Identification of a major I-Ek-restricted determinant of hen egg lysozyme: limitations of lymph node proliferation studies in defining immunodominance and crypticity

On Peptides and Altered Peptide Ligands: From Origin, Mode of Action and Design to Clinical Application (Immunotherapy)

T lymphocytes equipped with clonotypic T cell antigen receptors (TCR) recognize immunogenic peptides only when presented in the context of their own major histocompatibility complex (MHC) molecules. Peptide loading to MHC molecules occurs in intracellular compartments (ER for class I and MIIC for class II molecules) and relies on the interaction of the respective peptides and peptide binding pockets on MHC molecules. Those peptide residues not engaged in MHC binding point towards the TCR screening for possible peptide MHC complex binding partners. Natural or intentional modification of both MHC binding registers and TCR interacting residues of peptides - leading to the formation of altered peptide ligands (APLs) - might alter the way peptides interact with TCRs and hence influence subsequent T cell activation events, and consequently T cell effector functions. This review article summarizes how APLs were detected and first described, current concepts of how APLs modify T cellular signaling, which biological mechanisms might force the generation of APLs in vivo, and how peptides and APLs might be used for the benefit of patients suffering from allergic or autoimmune diseases.

CD4+ T Cells Targeting Dominant and Cryptic Epitopes from Bacillus anthracis Lethal Factor

Anthrax is an endemic infection in many countries, particularly in the developing world. The causative agent, Bacillus anthracis, mediates disease through the secretion of binary exotoxins. Until recently, research into adaptive immunity targeting this bacterial pathogen has largely focused on the humoral response to these toxins. There is, however, growing recognition that cellular immune responses involving IFNγ producing CD4+ T cells also contribute significantly to a protective memory response. An established concept in adaptive immunity to infection is that during infection of host cells, new microbial epitopes may be revealed, leading to immune recognition of so called ‘cryptic’ or ‘subdominant’ epitopes. We analyzed the response to both cryptic and immunodominant T cell epitopes derived from the toxin component lethal factor and presented by a range of HLA-DR alleles. Using IFNγ-ELISpot assays we characterized epitopes that elicited a response following immunization with synthetic peptide and the whole protein and tested their capacities to bind purified HLA-DR molecules in vitro. We found that DR1 transgenics demonstrated T cell responses to a greater number of domain III cryptic epitopes than other HLA-DR transgenics, and that this pattern was repeated with the immunodominant epitopes, as a greater proportion of these epitopes induced a T cell response when presented within the context of the whole protein. Immunodominant epitopes LF_457-476 and LF_467-487 were found to induce a T cell response to the peptide, as well as to the whole native LF protein in DR1 and DR15, but not in DR4 transgenics. The analysis of Domain I revealed the presence of several unique cryptic epitopes all of which showed a strong to moderate relative binding affinity to HLA-DR4 molecules. However, none of the cryptic epitopes from either domain III or I displayed notably high binding affinities across all HLA-DR alleles assayed. These responses were influenced by the specific HLA alleles presenting the peptide, and imply that construction of future epitope string vaccines which are immunogenic across a wide range of HLA alleles could benefit from a combination of both cryptic and immunodominant anthrax epitopes.

Unconventional T-cell recognition of an arthritogenic epitope of proteoglycan aggrecan released from degrading cartilage

It has been proposed that peptide epitopes bind to MHC class II molecules to form distinct structural conformers of the same MHC II-peptide complex termed type A and type B, and that the two conformers of the same peptide-MHC II complex are recognized by distinct CD4 T cells, termed type A and type B T cells. Both types recognize short synthetic peptides but only type A recognize endosomally processed intact antigen. Type B T cells that recognize self peptides from exogenously degraded proteins have been shown to escape negative selection during thymic development and so have the potential to contribute to the pathogenesis of autoimmunity. We generated and characterized mouse CD4 T cells specific for an arthritogenic epitope of the candidate joint autoantigen proteoglycan aggrecan. Cloned T-cell hybridomas specific for a synthetic peptide containing the aggrecan epitope showed two distinct response patterns based on whether they could recognize processed intact aggrecan. Fine mapping demonstrated that both types of T-cell recognized the same core epitope. The results are consistent with the generation of aggrecan-specific type A and type B T cells. Type B T cells were activated by supernatants released from degrading cartilage, indicating the presence of antigenic extracellular peptides or fragments of aggrecan. Type B T cells could play a role in the pathogenesis of proteoglycan-induced arthritis in mice, a model for rheumatoid arthritis, by recognizing extracellular peptides or protein fragments of joint autoantigens released by inflamed cartilage.

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).

Amino-terminal flanking residues determine the conformation of a peptide-class II MHC complex

The peptide spanning residues 48-62 of hen egg white lysozyme presented by I-A(k) molecules gives rise to two T cell populations, types A and B, that recognize distinct conformers of the complex generated in late and recycling endosomes. The class II-like accessory molecule H2-DM functions as a conformational editor, eliminating the type B conformer in late endosomes. Here, we show that the conformation of the complex, and its susceptibility to editing by H2-DM, are determined by peptide amino-terminal flanking residues. Elimination of these residues abolished editing, permitting formation of the type B conformer in late endosomes. Substitutions at P(-2) affected the stability of the type B conformer, preventing its formation and/or editing, without hindering peptide binding or formation of the type A conformer of the complex. We conclude that interactions involving amino-terminal flanking residues stabilize peptide-MHC conformers and confer resistance to editing by H2-DM, influencing the nature of the T cell repertoire.

Conformational isomers of a peptide-class II major histocompatibility complex

The relative plasticity of peptide binding to class II major histocompatibility complex (MHC) molecules permits formation of multiple conformational isomers by the same peptide and MHC molecule; such conformers are specifically recognized by distinct subsets of T cells. Here, we review current knowledge and recent advances in our understanding of peptide-class II MHC conformational isomerism and the mechanisms that generate distinct MHC-peptide conformers. We focus on our studies of two T-cell subsets, type A and B, which recognize distinct conformers of the dominant epitope of hen egg white lysozyme presented by I-A(k). These conformers form via different pathways and in distinct intracellular vesicles: the type A conformer forms in late endosomes upon processing of native protein, while the more flexible type B conformer forms in early endosomes and at the cell surface. In this process, H2-DM acts as a conformational editor, eliminating the type B conformer in late endosomes. Type B T cells constitute a significant component of the naïve T-cell repertoire; furthermore, self-reactive type B T cells escape negative selection and are present in abundance in the periphery. Ongoing studies should elucidate the role of type B T cells in immunity to pathogens and in autoimmune pathology.

CD4+ T cell responses to SSX-4 in melanoma patients

Genes of the synovial sarcoma X breakpoint (SSX) family are expressed in different human tumors, including melanomas, but not in adult somatic tissues. Because of their specific expression at the tumor site, SSX-encoded Ags are potential targets for anticancer immunotherapy. In this study, we have analyzed CD4+ T cell responses directed against the Ag encoded by SSX-4. Upon in vitro stimulation of PBMC from four melanoma patients bearing Ag-expressing tumors with a pool of long peptides spanning the protein sequence, we detected and isolated SSX-4-specific CD4+ T cells recognizing several distinct antigenic sequences, mostly restricted by frequently expressed HLA class II alleles. The majority of the identified sequences were located within the Krüppel-associated box domain in the N-terminal region of the protein, indicating a high potential immunogenicity of this region. Together our data document the existence of CD4+ T cells specific for multiple SSX-4 derived sequences in circulating lymphocytes from melanoma patients and encourage further studies to assess the impact of SSX-4-specific T cell responses on disease evolution in cancer patients.

Identification of an I-Ed-restricted T-cell epitope of Escherichia coli outer membrane protein F

A predominant T-cell epitope of Escherichia coli outer membrane protein F (OmpF) that encompasses amino acids 295 to 314 was identified in H-2(d) mice. BALB/c-derived T-cell hybridomas generated against this region were CD3(+), CD4(+), CD8(-), and T-cell receptor alphabeta(+) and secreted TH-1-associated cytokines (interleukin-2 [IL-2] and gamma interferon), but not a TH-2-associated cytokine (IL-4), when restimulated with peptide 295-314. Class II(+) mouse lymphoma (A20) cells, but not class II(-) mouse mastocytoma (P815) cells, supported IL-2 secretion of hybridomas when substituted for syngeneic splenocytes as antigen-presenting cells (APCs). Antibodies specific for I-E(d) blocked IL-2 secretion by hybridomas, but I-A(d)-specific antiserum did not. When transfected L cells expressing I-A(d) (AalphaAbeta(d)), I-E(d) (EalphaEbeta(d)), or the hybrid molecule I-EalphaAbeta(d) were used as APCs, hybridomas recognized peptide only when presented by the I-E(d)-transfected cells. When peptide 295-314 truncated at either the C or the N terminus of the sequence was used, the minimal epitope was determined. Critical residues were determined by using alanine-substituted peptide analogues. T-cell hybridomas were only stimulated by peptides that encompassed amino acids 295 to 303 (9-mer), and the core sequence required a minimum of three additional amino acids at either the amino or the carboxy terminus to induce IL-2 secretion. Critical residues were determined to be phenylalanine at position 295, threonine at position 300, and tyrosines at positions 301 and 302. This study is the first to identify a minimal T-cell epitope and major histocompatibility complex restriction element of the OmpF protein and confirms previous observations that there is considerable degeneracy in the length of peptides that can bind I-E(d) and variability in the amino acid composition of the C and N termini of these peptides.

Identification of Multiple HLA-DR-Restricted Epitopes of the Tumor-Associated Antigen CAMEL by CD4+Th1/Th2 Lymphocytes

CD4(+) Th cells play an important role in the induction and maintenance of adequate CD8(+) T cell-mediated antitumor responses. Therefore, identification of MHC class II-restricted tumor antigenic epitopes is of major importance for the development of effective immunotherapies with synthetic peptides. CAMEL and NY-ESO-ORF2 are tumor Ags translated in an alternative open reading frame from the highly homologous LAGE-1 and NY-ESO-1 genes, respectively. In this study, we investigated whether CD4(+) T cell responses could be induced in vitro by autologous, mature dendritic cells pulsed with recombinant CAMEL protein. The data show efficient induction of CAMEL-specific CD4(+) T cells with mixed Th1/Th2 phenotype in two healthy donors. Isolation of CD4(+) T cell clones from the T cell cultures of both donors led to the identification of four naturally processed HLA-DR-binding CAMEL epitopes: CAMEL(1-20), CAMEL(14-33), CAMEL(46-65), and CAMEL(81-102). Two peptides (CAMEL(1-20) and CAMEL(14-33)) also contain previously identified HLA class I-binding CD8(+) T cell epitopes shared by CAMEL and NY-ESO-ORF2 and are therefore interesting tools to explore for immunotherapy. Furthermore, two CD4(+) T cell clones that recognized the CAMEL(14-33) peptide with similar affinities were shown to differ in recognition of tumor cells. These CD4(+) T cell clones recognized the same minimal epitope and expressed similar levels of adhesion, costimulatory, and inhibitory molecules. TCR analysis demonstrated that these clones expressed identical TCR beta-chains, but different complementarity-determining region 3 loops of the TCR alpha-chains. Introduction of the TCRs into proper recipient cells should reveal whether the different complementarity-determining region 3 alpha loops are important for tumor cell recognition.

T Cells Distinguish MHC-Peptide Complexes Formed in Separate Vesicles and Edited by H2-DM

The peptide spanning residues 48-61 of hen egg white lysozyme (HEL) presented by I-A(k) gives rise to two T cell populations, referred to as type A and B, that distinguish the complex generated intracellularly upon processing of HEL from that formed with exogenous peptide. Here, we ascribe this difference to recognition of distinct conformers of the complex and show that formation of the two complexes results from antigen processing in different intracellular compartments and is dependent upon H2-DM. While the type A complex preferentially formed in a lysosome-like late vesicle, the type B complex failed to form in this compartment; this distinction was abolished in antigen-presenting cells lacking DM. Experiments in vitro indicated that H2-DM acts directly on the complex to eliminate the type B conformation. We conclude that different antigen-processing pathways generate distinct MHC-peptide conformers, priming T cells with distinct specificity that may play unique roles in immunity.

Characterization of the antigen specificity and TCR repertoire, and TCR-based DNA vaccine therapy in myelin basic protein-induced autoimmune encephalomyelitis in DA rats

Like Lewis rats, DA rats are an experimental autoimmune encephalomyelitis (EAE)-susceptible strain and develop severe EAE upon immunization with myelin basic protein (MBP). However, there are several differences between the two strains. In the present study we induced acute EAE in DA rats by immunization with MBP and MBP peptides and examined the Ag specificity and TCR repertoire of encephalitogenic T cells. It was found that although immunization with MBP and a peptide corresponding to its 62-75 sequence (MBP(62-75)) induced clinical EAE, the responses of lymph node T cells isolated from MBP-immunized rats to MBP(62-75) was marginal, indicating that this peptide contains major encephalitogenic, but not immunodominant, epitopes. The TCR analysis by CDR3 spectratyping of spinal cord T cells revealed that Vbeta10 and Vbeta15 spectratype expansion was always found in MBP(62-75)-immunized symptomatic rats. On the basis of these findings, we examined the encephalitogenicity of Vbeta10- and Vbeta15-positive T cells. First, the adoptive transfer experiments revealed that Vbeta10-positive T line cells derived from MBP(62-75)-immunized rats induced clinical EAE in recipients. Second, administration of DNA vaccines encoding Vbeta10 and Vbeta15, alone or in combination, ameliorated MBP(62-75)-induced EAE. Collectively, it was strongly suggested that Vbeta10- and Vbeta15-positive T cells are encephalitogenic. Analyses of the Ag specificity and T cell repertoire of pathogenic T cells performed in this study provide useful information for designing specific immunotherapies against autoimmune diseases.

APCs present A beta(k)-derived peptides that are autoantigenic to type B T cells

Type B T cells recognize peptide provided exogenously but are ignorant of the same epitope derived from intracellular processing. In this study, we demonstrate the existence of type B T cells to an abundant autologous peptide derived from processing of the I-A(k) beta-chain. T cell hybridomas raised against this peptide fail to recognize syngeneic APC despite abundant presentation of the naturally processed epitope but react in a dose-dependent manner to exogenous peptide. Moreover, these hybridomas respond to Abeta(k) peptide extracted from the surface of I-A(k)-expressing APC. This peptide was isolated from B cell lines where it was found in high abundance; it was also present in lines lacking HLA-DM, but in considerably lower amounts. Therefore, type B T cells exist in the naive repertoire to abundant autologous peptides. We discuss the implications of these findings to the potential biological role of type B T cells in immune responses and autoimmune pathology.

Role of disulfide bonds in regulating antigen processing and epitope selection

Knowledge of the events governing Ag processing and epitope selection within APC is key to the development of novel immunotherapeutic strategies for infectious diseases, cancer, and autoimmunity. The influence of disulfides and Ag reduction on the hierarchy of epitope presentation via MHC class II molecules was investigated through studies of a self Ag, IgG kappa. HLA-DR4(+) B cells preferentially present an immunodominant IgG-derived epitope, kappaI, relative to a subdominant kappaII peptide. kappaI contains a cysteine masked within the native Ag via an intrachain disulfide, the latter of which is reduced during Ag processing. Mutagenesis of this cysteine as well as others within kappa minimally perturbed the abundance and overall conformation of IgG. Yet, disruptions in disulfide bonding within this Ag influenced the selective display of class II-restricted dominant and subdominant T cell epitopes. Presentation of the kappaI epitope from both native and variant IgG was dependent upon cellular expression of IFN-gamma-inducible lysosomal thiol reductase. These studies indicate that disulfide bonds regulate Ag processing both locally and at distant sites, thus influencing epitope selection within the class II pathway.

Induction of transient arthritis by the adoptive transfer of a collagen II specific Th1 clone to HLA-DR4 (B1*0401) transgenic mice

Collagen II arthritis (CIA) represents an animal model of human RA that can be induced in DBA/1J (H-2(q)) but not in C57BL/6 mice (H-2(b)). A vigorous CII specific CD4 Th1-cell response but not IgG2 anti-CII antibody or CIA could be induced in C57BL/6 mice made transgenic for the RA shared epitope DR4 (B1*0401). We developed CD4 Th1-cell clones specific for CII from these transgenic (tg) mice in order to determine if the adoptive transfer of these clones into syngeneic tg C57BL/6 recipients could induce CIA. Three bovine CII specific (bCII) CD4 Th1-cell clones and one T-cell line specific for an immunodominant region of bCII (p261-273) were generated. Among these only one clone that could up-regulate anti-CII, IgG2 antibody in the recipient mice was able to induce transient arthritis. However, this level of IgG2 anti-CII antibody was only one-third of that seen in CII immunized DBA/1J mice that develop persistent arthritis. These results confirm our previous observations that the induction of CIA requires a sustained IgG2 antibody response to CII, an effect difficult to achieve even in DR4 (B1*0401) tg mice reconstituted with CD4 Th1 cells. This suggests that a rate limiting step in the development of human RA among those individuals expressing the RA shared epitope may be the requirement to generate sustained levels of complement fixing antibody to arthritogenic antigens.

The majority of immunogenic epitopes generate CD4+ T cells that are dependent on MHC class II-bound peptide-flanking residues

Peptides bind to MHC class II molecules with a defined periodicity such that the peptide-flanking residues (PFRs) P-1 and P11, which lie outside the core binding sequence (P1-P9), are solvent exposed and accessible to the TCR. Using a novel MHC class II:peptide binding assay, we defined the binding register for nine immunogenic epitopes to formally identify the flanking residues. Seven of the nine epitopes, restricted by H-2A(k), H-2A(g7), or H-2E(k), were found to generate T cells that were completely dependent on either P-1 or P11, with dependency on P-1 favored over P11. Such PFR dependency appears to be influenced by the type of amino acid exposed, in that residues that can form salt bridges or hydrogen bonds are favored over small or hydrophobic residues. Peptides containing alanine substitutions at P-1 or P11 in place of PFRs that mediate dependency were considerably less immunogenic and mediated a substantially reduced in vitro recall response to the native protein, inferring that PFR recognition increases immunogenicity. Our data suggest that PFR recognition is a common event characteristic of all MHC class II-restricted T cell responses. This key feature, which is not shared by MHC class I-restricted responses, may underlie the broad functional diversity displayed by MHC class II-restricted T cells.

Perspective on antigen processing and presentation

The phenomenon of antigen processing and presentation and the concept that T cells recognize peptides resulting from the partial catabolism of proteins, are relatively new. These concepts were first recognized and developed at a time when lymphocyte immunity - the adaptive system - and cellular immunity, with its major component of activated macrophages, were not perceived as part of one integrated system. To me, it was the fundamental findings on the role of major histocompatibility (MHC) molecules that set the framework for understanding how phagocytes and the antigen presenting cell (APC) system interact with the adaptive cellular system, in a truly symbiotic relationship (1). In this chapter we make a historical review of the developments that, in my biased opinion, led to the understanding of antigen presentation as a central event. I emphasize my own work, placing it in my perspective of how I saw the field moving.

Distinct recognition by two subsets of T cells of an MHC class II-peptide complex

We examine here the nature of the differential recognition by CD4+ T cells of a single peptide from hen-egg white lysozyme (HEL) presented by I-A(k) class II MHC molecules. Two subsets of T cells (called A and B) interact with the same peptide, each in unique ways that reflect the nature of the complex of peptide and MHC. We show that the A and B set of T cells can be distinguished by their functional interaction with the three T cell receptor (TCR) contact residues of the bound peptide. The dominant peptide of HEL selected from processing is bound in a single register where a critical TCR contact residue is situated about the middle of the core segment of the peptide: all T cells establish functional contact with it. Three sets of T cells, however, can be distinguished by their differential recognition of two TCR contacts situated at the amino and carboxyl sides of the central TCR contact residue. Type A T cells, the conventional cells that see the peptide after processing of HEL, need to recognize all three TCR contact residues. In contrast, the type B T cells recognize the peptide given exogenously, but not when processed: these T cells recognize either one of the peripheral TCR contact residues, indicating a much more flexible interaction of peptide with I-A(k) molecules. We discuss the mode of generation of the various T cells and their biological relevance.

Humanized mice as a model for rheumatoid arthritis

Genetic susceptibility to rheumatoid arthritis (RA), a common autoimmune disease, is associated with certain HLA-DR4 alleles. Treatments are rarely curative and are often tied to major side effects. We describe the development of a humanized mouse model wherein new, less toxic, vaccine-like treatments for RA might be pretested. This model includes four separate transgenes: HLA-DR*0401 and human CD4 molecules, a RA-related human autoantigenic protein (HCgp-39), and a T-cell receptor (TCRalphabeta) transgene specific for an important HCgp-39 epitope, eliciting strong Th1 responses in the context of HLA-DR*0401.

Deamidation of asparagine in a major histocompatibility complex-bound peptide affects T cell recognition but does not explain type B reactivity

We have analyzed a panel of T cell hybridomas specific for the chemically dominant epitope of hen egg-white lysozyme 48-61 which has asparagine 59 as an important T cell receptor contact residue. A number of T cells recognize 48-61 with asparagine at position 59, but not the aspartic acid or isoaspartic acid derivatives. Conversely, we find T cells that specifically recognize 48-61 bearing an isoaspartic acid at residue 59, but not asparagine. For other T cells, asparagine, aspartic acid, or isoaspartic acid at residue 59 is irrelevant. We present evidence that our previous distinction between type A and type B T cells is not explained by asparagine deamidation at residue 59.

T cell recognition of the dominant I-A(k)-restricted hen egg lysozyme epitope: critical role for asparagine deamidation

Type-B T cells raised against the immunodominant peptide in hen egg lysozyme (HEL(48-62)) do not respond to whole lysozyme, and this has been thought to indicate that peptide can bind to l-A(k) in different conformations. Here we demonstrate that such T cells recognize a deamidated form of the HEL peptide and not the native peptide. The sequence of the HEL epitope facilitates rapid and spontaneous deamidation when present as a free peptide or within a flexible domain. However, this deamidated epitope is not created within intact lysozyme, most likely because it resides in a highly structured part of the protein. These findings argue against the existence of multiple conformations of the same peptide-MHC complex and have important implications for the design of peptide-based vaccines. Furthermore, as the type-B T cells are known to selectively evade induction of tolerance when HEL is expressed as a transgene, these results suggest that recognition of posttranslationally modified self-antigen may play a role in autoimmunity.

Quantitation of lysozyme peptides bound to class II MHC molecules indicates very large differences in levels of presentation

Knowing the abundance of peptides presented by MHC molecules is a crucial aspect for understanding T cell activation and tolerance. In this report we determined the relative abundance of four distinct peptide families after the processing of the model Ag hen egg-white lysozyme. The development of a sensitive immunochemical approach reported here made it possible to directly quantitate the abundance of these four epitopes presented by APCs, both in vitro and in vivo. We observed a wide range of presentation among these four different epitopes presented on the surface of APCs, with 250-fold differences or more between the most abundant epitope (48-63) and the least abundant epitopes. Importantly, we observe similar ratios of presentation from APCs in vitro as well as from APCs from the spleens and thymi of hen egg-white lysozyme transgenic mice. We discuss the relationship between the amount of peptide presented and their binding to I-A(k) molecules, immunogenicity, and tolerogenicity.

Conformational alterations during biosynthesis of HLA-DR3 molecules controlled by invariant chain and HLA-DM

HLA-DM is known to catalyze the exchange of class II-associated invariant chain (Ii) peptide (CLIP) for cognate peptide during biosynthesis. In DM-negative cells HLA-DR3 molecules have been shown to predominantly present CLIP and to lack the DR3-specific mAb epitope 16.23, which has led to the assumption that CLIP prevents binding of mAb 16.23. In the present study we show that CLIP does not prohibit 16.23 epitope expression, but that the formation of this epitope is directly influenced by interactions of the DR molecule with Ii and DM. Detergent solubilized DR3 from wild-type as well as DM(-) cells bound CLIP in a 16.23(+) mode. On cells, however, neither CLIP nor antigenic peptide bound to DR3 in a 16.23(+) conformation, unless HLA-DM was expressed. Thus, HLA-DM appears to alter the conformation of DR3 in a peptide-independent fashion. Since in DM-deficient cells that also lack Ii, DR3 molecules assembled in a 16.23(+) conformation, we conclude that during biosynthesis Ii and DM exert opposing conformational constraints, characterized by suppressing or releasing 16.23 epitope expression. These results imply that DR3/peptide complexes, including DR3/ CLIP, can exist in two conformations depending on previous interaction with DM, but independent of the nature of the peptide bound. We show that these naturally occurring class II conformers can be selectively recognized by T cells.

Frequencies of neuroantigen-specific T cells in the central nervous system versus the immune periphery during the course of experimental allergic encephalomyelitis

Direct measurements of the frequency and the cytokine signature of the neuroantigen-specific effector cells in experimental allergic encephalomyelitis (EAE) are a continuing challenge. This is true for lymphoid tissues, and more importantly, for the CNS itself. Using enzyme-linked immunospot analysis (ELISPOT) assays, we followed proteolipid protein (PLP) 139--151-specific T cells engaged by active immunization of SJL mice. The total numbers of PLP(139--151)-specific CD4 cells were highest before disease onset. At this time, these cells resided in lymphoid and nonlymphoid tissues, but were not detected in the CNS. While the PLP(139--151)-specific cells reached high frequencies in the CNS during clinical EAE, in absolute numbers, less than 20% of them were present in the target organ, with the majority residing in the periphery throughout all stages of the disease. The numbers of PLP(139--151)-specific cells gradually declined in both compartments with time. While eventually this first wave of effector cells completely disappeared from the CNS, PLP(178--191)-specific cells became engaged, being detected first in the CNS. These data suggest that throughout all stages of EAE, the effector cells in the CNS are recruited from a vast peripheral reservoir, and that the second wave of effector cells is engaged while the first wave undergoes exhaustion.

Invariant chains with the class II binding site replaced by a sequence from influenza virus matrix protein constrain low-affinity sequences to MHC II presentation

Presentation of antigenic peptides by MHC II molecules is required to initiate CD4 T(h) cell responses. Some peptides, however, because of low affinity for MHC II, are not efficiently presented. A segment of the MHC II chaperon molecule, invariant chain (Ii), is known to bind early in biosynthesis with low affinity to the peptide binding groove. Here we have exploited the properties of Ii to manipulate the MHC II-loading pathway and to present low-affinity sequences. We used a deletion mutant of Ii where the promiscuous binding site to MHC II, which is adjacent to the groove binding segment, was deleted. A recombinant Ii (rIi) chimera, derived from this construct, was made in which the class II binding segment was exchanged for wild-type or single amino acid substitution variants of an HLA-DR1-restricted sequence from influenza matrix protein (MAT), which leads to MHC II allotype-specific binding. This rIi was expressed in antigen-presenting cells (APC) and introduced the MAT sequence into the MHC II-processing pathway. As expected, rIiMAT elicited antigen-specific, DR1-restricted T cell cytokine production and proliferation. Significantly, rIiMAT, that binds the HLA-DR4 allele with low affinity, elicited DR4-restricted IL-2 production but not proliferation. In contrast, exogenously provided MAT peptide failed to elicit any responses from DR4-restricted T cells. Compatible results were obtained with a single amino acid substitution variant (MAT(T)), which binds with high affinity to DR4 but low affinity to DR1. We conclude that loading of MHC II with antigenic peptides from endogenously synthesized rIi chimeras allows presentation of low-affinity sequences that cannot be presented if provided exogenously as peptides. Ii fusion proteins containing low-affinity antigenic sequences might be useful for vaccination with tumor antigens to overcome deficiencies in antigen presentation.

Characterization of a recombinant murine 18.5-kDa myelin basic protein

A recombinant hexahistidine-tagged 18.5-kDa isoform of murine myelin basic protein has been characterized biochemically and immunogenically, by mass spectrometry, by circular dichroism under various conditions (in aqueous solution, with monosialoganglioside G(M1), and in 89% 2-propanol), and by transmission electron microscopy. The preparations of this protein indicated a high degree of purity and homogeneity, with no significant posttranslational modifications. Circular dichroic spectra showed that this preparation had the same degree of secondary structure as the natural bovine 18.5-kDa isoform of myelin basic protein. Incubation of the recombinant protein with lipid monolayers containing a nickel-chelating lipid resulted in the formation of fibrous assemblies that formed paracrystals of spacings 4.8 nm between fibers and 3-4 nm along them.

Cutting edge: introduction of an endopeptidase cleavage motif into a determinant flanking region of hen egg lysozyme results in enhanced T cell determinant display

The choice of which determinants of a whole Ag will be presented on cell surface MHC class II molecules after uptake and processing by APC is the result of the interplay between structural characteristics of the Ag and the processing machinery of the APC. In this study, we demonstrate that introduction of a dibasic motif adjacent to a subdominant determinant enhances the presentation of this determinant from the whole molecule. This is the first report showing that a single amino acid substitution in a whole Ag, designed to introduce an endopeptidase recognition site, enhances display of class II-restricted determinants, most likely by creating a peptide chain cleavage in the antigenic molecule. Our findings have important implications for the understanding of immunodominance and for vaccine design.

Enhanced growth of primary tumors in cancer-prone mice after immunization against the mutant region of an inherited oncoprotein

One major objective of tumor immunologists is to prevent cancer development in individuals at high risk. (TG.AC x C57BL/6)F1 mice serve as a model for testing the feasibility of this objective. The mice carry in the germline a mutant ras oncogene that has an arginine at codon 12 instead of glycine present in the wild-type, and after physical (wounding) or chemical promotion, these mice have a high probability for developing papillomas that progress to cancer. Furthermore, F1 mice immunized with Arg(12) mutant ras peptide in complete Freund's adjuvant (CFA) develop T cells within 10 d that proliferate in vitro on stimulation with the Arg(12) mutant ras peptide. Within 14 d, these mice have delayed-type hypersensitivity to the peptide. Immunization with CFA alone or with a different Arg(12) mutant ras peptide in CFA induced neither response. To determine the effect of immunization on development of tumors, mice immunized 3 wk earlier were painted on the back with phorbol 12-myristate 13-acetate every 3 d for 8 wk. The time of appearance and the number of papillomas were about the same in immunized and control mice, but the tumors grew faster and became much larger in the mice immunized with the Arg(12) mutant ras peptide. Thus, the immunization failed to protect against growth of papillomas. The peptide-induced CD4(+) T cells preferentially recognized the peptide but not the native mutant ras protein. On the other hand, mice immunized with Arg(12) mutant ras peptide and bearing papillomas had serum antibodies that did bind native mutant ras protein. Together, these studies indicate that active immunization of cancer-prone individuals may result in immune responses that fail to eradicate mutant oncogene-expressing tumor cells, but rather induce a remarkable enhancement of tumor growth.

Helper T-cell epitope immunodominance associated with structurally stable segments of hen egg lysozyme and HIV gp120

Although many antigen sequences potentially can bind to the MHCII proteins, only a small number of epitopes dominate the immune response. Additional mechanisms of processing, presentation or recognition must restrict the immune response against a large portion of the antigen. A highly significant correlation is found between epitope immunodominance and local structural stability in hen egg lysozyme. Since antigen proteins are likely to retain substantial native-like structure in the processing compartment, protease action may be focused on regions that are most readily accommodated in the protease active sites, and thus, the intervening sequence are preferentially presented. Immunodominance also correlates with sequence conservation as expected from the constraints imposed by structure. These results suggest that the three-dimensional structure of the antigen limits the immune response against some antigen segments. For HIV gp120, a substantial improvement in the accuracy of epitope prediction is obtained by combining rules for MHCII binding with a restriction of the predicted epitopes to well-conserved sequences.

Hb(64-76) epitope binds in different registers and lengths to I-Ek and I-Ak

The nature of peptide binding to MHC molecules is intrinsically degenerate, in what, one given MHC molecule can accommodate numerous peptides which are structurally diverse, and one given peptide can bind to different alleles. The structure of the MHC class II molecules allows peptides to extend out of the binding groove at both ends and these residues can potentially influence the stability and persistence of peptide/class II complexes. We have previously shown that both I-E(k) and I-A(k)-restricted T cell hybridomas could be generated against the Hb(64-76) epitope. In this study, we characterized the binding register of the Hb(64-76) epitope to I-A(k), and showed that it was shifted by one residue in comparison to its binding to I-E(k), and did not use a dominant anchor residue at P1. This conclusion was further supported by the modeling of the Hb(64-76) epitope bound to I-A(k), which revealed that all of its putative anchor residues fit into their corresponding pockets. We identified the naturally processed Hb epitopes presented by both I-E(k) and I-A(k), and found that they consisted of different species. Those associated with I-A(k) being 20-22 residues long, whereas, those found to I-E(k) contained 14-16 residues. These findings suggested that the lack of a dominant P1 anchor could be compensated by the selection of longer peptides. Overall, these studies revealed the Hb(64-76) epitope bound to I-E(k) and I-A(k) in distinct registers and lengths, demonstrating the plasticity MHC molecules have in generating distinct TCR ligands from the same amino acid sequence.

Increasing the frequency of T-cell precursors specific for a cryptic epitope of hen-egg lysozyme converts it to an immunodominant epitope

Efforts to understand the mechanisms that govern how immunodominant T-cell epitopes are selected from protein antigens have focused mostly on differences in the efficiency of processing and presentation of peptide/major histocompatibility complex (MHC) complexes by antigen-presenting cells, while little attention has been directed at the role of the T-cell repertoire. In this report, the influence of the T-cell repertoire on immunodominance was investigated using transgenic mice that express the beta chain from a T-cell receptor specific for a cryptic Ek restricted epitope of hen-egg lysozyme, HEL85-96. In these mice, the frequency of HEL85-96-specific T-cell precursors is increased 10-20-fold over non-transgenic mice. Transgenic mice respond as well as non-transgenic controls to intact HEL, even though they respond poorly or not at all to a variety of other antigens, including the dominant H-2k restricted epitopes of HEL. Following immunization with native HEL, the only HEL peptide that could recall a response in vitro in the transgenic mice was HEL85-96. Therefore, this normally cryptic epitope is the sole immunodominant epitope in the transgenic mice, and this alteration in immune response is due solely to an increase in the frequency of specific T-cell precursors. An analysis of four additional H-2k restricted cryptic epitopes of HEL suggests that three are similarly limited by T-cell frequency, and that only one is consistent with a defect in efficient antigen presentation. This indicates that there are at least two different types of cryptic epitopes, one in which crypticity is caused by inefficient processing or presentation, and another in which the frequency of specific T-cell progenitors is limiting.

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.

Mechanisms and consequences of peptide selection by the I-Ak class II molecule

Important quantitative parameters can be utilized to define the selection and the immunogenicity of protein antigens precisely at a biochemical and a cellular level. Here we describe a naturally processed family of peptides comprising the dominant hen egg white lysozyme epitope, its major contribution to surface I-Ak molecules, the primary and auxiliary peptide anchors involved in its selection, and its display of T-cell receptor contacts. In addition, we explore the importance of the processing events that lead to the generation of residues flanking the minimal core epitope, the quantification of T-cell responses directed toward the epitope, and the ability of the dominant epitope to form two unique conformations within the binding groove. Lastly, we address the relationship between this dominant and a minor lysozyme epitope.

Unexpected Reactivities of T Cells Selected by a Single MHC-Peptide Ligand

In H2-DM mutant mice, most MHC class II molecules are bound by a single peptide, CLIP, derived from the class II-associated invariant chain. Previous studies showed that H2-DM− cells are defective in presenting synthetic peptides to class II-restricted T cells. In sharp contrast, however, the same peptides elicited strong CD4+ T cell responses in H2-DM− animals. We now provide an explanation for this apparent discrepancy. Peptide-specific CD4+ T cells from wild-type mice were efficiently stimulated by H2-DM+, but not by H2-DM− cells pulsed with the cognate peptide. In sharp contrast, CD4+ T cells from mutant animals specific for the same MHC-peptide combination recognized peptide-pulsed H2-DM+ and H2-DM− cells equally well. In addition, unlike Ag-specific T cells from wild-type animals, the reactivities of peptide-specific T cells from mutant animals could not be efficiently blocked by Abs specific for the cognate MHC class II-peptide combination. We further demonstrated that the distinct reactivities of CD4+ T cells from H2-DM+ and H2-DM− mice are due to differences in thymic selection. Collectively, these findings indicate that the CD4+ T cell repertoires of H2-DM+ and H2-DM− mice are remarkably different.

Oligodendrocyte-Specific Protein Peptides Induce Experimental Autoimmune Encephalomyelitis in SJL/J Mice

Oligodendrocyte-specific protein (OSP) is a recently isolated and cloned, 207-aa, hydrophobic, four-transmembrane protein found in CNS myelin. It represents ∼7% of total myelin protein. The OSP cDNA sequence has no significant homology with previously reported genes, but the predicted protein structure suggests that OSP is a CNS homologue of peripheral myelin protein-22. We previously reported the presence of anti-OSP Abs in the cerebrospinal fluid of relapsing-remitting multiple sclerosis (MS) patients, but not control patient groups. In this study, we tested the ability of a panel of 20-mer peptides with 10-aa overlaps, representing the sequence of murine OSP, to induce experimental autoimmune encephalomyelitis (EAE), an animal model for MS. SJL mice challenged with murine OSP peptides 52–71, 82–101, 102–121, 142–161, 182–201, and 192–207 exhibited clinical EAE. OSP:52–71 elicited severe relapsing-remitting EAE in some individuals. All other encephalitogenic peptides elicited, at most, a loss of tail tonicity from which the mice most often completely recovered. Mononuclear cell infiltrates and focal demyelination characteristic of EAE were evident. T cell proliferative responses were seen with all encephalitogenic peptides except 142–161 and 182–201. OSP peptides 72–91 and 132–151 did not cause clinical EAE, but did elicit robust proliferative responses. B10.PL and PL/J mice challenged with the same OSP peptide doses as SJL mice did not exhibit clinical EAE. These results in the SJL EAE model, together with the results from MS patient clinical samples, make OSP a promising candidate for autoantigenic involvement in MS.

Prevalent class I-restricted T-cell response to the Theiler's virus epitope Db:VP2121-130 in the absence of endogenous CD4 help, tumor necrosis factor alpha, gamma interferon, perforin, or costimulation through CD28

C57BL/6 mice mount a cytotoxic T-lymphocyte (CTL) response against the Daniel's strain of Theiler's murine encephalomyelitis virus (TMEV) 7 days after infection and do not develop persistent infection or the demyelinating syndrome similar to multiple sclerosis seen in susceptible mice. The TMEV capsid peptide VP2121-130 sensitizes H-2Db+ target cells for killing by central-nervous-system-infiltrating lymphocytes (CNS-ILs) isolated from C57BL/6 mice infected intracranially. Db:VP2121-130 peptide tetramers were used to stain CD8(+) CNS-ILs, revealing that 50 to 63% of these cells bear receptors specific for VP2121-130 presented in the context of Db. No T cells bearing this specificity were found in the cervical lymph nodes or spleens of TMEV-infected mice. H-2(b) mice lacking CD4, class II, gamma interferon, or CD28 expression are susceptible to persistent virus infection but surprisingly still generate high frequencies of CD8(+), Db:VP2121-130-specific T cells. However, CD4-negative mice generate a lower frequency of Db:VP2121-130-specific T cells than do class II negative or normal H-2(b) animals. Resistant tumor necrosis factor alpha receptor I knockout mice also generate a high frequency of CD8(+) CNS-ILs specific for Db:VP2121-130. Furthermore, normally susceptible FVB mice that express a Db transgene generate Db:VP2121-130-specific CD8(+) CNS-ILs at a frequency similar to that of C57BL/6 mice. These results demonstrate that VP2121-130 presented in the context of Db is an immunodominant epitope in TMEV infection and that the frequency of the VP2121-130-specific CTLs appears to be independent of several key inflammatory mediators and genetic background but is regulated in part by the expression of CD4.

Noncompetitive expansion of cytotoxic T lymphocytes specific for different antigens during bacterial infection

Listeria monocytogenes is an intracellular bacterium that elicits complex cytotoxic T-lymphocyte (CTL) responses in infected mice. The responses of CTL populations that differ in antigen specificity range in magnitude from large, dominant responses to small, subdominant responses. To test the hypothesis that dominant T-cell responses inhibit subdominant responses, we eliminated the two dominant epitopes of L. monocytogenes by anchor residue mutagenesis and measured the T-cell responses to the remaining subdominant epitopes. Surprisingly, the loss of dominant T-cell responses did not enhance subdominant responses. While mice immunized with bacteria lacking dominant epitopes developed L. monocytogenes-specific immunity, their ability to respond to dominant epitopes upon rechallenge with wild-type bacteria was markedly diminished. Recall responses in mice immunized with wild-type or epitope-deficient L. monocytogenes showed that antigen presentation during recall infection is sufficient for activating memory cells yet insufficient for optimal priming of naive T lymphocytes. Our findings suggest that T-cell priming to different epitopes during L. monocytogenes infection is not competitive. Rather, T-cell populations specific for different antigens but the same pathogen expand independently.

In vivo priming of T cells against cryptic determinants by dendritic cells exposed to interleukin 6 and native antigen

T cells recognizing poorly displayed self determinants escape tolerance mechanisms and persist in the adult repertoire. The process by which these T cells are primed is not clear, but once activated, they can cause autoimmunity. Here, we show that dendritic cells treated with interleukin 6 (IL-6) process and present determinants from a model native antigen in a qualitatively altered hierarchy, activating T cells in vitro and in vivo against determinants that were previously cryptic because of poor display. IL-6 does not induce conventional maturation of dendritic cells but alters the pH of peripheral, early endosomal compartments and renders the cells more susceptible to killing by chloroquine. Acidification of endosomes by ouabain mimics the effect of IL-6 and allows processing of the same cryptic determinant. These results suggest that cytokines such as IL-6 could initiate and help to propagate an autoimmune disease process by differentiating dendritic cells into a state distinct from that induced by normal maturation.

Isolation and Quantitation of a Minor Determinant of Hen Egg White Lysozyme Bound to I-Ak by Using Peptide-Specific Immunoaffinity

We report here the identification and quantitation of a minor epitope from hen egg white lysozyme (HEL) isolated from the class II MHC molecule I-Ak of APCs. We isolated and concentrated the peptides from the I-Ak extracts by a peptide-specific mAba, followed by their examination by electrospray mass spectrometry. This initial step improved the isolation, recovery, and quantitation and allowed us to identify 13 different minor peptides using the Ab specific for the HEL tryptic fragment 34–45. The HEL peptides varied on both the amino and carboxy termini. The shortest peptide was a 13-mer (residues 33–45), and the longest peptide was a 19-mer (residues 31–49). The two most abundant were 31–47 (1.3 pmol) and 31–46 (1 pmol), while the least abundant were 31–45 (40 fmol) and 32–45 (4 fmol). Only 0.3% of the total class II molecules were occupied by this family of HEL peptides. The amount of the 31–47 peptide, the predominant member of this series, was 22 times lower than that of 48–62, the major epitope of HEL. The 31–47 peptide bound about 20-fold weaker to I-Ak compared with the dominant 48–62 peptide. Thus, the lower abundance of the minor epitope correlated with its weaker binding strength.

Determinant spreading during experimental autoimmune encephalomyelitis: is it potentiating, protecting or participating in the disease?

During many autoimmune conditions, a T-cell response initially focused to a self-antigen evolves towards the recruitment of T cells to multiple antigenic determinants. Here we discuss whether such determinant spreading involves T cells activated in the peripheral lymphoid organs, or alternatively, whether the diversification occurs after infiltration of the initiating T cells into the target organ, for example, into the central nervous system during experimental autoimmune encephalomyelitis, a prototype for multiple sclerosis. The expression of myelin antigens in the thymus and spleen may not only contribute to the induction of tolerance but also to determinant spreading. In this case, the outcome of in vivo diversification may ultimately be determined by the balance between type 1 and type 2 responses to antigenic determinants derived from myelin components. Thus, spreading T cells could modulate disease progression positively or negatively, depending upon the nature of the accompanying cytokine secretion profile.

Epitope dominance: evidence for reciprocal determinant spreading to glutamic acid decarboxylase in non-obese diabetic mice

Autoimmune T-cell responses to peptide determinants of several autoantigens have recently been characterized. These data suggest that, in some autoimmune models, such as experimental autoimmune encephalomyelitis, T-cell responses may diversify from a nested set of peptides to include many other peptide regions. A similar immune phenomenon pertaining to autoimmune diabetes (IDDM) is observed in NOD mice. We have explored a similar pattern of T-cell responses related to age and disease status in NOD mice termed epitope dominance, which describes immune responses toward a pronounced subset of determinants of the autoantigen glutamic acid decarboxylase (GAD). Our studies have identified a total of five GAD epitopes between the 65 and 67 kDa isoforms. The magnitude of T-cell responses to these various determinants was dependent on the stage of disease as well as on whether mice were protected from disease. The T-cell responses of these epitopes in NOD mice correlated with the predicted binding of these peptides to the NOD class II molecule I-Ag7. We therefore propose a model which implicates antigen presenting cells as critical entities in the propagation of dominant responses to the presentation of autoantigens to T cells, particularly in the Th 1 environment of the NOD mouse. This hypothesis presents a new framework for the discussion and interpretation of the kinetics of T-cell responses to different peptide epitopes in autoimmune diseases such as IDDM.

Differential tolerance is induced in T cells recognizing distinct epitopes of myelin basic protein

Experimental allergic encephalomyelitis (EAE) is induced by T cell-mediated immunity to central nervous system antigens. In H-2u mice, EAE is mediated primarily by T cells specific for residues 1-11 of myelin basic protein (MBP). We demonstrate that differential tolerance to MBP1-11 versus epitopes in MBP121-150 is induced by expression of endogenous MBP, reflecting extreme differences in stability of peptide/MHC complexes. The diverse MBP121-150-specific TCR repertoire can be divided into three fine specificity groups. Two groups were identified in wild-type mice despite extensive tolerance, but the third group was not detected. Activated MBP121-150-specific T cells induce EAE in wild-type mice. Thus, encephalitogenic T cells that escape tolerance either recognize short-lived peptide/MHC complexes or express TCRs with unique specificities for stable complexes.

Effect of Antigen-Processing Efficiency on In Vivo T Cell Response Magnitudes

T lymphocytes eradicate and provide long-term immunity to infections caused by intracellular pathogens. The mechanisms that determine in vivo T cell response sizes are poorly understood. Although it is speculated that the relative processing efficiency of different epitopes determines the hierarchy of T cell responses following immunization, this hypothesis has not been rigorously tested. We therefore mutagenized the secreted p60 Ag of Listeria monocytogenes to alter the efficiency of T cell epitope generation. Ag-processing efficiencies in cells infected with the different L. monocytogenes mutants ranged from one H2-Kd-associated p60 217–225 epitope generated per 15 intracellularly degraded p60 molecules (1/15) to one epitope per 350 degraded p60 molecules (1/350), i.e., a spectrum encompassing a 20-fold range of efficiencies. Mice infected with L. monocytogenes secreting inefficiently processed p60 (1/350) did not mount p60 217–225-specific T cell responses. However, increasing the efficiency of Ag processing by a factor of 5 to 1/70 restored the T cell response size to normal, while further increases in the efficiency of p60 217–225 generation to 1/50, 1/35, and 1/17 did not further augment specific T cell responses. Our studies demonstrate an Ag-processing threshold for in vivo T cell activation. Surprisingly, once this threshold is achieved, further enhancement of Ag-processing efficiency does not enhance the size of T cell responses.

Coordinate regulation of complex T cell populations responding to bacterial infection

Bacterial infections activate complex T cell populations that differ in size and antigen specificity. We used tetramerized MHC class I molecules complexed with Listeria monocytogenes-derived epitopes to characterize four distinct CD8+ T lymphocyte populations during bacterial infection. Surprisingly, T cell populations differing in antigen specificity expand, contract, and enter the T cell memory compartment synchronously. Because the four L. monocytogenes epitopes are presented with different efficiencies and have distinct stabilities in infected cells, our findings suggest that these factors do not determine in vivo T cell dynamics. While T cell activation requires antigen presentation, the timing and extent of T cell expansion appear to be regulated in a coordinated fashion independent of antigen quantity and stability.

Immunodominance Does Not Result from Peptide Competition for MHC Class II Presentation

Competition for binding to MHC class II molecules between processed peptides derived from a single protein Ag is considered an important parameter leading to the presentation of a limited set of peptides by APCs. We tested the relevance of this competition process in a model Ag, the MalE protein, by deleting T cell epitopes or by introducing a competitor T cell peptide. We identified in DBA/1 (I-Aq) mice six immunodominant T cell determinants in the MalE sequence, 89–95, 116–123, 198–205, 211–219, 274–281, and 335–341. Synthetic peptides carrying these determinants were classified in three groups as weak, intermediate, or strong I-Aq binders in competition experiments with the PreS:T peptide of hepatitis B surface Ag. In vivo, synthetic MalE peptides with weak and intermediate MHC binding capacity were inhibited in their capacity to stimulate proliferative response in the presence of the PreS:T competitor peptide, whereas the strongest MHC binder was not. Strikingly, the insertion of the potent competitor PreS:T peptide into the MalE sequence, as a single copy or as four copies, did not inhibit the proliferative response to the six immunodominant peptides of the recipient protein. Moreover, deletion in the protein sequence disrupting either the weak (198–205) or strong (335–341) MHC binding determinant of MalE did not modify the proliferative response to the remaining T cell determinants as compared with wild-type MalE protein. Altogether, these results show that peptide competition for MHC binding may not represent the most important event in processes leading to immunodominance.