Interaction of an immunodominant epitope with Ia molecules in T-cell activation

A Plausible Framework Reveals Potential Similarities in the Regulation of Immunity against Some Cancers and Some Infectious Agents: Implications for Prevention and Treatment

Different frameworks, which are currently employed to understand how immune responses are regulated, can account for different observations reported in the classical literature. I have argued that the predominant frameworks, employed over the last two/three decades to analyze the circumstances that determine whether an immune response is generated or this potential is ablated, and that determine the class of immunity an antigen induces, are inconsistent with diverse classical observations. These observations are "paradoxical" within the context of these frameworks and, consequently, tend to be ignored by most contemporary researchers. One such observation is that low and high doses of diverse types of antigen result, respectively, in cell-mediated and IgG antibody responses. I suggest these paradoxes render these frameworks implausible. An alternative framework, The Threshold Hypothesis, accounts for the paradoxical observations. Some frameworks are judged more plausible when found to be valuable in understanding findings in fields beyond their original compass. I explore here how the Threshold Hypothesis, initially based on studies with chemically well-defined and "simple antigens", most often a purified protein, can nevertheless shed light on diverse classical and more recent observations in the fields of immunity against cancer and against infectious agents, thus revealing common, immune mechanisms. Most cancers and some pathogens are best contained by cell-mediated immunity. The success of the Threshold Hypothesis has encouraged me to employ it as a basis for proposing strategies to prevent and to treat cancer and those infectious diseases caused by pathogens best contained by a cell-mediated attack.

Epitope Mapping of Lysozyme Using the Chinese Egg-Allergic Sera at Both Pooled and Individual Levels

To accurately map the B-cell linear epitopes of lysozyme (LYS) in eggs, five bioinformatics tools were first used to obtain the mimotopes. Afterward, based on the Chinese egg-allergic sera samples screened by the indirect enzyme-linked immunosorbent, the epitopes possessing the capability of binding to IgG/IgE were mapped at both pooled and individual levels by using overlapping peptides covering the complete amino acid sequence of LYS. Six B-cell linear epitopes and two dominant B-cell linear epitopes that could bind to LYS-sIgG were mapped for the first time. Seven IgE-binding epitopes and three dominant IgE-binding epitopes were also obtained. Furthermore, AA31-34 and AA88-91 were the shared dominant epitopes of LYS-sIgG and LYS-sIgE at pooled and individual levels. Overall, the mapped B-cell linear epitopes filled in the gaps in the study of LYS epitopes, and the results may provide theoretical support for the following immunotherapy of egg allergy.

Adsorption of protein antigen to the cationic liposome adjuvant CAF®01 is required for induction of Th1 and Th17 responses but not for antibody induction

The degree of antigen adsorption to adjuvants in subunit vaccines may significantly influence the immune responses they induce upon vaccination. Commonly used approaches for studying how the level of adsorption affects the induction of antigen-specific immune responses include (i) using adjuvants with different abilities to adsorb antigens, and (ii) comparing different antigens selected based on their ability to adsorb to the adjuvant. A weakness of these approaches is that not only the antigen adsorption level is varied, but also other important functional factors such as adjuvant composition and/or the B/T cell epitopes, which may affect immunogenicity. Hence, we investigated how changing the adsorption capabilities of a single antigen to an adjuvant influenced the vaccine-induced immune responses. The model antigen lysozyme, which displays a positive net charge at physiological pH due to an isoelectric point (pI) of 11, was succinylated to different extents, resulting in a reduction of the pI value to 4.4–5.9, depending on the degree of succinylation. A pronounced inverse correlation was found between the pI value of the succinylated lysozyme analogues and the degree of adsorption to a cationic liposomal adjuvant consisting of dimethyldioctadecylammonium bromide (DDA) and trehalose dibehenate (TDB) (CAF®01). Furthermore, increased adsorption to this adjuvant correlated directly with the magnitude of lysozyme-specific Th1/Th17 immune responses induced by the vaccine in mice, while there was an inverse correlation with antibody induction. However, high lysozyme-specific antibody titers were induced with an increased antigen dose, even upon vaccination with a strongly adsorbed succinylated lysozyme analogue. Hence, these data illustrate that the degree of lysozyme adsorption to CAF®01 strongly affects the quality of the resulting immune responses.

The peptide specificity of the endogenous T follicular helper cell repertoire generated after protein immunization

T follicular helper (Tfh) cells potentiate high-affinity, class-switched antibody responses, the predominant correlate of protection from vaccines. Despite intense interest in understanding both the generation and effector functions of this lineage, little is known about the epitope specificity of Tfh cells generated during polyclonal responses. To date, studies of peptide-specific Tfh cells have relied on either the transfer of TcR transgenic cells or use of peptide∶MHC class II tetramers and antibodies to stain TcR and follow limited peptide specificities. In order to comprehensively evaluate polyclonal responses generated from the natural endogenous TcR repertoire, we developed a sorting strategy to separate Tfh cells from non-Tfh cells and found that their epitope-specific responses could be tracked with cytokine-specific ELISPOT assays. The immunodominance hierarchies of Tfh and non-Tfh cells generated in response to immunization with several unrelated protein antigens were remarkably similar. Additionally, increasing the kinetic stability of peptide-MHC class II complexes enhanced the priming of both Tfh and conventional CD4 T cells. These findings may provide us with a strategy to rationally and selectively modulate epitope-specific Tfh responses. By understanding the parameters that control epitope-specific priming, vaccines may be tailored to enhance or focus Tfh responses to facilitate optimal B cell responses.

Direct demonstration of CD4 T cell cooperation in the primary in vivo generation of CD4 effector T cells

Many observations bear upon the cellular and molecular requirements for CD4 T cell activation. The interaction of CD4 T cells with dendritic cells (DC), central to the induction of most immune responses, is the most studied. However, leukocytes other than DC can dramatically affect the induction and differentiation of CD4 T cells into effector cells. We recently provided indirect evidence that in vivo CD4 T cooperation facilitates the activation of CD4 T cells. Here, we demonstrate that the activation of CD4 T cells, specific for the hen egg lysozyme (HEL)(105) (-120) peptide, is optimally achieved when BALB/c mice are immunized with additional MHC class II-binding HEL peptides in incomplete Freund's adjuvant. This cooperation cannot be mimicked by the coadministration of LPS or of an agonistic antibody to CD40, at the time of immunization. In contrast, OX40-OX40L interactions are necessary for CD4 T cell cooperation in that an OX40 agonistic antibody can replace, and an OX40L-blocking antibody can abrogate, CD4 T cell cooperation in situations where such cooperation would otherwise enhance the activation of CD4 T cells.

Ligand binding but undetected functional response of FcR after their capture by T cells via trogocytosis

Intercellular transfer of cell surface proteins by trogocytosis is common and could affect T cell responses. Yet, the role of trogocytosis in T cell function is still elusive, and it is unknown whether a molecule, once captured by T cells, harbors the same biological properties as in donor APC. In this study, we showed that FcgammaR as well as the associated FcRgamma subunit could be detected at high levels on murine and human T cells after their intercellular transfer from FcgammaR-expressing APC. Capture of FcgammaR occurred during coculture of T cells with FcgammaR-expressing APC upon Ab- or Ag-mediated T cell stimulation. Once captured by T cells, FcgammaR were expressed in a conformation compatible with physiological function and conferred upon T cells the ability to bind immune complexes and to provision B cells with this source of Ag. However, we were unable to detect downstream signal or signaling-dependent function following the stimulation of FcgammaR captured by T cells, and biochemical studies suggested the improper integration of FcgammaR in the recipient T cell membrane. Thus, our study demonstrates that T cells capture FcgammaR that can efficiently exert ligand-binding activity, which, per se, could have functional consequences in T cell-B cell cooperation.

Abortive activation of CD4 T cell responses during competitive priming in vivo

Immunodominance refers to the highly selective peptide reactivity of T cells during an immune response. In this study, we tested the hypothesis that persistence of peptide:class II complexes is one key parameter that selects the final specificity of CD4 T cells. We found that low-stability peptide:class II complexes support the initial priming and expansion of CD4 T cells, but the expansion becomes strikingly aborted in the presence of competitive T cell responses to unrelated peptides. Our experiments revealed that for inhibition to occur, the competitive responses must be initiated by the same antigen presenting cell, and it is not because of competition for MHC binding. These studies not only provide an insight into the events that regulate competitive CD4 T cell priming in vivo, but also provide a previously undescribed conceptual framework to understand the parameters that select the final specificity of the T cell repertoire during pathogen or vaccine-induced immune responses.

Recent advances in the understanding of egg allergens: basic, industrial, and clinical perspectives

The emergence of egg allergy has had both industrial and clinical implications. In industrialized countries, egg allergy accounts for one of the most prevalent food hypersensitivities, especially in children. Atopic dermatitis represents the most common clinical manifestation in infancy; however, the range of clinical signs is broad and encompasses life-threatening anaphylaxis. The dominant egg allergens are proteins and are mainly present in the egg white, for example, ovalbumin, ovomucoid, ovotransferrin, and lysozyme. However, egg yolk also displays low-level allergenicity, for example, alpha-livetin. Strict avoidance of the offending food remains the most common recommendation for egg-allergic individuals. Nevertheless, the omnipresence of egg-derived components in prepackaged or prepared foods makes it difficult. Therefore, more efficient preventive approaches are investigated to protect consumers from inadvertent exposure and ensuing adverse reactions. On the one hand, commercial kits have become readily available that allow for the detection of egg contaminants at trace levels. On the other hand, attempts to produce hypoallergenic egg-containing products through food-processing techniques have met with promising results, but the approach is limited due to its potentially undesirable effects on the unique functional and sensory attributes of egg proteins. Therefore, the development of preventive or curative strategies for egg allergy remains strongly warranted. Pilot studies have suggested that oral immunotherapy (IT) with raw or cooked preparations of egg may represent a safe alternative, immediately available to allergic subjects, but remains applicable to only nonanaphylactic patients. Due to the limitations of conventional IT, novel forms of immunotherapy are sought based on information obtained from the molecular characterization of major egg allergens. In the past decade, promising approaches to the treatment and prevention of egg allergy have been explored and include, among others, the production of hypoallergenic recombinant egg proteins, the development of customized peptides, and bacterial-mediated immunotherapy. Nonspecific approaches have also been evaluated, and preliminary trials with the use of probiotic bacteria have yielded encouraging results. The current understanding of egg allergens offers novel approaches toward the making of food products safe for human consumption and the development of efficient immunotherapeutic strategies.

Structural parameterization and functional prediction of antigenic polypeptome sequences with biological activity through quantitative sequence-activity models (QSAM) by molecular electronegativity edge-distance vector (VMED)

Only from the primary structures of peptides, a new set of descriptors called the molecular electronegativity edge-distance vector (VMED) was proposed and applied to describing and characterizing the molecular structures of oligopeptides and polypeptides, based on the electronegativity of each atom or electronic charge index (ECI) of atomic clusters and the bonding distance between atom-pairs. Here, the molecular structures of antigenic polypeptides were well expressed in order to propose the automated technique for the computerized identification of helper T lymphocyte (Th) epitopes. Furthermore, a modified MED vector was proposed from the primary structures of polypeptides, based on the ECI and the relative bonding distance of the fundamental skeleton groups. The side-chains of each amino acid were here treated as a pseudo-atom. The developed VMED was easy to calculate and able to work. Some quantitative model was established for 28 immunogenic or antigenic polypeptides (AGPP) with 14 (1-14) A(d) and 14 other restricted activities assigned as "1"(+) and "0"(-), respectively. The latter comprised 6 A(b)(15-20), 3 A(k)(21-23), 2 E(k)(24-26), 2 H-2(k)(27 and 28) restricted sequences. Good results were obtained with 90% correct classification (only 2 wrong ones for 20 training samples) and 100% correct prediction (none wrong for 8 testing samples); while contrastively 100% correct classification (none wrong for 20 training samples) and 88% correct classification (1 wrong for 8 testing samples). Both stochastic samplings and cross validations were performed to demonstrate good performance. The described method may also be suitable for estimation and prediction of classes I and II for major histocompatibility antigen (MHC) epitope of human. It will be useful in immune identification and recognition of proteins and genes and in the design and development of subunit vaccines. Several quantitative structure activity relationship (QSAR) models were developed for various oligopeptides and polypeptides including 58 dipeptides and 31 pentapeptides with angiotensin converting enzyme (ACE) inhibition by multiple linear regression (MLR) method. In order to explain the ability to characterize molecular structure of polypeptides, a molecular modeling investigation on QSAR was performed for functional prediction of polypeptide sequences with antigenic activity and heptapeptide sequences with tachykinin activity through quantitative sequence-activity models (QSAMs) by the molecular electronegativity edge-distance vector (VMED). The results showed that VMED exhibited both excellent structural selectivity and good activity prediction. Moreover, the results showed that VMED behaved quite well for both QSAR and QSAM of poly-and oligopeptides, which exhibited both good estimation ability and prediction power, equal to or better than those reported in the previous references. Finally, a preliminary conclusion was drawn: both classical and modified MED vectors were very useful structural descriptors. Some suggestions were proposed for further studies on QSAR/QSAM of proteins in various fields.

Toward prediction of class II mouse major histocompatibility complex peptide binding affinity: in silico bioinformatic evaluation using partial least squares, a robust multivariate statistical technique

The accurate identification of T-cell epitopes remains a principal goal of bioinformatics within immunology. As the immunogenicity of peptide epitopes is dependent on their binding to major histocompatibility complex (MHC) molecules, the prediction of binding affinity is a prerequisite to the reliable prediction of epitopes. The iterative self-consistent (ISC) partial-least-squares (PLS)-based additive method is a recently developed bioinformatic approach for predicting class II peptide-MHC binding affinity. The ISC-PLS method overcomes many of the conceptual difficulties inherent in the prediction of class II peptide-MHC affinity, such as the binding of a mixed population of peptide lengths due to the open-ended class II binding site. The method has applications in both the accurate prediction of class II epitopes and the manipulation of affinity for heteroclitic and competitor peptides. The method is applied here to six class II mouse alleles (I-Ab, I-Ad, I-Ak, I-As, I-Ed, and I-Ek) and included peptides up to 25 amino acids in length. A series of regression equations highlighting the quantitative contributions of individual amino acids at each peptide position was established. The initial model for each allele exhibited only moderate predictivity. Once the set of selected peptide subsequences had converged, the final models exhibited a satisfactory predictive power. Convergence was reached between the 4th and 17th iterations, and the leave-one-out cross-validation statistical terms--q2, SEP, and NC--ranged between 0.732 and 0.925, 0.418 and 0.816, and 1 and 6, respectively. The non-cross-validated statistical terms r2 and SEE ranged between 0.98 and 0.995 and 0.089 and 0.180, respectively. The peptides used in this study are available from the AntiJen database (http://www.jenner.ac.uk/AntiJen). The PLS method is available commercially in the SYBYL molecular modeling software package. The resulting models, which can be used for accurate T-cell epitope prediction, will be made freely available online (http://www.jenner.ac.uk/MHCPred).

T Cell Epitope Mimicry in Antiglomerular Basement Membrane Disease

Antiglomerular basement membrane (GBM) disease or Goodpasture's syndrome is among the earliest recognized human autoimmune diseases. Although collagen 4alpha3 NC1 (Col4alpha3NC1) has been identified as the responsible autoantigen, it remains unknown how autoimmunity to this autoantigen is provoked. We have demonstrated in our rat model that a single nephritogenic T cell epitope pCol28-40 of Col4alpha3NC1 induces glomerulonephritis. We hypothesized that microbial peptides that mimic this T cell epitope could induce the disease. Based on the critical residue motif (xxtTxNPsxx) of pCol28-40, seven peptides derived from human infection-related microbes were chosen through GenBank search and synthesized. All peptides showed cross-reactivity with pCol28-40-specific T cells at various levels. Only four peptides induced transient proteinuria and minor glomerular injury. However, the other three peptides induced severe proteinuria and modest to severe glomerulonephritis in 16-25% of the immunized rats. Unexpectedly, the most nephritogenic peptide, pCB, derived from Clostridium botulinum, also induced modest (25%) to severe (25%) pulmonary hemorrhage, another important feature of anti-GBM disease; this was not correlated with the severity of glomerulonephritis. This finding suggests that subtle variations in T cell epitope specificity may lead to different clinical manifestations of anti-GBM disease. In summary, our study raises the possibility that a single T cell epitope mimicry by microbial Ag may be sufficient to induce the anti-GBM disease.

Selection of similar naive T cell repertoires but induction of distinct T cell responses by native and modified antigen

To study the T cell responses induced by native and modified Ag, we have followed in vivo TCR selection and cytokine profile of T cells, as well as the isotype of induced Abs, in response to the model Ag hen egg-white lysozyme (HEL) and its reduced and carboxymethylated form (RCM-HEL). RCM-HEL induces in vivo a T cell response focused on the same immunodominant determinant characterizing the response to native HEL, but further skewed to the Th1 pathway. No difference between HEL and RCM-HEL could be observed in the efficiency of processing, nor in the type of APCs involved. In vivo experiments show that coimmunization with HEL and RCM-HEL generates distinct Th2 or Th1 responses in naive mice, but the two forms of Ag expand the same HEL-specific public clonotype, characterized by the Vbeta8.2-Jbeta1.5 rearrangement, indicating that the populations of naive T cells activated by the two Ag forms overlap. T cells primed by RCM-HEL are restimulated by soluble HEL in vivo, but divert the phenotype of the HEL-specific response to Th1, implying that priming of naive T cells by a structurally modified Ag can induce Th1-type memory/effector T cells more efficiently than native Ag.

Farnesyltransferase inhibition: a novel method of immunomodulation

Farnesyltransferase inhibitors (FTIs) are anticancer compounds that inhibit Ras GTPases. Since Ras GTPases play key roles in T cell activation and function, we hypothesized that FTIs have immunomodulatory properties and are potential antirejection agents. An investigation was performed on a potent FTI to evaluate this hypothesis in the in vitro setting. The in vitro effects of the FTI A-228839 were evaluated. Lectin- or antigen presenting cell (APC)-induced lymphocyte proliferation in the presence of A-228839 was measured. The effects of A-228839 on 1E5 T cell polarity were assessed by microscopy. Intracellular calcium ([Ca(2+)](i)) kinetics of lectin-activated lymphocytes was monitored by flow cytometry. The effects of A-228839 on peripheral blood mononuclear cell (PBMC) cytokine production was assessed by a cytometric bead array method. Activation-induced apoptosis was measured with an annexin V staining assay.A-228839 inhibited lectin-induced proliferation (IC(50)=0.24+/-0.11 microM). The inhibitory effects of A-228839 on lectin induced lymphocyte proliferation were additive to those of CsA. A-228839 was more effective in inhibiting APC-induced T cell proliferation (IC(50)=0.10+/-0.09 microM). A-228839 significantly disrupted the polarized shape of 1E5 T cells at physiologic concentrations. A-228839 altered PBMC baseline [Ca(2+)](i) but did not affect [Ca(2+)](i) kinetics during lectin-induced lymphocyte activation. A-228839 inhibited lymphocyte Th1 cytokine production at submicromolar levels and promoted apoptosis in lectin-activated lymphocytes.A-228839 potently inhibits lymphocyte activation and function. Our results suggest that FTIs may represent a new class of clinically useful immunomodulatory agents. A-228839 has potent in vitro immunomodulatory properties that warrant in vivo evaluation as an antirejection agent.

Tracking T cell clonotypes in complex T lymphocyte populations by real-time quantitative PCR using fluorogenic complementarity-determining region-3-specific probes

The T cell receptor (TCR) alpha and beta chains are encoded by a series of stochastic rearrangements between variable (V), diversity (D) for TCR beta chain only, and joining (J) gene segments, creating hypervariable complementarity-determining region 3 (CDR3) regions that contact the peptide/MHC complex and confer specificity. In the present paper, we applied the recently developed real-time quantitative RT-PCR technique to the detection of rearranged TCR beta chain mRNA transcripts. We designed BV- and BJ-specific primers together with TaqMan probes specific for the CDR3 regions of the clones of interest. As an external reference, we used plasmids containing the entire TCR beta chains, making it possible to normalize the number of specific rearranged BV-J mRNA copies among the total number of TCR beta chains. Here, we present data validating this fluorogenic PCR-based method for the quantification of several TCR clonotypes characteristic of the CD4 T cell response to hen egg white lysozyme (HEL) in mice of the H-2d haplotype. This accurate and sensitive procedure permits the precise determination of T cell clone frequencies ranging from 10(-2) to less than 10(-5) in normal biological samples; it may provide an alternative approach when frequencies are too low to be assessed by flow cytometry.

Immunological adjuvants efficiently induce antigen-specific T cell responses in old mice: implications for vaccine adjuvant development in aged individuals

The morbidity and mortality of infectious diseases are significantly increased in aged humans. Hence, vaccination has been suggested as a means to reduce or prevent the impact of infections on old individuals. However, it has remained unresolved whether or not standard vaccine adjuvants such as aluminum hydroxide (Alum) are similarly efficacious in old individuals, as compared to young adults. Here, we have investigated the effects of prototypic immunological adjuvants, complete Freund's adjuvant (CFA), incomplete Freund's adjuvant (IFA), or Alum on HEL-specific T cell responses in young adult and old mice. We report that independent of the adjuvant used, the induced T cell responses to the prototypic protein antigen hen eggwhite lysozyme (HEL) were similar in young adult and old mice in terms of cytokine production, T cell frequencies, determinant specificity, and T cell repertoire. The results suggest that vaccine adjuvants developed in young adults should be equally effective in inducing T cell immunity in old individuals.

Potential biologic agents for treating rheumatoid arthritis

The encouraging clinical results observed in trials using anti-TNF therapy clearly warrant further studies to determine whether TNF inhibitors are capable of modifying the destructive component of this disease in long-term follow-up studies as well as to assess the safety of long-term use (see the article by Keystone in this issue). It is also reasonable to propose that interfering with the cytokine cascade earlier in the course of disease may be of even greater therapeutic benefit. As the pathogenetic mechanisms in RA are more clearly defined, especially in early disease and in those individuals destined to develop severe disease, the potential of other biologic agents to specifically inhibit these critical pathways may provide better treatments for our patients. Many potential targets in the immune-mediated process of RA are currently being rigorously evaluated in clinical trials. Use of combinations of biologic therapies, perhaps in human patients with RA, should be of considerable interest in future trials.

An example of immunodominance: engagement of synonymous TCR by invariant CDR3 beta

The structural basis of the T cell response against immunodominant tetanus toxin (TT)-derived peptides was investigated using TT-specific T cell clones raised from a DRB1*0301 homozygous donor. Three peptides forming T cell epitopes were identified, including one, TT(1272-1284), that stimulated four different TT-specific T cell clones. TCR sequence analysis revealed that these synonymous TCR shared only arginine at the third position of the CDR3 beta loop. This prominent residue may form a salt bridge with a corresponding aspartate at the relative position 8 (P8) of the antigenic peptide TT(1272-1284) as suggested from amino acid replacement analysis. A similar scenario was observed for a second TT epitope, TT(279-296), and its corresponding TCR. These examples show that immunodominance may result from a single strong amino acid interaction between TCR CDR3 beta loops here in contact with the C-terminus of the antigenic peptide. Such a dominant interaction could compensate for weaker contacts between other residues of the TCR and the antigenic peptide, and would allow the recognition of a single peptide-MHC complex by a broader synonymous TCR repertoire and could thus contribute to its immunodominance.

Perillyl alcohol inhibits TCR-mediated [Ca(2+)](i) signaling, alters cell shape and motility, and induces apoptosis in T lymphocytes

Perillyl alcohol (POH) inhibits isoprenylation and has shown anticancer and chemopreventive properties in rodent models. The mechanism that underlies the anticancer activity of POH and other isoprenylation inhibitors is unknown but has been postulated to involve decreased levels of isoprenylated Ras and Ras-related proteins. Previously we demonstrated that POH effectively inhibits human T cell proliferation in vitro and can prevent acute and chronic rejection in a rat cardiac transplant model. In this report, we investigate the effects of POH on T lymphocytes at the single-cell level. POH disrupts the polarized shape and motility of antigen-specific murine 1E5 T cells. Using an optical trap to position anti-CD3-coated beads in contact with 1E5 T cells, we demonstrate that POH inhibits their TCR-mediated calcium response. Furthermore, we show that POH preferentially induces apoptosis in PHA-activated human T cells as well as in 1E5 T cells.

Interleukin 4-producing CD4 T cells arise from different precursors depending on the conditions of antigen exposure in vivo

The precursor origin of T helper (Th) cell subsets in vivo has been difficult to study and remains poorly investigated. We have previously shown that chronic administration of soluble protein antigen induces selective development of antigen-specific CD4 Th2 cells in genetically predisposed mouse strains. To analyze the origin of effector T cells in this model, we designed a competitive polymerase chain reaction-based approach to track public BV-J rearrangement expressed by CD4 T cells specific for hen egg white lysozyme (HEL) in BALB/c mice. We show that public T cell clones are predominantly associated with type 1 or 2 effector Th cells recovered after primary immunization in complete or incomplete Freund's adjuvant, respectively. Conversely, continuous administration of soluble antigen, which induces strong memory Th2 response, is associated with a dose-dependent reduction of public clone size by a mechanism resembling clonal anergy. Thus, soluble HEL-induced Th2 cells do not express the public complementarity determining region 3 motifs characteristic of immunogenic challenge in the presence of adjuvant. These results demonstrate that there are multiple pathways of induction of Th2 responses depending on the condition of antigen exposure in vivo, i.e., clonal immune deviation versus recruitment of a different pool of precursor cells.

Role of APC in the Selection of Immunodominant T Cell Epitopes

Following antigenic challenge, MHC-restricted T cell responses are directed against a few dominant antigenic epitopes. Here, evidence is provided demonstrating the importance of APC in modulating the hierarchy of MHC class II-restricted T cell responses. Biochemical analysis of class II:peptide complexes in B cells revealed the presentation of a hierarchy of peptides derived from the Ig self Ag. Functional studies of κ peptide:class II complexes from these cells indicated that nearly 20-fold more of an immunodominant epitope derived from κ L chains was bound to class II DR4 compared with a subdominant epitope from this same Ag. In vivo, T cell responses were preferentially directed against the dominant κ epitope as shown using Ig-primed DR4 transgenic mice. The bias in κ epitope presentation was not linked to differences in class II:κ peptide-binding affinity or epitope editing by HLA-DM. Rather, changes in native Ag structure were found to disrupt presentation of the immunodominant but not the subdominant κ epitope; Ag refolding restored κ epitope presentation. Thus, Ag tertiary conformation along with processing reactions within APC contribute to the selective presentation of a hierarchy of epitopes by MHC class II molecules.

On the diversity and heterogeneity of H-2(d)-restricted determinants and T cell epitopes from the major bee venom allergen

One of the main limitations of using synthetic peptides for immunotherapy in allergic patients is the difficulty to delineate the immunodominant T cell epitopes which are necessarily dependent on HLA molecules. We have thus addressed the question of the role of MHC II molecules in immunodominant epitopes selection in the particular case of the major bee venom allergen (API m1). To exhaustively and easily explore it, we used BALB/c mice whose H-2 haplotype is associated with high IgE and IgG responses to API m1. By means of extensive sets of synthetic peptides, we investigated the specificity of polyclonal T cells and monoclonal hybridomas from mice immunized with API m1 and delineated four immunodominant regions, restricted to either the I-E(d) or the I-A(d) molecule. All the peptides were also tested for their capacity to bind to immunopurified MHC II molecules. Eight determinants of high affinity were identified. They clustered into three distinct regions and were largely overlapping. They included all the immunodominant epitopes, but half of them were not capable of stimulating T cells. Strikingly, interacting surfaces with either the TCR or MHC II molecule greatly differed from one determinant to another. In one case, we observed that flanking regions exerted a particular action on T cell stimulation which prevented the fine epitope localization. Our results underline the diversity and complexity of MHC II-restricted determinants and T cell epitopes from the major bee venom allergen, even in a single haplotype. These data also participate in the development of alternative approaches to conventional immunotherapy.

Mapping the sensitivity of T cells with an optical trap: polarity and minimal number of receptors for Ca(2+) signaling

Contact with antigen-presenting cells (APCs) initiates an activation cascade within T lymphocytes, including a rise in cytosolic calcium, lymphokine production, and cell division. Although T cell-APC physical contact is required for an immune response, little is known about the patterns of cellular interactions and their relation to activation. Calcium imaging combined with an optical trap enabled the T cell contact requirements and polarity to be investigated at the single-cell level. APCs or anti-CD3 mAb-coated beads were trapped with a laser and placed at different locations along the T cell, which has a polarized appearance defined by the shape and direction of crawling. T cells were 3-fold more sensitive to APC contact made at the leading edge of the T cell than with contact made at the tail. Anti-CD3 mAb-coated 6-micrometer beads induced calcium signaling with approximately 10-fold higher frequency and approximately 4-fold shorter latency on contact with the leading edge of the T cell than on contact with the trailing edge. Alterations in antibody density (2 to 500 per micrometer(2)) and bead size (1 to 6 micrometer in diameter) were used to determine the spatial requirements and the minimal number of receptors which must be engaged to transmit a positive signal. T cell response percentage, latency, and calcium-signaling pattern (transient vs. sustained or oscillatory) depended on antibody density on the bead. The presence of approximately 170 anti-CD3 mAb within the contact area elicited a detectable T cell calcium response. We propose here that engagement of no more than 340 T cell receptors (approximately 1% of the total on the cell) is sufficient to initiate Ca(2+) signaling. The minimal contact area was approximately 3 micrometer(2).

Cell Viability and DNA Denaturation Measurements by Two-Photon Fluorescence Excitation in CW Al:GaAs Diode Laser Optical Traps

Cell viability and DNA denaturation are measured through two-photon fluorescence excitation using a single diode laser beam as the trapping and exciting source simultaneously. Two-photon fluorescence emission spectra are presented for CHO cells and T lymphocytes loaded with various fluorescent probes. This single beam method is demonstrated to be a safe tool to monitor the viability of optically trapped cells, even under intense 809 nm diode laser illumination (∼106 W/cm2). The dynamics of cellular necrosis is monitored by adding ethanol to the cell suspension during trapping. Thermal damage to heat-treated samples is assessed by recording shifts in the emission spectra from trapped cells loaded with the nucleic acid probe, acridine orange. © 1999 Society of Photo-Optical Instrumentation Engineers.

Presentation of Antigens Internalized Through the B Cell Receptor Requires Newly Synthesized MHC Class II Molecules

Exogenous Ags taken up from the fluid phase can be presented by both newly synthesized and recycling MHC class II molecules. However, the presentation of Ags internalized through the B cell receptor (BCR) has not been characterized with respect to whether the class II molecules with which they become associated are newly synthesized or recycling. We show that the presentation of Ag taken up by the BCR requires protein synthesis in splenic B cells and in B lymphoma cells. Using B cells transfected with full-length I-Ak molecules or molecules truncated in cytoplasmic domains of their α- or β-chains, we further show that when an Ag is internalized by the BCR, the cytoplasmic tails of class II molecules differentially control the presentation of antigenic peptides to specific T cells depending upon the importance of proteolytic processing in the production of that peptide. Integrity of the cytoplasmic tail of the I-Ak β-chain is required for the presentation of the hen egg lysozyme determinant (46–61) following BCR internalization, but that dependence is not seen for the (34–45) determinant derived from the same protein. The tail of the β-chain is also of importance for the dissociation of invariant chain fragments from class II molecules. Our results demonstrate that Ags internalized through the BCR are targeted to compartments containing newly synthesized class II molecules and that the tails of class II β-chains control the loading of determinants produced after extensive Ag processing.

Th1 cells induce and Th2 inhibit antigen-dependent IL-12 secretion by dendritic cells

Dendritic cells are the most relevant antigen-presenting cells (APC) for presentation of antigens administered in adjuvant to CD4+ T cells. Upon interaction with antigen-specific T cells, dendritic cells (DC) expressing appropriate peptide-MHC class II complexes secrete IL-12, a cytokine that drives Th1 cell development. To analyze the T cell-mediated regulation of IL-12 secretion by DC, we have examined their capacity to secrete IL-12 in response to stimulation by antigen-specific Th1 and Th2 DO11.10 TCR-transgenic cells. These cells do not differ either in TCR clonotype or CD40 ligand (CD40L) expression. Interaction with antigen-specific Th1, but not Th2 cells, induces IL-12 p40 and p75 secretion by DC. The induction of IL-12 production by Th1 cells does not depend on their IFN-gamma secretion, but requires direct cell-cell contact mediated by peptide/MHC class II-TCR and CD40-CD40L interactions. Th2 cells not only fail to induce IL-12 secretion, but they inhibit its induction by Th1 cells. Unlike stimulation by Th1, inhibition of IL-12 production by Th2 cells is mediated by soluble molecules, as demonstrated by transwell cultures. Among Th2-derived cytokines, IL-10, but not IL-4 inhibit Th1-driven IL-12 secretion. IL-10 produced by Th2 cells appears to be solely responsible for the inhibition of Th1 -induced IL-12 secretion, but it does not account for the failure of Th2 cells to induce IL-12 production by DC. Collectively, these results demonstrate that Th1 cells up-regulate IL-12 production by DC via IFN-gamma-independent cognate interaction, whereas this is inhibited by Th2-derived IL-10. The inhibition of Th1 -induced IL-12 production by Th2 cells with the same antigen specificity represents a novel mechanism driving the polarization of CD4+ T cell responses.

HgCl2-induced interleukin-4 gene expression in T cells involves a protein kinase C-dependent calcium influx through L-type calcium channels

Mercuric chloride (HgCl2) induces T helper 2 (Th2) autoreactive anti-class II T cells in Brown Norway rats. These cells produce interleukin (IL)-4 and induce a B cell polyclonal activation that is responsible for autoimmune disease. In Brown Norway rats, HgCl2 triggers early IL-4 mRNA expression both in vivo and in vitro by T cells, which may explain why autoreactive anti-class II T cells acquire a Th2 phenotype. The aim of this study was to explore the transduction pathways by which this chemical operates. By using two murine T cell hybridomas that express IL-4 mRNA upon stimulation with HgCl2, we demonstrate that: 1) HgCl2 acts at the transcriptional level without requiring de novo protein synthesis; 2) HgCl2 induces a protein kinase C-dependent Ca2+ influx through L-type calcium channels; 3) calcium/calcineurin-dependent pathway and protein kinase C activation are both implicated in HgCl2-induced IL-4 gene expression; and 4) HgCl2 can activate directly protein kinase C, which might be one of the main intracellular target for HgCl2. These data are in agreement with an effect of HgCl2 which is independent of antigen-specific recognition. It may explain the T cell polyclonal activation in the mercury model and the expansion of pathogenic autoreactive anti-class II Th2 cells in this context.

Depression of T-cell epitope generation by stabilizing hen lysozyme

Conformational stability of proteins is an important factor that determines their resistance/susceptibility to proteolytic digestion. Intracellular proteolysis is the key step in antigen presentation events for protein antigens; hence, it is likely that increasing protein stability reduces the antigenicity of proteins. We prepared three hen egg white lysozyme derivatives possessing different stabilities by chemical modification to clarify the relationship between conformational stability and the antigenicity of the protein. One of the derivatives was conformationally unstabilized by removing one intramolecular disulfide bond, whereas the two others were stabilized by the addition of an intramolecular cross-link. The antigenicity of these derivatives was evaluated using hen egg white lysozyme-specific T-cell hybridoma cells and a B-lymphoma cell line, A20, as antigen-presenting cells. With an increase in conformational stability, the T-cell response decreased. However, the reduction was not derived from the inefficiency of internalization to A20 cells nor the alteration of antigenicity by chemical modifications. Moreover, from analyses of their susceptibility to proteolysis and the kinetics of presentation of the T-cell epitope, it was confirmed that increasing protein stability led to the depression of T-cell epitope generation by increasing resistance to proteolysis. These results have an important implication in devising a new strategy for manipulating T-cell response by the stability of protein antigen.

Antigen-specific T cell receptor antagonism by antigen-presenting cells treated with the hemolysin of Listeria monocytogenes: a novel type of immune escape

We have examined the influence of listeriolysin O (LLO), the hemolysin secreted by the pathogenic bacterium Listeria monocytogenes, on major histocompatibility complex class II-dependent T cell activation. Stimulation of T cells by native antigens but not by peptides is inhibited upon pretreatment of antigen-presenting cells (APC) with LLO. Experiments presented here reveal that this inhibition is not due to a lack in processing of antigen by APC but is the result of an irreversible inactivation of T cells that recognize antigen on LLO-treated APC. Incubation of mixtures of two different T cells where only one antigen was presented on LLO-treated APC suggested that T cell inactivation is antigen specific. The inactivation was dominant and could be observed even in the presence of amounts of synthetic peptides that normally lead to T cell responses. This condition is reminiscent of the T cell inhibition observed when antagonistic and stimulatory peptides are added to APC at the same time. Our results thus reveal a novel type of interference by pathogens with antigen presentation and T cell stimulation that could give the pathogen a decisive advantage in dissemination and disease.

Normal B cells fail to secrete interleukin-12

Interleukin-12 is a key regulatory cytokine produced by antigen-presenting cells (APC) which drives the development of interferon-gamma (IFN-gamma)-producing cells and promotes cell-mediated immunity. Following subcutaneous immunization with protein antigen in adjuvant, dendritic cells (DC) but not small nor large B cells in immune lymph nodes express antigenic complexes and secrete substantial amounts of bioactive IL-12 p75 upon antigen-specific interaction with T cells. We have analyzed secretion of IL-12 p40 and p75 by cell populations enriched in DC, macrophages or B cells in response to nonspecific stimulation or to interaction with antigen-specific CD4+ cells. These APC populations do not produce IL-12 constitutively but, upon stimulation with heat-fixed Staphylococcus aureus and IFN-gamma, IL-12 p40 and p75 are secreted by DC and macrophages, whereas B cells fail to produce IL-12. B cells also fail to secrete IL-12 in response to stimulation with LPS and IFN-gamma. Co-culture with CD4+ T hybridoma cells and antigen induces IL-12 secretion by DC. Up-regulation of IL-12 secretion by interaction with antigen-specific CD4+ T cells is abrogated by anti-class II monoclonal antibodies (mAb), by soluble CD40 molecules and by anti-CD40 ligand mAb, demonstrating a positive feedback between T cells and DC mediated by TCR-peptide/class II and by CD40-CD40 ligand interactions. Expression of class II and CD40 molecules is comparable in B cells and DC, and both APC types activate CD4+ T cells. Yet, even upon interaction with antigen-specific T cells, B cells fail to secrete IL-12. The capacity of B cells to present antigen but not to secrete IL-12 may explain their propensity to selectively drive T helper type 2 cell development.

Determinant selection for T-cell tolerance in HEL-transgenic mice: dissociation between immunogenicity and tolerogenicity

The induction of T-cell tolerance to self-antigens has been extensively characterized for immunodominant (ID) regions. However, tolerance toward other minor self-determinants has received less attention. In the H-2(d) haplotype, HEL contains a single ID determinant (region 102-120) presented by I-E(d) MHC class II molecules. The present study evaluates the role of subdominant and cryptic HEL regions in maintaining tolerance. We have generated a mutated HEL antigen, HEL mu, whose ID region does not bind to I-E(d). Lymph node cells from HEL-immunized mice proliferated strongly to HEL mu in vitro. Two new stimulatory regions common to HEL and HEL mu were uncovered. They are produced during antigen processing and prime specific T lymphocytes. HEL-Tg mice were tolerant to these determinants, thus confirming their in vivo presentation. These HEL regions were as tolerogenic as the HEL ID determinant, despite their poor immunogenicity. These results demonstrate that there is not always a correlation between tolerogenicity and immunogenicity, a finding that may be critical for understanding T-cell tolerance.

A peptide-binding motif for I-A(g7), the class II major histocompatibility complex (MHC) molecule of NOD and Biozzi AB/H mice

The class II major histocompatibility complex molecule I-A(g7) is strongly linked to the development of spontaneous insulin-dependent diabetes mellitus (IDDM) in non obese diabetic mice and to the induction of experimental allergic encephalomyelitis in Biozzi AB/H mice. Structurally, it resembles the HLA-DQ molecules associated with human IDDM, in having a non-Asp residue at position 57 in its beta chain. To identify the requirements for peptide binding to I-A(g7) and thereby potentially pathogenic T cell epitopes, we analyzed a known I-A(g7)-restricted T cell epitope, hen egg white lysozyme (HEL) amino acids 9-27. NH2- and COOH-terminal truncations demonstrated that the minimal epitope for activation of the T cell hybridoma 2D12.1 was M12-R21 and the minimum sequence for direct binding to purified I-A(g7) M12-Y20/K13-R21. Alanine (A) scanning revealed two primary anchors for binding at relative positions (p) 6 (L) and 9 (Y) in the HEL epitope. The critical role of both anchors was demonstrated by incorporating L and Y in poly(A) backbones at the same relative positions as in the HEL epitope. Well-tolerated, weakly tolerated, and nontolerated residues were identified by analyzing the binding of peptides containing multiple substitutions at individual positions. Optimally, p6 was a large, hydrophobic residue (L, I, V, M), whereas p9 was aromatic and hydrophobic (Y or F) or positively charged (K, R). Specific residues were not tolerated at these and some other positions. A motif for binding to I-A(g7) deduced from analysis of the model HEL epitope was present in 27/30 (90%) of peptides reported to be I-A(g7)-restricted T cell epitopes or eluted from I-A(g7). Scanning a set of overlapping peptides encompassing human proinsulin revealed the motif in 6/6 good binders (sensitivity = 100%) and 4/13 weak or non-binders (specificity = 70%). This motif should facilitate identification of autoantigenic epitopes relevant to the pathogenesis and immunotherapy of IDDM.

Identification of an immunodominant T cell epitope on cholera toxin

Cholera toxin (CT), the enterotoxin of Vibrio cholerae, is a potent mucosal immunogen as well as a strong mucosal adjuvant to related and unrelated antigens. The mucosal immune response to CT is T cell dependent and MHC class II restricted. The epitopes on CT recognized by T cells have not been identified. The purpose of this study was to determine the fine specificity of T cell recognition of both the CT A subunit (CT-A) and the CT B subunit (CT-B) by using a range of synthetic peptides. After immunization with CT-B or CT-A in CFA subcutaneously, the peripheral lymph node T cells were stimulated with different synthetic peptides in vitro. The peptide specificity of T cell recognition was identified by assaying T cell proliferation and interleukin-3 production. T cells from C57BL/6 (H-2b) high responder mice recognized one immunodominant epitope (peptide 89-100) and one weak epitope (peptide 31-50) on CT-B and two epitopes (peptide 21-39 and 180-194) on CT-A. The immunization of C57BL/6 mice with synthetic immunodominant CT-B peptide 89-100 induced T cell immunity to the pentameric CT-B. Induction of tolerance to CTB peptide 89-100 by i.v. injection in high responder C57BL/6 mice induced unresponsiveness to mucosal immunization with CT, compatible with an immunodominant role for this T cell epitope.

Cell type-specific processing of the I-Ed-restricted hen egg lysozyme determinant 107-116

Class II major histocompatibility complex heterodimers present to T cells determinants as sets of antigen fragments with ragged N and C termini. It is not yet elucidated whether different types of antigen-presenting cells generate identical sets of peptides containing the same determinant. Taking advantage of recombinant I-Ed molecules produced by insect cells as empty heterodimers, a sensitive T cell stimulation assay was developed to analyze naturally processed hen egg lysozyme (HEL) peptides. I-Ed preparations were isolated from antigen-presenting cells cultured with HEL. Following acid treatment, peptides eluted from I-Ed were chromatographed and the fractions incubated at acidic pH with purified recombinant I-Ed molecules, conditions which favor peptide binding. The stimulatory capacity of the reconstituted peptide-I-Ed complexes adsorbed on the well surface of cell culture plates was then evaluated by measuring interleukin-2 secreted by an HEL 107-116-specific, I-Ed-restricted T cell hybridoma. We found that the B lymphoma A20 and an I-Ed-transfected fibroblast cell line generated distinct sets of peptides containing the HEL sequence 107-116. Our results suggest the possibility that presentation of one determinant by different types of antigen-presenting cells stimulates populations of T cells with distinct fine antigen specificities.

Polarity of T cell shape, motility, and sensitivity to antigen

T cell activation requires contact with APCs. We used optical techniques to demonstrate T cell polarity on the basis of shape, motility, and localized sensitivity to antigen. An intracellular Ca2+ clamp showed that T cell shape and motility are extremely sensitive to changes in [Ca2+]i (Kd = 200 nM), with immobilization and rounding occurring via a calcineurin-independent pathway. Ca2+ dependent immobilization prolonged T cell contact with the antigen-presenting B cell; buffering the [Ca2+]i signal prevented the formation of stable cell pairs. Optical tweezers revealed spatial T cell sensitivity to antigen by controlling placement on the T cell surface of either B cells or alpha-CD3 MAb-coated beads. T cells were 4-fold more sensitive to contact made at the leading edge of the T cell compared with the tail. We conclude that motile T cells are polarized antigen sensors that respond physically to [Ca2+]i signals to stabilize their interaction with APCs.

Dendritic cells but not B cells present antigenic complexes to class II-restricted T cells after administration of protein in adjuvant

We have analyzed the relative contribution of dendritic cells (DC) and B cells in the presentation of peptide-class II complexes in an inflammatory situation in vivo. Draining lymph node cells from mice immunized subcutaneously with hen egg-white lysozyme (HEL) in adjuvant display HEL peptide-major histocompatibility complex class II complexes able to stimulate, in the absence of any further antigen addition, specific T hybridoma cells. The antigen-presenting capacity of three different antigen-presenting cell (APC) populations recruited in lymph nodes, DC (N418+, class II+, B220-, low buoyant density), large B cells (B220+, low buoyant density), and small B cells (B220+, high buoyant density), was analyzed. After immunization with HEL in adjuvant, DC are the only lymph node APC population expressing detectable HEL peptide-class II complexes. These results indicate that lymph node DC and not B cells are the APC initiating the immune response in vivo after administration of antigen in adjuvant.

Selective development of T helper (Th)2 cells induced by continuous administration of low dose soluble proteins to normal and beta(2)-microglobulin-deficient BALB/c mice

Continuous administration of soluble proteins, delivered over a 10-d period by a mini-osmotic pump implanted subcutaneously, induces a long-lasting inhibition of antigen-specific T cell proliferation in lymph node cells from BALB/c mice subsequently primed with antigen in adjuvant. The decreased T cell proliferative response is associated with a down-regulation of the T helper cell (Th)1 cytokines interleukin (IL)-2 and interferon (IFN)-gamma and with a strong increase in the secretion of the Th2 cytokines IL-4 and IL-5 by antigen specific CD4+ T cells. This is accompanied by predominant inhibition of antigen-specific antibody production of IgG2a and IgG2b, rather than IgG1 isotype. Interestingly, inhibition of Th1 and priming of Th2 cells is also induced in beta(2) microglobulin-deficient BALB/c mice, indicating that neither CD8+ nor CD4+ NK1.1+ T cells, respectively, are required. The polarization in Th2 cells is stably maintained by T cell lines, all composed of CD4+/CD8- cells expressing T cell receptor for antigen (TCR) alpha/beta chains, derived from BALB/c mice treated with continuous antigen administration, indicating that they originate from Th2 cells fully differentiated in vivo. This polarization is induced in BALB/c mice by continuous administration of any protein antigen tested, including soluble extracts from pathogenic microorganisms. Priming of Th2 cells is dose dependent and it is optimal for low rather than high doses of protein. Blocking endogenous IL-4 in vivo inhibits expansion of antigen-specific Th2 cells, but does not restore IFN-gamma production by T cells from mice treated with soluble antigen-specific Th2 cells, but does not restore IFN-gamma production by T cells from mice treated with soluble antigen, indicating the involvement of two independent mechanisms. Consistent with this, Th2 cell development, but not inhibition of Th1 cells, depends on non-major histocompatibility complex genetic predisposition, since the Th2 response is amplified in BALB/c as compared to DBA/2, C3H, or C57BL/6 mice whereas tested. These findings support the hypothesis that continuous release of low amounts of protein antigens from pathogenic microorganisms may polarize the immune response toward a Th2 phenotype in susceptible mouse strains.

The accessibility of peptides bound to the mouse MHC class II molecule IEd studied by fluorescence

The accessibility of fluorescently labeled (antigenic) peptides bound to the detergent-solubilized mouse MHC class II protein IEd has been studied by fluorescence techniques. Based on the efficiency of fluorescence quenching by the aqueous quenchers iodide and TEMPOL, different degrees of accessibility of the peptide-attached fluorescein moiety are distinguished in the IEd-bound state, which depend on the nature of the peptide and on the site of attachment. These different extents of sequestration from the aqueous phase are reflected in the fluorescence properties of the corresponding NBD-labeled peptides bound to IEd. The results provide information on the topology of class II bound peptides.

H-2b restriction of the immune response to the p126 Plasmodium falciparum antigen

Inbred BALB/c (H-2d), CBA (H-2k) and C57B1/6 (H-2b) mice immunized with Plasmodium falciparum schizonts or culture supernates develop antibodies of different antigenic specificities. It has been observed that C57B1/6 mice were unable to produce detectable antibodies against the p126 antigen (native molecule and p73 or p50 processed fragments) compared with other inbred mice. Similar results were obtained using BALB congenic mice with a lack of p126 antibody response in H-2b mice, while H-2d and H-2k mice produced antibodies against the p126. Lymphocyte proliferation assays performed by incubation of spleen cells with immunopurified p126 were positive for immunized BALB/c (H-2d) and congenic H-2d or H-2k mice. On the other hand, no lymphocyte stimulation was observed with either C57B1/6 (H-2b) or congenic H-2b mice. These results suggest an MHC restriction of the immune response against the entire p126 (found in schizonts) and its p73 and p50 naturally processed fragments (found in culture supernates).

Induction of T cell responses by chimeric bacterial proteins expressing several copies of a viral T cell epitope

A viral T cell epitope was genetically inserted within the periplasmic MalE protein of Escherichia coli in two different permissive insertion sites and resulting hybrid proteins were used to study the in vitro and in vivo immunogenicity of the foreign T cell epitope. Purified hybrid MalE proteins containing the T cell epitope 120-132 (PreS:T) from PreS2 region of hepatitis B virus HBsAg inserted alone or with its adjacent B cell epitope (132-145) were able to induce strong peptide-specific T cell responses in mice. In vitro stimulation of primed lymph node cells or specific T cell hybridomas by the hybrid proteins required processing of the inserted T cell epitope and was inhibited by antigen-presenting cells fixation. The inserted T cell epitope was presented in vitro, in association with appropriate major histocompatibility complex molecules, as efficiently as free synthetic peptide. The in vitro immunogenicity of MalE hybrid proteins was increased by inserting four tandemly repeated copies of PreS:T, either at site 133 or 303. These results were confirmed in vivo by comparing the proliferative responses of lymph node cells from DBA/1 mice primed with MalE hybrid proteins containing one or four copies of PreS:T. Thus, the use of MalE hybrid proteins expressing multiple copies of a given foreign T cell epitope allows the induction of peptide-specific T cell response with a lower dose of priming antigen.

Determinant capture as a possible mechanism of protection afforded by major histocompatibility complex class II molecules in autoimmune disease

How peptide-major histocompatibility complex (MHC) class II complexes are naturally generated is still unknown, but accumulating evidence suggests that unfolding proteins or long peptides can become bound to class II molecules at the dominant determinant before proteolytic cleavage. We have compared the immunogenicity of hen egg-white lysozyme (HEL) in nonobese diabetic (NOD), (NOD x BALB/c)F1, and E(d) alpha transgenic NOD mice. We find that a response to the subdominant ANOD-restricted determinant disappears upon introduction of an E(d) molecule, and is restored when scission of HEL separates this determinant from its adjoining, competitively dominant, E(d)-restricted determinant. This suggests that the E(d) molecule binds and protects its dominant determinant on a long peptide while captured neighboring determinants are lost during proteolysis. These results provide clear evidence for "determinant capture" as a mechanism of determinant selection during antigen processing and a possible explanation for MHC-protective effects in insulin-dependent diabetes mellitus.

Enhancement of peptide antigen presentation by a second peptide

The influence of nonstimulatory "competitor" peptides on the binding of an antigenic peptide to a major histocompatibility complex (MHC) class II molecule was investigated. Using high-performance size-exclusion chromatography and fluorescein-labeled peptides, we show that the presence of the peptides dynorphin A-(1-13) and poly(L-lysine) results in enhancement rather than inhibition of the binding of hen egg lysozyme peptide-(107-116) [HEL-(107-116)] to the detergent-solubilized mouse class II molecule IEd. In parallel, dynorphin A-(1-13) and poly(L-lysine) were found to enhance the specific activation of an IEd-restricted T-cell hybridoma by HEL-(107-116). A molecular mechanism involving an intermediate two peptide-MHC class II protein complex is proposed to explain the enhancement of peptide binding to class II molecules by an irrelevant second peptide.

Prevention of autoimmune diabetes in non-obese diabetic mice by treatment with a class II major histocompatibility complex-blocking peptide

The role of antigen presentation as a possible mechanism underlying major histocompatibility complex (MHC) association of autoimmune disease has been studied in non-obese diabetic (NOD) mice. By screening for inhibition of antigen presentation to NOD T cell hybridoma, we have selected a synthetic peptide, yTYTVHAAHAYTYt (small letters denote D amino acids), that efficiently blocks antigen presentation by the NOD class II MHC molecule A alpha g7A beta g7 (Ag7) in vitro. The inhibition is MHC selective, in that it does not affect antigen presentation by the E(d) and E(k) molecules, and has only a marginal effect on presentation by the A(d) molecule. This peptide also inhibits the priming for Ag7-restricted T cell responses in vivo, and prevents the spontaneous development of diabetes in female NOD mice, when administered chronically from 3 wk of age on. Chronic treatment with a control peptide, KMKMVHAAHAKMKM, that fails to bind to Ag7 has no effect on the disease. These data indicate that antigen presentation by the Ag7 molecule plays a pivotal role in the induction of autoimmune diabetes. Furthermore, the results demonstrate that interference with antigen presentation by a class II molecule can prevent the onset of spontaneous autoimmune disease associated with the same molecule.

Selective immunosuppression by administration of major histocompatibility complex class II-binding peptides. II. Preventive inhibition of primary and secondary in vivo antibody responses

The self-mouse lysozyme peptide corresponding to residues 46-62 (ML46-62) binds to the major histocompatibility complex (MHC) class II molecules I-A(k) and it selectively inhibits, when coinjected with antigen, priming of I-A(k)-restricted, antigen-specific T cells. We demonstrate that administration of ML46-62 also inhibits in vivo antibody responses induced by I-A(k)-restricted T helper cells. ML46-62 is able to prevent the primary anti-hen egg white lysozyme (HEL) antibody response induced by the entire HEL molecule in B10.A(4R) mice, expressing only I-A(k) molecules, but not in mice of H-2d haplotype. ML46-62 also strongly decreases, in B10.A(4R) mice, the antibody response to ribonuclease A, a protein antigen unrelated to the MHC blocker, indicating that MHC blockade is the mechanism leading to inhibition of antibody response. This is further supported by the concomitant decrease, in vivo, of complex formation between immunodominant HEL peptides and I-A(k) molecules, preventing I-A(k)-restricted T cell induction. Administration of ML46-62 after antigen priming does not affect ongoing antibody responses, as expected from MHC blockade. A single injection of ML46-62 at the time of protein antigen priming precludes not only the primary, but also the secondary antibody response to a subsequent challenge with soluble protein, even when the challenge is performed several months after priming. Coinjection of antigen and MHC antagonist inhibits production of all antibody isotypes equally well, suggesting that MHC class II blockade affects both Th1- and Th2-type T helper cells. Therefore, these results indicate that administration of MHC class II-binding peptides can efficiently and selectively prevent the induction of T cell-dependent primary and secondary in vivo antibody responses by blocking antigen presentation to class II-restricted T helper cells.

Human T cell autoimmunity against myelin basic protein: CD4+ cells recognizing epitopes of the T cell receptor beta chain from a myelin basic protein-specific T cell clone

We have investigated whether the normal immune system contains T cells that are able to recognize T cell receptor (TcR) determinants of autologous autoantigen-specific T cells. The T cell clone HW.BP3, specific for myelin basic protein (MBP) was isolated from a healthy donor. HW.BP3 is restricted by HLA-DR2a, and reacts to human MBP 139-153. The expressed alpha beta TcR genes of HW.BP3 were cloned and sequenced, and the sequences analyzed for potential T cell epitopes. Two synthetic peptides, one from the VDJ beta junctional (beta 1) and one from the V beta region (beta 2) of the TcR of HW.BP3, were used to select four TcR peptide-specific T cell lines from the donor of HW.BP3. All anti-TcR lines had the phenotype CD3+/CD4+/HLA-DR+/CD25+/CD45RO+, and recognized the antigen in the context of HLA-DR. Three anti-TcR lines, which had been selected for reactivity to peptide beta 1, recognized exclusively this peptide restricted by HLA-DR2b. One anti-TcR line, selected for peptide beta 2, responded to both peptides beta 1 and beta 2 when presented by autologous blood mononuclear cells, but not by HLA-DR2a- or HLA-DR2b-transfected L cells. All TcR peptide-specific T cell lines were efficiently cytotoxic. They specifically lysed autologous macrophages or HW.BP3 line cells in the presence of exogenous peptide antigen. In contrast, HW.BP3 did not present endogenous TcR peptides to the anti-TcR lines. The results demonstrate that the normal human immune system contains not only autoantigen-specific T cells, but also T cells that recognize antigenic determinants of autologous autoreactive TcR.

Factors governing the binding and recognition of foreign and self-peptides by MHC class II

Considerable progress has been made in understanding the basis of T cell recognition and T cell activation. This knowledge has recently been used to modulate T cell activation in animal models of experimental autoimmune disease by two means--selective MHC blockade and peptide-induced tolerance. The use of peptides to interfere with the binding of autoantigenic peptides to MHC requires knowledge of both the class II allele which presents the immunodominant peptide to autoimmune T cells and the identification of peptide analogs that bind with high affinity to that allele. The alternative strategy of peptide-induced tolerance will require identification of the autoantigen and its immunodominant peptides. While the latter approach holds great promise for immunointervention, its wide application will require full knowledge of the mechanisms by which tolerance to self is maintained and how it can be broken.

Selective immunosuppression by administration of major histocompatibility complex (MHC) class II-binding peptides. I. Evidence for in vivo MHC blockade preventing T cell activation

Draining lymph node cells (LNC) from mice immunized with hen egg white lysozyme (HEL) display at their surface antigen-MHC complexes able to stimulate, in the absence of any further antigen addition, HEL peptide-specific, class II-restricted T cell hybridomas. Chloroquine addition to these LNC cultures fails to inhibit antigen presentation, indicating that antigenic complexes of class II molecules and HEL peptides are formed in vivo. MHC class II restriction of antigen presentation by LNC from HEL-primed mice was verified by the use of anti-class II monoclonal antibodies. Coinjection of HEL and the I-Ak-binding peptide HEL 112-129 in mice of H-2k haplotype inhibits the ability of LNC to stimulate I-Ak-restricted, HEL 46-61-specific T cell hybridomas. Similar results are obtained in mice coinjected with the HEL peptides 46-61 and 112-129. Inhibition of T hybridoma activation can also be observed using as antigen-presenting cells irradiated, T cell-depleted LNC from mice coinjected with HEL 46-61 and HEL 112-129, ruling out the possible role of either specific or nonspecific suppressor T cells. Inhibition of T cell proliferation is associated with MHC-specific inhibition of antigen presentation and with occupancy by the competitor of class II binding sites, as measured by activation of peptide-specific T cell hybridomas. These results demonstrate that administration of MHC class II binding peptide competitors selectively inhibits antigen presentation to class II-restricted T cells, indicating competitive blockade of class II molecules in vivo.