Coexpression and functional cooperation of CTLA-4 and CD28 on activated T lymphocytes

Molecular determinants of inverse agonist activity of biologicals targeting CTLA-4

Ligation of CD28 or CTLA-4 with some biologicals can activate T cells due to an unexpected superagonist or inverse agonist activity, respectively. The risk of such an outcome limits the therapeutic development of these reagents. Thus, identifying the molecular determinants of superagonist/inverse agonist properties for biologicals targeting costimulatory/inhibitory receptors has not only fundamental value but also important therapeutic implications. In this study, we show that ligation of CTLA-4 with either soluble B7.1 Ig (but not B7.2 Ig) or with a recombinant bispecific in-tandem single chain Fv known as 24:26 induces TCR-independent, T cell activation. Such an inverse agonist activity requires CD28 expression and high CTLA-4 expression and is not seen when CTLA-4 is ligated by membrane-bound B7.1 or B7.2. At the molecular level, the inverse agonist activity of B7.1 Ig or 24:26 correlates with their ability to induce the formation of unique dimer-based, CTLA-4 oligomers on the T cell surface and involves CTLA-4 signaling through its cytoplasmic domain. Our results provide a potential mechanism to explain and to predict inverse agonist activity for CTLA-4 ligands.

CD86 has sustained costimulatory effects on CD8 T cells

CD80 and CD86 both costimulate T cell activation. Their individual effects in vivo are difficult to study as they are coordinately up-regulated on APCs. We have studied mice expressing rat insulin promoter (RIP)-CD80 and RIP-CD86 on the NOD and NOD.scid genetic background to generate in vivo models, using diabetes as a readout for cytotoxic T cell activation. Accelerated spontaneous diabetes onset was observed in NOD-RIP-CD80 mice and the transfer of diabetes from 6-wk-old NOD mice to NOD.scid-RIP-CD80 mice was greater compared with NOD-RIP-CD86 and NOD.scid-RIP-CD86 mice, respectively. However, the secondary in vivo response was maintained if T cells were activated through CD86 costimulation compared with CD80. This was demonstrated by greater ability to cause recurrent diabetes in NOD-RIP-CD86 diabetic mice transplanted with 6-wk-old NOD islets and adoptively transferred diabetes from diabetic NOD-RIP-CD86 mice to NOD.scid mice. In vitro, CD80 costimulation enhanced cytotoxicity, proliferation, and cytokine secretion in activated CD8 T cells compared with CD86 costimulation. We demonstrated increased CTLA-4 and programmed death-1 inhibitory molecule expression following costimulation by both CD80 and CD86 (CD80 > CD86). Furthermore, T cells stimulated by CD80 were more susceptible to inhibition by CD4(+)CD25(+) T cells. Overall, while CD86 does not stimulate an initial response as strongly as CD80, there is greater sustained activity that is seen even in the absence of continued costimulation. These functions have implications for the engineered use of costimulatory molecules in altering immune responses in a therapeutic setting.

Spontaneous allograft tolerance in B7-deficient mice independent of preexisting endogenous CD4+CD25+ regulatory T-cells

BACKGROUND

Acute cardiac allograft rejection requires host, but not donor, expression of B7-1/B7-2 costimulatory molecules. However, acute cardiac rejection requires direct antigen presentation by donor-derived antigen presenting cells to CD4 T-cells and does not require indirect antigen presentation to CD4 T-cells. Given this discrepancy in the literature and that the consequence of allograft exposure in B7-deficient mice is unknown; the goal of the study was to examine the antidonor status of allografted B7-1/B7-2-deficient hosts.

METHODS

C57Bl/6 B7-1/B7-2-/- mice were grafted with heterotopic BALB/c hearts. Recipients bearing long-term surviving allografts were used to examine the status of antidonor reactivity in vitro and in vivo. Tolerance was examined in vivo through adoptive transfer of splenocytes from graft-bearing animals to secondary immune-deficient Rag-1-/- hosts bearing donor-type or third-party cardiac allografts and by regulatory T-cell depletion with anti-CD25 antibody.

RESULTS

When transferred to B7-replete Rag-1-/- recipients, cells from naïve B7-1/B7-2-/- mice readily initiated cardiac allograft rejection. However, splenocytes transferred from long-term allograft acceptor B7-1/B7-2-/- hosts failed to reject donor-type hearts but acutely rejected third-party allografts. In addition, such cells did not reject (donorxthird-party) F1 allografts. Finally, in vivo depletion of regulatory T-cells did not prevent long-term acceptance.

CONCLUSIONS

Results demonstrate that B7-deficient T-cells are capable of acute cardiac allograft rejection in a B7-replete environment. Importantly, results also show that B7-deficient hosts do not simply ignore cardiac allografts, but rather spontaneously develop transferable, donor-specific tolerance and linked suppression in vivo. Interestingly, this tolerant state does not require endogenous CD4+CD25+ regulatory T-cells.

T cell costimulation: a rational target in the therapeutic armamentarium for autoimmune diseases and transplantation

T cells are central mediators of adaptive immunity. As such, they are involved in both normal immune responses (e.g., rejection of a transplanted organ) and abnormal ones (e.g., rheumatoid arthritis). T cells require both antigen-specific and costimulatory signals for their full activation. Advances in protein engineering and an increased understanding of the immune response have culminated in the evolution and creation of protein therapeutics that target specific costimulatory molecules. The selective costimulation modulator abatacept (CTLA-4Ig) binds to CD80 and CD86, blocking interaction with CD28, and is approved for the treatment of moderate to severe rheumatoid arthritis. Belatacept, currently enrolling phase III trials in renal transplantation, was rationally designed from abatacept to bind with more avidity to CD86, providing the more potent immunosuppressive properties required for immunosuppression in transplantation. This review describes the relevant immunology and summarizes recent clinical findings on these two molecules. Although both inhibit the CD28 costimulatory pathway, they are tailored for specific disease states--abatacept for autoimmune diseases and belatacept for transplantation.

The CD40, CTLA-4, thyroglobulin, TSH receptor, and PTPN22 gene quintet and its contribution to thyroid autoimmunity: back to the future

Autoimmune thyroid diseases (AITD) are common autoimmune diseases, affecting up to 5% of the general population. Thyroid-directed autoimmunity is manifested in two classical autoimmune conditions, Hashimoto's thyroiditis, resulting in hypothyroidism and Graves' disease resulting in hyperthyroidism. Autoimmune thyroid diseases arise due to an interplay between environmental and genetic factors. In the past decade significant progress has been made in our understanding of the genetic contribution to the etiology of AITD. Indeed, several AITD susceptibility genes have been identified. Some of these susceptibility genes are specific to either Graves' disease or Hashimoto's thyroiditis, while others confer susceptibility to both conditions. Both immunoregulatory genes and thyroid specific genes contribute to the pathogenesis of AITD. The time is now ripe to examine the mechanistic basis for the contribution of genetic factors to the etiology of AITD. In this review, we will focus on the contribution of non-MHC II genes.

Pivotal advance: CTLA-4+ T cells exhibit normal antiviral functions during acute viral infection

Previous studies have shown that T cells, which are genetically deficient in CTLA-4/CD152 expression, will proliferate uncontrollably, resulting in lethal autoimmune disease. This and other evidence indicate that CTLA-4 plays a critical role in the negative regulation of effector T cell function. In contrast to expectations, BrdU incorporation experiments demonstrated that CTLA-4 expression was associated with normal or even enhanced in vivo proliferation of virus-specific CD4+ and CD8+ T cells following acute lymphocytic choriomeningitis virus or vaccinia virus infection. When compared with CTLA-4- T cells directly ex vivo, CTLA-4+ T cells also exhibited normal antiviral effector functions following stimulation with peptide-coated cells, virus-infected cells, plate-bound anti-CD3/anti-CTLA-4, or the cytokines IL-12 and IL-18. Together, this indicates that CTLA-4 does not directly inhibit antiviral T cell expansion or T cell effector functions, at least not under the normal physiological conditions associated with either of these two acute viral infections.

Does CTLA4 influence the suppressive effect of CD25+CD4+ regulatory T cells?

BACKGROUND

Members of the B7 costimulatory protein family (CD80 and CD86) play a determining role in allograft rejection. Both CD80 and CD86 have naturally occurring splice variants whose roles in transplantation are unknown. Full length CD80 has two immunoglobulin (Ig)-like domains in the extracellular portion, IgC and IgV. In mouse, the isoform IgV-CD80 lacks the IgC-like domain. Here we analyzed the role of mouse IgV-CD80 in heart allograft rejection and search for equivalent splice variants in human.

METHODS

Mice made deficient for full-length CD80 but which retain expression of the shorter IgV-CD80 (CD80 mice) were used as donor or recipient of a heart allograft. Recipient animals were untreated or pretreated with alloantigen expressing cells and/or treated with CD80 and CTLA4 monoclonal antibodies (mAbs).

RESULTS

Recipients expressing IgV-CD80 but not full length CD80 exhibited a slight prolongation in survival of either wild-type (Wt) or CD80 grafts. More dramatically, CD80 animals pretreated with donor alloantigen exhibited permanent graft survival, whereas their Wt counterparts rejected their grafts with a median survival of 24 days. This prolonged survival was due to the expression of IgV-CD80 in recipients since treatment with CD80 mAb abrogated the beneficial effect observed. We identified and report here a similar isoform of CD80 from human cDNA encoding a putative soluble, IgV-containing protein.

CONCLUSIONS

IgV-CD80 bearing recipients show enhanced allograft survival especially after donor alloantigen pretreatment. This together with data from other species suggests that regulation delivered by splice variants of CD80 significantly modulates immunity and may be common across the species.

Erythropoietin treatment in advanced multiple myeloma is associated with improved immunological functions: could it be beneficial in early disease?

Erythropoietin (Epo) is the main growth regulator of red blood cells, and recombinant human erythropoietin (rHuEpo) is thus used in clinical practice for the treatment of anaemia, primarily in kidney disease and cancer. rHuEpo treatment was found to be associated with prolonged survival of multiple myeloma (MM) patients. This clinical observation was then supported by studies on murine myeloma models. It thus appeared that rHuEpo had an anti-myeloma effect, causally related to an immunomodulatory function of rHuEpo. The present study investigated whether rHuEpo-treated MM patients acquire improved immunological functions. Treatment with rHuEpo, prescribed for anaemia that occurs in advanced disease, was associated with effects on a variety of immunological parameters and functions. This was expressed in an actual normalisation of the CD4:CD8 cell ratio, enhanced T cell phytohaemagglutinin-mediated activation and proliferation potential, T cell expression of the costimulatory CD28 and inhibitory CTLA-4 molecules, as well as reduced interleukin-6 serum values to normal levels. Furthermore, it was demonstrated that immunological abnormalities manifest in patients even in the early stages of MM. Our findings thus suggest that rHuEpo treatment might be effective in the early stages of MM, before anaemia develops. It is expected that this would boost the immune system, consequently achieving an anti-myeloma function; affecting disease progression and improving the prognosis.

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

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

Modeling CTLA4-linked autoimmunity with RNA interference in mice

The CTLA4 gene is important for T lymphocyte-mediated immunoregulation and has been associated with several autoimmune diseases, in particular, type 1 diabetes. To model the impact of natural genetic variants of CTLA4, we constructed RNA interference (RNAi) "knockdown" mice through lentiviral transgenesis. Variegation of expression was observed in founders but proved surmountable because it reflected parental imprinting, with derepression by transmission from male lentigenics. Unlike the indiscriminate multiorgan autoimmune phenotype of the corresponding knockout mice, Ctla4 knockdown animals had a disease primarily focused on the pancreas, with rapid progression to diabetes. As with the human disease, the knockdown phenotype was tempered by genetic-modifier loci. RNAi should be more pertinent than gene ablation in modeling disease pathogenesis linked to a gene-dosage variation.

CTLA4-Ig-based conditioning regimen to induce tolerance to cardiac allografts

BACKGROUND

Transplant rejection and toxicity associated with chronic immunosuppressive therapy remain a major problem. Mixed hematopoietic chimerism has been shown to produce tolerance to solid organ transplants. However, currently available protocols to induce mixed hematopoietic chimerism invariably require toxic pre-conditioning. In this study, we investigated a non-toxic CTLA4-Ig-based protocol to induce donor-specific tolerance to cardiac allografts in rats.

METHODS

Fully mismatched, 4 to 6 week old ACI (RT1.A(a)) and Wistar Furth (RT1.A(u)) rats were used as cell/organ donors and recipients, respectively. Recipients were treated with CTLA4-Ig 2 mg/kg/day (on days 0, 2, 4, 6, 8), tacrolimus 1 mg/kg/day (daily, from days 0 to 9), and a single dose of anti-lymphocyte serum (10 mg) on day 10, soon after total body irradiation (300 cGy) and donor bone marrow (100 x 10(6) T-cell depleted cells) transplantation (BMT). Six weeks after BMT, chimeric animals received heterotopic heart transplants.

RESULTS

Hematopoietic chimerism was 18.8 +/- 10.6% at day 30, and was stable (24 +/- 10%) at 1 year post-BMT; there was no graft versus host disease. Chimeric recipients (RT1.A(u)) permanently accepted (>360 days) donor-specific (RT1.A(a); n = 6) hearts, yet rapidly rejected (<9 days) third-party hearts (RT1.A(l); n = 5). Graft (heart) tolerant (>100 days) recipients accepted donor-specific secondary skin grafts (>200 days) while rejected the third-party skin grafts (<9 days). Lymphocytes of graft tolerant animals demonstrated hyporesponsiveness in mixed lymphocyte cultures in a donor-specific manner. Tolerant graft histology showed no obliterative arteriopathy or chronic rejection.

CONCLUSIONS

The CTLA4-Ig based conditioning regimen with donor BMT produced mixed chimerism and induced donor- specific tolerance to cardiac allografts.

Blockade of CTLA-4 on CD4+CD25+ regulatory T cells abrogates their function in vivo

Naturally occurring CD4+ regulatory T cells (T(R)) that express CD25 and the transcription factor FoxP3 play a key role in immune homeostasis, preventing immune pathological responses to self and foreign Ags. CTLA-4 is expressed by a high percentage of these cells, and is often considered as a marker for T(R) in experimental and clinical analysis. However, it has not yet been proven that CTLA-4 has a direct role in T(R) function. In this study, using a T cell-mediated colitis model, we demonstrate that anti-CTLA-4 mAb treatment inhibits T(R) function in vivo via direct effects on CTLA-4-expressing T(R), and not via hyperactivation of colitogenic effector T cells. Although anti-CTLA-4 mAb treatment completely inhibits T(R) function, it does not reduce T(R) numbers or their homing to the GALT, suggesting the Ab mediates its function by blockade of a signal required for T(R) activity. In contrast to the striking effect of the Ab, CTLA-4-deficient mice can produce functional T(R), suggesting that under some circumstances other immune regulatory mechanisms, including the production of IL-10, are able to compensate for the loss of the CTLA-4-mediated pathway. This study provides direct evidence that CTLA-4 has a specific, nonredundant role in the function of normal T(R). This role has to be taken into account when targeting CTLA-4 for therapeutic purposes, as such a strategy will not only boost effector T cell responses, but might also break T(R)-mediated self-tolerance.

Immune privilege induced by regulatory T cells in transplantation tolerance

Immune privilege was originally believed to be associated with particular organs, such as the testes, brain, the anterior chamber of the eye, and the placenta, which need to be protected from any excessive inflammatory activity. It is now becoming clear, however, that immune privilege can be acquired locally in many different tissues in response to inflammation, but particularly due to the action of regulatory T cells (Tregs) induced by the deliberate therapeutic manipulation of the immune system toward tolerance. In this review, we consider the interplay between Tregs, dendritic cells, and the graft itself and the resulting local protective mechanisms that are coordinated to maintain the tolerant state. We discuss how both anti-inflammatory cytokines and negative costimulatory interactions can elicit a number of interrelated mechanisms to regulate both T-cell and antigen-presenting cell activity, for example, by catabolism of the amino acids tryptophan and arginine and the induction of hemoxygenase and carbon monoxide. The induction of local immune privilege has implications for the design of therapeutic regimens and the monitoring of the tolerant status of patients being weaned off immunosuppression.

The association of CTLA-4 and HLA class II autoimmune risk genotype with regulatory T cell marker expression in 5-year-old children

Regulatory T cells (Treg) are involved in the maintenance of peripheral tolerance by suppression of autoreactive lymphocytes that have avoided thymic depletion. The defective function of Treg cells has recently attracted attention in autoimmune diseases such as type 1 diabetes (T1D), rheumatoid arthritis and multiple sclerosis. Susceptibility to these diseases is associated with specific human leucocyte antigen (HLA) class II and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) gene polymorphisms. This study aimed to investigate the relationship between HLA class II and CTLA +49 A/G polymorphisms associated with susceptibility to T1D and the number and characteristics of Treg cells in children. Samples from 47 5-year-old children who participated in the All Babies in South-east Sweden (ABIS) follow-up study were grouped according to the presence of the T1D risk-associated HLA genotype (DQA1*0501-DQB1*0201, DQA1*0301-DQB1*0302) or neutral HLA genotypes. Lower percentages of CD4+ T cells (P = 0.03) and CD4+ CD25high cells (P = 0.06) expressing intracellular CTLA-4 were detected in samples from children with CTLA-4 +49GG compared to children with the +49AA genotype. Similarly, lower percentages of CD4+ (P = 0.002) and CD4+ CD25high (P = 0.002) cells expressing CTLA-4 were observed in children positive for HLA DQA1*0501-DQB1*0201 and DQA1*0301-DQB1*0302 (P = 0.04 for CD4+ and P = 0.02 for CD4+ CD25high) risk haplotypes when compared to children without these alleles. The percentage of CD25high cells among CD4+ cells was correlated inversely with CTLA-4 mRNA expression in PBMC (r = -0.56, P = 0.03). Decreased levels of CTLA-4 in CD4+ and CD4+ CD25high cells in individuals with CTLA-4 and HLA class II alleles associated with T1D may contribute to the initiation and/or progression of autoimmune response.

Activation-associated phenotype of CD3 T cells in acute graft-versus-host disease

During the effector phase of graft-versus-host disease (GvHD) response, donor T cells play an essential role and they are believed to change the expression of activation and co-stimulatory markers associated with functional alloreactivity. We analysed the expression of CD25, CD69, HLA-DR, CD154 and CD134 on CD4+ and CD8+ T cells by flow cytometry during acute GvHD (aGvHD) in 24 patients receiving human leucocyte antigen (HLA)-identical stem cell transplants. Expression of these molecules in nine patients with stages I-IV aGvHD was compared with 15 patients without aGvHD (n = 15). Serial analysis showed that peripheral blood of aGvHD patients presented a significant increase of CD4+ CD25+ cells (P < 0.03), CD4+ CD69+ (P < 0.04) and CD4+ CD134+ cells (P < 0.01). Additionally, there was a significant increase in CD8+ cells expressing CD134 (P = 0.007) and CD154 (P = 0.02). After resolution of aGvHD, the increased expression of these molecules returned to values comparable to patients without aGvHD. Only two of the 15 patients without clinical signs of aGvHD presented activated T cells that could not be attributed to development of aGvHD. In summary, our data show that multiple activation molecules are preferentially up-regulated on CD4+ and CD8+ T cells from patients with aGvHD. These patients had a significant increase in the expression of the co-stimulatory molecules CD134 and CD154.

A molecular perspective of CTLA-4 function

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

Analysis of CD28 and bcl-2 Expression on Peripheral Blood and Liver-Infiltrating Mononuclear Cells in Patients with Autoimmune Hepatitis

Because the underlying mechanism of hepatocellular damages in autoimmune hepatitis (AIH) still remains unclear, analysis of CD28 and bcl-2 molecules, which are critical for T cell activation and survival, was performed in patients with AIH. The number of CD28(+)CD4(+) peripheral blood mononuclear cells (PBMC) in corticosteroid (CS)-treated patients was comparable to normal control individuals but decreased in untreated AIH patients. In contrast, the number of CD28(+)CD8(+) PBMC was decreased in both CS-treated and untreated AIH patients. Analysis of liver-infiltrating mononuclear cells (LIMC) showed that the number of CD28(+)CD4(+) and CD28(-)CD8(+) LIMC were positively correlated with the histology activity index score. Bcl-2(+)CD4(+) LIMC were observed in the portal area of the liver and the numbers fluctuated with disease activity during the time course after CS administration. By contrast, CD8(+) LIMC were shown not to express bcl-2. Taken collectively, these results suggest that bcl-2(+)CD28(+)CD4(+) and bcl-2(-)CD28(-)CD8(+) cells may play critical and distinct roles in hepatocellular damage in AIH.

Engagement of glucocorticoid-induced TNF receptor costimulates NKT cell activation in vitro and in vivo

Glucocorticoid-induced TNF receptor (GITR) is known to provide costimulatory signals to CD4+CD25- and CD4+CD25+ T cells during immune responses in vivo. However, the functional roles of GITR expressed on NKT cells have not been well characterized. In this study, we have explored the functions of GITR as a costimulatory factor on NKT cells. GITR was found to be constitutively expressed on NKT cells and its expression was enhanced by TCR signals. GITR engagement using DTA-1, an agonistic mAb against GITR, in the presence of TCR signals, augmented IL-2 production, the expression of activation markers, cell cycle progression, and the nuclear translocations of NF-kappaB p50 and p65. Furthermore, GITR engagement enhanced the production of IL-4, IL-10, IL-13, and IFN-gamma by NKT cells and the expression level of phosphorylated p65 in NKT cells in the presence of TCR engagement, indicating that GITR provides costimulatory signals to NKT cells. The costimulatory effects of GITR on NKT cells were comparable to those of CD28 in terms of cytokine production. Moreover, the coinjection of DTA-1 and alpha-galactosylceramide into B6 mice induced more IL-4 and IFN-gamma production than the coinjection of control mAbs and alpha-galactosylceramide. In addition, the adoptive transfer of DTA-1-pretreated NKT cells into CD1d(-/-) mice attenuated hypersensitivity pneumonitis more than control IgG pretreated NKT cells in these mice. These findings demonstrate that GITR engagement on NKT cells modulates immune responses in hypersensitivity pneumonitis in vivo. Taken together, our findings suggest that GITR engagement costimulates NKT cells and contributes to the regulation of immune-associated disease processes in vivo.

Expression and function of the B and T lymphocyte attenuator (BTLA/CD272) on human T cells

Co-signal receptors provide crucial activating or attenuating signals for T cells. The B and T lymphocyte attenuator (BTLA/CD272) is a third member of co-inhibitory receptors, which belongs to the CD28 immunoglobulin-superfamily. Using monoclonal antibodies (mAbs) against human BTLA, we show that BTLA is constitutively expressed on most CD4+ and CD8+ T cells and its expression progressively decreases upon T cell activation. Polarized Th1 and Th2 cells contained both BTLA-positive and BTLA-negative populations, but the extended culture diminished BTLA expression. Cross-linking BTLA with an agonistic mAb inhibited T cell proliferation and the production of the cytokines IFN-gamma and IL-10 in response to anti-CD3 stimulation. BTLA-mediated inhibition of T cell activation occurred during both primary CD4+ T cell responses and secondary CD4+ and CD8+ T cell responses, suggesting that BTLA ligation sends a constitutive "off" signal to T cells and thus might play an important role in the maintenance of T cell tolerance.

B7-1 and B7-2: Similar costimulatory ligands with different biochemical, oligomeric and signaling properties

B7-1 and B7-2 are homologous costimulatory ligands expressed on the surface of antigen presenting cells (APCs). Binding of these molecules to the T cell costimulatory receptors, CD28 and CTLA-4, is essential for the activation and regulation of T cell immunity. Despite strong structural similarities, B7-1 and B7-2 exhibit different biochemical features, and their binding to the costimulatory receptors results in distinct T cell functional outcomes. Using photobleaching based fluorescence resonance energy transfer (FRET), our previous studies have demonstrated that B7-1 and B7-2 have different cell surface oligomeric states. While B7-1 is present as a dimer, B7-2 exists as a monomer on the cell surface suggesting that the unique cell surface oligomeric states of the costimulatory ligands may play a key role in the regulation of T cell responses. Moreover, signaling via B7-1 and B7-2 in dendritic cells has been reported to be dependent on their simultaneous expression, raising the possibility that their direct interaction or their involvement in synergistic signaling pathways may play a role in the function of antigen presenting cells. We discuss physiological relevance of distinct oligomeric states of B7-1 and B7-2 and address whether these molecules can associate with one another on the cell surface to form hetero-oligomers. Our findings suggest that B7-1 and B7-2 do not form hetero-oligomers, underscoring the biological relevance of dimeric and monomeric state of B7-1 and B7-2, respectively.

CD80 cytoplasmic domain controls localization of CD28, CTLA-4, and protein kinase Ctheta in the immunological synapse

The binding of costimulatory ligand CD80 to CD28 or CTLA-4 on T cells plays an important role in the regulation of the T cell response. We have examined the role of the cytoplasmic domain of CD80 in murine T cell costimulation and its organization in the immunological synapse (IS). Removal of CD80 cytoplasmic tail decreased its effectiveness in costimulating T cell proliferative response and early IL-2 production in response to agonist MHC-peptide complexes. Immunofluorescent study showed a decreased tailless CD80 accumulation in the IS of naive T cells. The two forms of CD80 accumulated differently at the IS; the tailless CD80 was colocalized with the TCR whereas the full-length CD80 was segregated from the TCR. In addition, we showed that CD80, CD28, and protein kinase Ctheta colocalized in the presence or absence of the CD80 cytoplasmic tail. Thus, the cytoplasmic tail of CD80 regulates its spatial localization at the IS and that of its receptors and T cell signaling molecules such as protein kinase Ctheta, and thereby facilitates full T cell activation.

CTLA-4 (CD152) and its involvement in autoimmune disease

Autoimmune diseases (AID) are inherited as complex genetic diseases. Different Autoimmune diseases have been found to cluster in families and are believed to share some common etiological factors. With the exception of major histocompatibility complex (MHC) genes contributing susceptibility to these diseases have been difficult to identify. CD152 has emerged as one such candidate unifying several autoimmune diseases. We here review the evidence that CD152 constitutes a general susceptibility factor for multiple autoimmune diseases and discuss how CD152 and other co-stimulatory pathways may contribute to autoimmune pathogenesis.

CTLA-4 blockade augments human T lymphocyte-mediated suppression of lung tumor xenografts in SCID mice

Previous studies by others using transplantable murine tumor models have demonstrated that the administration of antibodies that block CTLA-4 interaction with B7 can provoke the elimination of established tumors, and that the tumor suppression is mediated by T-cells and/or cells expressing NK1.1. Studies from our lab have established in a human/severe combined immunodeficient (SCID) mouse chimeric model that autologous peripheral blood leukocytes (PBL) can suppress the growth of tumor xenografts in a PBL dose-dependent fashion, and that this suppression is dependent upon the patient's T and NK cells. Using this human/mouse chimeric model, we sought to determine whether an antibody blockade of CTLA-4 would enhance the anti-tumor response of a patient's PBL. It was first important to determine whether the tumor suppression observed in the SCID model was dependent upon CD28/B7 co-stimulation. Blockade of B7 with a human CTLA-4-Ig fusion protein completely abrogated the lymphocyte-mediated tumor suppression, confirming in this model that tumor suppression is dependent upon a CD28/B7 co-stimulation. Using two different CTLA-4 specific monoclonal antibodies, we observed that CTLA-4 blockade significantly enhanced the human lymphocyte-mediated tumor suppression in mice co-engrafted with PBL and tumor cells. This enhancement was observed in both an allogeneic setting (in which the PBL were allogeneic with respect to the tumor) and an autologous setting (in which the PBL and tumor were from the same patient). These results sustain the notion that human anti-tumor immune response can be augmented (in vivo) by blocking the interaction between CTLA-4 and B7.

Blockade of CTLA-4 (CD152) enhances the murine antibody response to pneumococcal capsular polysaccharides

The capsular polysaccharides (caps-PS) of Streptococcus pneumoniae are classified as thymus-independent antigens. Nevertheless, T lymphocytes can modulate the antibody response to caps-PS. In this study, we show that anticytotoxic T lymphocyte-associated antigen 4 (CTLA-4) treatment, along with administration of caps-PS to BALB/c mice, resulted in a dose-dependent generation of a strong caps-PS-specific antibody response. Anti-CTLA-4 treatment had no effect on the immunoglobulin G (IgG) antibody production in athymic nu/nu mice. Anti-CTLA-4 treatment stimulated the IgG antibody production in severe combined immunodeficiency (SCID)/SCID mice reconstituted with CTLA-4(-/-) B lymphocytes and wild-type T lymphocytes. This excluded the possibility that anti-CTLA-4 enhanced antibody production by direct interaction with B lymphocytes. Anti-CTLA-4 treatment enhanced the antibody production in SCID/SCID mice reconstituted with B lymphocytes and CD4(+) and CD8(+) T lymphocytes but not in SCID/SCID mice reconstituted with B lymphocytes in the absence of CD4(+) and/or CD8(+) cells. Administration of anti-CTLA-4 in BALB/c mice but not in nu/nu mice resulted in a markedly increased production of interleukin (IL)-2, IL-4, and interferon-gamma. Taken together, these data strongly suggest a role of T lymphocytes and CTLA-4 in the regulation of the antibody response to caps-PS.

Decreased Alloreactivity Using Donor Cells from Mice Expressing a CD200 Transgene Under Control of a Tetracycline-Inducible Promoter

BACKGROUND

CD200 delivers an immunsuppressive signal that augments allograft survival following interaction with its receptor, CD200R1. We hypothesized that mice overexpressing CD200 as a trangene would also show a diminished alloresponsiveness and decreased allograft rejection.

METHODS

A transgenic mouse on a C57BL/6 background, expressing a murine CD200 cDNA genetically linked to a green fluorescent protein tag (GFP) under control of a tetracycline response element (TRE), was mated with a commercial transgenic mouse carrying the reverse tetracycline regulated transactivator gene under control of a human CMV promoter. F1 mice were examined for induction of alloimmunity in vivo/in vitro, and for their ability to reject skin allografts in vivo.

RESULTS

The F1 hybrid expressed CD200 after exposure to doxycyline (DOX), as assessed both by enhanced GFP expression in multiple organs and CD200-GFP expression. Splenocytes from F1 mice stimulated with LPS or allogeneic cells in vitro in the presence/absence of DOX showed reduced production of TNFalpha, and of allospecific CTL. Splenocytes from F1 mice used as stimulator cells in allogeneic MLCs in the presence of DOX were inefficient at induction of cytokines or CTL in vitro from normal allogeneic responder cells. Skin grafts from transgenic mice were inefficient at induction of CTL in vivo. Transgenic mice receiving DOX showed prolonged acceptance of skin allografts, which was abolished by infusion of anti-CD200 mAb.

CONCLUSIONS

Our data confirmed that overexpression of CD200 in transgenic mice, or in skin grafts from these mice, decreases alloimmunity. This has potential clinical utility in transplantation and other diseases.

High-affinity small molecule inhibitors of T cell costimulation: compounds for immunotherapy

Costimulatory molecules are important regulators of T cell activation and thus favored targets for therapeutic manipulation of immune responses. One of the key costimulatory receptors is CD80, which binds the T cell ligands, CD28, and CTLA-4. We describe a set of small compounds that bind with high specificity and low nanomolar affinity to CD80. The compounds have relatively slow off-rates and block both CD28 and CTLA-4 binding, implying that they occlude the shared ligand binding site. The compounds inhibit proinflammatory cytokine release in T cell assays with submicromolar potency, and as such, they represent promising leads for the development of novel therapeutics for immune-mediated inflammatory disease. Our results also suggest that other predominantly beta proteins, such as those that dominate the cell surface, may also be accessible as potentially therapeutic targets.

Dysregulation of CD28 and CTLA-4 expression by CD4 T cells from previously immunodeficient HIV-infected patients with sustained virological responses to highly active antiretroviral therapy

OBJECTIVES

Current guidelines recommend commencing highly active antiretroviral therapy (HAART) in HIV-infected patients when CD4 T-cell counts reach 350 cells/microL. However, late-presenting HIV-infected patients with CD4 T-cell counts<50 cells/microL are still common. The ability of long-term HAART to normalize immune dysregulation in severely immunodeficient HIV-infected patients remains unclear. Here we address indices of immune dysregulation in previously severely immunocompromised HIV-infected patients treated with long-term HAART who had achieved increased CD4 T-cell counts and complete suppression of HIV viraemia.

METHODS

We examined expression of CD28, cytotoxic T-lymphocyte antigen-4 (CTLA-4) and intracellular perforin by CD4 and CD8 lymphocytes from 25 highly selected HIV-infected patients [nadir CD4 T-cell counts <50 cells/microL, >4 years on HAART and >6 months of complete viral suppression (<50 HIV-1 RNA copies/mL)] and 18 HIV-seronegative age- and sex-matched controls.

RESULTS

HIV-infected patients had lower percentages of CD28-expressing CD4 lymphocytes and higher percentages of CTLA-4-expressing CD4 lymphocytes than controls. The percentage of CTLA-4-expressing CD4 lymphocytes correlated inversely with that of CD28-expressing CD4 lymphocytes. The proportion of CD4 lymphocytes expressing perforin was generally low. However, more HIV-infected patients than controls had >1% of CD4 lymphocytes expressing perforin [11 of 25 (44%) vs. one of 18 (5.5%)]. The percentage of CD8 lymphocytes expressing perforin did not differ between HIV-infected patients and controls. Amongst HIV-infected patients, the percentage of perforin-expressing CD8 lymphocytes correlated inversely with nadir but not current CD4 T-cell count.

CONCLUSIONS

Expression of CD28, CTLA-4 and perforin by CD4 lymphocytes remain dysregulated in HIV-infected patients with previous severe immunodeficiency, despite increased CD4 T-cell counts and control of HIV viraemia by HAART.

Creation of tolerogenic human dendritic cells via intracellular CTLA4: a novel strategy with potential in clinical immunosuppression

Activation of T lymphocytes requires the recognition of peptide-major histocompatibility complexes (MHCs) and costimulatory signals provided by antigen-presenting cells (APCs). It has been shown that T-cell activation without costimulation can lead to anergy. In this study, we developed a novel strategy to inhibit expression of B7 molecules (CD80/86) by transfecting APCs with a gene construct encoding a modified cytotoxic T lymphocyte antigen 4 (CTLA4) molecule (CTLA4-KDEL) that is targeted to the endoplasmic reticulum (ER). APCs expressing this construct failed to express CD80/86 on their surface, were unable to stimulate allogeneic and peptide-specific T-cell responses, and induced antigen-specific anergy of the responding T cells. Cells expressing CTLA4-KDEL do not up-regulate the indoleamine 2, 3-dioxygenase enzyme, unlike cells treated with soluble CTLA4-immunoglobin (Ig). This gene-based strategy to knock out surface receptors is an attractive alternative to using immature dendritic cells for preventing transplant rejection and treating of autoimmune diseases.

Downregulation of IL-12 and a novel negative feedback system mediated by CD25+CD4+ T cells

CD25(+)CD4(+) regulatory T cells suppress immune responses and are believed to play roles in preventing autoimmune diseases. However, the mechanism(s) underlying the suppression and the regulation of their homeostasis remain to be elucidated. Here we show that these regulatory T cells downregulated CD25(-)CD4(+) T-cell-mediated production of IL-12 from antigen-presenting cells, which can act as a growth factor for CD25(-)CD4(+) T cells. We further found that CD25(+)CD4(+) T cells, despite their well-documented "anergic" nature, proliferate significantly in vitro only when CD25(-)CD4(+) T cells are present. Notably, this proliferation was strongly dependent on IL-2 and relatively independent of IL-12. Thus, CD25(+)CD4(+) T cells suppress CD25(-)CD4(+) T-cell responses, at least in part, by inhibiting IL-12 production while they themselves can undergo proliferation with the mediation of CD25(-)CD4(+) T cells in vitro. These results offer a novel negative feedback system involving a tripartite interaction among CD25(+)CD4(+) and CD25(-)CD4(+) T cells, and APCs that may contribute to the termination of immune responses.

Age associated alterations in costimulatory and adhesion molecule expression in lupus-prone mice are attenuated by food restriction with n-6 and n-3 fatty acids

Costimulatory and adhesion molecules are known to play a major role in the pathogenesis of systemic lupus erythematosus. Since fish oil and calorie restriction have been reported to attenuate the development of disease in lupus prone (NZBxNZW)F1 mice, the objective of this study was to assess the expression of these key inflammatory molecules in these mice fed diets differing in n-6 and n-3 fatty acid content and fed either food restricted or ad libitum. Age-associated increases in the expression of CD28, ICAM-1, and PGP-1 molecules that are involved in the recruitment of inflamed lymphocytes into the kidney were attenuated in mice restricted in food intake. The increase in costimulatory (CD80 and CD86) and adhesion (ICAM-1, PGP-1, LFA-1, and Mac-1) in peripheral blood mononuclear cells was also attenuated by food restriction and to a lesser extent by fish oil alone. Interestingly, amelioration of lupus (laminin expression and proteinuria) correlated with the above beneficial effects and could be seen even in 24-month-old mice. In summary, food restriction and fish oil delay the onset of lupus disease and increase life span in B/W mice by prolonging the maintenance of a youthful immune phenotype.

Rational development of LEA29Y (belatacept), a high-affinity variant of CTLA4-Ig with potent immunosuppressive properties

Current success in organ transplantation is dependent upon the use of calcineurin-inhibitor-based immunosuppressive regimens. Unfortunately, current immunotherapy targets molecules with ubiquitous expression resulting in devastating non-immune side effects. T-cell costimulation has been identified as a new potential immunosuppressive target. The best characterized pathway includes CD28, its homologue CTLA4 and their ligands CD80 and CD86. While an immunoglobulin fusion protein construct of CTLA4 suppressed rejection in rodents, it lacked efficacy in primate transplant models. In an attempt to increase the biologic potency of the parent molecule a novel, modified version of CTLA4-Ig, LEA29Y (belatacept), was constructed. Two amino acid substitutions (L104E and A29Y) gave rise to slower dissociation rates for both CD86 and CD80. The increased avidity resulted in a 10-fold increase in potency in vitro and significant prolongation of renal allograft survival in a pre-clinical primate model. The use of immunoselective biologics may provide effective maintenance immunosuppression while avoiding the collateral toxicities associated with conventional immunsuppressants.

Defective suppressor function in CD4(+)CD25(+) T-cells from patients with type 1 diabetes

Type 1 diabetes is a T-cell-mediated disease that is associated with loss of immunological tolerance to self-antigens. The mechanisms involved in maintenance of peripheral tolerance include a specialized subset of regulatory T-cells (Treg) within the CD4(+)CD25(+) T-cell population, but the function and phenotype of these cells in type 1 diabetes have not been investigated. We hypothesized that a deficiency in the CD4(+)CD25(+) Treg population or its function could contribute to the lack of self-tolerance evident in patients with type 1 diabetes. We show that although levels of CD4(+)CD25(+) T-cells are normal in patients with recent-onset adult type 1 diabetes, the ability of the Tregs in this population to suppress T-cell proliferation during in vitro cocultures is markedly reduced compared with control subjects (P = 0.007). Moreover, in patients with type 1 diabetes, these cocultures display a more proinflammatory phenotype, with increased secretion of interferon-gamma (P = 0.005) and decreased interleukin-10 production (P = 0.03). These deficiencies may reflect a disturbance in the balance of the CD4(+)CD25(+) population, because in patients with type 1 diabetes, a higher proportion of these cells coexpress the early activation marker CD69 (P = 0.007) and intracellular CTLA-4 (P = 0.01). These data demonstrate deficiency in function of the CD4(+)CD25(+) Treg population that may influence the pathogenesis of type 1 diabetes.

The CD28 peptidemimic can induce mixed chimerism and prolong the survival of cardiac allografts

Costimulatory blockade with CD28 peptidemimic (CD28PM, CD28 PM was synthesized by solid phase synthetic methods) prolongs cardiac allograft survival in mice, but has not reliably induced tolerance when used alone. In the current studies, we evaluated the effect of adding B7 blockade to a chimerism inducing nonmyeloablative regimen in mice and observed a significant improvement of donor bone marrow cells (BMC) engraftment, which had been associated with mixed chimerism and long-term survival of cardiac allografts. The mixed lymphocyte reaction (MLR) and the ear pinna cardiac transplantation model were performed to evaluate the effects of CD28PM in induction of specific immune hypo-response and extension of allograft survival. The expressed rates of B7.1 and B7.2 on the C57BL/6 splenocytes were 56.25% and 20.52%, respectively. The specific hypo-response status was established after immunization with CD28PM pre-treated donor splenocytes and the average inhibition rate was only 43% compared with normal control. Subsequently, a total number of 2 x 10(7) bone marrow cells per mouse were implanted to the recipients. The allogenic chimerism was obviously observed with the rate as high as 8.84% (mean) at the time point of day 14. During the first 50 days post bone marrow transfusion (BMT) the chimerism rate declined stepwise. But from 50 to 100 days, the chimerism rate sustained in a range of 3.35% to 4.6%. The results of transplantation experiments showed the survival of allgenic cardiac grafts were maintained over 100 days in recipients. Thus, donor BMC engraftment with mixed chimerism appears essential for induction of allograft tolerance using this conditioning regimen. Mixed chimerism approach, by the addition of CD28-B7 costimulatory blockade with CD28PM, has been shown to establish mixed chimerism and induce cardiac allograft tolerance in mice.

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.

Two different forms of human CTLA-4 proteins following peripheral T cell activation

To study expression of human CTLA-4 in peripheral T cells, we developed a new anti-peptide (anti-CTLA4pB) antibody. Both denatured and native forms of CTLA-4 can be recognized by anti-CTLA4pB antibody. After peripheral T cell activation, we observed that most of CTLA-4 is localized primarily to intracellular compartment rather than to the cell surface. Biochemical study using anti-CTLA4pB antibody was revealed two forms of CTLA-4 on activated peripheral T cells. The majority of CTLA-4 was detected as a 34 kDa moiety in cytosolic fraction under reducing conditions. The presence of a 30 kDa CTLA-4 moiety was observed simultaneously in membrane fraction under reducing condition. The post-induction ratio of cytosolic:membrane-bound forms of CTLA-4 was approximately 50:1. Further analysis was indicated that the 34 kDa moiety in cytosolic fraction is part of a 250 kDa multi-protein complex. Antibodies raised against this complex recognized 54 and 75 kDa proteins together with a 34 kDa form of CTLA-4. In contrast, membrane-bound forms of CTLA-4 were 60 and 120 kDa under non-reducing condition. The molar ratio of 60 and 120 kDa oligomers was approximately 1:1. These results may provide evidence for post-translational modification of human CTLA-4 after T cell activation.

Progression of multiple sclerosis is associated with exon 1 CTLA-4 gene polymorphism

OBJECTIVES

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system which is widely believed to have a T-cell-mediated etiology. The cytotoxic T-lymphocyte antigen-4 (CTLA-4) antigen molecule plays a key role in the downregulation of T-cell responses. To examine the genetic association of the CTLA-4 gene locus with MS, we analyzed an exon 1 (A49G) transition.

MATERIAL AND METHODS

One hundred and fifty-two MS patients and 154 controls were examined. The A/G transition was genotyped by a polymerase chain reaction followed by labeling with a SNaPshot kit and detection using a capillary genetic analyzer.

RESULTS

The genotype, allele and phenotype frequencies did not differ significantly between MS patients and controls. Those MS patients with AA and AG genotypes had 4.36 times greater risk of progressing from the relapsing-remitting to the secondary progressive form of the disease than those with the GG genotype.

CONCLUSION

The results of our study indicate that CTLA-4 (A49G) exon 1 polymorphism is associated with MS progression.

Insertion of host-derived costimulatory molecules CD80 (B7.1) and CD86 (B7.2) into human immunodeficiency virus type 1 affects the virus life cycle

Human immunodeficiency virus type 1 (HIV-1) carries virus-encoded and host-derived proteins. Recent advances in the functional characterization of host molecules inserted into mature virus particles have revealed that HIV-1 biology is influenced by the acquisition of host cell membrane components. The CD28/B7 receptor/ligand system is considered one of the fundamental elements of the normal immune response. Two major cell types that harbor HIV-1 in vivo, i.e., monocytes/macrophages and CD4+ T cells, express the costimulatory molecules CD80 (B7.1) and CD86 (B7.2). We investigated whether CD80 and CD86 are efficiently acquired by HIV-1, and if so, whether these host-encoded molecules can contribute to the virus life cycle. Here we provide the first evidence that the insertion of CD80 and CD86 into HIV-1 increases virus infectivity by facilitating the attachment and entry process due to interactions with their two natural ligands, CD28 and CTLA-4. Moreover, we demonstrate that NF-kappaB is induced by CD80- and CD86-bearing virions when they are combined with the engagement of the T-cell receptor/CD3 complex, an event that is inhibited upon surface expression of CTLA-4. Finally, both CD80 and CD86 were found to be efficiently incorporated into R5- and X4-tropic field strains of HIV-1 expanded in cytokine-treated macrophages. Thus, besides direct interactions between the virus envelope glycoproteins and cell surface constituents, such as CD4 and some specific chemokine coreceptors, HIV-1 may attach to target cells via interactions between cell-derived molecules incorporated into virions and their natural ligands. These findings support the theory that HIV-1-associated host proteins alter virus-host dynamics.

Differential expression of CTLA-4 among T cell subsets

CTLA-4 (CD152), the CD28 homologue, is a costimulatory molecule with negative effects on T cell activation. In addition to its role in the termination of activation, CTLA-4 has been implicated in anergy induction and the function of regulatory cells. As an intracellular molecule, it must first relocate to the cell surface and be ligated, in order to inhibit activation. Although some studies have investigated CTLA-4 expression on CD4(+) T cells, evidence is lacking regarding the kinetics of expression, and expression on T cell subpopulations. We have investigated CTLA-4 kinetics on human purified peripheral CD4(+), naïve, memory, CD4(+)CD25(-), CD4(+)CD25(+) regulatory T cells, and T cell clones. Intracellular stores of CTLA-4 were shown to be very low in naïve T cells, whilst significant amounts were present in memory T cells and T cell clones. Cell surface CTLA-4 expression was then investigated on CD4(+)CD45RA(+) (naïve), CD4(+)CD45RO(+) (memory), CD4(+)CD25(-), and CD4(+)CD25(+) T cells. CD25 and CD45RO are both expressed by regulatory T cells. On naïve and CD4(+)CD25(-) T cells, CTLA-4 expression declined after four hours. In contrast, on memory and CD4(+)CD25(+) T cells, high levels of expression were maintained until at least 48 hours. In addition, significant CTLA-4 expression was observed on T cell clones following anergy induction, indicating the potential involvement of CTLA-4 also in this form of tolerance.

Importance of the inducible costimulator molecule for the induction of allergic immune responses and its decreased expression on T helper cells after venom immunotherapy

The inducible costimulator (ICOS), a newly identified member of the CD28 receptor family that is induced after T-cell activation, and its ligand (ICOSL), being expressed on activated monocytes and dendritic cells play a key role in T-cell-mediated immune responses. As ICOS costimulation also seems to regulate T helper 2 effector cells, the aim of this study was to analyse the function of this molecule in allergic immune responses and their specific therapy, mainly venom immunotherapy (VIT). CD4+ T cells from grass pollen-, or bee or wasp venom-allergic donors were stimulated in the presence of autologous mature dendritic cells, which were pulsed with different allergen doses. In this system, costimulation of ICOS strongly enhanced the production of the T helper 2 cytokines interleukin (IL)-4, IL-5 and IL-10 and, to a lesser extent, secretion of the T helper 1 cytokine, interferon-gamma. Expression of ICOS on CD4+ T cells was induced, in a dose-dependent manner, after a few days of stimulation with allergen-pulsed dendritic cells, reaching a peak on day 6. The upregulation of ICOS after stimulation with venom allergens was significantly reduced after VIT. Addition of exogenous IL-10 (which is induced during VIT) to the co-cultures before VIT also led to an inhibition of ICOS expression, while blocking of IL-10 in co-cultures after VIT partially restored the expression of ICOS. These data indicate that the inhibition of T cells after immunotherapy also involves decreased induction of the costimulatory molecule ICOS, which, in turn, seems to be dependent on the presence of IL-10, also associated with the inhibited status of T cells after VIT. This makes the ICOS-ICOSL pathway a potential target for therapeutic intervention in T helper 2-mediated diseases, such as allergic diseases.

CTLA-4 blockade inhibits induction of Helicobacter pylori-associated gastritis in mice

The balance between Th1 and Th2 response determines the outcome of Helicobacter pylori infection. Interferon (IFN)-gamma plays an inductive role in gastric inflammation, whereas interleukin (IL)-4 counterbalances Th1 response and suppresses the development of gastritis. Th cell response is regulated by co-stimulatory factors. A co-stimulatory molecule, cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), plays an inhibitory role in IL-2-dependent cell growth and mediates an optimal inhibitory signal to Th1 and Th2 cells. We administered anti-CTLA-4 monoclonal antibody (MoAb), which blocks CTLA-4 signalling, to examine the relative role for this signalling during maturation of Th1 and Th2 cells in H. pylori infection in mice. Mice treated by anti-CTLA-4 MoAb within the first week of infection showed an inhibition of gastric inflammation, accompanied by an increasing ratio of H. pylori-specific IgG1/IgG2a in serum following infection. Furthermore, the treatment resulted in the higher ratio of IL-4/IFN-gamma by splenocytes in response to H. pylori antigen at 6 weeks after infection, compared with untreated mice. These results suggest that the predominance of Th2 response by CTLA-4 blockade leads to an inhibition of the development of gastric inflammation. CTLA-4 signalling could contribute to the regulation of Th subsets and the development of gastric inflammation in H. pylori infection.

Porins and lipopolysaccharide from Salmonella typhimurium regulate the expression of CD80 and CD86 molecules on B cells and macrophages but not CD28 and CD152 on T cells

OBJECTIVE

The aim of this study was to evaluate the effect of porins from Salmonella typhimurium on costimulatory molecules such as CD80/CD86 and CD28/CD152. The interactions between these molecules are able to influence the immune response through the regulation of cytokines release which, on their own, are able to regulate the immunological response by a feedback mechanism.

METHODS

S. typhimurium strain SH5014 (a rough lipopolysaccharide (LPS) producing strain) was used as the source of porins and LPS. Peripheral blood mononuclear cells were obtained from healthy adult donors. THP1 cells were obtained from ATCC (Rockville, MD, USA). Immunofluorescence and flow cytometry were performed using a FACS IV (Becton-Dickinson, Mountain View, CA, USA).

RESULTS

Our results show that porins of S. typhimurium increase the expression of CD86 and the expression of CD80 both on B lymphocytes and macrophages, while the expression of CD28 and CD152 on T lymphocytes was unaltered. The expression of CD80 and CD86 is dose-dependent and starts after 24 h post treatment, peaks at 48 h and goes back to the basal value after 72 h.

CONCLUSIONS

S. typhimurium porins are able to induce a high expression of costimulatory molecules (CD80 and CD86) on lymphocytes and macrophages.

Diverse roles of integrins in human T lymphocyte biology

T lymphocytes are the primary cells responsible for maintaining the immune system. There are many intricate mechanisms involved in the regulation of T cells and the integrin family of adhesive surface proteins plays a pivotal role in the control of T lymphocyte activation and functions. Integrins are heterodimeric transmembrane proteins that are not merely adhesion molecules but also function in T cell coactivation by providing a scaffold for signaling and cytoskeletal proteins that are adept at transmitting signals from the inside of the cell to the outside ("inside-out signaling") or from the outside of the cell to the inside ("outside-in signaling"). The signaling property of integrins allows for rapid responses to changes in the microenviroment of the lymphocyte. Therefore, whether the T cell needs to adhere or detach, integrins can quickly accommodate either state of the cell. Once cells are guided to sites of infection, inflammation, or antigen presentation, integrins can also participate in the initiation, maintenance, or termination of the response. This review will focus on the aspects of integrin-mediated T cell coactivation, affinity and avidity control of integrins, signaling molecules involved with integrins, association of integrins in lipid microdomains, and negative regulation of integrins.