Essential flexibility in the T-cell recognition of antigen

Cutting Edge: Predictable TCR Antigen Recognition Based on Peptide Scans Leads to the Identification of Agonist Ligands with No Sequence Homology

The potential of CD4+ T cells for cross-recognition of self and foreign Ags has important implications for the understanding of thymic selection, lymphocyte survival, and the occurrence of autoimmune diseases. Here, we define the extensive flexibility of Ag recognition for three human CD4+ autoreactive T cell clones (TCC) by using ligands with single and multiple amino acid (aa) substitutions. Our results demonstrate that the spectrum of tolerated ligands and the resulting stimulatory potency of peptides for a TCC can be predicted by the relative influence of each aa. Using this approach, we have identified stimulatory ligands not sharing a single aa in corresponding positions with the Ag used to establish the TCC. These results argue for an independent contribution of each aa in the peptide sequence to the affinity of the MHC/peptide complex to the TCR.

Inhibition of an In Vitro CD4+ T Cell Alloresponse Using Altered Peptide Ligands

In this study, we explore the potential of altered peptide ligands (APLs) to modulate the alloresponse of CD4+ T cells using elements of the murine hemoglobin (Hb) Ag model. We first demonstrated that the T cell 2.102, specific for the Hb(64-76)/I-Ek complex, was alloreactive against splenocytes of the H-2p haplotype. Using Ab-blocking and transfection experiments, we further showed that this alloreactivity was restricted to the class II molecule I-Ep. We tested a panel of APLs previously shown to antagonize the Hb response of 2.102 and found that these peptides could also effectively inhibit the alloresponse to I-Ep. Importantly, these peptides were able to antagonize the alloresponse of naive T cells derived from mice transgenic for the 2.102 TCR, as well as Th1 and Th2 cell lines. The antagonism required the presence of both I-Ep and I-Ek on the same APC. Our study demonstrates the effectiveness of APLs to antagonize the primary alloresponse of specific T cells and provides a basis for the development of immunotherapeutics for use in transplantation and immune-mediated diseases.

Cytotoxic T cells from human immunodeficiency virus type 2-infected patients frequently cross-react with different human immunodeficiency virus type 1 clades

Knowledge of immune mechanisms responsible for the cross-protection between highly divergent viruses such as human immunodeficiency virus type 1 (HIV-1) and HIV-2 may contribute to an understanding of whether virus variability may be overcome in the design of vaccine candidates which are broadly protective across the HIV subtypes. We demonstrate that despite the significant difference in virus amino acid sequence, the majority of HIV-2-infected individuals with different HLA molecules possess a dominant cytotoxic T-cell response which is able to recognize HIV-1 Gag protein. Furthermore, HLA-B5801-positive subjects show broad cross-recognition of HIV-1 subtypes since they mounted a T-cell response that tolerated extensive amino acid substitutions within HLA-B5801-restricted HIV-1 and HIV-2 epitopes. These results suggests that HLA-B5801-positive HIV-2-infected individuals have an enhanced ability to react with HIV-1 that could play a role in cross-protection.

Identification of core structure and critical T cell receptor contact residues in an antigenic peptide by measuring acidification rates

A silicon-based biosensor microphysiometer measures real time cell response by monitoring an increase in extracellular acidification rate in response to ligands for specific membrane receptors. We used the microphysiometer to identify the minimal structure and critical residues of an antigenic peptide for its interaction with T cell receptor (TCR) using a synthetic peptide analog of human myelin basic protein (MBP) corresponding to residues 84-102 [MBP(83-102)Y83]. MBP(83-102)Y83 peptide analogs were allowed to interact with TCRs on a DRB5*0101-restricted Herpes virus saimiri (HVS) transformed human T cell clone (SS8T) which also contains major histocompatibility complexes (MHC) class II (DR2) molecules. Cultured SS8T cells were exposed to 11 N-terminus and 11 C-terminus truncated peptides separately in the microphysiometer chambers to determine the minimal amino acid residues required for the T cell response. In parallel, 13 analogs of the MBP(83-102)Y83 peptide with single alanine substitutions were tested in this assay to identify critical amino acid residues involved in TCR interactions. A minimal core length of MBP(91-100) peptide and residues F-91, K-93, N-94, I-95 and V-96 were essential for TCR interaction. Acidification rate measurements correlated well with enhanced levels of gamma-IFN (interferon gamma) and TNF-beta cytokine production and suggested that the increase in the extracellular acidification rate is a direct result of early T cell signaling events.

Biochemical features of anergic T cells

T cell anergy is a functionally defined state of hyporesponsiveness in which T cells neither proliferate nor produce IL2 following subsequent TCR ligation. Recent biochemical data from in vitro studies suggest that anergic cells do not utilize all of the signaling pathways normally initiated by TCR triggering. These findings appear to hold true for T cells rendered anergic in vivo, as well; however, biochemical studies on clonal anergy in vivo have been limited by the inability to recover a homogeneous population of anergic T cells. Here we review progress on TCR mediated signaling pathways as well as the description of surface marker phenotypes specific to T cell anergy.

Structural basis of plasticity in T cell receptor recognition of a self peptide-MHC antigen

The T cell receptor (TCR) inherently has dual specificity. T cells must recognize self-antigens in the thymus during maturation and then discriminate between foreign pathogens in the periphery. A molecular basis for this cross-reactivity is elucidated by the crystal structure of the alloreactive 2C TCR bound to self peptide-major histocompatibility complex (pMHC) antigen H-2Kb-dEV8 refined against anisotropic 3.0 angstrom resolution x-ray data. The interface between peptide and TCR exhibits extremely poor shape complementarity, and the TCR beta chain complementarity-determining region 3 (CDR3) has minimal interaction with the dEV8 peptide. Large conformational changes in three of the TCR CDR loops are induced upon binding, providing a mechanism of structural plasticity to accommodate a variety of different peptide antigens. Extensive TCR interaction with the pMHC alpha helices suggests a generalized orientation that is mediated by the Valpha domain of the TCR and rationalizes how TCRs can effectively "scan" different peptides bound within a large, low-affinity MHC structural framework for those that provide the slight additional kinetic stabilization required for signaling.

Characterization of the synovial T cell response to various recombinant Yersinia antigens in Yersinia enterocolitica-triggered reactive arthritis. Heat-shock protein 60 drives a major immune response

OBJECTIVE

In Yersinia enterocolitica-triggered reactive arthritis (Yersinia ReA), the synovial T cell response is primarily directed against bacterial components, which are mostly unknown. This study was performed to investigate the synovial proliferative T cell response to a panel of recombinant Yersinia antigens in patients with Yersinia ReA and in controls.

METHODS

Synovial fluid mononuclear cells (SFMC) were obtained from 4 patients with Yersinia ReA and from 14 patients with arthritides of different etiology. SFMC were stimulated with 5 recombinant Yersinia antigens (the 19-kd urease beta subunit, 13-kd ribosomal L23 protein, 32-kd ribosomal L2 protein, 18-kd outer membrane protein H, and Y. enterocolitica heat-shock protein 60 [hsp60]), and with human, Chlamydia trachomatis, and Borrelia burgdorferi hsp60. Three T cell clones specific for Y. enterocolitica hsp60 were generated from 1 patient with Yersinia ReA. Antigen-induced cytokine release was measured by enzyme-linked immunosorbent assay.

RESULTS

SFMC from all 4 patients with Yersinia ReA responded to each of the Yersinia antigens except the 13-kd protein. These antigens were also recognized by SFMC from a subgroup of patients with undifferentiated arthritis (n = 4), but not by SFMC from other patients with arthritis of different etiology (n = 10). Y. enterocolitica hsp60 induced the strongest proliferative response in all cases. Two types of hsp60-reactive T cell clones could be obtained. One clone responded to all hsp60 variants, including the human variant, and showed a type 2 T helper (Th2)-like cytokine-secretion pattern. In contrast, another clone with specificity for the bacterial hsp60 proteins, but not the human equivalent, reacted with a more Th1-like pattern.

CONCLUSION

In Y. enterocolitica-triggered ReA, at least 4 immunodominant T cell antigens exist, which might be used in lymphocyte proliferation assays to identify patients with Yersinia ReA. The hsp60 is a strong antigen, inducing both bacteria-specific and potentially autoreactive CD4+ T cells of both the Th1 and Th2 type.

Heteroclitic proliferative responses and changes in cytokine profile induced by altered peptides: implications for autoimmunity

Productive engagement of T cell receptors (TCRs) by cognate ligand (major histocompatibility complex plus peptide) leads to proliferation, differentiation, and the elaboration of effector functions. Altered peptides generated by single amino acid substitutions in the antigenic peptide have diverse effects on the outcome of the T cell response. We have generated an altered peptide (Q144) from an autoantigenic peptide of myelin proteolipid protein 139-151 by a single amino acid substitution (from tryptophan to glutamine) in the primary TCR contact at position 144 that is capable of inducing CD4(+) T cell responses in H-2(s) mice. By using a Q144-specific T cell clone (Q1.1B6), we see a hierarchy in T cell proliferation and cytokine production with various position 144 substituted peptides and have identified a peptide (L144) that hyperstimulates this T cell clone. In contrast to Q144, L144 induces maximal proliferation at 7 logs lower antigen concentration, induces greater cell death at higher antigen dose, and induces the secretion of cytokines not detected following stimulation with the cognate ligand. This heteroclitic T cell response associated with changes in cytokine profile was observed with several other T cell clones of different specificities. The L144 peptide also induces costimulation independent proliferation and cytokine production from the Q1.1B6 T cell clone. We describe this as a superagonist response. Such responses may have a role in the initiation of autoimmunity by promoting a proinflammatory environment following ligation of a cross-reactive TCR on autoreactive T cells.

Role of CD8 in aberrant function of cytotoxic T lymphocytes

Using H-2Kd-restricted photoprobe-specific cytotoxic T lymphocyte (CTL) clones, which permit assessment of T cell receptor (TCR)-ligand interactions by TCR photoaffinity labeling, we observed that the efficiency of antigen recognition by CTL was critically dependent on the half-life of TCR-ligand complexes. We show here that antigen recognition by CTL is essentially determined by the frequency of serial TCR engagement, except for very rapid dissociations, which resulted in aberrant TCR signaling and antagonism. Thus agonists that were efficiently recognized exhibited rapid TCR-ligand complex dissociation, and hence a high frequency of serial TCR engagement, whereas the opposite was true for weak agonists. Surprisingly, these differences were largely accounted for by the coreceptor CD8. While it was known that CD8 substantially decreases TCR-ligand complex dissociation, we observed in this study that this effect varied considerably among ligand variants, indicating that epitope modifications can alter the CD8 contribution to TCR-ligand binding, and hence the efficiency of antigen recognition by CTL.

Generation and functional characterization of anti-clonotype antibodies to human T-cell receptors

Monoclonal antibodies (mAb) directed against the clonotypic structure of the T-cell receptor (TCR) may be useful reagents in the study and therapy of T-cell-mediated diseases. In contrast to several reports concerning the generation of anti-clonotype mAb to mouse TCR, only very limited numbers of anti-clonotype mAb to human TCR have been described. So far, a suitable method for the generation of anti-clonotype mAb to a given TCR has not been available and in this report we describe a novel strategy for the generation of such mAb. Mice were immunized with intact human T-cells. Then. spleen cell populations were precleared from B-cells reactive to CD3 and the constant region of the TCR by adsorption to TCR/CD3 complexes derived from an irrelevant T-cell clone. Subsequently, clonotype-specific B-cells were selected with TCR/CD3 complexes from the T-cell clone of interest. The small number of B-cells resulting from this selection were clonally expanded in a B-cell culture system and then immortalized by mini-electrofusion. Ten clonotype-specific mAb were generated against a DRB1*0401-restricted T-cell clone recognizing an epitope of the human cartilage glycoprotein 39 (HC gp-39). All mAb immunoprecipitated a heterodimeric 85 kDa protein. Absolute specificity was demonstrated in a T-cell agglutination test with the T-cell clone of interest compared to a set of 16 defined, irrelevant T-cell clones or lines. In functional assays, the mAb were found to inhibit or block antigen-specific T-cell stimulation. In addition, crosslinked mAb were found to stimulate proliferation of the specific clone in the absence of antigen and antigen presenting cells (APC). Such mAb may have clinical relevance in deleting or modulating autoreactive T-cells in a clonotype-specific manner.

T-cell receptor structure and TCR complexes

The first crystal structures of intact T-cell receptors (TCRs) and their complexes with MHC peptide antigens (pMHC) were reported during the past year, along with those of a single-chain TCR Fv fragment and a beta-chain complexed with two different bacterial superantigens. These structures have shown the similarities and differences in the architecture of the antigen-binding regions of TCRs and antibodies, and how the TCR interacts with pMHC ligands as well as with superantigens.

Activation of autoreactive T cells by peptides from human pathogens

Activation of autoreactive T cells is a necessary-but not sufficient-step in the development of T cell mediated autoimmunity. Autoreactive T cells can be activated by viral and bacterial peptides that meet the structural requirements for MHC molecule binding and T cell receptor recognition. Due to the degenerate nature of MHC class II molecule binding motifs and a certain degree of flexibility in T cell receptor recognition, such microbial peptides have been found to be quite distinct in their primary sequence from the self-peptide they mimic.

Gene therapy with B7.1 and GM-CSF vaccines in a murine AML model

PURPOSE

Characterization of B7.1 and GM-CSF vaccines on the induction of anti-tumor immunity in a murine AML model.

MATERIALS AND METHODS

Primary AML cells were retrovirally transduced with the murine costimulatory molecule B7.1, a natural ligand for the T-cell receptors CD28 and CTLA-4, or the cytokine GM-CSF. Mice were vaccinated with irradiated AML cells expressing B7.1 or GM-CSF before or after inoculation of wild type AML cells.

RESULTS

Intravenous injection of irradiated B7.1 or GM-CSF expressing AML cells can provide long lasting systemic immunity against a subsequent challenge of wild type AML cells. Vaccination with irradiated B7.1 or GM-CSF expressing AML cells results in rejection of established leukemia when the vaccination occurs in the early stages of the disease. However, when the vaccines are administered > 2 weeks after leukemic inoculation, only mice which receive the GM-CSF vaccine are cured of leukemia.

CONCLUSIONS

These results suggest that tumor burden and vaccine efficiency are most likely to be the limiting factors in the curative potential of tumor vaccines. Novel approaches such as this experiment could provide improved therapeutic outcomes in patients with AML and other cancers.

The tetramer model: a new view of class II MHC molecules in antigenic presentation to T cells

Crystallographic studies suggest a plausible divalent interaction between T-cell receptor (TCR) and MHC class II molecules. In addition, biochemical data suggest that these divalent MHC molecules are preformed at the membrane of the antigen-presenting cell. The tetramer model is based on these preformed tetrameric class II molecules that can be loaded with identical or different peptides in their two grooves. This enables divalent class II molecules to deliver two different messages to T cell: 1) a two-peptide message, in which the tetramer with two identical peptides is able to cross-link two TCRs triggering full activation of a T cell. At the thymic level we propose that this message induces negative selection; or 2) a one-peptide message: only one of the peptides loaded in the class II tetramer is able to interact with that TCR. This message would be involved in triggering partial activation phenomena in mature lymphocytes, whereas in thymocytes this message would mediate positive selection. Since high concentrations of a peptide would favor the load of tetramers with identical peptides, the tetramer could therefore be viewed as a quantitative-qualitative transducer that would trigger different responses depending on the concentration of antigenic peptides.

A T cell receptor (TCR) antagonist competitively inhibits serial TCR triggering by low-affinity ligands, but does not affect triggering by high-affinity anti-CD3 antibodies

It has been demonstrated that modified peptides which fail to induce detectable T cell responses can act as T cell receptor (TCR) antagonists when presented together with agonist by the same antigen-presenting cell (APC). We report that a TCR antagonist competitively inhibits TCR triggering induced by low-affinity ligands such as agonistic peptides or bacterial superantigens. However, the same antagonist cannot inhibit TCR triggering and T cell activation induced by high-affinity anti-CD3 antibodies that engage most TCR at once. These results indicate that TCR antagonists inhibit T cell responses by interfering with the ongoing process of serial triggering, rather than by delivering an inhibitory signal to T cells.

CD4 augments the response of a T cell to agonist but not to antagonist ligands

The recognition of peptide variants by the T cell receptor (TCR) has revealed a wide range of possible responses. Here, using a series of CD4+ and CD4- variants of the same T cell hybridoma, we find that while the expression of CD4 converts weak agonists into full agonists, none of the antagonist peptides are efficiently recognized as agonists. Furthermore, in antagonist assays, little difference can be seen in the response of CD4+ and CD4- T cells. Together with previous work showing a marked difference in stability between TCR binding to agonist versus antagonist ligands, these data suggest that CD4 engagement occurs after a TCR-peptide/MHC complex has formed and that it requires a certain minimal half-life of the ternary complex to be fully engaged in signaling.

Topology of T cell receptor-peptide/class I MHC interaction defined by charge reversal complementation and functional analysis

The molecular interactions between the CD8 co-receptor dependent N15 and N26 T cell receptors (TCRs) and their common ligand, the vesicular stomatitis virus octapeptide (VSV8) bound to H-2Kb, were studied to define the docking orientation(s) of MHC class I restricted TCRs during immune recognition. Guided by the molecular surfaces of the crystallographically defined peptide/MHC and modeled TCRs, a series of mutations in exposed residues likely contacting the TCR ligand were analyzed for their ability to alter peptide-triggered IL-2 production in T cell transfectants. Critical residues which diminished antigen recognition by 1000 to 10,000-fold in molar terms were identified in both N15 Valpha (alphaE94A or alphaE94R, Y98A and K99) and Vbeta (betaR96A, betaW97A and betaD99A) CDR3 loops. Mutational analysis indicated that the Rp1 residue of VSV8 is critical for antigen recognition of N15 TCR, but R62 of H-2Kb is less critical. More importantly, the alphaE94R mutant could be fully complemented by a reciprocal charge reversal at Kb R62 (R62E). This result suggests a direct interaction between N15 TCR Valpha E94R and Kb R62E residues. As Rp1 of VSV8 is adjacent to R62 in the VSV8/Kb complex and essential for T cell activation, this orientation implies that the N15 Valpha CDR3 loop interacts with the N-terminal residues of VSV8 with the Valpha domain docking to the Kb alpha2 helix while the N15 Vbeta CDR3 loop interacts with the more C-terminal peptide residues and the Vbeta domain overlies the Kb alpha1 helix. An equivalent orientation is suggested for N26, a second VSV8/Kb specific TCR. Given that genetic analysis of two different class II MHC-restricted TCRs and two crystallographic studies of class I restricted TCRs offers a similar overall orientation of V domains relative to alpha-helices, these data raise the possibility of a common docking mode between TCRs and their ligands regardless of MHC restriction.

Fractionated dosing of cyclophosphamide for establishing long-lasting skin allograft survival, stable mixed chimerism, and intrathymic clonal deletion in mice primed with allogeneic spleen cells

BACKGROUND

Injection of allo-spleen cells (SC) followed by a single dose of cyclophosphamide (CP) can induce tolerance of tumor and/or skin allografts in mice. To minimize the damage caused by CP, fractionation of CP that can establish long-lasting skin graft survival, stable mixed chimerism, and intrathymic clonal deletion in the host was investigated in the present study.

METHODS

Allo-SC (10(8)) were given intravenously on day 0. CP at 200 mg/kg was given intraperitoneally on day 2 in a single dose (CP 200x1 group). CP at 100, 66, 50, 40, and 33 mg/kg was given daily from day 1 through days 2, 3, 4, 5 and 6, respectively, in the fractionated doses (CP 100x2, 66x3, 50x4, 40x5, and 33x6 groups; total dose=200 mg/kg). Allografting was performed on day 14.

RESULTS

In a fully allogeneic combination of C57BL/6 (H2b)-->AKR (H2k, Mls-1a), an EL-4 tumor (H2b) was specifically accepted to kill the AKR mice in all of the SC+CP 200x1, 100x2, 66x3, 50x4, 40x5, and 33x6 groups (n=6), but C57BL/6 skin graft survival was not prolonged in any of the tumor-tolerant groups. In an H2-identical combination of AKR-->C3H (H2k, Mls-1b), AKR skin graft survival was prolonged remarkably (80-90 days) in the SC+CP 200x1, 100x2, and 66x3 groups (n=5-11), but was prolonged moderately (20-60 days) in the SC+CP 50x4 and 40x5 groups. In both of the SC+CP 200x1 and 66x3 groups in the AKR-->C3H combination, mixed chimerism was maintained for as long as 100 days after tolerance induction in both the spleen and thymus, associated with intrathymic clonal deletion of Vbeta6+ T cells. The decreases in leukocyte count, hemoglobin level, spleen weight, SC count, and body weight were significantly smaller in the SC+CP 66x3 group than in the SC+CP 200x1 group.

CONCLUSIONS

Fractionated CP is effective in ameliorating the compromised state induced by a single dose of CP. To induce a long-lasting skin allograft survival associated with stable mixed chimerism and intrathymic clonal deletion in an H2-identical combination, 200 mg/kg of CP can be divided into three or fewer fractions.

Prevention of Graft-Versus-Host Disease in Mice Using a Suicide Gene Expressed in T Lymphocytes

Alloreactive T cells present in a bone marrow transplant are responsible for graft-versus-host disease (GVHD), but their depletion is associated with impaired engraftment, immunosuppression, and loss of the graft-versus-leukemia effect. We developed a therapeutic strategy against GVHD based on the selective destruction of these alloreactive T cells, while preserving a competent T-cell pool of donor origin. We generated transgenic mice expressing in their T lymphocytes the Herpes simplex type 1 thymidine kinase (TK) suicide gene that allows the destruction of dividing T cells by a ganciclovir treatment. T cells expressing the TK transgene were used to generate GVHD in irradiated bone marrow grafted mice. We show that a short 7-day ganciclovir treatment, initiated at the time of bone marrow transplantation, efficiently prevented GVHD in mice receiving TK-expressing T cells. These mice were healthy and had a normal survival. They maintained a T-cell pool of donor origin that responded normally to in vitro stimulation with mitogens or third party alloantigens, but were tolerant to recipient alloantigens. Our experimental system provides the proof of concept for a therapeutic strategy of GVHD prevention using genetically engineered T cells.

Localization of cryptic tolerogenic epitopes in the alpha1-helical region of the RT1.Au alloantigen

BACKGROUND

Transplantation tolerance is induced by perioperative administration of host class I major histocompatibility complex proteins bearing donor-type amino acid (a.a.) epitopes substituted for native residues. Herein we demonstrate that two cryptic tolerogenic a.a. epitopes are localized in the alpha1-helical region of the rat RT1.Au class I major histocompatibility complex alloantigen.

METHODS

Three allochimeric proteins were produced by superimposing the nucleotides encoding donor-type RT1.Au a.a. onto the host RT1.Aa backbone using the polymerase chain reaction-based method of gene splicing with overlap extension. We substituted nucleotide sequences encoding all nine (Arg62, Glu63, Gln65, Gly66, Gly69, His70, Val73, Asn74, and Asn77; alpha1h u62-77-RT1.Aa), the first four (Arg62, Glu63, Gln65, and Gly69; alpha1h u62-69-RT1.Aa), or the last four (His70, Val73, Asn74, and Asn77; alpha1h u70-77-RT1.Aa) alpha1-helical RT1.Au polymorphic a.a. in RT1.Aa cDNA. A baculovirus/Spodoptera frugiperda (Sf9) expression system was harnessed for production of the proteins.

RESULTS

Untreated ACI (RT1a) rats reject Wistar-Furth (WF; RT1u) heart allografts at a mean survival time of 8.9+/-1.0 days. A single portal vein injection of 10 microg of alpha1h u62-77-RT1.Aa protein had no effect on the survival of WF heart allografts (10.5+/-0.6 days) in ACI hosts. Interestingly, portal vein administration of 10 microg of alpha1h u70-77-RT1.Aa induced transplantation tolerance toward WF grafts in four of six untreated ACI recipients (>100 days; P<0.01). In contrast, 10 microg of alpha1h u62-69-RT1.Aa only prolonged the survival of WF heart allografts in ACI hosts (14.0+/-0.8 days; P<0.01). However, when combined with a 7-day course of cyclosporine (4.0 mg/kg; oral gavage), alpha1h u62-69-RT1.Aa induced tolerance toward WF allografts in five of seven ACI recipients (>170 days; P<0.01). Long-term survival of WF grafts was not achieved when a 7-day course of cyclosporine was administered alone (14.3+/-3.0 days) or with 10 microg of alpha1h u62-77-RT1.Aa (14.6+/-0.6 days; NS).

CONCLUSIONS

These findings suggest that the use of allochimeric proteins may provide a novel approach to the induction of tolerance.

Kinetics and extent of T cell activation as measured with the calcium signal

We have characterized the calcium response of a peptide-major histocompatibility complex (MHC)-specific CD4(+) T lymphocyte line at the single cell level using a variety of ligands, alone and in combination. We are able to distinguish four general patterns of intracellular calcium elevation, with only the most robust correlating with T cell proliferation. Whereas all three antagonist peptides tested reduce the calcium response to an agonist ligand, two give very different calcium release patterns and the third gives none at all, arguing that (a) antagonism does not require calcium release and (b) it involves interactions that are more T cell receptor proximal. We have also measured the time between the first T cell-antigen-presenting cell contact and the onset of the calcium signal. The duration of this delay correlates with the strength of the stimulus, with stronger stimuli giving a more rapid response. The dose dependence of this delay suggests that the rate-limiting step in triggering the calcium response is not the clustering of peptide-MHC complexes on the cell surface but more likely involves the accumulation of some intracellular molecule or complex with a half-life of a few minutes.

CD8+ T Cells Activated During the Course of Murine Acute Myelogenous Leukemia Elicit Therapeutic Responses to Late B7 Vaccines After Cytoreductive Treatment

We have previously shown in a murine acute myelogenous leukemia (AML) model that leukemic mice can be cured with a B7 vaccine if immunized early in the disease and that CD8+ T cells are necessary for tumor rejection. However, when B7 vaccine is administered 2 weeks after leukemia inoculation, the effect is only prolonged survival, ending in death virtually of all the mice. To distinguish between tumor kinetics and tumor-induced immunosuppression as potential mechanisms eliminating the therapeutic potential of late B7 vaccines, we performed in vitro T-cell studies during leukemia progression and in vivo studies on the clinical outcome of late B7 vaccines in combination with prior cytoreductive chemotherapy. Our results show that CD8+ T cells from leukemic mice 1 and 2 weeks after leukemia inoculation proliferate more vigorously in response to in vitro activation than cells from normal mice and produce Th1-type cytokines interleukin-2 and interferon-γ. Cytotoxic T lymphocyte (CTL) assays demonstrate that cells from week-2 vaccinated mice (which succumb to their leukemia), surprisingly develop a stronger CTL activity than cells from week-1 vaccinated mice (which reject their leukemia). Finally, the combination of late chemotherapy and late B7 vaccine administration can cure only 20% of leukemic mice, whereas early chemotherapy and the same late B7 vaccine administration cure 100% of leukemic mice. These results demonstrate that in murine AML tumor growth does not induce T-cell anergy or a Th2 cytokine profile and suggest that tumor growth is most likely to be the limiting factor in the curative potential of late B7 vaccines.

Different nuclear signals are activated by the B cell receptor during positive versus negative signaling

It is not known how immunogenic versus tolerogenic cellular responses are signaled by receptors such as the B cell antigen receptor (BCR). Here we compare BCR signaling in naive cells that respond positively to foreign antigen and self-tolerant cells that respond negatively to self-antigen. In naive cells, foreign antigen triggered a large biphasic calcium response and activated nuclear signals through NF-AT, NF-kappa B, JNK, and ERK/pp90rsk. In tolerant B cells, self-antigen stimulated low calcium oscillations and activated NF-AT and ERK/pp90rsk but not NF-kappa B or JNK. Self-reactive B cells lacking the phosphatase CD45 did not exhibit calcium oscillations or ERK/pp90rsk activation, nor did they repond negatively to self-antigen. These data reveal striking biochemical differences in BCR signaling to the nucleus during positive selection by foreign antigens and negative selection by self-antigens.

Effects of epitope modification on T cell receptor-ligand binding and antigen recognition by seven H-2Kd-restricted cytotoxic T lymphocyte clones specific for a photoreactive peptide derivative

We tested for antigen recognition and T cell receptor (TCR)-ligand binding 12 peptide derivative variants on seven H-2Kd-restricted cytotoxic T lymphocytes (CTL) clones specific for a bifunctional photoreactive derivative of the Plasmodium berghei circumsporozoite peptide 252-260 (SYIPSAEKI). The derivative contained iodo-4-azidosalicylic acid in place of PbCS S-252 and 4-azidobenzoic acid on PbCS K-259. Selective photoactivation of the N-terminal photoreactive group allowed crosslinking to Kd molecules and photoactivation of the orthogonal group to TCR. TCR photoaffinity labeling with covalent Kd-peptide derivative complexes allowed direct assessment of TCR-ligand binding on living CTL. In most cases (over 80%) cytotoxicity (chromium release) and TCR-ligand binding differed by less than fivefold. The exceptions included (a) partial TCR agonists (8 cases), for which antigen recognition was five-tenfold less efficient than TCR-ligand binding, (b) TCR antagonists (2 cases), which were not recognized and capable of inhibiting recognition of the wild-type conjugate, (c) heteroclitic agonists (2 cases), for which antigen recognition was more efficient than TCR-ligand binding, and (d) one partial TCR agonist, which activated only Fas (C1)95), but not perforin/granzyme-mediated cytotoxicity. There was no correlation between these divergences and the avidity of TCR-ligand binding, indicating that other factors than binding avidity determine the nature of the CTL response. An unexpected and novel finding was that CD8-dependent clones clearly incline more to TCR antagonism than CD8-independent ones. As there was no correlation between CD8 dependence and the avidity of TCR-ligand binding, the possibility is suggested that CD8 plays a critical role in aberrant CTL function.

On the mechanisms of immunodominance in cytotoxic T lymphocyte responses to minor histocompatibility antigens

Although there are numerous minor histocompatibility antigens (MiHA), T cell responses leading to graft-versus-host (GVH) and graft-versus-tumor effects involve only a small number of immunodominant MiHA. The goal of the present study was to analyze at the cellular and molecular levels the mechanisms responsible for MiHA immunodominance. Cytotoxic T lymphocytes (CTL) generated in eight combinations of H2b strains of mice were tested against syngeneic targets sensitized with HPLC-fractionated peptides eluted from immunizing cells. The number of dominant MiHA was found to range from as little as two up to ten depending on the strain combination used. The nature of dominant MiHA was influenced by both the antigen profile of the antigen-presenting cells (APC) and the repertoire of responding CTL. When C57BL/6 dominant MiHA (B6dom) and H-Y were presented on separate APC, they showed similar immunogenicity. In contrast, when they were presented on the same APC, B6dom MiHA totally dominated H-Y. B6dom MiHA did not suppress anti-H-Y responses by acting as T cell receptor antagonists for anti-H-Y CTL, nor were anti-B6dom CTL precursors more abundant than anti-H-Y CTL precursors. Dominance resulted from competition for the APC surface between anti-B6dom and anti-H-Y CTL; the crucial difference between the dominant and the dominated MiHA appears to depend on the differential avidity of their respective CTL for APC. The only B6dom epitope thus far identified is the nonapeptide AAPDNRETF presented by H2-D(b). We found that compared with other known D(b)-binding peptides, AAPDNRETF is expressed at very high levels on the cell surface, binds to the D(b) molecule with very high affinity, and dissociates very slowly from its presenting class I molecule. These data indicate that one cannot predict which MiHA will be dominant or dominated based simply on their respective immunogenicity when presented on separate APC. Indeed, the avidity of T cell/APC interactions appears to determine which antigen(s) will trigger T cell responses when numerous epitopes are presented by the same APC.

Biophysical and structural studies of TCRs and ligands: implications for T cell signaling

The availability of soluble alphabeta TCRs and the individual chains has now made it possible to carry out structural studies of these molecules and analyze their molecular interactions with peptide-MHC ligands. Recent X-ray crystallographic structures of TCR alpha and beta chains have finally established their structural similarity with the lg molecules. Kinetic measurements of the interaction between TCRs and their ligands have provided strong evidence in favour of an affinity/avidity model for T cell activation in the periphery as well as during development in the thymus.

Antigen receptor-induced apoptosis of human germinal center B cells is targeted to a centrocytic subset

The outcome of the signals transduced through the B cell antigen receptor (BCR) depends both on their maturational stage and on the extent of receptor cross-linking. It is established that the BCR-mediated apoptosis of immature B cells represents an important mechanism for tolerance induction in the pre-immune B cell compartment. We show here that mature germinal center (GC) B cells can re-acquire sensitivity to BCR-induced cell death following CD40 ligation. In contrast, neither virgin nor memory B cells become susceptible to antigen receptor-triggered apoptosis upon CD40 stimulation, suggesting that this phenomenon may play a role in the shaping of the mature B cell repertoire in GC. Our data reveal that the death signal evoked through the BCR does not involve the Fcgamma receptors, does not operate through the Fas/Fas ligand system, and can be blocked by interleukin-4. Finally, we found that the acquisition of sensitivity to the death-promoting effect of anti-Ig antibodies in CD40-stimulated GC B cell cultures correlates with the induction of a centrocytic phenotype. We propose that negative regulatory signals via the BCR may delete somatically mutated centrocytes with self-reactivity.

The efficiency of CD4 recruitment to ligand-engaged TCR controls the agonist/partial agonist properties of peptide-MHC molecule ligands

One hypothesis seeking to explain the signaling and biological properties of T cell receptor for antigen (TCR) partial agonists and antagonists is the coreceptor density/kinetic model, which proposes that the pharmacologic behavior of a TCR ligand is largely determined by the relative rates of (a) dissociation ofligand from an engaged TCR and (b) recruitment oflck-linked coreceptors to this ligand-engaged receptor. Using several approaches to prevent or reduce the association of CD4 with occupied TCR, we demonstrate that consistent with this hypothesis, the biological and biochemical consequence of limiting this interaction is to convert typical agonists into partial agonist stimuli. Thus, adding anti-CD4 antibody to T cells recognizing a wild-type peptide-MHC class II ligand leads to disproportionate inhibition of interleukin-2 (IL-2) relative to IL-3 production, the same pattern seen using a TCR partial agonist/antagonist. In addition, T cells exposed to wild-type ligand in the presence of anti-CD4 antibodies show a pattern of TCR signaling resembling that seen using partial agonists, with predominant accumulation of the p21 tyrosine-phosphorylated form of TCR-zeta, reduced tyrosine phosphorylation of CD3epsilon, and no detectable phosphorylation of ZAP-70. Similar results are obtained when the wild-type ligand is presented by mutant class II MHC molecules unable to bind CD4. Likewise, antibody coligation of CD3 and CD4 results in an agonist-like phosphorylation pattern, whereas bivalent engagement of CD3 alone gives a partial agonist-like pattern. Finally, in accord with data showing that partial agonists often induce T cell anergy, CD4 blockade during antigen exposure renders cloned T cells unable to produce IL-2 upon restimulation. These results demonstrate that the biochemical and functional responses to variant TCR ligands with partial agonist properties can be largely reproduced by inhibiting recruitment of CD4 to a TCR binding a wild-type ligand, consistent with the idea that the relative rates of TCR-ligand disengagement and of association of engaged TCR with CD4 may play a key role in determining the pharmacologic properties of peptide-MHC molecule ligands. Beyond this insight into signaling through the TCR, these results have implications for models of thymocyte selection and the use of anti-coreceptor antibodies in vivo for the establishment ofimmunological tolerance.

Functional inactivation in the whole population of human V gamma 9/V delta 2 T lymphocytes induced by a nonpeptidic antagonist

Nonpeptidic compounds stimulate human T cells bearing the TCR-gamma delta in the absence of major histocompatibility complex restriction. We report that one of these ligands, 2,3-diphosphoglyceric acid (DPG), which induces expansion of V gamma 9/V delta T cells ex vivo, antagonizes the same cell population after repetitive activation. Stimulation with DPG results in partial early protein tyrosine phosphorylation and a prolonged, but reversible, state of unresponsiveness to agonist ligands in V gamma 9/V delta 2, but not in other T cells. These findings show that TCR antagonism is a general phenomenon of T cells. However, in contrast to the clonal specificity of altered peptides antagonizing alpha beta T cells, all the tested V gamma 9/V delta 2 polyclonal cell lines and clones become unresponsive, a fact that may be relevant for the regulation of their response in vivo.

Differential signaling by lymphocyte antigen receptors

Studies performed during the past several years make plain that ligand occupancy of antigen receptors need not necessarily provoke identical responses in all instances. For example, ligation of antigen receptors may stimulate a proliferative response, induce a state of unresponsiveness to subsequent stimulation (anergy), or induce apoptosis. How does a single type of transmembrane receptor induce these very heterogeneous cellular responses? In the following pages, we outline evidence supporting the view that the nature of the ligand/receptor interaction directs the physical recruitment of signaling pathways differentially inside the lymphocyte and hence defines the nature of the subsequent immune response. We begin by providing a functional categorization of antigen receptor components, considering the ways in which these components interact with the known set of signal transduction pathways, and then review the evidence suggesting that differential signaling through the TCR is achieved by qualitative differences in the effector pathways recruited by TCR, perhaps reflecting the time required to bring complicated signal transduction elements into proximity within the cell. The time-constant of the interaction between antigen and receptor in this way determines, at least in part, the nature of the resulting response. Finally, although our review focuses substantially on T cell receptor signaling, we have included a less detailed description of B cell receptor signaling as well, simply to emphasize the parallels that exist in these two closely related systems.

Antigen-specific inhibition of CD4+ T-cell responses to beta-lactoglobulin by its single amino acid-substituted mutant form through T-cell receptor antagonism

T-cell responses can be antagonized by some single amino acid-substituted analogs of a peptide ligand for T-cell receptors (TCR), and these are called TCR antagonists. In this study, we addressed the question of whether TCR antagonism can be elicited by a whole protein antigen carrying a mutated T-cell determinant region corresponding to a TCR antagonist peptide. To clarify this, we examined the ability of a single amino acid-substituted mutant form of bovine beta-lactoglobulin (beta-Lg) to inhibit three CD4+ T-cell clones recognizing a peptide corresponding to an immunodominant determinant region 119-133 of beta-Lg (p119-133). First, we identified pD129A, an analog of p119-133 with a substitution of Ala for 129Asp, as an antagonist which can inhibit the response of two of the three T-cell clones. Then, using a yeast expression system, we prepared a mutant beta-Lg (mutD129A) with the same substitution of Ala for 129Asp as that in pD129A. This mutant protein could inhibit the proliferation of the two T-cell clones in a manner similar to the effect of pD129A. From these results we can demonstrate that TCR antagonism can be elicited by peptides naturally processed from a single-substituted mutant protein as well as by the corresponding peptides added exogenously.

Killer cell inhibitory receptor interactions with HLA class I molecules: implications for alloreactivity and transplantation

Human killer cell inhibitory receptors (KIR) are novel members of the immunoglobulin superfamily of cell surface glycoproteins, which are expressed by lymphocytes with natural killers (NK) and cytotoxic T-cell (CTL) phenotypes. These receptors have specificity for relatively conserved epitopes of HLA-A, -B, and -C class I antigens. Recent studies have identified KIR as being involved in the transmission of negative, inhibitory signaling events to the cytotoxic cell which prevent or diminish target cell lysis. KIR are thus likely to play an important role in the responses of alloreactive NK cells and CTL to allogeneic HLA antigens. In this article, we review the known structural and functional characteristics of KIR, suggest a possible mechanism for the transmission of intracellular negative signaling by these receptors, and discuss the relevance of KIR function and HLA specificity to the clinical transplantation of allogeneic tissues.

Exploration of requirements for peptidomimetic immune recognition. Antigenic and immunogenic properties of reduced peptide bond pseudopeptide analogues of a histone hexapeptide

We present a detailed analysis of the antigenic and immunogenic properties of a series of very stable peptidomimetics of a model hexapeptide corresponding to the C-terminal residues 130-135 of histone H3. Five pseudopeptide analogues of the natural sequence IRGERA were synthesized by systematically replacing, in each analogue, one peptide bond at a time by a reduced peptide bond Psi(CH2-NH). Three important features of the resulting analogues were examined. First, the analogues were tested in a biosensor system for their ability to bind monoclonal antibodies generated against the parent natural peptide, and their kinetic rate constants were measured. The results show that reduced peptide bond analogues can very efficiently mimic the parent peptide. The position of reduced bonds which were deleterious for the binding was found to depend on the antibody tested, and one monoclonal antibody recognized all five analogues. The equilibrium affinity constant toward reduced peptide bond analogues of four antibodies of IgG1 isotype induced against the parent hexapeptide was higher (up to 670 times) with certain analogues than toward the homologous peptide. Second, immunogenic properties of the five analogues were studied, and it was found that polyclonal antibodies induced against analogues in which Psi(CH2-NH) bonds were introduced between residues 130-131, 131-132, and 132-133 (R1-R2, R2-R3, and R3-R4) cross-reacted strongly with the cognate protein H3. Third, we tested the protease resistance of analogues. Altogether, the results provide a strong support for the potent applicability of reduced peptide bond pseudopeptides as components of synthetic vaccines and open a new field for the development of immunomodulatory agents.

T cell selection and autoimmunity: flexibility and tuning

Peptide-MHC interactions govern the fate of T cells in the thymus and the peripheral T cell repertoire. Recent progress has involved investigating how different peptides influence T cell selection and mature T cell function and the subsequent implications for tolerance and autoimmunity.

Immunologic mechanisms of cutaneous drug reactions

Idiosyncratic reactions (type B) are a major complication of drug therapy, because they are related to both the drug and to individual factors in the host. In comparison with other organs, the skin is quite frequently a target of allergic reactions, which are mainly elicited by small molecular weight compounds. This is the case in allergic contact dermatitis as well as in drug allergic reactions. In contrast to allergic contact dermatitis however, drug-induced hypersensitivity reactions of the skin have enormous variability with regard to their pathophysiological pathways, clinical signs of symptoms, severity, and the drugs which can elicit these reactions. Allergic reactions are mediated either by specific antibodies or a cellular immunocompetent immune response. About 25% to 30% of type B reactions are estimated to be allergic drug reactions, which are classified by the latency period between the ingestion of the responsible allergen and the onset of clinical symptoms. Studying the mechanisms of these hypersensitivity reactions improves our understanding of these diseases in general, and shows the importance of the skin as a signaling organ in these reactions.

Structural basis for T cell recognition of altered peptide ligands: a single T cell receptor can productively recognize a large continuum of related ligands

T cells recognize short linear peptides bound to major histocompatibility complex (MHC)-encoded molecules. Subtle molecular changes in peptide antigens produce altered peptide ligands (APLs), which induce different T cell responses from those induced by the antigenic ligand. A molecular basis for how these slight molecular variations lead to such different consequences for the T cell has not been described. To address this issue, we have made amino acid substitutions at the primary T cell receptor (TCR) contact residue of the murine hemoglobin determinant, Hb(64-76)/I-Ek and produced 12 peptides that interact with the TCR of the T cell clone 3.L2. The 3.L2 T cell responds to these peptides, which vary 1 million-fold in their activity, and enables them to be ranked according to their relative ability to signal through the 3.L2 TCR. Such a ranking reveals that the ability of the 3.L2 T cell to respond to these peptides depends on how well the structure of the side chain at the primary TCR contact site mimics that of the Asn residue present in the antigenic ligand. The reactivity of the 3.L2 T cell also depends on an MHC contact residue that is next to the primary TCR contact residue, suggesting that conformation of the Asn side chain is also important. By using nonnatural amino acids at a TCR contact residue, we have demonstrated that APLs can be rationally designed based on structure. These data are consistent with a model in which the affinity of a peptide-MHC complex for the TCR determines how the T cell will respond.

A TCR binds to antagonist ligands with lower affinities and faster dissociation rates than to agonists

T lymphocyte activation is mediated by the interaction of specific TCR with antigenic peptides bound to MHC molecules. Single amino acid substitutions are often capable of changing the effect of a peptide from stimulatory to antagonistic. Using surface plasmon resonance, we have analyzed the interaction between a complex consisting of variants of the MCC peptide bound to a mouse class II MHC (Ek) and a specific TCR. Using both an improved direct binding method as well as a novel inhibition assay, we show that the affinities of three different antagonist peptide-Ek complexes are approximately 10-50 times lower than that of the wildtype MCC-Ek complex for the TCR, largely due to an increased off-rate. These results suggest that the biological effects of peptide antagonists and partial agonists may be largely based on kinetic parameters.