Isolation and characterization of antigen-Ia complexes involved in T cell recognition

Regulatory mechanisms of antitumor T cell responses in the tumor-bearing state

Tumor-bearing hosts develop antitumor immune responses. However, a number of immunosuppressive mechanisms come into operation with the progression of tumor growth. This article will review the observations regarding the modulation of antitumor immune responses in the tumor-bearing state, and consider the mechanisms underlying tumor-induced immune defects, especially in the light of the induction of an abnormal cytokine network. We will also describe the restoration of suppressed antitumor immune responses by administration of a particular cytokine, interleukin-12.

Fine chemical modifications at N- and C-termini enhance peptide presentation to T cells by increasing the lifespan of both free and MHC-complexed peptides

We investigated the effect of modifying the N- and/or C-termini of the snake toxin peptide 24-36 on its presentation to T cells. Acetylation at the N-terminus as well as amidation at the C-terminus enhanced the capacity of the peptide to activate T cells. Simultaneous modifications further increased the stimulating activity, the peptide becoming approximately 100-fold more potent than the unmodified peptide. Clearly, the introduced modifications increased the lifetime of the peptide free in solution, by decreasing its proteolytic degradation, during the T cell stimulation assays. Paradoxically, however, at similar concentrations of free peptides, the modified ones, especially those having an acetylated N-terminus, were much more active than the unmodified peptide, irrespective of the experimental conditions. These observations suggested that components other than protection from proteolytic degradation should be associated with the higher stimulating activities of the modified peptides. Accordingly, chasing experiments with APC revealed that acetylation at N-terminus caused a higher persistence of the peptides at APC surface. Together, our data indicate that (i) the T cell stimulating capacity of a peptide is associated with its lifespans in the free and MHC II bound states; and (ii) these lifespans can be greatly enhanced by introducing fine chemical modifications at N- and C-termini. These data may have some implications in designing more potent peptidic immunomodulators.

DM enhances peptide binding to class II MHC by release of invariant chain-derived peptide

Major histocompatibility complex (MHC) class II molecules bind antigenic peptides rapidly after biosynthesis in antigen-presenting cells (APCs). By contrast, the rate of peptide binding to purified class II molecules is remarkably slow. We find that purified HLA-DR molecules bind peptides rapidly in the presence but not the absence of HLA-DM, a recently identified heterodimer required for efficient antigen processing. The same effect is seen with immunoprecipitated DM, suggesting that DM interacts directly with DR. Class II-associated invariant chain peptides (CLIP) are selectively and rapidly released from DR during incubation with DM at pH 5. We conclude that DM is a cofactor that enhances peptide binding to DR molecules through a mechanism involving peptide exchange.

Naturally processed peptides from two disease-resistance-associated HLA-DR13 alleles show related sequence motifs and the effects of the dimorphism at position 86 of the HLA-DR beta chain

HLA-DR13 has been associated with resistance to two major infectious diseases of humans. To investigate the peptide binding specificity of two HLA-DR13 molecules and the effects of the Gly/Val dimorphism at position 86 of the HLA-DR beta chain on natural peptide ligands, these peptides were acid-eluted from immunoaffinity-purified HLA-DRB1*1301 and -DRB1*1302, molecules that differ only at this position. The eluted peptides were subjected to pool sequencing or individual peptide sequencing by tandem MS or Edman microsequencing. Sequences were obtained for 23 peptides from nine source proteins. Three pool sequences for each allele and the sequences of individual peptides were used to define binding motifs for each allele. Binding specificities varied only at the primary hydrophobic anchor residue, the differences being a preference for the aromatic amino acids Tyr and Phe in DRB1*1302 and a preference for Val in DRB1*1301. Synthetic analogues of the eluted peptides showed allele specificity in their binding to purified HLA-DR, and Ala-substituted peptides were used to identify the primary anchor residues for binding. The failure of some peptides eluted from DRB1*1302 (those that use aromatic amino acids as primary anchors) to bind to DRB1*1301 confirmed the different preferences for peptide anchor residues conferred by the Gly-->Val change at position 86. These data suggest a molecular basis for the differential associations of HLA-DRB1*1301 and DRB1*1302 with resistance to severe malaria and clearance of hepatitis B virus infection.

Mediation by HLA-DM of dissociation of peptides from HLA-DR

Human leukocyte antigen (HLA)-DM is an unconventional major histocompatibility complex (MHC) class II heterodimer that is important for B-cell-mediated antigen processing and presentation to MHC class II-restricted T cells. HLA-DM is encoded by two genes, DMA and DMB, which map to the MHC class II region, and shares some homology with MHC class I and class II proteins. Here we define the biochemical role of HLA-DM. Recombinant soluble HLA-DM heterodimers have been purified from culture supernatants of insect cell transformants. At pH 5.0, they induce the dissociation of a subset of peptides bound to HLA-DR, including a nested set of class-II-associated invariant chain peptides (CLIP). This process liberates HLA-DR and leads to the enhanced binding of exogenous peptides.

A syngeneic idiotype is immunogenic when borne by IgM but tolerogenic when joined to IgG

Some syngeneic monoclonal antibodies (mAb) elicit immune responses like conventional T-dependent antigens. To find out whether the heavy chain class (isotype) plays a role for the immunogenicity of an idiotype (Id), we isolated rare subclones of an IgM mAb (termed Id3) in which the variable region of the heavy chain (VH) is associated with a new constant region (CH). The VH-Id3 gene is a member of the murine 36-60 family and probably has three replacement mutations. The light chain V gene is germ-line V lambda 2. IgM, IgG1, IgG2a and IgG2b variants of Id3 were purified from protein-free medium and injected without adjuvant into BALB/c mice. The parental 19S IgM mAb given subcutaneously (s.c.) elicited a vigorous humoral response against Id3; in comparison, monomeric 8S IgM was a much weaker immunogen. Unlike IgM, multiple challenges with the IgG switch variants failed to induce anti-Id3 Ab. IgG variants gained immunogenicity if they were purified from medium containing fetal calf serum, mixed with complete Freund's adjuvant or injected into mice primed with IgM-Id3. Pretreatment with 100 micrograms s.c. + 50 micrograms of the IgG2a variant extinguished the Ab response to parental IgM, but the response to adjuvant-free bovine serum albumin was intact. Therefore, the tolerance induced by the IgG2a switch variant is antigen-specific and not due to toxicity. Significant inhibition of the Ab response to parental IgM was observed after treatment with 4 micrograms of the IgG2a switch variant. Administration of the IgG1 and IgG2b switch variants also inhibited this response significantly. Thus, the outcome of an encounter with Id3 is strongly influenced by the CH isotype to which the Id is joined. This suggests novel ways to minimize unwanted Ab responses against Id of human therapeutic mAb. In the context of the theory of Id networks, we suggest that dominant B cell clones can preempt anti-Id Ab responses against themselves by early switching from IgM to IgG secretion, before immunogenic IgM Ab have had time to activate anti-Id B cells.

pH dependent binding of high and low affinity myelin basic protein peptides to purified HLA-DR2

Major histocompatibility complex (MHC) class II molecules are cell surface glycoproteins and are known to display processed antigens on the surface of antigen presenting cells (APC). Within the APC, the loading of processed antigenic peptides to MHC class II molecules is known to take place in the endosomal compartment at acidic pH environment. The present study describes the in vitro effect of pH on binding of four biotinylated myelin basic protein (MBP) peptides to affinity purified HLA-DR2 containing a mixture of DRB1*1501 and DRB5*0101 beta chain. The binding affinity of the selected peptides are in the order of MBP(83-102)Y83 > MBP(124-143) > MBP(143-168) > MBP(1-14). Most of these peptides in association with HLA-DR2 are considered as immunodominant epitopes for human multiple sclerosis autoimmune disorder. One epitope, MBP(1-14), had almost no affinity to purified HLA-DR2 and was used as a control peptide in all binding assays. The quantitation of the bound peptide at various pH was carried out by antibody capture of complexes followed by avidin-alkaline phosphatase detection system. Among four peptides tested, only the highest affinity MBP(83-102)Y83 peptide showed maximum binding to purified HLA-DR2 at acidic pH. Two other epitopes, MBP(124-143) and MBP(143-168), showed maximum binding at basic and neutral pH values, respectively. The binding of only high affinity peptides, MBP(83-102)Y83 and MBP(124-143), was significantly affected by changing the pH of the binding buffer. Such alteration in pH of the binding buffer resulted in 100% occupancy of DR2 with both high affinity MBP peptides. In contrast, no significant increase in binding of the low affinity MBP(143-168) peptide was observed at altered pH values. The specificity of the increased binding of high affinity peptides to HLA-DR2 at optimum pH was demonstrated by competitive binding assays using non-biotinylated peptides. Finally, the stability of various MBP peptide bound complexes was tested at 4 degrees, 25 degrees and 37 degrees C which correlates well with their affinity to HLA-DR2. These results suggest that pH plays an important role in in vitro binding of antigenic peptides and such manipulation of binding conditions can be utilized in generating 100% loaded MHC class II with high affinity antigenic peptides. Since high affinity peptides are generally considered as major immunodominant epitopes, the in vitro pH dependent binding can be utilized in screening immunodominant epitopes of various autoantigens and generating complexes of defined composition.

Detection of functional class II-associated antigen: role of a low density endosomal compartment in antigen processing

We have developed a functional assay to identify processed antigen in subcellular fractions from antigen-presenting cells; stimulatory activity in this assay may be caused by either free peptide fragments or by complexes of peptide fragments and class II molecules present on organellar membrane sheets and vesicles. In addition, we have developed a functional assay to identify proteolytic activity in subcellular fractions capable of generating antigenic peptides from intact proteins. These techniques permit the direct identification of intracellular sites of antigen processing and class II association. Using a murine B cell line stably transfected with a phosphorylcholine (PC)-specific membrane-bound immunoglobulin (Ig), we show that PC-conjugated antigens are rapidly internalized and efficiently degraded to generate processed antigen within an early low density compartment. Proteolytic activity capable of generating antigenic peptide fragments from intact proteins is found within low density endosomes and a dense compartment consistent with lysosomes. However, neither processed peptide nor peptide-class II complexes are detected in lysosomes from antigen-pulsed cells. Furthermore, blocking the intracellular transport of internalized antigen from the low density endosome to lysosomes does not inhibit the generation of processed antigen. Therefore, antigens internalized in association with membrane Ig on B cells can be efficiently processed in low density endosomal compartments without the contribution of proteases present within denser organelles.

Receptor-ligand interactions measured by an improved spun column chromatography technique. A high efficiency and high throughput size separation method

Size exclusion chromatography may under the right circumstances be an easy and powerful way to measure in solution the interaction between a receptor an dits ligand. Spun column chromatography is a fast size exclusion technique of increasing popularity, however, little information exists on the method development essential to obtain efficient separation in particular when used for analytical purposes. In this paper we describe a systematic approach to select the optimal parameters for spun column separation including a simple modification of the technique whereby the spun columns are eluted by high-speed gradient centrifugation. This modification is easy to implement and it considerably improves spun column performance. We hypothesize that the high-speed centrifugation step leads to the release of additional buffer which assists in the complete elution of excluded molecules while the gradient centrifugation helps to achieve equilibrium across the gel matrix during the elution. The new method has been used successfully for several different receptor-ligand interactions, and this paper describes a general approach on how to develop new applications of the technique.

Normal HBsAg presentation and T-cell defect in the immune response of nonresponders

Lymphocytes from nonresponders to HBsAg fail to proliferate in vitro in the presence of HBsAg-pulsed antigen presenting cells. We studied four pairs of major histocompatibility complex (MHC)-matched, mixed lymphocyte reaction-negative individuals discordant for HBsAg response. For each pair, responder lymphocytes proliferated in the presence of nonresponder antigen-pulsed antigen presenting cells. Responder and nonresponder antigen presenting cells were equally effective. There was no evidence for inhibition of responder T-cell proliferation by nonresponder lymphocytes or antigen presenting cells. The defect is thus in the helper T cells of nonresponders and not in the antigen processing or binding of processed peptides to MHC molecules on antigen presenting cells.

Hindrance of binding to class II major histocompatibility complex molecules by a single amino acid residue contiguous to a determinant leads to crypticity of the determinant as well as lack of response to the protein antigen

The immune system has evolved the potential to respond to a wide variety of antigens, yet unresponsiveness to many foreign determinants is encountered frequently. Here, we report a lack of response to a particular determinant, hen egg lysozyme (HEL)-(46-61)-peptide (p46-61), in C57BL/6 (H-2b) mice, whereas a strong T-cell response to this determinant is obtained in major histocompatibility complex (MHC)-identical C3H.SW mice. However, (C3H.SW x C57BL/6)F1 mice respond well to p46-61, suggesting the absence of a p46-61-specific "hole" in the T-cell repertoire in C57BL/6 mice. We further show that p46-61 cannot bind the I-Ab class II MHC molecule, whereas p46-60 lacking Arg61 exhibits good binding and is immunogenic in both strains. Thus, the presence of the hindering residue, Arg61, renders p46-61, a dominant determinant in C3H.SW, into a silent, cryptic determinant in C57BL/6 mice. Upon i.p. immunization with HEL, no T-cell responses to either HEL or p46-61 could be demonstrated in spleens of HEL-primed C57BL/6 mice, whereas a predominant response to p46-61 and HEL was demonstrated in C3H.SW mice. Evidently, C57BL/6 mice differ from C3H.SW in their ability to process p46-61 into an actual I-Ab binding determinant, indicating a putative enzymatic defect in the C57BL/6 strain. Furthermore, our results suggest that the inability of C57BL/6 mice to respond in the spleen to HEL is based upon its failure to generate a dominant immunogenic determinant from HEL, coupled with its pattern of susceptibility to regulatory effects.

Binding of an invariant-chain peptide, CLIP, to I-A major histocompatibility complex class II molecules

Invariant chain (Ii) associates with major histocompatibility complex (MHC) class II molecules and is crucial for antigen presentation by class II molecules. The exact nature of Ii interaction with MHC class II molecules remains undefined. A nested set of Ii peptides, CLIPs (class II-associated Ii peptides), have been eluted from various MHC class II molecules, suggesting that CLIPs correspond, at least in part, to the Ii motif which blocks the conventional peptide binding site in MHC class II molecules. Here we report how CLIPs interact with class II MHC molecules, I-A. We have identified regions critical for binding of CLIPs and I-A class II molecules. In most cases, the binding of CLIPs to a number of I-A molecules is modulated by the steric bulk of methionine residues at positions 93 and 99. In addition, the binding of CLIPs to an I-A molecule, I-Au, is sensitive to substitutions at aspartic acid-59 in the alpha chain and threonine-86 in the beta chain, whereas the binding of an antigen-derived peptide is not. Taken together, these results provide an insight as to how CLIPs bind to MHC class II heterodimers.

Short-lived complexes between myelin basic protein peptides and IAk

Kinetic rate constants and the equilibrium dissociation constant have been determined for the reaction between an affinity-purified class II major histocompatibility complex molecule IAk and a myelin basic protein analogue peptide, fluorescein-labeled Ac(1-14)A4C15. Under the experimental conditions used, the lifetime of the peptide-free IAk molecule with respect to inactivation is 3.1 hr. The equilibrium dissociation constant, 3.3 +/- 1.7 microM, is determined from measurements of the kinetics of peptide inhibition of IAk inactivation. The measured peptide dissociation halftime is relatively short, 30 min, and the deduced association rate is 100 M-1.s-1. The rate constants and the equilibrium constant are similar to those characteristic of kinetic intermediates in reactions of peptides and class II proteins that lead to long-lived terminal complexes.

Recognition of a lipid antigen by CD1-restricted alpha beta+ T cells

Major histocompatibility complex (MHC) class I and class II molecules bind immunogenic peptides and present them to lymphocytes bearing the alpha beta T-cell antigen receptor (TCR). An analogous antigen-presenting function also has been proposed for the non-MHC-encoded CD1 molecules, a family of non-polymorphic, beta 2-microglobulin-associated glycoproteins expressed on most professional antigen-presenting cells. In support of this hypothesis, CD1 molecules are recognized by selected CD4-CD8- alpha beta or gamma delta TCR+ T-cell clones, and we have recently shown that CD1 molecules restrict the recognition of foreign microbial antigens by alpha beta TCR+ T cells. But the substantial structural divergence of CD1 from MHC class I and class II molecules, raises the possibility that the antigens presented by the CD1 system may differ fundamentally from those presented by MHC-encoded molecules. Here we report that a purified CD1b-restricted antigen of Mycobacterium tuberculosis presented to alpha beta TCR+ T cells is mycolic acid, a family of alpha-branched, beta-hydroxy, long-chain fatty acids found in mycobacteria. This example of non-protein microbial antigen recognition suggests that alpha beta TCR+ T cells recognize a broader range of antigens than previously appreciated and that at least one member of the CD1 family has evolved the ability to present lipid antigens.

Endogenous antigen presentation by MHC class II molecules

T cell recognition of antigen requires that a complex form between peptides derived from the protein antigen and cell surface glycoproteins encoded by genes within the major histocompatibility complex (MHC). MHC class II molecules present both extracellular (exogenous) and internally synthesized (endogenous) antigens to the CD4 T cells subset of lymphocytes. The mechanisms of endogenous antigen presentation are the subject of this review. Isolation and amino acid sequencing of peptides bound to the class II molecule indicate that a very high proportion (70-90%) of the total peptides presented by the class II molecule are in fact derived from the pool of proteins that are synthetized within the antigen-presenting cell (APC). This type of sequence information as well as the study of model antigens has indicated that proteins expressed in a diversity of intracellular sites, including the cell surface, endoplasmic reticulum and cytosol can gain access to the class II molecule, albeit with different efficiencies. The main questions that remain to be answered are the intracellular trafficking patterns that allow colocalization of internally synthesized antigens with the class II molecule, the site(s) within the cell where peptide:class II molecule complex formation can take place and whether presentation of 'foreign' as well as 'self' antigens takes place by mechanisms that vary from one cell type to another or that vary with the metabolic state of the APC. If such variability exists, is would imply that the array of peptides displayed by class II molecules at the cell surface has similar variability, a possibility that would impact on self tolerance and autoimmunity.

Development of high potency universal DR-restricted helper epitopes by modification of high affinity DR-blocking peptides

Pan DR-binding peptides engineered by introducing anchor residues for different DR motifs within a polyalanine backbone bound 10 of 10 DR molecules tested, with affinities, in most cases, in the nanomolar range. Because of the small methyl group exposed for T cell recognition, these peptides were poor immunogens but effective blockers of DR-restricted antigen presentation. Introduction of bulky and charged residues at positions accessible for T cell recognition yielded extremely powerful Pan DR epitope peptides (PADRE). These peptides elicited powerful responses in vitro from human peripheral blood mononuclear cells (PBMC). Because these cells also cross-react on certain mouse class II alleles, we could also demonstrate that PADRE peptides are active in vivo. In one example of their capacity to elicit T help, they were approximately 1000 times more powerful than natural T cell epitopes. We propose that PADRE peptides may be useful in the development of subunit vaccines.

DNA typing of HLA class II genes; DRB1*0803 increases the susceptibility of Japanese to primary biliary cirrhosis

The association between human leukocyte antigens and primary biliary cirrhosis is controversial, but major histocompatibility complex class II antigen DR8 was recently reported to be associated with increased susceptibility for primary biliary cirrhosis in some Caucasians and Japanese. Accordingly, we performed DNA typing of HLA class II genes in Japanese patients with primary biliary cirrhosis. The genotypes of HLA DRB1, DRB3-5, DQA and DQB were determined by polymerase chain reaction and subsequent hybridization with sequence specific oligonucleotides in 31 primary biliary cirrhosis patients and 215 racially matched local controls. DR8 was found in 24 of the 31 primary biliary cirrhosis patients and was highly concentrated in DRB1*0803. The gene frequency of DRB1*0803 was significantly increased in the patients (35.5% vs 7.4%, relative risk = 6.84, p < 0.0001). DQA1*0103 and DQB1*0601 were also increased in the primary biliary cirrhosis patients, in relation to linkage disequilibrium with DRB1*0803 on the same haplotype. In contrast, DQA1*0102 showed a significantly lower frequency in the primary biliary cirrhosis patients (p < 0.05). These data suggest that DRB1*0803 is one of the HLA class II genes related to an increased risk of primary biliary cirrhosis in Japanese individuals.

Effects of MHC II conformation and pH on the recognition of peptide by T cells

In this study we analysed the binding of the peptide HEL46-61 to purified Ak molecules which have been altered by site-directed mutagenesis at polymorphic positions to include amino acids from the Ad alpha-chain. We find that changes in the floor of the peptide binding groove, at positions 11, 14 and 28, abolish T cell recognition without changing peptide binding affinity. We further show that amino acid changes at these positions in the Ad molecule result in a conformationally altered molecule as evidenced by loss of binding of the Ad alpha specific monoclonal antibody K24. Thus the T cell receptor is highly sensitive to subtle changes in MHC II structure induced at sites that are unlikely to be involved in direct T cell contact. This has important implications with respect to allorecognition. The binding studies reported here were performed both at pH 7, to reflect binding of peptides at the cell surface, and at pH 5.5, to mimic binding in an intracellular acidic compartment. Binding to wild-type Ak was increased 2-3-fold at pH 5.5, whereas binding to some MHC II mutants was increased by greater than 20-fold at pH 5.5 relative to pH 7. These results show that the apparent peptide binding specificity for the mutants differs at pH 7 and 5.5, and suggest that caution should be used in defining the MHC-restriction of peptide epitopes at neutral pH.

Molecular chaperones in the processing and presentation of antigen to helper T cells

Helper T lymphocytes recognize peptide fragments of antigen bound to Major Histocompatibility Complex (MHC) class II molecules on the surfaces of antigen presenting cells (APC). Antigen processing involves internalization of the antigen into an acidic compartment where the antigen is degraded and the resulting peptide fragments of the antigen are bound to MHC class II molecules and the complexes subsequently displayed at the APC surface. Thus, antigen processing represents a complex, intracellular assembly process which may, like many intracellular protein folding and assembly processes, require the function of molecular chaperones. This contribution focuses on the evidence which suggests that members of the heat shock protein family of molecular chaperones play a role in this pathway.

T cell epitope analysis with peptides simultaneously synthesized on cellulose membranes: fine mapping of two DQ dependent epitopes

Several MHC class II restricted, CD4+ human T cell clones from three donors were induced with soluble matrix protein of influenza virus. The epitopes recognized by these clones were mapped using a complete set of overlapping 15-mer peptides synthesized with the newly developed SPOT method which allows the simple simultaneous synthesis of numerous peptides on cellulose membranes. Fine analysis of two clones by truncation of the stimulatory peptide by single subsequent amino acids from either the NH2- or the COOH-terminus revealed the minimal stimulatory determinants of two DQ dependent T cell epitopes.

Advances in immunology

Much was accomplished in the last decade in understanding how the adaptive immune system evolved to combat pathogens. Essential features of antigen presentation and T lymphocyte recognition were deciphered, setting the stage for further studies that elucidated basic elements of lymphocyte differentiation (including positive and negative selection during lymphocyte ontogeny) and the major interactions that occur among cells in secondary lymphoid organs in an ongoing immune response. The major challenges of today are found in the burgeoning fields of programmed cells death, enzymology of recombination and somatic mutation, development of memory, and the recognition of pathogens by unconventional lymphocytes.

Allele specificity of structural requirement for peptides bound to HLA-DRB1*0405 and -DRB1*0406 complexes: implication for the HLA-associated susceptibility to methimazole-induced insulin autoimmune syndrome

Self-peptides bound to HLA-DR4 (DRA-DRB1*0405 complex) were eluted from the purified DR4 complex, fractionated on reverse-phase HPLC, and subjected to NH2-terminal sequencing. Seven independent sequences were obtained, and all putative peptides synthesized bound to DRB1*0405 as well as DRB1*0406 complex, which differ only at DR beta residues 37, 57, 74, and 86. Binding assay using analogue peptides of a DR4 binder GSTVFDNLPNPE revealed that FxxLxN is an important anchor motif necessary for binding (where x is any amino acid), which was common to DRB1*0405 and 0406. Determination of the binding affinity of 60 synthetic AAFAALANAA-based analogue peptides showed that substituting F to W or C; L to F, W, or Y; and N to Q or S on AAFAALANAA changed the affinity substantially between DRB1*0405 and DRB1*0406. It is noteworthy that all patients with methimazole-induced insulin autoimmune syndrome are positive for DRB1*0406 and negative for DRB1*0405. Interestingly, the quantitative structural motif identified in this study predicted that 8TSICSLYQLE17 of human insulin alpha chain may bind specifically to DRB1*0406 using its 10IxxLxQ15 motif. Indeed, DRB1*0406 complex bound 8TSICSLYQLE17 with a high affinity, and in striking contrast, DRB1*0405 complex did not. Furthermore, a short-term T cell line specific to human insulin established from a DRB1*0406-bearing individual did show reactivity with a peptide fragment containing the 10IxxLxQ15 motif. Although this fragment probably exists at a very low level under normal physiological conditions due to the disulfide bond between flanking cysteine residues (6Cys-11Cys), a reducing compound such as methimazole may cleave the disulfide bond in vivo and allow DR alpha-DRB1*0406 complex on antigen-presenting cells to bind much of the linear fragment of insulin alpha chain, which may lead to the activation of self-insulin-specific T-helper cells.

Binding of a soluble alpha beta T-cell receptor to superantigen/major histocompatibility complex ligands

The genes for the alpha and beta chains of a murine T-cell receptor were truncated just prior to the portions encoding the transmembrane regions and introduced into baculovirus by recombination. Insect cells infected with the virus secreted a soluble form of the receptor that could be purified to homogeneity. This soluble receptor reacted with a set of six monoclonal antibodies originally raised to different epitopes on the natural transmembrane-region-containing receptor and bound with appropriate specificity to a cell surface complex of the human major histocompatibility complex class II molecule DR1 with the bacterial superantigen staphylococcal enterotoxin B.

MHC class II function preserved by low-affinity peptide interactions preceding stable binding

Major histocompatibility complex class II molecules and their peptide ligands show unusual interaction kinetics, with slow association and dissociation rates that yield an apparent equilibrium constant of approximately 10(-6)-10(-8) M (refs 1-5). However, there is evidence for a specific, rapidly formed, short-lived complex. The altered migration on SDS-polyacrylamide gel electrophoresis of class II molecules upon stable peptide binding has led to the hypothesis that the two kinetically distinguishable types of class II-peptide complexes correspond to different structures. In accord with this model, we demonstrate here that insect cell-derived HLA-DR1 class II molecules show fast, almost stoichiometric occupancy with rapidly dissociating peptide while remaining sensitive to SDS-induced chain dissociation. The same DR1 molecules slowly and quantitatively form long-lived complexes resistant to SDS-induced denaturation. Surprisingly, low-affinity interaction with peptide protects class II from denaturation at physiological temperature, a finding that has implications for understanding the role of invariant chain in the intracellular behaviour of class II molecules.

T cell receptor-MHC class I peptide interactions: affinity, kinetics, and specificity

The critical discriminatory event in the activation of T lymphocytes bearing alpha beta T cell receptors (TCRs) is their interaction with a molecular complex consisting of a peptide bound to a major histocompatibility complex (MHC)-encoded class I or class II molecule on the surface of an antigen-presenting cell. The kinetics of binding were measured of a purified TCR to molecular complexes of a purified soluble analog of the murine MHC class I molecule H-2Ld (sH-2Ld) and a synthetic octamer peptide p2CL in a direct, real-time assay based on surface plasmon resonance. The kinetic dissociation rate of the MHC-peptide complex from the TCR was rapid (2.6 x 10(-2) second-1, corresponding to a half-time for dissociation of approximately 27 seconds), and the kinetic association rate was 2.1 x 10(5) M-1 second-1. The equilibrium constant for dissociation was approximately 10(-7) M. These values indicate that TCRs must interact with a multivalent array of MHC-peptide complexes to trigger T cell signaling.

Inhibition of class II MHC-peptide complex formation by protease inhibitors

Studies on the kinetics of antigenic peptide binding to major histocompatibility complex class II molecules have been used extensively to probe major histocompatibility complex (MHC) structure as well as to investigate the molecular mechanism of peptide recognition. Previous experiments have frequently been carried out in the presence of a cocktail of protease inhibitors to inhibit the proteolysis of MHC heterodimers. By using high performance size exclusion chromatography to measure fluorescent peptide binding to MHC protein, we have found that the addition of a commonly used mixture of protease inhibitors leads to a significant reduction in peptide binding to the class II heterodimer.

Binding of ras oncogene peptides to purified HLA-DQ(alpha 1*0102,beta 1*0602) and -DR(alpha,beta 1*0101) molecules

Mutated oncogene peptides may be presented to T cells by HLA molecules. To be able to design the optimal peptides for stimulation of T cells in individuals with different HLA molecules, it is important to analyse the binding characteristics of oncogene peptides to HLA. HLA-DQ6 (DQ(alpha 1*0102,beta 1*0602)) and HLA-DR1 (DR(alpha,beta 1*0101)) molecules were purified from lysates of homozygous EBV-transformed cell lines. Purified HLA molecules were then tested for their ability to bind synthetic peptides in gel filtration assays. A p21 ras oncogene peptide (previously found to stimulate DQ6-restricted T-cell clones) and an influenza matrix peptide were labelled with 125I and served as indicator peptides for binding to DQ6 and DR1 respectively. Binding of homologous truncated and mutated p21 ras peptides and unrelated peptides was then evaluated by their capacity to inhibit binding of the indicator peptides. p21 ras-derived peptides were found to bind to both DQ6 and DR1 molecules indicating the existence of a promiscuous binding motif in these peptides. The binding affinities seemed to vary between the different peptides, but the amino acid substitutions resulting from natural mutations were not critical for binding. Notably, the results obtained for DQ6 in the biochemical peptide binding assay correlated well with results obtained in a functional assay using T-cell clones as probes.

Preformed purified peptide/major histocompatibility class I complexes are potent stimulators of class I-restricted T cell hybridomas

A panel of antigen-specific, major histocompatibility complex class I-restricted T cell hybridomas has been generated to examine the capacity of peptide/class I complexes to stimulate T cells at the molecular level. Peptide/class I complexes were generated in detergent solution, purified and quantitated. Latex particles were subsequently coated with known amounts of preformed complexes and used to stimulate the T cell hybridomas. Stimulation was specific, i.e. only the appropriate peptide/class I combination were stimulatory, and quite sensitive, i.e. as little as 300 complexes per bead could be detected by the T cells. Preformed complexes were about 500,000 times more potent than free peptide in terms of T cell stimulation, demonstrating the physiological relevancy of the biochemically generated complexes. Surprisingly, the majority (including the most sensitive of the hybridomas) had lost CD8 expression, suggesting that antigen-specific stimulation of class I-restricted T cell hybridomas, as assessed by IL-2 release, does not depend on CD8.

Attachment of oligosaccharides to peptide antigen profoundly affects binding to major histocompatibility complex class II molecules and peptide immunogenicity

To investigate the immunogenicity of glycopeptides, a peptide fragment from hen egg lysozyme, HEL(81-96)-Y (here named 1) which is immunogenic in H-2k mice and known to bind to the murine major histocompatibility complex (MHC) class II molecule Ek, was synthesized in five different glycosylated forms. The N-terminal serine of HEL(81-96)-Y was derivatized with D-glucose (2), maltotriose (3), and a branched D-glucose pentasaccharide (4). Furthermore, 1 was prepared with a central serine or asparagine derivatized with the branched D-glucose pentasaccharide (5) and GlcNAc (6), respectively. The ability of the five glycopeptides and the non-glycosylated peptide, labeled with 125I, to bind to the two MHC class II molecules, Ak and Ek, was studied using a gel filtration assay. None of them could bind to Ak. Neither 5 nor 6 were able to bind to Ek. Surprisingly 2, 3 and 4 bound better to Ek than did the non-glycosylated peptide 1. The increased binding varied depending on the type of oligosaccharide attached to the N terminus of the peptide. The better binding to Ek of glycopeptide 4 was found to be due to an increased association rate. The binding of 1 as well as 4 was optimal at pH 5.0. Functional studies showed that 4 was able to elicit a heteroclitic proliferative response from T cells of mice immunized with the native non-glycosylated peptide. Circular dichroism studies of 1 and 4 indicated a more unordered structure of 4 and a predominant alpha-helical conformation of 1, suggesting that the MHC class II molecule may bind to peptides which are in a non-alpha-helical conformation. These results demonstrate that glycosylation has considerable influence on peptide immunogenicity for T lymphocytes.

Separation of subcellular compartments containing distinct functional forms of MHC class II

Antigen processing in B lymphocytes entails initial binding of antigen to the surface Ig and internalization of the antigen into acidic compartments where the antigen is degraded, releasing peptides for binding to major histocompatibility complex class II molecules. Using subcellular fractionation techniques we show that functional, processed antigen-class II complexes capable of activating antigen-specific T cells in vitro are first formed in dense vesicles cosedimenting with lysosomes which are distinct from early endosomes and the bulk of late endosomes. With time, processed antigen-class II complexes appear in vesicles sedimenting with early endosomes and finally cofractionate with plasma membrane. A separate compartment is identified which contains major histocompatibility complex class II receptive to peptide binding but which does not have access to processed antigen in the B cell. These class II molecules are in the so-called "floppy" form in contrast to the class II molecules in the very dense vesicles which are in the "compact" form. These results demonstrate a correlation between the floppy and compact forms of class II molecules and their association with processed antigen and show that floppy and compact forms of class II reside in distinct and physically separable subcellular compartments.

Rules for peptide binding to MHC class II molecules

CD4+ T lymphocytes recognize peptide fragments of antigens that lie in the antigen binding pocket of class II major histocompatibility complex (MHC) molecules expressed on antigen-presenting cells. Specificity of T cells is determined by structural features of both the MHC molecule and antigenic peptide. MHC class II amino acid sequences are highly polymorphic within a population, and correlate with individual differences in response to infectious agents, vaccines, tumour antigens, and autoantigens. In the last few years several important breakthroughs and technological advances have made it possible to clarify the role of polymorphism and the molecular events in peptide interaction with major histocompatibility complex (MHC) class II proteins. The X-ray structural analysis of a MHC class II molecule together with the use of peptide libraries has permitted determination of the structural features of the complex between peptide antigen fragments and MHC class II proteins. The purpose of this article is to review our own studies and those of others on the requirements for peptide-class II molecule interaction and discuss possible implications for active immune intervention.

More efficient peptide binding to MHC class II molecules during cathepsin B digestion of Ii than after Ii release

The binding of a T cell-presented peptide to MHC class II alpha,beta chains occurs as a concurrent process with the release of the associated invariant chain (Ii) by cathepsin B. Ii was digested by cathepsin B from solubilized, MHC class II alpha,beta,Ii complexes in the presence of N-hydroxysuccinimidyl-4-azidobenzoate-conjugated, 125I-labeled, influenza virus matrix (18-29) peptide. The peptide was crosslinked where it became bound. This HLA-DR1-restricted peptide bound about three times more efficiently to class II alpha,beta chains of DR1-positive B cells when present during cathepsin B digestion of Ii than when added afterward, also at pH 5.0. Binding was competed by similarly DR-restricted peptides. Cathepsin D cleaved Ii but did not enhance peptide binding. However, a trace level of cathepsin D, added to the assay for peptide binding in the presence of cathepsin B, further enhanced peptide binding about three times. These experiments support an hypothesis for the staged release of Ii fragments by cathepsin D and cathepsin B, catalyzing at one point the insertion of a peptide into the antigen binding site formed by class II alpha and beta chains.

A quantitative assay to measure the interaction between immunogenic peptides and purified class I major histocompatibility complex molecules

A direct and sensitive biochemical assay to measure the interaction in solution between peptides and affinity-purified major histocompatibility complex (MHC) class I molecules has been generated. Specific binding reflecting the known class I restriction of cytotoxic T cell responses was obtained. Adding an excess of beta 2-microglobulin (beta 2m) significantly increased the rate of peptide association, but it did not affect the rate of dissociation. Binding was complicated by a rapid and apparently irreversible loss of functional MHC class I at 37 degrees C which might limit the life span of empty MHC class I thereby preventing the inadvertent exchange of peptides at the target cell surface. All class I molecules tested bound peptides of the canonical octa- to nona-meric length. However, one class I molecule, Kk, also bound peptides, which were much longer suggesting that the preference of class I molecules for short epitopes is not absolute and may be caused by factors other than the peptide-MHC class I binding event itself.

Minimum structural requirements for peptide presentation by major histocompatibility complex class II molecules: implications in induction of autoimmunity

The precise mechanisms of failure of immunological tolerance to self proteins are not known. Major histocompatibility complex (MHC) susceptibility alleles, the target peptides, and T cells with anti-self reactivity must be present to cause autoimmune diseases. Experimental autoimmune encephalomyelitis (EAE) is a murine model of a human autoimmune disease, multiple sclerosis. In EAE, residues 1-11 of myelin basic protein (MBP) are the dominant disease-inducing determinants in PL/J and (PL/J x SJL/J)F1 mice. Here we report that a six-residue peptide (five of them native) of MBP can induce EAE. Using peptide analogues of the MBP-(1-11) peptide, we demonstrate that only four native MBP residues are required to stimulate MBP-specific T cells. Therefore, this study demonstrates lower minimum structural requirements for effective antigen presentation by MHC class II molecules. Many viral and bacterial proteins share short runs of amino acid similarity with host self proteins, a phenomenon known as molecular mimicry. Since a six-residue peptide can sensitize MBP-specific T cells to cause EAE, these results define a minimum sequence identity for molecular mimicry in autoimmunity.

The nature of major histocompatibility complex recognition by gamma delta T cells

Despite intensive efforts, the general rules for gamma delta T cell recognition remain undefined. Here, we take advantage of the detailed knowledge of the molecular structure and biosynthetic pathways of major histocompatibility complex (MHC) molecules to analyze the recognition properties of the gamma delta T cell clones LBK5 (specific for the class II MHC, IEk) and G8 (specific for the nonclassical class I MHC, TL10b). We find that the activation of these clones requires neither class I nor class II antigen-processing and that peptides do not confer specificity. Epitope mapping also shows that the topology of gamma delta T cell receptor interaction with the MHC is distinct from that of alpha beta T cells. These results suggest that the molecular nature of gamma delta T cell recognition is fundamentally different than that of alpha beta T cells.

The interaction between beta 2-microglobulin (beta 2m) and purified class-I major histocompatibility (MHC) antigen

The function of MHC class-I molecules is to sample peptides from the intracellular environment and present them to CD8+ cytotoxic T lymphocytes. To understand the molecular details of the assembly (and disassembly) of peptide-beta 2m-class-I complexes a biochemical peptide-class-I binding assay has been generated recently and this paper reports on a similar assay for the interaction between beta 2m and class I. As a model system human beta 2m binding to mouse class I was used. The assay is strictly biochemical using purified reagents which interact in solution and complex formation is determined by size separation. It is specific and highly sensitive. The observed affinity of the interaction, KD, is close to 0.4 nM. The rate of association at 37 degrees C is very fast (the ka is around 5 x 10(4)/M/s) whereas the dissociation is slow (the kd is around 8 x 10(-6)/s); the ratio of dissociation to association yields a calculated KD close to the observed value. At 37 degrees C almost all of the purified class I participates in binding of the exogenously offered beta 2m showing that a considerable exchange of the endogenous beta 2m occurs. Finally, it was demonstrated that exogenous beta 2m enhances binding to MHC class-I of short perfectly-matching peptides as well as longer peptides.

Membrane interactions influence the peptide binding behavior of DR1

We analyzed the binding of an influenza matrix protein-derived peptide, MAT(17-31), to cell surface and purified DR1. The pH dependence of peptide binding was dramatically influenced by the membrane environment. Cell surface binding was enhanced at low pH, with little or no binding detected at neutral pH and optimal binding at pH 4. By contrast, hydrogen ion concentration had minimal effect on peptide binding to purified DR1. Exposure to low pH in the absence of peptide did not affect the peptide binding capacity of cell-associated DR1. Purified DR1 was stable at low pH, excluding the possibility that enhanced binding was offset by a competing denaturation event at low pH. The striking effect of pH on peptide binding characteristic of cell surface DR1 was recovered after reconstitution of purified DR1 in B cell membranes by detergent dialysis. This behavior was partially recovered by reconstitution of full-length, but not truncated DR1 in vesicles containing purified lipid. Our results demonstrate that interactions involving membrane components influence the peptide-binding behavior of DR1.

Deficient antigen processing of a protein quaternary structure can be overcome by receptor-mediated uptake

Human chorionic gonadotropin (hCG) is a dimer of non-covalently associated alpha (hCG-alpha) and beta (hCG-beta) subunits. This molecule was used to study whether receptor-mediated uptake influences the presentation of a protein quaternary structure. Unprimed splenocytes and a B cell lymphoma were capable of presenting only the free (hCG-alpha) but not the combined (hCG) alpha subunit to hCG-alpha T cell hybridomas, while hCG-alpha-primed lymph node cells (LNC) responded to both hCG-alpha and hCG. As antigen (Ag)-specific antigen-presenting cells (APC) present in the hCG-alpha-primed LNC population may be potentially effective for presenting hCG, we investigated the role of specific Ag capture, through mIg and Fc gamma R, in the processing and presentation of hCG and hCG-alpha to HAG5, a T cell hybridoma directed against the immunodominant region (amino acids 61-81) of hCG-alpha. Results showed that only B cells bearing membrane immunoglobulin capable of recognizing hCG-alpha and hCG, and present in hCG-alpha-primed mice, were extremely effective in presenting the free as well as the combined alpha subunit. The effect of FcR-mediated uptake was analyzed using a B cell line transfected with the Fc gamma RII-B2 gene to present immune complexes of either hCG-alpha or hCG. We found that hCG-alpha and hCG were presented equally well, whatever the Ag-binding site of each antibody to hCG or its alpha subunit. Using HBG 6, an hCG-beta T cell hybridoma, we performed similar experiments with the Fc gamma RII-B2 cell line and determined that the potentiation of hCG presentation to HBG 6 was similar to that observed with HAG 5. Then kinetic experiments were performed to examine the effect of Ag uptake through FcR on processing. Results demonstrated that the uptake pathway drastically influenced the expression of alpha T cell determinants in the alpha/beta dimer. In addition, treatment with cycloheximide, a protein synthesis inhibitor, only impaired the ability of APC to present specifically captured Ag. Thus, the processing pathway for specifically captured Ag might be different from the pathway used to process nonspecifically captured Ag. This observation might explain why receptor-enhanced uptake bypasses the inefficient processing of the hCG quaternary structure and enables similar efficiency in the presentation of alpha and beta T cell specificities. These findings provide new insight into the antigenicity of oligomeric molecules, which is modified whether antigen capture is specific or not.

Release of DR molecules from complexes with invariant chain through the formation of a C-terminal 25 kDa invariant chain fragment

To investigate how class II major histocompatibility complex (MHC) molecules are released from complexes with invariant chain (Ii), we studied a 25 kDa Ii fragment (p25) detected by Western blotting in affinity chromatographed DR preparations. The p25 species corresponds to the non-transmembrane, C-terminal Ii fragment 107-232. It was determined by gel filtration chromatography that the p25 fragment has a relative molecular mass (M(r)) of 46 kDa, indicating that this Ii fragment is present as dimers in B cell lysate. Two independent approaches were followed to demonstrate that generation of the p25 fragment takes place shortly before, or concomitantly to, loading of class II MHC molecules with antigen fragments. First, it was shown that a fraction of the p25 molecules is resistant to endoglycosidase H digestion, indicating that the p25 polypeptide can exit the endoplasmic reticulum (ER) and is transported at least to the cis-Golgi compartment. Second, treatment of class II MHC-positive B cells with leupeptin blocks the formation of p25, further indicating that this Ii fragment is generated in the endosomal compartment. The role of the p25 Ii species in the assembly of complexes between peptides and DR molecules was then investigated. While the p25 fragment was totally unable to prevent binding of a synthetic tetanus toxin peptide to DR molecules, the full-length Ii species (p33/35) effectively inhibited peptide binding, indicating that, by contrast with the p33/35 species, the p25 fragment does not occlude the peptide binding site of DR molecules. We concluded that the p25 fragment, which is produced by proteolytic cleavage at the N-terminal side of Methionine 107, has a decreased affinity for DR molecules as compared with the p33/35 species. Dissociation of the p25 fragment from DR molecules exposes the peptide binding site, which is thus made accessible for antigen fragments. This model of the complexes between DR and antigen fragments proposes that a stretch of Ii prevents peptide binding by occluding the peptide binding site without directly occupying it.

Long-term anti-nucleoprotein cellular and humoral immunity is induced by intramuscular injection of plasmid DNA containing NP gene

Cytolytic T-lymphocyte-mediated killing is thought to be an important effector mechanism in controlling viral infections. Recently, we reported that intramuscular injection of plasmid DNA containing the nucleoprotein (NP) gene of the influenza virus resulted in generating nucleoprotein-specific cytolytic T cells and antibodies. Gene-injected mice were subsequently protected from a lethal challenge with live influenza virus. Here we show that a single intramuscular injection of a small dose of nucleoprotein plasmid DNA generates nucleoprotein-specific cellular and humoral immune responses that last 1 year. The cellular response is associated with the CD8+ subpopulation of T cells. Thus, plasmid DNA injections can be used to induce long-lasting immune responses against the viral gene product without an exposure to live virus itself.