Primary B-cell response to neuropeptide Y and bovine pancreatic polypeptide

Cross‐reactivity in T‐cell antigen recognition

The molecular interactions between the T-cell receptor (TCR) and peptide-MHC (pMHC) have been elucidated in recent years. Nevertheless, the fact that binding of only slightly different ligands by a TCR, or ligation of the same pMHC at different developmental stages of the T cell, can have opposing consequences, continues to pose intellectual challenges. Kinetic proofreading models, which have at their core the dissociation rates of pMHC from the TCR, are best suited to account for these observations. However, T cells can be triggered by peptides with often minimal homology to the primary immunogenic peptide. This cross-reactivity of the TCR is manifest at several levels, from positive selection of immature thymocytes to homeostasis and antigen-cross- reactive immune responses of mature peripheral T cells. The implications of the high cross-reactivity of T-cell antigen recognition for self-tolerance and T-cell memory are discussed.

Molecular determinants of polyreactive antibody binding: HCDR3 and cyclic peptides

Human monoclonal antibody 63 (mAb63) is an IgM/lambda polyreactive antibody that binds to multiple self and non-self antigens. The molecular basis of polyreactivity is still unclear. The present study was initiated to prepare a recombinant Fab of mAb63 and use it to study the determinants involved in polyreactivity. The baculovirus system was employed to express large amounts of mAb63 Fab in Sf9 cells. Our experiments showed that infected Sf9 cells secreted a soluble 50-kD Fab heterodimer that bound to multiple self and non-self antigens. The antigen-binding activity of mAb63 Fab was inhibited by both homologous and heterologous antigens. To study in more detail the molecular determinants involved in polyreactivity, the heavy chain complementarity-determining region 3 (HCDR3), which is known to play a key role in the binding of monoreactive antibodies to antigens, was subjected to site-directed mutagenesis. A single substitution, alanine for arginine, at position 100A resulted in complete loss of antigen-binding activity. The 19 amino acids comprising the HCDR3 of mAb63 were then synthesized and a cyclic peptide prepared. The cyclic peptide showed the same antigen-binding pattern as the parental mAb63 and the recombinant mAb63 Fab. A five amino acid motif (RFLEW), present in the HCDR3 of mAb63, was found by searching the GenBank in three of 50 other human polyreactive antibodies, but in none of nearly 2500 human antibodies thought to be monoreactive. It is concluded that HCDR3 plays a major role in polyreactivity and that in some cases cyclic peptides comprising the HCDR3, by themselves, may be polyreactive.

Unusual amino acid usage in the variable regions of mercury-binding antibodies

Monoclonal antibodies (mAb) specific for mercuric ions were isolated from BALB/c mice injected with a mercury-containing, hapten-carrier complex. The antibodies reacted by enzyme-linked immunosorbent assay with bovine serum albumin-glutathione-mercuric chloride (BSA-GSH-HgCl) but not with BSA-GSH without mercury. Nucleotide sequences from polymerase chain reaction products encoding six of the antibody heavy-chain variable regions and seven light-chain variable regions revealed that all the antibodies contained an unpaired cysteine residue in one hypervariable region, which is unusual for murine antibodies. Mutagenesis of the cysteine to either tyrosine or serine in one of the Hg-binding antibodies, mAb 4A10, eliminated mercury binding. However, of two influenza-specific antibodies that contain cysteine residues at the same position as mAb 4A10, one reacted with mercury, although not so strongly as 4A10, whereas the other did not react at all. These results suggested that, in addition to an unpaired cysteine, there are other structural features, not yet identified, that are important for creating an appropriate environment for mercury binding. The antibodies described here could be useful for investigating mechanisms of metal-protein interactions and for characterizing antibody responses to structurally simple haptens.

A Molecular Basis for How a Single TCR Interfaces Multiple Ligands

CD8+ T cells respond to Ags when their clonotypic receptor, the TCR, recognizes nonself peptides displayed by MHC class I molecules. The TCR/ligand interactions are degenerate because, in its life time, the TCR interacts with self MHC class I-self peptide complexes during ontogeny and with self class I complexed with nonself peptides to initiate Ag-specific responses. Additionally, the same TCR has the potential to interact with nonself class I complexed with nonself peptides. How a single TCR interfaces multiple ligands remains unclear. Combinatorial synthetic peptide libraries provide a powerful tool to elucidate the rules that dictate how a single TCR engages multiple ligands. Such libraries were used to probe the requirements for TCR recognition by cloned CD8+ T cells directed against Ags presented by H-2Kb class I molecules. When H-2Kb contact residues were examined, position 3 of the peptides proved more critical than the dominant carboxyl-terminal anchor residue. Thus, secondary anchor residues can play a dominant role in determining the antigenicity of the epitope presented by class I molecules. When the four solvent-exposed potential TCR contact residues were examined, only one or two of these positions required structurally similar residues. Considerable structural variability was tolerated at the remaining two or three solvent-exposed residues of the Kb-binding peptides. The TCR, therefore, requires close physico-chemical complementarity with only a few amino acid residues, thus explaining why TCR/MHC interactions are of low affinity and degenerate.

Epitopic characterization of neuropeptide Y (NPY) by alanine-scanning mutagenesis

Characterization of the epitopic structures of neuropeptide Y (NPY) has been studied by alanine-scanning mutagenesis, based on our previous investigation of a panel of six murine anti-NPY IgM monoclonal antibodies. To evaluate the structural requirement for these anti-NPY IgM antibodies, recognition variants of the native sequences of the NPY fragment (19-36) were prepared by single alanine substitutions in residues 22 and 25-36. Their binding to these antibodies was examined by competitive inhibition assays. The results demonstrated that the epitopic structures are largely confined to residues 25-36 of NPY and the C-terminal residues of NPY are essential for these anti-NPY IgM antibodies recognition. It emphasizes the notion that even small regions of a protein consisting of as few as 15 residues (22-36) can exhibit multiple epitopic structures. In several anti-NPY IgM antibodies, pairs of residues on opposite faces of the alpha-helix interact with the antibody site, which indicates that the antibody site consists either of a cavity or a deep groove either of which encompasses the alpha-helical segment sufficiently to allow simultaneous contact with most of the residues of this segment.

D-penicillamine- and quinidine-induced antinuclear antibodies in A.SW (H-2s) mice: similarities with autoantibodies in spontaneous and heavy metal-induced autoimmunity

Ten percent of human lupus syndromes occur in patients as a result of treatment with certain medications. H-2s mice can produce autoantibodies following treatment with various drugs or heavy metals and they are a potential animal model of drug-induced lupus. We have examined nine anti-chromatin monoclonal antibodies (mAb) from A.SW mice that had been treated with either D-penicillamine or quinidine, two lupus-inducing drugs in humans. These mAb are specific either for DNA or histone-DNA complexes corresponding to nucleo-specific either for DNA or histone-DNA complexes corresponding to nucleosomes or subnucleosome particles. Only one mAb reacts with an unknown chromatin antigen. The V region sequences of six of these mAb were studied and are notable by several features. As previously observed in spontaneous autoantibodies to DNA or histone-DNA complexes, arginine or asparagine residues are found at critical locations throughout the V regions. Many of these residues, potentially important for binding to DNA or DNA-histone complexes, result either from somatic mutations or atypical VH-D-JH rearrangements. Another significant characteristic is that the VH genes of several D-penicillamine- or quinidine-induced mAb are most similar to those of anti-nucleolar mAb obtained from mercury-injected A.SW mice. The implications of these findings for the pathogenesis of spontaneous or induced autoimmunity are discussed.

Somatic diversification and affinity maturation of IgM and IgG anti-DNA antibodies in murine lupus

Molecular events occurring during the process of generation of pathogenic immunoglobulin (Ig)G anti-DNA antibodies in systemic lupus erythematosus (SLE) were studied using a newly established method. We analyzed the Ig variable (V) region gene sequence and DNA-binding activity of IgM and IgG anti-DNA monoclonal antibodies (mAb) from individual SLE-prone (NZB x NZW) F1 mice. The first event appeared to be clonal selection and expansion of IgM anti-DNA clones, in which several clones had intraclonal V gene mutations. Although the number of mutations was small, the mutated IgM clones were associated with an increase in DNA-binding activity. The somatic mutations located in complementarity-determining regions (CDR) and in framework regions (FR) of V genes were apparently related to changes in DNA-binding activity. IgG anti-DNA clones that progressively increased in number with aging had numerous somatic mutations in the V region genes and there was a pair of clones which showed an intraclonal accumulation of mutations, in association with increase in the DNA-binding activity. All these findings show that somatic mutations associated with affinity maturation of the V region begin immediately before isotype-switching from IgM to IgG of the clones that have been selected and expanded, in an antigen-driven manner and/or by other forces. We propose that further accumulations of intraclonal somatic hypermutation, in association with selection and expansion of high affinity IgG clones, may lead to formation of highly pathogenic anti-DNA antibodies.