Critical role for the Val/Gly86 HLA-DR beta dimorphism in autoantigen presentation to human T cells

Crystal structure of the human class II MHC protein HLA-DR1 complexed with an influenza virus peptide

An influenza virus peptide binds to HLA-DR1 in an extended conformation with a pronounced twist. Thirty-five per cent of the peptide surface is accessible to solvent and potentially available for interaction with the antigen receptor on T cells. Pockets in the peptide-binding site accommodate five of the thirteen side chains of the bound peptide, and explain the peptide specificity of HLA-DR1. Twelve hydrogen bonds between conserved HLA-DR1 residues and the main chain of the peptide provide a universal mode of peptide binding, distinct from the strategy used by class I histocompatibility proteins.

Combinations of HLA-DPB1 and HLA-DQB1 alleles determine susceptibility to early-onset myasthenia gravis in Japan

HLA class II alleles in the DQA1, DQB1, DRB1, and DPB1 genes were investigated in Japanese patients with myasthenia gravis (MG) by digestion of polymerase chain reaction amplified DNAs with allele specific restriction endonucleases (PCR-RFLP). A significantly higher frequency of DQB1*03, which includes *0301, *0302, *0303 and determines the serological DQ3 specificity, was observed in female patients less than 30 years in age at onset of disease compared with healthy controls (90.5 vs. 53.2%). This study also confirms the high incidence of DPB1*0201 in early-onset female patients compared to the controls (85.7 vs. 40.3%). Moreover, 81.0% of the early onset female patients were found to carry both DQB1*03 and DPB1*0201, compared to 17.7% of the controls. Since DQB1*03 and DPB1*0201 are not in linkage disequilibrium, both these alleles are supposed to be synergistically involved in disease development in early-onset female MG. In contrast, no obvious association of HLA-DQA1, DQB1, DRB1 and DPB1 alleles with either late-onset patients or patients with thymoma was observed. Clearly, the genetic background of Japanese females with early onset MG is different from that of other patients with MG.

Detection of alpha-subunit isoforms in human muscle acetylcholine receptor by specific T cells from a myasthenia gravis patient

The nicotinic acetylcholine receptor (AChR) is both the best-characterized transmitter receptor-ion channel and the target for the pathogenic antibodies in the human autoimmune disease myasthenia gravis (MG). In cloning and sequencing its components in man, we found that the alpha-subunit was transcribed in two isoforms, with (P3A+) or without (P3A-) a 75 base pair exon that had not been described in other species. While studying the human T lymphocyte response to recombinant AChR, we found that part of this P3A insert was recognized by one T cell line (from an MG patient), whereas another line only recognized the uninterrupted insertion site. To establish whether this exon is also translated in normal human muscle, we initially raised anti-peptide antibodies to the relevant amino acid sequences, but these failed to bind native AChR (affinity-purified from muscle on alpha-neurotoxin columns). We therefore exploited the great sensitivity and specificity of these T cells to detect the two isoforms after unfolding by antigen-presenting cells, and have been able to show that both are expressed in affinity-purified human muscle AChR.

Identification of T-cell stimulatory antigens of Chlamydia trachomatis using synovial fluid-derived T-cell clones

Chlamydia trachomatis is a major cause of sexually transmitted disease, infertility and reactive arthritis in the Western world, and of trachoma in the developing world. There is evidence that the chronic inflammatory reaction seen in diseases associated with chlamydiae represents a delayed-type hypersensitivity response to chlamydial antigens. Little is known about which chlamydial antigens elicit T-cell responses yet such information could have important implications in terms of both immunopathological understanding of these diseases and immunoprophylaxis design. In this study, 61 chlamydia-specific T-cell clones have been produced from the synovial fluid of an individual with sexually acquired reactive arthritis (SARA). Ten clones have been characterized in detail and used to identify T-cell stimulatory antigens of chlamydiae by means of T-cell immunoblotting. Two distinct antigenic fractions have been identified, one recognized by three of the clones (molecular weight 18,000), the other recognized by six of the clones (molecular weight 30,000). The fractions are distinct from the major outer membrane protein, the 57,000 MW stress protein and the 60,000 MW cysteine-rich membrane protein of chlamydiae. The major histocompatibility complex (MHC) restriction of the response to these antigens differed: clones recognizing the 18,000 MW antigen required antigen-presenting cells expressing DR1 subtype DRB1*0101 or DRB1*0102 which only differ at amino acids 85 and 86 on the DR beta-chain; by contrast clones recognizing the 30,000 MW antigen were presented to only by antigen-presenting cells from DRB1*0101 individuals, reflecting extreme sensitivity of these clones to the polymorphism at positions 85 and 86 on the DR beta-chain.

Primary structure of the human muscle acetylcholine receptor. cDNA cloning of the gamma and epsilon subunits

cDNA sequences encompassing the full coding region for the human muscle acetylcholine receptor (AChR) epsilon and gamma subunits have been isolated. The deduced amino-acid sequences indicate that the mature epsilon subunit contains 473 amino acids and is preceded by a 20-amino-acid signal peptide. As predicted from genomic clones, the gamma subunit contains 495 amino acids preceded by a 22-amino-acid signal peptide. In common with the human alpha, beta, gamma and delta subunits the epsilon subunit is highly conserved between mammalian species. The epsilon subunit gene is not closely linked to the gamma and delta subunits on chromosome 2 but rather is located with the beta subunit on chromosome 17. Expression of the alpha-, beta-, gamma-, delta- and epsilon-subunit cRNAs in rabbit-reticulocyte lysates followed by analysis on SDS/PAGE show glycosylated proteins with apparent molecular masses of 44-60 kDa.

Approaches for studying the pathogenic T cells in autoimmune patients

Our provisional conclusions from this work are as follows. (1) For screening responses of established lines, native human AChR is not prohibitively scarce, especially if it is concentrated onto beads, and class II-transfected TE671 cells may be useful too; both may give vital evidence of AChR-specificity, but it is still crucial to confirm that with synthetic peptides. (2) For mapping epitopes, panels of full-length and shorter recombinant human polypeptides, and of synthetic peptides, are invaluable complementary material: longer peptides tend to stimulate particularly strongly. (3) Initial selection with pooled synthetic peptides can easily generate interesting lines from both patients and controls, but they may depend on the artificial processing sites that are an inevitable consequence of arbitrarily chosen start and stop points. Of course, these might conceivably be employed in unusual antigen-presenting cells (such as thymic myoid cells), so we cannot totally dismiss such "cryptic" epitopes. This system can sometimes select T cells responding to "natural" epitopes too, as now reported for tetanus toxin. Nevertheless, for these and other reasons, at present, we strongly favor using the longest human recombinant material possible, because it is apparently processed more naturally. This must be combined with rigorous screening for reactivity to E. coli-derived contaminants plus concomitant mapping of epitopes as above. Use of intact AChR for initiating lines may yet become feasible. (4) The T cells thus isolated and characterized so far are proving to be heterogeneous in the epitopes and presenting class II molecules they recognize, and in their T-cell receptor gene usage. It is premature to claim key myasthenogenic epitopes or clonotypes, but HLA-DR3 and the linked -DQw2 do not appear to monopolize presentation. (5) Assessing the disease-relevance of these T cells is a separate problem, highlighted by their apparent similarity in healthy controls. In the meantime, to test their potential pathogenicity, we are assaying their cytokine profiles and ability to help specific antibody production in vitro. In the hope that they do prove to be relevant, we are also using some of them to test possible therapeutic strategies that might prove applicable in the patients.

HLA-DR beta chain residue 86 controls DR alpha beta dimer stability

Major histocompatibility complex class II molecules exist in two forms, which can be distinguished on the basis of their stability in sodium dodecyl sulfate (SDS) as SDS-stable and SDS-unstable alpha beta dimers. The ratio of stable vs. unstable alpha beta dimers varies between murine H-2 alleles and isotypes, but the molecular basis for this observation is unknown. Here we show that for the human HLA-DRB1 and HLA-DRB3 gene products this ratio is controlled by the valine/glycine dimorphism at position 86. Haplotypes coding for DR beta chains with a valine at position 86 express higher numbers of stable dimers compared to similar haplotypes expressing DR beta chains with a glycine at that position. Reverse-phase high-performance liquid chromatography analysis of iodinated peptides, which were eluted from DR dimers with either a DRB1*1101 or a DRB1*1104 beta chain which differ only at position 86, indicated that these DR dimers contain (partially) distinct sets of peptides. The valine/glycine dimorphism is highly conserved, present in most HLA-DR alleles and influences peptide-binding. Analysis of the occurrence of the Val86 and the Gly86 gene products revealed that these are not equally present in the population. Depending on the DR specificity either the Val86 of Gly86 allelic variant is favored. Thus, the natural, highly conserved dimorphism at HLA-DR beta chain position 86 influences peptide selection. The dimorphism is therefore likely to influence antigen presentation and forms the molecular basis for the observed differences in stability of Val86- and Gly86-containing DR dimers in the presence of SDS.

HLA-DRB1 gene sequences in HLA-DR4 positive and negative patients with rheumatoid arthritis

The second exon of the DRB1 gene encoding for the first domain of the HLA-DR beta 1-chain was sequenced in 16 patients (10 DR4/DR1 positive, 6 DR4/DR1 negative) with seropositive rheumatoid arthritis (RA). We could confirm the strong association of susceptibility to RA with functionally equivalent conformations on otherwise distinct MHC molecules. At least one HLA-DR allele in all of the analysed DR4 or DR1 positive patients showed such an epitope with a minimal variability limited to residue 71. However, in HLA-DR4 and -DR1 negative patients such a similar epitope could not be detected.

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.

The set of naturally processed peptides displayed by DR molecules is tuned by polymorphism of residue 86

The response to tetanus toxoid (TT) was studied in immune donors that carry two alleles of DR5 that differ only at DR beta residue 86: DRB1*1101 (G86, abbreviated 1101) and DRB1*1104 (V86, abbreviated 1104). A large number of TT-specific T cell clones was isolated and the epitopes recognized in association with 1101 and 1104 were mapped. We found that two epitopes (p2 and p32) can be recognized in association with both 1101 and 1104 while three epitopes (p23, p27 and p30) are recognized in association with 1101, but never in association with 1104. The sets of naturally processed self peptides displayed by 1101 and 1104 were characterized using alloreactive T cell clones. We found that all 1104 alloreactive clones recognize both 1104 and 1101, while approximately 30% of the alloreactive 1101 clones fail to recognize 1104. Thus it is apparent that both naturally processed TT and self peptides displayed on 1104 molecules represent a fraction of those displayed on 1101 molecules. The mechanism responsible for this differential presentation was investigated by comparing the capacity of 1101 and 1104 antigen-presenting cells to present TT or synthetic peptides to specific T cells and by measuring the binding of these peptides to DR molecules. Three sets of results suggest that V86 acts as a constraint to the binding of naturally processed peptides: (i) all 1104-restricted or alloreactive T cell clones recognize TT- or allo-epitopes presented by 1101 as well, thus ruling out a major effect of V86 as a residue determining T cell restriction specificity; (ii) presentation of naturally processed peptides correlates in general with the capacity of long synthetic peptides to bind to 1101 or 1104 and (iii) while the naturally processed p30 epitope discriminates between 1101 and 1104, a short synthetic peptide binds equally well to and is comparably recognized in association with both 1101 and 1104. Taken together these results suggest that the natural polymorphism at residue 86 might be a molecular switch that finely tunes the complexity of the peptide collection presented on DR molecules.

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

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

Self-reactive and antigen-specific T cell clones derived from a HLA-DR4+/DR5+ donor: T cell receptors and MHC-restriction patterns

The relationship of heat shock proteins and rheumatoid arthritis as well as the relevance of autoreactivity in this disease is unclear. T cells of six individuals (four expressing the DRB1*0401 allele, one harboring DRB1*0404 and one the DRB1*0407 allele) were cloned in the presence of 65kD mycobacterial heat shock protein (HSP60) in order to determine T cell receptors (TcR) used and the MHC class II restriction patterns of potentially relevant T cell clones (TcC). All TcC obtained were not specific for HSP60, but six TcC of one donor (HLA-DR4/HLA-DR5) were responsive towards autologous antigen-presenting cells. One TcC displayed authentic autoreactivity whereas five TcC reacted specifically to serum proteins. The amino acids (aa) of the MHC molecule, crucial for immune recognition were mapped to aa #71 or #86 of either maternal or paternal origin. The strictly autoreactive TcC did not recognize transfected L cells implicating specificity for self-peptides not presented by L cells or the involvement of adhesion molecules. Correlations between autoreactivity and TcR V(D)J sequences or N nucleotides of various "autoreactive" TcC were not evident.

Two new HLA DRB1 alleles found in African Americans: implications for balancing selection at positions 57 and 86

Two new DRB1 alleles have been identified (DRB1*0303 and DRB1*0805) in African Americans that differ from known DRB1 alleles only by a glycine to valine exchange at position 86. The novel DRB1*0303 allele, found in one individual, has the same DQA1*0401-DQB1*04 haplotype as DRB1*0302, suggesting that it may be a recent diversification of *0302. The novel DRB1*0805 allele, identified in 4 individuals, was found on two haplotypes, sharing a DQA1*0501-DQB1*0301 haplotype with DRB1*0804, and a DQA1*0102-DQB1*0602 haplotype found with DRB1*0801 in some African populations. DRB1*0805 differs from *0804 only at position 57 and differs from *0801 only at position 86. Assuming that DRB1*0801 and DRB1*0802 are ancestral, based on their distribution in various human populations, DRB1*0805 may have been generated twice by two independent mutations or gene conversion events at each of these positions. Alternatively, DRB1*0805 may have arisen from a single gene conversion event (or mutation) and recombined to generate multiple DR-DQ haplotypes. These findings increase the number of DRB1 allelic pairs that differ only at position 86 to 9, suggesting strong balancing selection at this position. A number of DRB1 alleles for DR8 and DR4 also differ only at position 57, a site previously postulated to be strongly influenced by balancing selection in DQB1 alleles by phylogenetic analysis.

HLA-DR peptide-induced alloreactive T cell lines reveal an HLA-DR sequence that can be both "dominant" and "cryptic": evidence for allele-specific processing

Previously, we reported on a T cell line, ThoU6, which we obtained through stimulation of DPw3+ cells with a synthetic "DR3 peptide" with a sequence identical to the third hypervariable region of the DRB1*0301 chain. This T cell line recognizes both the synthetic peptide presented by DPw3 as well as DR3+ DPw3+ stimulator cells. This implies that the synthetic DR3 peptide has a natural counterpart in DR3-positive cells. Here we describe the recognition pattern of another T cell line that was sensitized with the same synthetic DR3 peptide. This T cell line, BieU6, shows both HLA-DRw13/Dw18 (self)-restricted recognition of the synthetic DR3 peptide and allorecognition towards DR13/Dw19, a molecule which is highly homologous to Dw18, in the absence of synthetic peptide. These results suggest that the epitope formed by the Dw18 molecule plus the synthetic DR3 peptide and recognized by T cell line BieU6 mimics the Dw19 molecule. The potential role for a Dw19-specific peptide is discussed. The inability of T cell line BieU6 to recognize Dw18+ DR3+ cells indicates that, in this case, the synthetic DR3 peptide is "cryptic", i.e. does not have a natural counterpart that is effectively presented to T cells. Mapping of the shortest peptides recognized by T cell lines ThoU6 and BieU6 indicate that these sequences are fully overlapping. We, therefore, suggest that the antigen-presenting molecules, HLA-DPw3 and HLA-Dw18, differ in their accessibility for self peptides derived from the third hypervariable region of DR molecules. These observations may be explained by allele-specific processing.