The cross-reactive idiotopes recognized by the monoclonal antibodies 9G4 and LC1 are located in framework region 1 of two non-overlapping subsets of human VH4 family encoded antibodies

Human VH4-34 antibodies derived from B1 cells are more frequently autoreactive than VH4-34 antibodies derived from memory cells

Human B1 cells produce natural antibodies characterized by overutilization of heavy chain variable region VH4-34 in comparison to other B cell populations. VH4-34-containing antibodies have been reported to be autoreactive and to be associated with lupus and other autoimmune dyscrasias. However, it has been unclear to what extent VH4-34 antibodies manifest autoreactivity in B1 cells or other B cell populations-in other words, are VH4-34 containing antibodies autoreactive wherever found, or mainly within the B1 cell population? To address this issue we sort purified single human B1 and memory B cells and then amplified, sequenced, cloned and expressed VH4-34-containing antibodies from 76 individual B cells. Each of these antibodies was tested for autoreactivity by HEp-2 IFA and autoantigen ELISA. Antibodies were scored as autoreactive if positive by either assay. We found VH4-34 antibodies rescued from B1 cells were much more frequently autoreactive (14/48) than VH4-34 antibodies rescued from memory B cells (2/28). Among B1 cell antibodies, 4 were HEp-2+, 6 were dsDNA+ and 4 were positive for both. Considering only HEp-2+ antibodies, again these were found more frequently among B1 cell VH4-34 antibodies (8/48) than memory B cell VH4-34 antibodies (1/28). We found autoreactivity was associated with greater CDR3 length, as expected; however, we found no association between autoreactivity and a previously described FR1 "hydrophobic patch". Our results indicate that autoreactive VH4-34-containing antibodies tend to reside within the human B1 cell population.

IgG Subclasses and Isotypes of VH4-34 Encoded Antibodies

VH4-34 gene encoded autoantibodies are elevated in systemic lupus erythematosus (SLE) and in other diseases associated with B-cell hyperproliferation/dysfunction. One of the autoantigens recognized by VH4-34-encoded antibodies are branched/linear poly N-acetyl lactosamine chains. Since the anti-carbohydrate response in humans is dominated by the IgG2 subclass, here we tested whether VH4-34 encoded IgG showed similar subclass segregation. Serum samples from SLE, infectious mononucleosis, nasopharyngeal carcinoma and hepatitis-C were analyzed. Levels of VH4-34-encoded IgM and IgA isotypes were also tested. VH4-34-IgM and IgA were elevated in all four clinical conditions. VH4-34-IgG was detected in the IgG1 and IgG3 subclass but not in the IgG2 and IgG4 subclass. Interestingly, VH4-34-IgG3 was also detected in serum samples of normal healthy adults. These observations are discussed in context of the VH4-34 gene regulation. VH4-34 repertoire development is of interest since it is the only human VH gene profoundly overrepresented in the naïve repertoire but counter-selected for antibody secretion. VH4-34 B-cell could thus become a unique tool to inspect germinal center independent/dependent pathways of subclass and isotype-specific antibody secretion.

Expression of the inherently autoreactive idiotope 9G4 on autoantibodies to citrullinated peptides and on rheumatoid factors in patients with early and established rheumatoid arthritis

BACKGROUND

The pre-symptomatic stage of Rheumatoid arthritis (RA) is associated with pro-inflammatory cytokines and autoantibodies. High levels and epitope spread by Rheumatoid factors (RhF) and autoantibodies to citrullinated proteins signify progression towards disease expression. In established RA, the persistence of high autoantibody levels reflects production by both long-lived plasma cells and short-lived plasmablasts. Neither the relative contributions to pathogenesis by autoantibodies from either source, nor the factors responsible for deciding the fate of autoantigen specific 'parent' B-cells, is understood. Phenotypic markers identifying subsets of autoreactive B-cells are therefore of interest in understanding the origin and perpetuation of the autoimmune response in RA. One such phenotypic marker is the rat monoclonal antibody, 9G4, which recognises an idiotope on immunoglobuins derived from the inherently autoreactive VH-gene, VH4-34. We therefore investigated whether the 9G4 idiotope was expressed on autoantibodies in patients with RA.

METHODOLOGY/PRINCIPAL FINDINGS

Sera from 19 patients with established RA and those with <1year history of untreated polyarthritis either resolving into RA (n = 42) or non-RA diagnosis (n = 31) were included. Autoantibodies to cyclic citrullinated peptides (CCP), RhF and co-expression of the 9G4 idiotope were measured by ELISA. 9G4 recognised a population of anti-CCP antibodies in the majority of sera from patients with established disease and also in samples from patients with early disaese. 9G4+RhF levels were generally lower and not associated with positivity for, or levels of 9G4+CCP.

CONCLUSIONS/SIGNIFICANCE

The persistence of 9G4+ immunoglobulins, of any isotype, in serum is rare. We describe here the novel finding of 9G4 expression on anti-CCP antibodies in patients from the earliest symptoms of RA through to established disease. Our results suggest that 9G4 expression on anti-CCP autoantibodies was not due to polyclonal expansion of VH4-34-encoded immunoglobulins. These studies may therefore provide a new focus for investigation into the evolution of the autoimmune response in RA patients.

Molecular basis of 9G4 B cell autoreactivity in human systemic lupus erythematosus

9G4(+) IgG Abs expand in systemic lupus erythematosus (SLE) in a disease-specific fashion and react with different lupus Ags including B cell Ags and apoptotic cells. Their shared use of VH4-34 represents a unique system to understand the molecular basis of lupus autoreactivity. In this study, a large panel of recombinant 9G4(+) mAbs from single naive and memory cells was generated and tested against B cells, apoptotic cells, and other Ags. Mutagenesis eliminated the framework-1 hydrophobic patch (HP) responsible for the 9G4 idiotype. The expression of the HP in unselected VH4-34 cells was assessed by deep sequencing. We found that 9G4 Abs recognize several Ags following two distinct structural patterns. B cell binding is dependent on the HP, whereas anti-nuclear Abs, apoptotic cells, and dsDNA binding are HP independent and correlate with positively charged H chain third CDR. The majority of mutated VH4-34 memory cells retain the HP, thereby suggesting selection by Ags that require this germline structure. Our findings show that the germline-encoded HP is compulsory for the anti-B cell reactivity largely associated with 9G4 Abs in SLE but is not required for reactivity against apoptotic cells, dsDNA, chromatin, anti-nuclear Abs, or cardiolipin. Given that the lupus memory compartment contains a majority of HP(+) VH4-34 cells but decreased B cell reactivity, additional HP-dependent Ags must participate in the selection of this compartment. This study represents the first analysis, to our knowledge, of VH-restricted autoreactive B cells specifically expanded in SLE and provides the foundation to understand the antigenic forces at play in this disease.

B cell depletion therapy in systemic lupus erythematosus: Effect on autoantibody and antimicrobial antibody profiles

OBJECTIVE

Autoantibody production in patients with systemic lupus erythematosus (SLE) is associated with abnormalities of B cell function and phenotype. Clinical responses to B cell depletion therapy (BCDT), based on rituximab, are encouraging. Therefore, we undertook this study to investigate the effect of BCDT on antibody profiles.

METHODS

Serial sera from 16 patients with active, refractory SLE were assayed for antinucleosome antibodies, anti-double-stranded DNA (anti-dsDNA), anti-extractable nuclear antigen, anti-tetanus toxoid, and antibodies to pneumococcal capsular polysaccharide for at least 1 year following BCDT. Anti-dsDNA antibodies derived from the V(H)4.34 immunoglobulin germ line gene (9G4+) were also measured.

RESULTS

All patients achieved peripheral B cell depletion and improved clinically for at least 3 months. Antinucleosome and anti-dsDNA antibodies decreased to a mean +/- SD of 64 +/- 37% and 38 +/- 33% of baseline values, respectively, by 6-8 months post-BCDT. Levels of other autoantibodies and antimicrobial antibodies were generally unchanged. In the 9 of 16 patients who were still well at 1 year, anti-dsDNA antibodies fell to 42 +/- 36% of baseline values at 6-8 months and to 37 +/- 33% at 10-14 months. In patients who had disease flares within 1 year of BCDT, levels of these antibodies decreased to 60 +/- 40% and 83 +/- 93% of baseline values at 6-8 months and at 10-14 months, respectively. Circulating anti-dsDNA antibodies were positive for 9G4 expression in 4 of 6 patients tested, and flares in 2 of these patients were accompanied by rises in 9G4+ anti-dsDNA antibodies.

CONCLUSION

These observations suggest that B cell clones committed to producing antinucleosome and anti-dsDNA antibodies, including the V(H)4.34 subpopulation of anti-dsDNA antibodies, have a relatively rapid turnover compared with B cell clones producing other antibodies. There was also a trend toward a greater and more sustained decrease in anti-dsDNA antibodies in patients with clinical benefit lasting >1 year.

Germinal center exclusion of autoreactive B cells is defective in human systemic lupus erythematosus

Breach of B cell tolerance is central to the pathogenesis of systemic lupus erythematosus (SLE). However, how B cell tolerance is subverted in human SLE is poorly understood due to difficulties in identifying relevant autoreactive B cells and in obtaining lymphoid tissue. We have circumvented these limitations by using tonsil biopsies to study autoreactive B cells (9G4 B cells), whose regulation is abnormal in SLE. Here we show that 9G4 B cells are physiologically excluded during the early stages of the GC reaction before acquiring a centroblast phenotype. Furthermore, we provide evidence to indicate that an anergic response to B cell receptor stimulation may be responsible for such behavior. In contrast, in SLE, 9G4 B cells progressed unimpeded through this checkpoint, successfully participated in GC reactions, and expanded within the post-GC IgG memory and plasma cell compartments. The faulty regulation of 9G4 B cells was not shared by RA patients. To our knowledge, this work represents the first comparative analysis of the fate of a specific autoreactive human B cell population. The results identify a defective tolerance checkpoint that appears to be specific for human SLE.

Human innate B cells: a link between host defense and autoimmunity?

B cells play a variety of immunoregulatory roles through their antigen-presentation ability and through cytokine and chemokine production. Innate immune activation of B cells may play a beneficial role through the generation of natural cross-reactive antibodies, by maintaining B cell memory and by exercising immunomodulatory functions that may provide protection against autoimmunity. In this article, we review human B cell populations and their functional properties, with a particular focus on a population of inherently autoreactive B cells, which seem to play an important physiological role in innate immunity, but which, if selected into adaptive immune responses, appear to become pathogenic agents in systemic lupus erythematosus.

Human immunoglobulin selection associated with class switch and possible tolerogenic origins for C delta class-switched B cells

Current paradigms of peripheral B cell selection suggest that autoreactive B cells are controlled by clonal deletion, anergy, and developmental arrest. We report that changes to the human antibody repertoire likely resulting from these mechanisms both for a well-characterized autoreactivity from antibodies encoded by the V(H)4-34 gene and for other hallmarks of an autoreactive repertoire are apparent mainly for class-switched B cells and not for IgM germinal center, IgM memory, or IgM plasma cells. Other possible indicators of autoreactivity found selected with immunoglobulin class include J(H)6 gene segment usage, increased frequency of B cells with long third hypervariable regions, and distal J(kappa) gene segment bias. Of particular interest is the finding that B cells with these same characteristics are selected into the lineage of B cells that have undergone the unusual class switch from constant region C mu to C delta (C delta-CS). The C delta-CS population also displays an increased frequency of charged amino acids localized to the complementarity-determining regions, further suggesting autoreactivity, and evidence is presented that these B cells had undergone extensive receptor editing. Thus, the C delta-CS lineage may be a "sink" for B cells harboring autoreactive specificities in normal humans. A model for a new tolerizing mechanism that could account for the C delta-CS lineage is presented.

Lupus IgG VH4.34 Antibodies Bind to a 220-kDa Glycoform of CD45/B220 on the Surface of Human B Lymphocytes

Anti-lymphocyte autoantibodies are a well-recognized component of the autoimmune repertoire in human systemic lupus erythematosus (SLE) and have been postulated to have pathogenic consequences. Early studies indicated that IgM anti-lymphocyte autoantibodies mainly recognized T cells and identified CD45, a protein tyrosine phosphatase of central significance in the modulation of lymphocyte function, as the main antigenic target on T cells. However, more recent work indicates that lupus autoantibodies can also recognize B cells and that CD45 may also represent their antigenic target. In particular, IgM Abs encoded by V(H)4.34 appear to have special tropism for B cells, and strong, but indirect evidence suggests that they may recognize a B cell-specific CD45 isoform. Because V(H)4.34 Abs are greatly expanded in SLE, in the present study we investigated the antigenic reactivity of lupus sera V(H)4.34 IgG Abs and addressed their contribution to the anti-lymphocyte autoantibody repertoire in this disease. Our biochemical studies conclusively demonstrate that lupus IgG V(H)4.34 Abs target a developmentally regulated B220-specific glycoform of CD45, and more specifically, an N-linked N-acetyllactosamine determinant preferentially expressed on naive B cells that is sterically masked by sialic acid on B220-positive memory B cells. Strikingly, our data also indicate that this reactivity in SLE sera is restricted to V(H)4.34 Abs and can be eliminated by depleting these Abs. Overall, our data indicate that V(H)4.34 Abs represent a major component of the lupus IgG autoantibody repertoire and suggest that the carbohydrate moiety they recognize may act as a selecting Ag in SLE.

Analysis of the expressed immunoglobulin variable region heavy chain gene products in paraproteins from Iranian patients with multiple myeloma

The frequency of expression of immunoglobulin (Ig) variable region heavy (VH ) chain gene products was studied in 43 Iranian patients with mutiple myeloma (MM). The expressed VH gene families and associated cross-reactive idiotypes (CRI) were analysed by immunoblotting and ELISA, using peptide-induced polyclonal antibodies specific for VH 1-VH 6 gene families and monoclonal antibodies (MAb) recognising CRI linked to theVH 1, VH 3, VH 4 and VH 6 gene families. The results revealed that the VH 3 family (60. 5%) was the most predominant gene family. In contrast, no paraproteins were encoded by genes from the VH 2 gene family and only 2.3% were encoded by the VH 5 family. The panel of paraproteins tested rarely expressed the probed VH -associated CRI. Our results suggest that: 1-The Ig VH genes, may not be randomly expressed in the malignant plasma cells from Iranian patients with MM. 2- Some of the genes seem to be negatively selected or highly mutated, as evidenced by the lack of expression of the probed CRI.

The molecular specificity of insulin autoantibodies

Insulin autoantibodies (IAA) are one of several markers for Type I (autoimmune) diabetes, but alone deserve special attention. Unlike the other markers, their ligand is unique to the beta cell. IAA are the first markers to appear during the symptomless period which precedes diabetes and they are present in the vast majority of young children destined to develop diabetes. The primary and tertiary structures of insulin have been known for decades. Binding studies with insulin variants have shown epitope restriction that can distinguish Type 1 diabetes-predictive from non-predictive IAA-positive sera, thereby improving specificity for the test. With two major international Type 1 diabetes prevention trials underway, there is a pressing need to refine markers that reliably indicate the presence of, and remission from, autoimmune insulitis. The binding regions of antibodies are assembled from three multi-gene families, and some of their diversity derives from random mutation during their antigen-driven maturation. There is evidence that mature IAA derive from germline-encoded 'natural' antibodies, and that the gene segments utilised by IAA may be influenced by clinical context. Monoclonal anti-idiotypic (anti-Id) antibodies can serve as probes for antibody variable region determinants, and antibodies to the different epitopes of beef and porcine insulins have already been analysed with monoclonal reagents. Used as antibodies in a radioimmunoassay format, monoclonal anti-Ids will identify and measure autoantibody idiotopes as if they were ligands. The challenge now is to replace the conventional radiobinding assays for IAA, which only detect and titrate, with radioimmunoassays that can be standardised in absolute units. There is sufficient evidence for the existence of Type 1 diabetes-predictive IAA idiotopes to justify the development of idiotope-specific radioimmunoassays which ignore Type 1 diabetes-unrelated IAA.

Preferential IgH V4-34 gene segment usage in particular subtypes of B-cell lymphoma detected by antibody 9G4

Immunoglobulin heavy chain locus variable gene segment V4-34 (V(H)4.21) use in productive heavy chain (IgH) gene rearrangements has been described in a number of human reactive and autoimmune B cell responses, and has been shown to be frequently used in some series of cases of diffuse large cell lymphoma (DLCL). The 9G4 antibody is relatively specific for the V4-34 gene product and can be used to screen for cells that use V4-34 in their productive IgH locus rearrangements. The purposes of this study were to determine the sensitivity of the 9G4 antibody against DLCL cases known to use V4-34, then to screen a variety of human lymphoma types for 9G4 reactivity. Frozen tissue sections were cut from 118 cases of various human lymphomas. Generalized 9G4 membrane reactivity was identified in 78% of DLCL cases known to use V434. 9G4 reactivity varied by lymphoma type for the unknown cases, with diffuse large cell lymphoma (30%) and mantle cell lymphoma (28%) showing statistically significant differences (P < .001) from the expected value of 6% V4-34 positivity in peripheral blood B cells. This nonrandom increased utilization of V4-34 in productive IgH locus rearrangements supports the hypothesis that Ig binding specificity may play a role in lymphomagenesis.

The frequent expansion of a subpopulation of B cells that express RF-associated cross-reactive idiotypes: evidence from analysis of a panel autoreactive monoclonal antibodies

Preferential expression of VH gene segments is evident within the adult human primary B-cell repertoire. The repertoire may be influenced by genetic factors, e.g. VH gene segment polymorphisms, or in a temporal manner due to the exposure to environmental antigens. The molecular characteristics of 15 autoreactive human monoclonal antibodies (MoAbs) are reported. All antibodies were of the IgM isotype, and 12 of the 15 were polyreactive and included rheumatoid factor type specificity, i.e. reactivity with IgG. Nine of the 15 MoAbs are products of VH3 gene segments, as evidenced by staphylococcal protein A binding; four of these express the cross-reactive idiotype recognized by the mouse MoAb 3H7 and are thus products of the VH26 gene segment. One of the five remaining VH3 gene products expresses the cross-reactive idiotypes recognized by the mouse MoAbs B6 and D12. V-gene family usage, determined by polymerase chain reaction (PCR) amplification of cDNA and further hybridization with family-specific oligonucleotide probes, confirmed the cross-reactive idiotype studies and showed that only VH3-gene-encoded proteins bound staphylococcal protein A. Five of the six non-VH3 gene segment products express the cross-reactive idiotype recognized by the mouse MoAb LC1 and could be assumed to be products of the VH4.21 gene segment; however, one human MoAb is shown to be the product of a VH2 gene segment. This is interesting because it turns LC1 from being an anti-cross-reactive idiotype antibody into an anticlan reagent.

Somatic hypermutation introduces insertions and deletions into immunoglobulin V genes

During a germinal center reaction, random mutations are introduced into immunoglobulin V genes to increase the affinity of antibody molecules and to further diversify the B cell repertoire. Antigen-directed selection of B cell clones that generate high affinity surface Ig results in the affinity maturation of the antibody response. The mutations of Ig genes are typically basepair substitutions, although DNA insertions and deletions have been reported to occur at a low frequency. In this study, we describe five insertion and four deletion events in otherwise somatically mutated VH gene cDNA molecules. Two of these insertions and all four deletions were obtained through the sequencing of 395 cDNA clones (approximately 110,000 nucleotides) from CD38+IgD- germinal center, and CD38-IgD- memory B cell populations from a single human tonsil. No germline genes that could have encoded these six cDNA clones were found after an extensive characterization of the genomic VH4 repertoire of the tonsil donor. These six insertions or deletions and three additional insertion events isolated from other sources occurred as triplets or multiples thereof, leaving the transcripts in frame. Additionally, 8 of 9 of these events occurred in the CDR1 or CDR2, following a pattern consistent with selection, and making it unlikely that these events were artifacts of the experimental system. The lack of similar instances in unmutated IgD+CD38- follicular mantle cDNA clones statistically associates these events to the somatic hypermutation process (P = 0.014). Close scrutiny of the 9 insertion/deletion events reported here, and of 25 additional insertions or deletions collected from the literature, suggest that secondary structural elements in the DNA sequences capable of producing loop intermediates may be a prerequisite in most instances. Furthermore, these events most frequently involve sequence motifs resembling known intrinsic hotspots of somatic hypermutation. These insertion/deletion events are consistent with models of somatic hypermutation involving an unstable polymerase enzyme complex lacking proofreading capabilities, and suggest a downregulation or alteration of DNA repair at the V locus during the hypermutation process.

Lack of allelic exclusion in B cell chronic lymphocytic leukemia

We determined the immunoglobulin (Ig) V(H) subgroup expressed by the leukemia cells of 108 patients with B cell chronic lymphocytic leukemia (CLL). Surprisingly, we found that six samples (5%) each expressed Ig of more than one V(H) subgroup. Southern blot analysis demonstrated that these samples each had rearrangements involving both Ig heavy chain alleles. Nucleic acid sequence analyses of the Ig cDNA revealed each to express two functional Ig V(H) genes: V(H)3-33 and V(H)4-39; V(H)3-7 and V(H)4-39; V(H)3-23 and V(H)4-61; V(H)2-70 and V(H)3-30.3; or V(H)3-30 and V(H)4-b (DP67). One sample expressed three Ig V(H) genes: V(H)2-70, V(H)3-7, and V(H)4-59. Despite having more than one Ig heavy chain transcript, each sample was found to express only one functional Ig light chain. From the primary sequence, we deduced that the Ig of some of these CLL samples should react with Lc1, a monoclonal antibody (mAb) reactive with a supratypic cross-reactive idiotype present on Ig encoded by a subgroup of Ig V(H)4 genes (namely, V(H)4-39, V(H)4-b [DP-67], V(H)4-59, or V(H)4-61), and B6, an mAb that reacts with Ig encoded by certain Ig V(H)3 genes (namely, V(H)3-23, V(H)3-30, or V(H)3-30.3), and/or modified staphylococcal protein A (SpA), a 45-kilodalton bacterial "superantigen" that reacts with most Ig of the V(H)3 subgroup. Flow cytometric analyses revealed that such samples did in fact react with Lc1 and B6 and/or SpA, but not with control mAbs of irrelevant specificity. This study demonstrates that a subset of CLL patients have leukemic B cells that express more than one functional Ig heavy chain.

Staphylococcal protein A binding to VH3 encoded immunoglobulins

Staphylococcal protein A (SPA) is a B-cell superantigen which binds specifically to the variable region of human VH3 encoded antibodies. We undertook to identify the VH3 regions involved in the interaction with SPA by producing mutant antibodies in the baculovirus expression system. We had previously shown that a single amino acid change at position 57 in the CDR2 of a human SPA nonbinding VH3 encoded rheumatoid factor converted it to an SPA binder, implicating CDR2 in SPA binding. When regions of the mutated binder were exchanged with those from a mouse nonbinding antibody, the pattern of SPA binding indicated that residues in FR1, CDR2 and FR3 are involved in the interaction between VH3 encoded antibodies and SPA. In addition, all three regions are simultaneously required for SPA binding to occur. When any one of the three regions was altered, SPA binding was severely disrupted.

Molecular characterization of monoclonal IgM derived from human B cell lines expressing the 4C9 rheumatoid factor associated idiotype

Ten human monoclonal B cell lines that express the RF associated Id 4C9 were analyzed using an immunogenetic approach. Five of eight tested lines were also strongly positive for the 6B6.6 Id. We found that all the 4C9/6B6.6 positive lines expressed VkIIIa light chain genes. In contrast, 4C9 reactivity was also found on a cell line expressing a VkIIIb light chain gene and on a line expressing a V light chain gene. The two anti-Ids recognized a linear light chain determinant on Humkv328 encoded light chains but also a conformational determinant on Vg encoded light chains that appeared to be dependent on the presence of a heavy chain. Idiotypic reactivity occurred on both RF positive and RF negative antibodies. Within this idiotypic system, the basis for idiotypic reactivity and RF reactivity is complex, subject to both heavy and light chain gene usage and sensitive to small numbers of somatic mutations.