Complexities of CD28/B7: CTLA-4 costimulatory pathways in autoimmunity and transplantation

Mice deficient in perforin, CD4+ T cells, or CD28-mediated signaling maintain the typical immunodominance hierarchies of CD8+ T-cell responses to influenza virus

CD8 T-cell (T(CD8+)) responses elicited by viral infection demonstrate the phenomenon of immunodominance: the numbers of T(CD8+) responding to different viral peptides vary over a wide range in a reproducible manner for individuals with the same major histocompatibility complex class I alleles. To better understand immunodominance, we examined T(CD8+) responses to multiple defined viral peptides following infection of mice with influenza virus. The immunodominance hierarchy of influenza virus-specific T(CD8+) was not greatly perturbed by the absence of either perforin or T-helper cells or by interference with B7 (CD80)-mediated signaling. These findings indicate that costimulation by antigen-presenting cells (APCs) or killing of APCs by T(CD8+) plays only a minor role in establishing the immunodominance hierarchy of antiviral T(CD8+) in this system. This points to intrinsic features of the T(CD8+) repertoire as major contributors to immunodominance.

Cutting edge: the relative distribution of T cells responding to chemically dominant or minor epitopes of lysozyme is not affected by CD40-CD40 ligand and B7-CD28-CTLA-4 costimulatory pathways

We examined the frequencies and specificities of the CD4+ T cell responses to the protein hen egg white lysozyme in mice deficient in the CD40-CD40 ligand or B7-CD28 costimulatory pathways. The frequency of T cells was decreased by between 3- and 4-fold in CD40-/- mice, and 12-fold in B7-1/B7-2-/- mice, but surprisingly, the relative distribution of T cells responding to peptides that were presented at levels that differed by >250-fold was similar. We also examined the CD4 response after blocking the regulatory molecule CTLA-4 during immunization. We observed no difference in either the frequency or specificity of the CD4+ T cell response if CTLA-4 was blocking during priming. Thus, the T cell response was generated toward the constellation of chemically dominant and subdominant epitopes as a whole, and did not discriminate among them based on their relative abundance.

Modulation of TCR-induced transcriptional profiles by ligation of CD28, ICOS, and CTLA-4 receptors

Signals generated by T cell receptor (TCR) and CD28 engagement are required for optimal T cell activation, but how these signals integrate within the cell is still largely unknown. We have used near genome-scale expression profiling to monitor T cell signal transduction pathways triggered via TCR and/or costimulatory receptors. Ligation of CD28 alone induced a set of short-lived early response transcripts in both Jurkat T cells and primary CD4 T cells, thus providing evidence that CD28 engagement can affect gene regulation independently of TCR engagement. Simultaneous signaling through both the TCR and CD28 resulted in altered expression of several thousand genes following several distinct temporal patterns. Most of these gene regulations were induced by TCR signaling alone and were augmented to varying degrees by CD28 costimulation. CD28 and ICOS costimulation had nearly identical effects on gene regulation, but a few transcripts (e.g., IL2, IL9) were significantly more affected by CD28. Therefore, the distinctive functional outcomes of costimulation via CD28 and ICOS are accompanied by relatively few distinct differences in gene expression. Cytotoxic T lymphocyte antigen 4 (CTLA-4) engagement selectively blocked augmentation of gene regulations by CD28-mediated costimulation, but did not ablate gene regulation induced by TCR triggering alone.

Genomic expression programs and the integration of the CD28 costimulatory signal in T cell activation

Optimal activation of T cells requires effective occupancy of both the antigen-specific T cell receptor and a second coreceptor such as CD28. We used cDNA microarrays to characterize the genomic expression program in human peripheral T cells responding to stimulation of these receptors. We found that CD28 agonists alone elicited few, but reproducible, changes in gene expression, whereas CD3 agonists elicited a multifaceted temporally choreographed gene expression program. The principal effect of simultaneous engagement of CD28 was to increase the amplitude of the CD3 transcriptional response. The induced genes whose expression was most enhanced by costimulation were significantly enriched for known targets of nuclear factor of activated T cells (NFAT) transcription factors. This enhancement was nearly abolished by blocking the nuclear translocation of NFATc by using the calcineurin inhibitor FK506. CD28 signaling promoted phosphorylation, and thus inactivation, of the NFAT nuclear export kinase glycogen synthase kinase-3 (GSK3), coincident with enhanced dephosphorylation of NFATc proteins. These results provide a detailed picture of the transcriptional program of T cell activation and suggest that enhancement of transcriptional activation by NFAT, through inhibition of its nuclear export, plays a key role in mediating the CD28 costimulatory signal.

The Th2-response in mercuric chloride-induced autoimmunity requires continuing costimulation via CD28

Mercuric chloride (HgCl2)-induced autoimmunity in Brown Norway (BN) rats is a highly polarized polyclonal Th2-driven autoimmune response with increased IgE production, lymphoproliferation, vasculitis and proteinuria. The increase in serum IgE concentration is clearly measurable by day 4 after the first HgCl2 injection and peaks between days 15 and 20. Treatment with CD80 and CD86 antibodies prior to administration of HgCl2 completely suppresses the autoimmune process. To determine whether interruption of CD28 signalling after initial stimulation of the Th2-response would be suppressive, antibody treatment was delayed. BN rats were given 5 doses of HgCl2 subcutaneously on alternate days. CD80 and CD86 antibodies, or an isotype control, were given daily for 3 days and then on alternate days until day 12 commencing either on the day of the first HgCl2 injection (day 0) or on days 4 or 8. Treatment from day 0 reduced serum IgE concentrations to below baseline (median 9.34 microg/ml on day 0 versus 4.6 microg/ml, on day 5, P = 0.03) suggesting that ongoing costimulation via CD28 is required to maintain basal serum IgE production. Delaying treatment until day 4 or day 8 after the first HgCl2 injection resulted in significant inhibition of IgE secretion, lymphoproliferation, and vasculitis, although less markedly than when treatment was commenced on day 0. These data indicate that CD28-mediated costimulation is not only required for the initiation of the Th2-response but is required for maintenance of a maximal response, making this an attractive therapeutic target for antibody-mediated autoimmune diseases.

CD28-dependent activation of protein kinase B/Akt blocks Fas-mediated apoptosis by preventing death-inducing signaling complex assembly

The T cell costimulatory molecule CD28 is important for T cell survival, yet both the signaling pathways downstream of CD28 and the apoptotic pathways they antagonize remain poorly understood. Here we demonstrate that CD4(+) T cells from CD28-deficient mice show increased susceptibility to Fas-mediated apoptosis via a phosphatidylinositol 3-kinase (PI3K)-dependent pathway. Protein kinase B (PKBalpha/Akt1) is an important serine/threonine kinase that promotes survival downstream of PI3K signals. To understand how PI3K-mediated signals downstream of CD28 contribute to T cell survival, we examined Fas-mediated apoptosis in T cells expressing an active form of PKBalpha. Our data demonstrate that T cells expressing active PKB are resistant to Fas-mediated apoptosis in vivo and in vitro. PKB transgenic T cells show reduced activation of caspase-8, BID, and caspase-3 due to impaired recruitment of procaspase-8 to the death-inducing signaling complex (DISC). Similar alterations are seen in T cells from mice which are haploinsufficient for PTEN, a lipid phosphatase that regulates phosphatidylinositol-3,4,5-trisphosphate (PIP(3)) and influences PKBalpha activity. These findings provide a novel link between CD28 and an important apoptosis pathway in vivo, and demonstrate that PI3K/PKB signaling prevents apoptosis by inhibiting DISC assembly.

Myeloid dendritic cells in non-obese diabetic mice have elevated costimulatory and T helper-1-inducing abilities

Type 1 diabetes (T1D) in the non-obese diabetic (NOD) mouse begins with activation of islet-reactive T helper-1 (Th1) cells by dendritic cells (DCs). Since multiple genetic loci contribute to T1D, we evaluated the hypothesis that NOD DCs possess inherent characteristics that contribute to the autoimmune phenotype. When compared to a representative Th1 (C57BL/6) and Th2 (BALB/C) control strain, in vitro generated NOD myeloid DCs matured normally. Functionally, NOD DCs exhibited higher expression of CD80/86 and IL-12 production during stimulation of nai;ve T cells, even in comparison to C57BL/6 DCs, the prototype strain for vigorous, Th1-biased immunity. These features of NOD DCs translated into aberrantly elevated IFN-gamma synthesis, enhanced T-cell proliferation, and heightened CD69 expression. Further, NOR DCs, from an NOD-related, autoimmune-resistant strain, did not display this hyper-responsiveness, suggesting that these abnormalities are genetic features of NOD DCs that are related to disease pathogenesis. Cumulatively, these results indicate that NOD DCs are inherently biased towards abnormally high costimulation and Th1-induction, two features that would be expected to confer activation and persistence of autoreactive T cells.

It's all Rel-ative: NF-kappaB and CD28 costimulation of T-cell activation

Costimulatory signals complement or modify the signals provided to a lymphocyte through antigen receptors. For productive T-cell activation, the CD28 molecule is apparently the most important, although not the only, costimulatory receptor. CD28 can provide a signal that is at least partially distinct from that delivered by the T cell receptor (TCR)-CD3 complex. Several lines of evidence indicate that the nuclear factor (NF)-kappaB pathway is perhaps the most relevant biochemical or transcriptional target for the costimulatory activity of CD28. Although many questions remain, recent years have witnessed significant progress in understanding the signal transduction pathways leading from the TCR and CD28 to Rel/NF-kappaB-dependent transcription.

Gene array profiling and immunomodulation studies define a cell-mediated immune response underlying the pathogenesis of alopecia areata in a mouse model and humans

Alopecia areata is a suspected autoimmune hair loss disease. In a rodent model, alopecia areata can be induced in normal haired C3H/HeJ mice by transfer of skin grafts from mice with spontaneous alopecia areata. At weeks 2, 4, 6, and 10 after surgery, grafted mice were euthanized, skin collected and processed for histology, and RNA extracted. Age-matched sham-grafted mice, and mice with and without spontaneous alopecia areata, were similarly processed. For comparison, skin biopsies from alopecia areata and androgenetic alopecia affected humans were also collected. Skin mRNA processed to cDNA was analyzed using Affymetrix mouse 11K and human 6800 gene chip(R) array technology. Microarray results indicated 42 known genes upregulated or downregulated during onset of mouse alopecia areata consistent with an inflammatory cell-mediated disease pathogenesis involving antigen presentation, costimulation, and a T helper 1 lymphocyte response. In contrast, 114 genes, many regulating immunoglobulin response, were altered late in disease development. In alopecia areata affected humans, 95 genes were significantly modulated. As confirmation of microarray analysis results, lymph node and spleen cells from alopecia areata affected mice injected into normal haired littermates transferred the alopecia areata phenotype. Alopecia areata onset could be inhibited in skin-grafted mice by modulation with B7.1- and B7.2-specific monoclonal antibodies. In addition, depletion of CD4+ CD8+ expressing cells in chronic alopecia areata affected mice using monoclonal antibodies permitted hair regrowth. The results consistently demonstrated the importance of an immune cell-mediated disease mechanism in alopecia areata pathogenesis and suggested targeting antigen-presenting cells and reactive lymphocytes may be effective in alopecia areata treatment.

The interaction properties of costimulatory molecules revisited

B7-1 and B7-2 are generally thought to have comparable structures and affinities for their receptors, CD28 and CTLA-4, each of which is assumed to be bivalent. We show instead (1) that B7-2 binds the two receptors more weakly than B7-1, (2) that, relative to its CTLA-4 binding affinity, B7-2 binds CD28 2- to 3-fold more effectively than B7-1, (3) that, unlike B7-1, B7-2 does not self-associate, and (4) that, in contrast to CTLA-4 homodimers, which are bivalent, CD28 homodimers are monovalent. Our results indicate that B7-1 markedly favors CTLA-4 over CD28 engagement, whereas B7-2 exhibits much less bias. We propose that the distinct structures and binding properties of B7-1 and B7-2 account for their overlapping but distinct effects on T cell responses.

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

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

Antisense deoxyoligonucleotides or antibodies to murine MD-1 inhibit rejection of allogeneic and xenogeneic skin grafts in C3H mice

BACKGROUND

Altered expression of murine MD-1, a molecule controlling expression of members of the interleukin (IL)-1 receptor family of signaling proteins, regulates antigen-presenting cell-induced alloreactions. We investigated the effect of treatment with antisense deoxyoligonucleotides or antibodies to MD-1 in vivo on allogeneic and xenogeneic skin graft survival and the immune responses in transplanted mice.

METHODS

C3H mice received C57BL/6 or Lewis rat skin grafts, followed by i.v. injections of anti-MD-1 antibody or antisense oligonucleotides or control reagents at 48-hr intervals. Survival was monitored. In separate studies, mice were sacrificed at 5-day intervals. Serum was analyzed for circulating MD-1 antigen, and peritoneal cells for surface expression of MD-1. The proliferative and cytolytic response of lymphocytes harvested from treated animals and restimulated in vitro with allo- or xenogeneic cells, and the cytokines produced, was measured. Graft histology was assessed at 11 days after transplantation.

RESULTS

Treatment with anti-MD-1 oligonucleotides or antibodies suppressed rejection of both xeno- and allogeneic grafts, decreased induction of graft-specific cytotoxic T cells, increased production of type-2 cytokines (IL-4 and IL-10), and decreased production of type-1 cytokines (IL-2 and interferon-gamma). Serum levels of MD-1 were suppressed, as was expression of MD-1 on the surface of antigen-presenting cells. Grafts from MD-1-treated mice showed little lymphocyte infiltration, and no signs of graft necrosis.

CONCLUSION

Our data suggest a critical in vivo role for MD-1 expression in regulating graft rejection, as well as in the concomitant sensitization of T cells and their cytokine production profile, which parallels the rejection response.

The B7 family of ligands and its receptors: new pathways for costimulation and inhibition of immune responses

T cell activation is dependent upon signals delivered through the antigen-specific T cell receptor and accessory receptors on the T cell. A primary costimulatory signal is delivered through the CD28 receptor after engagement of its ligands, B7-1 (CD80) or B7-2 (CD86). Engagement of CTLA-4 (CD152) by the same B7-1 or B7-2 ligands results in attenuation of T cells responses. Recently, molecular homologs of CD28 and CTLA-4 receptors and their B7-like ligands have been identified. ICOS is a CD28-like costimulatory receptor with a unique B7-like ligand. PD-1 is an inhibitory receptor, with two B7-like ligands. Additional members of B7 and CD28 gene families have been proposed. Integration of signals through this family of costimulatory and inhibitory receptors and their ligands is critical for activation of immune responses and tolerance. Understanding these pathways will allow development of new strategies for therapeutic intervention in immune-mediated diseases.

Expression, refolding, purification, molecular characterization, crystallization, and preliminary X-ray analysis of the receptor binding domain of human B7-2

The cell-mediated immune response involves a series of specific molecular interactions between cell surface molecules on T cells and antigen-presenting cells. Of particular importance for the regulation of T cell activity is the interaction of the B7 isoforms, B7-1 and B7-2, with the T cell surface costimulatory receptors, CD28 and CTLA-4. The binding of CD28 by B7-1/B7-2 results in an enhancement of T cell responses initiated by the interaction between a clonotypic T cell receptor and its specific, antigenic MHC-peptide complex, whereas the subsequent engagement of CTLA-4 by B7-1/B7-2 leads to a down-regulation of the response. Here we report the expression, refolding, purification, characterization, and crystallization of the receptor-binding domain of human B7-2. The receptor-binding domain of human B7-2 was overexpressed in Escherichia coli as inclusion bodies, solubilized in 6 M guanidine-hydrochloride, and then refolded in vitro by rapid dilution into a renaturing buffer. Refolded B7-2 was subsequently purified to homogeneity by anion-exchange chromatography. Gel-filtration chromatography and native PAGE analysis showed that the receptor-binding domain of B7-2 is exclusively monomeric in solution. Purified B7-2 binds tightly to bacterially expressed monomeric and disulfide-linked homodimeric human CTLA-4 as shown by gel-filtration chromatography and native PAGE. This suggests that glycosylation is not important for the proper folding of the receptor-binding domain of B7-2 nor for its binding to CTLA-4. In addition, these results suggest that refolded B7-2 is biologically active and may be a useful therapeutic and experimental reagent for regulating T cell activity. Refolded and purified B7-2 was crystallized by the hanging-drop vapor diffusion method, allowing for the initiation of an X-ray crystallographic study.

Effects of CTLA4-Fc on glomerular injury in humorally-mediated glomerulonephritis in BALB/c mice

The effect of cytotoxic T-lymphocyte-associated molecule 4-immunoglobulin fusion protein (CTLA4-Fc) on humorally-mediated glomerulonephritis was studied in accelerated anti-glomerular basement membrane (anti-GBM) glomerulonephritis induced in BALB/c mice. This strain of mice develops antibody and complement dependent glomerulonephritis under this protocol. Sensitized BALB/c mice developed high levels of circulating autologous antibody titres, intense glomerular deposition of mouse immunoglobulin and complement, significant proteinuria, renal impairment, significant glomerular necrosis and a minor component of crescent formation 10 days after challenge with a nephritogenic antigen (sheep anti-GBM globulin). Early treatment during the primary immune response, or continuous treatment throughout the disease with CTLA4-Fc, significantly suppressed mouse anti-sheep globulin antibody titres in serum, and immunoglobulin and complement deposition in glomeruli. The degree of glomerular necrosis was improved and proteinuria was reduced, particularly in the earlier stages of disease. Late treatment by CTLA4-Fc starting one day after challenge with sheep anti-mouse GBM did not affect antibody production and did not attenuate glomerulonephritis. The low level of crescent formation found in BALB/c mice developing glomerulonephritis was not prevented by the administration of CTLA4-Fc. These results demonstrate that CTLA4-Fc is of benefit in this model of glomerulonephritis by its capacity to attenuate antibody production, without affecting the minor degree of cell-mediated glomerular injury.

Negative co-receptors on lymphocytes

The past year has seen significant advances in our understanding of critical roles of negative immunoregulatory signals delivered through the B7-CD28 superfamily in regulating T cell activation and tolerance. Structural data on CTLA-4 have provided novel insights into the inhibitory functions of CTLA-4. Initial characterization of the PD-1-PD-1-ligand pathway has revealed that this pathway can downregulate TCR- and CD28-mediated signals. Recent studies indicate that ICOS exerts distinct effects at different phases of an immune response: ICOS can inhibit as well as stimulate T cell responses.

T-cell activation through the antigen receptor. Part 2: role of signaling cascades in T-cell differentiation, anergy, immune senescence, and development of immunotherapy

Part 2 of this review on cellular activation by the T-cell antigen receptor (TCR) will highlight how TCR signaling pathways are adapted to achieve specific biologic outcomes, including different states of T-cell differentiation and the induction of T-cell tolerance. We will also explore how treatment with altered peptide ligands affects TCR signaling to change T-cell differentiation or to induce an anergy state. These changes are accomplished through alteration of protein tyrosine kinase activity, the stoichiometry of phosphorylation of immunoreceptor tyrosine-based activation motifs, intracellular free ionized calcium flux, mitogen-activated protein kinase activity, and transcriptional activation of key cytokine promoters. The CTLA-4 plays an important role in the induction and maintenance of anergy. The second theme will highlight how altered TCR signal transduction, including changes in the compartmentalization of signaling components at the TCR synapse, contributes to decreased T-cell activation during immune senescence. Finally, we will illustrate how the molecular details of TCR activation can be used to modify the function of the immune system. This includes a description of the mechanism of action of altered peptide ligands, CTLA-4Ig, and pharmacologic inhibitors of mitogen-activated protein kinases, nuclear factor kappaB, and protein kinase C cascades.

Transient CD44 variant isoform expression and reduction in CD4(+)/CD25(+) regulatory T cells in C3H/HeJ mice with alopecia areata

Alopecia areata, an autoimmune disease affecting anagen stage hair follicles, can be induced by grafting spontaneous alopecia areata affected skin to normal-haired C3H/HeJ mice. As the onset of alopecia areata can be significantly retarded by anti-CD44 variant isoform 10 treatment, it was interesting to explore the underlying disease mechanism. Two weeks after transplanting alopecia areata affected skin, expression of CD44 variant isoforms 3, 6, 7, and 10 was strikingly upregulated as compared with sham-grafted mice. By 6 wk after grafting, CD44 variant isoform levels had returned to normal, whereas in draining lymph nodes, CD44 variant isoform expression was slightly decreased. Leukocytes in the skin of mice with chronic alopecia areata expressed a hematopoietic isoform of CD44 and CD44 variant isoform 6 at an elevated level, but CD44 variant isoform 3 expression was reduced. Cytokine expression in leukocytes of chronic alopecia areata affected skin was higher than in normal-haired controls. Cytokine expression also increased postsurgery in sham and alopecia areata grafted mice, but remained elevated only in mice receiving alopecia areata affected skin. Finally, from the skin of mice with chronic alopecia areata and of mice transplanted with alopecia areata affected skin, an increased number of CD4(+) and CD8(+) cells, but a strongly decreased number of CD4(+)/CD25(+) regulatory T cells was recovered. Thus, expression of CD44 variant isoforms is important for the migration of leukocytes during the initial period of alopecia areata. CD44, however, is apparently not involved in the maintenance of the disease state, which is characterized by high cytokine expression levels, an increased number of CD4(+) and CD8+ cells, but a low level of CD4(+)/CD25(+) suppressor cells.

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

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

Resident skin-specific gammadelta T cells provide local, nonredundant regulation of cutaneous inflammation

The function of the intraepithelial lymphocyte (IEL) network of T cell receptor (TCR) gammadelta(+) (Vgamma5(+)) dendritic epidermal T cells (DETC) was evaluated by examining several mouse strains genetically deficient in gammadelta T cells (delta(-/-) mice), and in delta(-/-) mice reconstituted with DETC or with different gammadelta cell subpopulations. NOD.delta(-/-) and FVB.delta(-/-) mice spontaneously developed localized, chronic dermatitis, whereas interestingly, the commonly used C57BL/6.delta(-/-) strain did not. Genetic analyses indicated a single autosomal recessive gene controlled the dermatitis susceptibility of NOD.delta(-/-) mice. Furthermore, allergic and irritant contact dermatitis reactions were exaggerated in FVB.delta(-/-), but not in C57BL/6.delta(-/-) mice. Neither spontaneous nor augmented irritant dermatitis was observed in FVB.beta(-/-) delta(-/-) mice lacking all T cells, indicating that alphabeta T cell-mediated inflammation is the target for gammadelta-mediated down-regulation. Reconstitution studies demonstrated that both spontaneous and augmented irritant dermatitis in FVB.delta(-/-) mice were down-regulated by Vgamma5(+) DETC, but not by epidermal T cells expressing other gammadelta TCRs. This study demonstrates that functional impairment at an epithelial interface can be specifically attributed to absence of the local TCR-gammadelta(+) IEL subset and suggests that systemic inflammatory reactions may more generally be subject to substantial regulation by local IELs.

Transcription: tantalizing times for T cells

The T helper lymphocyte is responsible for orchestrating an appropriate immune response to pathogens. To do so, it has evolved into two specialized subsets that direct type 1 and type 2 immunity. Here, we discuss the genetic programs that control lineage commitment of progenitor T helper cells along each of these pathways.

Critical role for activation of antigen-presenting cells in priming of cytotoxic T cell responses after vaccination with virus-like particles

Virus-like particles (VLPs) are known to induce strong Ab responses in the absence of adjuvants. In addition, VLPs are able to prime CTL responses in vivo. To study the efficiency of this latter process, we fused peptide p33 derived from lymphocytic choriomeningitis virus to the hepatitis B core Ag, which spontaneously assembles into VLPs (p33-VLPs). These p33-VLPs were efficiently processed in vitro and in vivo for MHC class I presentation. Nevertheless, p33-VLPs induced weak CTL responses that failed to mediate effective protection from viral challenge. However, if APCs were activated concomitantly in vivo using either anti-CD40 Abs or CpG oligonucleotides, the CTL responses induced were fully protective against infection with lymphocytic choriomeningitis virus or recombinant vaccinia virus. Moreover, these CTL responses were comparable to responses generally induced by live vaccines, because they could be measured in primary ex vivo (51)Cr release assays. Thus, while VLPs alone are inefficient at inducing CTL responses, they become very powerful vaccines if applied together with substances that activate APCs.

A cure for murine sickle cell disease through stable mixed chimerism and tolerance induction after nonmyeloablative conditioning and major histocompatibility complex-mismatched bone marrow transplantation

The morbidity and mortality associated with sickle cell disease (SCD) is caused by hemolytic anemia, vaso-occlusion, and progressive multiorgan damage. Bone marrow transplantation (BMT) is currently the only curative therapy; however, toxic myeloablative preconditioning and barriers to allotransplantation limit this therapy to children with major SCD complications and HLA-matched donors. In trials of myeloablative BMT designed to yield total marrow replacement with donor stem cells, a subset of patients developed mixed chimerism. Importantly, these patients showed resolution of SCD complications. This implies that less toxic preparative regimens, purposefully yielding mixed chimerism after transplantation, may be sufficient to cure SCD without the risks of myeloablation. To rigorously test this hypothesis, we used a murine model for SCD to investigate whether nonmyeloablative preconditioning coupled with tolerance induction could intentionally create mixed chimerism and a clinical cure. We applied a well-tolerated, nonirradiation-based, allogeneic transplantation protocol using nonmyeloablative preconditioning (low-dose busulfan) and costimulation blockade (CTLA4-Ig and anti-CD40L) to produce mixed chimerism and transplantation tolerance to fully major histocompatibility complex-mismatched donor marrow. Chimeric mice were phenotypically cured of SCD and had normal RBC morphology and hematologic indices (hemoglobin, hematocrit, reticulocyte, and white blood cell counts) without evidence of graft versus host disease. Importantly, they also showed normalization of characteristic spleen and kidney pathology. These experiments demonstrate the ability to produce a phenotypic cure for murine SCD using a nonmyeloablative protocol with fully histocompatibility complex-mismatched donors. They suggest a future treatment strategy for human SCD patients that reduces the toxicity of conventional BMT and expands the use of allotransplantation to non-HLA-matched donors.

Immunoinhibitory DNA vaccine protects against autoimmune diabetes through cDNA encoding a selective CTLA-4 (CD152) ligand

Cytotoxic T lymphocyte antigen 4 (CTLA-4 or CD152) is a strong negative regulator of T cell activity. Like CD28 (a positive regulator) it binds to B7-1 and B7-2, and there is no known natural selective ligand. Monoclonal antibodies to CTLA-4 generally have a masking effect, enhancing rather than suppressing responses. However, a single amino acid substitution in B7-1 (W88 > A; denoted B7-1wa) abrogates binding to CD28 but not to CTLA-4. We constructed plasmids encoding B7-1 or B7-1wa, as cell-surface or Ig fusion proteins. In a bound state, B7-1-Ig enhanced CD3-mediated T cell activation, but B7-1wa-Ig was inhibitory, as expected of a CTLA-4 ligand. To alter immunity in vivo, we inoculated mice intramuscularly (i.m.) with a carcinoembryonic antigen (CEA) plasmid. Gene transfer was amplified by electroporation. Co-injection of a B7-1wa (membrane-bound form) plasmid blocked induction of anti-CEA immunity, whereas a B7-1 plasmid was stimulatory. We studied this DNA covaccination method in nonobese diabetic (NOD) mice with autoimmune diabetes. Delivery of either preproinsulin I (PPIns) or B7-1wa cDNA alone did not suppress the autoimmune anti-insulin response of spleen cells. However, co-delivery of B7-1wa and PPIns cDNA abrogated reactivity to insulin and ameliorated disease. Interferon-gamma and interleukin-4 were both depressed, arguing against a Th2 bias. Reactivity to glutamic acid decarboxylase 65, another major islet autoantigen, was not altered and suppressor cells were not identified, suggesting induction of tolerance to insulin by either T cell anergy or deletion. Selective engagement of CTLA-4 through gene transfer represents a novel and powerful way to block autoimmunity specifically.

CTLA-4 and autoimmune thyroid disease: lack of influence of the A49G signal peptide polymorphism on functional recombinant human CTLA-4

A single nucleotide A49G polymorphism (SNP) of CTLA-4 has been linked and associated with the autoimmune thyroid diseases (AITD) and thyroid autoantibody secretion. We have explored the functional mechanisms of CTLA-4 by means of recombinant human CTLA-4 expressed on transfected Jurkat T cells. Analysis of CTLA-4 transcripts with quantitative real-time PCR demonstrated similar baseline and PHA-stimulated levels for both the A49 and G49 alleles, which were markedly enhanced by anti-CTLA-4 engagement. Both alleles also coded for proteins which were expressed on the cell membrane, as measured by FACS analysis using anti-CTLA-4 (G: 34.4+/-11.9% cells, A: 27.6+/-8.6% cells) (p=ns). Baseline and PHA-stimulated IL-2 production were also similar among control and CTLA-4 clones expressing both alleles. After anti-CTLA-4 engagement, IL-2 production was markedly inhibited in a dose- and time-dependent manner but this also appeared to be similar in the A and G allele expressing cells (95.7+/-1.2% inhibition and 94.9+/-1.1% inhibition, respectively). In conclusion, both the extrinsic and intrinsic actions of human CTLA-4 were not affected by the signal peptide A49G polymorphism. Therefore, the linkage of the CTLA-4 A49G SNP to AITD is most likely secondary to linkage disequilibrium.

The B7-CD28 superfamily

The B7-1/B7-2-CD28/CTLA-4 pathway is crucial in regulating T-cell activation and tolerance. New B7 and CD28 molecules have recently been discovered and new pathways have been delineated that seem to be important for regulating the responses of previously activated T cells. Several B7 homologues are expressed on cells other than professional antigen-presenting cells, indicating new mechanisms for regulating T-cell responses in peripheral tissues. Some B7 homologues have unknown receptors, indicating that other immunoregulatory pathways remain to be described. Here, we summarize our current understanding of the new members of the B7 and CD28 families, and discuss their therapeutic potential.

CD8+CD28- T cells: certainties and uncertainties of a prevalent human T-cell subset

Human peripheral blood CD8+ T cells comprise cells that are in different states of differentiation and under the control of complex homeostatic processes. In a number of situations ranging from chronic inflammatory conditions and infectious diseases to ageing, immunodeficiency, iron overload and heavy alcohol intake, major phenotypic changes, usually associated with an increase in CD8+ T cells lacking CD28 expression, take place. CD8+CD28- T cells are characterized by a low proliferative capacity to conventional stimulation in vitro and by morphological and functional features of activated/memory T cells. Although the nature of the signals that give origin to this T-cell subset is uncertain, growing evidence argues for the existence of an interplay between epithelial cells, molecules with the MHC-class I fold and CD8+ T cells. The possibility that the generation of CD8+CD28- T cells is the combination of TCR/CD3zeta- and regulatory factor-mediated signals as a result of the sensing of modifications of the internal environment is discussed.

CD28 function: a balance of costimulatory and regulatory signals

Both the recognition of MHC/antigen complex by the T-cell receptor and engagement of costimulatory molecules are necessary for efficient T-cell activation. CD28 has been widely recognized as the major costimulation pathway for naive T-cell activation, and the CD28/B7 pathway plays a central role in immune responses against pathogens, autoimmune diseases, and graft rejection. In this review, we will summarize evidence that CD28 is also prominent in the regulation of immune responses and the maintenance of peripheral tolerance. Indeed, CD28 engagement increases the expression of the down-modulatory molecule CTLA-4, induces the differentiation of Th2 cells that have a protective function in autoimmunity, and has an obligatory role in the homeostasis of regulatory T cells. Therefore, CD28/B7 interactions induce a balance of costimulatory and regulatory signals that have opposite outcomes on immune responses. This new perspective on CD28 function suggests that caution should be taken in the development of immunotherapies targeting costimulatory pathways.

T-cell education in autoimmune diabetes: teachers and students

Type 1 diabetes mellitus is a classical example of a T-cell-mediated autoimmune disease. Several aberrations in immune regulation have been described in both human diabetes patients and animal models of type 1 diabetes. In this review, we summarize how proposed immune defects might be implicated in the loss of T-cell tolerance towards self in autoimmune diabetes in humans, nonobese diabetic (NOD) mice and Biobreeding (BB) rats. For this purpose, we will discuss the tolerance-inducing mechanisms that an autoreactive T cell should encounter from its genesis to its pathogenic role in the pancreas, in order of appearance. These comprise central tolerance mechanisms (i.e. positive and negative selection in the thymus) and those mechanisms operative in the periphery (i.e. activation-induced cell death and regulatory T cells).

The regulation of T cell homeostasis and autoimmunity by T cell-derived LIGHT

Costimulatory molecules on antigen-presenting cells (APCs) play an important role in T cell activation and expansion. However, little is known about the surface molecules involved in direct T-T cell interaction required for their activation and expansion. LIGHT, a newly discovered TNF superfamily member (TNFSF14), is expressed on activated T cells and immature dendritic cells. Here we demonstrate that blockade of LIGHT activity can reduce anti-CD3-mediated proliferation of purified T cells, suggesting that T cell-T cell interaction is essential for this proliferation. To test the in vivo activity of T cell-derived LIGHT in immune homeostasis and function, transgenic (Tg) mice expressing LIGHT in the T cell lineage were generated. LIGHT Tg mice have a significantly enlarged T cell compartment and a hyperactivated peripheral T cell population. LIGHT Tg mice spontaneously develop severe autoimmune disease manifested by splenomegaly, lymphadenopathy, glomerulonephritis, elevated autoantibodies, and severe infiltration of various peripheral tissues. Furthermore, the blockade of LIGHT activity ameliorates the severity of T cell-mediated diseases. Collectively, these findings establish a crucial role for this T cell-derived costimulatory ligand in T cell activation and expansion; moreover, the dysregulation of T cell-derived LIGHT leads to altered T cell homeostasis and autoimmune disease.

Without CD4 help, CD8 rejection of pig xenografts requires CD28 costimulation but not perforin killing

Although CD4 cells are major mediators in cellular rejection of fetal pig pancreas (FPP) in the mouse, rejection still occurs in the absence of CD4 cells, albeit with delayed kinetics. CD4 cell-independent mechanisms of cellular rejection are poorly understood. To investigate the involvement of CD8 T cells in FPP rejection and their activation requirements, we used mice transgenic for anti-CD4 Ab; this is the most complete model of CD4 cell deficiency. We showed that in such mice FPP was infiltrated with CD8 cells starting from 2 wk posttransplantation and FPP was eventually rejected 8 wk posttransplantation. Ab depletion of CD8 cells greatly improved the survival of FPP and reduced cell infiltration at the graft site. This suggests that CD8 cells can mediate the rejection of porcine xenografts in the absence of CD4 cells. This CD8-mediated rejection of FPP is independent of their perforin-mediated lytic function, as graft survival was not affected in mice deficient in perforin. The production of IFN-gamma and IL-5 by the graft infiltrates indicates that CD8 cells may act through cytokine-mediated mechanisms. Remarkably, in the absence of CD4 cells, lymphocyte infiltration at the graft site was absent in mice transgenic for CTLA4Ig such that the islet grafts flourished beyond 24 wk. In contrast, rejection was little affected by CD40 ligand deficiency. Therefore, we show that CD8 cells are activated to mediate FPP rejection independent of perforin and that this CD4-independent activation of CD8 cells critically depends on B7/CD28 costimulation.

Opposing effects of anti-activation-inducible lymphocyte-immunomodulatory molecule/inducible costimulator antibody on the development of acute versus chronic graft-versus-host disease

The functional role of inducible costimulator (ICOS)-mediated costimulation was examined in an in vivo model of alloantigen-driven Th1 or Th2 cytokine responses, the parent-into-F(1) model of acute or chronic graft-vs-host disease (GVHD), respectively. When the Ab specific for mouse ICOS was injected into chronic GVHD-induced mice, activation of B cells, production of autoantibody, and development of glomerulonephritis were strongly suppressed. In contrast, the same treatment enhanced donor T cell chimerism and host B cell depletion in acute GVHD induced host mice. Blocking of B7-CD28 interaction by injection of anti-B7-1 and anti-B7-2 Abs inhibited both acute and chronic GVHD. These observations clearly indicate that the costimulatory signal mediated by CD28 caused the initial allorecognition resulting in the clonal expansion of alloreactive T cells, whereas the costimulatory signal mediated by ICOS played a critical role in the functional differentiation and manifestation of alloreactive T cells. Furthermore, treatment with anti-ICOS Ab selectively suppresses Th2-dominant autoimmune disease.

Homeostatic expansion occurs independently of costimulatory signals

Naive T cells undergo homeostatic proliferation in lymphopenic mice, a process that involves TCR recognition of specific self peptide/MHC complexes. Since costimulation signals regulate the T cell response to foreign Ags, we asked whether they also regulate homeostatic expansion. We report in this study that homeostatic expansion of CD4 and CD8 T cells occurs independently of costimulation signals mediated through CD28/B7, CD40L/CD40, or 4-1BB/4-1BBL interactions. Using DO11.10 TCR transgenic T cells, we confirmed that CD28 expression was dispensable for homeostatic expansion, and showed that the presence of endogenous CD4(+)CD25(+) regulatory cells did not detectably influence homeostatic expansion. The implications of these findings with respect to regulation of T cell homeostasis and autoimmunity are discussed.

Induction of cytotoxic T lymphocyte antigen 4 (CTLA-4) restricts clonal expansion of helper T cells

Cytotoxic T lymphocyte antigen (CTLA)-4 plays an essential role in immunologic homeostasis. How this negative regulator of T cell activation executes its functions has remained controversial. We now provide evidence that CTLA-4 mediates a cell-intrinsic counterbalance to restrict the clonal expansion of proliferating CD4(+) T cells. The regulation of CTLA-4 expression and function ensures that, after approximately 3 cell divisions of expansion, most progeny will succumb to either proliferative arrest or death over the ensuing three cell divisions. The quantitative precision of the counterbalance hinges on the graded, time-independent induction of CTLA-4 expression during the first three cell divisions. In contrast to the limits imposed on unpolarized cells, T helper type 1 (Th1) and Th2 effector progeny may be rescued from proliferative arrest by interleukin (IL)-12 and IL-4 signaling, respectively, allowing appropriately stimulated progeny to proceed to the stage of tissue homing. These results suggest that the cell-autonomous regulation of CTLA-4 induction may be a central checkpoint of clonal expansion of CD4(+) T cells, allowing temporally and spatially restricted growth of progeny to be dictated by the nature of the threat posed to the host.

Development of spontaneous autoimmune peripheral polyneuropathy in B7-2-deficient NOD mice

An increasing number of studies have documented the central role of T cell costimulation in autoimmunity. Here we show that the autoimmune diabetes-prone nonobese diabetic (NOD) mouse strain, deficient in B7-2 costimulation, is protected from diabetes but develops a spontaneous autoimmune peripheral polyneuropathy. All the female and one third of the male mice exhibited limb paralysis with histologic and electrophysiologic evidence of severe demyelination in the peripheral nerves beginning at 20 wk of age. No central nervous system lesions were apparent. The peripheral nerve tissue was infiltrated with dendritic cells, CD4(+), and CD8(+) T cells. Finally, CD4(+) T cells isolated from affected animals induced the disease in NOD.SCID mice. Thus, the B7-2-deficient NOD mouse constitutes the first model of a spontaneous autoimmune disease of the peripheral nervous system, which has many similarities to the human disease, chronic inflammatory demyelinating polyneuropathy (CIDP). This model demonstrates that NOD mice have "cryptic" autoimmune defects that can polarize toward the nervous tissue after the selective disruption of CD28/B7-2 costimulatory pathway.

Costimulation of memory T-cells by ICOS: a potential therapeutic target for autoimmunity?

Approaches that target costimulatory receptors are independent of T-cell receptor specificity and may be useful for T-cell-mediated diseases in which the antigens involved are not well defined. However, the proper costimulatory pathways need to be targeted. For example, therapies for human T-cell-mediated diseases need to be effective against previously activated memory cells. In this review, we use autoimmune demyelination as a paradigm for established immune-mediated pathogenesis. Studies with the human disease multiple sclerosis and the rodent model experimental autoimmune encephalomyelitis have suggested that the effectiveness of CD28 blockade, as a therapeutic strategy for established autoimmune demyelination, may be limited. ICOS, a receptor that appears to be involved in the costimulation of previously activated T-cells, may be an attractive alternative.

Tolerogenic strategies to halt or prevent type 1 diabetes

A variety of therapeutic strategies have been developed to tolerize autoreactive T cells and prevent autoimmune pathology. In terms of type 1 diabetes, prevention strategies can inhibit the priming and expansion of autoreactive T cells; however, a cure for diabetes would require tolerance to be established in the presence of primed effector cells together with replacement of the destroyed beta cell mass. Replacement of beta cells could be accomplished by transplantation of islets or stem cells or through islet regeneration. We will focus here on tolerogenic strategies that have been used to prevent onset of type 1 diabetes and discuss the potential for a cure.

Therapeutic potential of 4-1BB (CD137) as a regulator for effector CD8(+) T cells

A fundamental problem of antitumor immunity is tumor-induced immunosuppression. Tumor cells often down-regulate expression of co-stimulatory molecules, tumor antigens, and major histocompatibility complex (MHC) molecules on tumor cells, secrete immunosuppressive substance such as transforming growth factor-beta (TGF-beta) or interleukin-4 (IL-4), and induce apoptosis of effector T cells to escape surveillance. A major goal of antitumor or antivirus immunotherapy is to generate long-lived protective T cells that enable killing of target cells. In this review, we discuss the importance of 4-1BB for development or survival of functionally active effector CD8(+) T cells against tumors, virus infection, and allogeneic immune responses and for potential therapeutic application.

Reversal of spontaneous autoimmune insulitis in nonobese diabetic mice by soluble lymphotoxin receptor

One striking feature of spontaneous autoimmune diabetes is the prototypic formation of lymphoid follicular structures within the pancreas. Lymphotoxin (LT) has been shown to play an important role in the formation of lymphoid follicles in the spleen. To explore the potential role of LT-mediated microenvironment in the pathogenesis of insulin-dependent diabetes mellitus (IDDM), an LTbeta receptor-immunoglobulin fusion protein (LTbetaR-Ig) was administered to nonobese diabetic mice. Early treatment with LTbetaR-Ig prevented insulitis and IDDM, suggesting that LT plays a critical role in the insulitis development. LTbetaR-Ig treatment at a late stage of the disease also dramatically reversed insulitis and prevented diabetes. Moreover, LTbetaR-Ig treatment prevented the development of IDDM by diabetogenic T cells in an adoptive transfer model. Thus, LTbetaR-Ig can disassemble the well established lymphoid microenvironment in the islets, which is required for the development and progression of IDDM.