis (EAMG) in C57BL/6 mice. To study the contributions of B7-1 and B7-2 costimulatory molecules in EAMG, B7-1, B7-2, and B7-1/B7-2 gene knockout (KO) mice were immunized with Torpedo AChR in CFA. Compared with wild-type C57BL6 mice, B7-1 and B7-1/2 KO mice were resistant to EAMG development. B7-1 KO mice had reduced anti-AChR Ab compared with C57BL/6 mice. However, neither B7-1 nor B7-2 gene disruption impaired AChR-induced or dominant alpha(146-162) peptide-induced in vitro lymphoproliferative responses. Blocking of the B7-1 or B7-2 molecule by specific mAbs in vivo led to a reduction in the AChR-specific lymphocyte response, and the reduction was more pronounced in mice treated with anti-B7-2 Ab. The findings implicate B7-1 molecules as having a critical role in the induction of EAMG, and the resistance of B7-1 KO mice is associated with suppressed humoral, rather than suppressed AChR-specific, T cell responses. The data also point to B7-2 molecules as being the dominant costimulatory molecules required for AChR-induced lymphocyte proliferation.

Blockade of CD86 in BALB/c mice infected with Leishmania major does not prevent the expansion of low avidity T cells

The interactions between CD28 and its ligand CD86 are critical for the regulation of T cell responses. However, it is not clear whether CD4+ T cells expressing low and high avidity TCR are equally dependent on CD28 costimulation for their activation and expansion. To address this issue, we have used multimers of I-Ad molecules linked to a peptide derived from the Leishmania major homolog for the receptor of activated C kinase (LACK) antigen to compare the fate of LACK-specific CD4+ T cells in Leishmania-infected BALB/c mice which have been treated or not with anti-CD86 mAb. Although the administration of anti-CD86 mAb did not completely prevent the expansion of LACK-specific T cells, their frequency and number were markedly reduced. In mice treated with anti-CD86 mAb as well as in control animals, L. major induced the clonal expansion of LACK-specific T cells which expressed a canonical low avidity Valpha8/Vbeta4 TCR. Taken together, our results suggest that the molecular interactions between CD28 on T cells and CD86 on APC serve to amplify and modulate T cell responses without promoting breadth in the TCR repertoire.

CD80+Gr-1+ myeloid cells inhibit development of antifungal Th1 immunity in mice with candidiasis

To find out whether polymorphonuclear neutrophils (PMN), abundantly recruited in disseminated Candida albicans infection, could directly affect the activation of Th cells we addressed the issues as to whether murine PMN, like their human counterparts, express costimulatory molecules and the functional consequence of this expression in terms of antifungal immune resistance. To this purpose, we assessed 1) the expression of CD80 (B7-1) and CD86 (B7-2) molecules on peripheral, splenic, and inflammatory murine Gr-1+ PMN; 2) its modulation upon interaction with C. albicans in vitro, in vivo, and in human PMN; 3) the effect of Candida exposure on the ability of murine PMN to affect CD4+ Th1 cell proliferation and cytokine production; and 4) the mechanism responsible for this effect. Murine PMN constitutively expressed CD80 molecules on both the surface and intracellularly; however, in both murine and human PMN, CD80 expression was differentially modulated upon interaction with Candida yeasts or hyphae in vitro as well as in infected mice. The expression of the CD86 molecule was neither constitutive nor inducible upon exposure to the fungus. In vitro, Gr-1+ PMN were found to inhibit the activation of IFN-gamma-producing CD4+ T cells and to induce apoptosis through a CD80/CD28-dependent mechanism. A population of CD80+Gr-1+ myeloid cells was found to be expanded in conventional as well as in bone marrow-transplanted mice with disseminated candidiasis, but its depletion increased the IFN-gamma-mediated antifungal resistance. These data indicate that alternatively activated PMN expressing CD80 may adversely affect Th1-dependent resistance in fungal infections.

Failure of B7.1-modified tumor to evoke full activation of CD8+ tumor-infiltrating lymphocytes in the central nervous system: prevention of parental tumor growth in the subcutaneous environment

OBJECT

It is well known that the central nervous system (CNS) is an immunologically privileged site. To characterize CD8+ tumor-infiltrating lymphocytes (TILs) recovered from the CNS, the authors compared these cells with TILs recovered from subcutaneous tissue by using a B7.1 gene-modified tumor implantation model.

METHODS

The authors established a B7.1 gene-modified EL4 murine lymphoma cell line (EL4-B7.1) and implanted the cells into the CNS to observe the duration of tumor-free survival. Although EL4-B7.1 cells were completely rejected in a subcutaneous implantation model, 40% of animals died after the CNS implantation (all animals in which the parent tumor was implanted died within 16 days). Therefore, the authors isolated TILs from each implantation site and analyzed the expressions of activation antigens CD25 and CD69 by performing the anti-CD8 magnetic beads separation method and flow cytometric analysis. After implantation of the parent tumor, there was no difference in the number of TILs from each site (CD25 1.7-3.2%, CD69 21.9-34.3%). After implantation of the B7.1-modified tumor, the CD25-expressing TIL population from the subcutaneous site was 4.68 times higher than that from the CNS site (17.8% compared with 3.8%). Based on these findings, the authors used a mitomycin C-treated EL4-B7.1 subcutaneous vaccination with various protocols. Vaccination before tumor challenge was sufficient to prevent the development of the tumor. For animals with established tumor, the vaccination protocol was able to prolong host survival (p = 0.0053).

CONCLUSIONS

The data clearly demonstrate that the CNS environment fails to activate CD8+ TILs fully. These are the first data indicating in detail a difference between CD8+ TILs from the CNS and those from other sites based on a B7.1-modified tumor model.

Immunotherapy for pancreatic cancer: current concepts

Despite advances in chemotherapy and surgical technique, patients with pancreatic cancer often succumb to local recurrence or metastatic spread. The need for new therapeutic strategies for this disease coupled with a better understanding of basic immunology have led to the development of novel anti-tumor vaccines. This review focuses on the historical development of tumor vaccines emphasizing the identification of potential pancreatic tumor antigens. The role of both B-cell and T-cell responses in tumor rejection will be reviewed. Methods for antigen presentation, including peptides, recombinant viral and bacterial vectors, dendritic cells, and whole cell approaches will be discussed. The use of immune adjuvants and improved methods of vaccine delivery will also be explored. The full potential for the immunotherapy of pancreatic cancer awaits the results of early phase clinical trials. The development of pancreatic cancer vaccines represents a useful paradigm for the translation of basic research into the clinical arena.

The immunologic basis for the treatment of psoriasis with new biologic agents

Psoriasis vulgaris is the most prevalent T-cell-mediated inflammatory disease in humans. The pathogenesis of psoriasis is linked to activation of several types of leukocytes that control cellular immunity and to a T-cell-dependent inflammatory process in skin that accelerates the growth of epidermal and vascular cells in psoriasis lesions. Critical steps in immunologic activation include Langerhans cell maturation (activation), T-cell activation, differentiation and expansion of type 1 T cells, selective trafficking of activated T cells to skin, and induction of an inflammatory cytokine and chemokine cascade in skin lesions. In turn, each of these steps offers an opportunity for intervention with engineered biologic therapeutics.

Expression of CTLA-4 by human monocytes

Cytotoxic T lymphocyte-associated molecule-4 (CTLA-4) is a receptor present on T cells that plays a critical role in the downregulation of antigen-activated immune responses. CTLA-4 interacts with the ligands CD80 and CD86 on antigen-presenting cells (APC), and also directs the assembly of inhibitory signalling complexes that lead to quiescence or anergy. In this study, we show that human monocytes constitutively express CTLA-4. About 3% of monocytes expressed CTLA-4 on the cell surface, whereas the intracellular expression was higher and present in about 20% of the monocytes. The sequences of the cDNAs from human monocytes were identical to the sequences of CTLA-4 from T cells. Expression of CTLA-4 was also confirmed in the activated myelomonocytic cell lines U937 and THP-1. Monocytes, but not T cells, activated by interferon (IFN)-gamma also secreted soluble CTLA-4 in vitro. The CTLA-4 expression was upregulated upon treatment with phorbol 12-myristate 13-acetate (PMA) and IFN-gamma. This increased expression could be partially abolished by staurosporine, an inhibitor of protein kinase C (PKC). Ligation of CTLA-4 in the monocyte-like cell-line U937 with antibodies against CTLA-4 partially inhibited the proliferation of cells and the upregulation of cell-surface markers CD86, CD54, HLA-DR and HLA-DQ induced by IFN-gamma and Staphylococcus aureus, Cowan I strain (SAC). Ligation of CTLA-4 suppressed the PMA-stimulated activation of transcription activator protein 1 (AP-1) and nuclear factor (NF)-kappaB in the U937 cell line, indicating the involvement of an inhibitory signal transduction. These data provide the first evidence that CTLA-4 is constitutively expressed by monocytes and thus might be important for the regulation of immune mechanisms associated with monocytes.

Downmodulation of CD18 and CD86 on macrophages and VLA-4 on lymphocytes in experimental tuberculosis

Development and evaluation of new vaccines and immunotherapy against tuberculosis demand a better understanding of the immune mechanisms in this disease. Costimulatory signals and intercellular contact seem to be pivotal in determining whether recognition of antigen by T cells leads to activation or anergy. In this paper, we show that virulent M. tuberculosis H37Rv downmodulates the ex vivo expression of CD18 and CD86 on peritoneal macrophages and VLA-4 on lymphocytes but does not disturb the in vitro production of interleukin (IL)-12 and interferon (IFN)-gamma after intraperitoneal infection. In addition, splenocytes from infected mice produce IL-10, while the expression of cell surface receptors is unchanged. The interplay among IL-12, IFN-gamma and IL-10 in vivo and the downmodulation of cell-surface receptors during the infection at the inflammatory site may contribute to the explanation of the maintenance of infection.

Inhibition of the function of the FcgammaRIIB by a monoclonal antibody to thymic shared antigen-1, a Ly-6 family antigen

Thymic shared antigen-1 (TSA-1) is a member of the Ly-6 family of glycosyl-phosphatidylinositol (GPI)-linked proteins. While it has been proposed that TSA-1 may play a role in thymic development, a physiological ligand for this antigen has not been identified. Here we report that a monoclonal antibody (mAb) to TSA-1, generated by immunizing a hamster with CD40 ligand (CD40L)-activated B cells, interferes with the function of FcgammaRIIB on splenic B cells and the B-cell lymphoma cell line, M12, by binding to TSA on the same cells. The interaction of anti-TSA with FcgammaRIIB resulted in an inhibition of the ability of the FcgammaRIIB to cross-link and/or aggregate soluble anti-CD3 or soluble anti-Cbeta T-cell receptor (TCR), leading to an inhibition of induction of expression of CD25 and CD69, interleukin (IL)-2 production and proliferation of naive T cells. Cross-blocking studies with mAbs strongly suggested that a physical association exists between TSA-1 and the FcgammaRIIB on the surface of activated B cells and favour the view that a functional intermolecular association exists between these two distinct membrane antigens.

Anti-Sm B cell differentiation in Ig transgenic MRL/Mp-lpr/lpr mice: altered differentiation and an accelerated response

To determine the regulation of B cells specific for the ribonucleoprotein Sm, a target of the immune system in human and mouse lupus, we have generated mice carrying an anti-Sm H chain transgene (2-12H). Anti-Sm B cells in nonautoimmune 2-12H-transgenic (Tg) mice are functional, but, in the absence of immunization, circulating anti-Sm Ab levels are not different from those of non-Tg mice. In this report, we compare the regulation of anti-Sm B cells in nonautoimmune and autoimmune MRL/Mp-lpr/lpr (MRL/lpr) and bcl-2-22-Tg mice. Activation markers are elevated on splenic and peritoneal anti-Sm B cells of both nonautoimmune and autoimmune genetic backgrounds indicating Ag encounter. Although tolerance to Sm is maintained in 2-12H/bcl-2-22-Tg mice, it is lost in 2-12H-Tg MRL/lpr mice, as the transgene accelerates and increases the prevalence of the anti-Sm response. The 2-12H-Tg MRL/lpr mice have transitional anti-Sm B cells in the spleen similar to nonautoimmune mice. However, in contrast to nonautoimmune mice, there are few if any peritoneal anti-Sm B-1 cells. These data suggest that a defect in B-1 differentiation may be a factor in the loss of tolerance to Sm and provide insight into the low prevalence of the anti-Sm response in lupus.

OX40 ligation enhances cell cycle turnover of Ag-activated CD4 T cells in vivo

OX40 costimulates T cells, increases activated T cell longevity, and promotes memory acquisition. T cells activated in vivo with agonist anti-OX40 and ovalbumin have a unique pattern of survival and cell division compared to control cells, but are able to respond to recall Ag equally well. BrdU incorporation shows that early cellular division rates of the anti-OX40-treated and the control groups are similar. Nevertheless, more BrdU(+) Ag-specific T cells accumulate in lymphoid tissue upon anti-OX40 administration. Thus, OX40 ligation does not necessarily lead to increased cell cycle entry, but promotes the accumulation of dividing cells. However, CFSE staining shows that OX40 ligation allows cells to progress through more cellular division cycles, while control cells stall or die. Moreover, OX40 ligation leads to a proportional decrease in apoptotic Ag-specific T cells. Thus, OX40 ligation boosts immunity by promoting an increase in the number cell cycles completed, thereby increasing the life span of Ag-activated CD4 T cells.

The expanding B7 superfamily: increasing complexity in costimulatory signals regulating T cell function

Upon encounter with specific antigen, naïveT helper precursor (THP) cells become activated. This event is regulated not only by engagement of the T cell receptor (TCR) with peptide presented in the context of major histocompatibility complex (MHC) class II molecules but by a number of costimulatory signals. CD28 engagement by B7-1 and B7-2 on resting THP cells provides a critical signal for initial cell cycle progression, interleukin 2 production and clonal expansion. However, largely as a consequence of the unraveling of the human genome, it is becoming clear that B7-1 and B7-2 are part of a larger family T of related counter-receptors that play an essential role in regulating the fate of primed, rather then resting,THP cells. These molecules play an important sequential role and act, together with B7-1- and B7-2-primed T cells, in the acquisition of effector function and/or tolerance induction.

Development of therapeutic vaccines by direct modification of cell membranes from surgically removed human tumor tissue with immunostimulatory molecules

The addition of immunostimulatory molecules to tumor cells has been proposed as a potentially useful strategy to induce anti-tumor immunity. In this report we have investigated the application of using isolated tumor membranes modified by transfer of a glycosyl-phosphatidylinositol (GPI)-anchored form of the costimulatory molecule, B7-1 (CD80), as a cell free cancer vaccine for clinical use. Isolated tumor cell membranes were prepared from established tumor cell lines and the optimum conditions necessary for modification and clinical application were determined. GPI-B7-1 transferred optimally onto isolated human tumor membranes at physiological temperature (37 degrees C) in a dose dependent manner. Transfer of GPI-B7-1 to isolated membranes resulted in stable expression and costimulatory function. These modified membranes could be stored for repeated immunizations while retaining expression of GPI-B7-1. Critically, isolated tumor membranes, prepared directly from surgically removed human tumor tissue, could be modified by GPI-B7-1 and costimulate T cells. Finally, membranes isolated from tumor tissue expressed MHC class II, unlike the cell line established in vitro from the same patient. This novel approach to express immunostimulatory molecules on isolated membranes derived from a patient's tumor tissue will make the preparation of autologous therapeutic cancer vaccines available to patients from which tumor cell lines can not be established.

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

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

Functional equivalency of B7-1 and B7-2 for costimulating plasmid DNA vaccine-elicited CTL responses

A costimulatory signal in addition to an Ag-specific stimulus is required for optimal activation of T lymphocytes. CD28, the primary positive costimulatory receptor on T cells, has two identified ligands, B7-1 and B7-2. Whether B7-1 and B7-2 have identical, overlapping, or distinct functions remains unresolved. In this study, we show that mice lacking B7-2 were unable to generate CTL responses following immunization with a plasmid DNA vaccine. The ability of these B7-2-deficient mice to generate CTL responses following plasmid gp120 DNA vaccination was fully reconstituted by coadministering either a plasmid expressing B7-2 or B7-1. Moreover, the ability to generate CTL responses following plasmid DNA vaccination in mice lacking both B7-1 and B7-2 could be reconstituted by administering either plasmid B7-1 or plasmid B7-2 with the vaccine construct. These data demonstrate that either B7-1 or B7-2 administered concurrently with a plasmid DNA vaccine can fully costimulate vaccine-elicited CTL responses. Functional differences between B7-1 and B7-2 observed in vivo therefore may not reflect inherent differences in the interactions of CD28 with these ligands.

Either B7 costimulation or IL-2 can elicit generation of primary alloreactive CTL

B7-1 and B7-2 are important costimulatory molecules in the activation of T cell immunity. We have used mice made genetically deficient in either or both B7 molecules to determine the role of B7 molecules in activation of primary alloreactive CTL. The absence of either B7-1 or B7-2 did not alter generation of CTL from unfractionated lymphocytes, but the absence of B7-2 greatly decreased CTL generation from purified CD8+ responder cells. However, if B7-1 was induced on the stimulating cells then CTL generation was restored to wild-type levels. Absence of both B7-1 and B7-2 from MLR using whole splenocytes resulted in a profound reduction in generation of CTL. This could completely be reversed by the addition of IL-2. B7 molecules could directly costimulate CD8+ cells, as purified CD8+ cells developed into mature CTL when stimulated with wild-type APC, but not with B7-deficient APC. Again, IL-2 could drive CTL generation from purified CD8+ cells, even in the absence of B7 molecules. Taken together, these results demonstrate an important role for B7 costimulation in CTL generation.

Autoantibody responses and pathology regulated by B7-1 and B7-2 costimulation in MRL/lpr lupus

The activation of T lymphocytes requires both Ag-mediated signaling through the TCR as well as costimulatory signals transmitted through B7-1 and/or B7-2 with CD28. The interference of B7-mediated costimulatory signals has been proposed as one immunotherapeutic intervention for the prevention autoimmune disease. This study has examined autoantibody responses and autoimmune pathology in a murine model of human systemic lupus erythematosus (SLE), the MRL-lpr/lpr mouse, genetically deficient in B7-1 or B7-2, or in mice treated with B7-1/B7-2 blocking Abs. In contrast to other studies of murine models of SLE, MRL-lpr/lpr mice treated with B7 blocking Abs exhibit strong anti-small nuclear ribonucleoprotein (snRNP) and anti-DNA autoantibody responses with some changes in isotype switching as compared with untreated animals. All MRL-lpr/lpr mice deficient in B7-1 or B7-2 produce anti-snRNP and anti-DNA titers with isotypes virtually identical with wild-type animals. However, the absence of B7-2 costimulation did interfere with the spontaneous activation and the accumulation of memory CD4+ or CD8+ T lymphocytes characteristic of wild-type MRL-lpr/lpr mice. IgG and C3 complement deposition was less pronounced in the kidneys of B7-2 deficient MRL-lpr/lpr mice, reflecting their lessor degree of glomerulonephritis. By comparison, B7-1-deficient MRL-lpr/lpr mice had more severe IgG and C3 deposits in glomeruli.

CD152 ligation by CD80 on T cells is required for the induction of unresponsiveness by costimulation-deficient antigen presentation

Two apparently contradictory observations have been made concerning peripheral T cell tolerance; costimulation-deficient Ag presentation leads to unresponsiveness, and CTLA4 (CD152) ligation is required for unresponsiveness to be induced. This issue was addressed using a CD80- CD86low B cell line to present Ag to DO.11.10 naive CD4+ T cells. Proliferation was substantially enhanced by anti-CD80 or anti-CD152, but was inhibited by anti-CD86. Furthermore, anti-CD80 partially, and anti-CD152 totally protected cloned DO.11.10 T cells from the induction of unresponsiveness following culture with peptide and Chinese hamster ovary H2-Ad+ CD80- CD86- cells. Fab of anti-CD80 caused similar enhancement, and coimmobilized anti-CD80 failed to costimulate the anti-CD3 response of purified T cells, indicating that direct signaling by anti-CD80 was not responsible for these effects. The possibility that anti-CD80 liberated CD28 molecules that were sequestered by the T cell-expressed CD80, enabling them to coaggregate with TCR:CD3 complexes was excluded by finding that anti-CD80 and anti-CD152 individually caused maximal enhancement, rather than having additive effects. These data suggest that T cell-expressed CD80 has a regulatory function and plays a key role in the induction of unresponsiveness due to costimulation-deficient Ag presentation by the ligation of CD152 on neighboring, or even the same, T cell.

Association of B7-1 co-stimulation with the development of graft arterial disease. Studies using mice lacking B7-1, B7-2, or B7-1/B7-2

To investigate the roles of B7-1 and/or B7-2 co-stimulatory molecule in the development of graft arterial disease (GAD), major histocompatibility complex (MHC) class II-mismatched allograft hearts were transplanted into wild-type, B7-1(-/-), B7-2(-/-), or B7-1/B7-2(-/-) recipient mice. Grafts were explanted at 4 or 8 weeks and used for histological and immunohistochemical analyses, RNase protection assay, and flow cytometry of graft infiltrating cells. Grafts in wild-type recipients showed macrophage, recipient MHC class II, and B7 molecule co-localization by immunohistochemistry to GAD lesions. Flow cytometry revealed that CD11b(+) and MHC class II(+) graft infiltrating cells expressed B7-1 more than B7-2, whereas B7-2 expression was predominant in CD11b(-) cells at 4 and 8 weeks. GAD was significantly attenuated in the allografts in B7-1(-/-) and B7-1/B7-2(-/-) but not in B7-2(-/-) recipients compared to wild-type hosts. Interferon-gamma mRNA levels were comparable in all graft combinations, whereas interleukin-4 mRNA levels decreased in grafts in B7-2 deficient hosts, but did not correlate with GAD attenuation. The findings indicate distinct roles for B7-1 and B7-2 co-stimulatory molecules in the development of GAD, potentially because of differential expression of B7-1 and B7-2 molecules on distinct stimulator and/or effector cell populations.

The role of costimulation in autoimmune demyelination

Experimental allergic encephalomyelitis (EAE) is a T cell-mediated, autoimmune disorder characterized by central nervous system (CNS) inflammation and demyelination, features reminiscent of the human disease, multiple sclerosis (MS). In addition to the signal the encephalitogenic T cell receives through the T cell receptor (TCR), a second signal, termed costimulation, is required for complete T cell activation. The B7 family of cell surface molecules expressed on antigen presenting cells (APC) is capable of providing this second signal to T cells via two receptors, CD28 and CTLA-4. Our studies have shown that costimulation provided by B7 molecules to its ligand CD28 is important in the initiation of the autoimmune response in EAE. Further, it appears the costimulation provided by B7-1 is important in disease development, while B7-2 may play an important regulatory role. We and others later showed that B7/CTLA-4 interaction plays a critical role in down-regulating the immune response. Previous work has shown that activated T cells and T cells of a memory phenotype are less dependent on costimulation than naive T cells. T cells reactive with myelin components that are involved in the pathogenesis of EAE and possibly MS would be expected to have been activated as part of the disease process. Building upon our prior work in the EAE model, we have tested the hypothesis that myelin-reactive T cells, which are relevant to the pathogenesis of CNS inflammatory demyelination, can be distinguished from naive myelin-reactive T cells by a lack of dependence upon costimulation for activation and that the costimulatory requirements of these myelin-reactive T cells change during the course of disease. Our studies in the EAE model have also addressed the mechanisms of extrathymic (peripheral) T cell tolerance following intravenous (i.v. ) administration of high dose antigen. It is believed that TCR signaling in the absence of costimulation is a vital component of peripheral tolerance mechanisms. However, recent evidence suggests that peripheral tolerance of antigen-specific T cells induced in vivo may require CTLA-4 engagement of the tolerized T cells. We have begun to examine the molecular mechanisms of tolerance induction following intravenous and intraperitoneal administration of myelin antigens in the EAE model and test the hypothesis that tolerance induction is dependent on the B7:CD28/CTLA-4 pathway. The results from our studies will enhance our understanding of the role that myelin-reactive T cells may play in the pathogenesis of MS. We have determined that MBP-reactive T cells in MS patients are less dependent upon CD28 costimulation than in normal controls, suggesting that these T cells were previously primed in vivo. Characterization of these CD28-independent myelin-specific T cells will have broad implications for a variety of immunologically based therapies in diseases such as MS.