Polyclonal human rheumatoid factors cross-reacting with histone H3: Characterization of an idiotope on the H3 binding site

The development of highly sensitive immunoassays has made the detection of the multireactivity of antibodies a relatively common phenomenon. Polyreactivity is frequent in human auto antibodies, especially in rheumatoid factors (RFs), but the structural basis and the significance of this phenomenon remain substantially unknown. Recently, we showed that the double reactivity of a human monoclonal RF with histones was probably due to two distinct binding sites. However, cross-reactivity seems more frequent among polyclonal RFs occurring during autoimmune diseases than with monoclonal RFs. We studied double-reactive (IgG and histone H₃) polyclonal RFs in a patient suffering from primary Sjögren's syndrome. We showed by means of affinity chromatographies that H₃ cross-reactive RFs were only a small subset of the total patient's RFs and that this subset was enriched in IgA class. Competitive inhibition experiments suggested the existence of two distinct binding sites for IgG and H₃. These results were confirmed by showing the selective sensitivity to acid treatment of the histone binding site and by producing a murine antiidiotope monoclonal antibody BII 2.1 defining an idiotope on bireactive RF apparently linked to the H₃ binding site. This idiotope was absent in a panel of monoclonal RF, one of them cross-reacting with histone H₃. This report extends previous results concerning a monoclonal RF to the polyclonal RFs which occur during autoimmune diseases.     

A common idiotope on human rheumatoid factors identified by a hybridoma antibody

Human monoclonal and polyclonal anti-IgG autoantibodies [rheumatoid factors (RFs)] are composed primarily of kappa light chains, and may display cross-reactive idiotypes. However, the nature of the shared idiotope(s) has remained unclear. We have prepared a murine hybridoma antibody (17-109) that recognizes an idiotope present on 30% (3/10) of human IgM-RF paraproteins, and absent on immunoglobulins without RF activity. The idiotope was measurable on isolated, intact kappa light chains, but not on light-chain tryptic peptides, nor on isolated heavy chains. A comparison of the binding to 17-109 of five IgM-RF paraproteins, with known kappa chain amino acid sequences, suggested a relationship between the idiotope recognized by the hybridoma and the complementarity-determining regions. The serum of patients with rheumatoid arthritis contained idiotope positive material that bound specifically to a 17-109 immunoadsorbent column. Moreover, the 17-109 anti-idiotope antibody partially inhibited the binding to IgG of IgM-RF and IgA-RF in serum, but did not effect the binding to antigen of IgM and IgA anti-tetanus toxoid antibodies. These results suggest that a significant proportion of IgM-RF paraproteins share an idiotope located at or near the complementarity-determining regions of the kappa light chain. Human serum RFs include a kappa light chain family that is idiotopically related to the kappa chains on IgM-RF paraproteins.     

Double reactivity of monoclonal and polyclonal rheumatoid factors for IgG and histones: mapping of binding sites by means of histone synthetic peptides and anti-Id antibodies.

Polyreactive antibodies able to bind various apparently unrelated structures represent a frequent antibody population in autoimmune diseases. In this work, the structural basis of the double reactivity of such autoantibodies was investigated using as models polyclonal and monoclonal human rheumatoid factors (RF) reacting with histones. Both direct ELISA binding and competitive inhibition experiments were performed. A more precise delineation of the histone regions recognized by the RFs was made by means of 27 synthetic peptides of these proteins. Anti-idiotope (Id) murine antibodies were used to map the binding sites involved on RF in the interaction with IgG and histones. Among the 13 polyclonal and six monoclonal RFs tested, four and two respectively were found to cross-react with IgG and histones. The fragments shown to be the most frequently recognized by RFs were located in residues 1-16 and 204-218 of H1, 1-20 and 65-85 of H2A, and 1-21 of H3. The results obtained by competitive ELISA assays using IgG, histone peptides and anti-Id monoclonal antibodies led us to confirm and characterize more precisely our previous finding suggesting the existence of topographically distinct binding sites for the different targets recognized by RFs.     

The Major Rheumatoid Factor Cross-Reactive Idiotype in Rheumatic Disease

The major rheumatoid factor cross-reactive idiotype (RCRI), a tertiary structure formed by both light and heavy chains, is found on 60% of all monoclonal IgM kappa RFs. To determine if the RCRI is expressed in patients with rheumatic disease, we used polyclonal rabbit anti-idiotypic antibodies to detect RCRI in sera and in pokeweed mitogen cultures of blood mononuclear cells (PBM) from patients with rheumatoid arthritis (RA) and juvenile rheumatoid arthritis (JRA). We detected increased expression of RCRI + plasma cells in PWM cultures, and in sera from these patients. We have determined that some 7S IgM molecules from RF+ RA patients are RCRI +, and can bind IgG in a sensitive RF ELISA. We have also observed that the CD5+ B cell subset, which is responsible for autoantibody production, generates RCRI+ antibodies. We review these data and discuss the relationship of the idiotypic network of interacting antibodies with rheumatic disease.     

Recognition of several idiotopes on a monoclonal anti-hepatitis B virus surface antigen using monoclonal anti-idiotypic antibodies.

A monoclonal murine antibody H3F5 directed to the a determinant of hepatitis B surface antigen (HBsAg) was used to raise several monoclonal anti-idiotypes. Cross-blocking experiments between the anti-idiotypes and the patterns of inhibition produced by a number of other monoclonal anti-HBsAg, generated in the same fusion as H3F5, identified three idiotopes on H3F5 which were shared to varying degrees with the other anti-HBsAg monoclonals. One behaved as a dominant cross-reacting idiotope (CRI) in that it appeared strongly in 5/7 monoclonal idiotypes. The CRI could represent an important target for regulation by anti-idiotope. Monoclonal antibodies have many advantages over polyclonal sera in the detailed analysis of idiotope structures.     

Monoclonal antibodies to restricted and cross-reactive idiotopes on monoclonal rheumatoid factors and their recognition of idiotope-positive cells

Four human monoclonal rheumatoid factors (MRF) were used to raise a panel of mouse monoclonal antibodies (mAb) which were selected in a solid-phase radioimmunoassay for binding to MRF but not normal IgM. Three mAb, each raised against a different MRF, bound to the majority of MRF and also to most polyclonal RF. Four other mAb bound selectively to the MRF against which they were raised and to no other MRF, and rarely to any polyclonal RF. Competition studies using cold and radiolabeled mAb further indicated that these mAb recognize distinct and different epitopes on MRF. RF activity of MRF was inhibited by 3 of the 4 mAb binding to a single MRF and 2 of the 3 mAb binding to multiple RF. It was thus concluded that of this panel of mAb 3 recognized cross-reactive idiotopes and the remainder demonstrated highly restricted idiotopes on MRF. These mAb identified MRF idiotopebearing cells in the peripheral blood of 3 of the MRF donors (and a further subject with type II essential cryoglobulinemia), with a frequency ranging from 0.3–10% of all mononuclear cells with the mAb to restricted idiotopes or 1.5–17% with mAb to cross-reactive idiotopes. These anti-idiotopic mAb should thus provide a highly specific means of identifying and monitoring MRF-producing cells in vivo.     

Isotype restriction of idiotopes associated with human anti-streptococcal A carbohydrate antibodies

Sharing of idiotopes between IgG and IgM antibodies has been described with antibodies of various specificities in the literature. However, this does not seem to be the rule for human IgG and IgM antibodies with specificity to streptococcal A carbohydrate. Six idiotopes were described which are associated either with IgM or with IgG antibodies to one of the two major epitopes of streptococcal A carbohydrate: N-acetyl-D-glucosamine or alpha(1,2), alpha(1,3)-linked rhamnose oligomers. Some of the idiotopes are widely cross-reactive with antibodies of the corresponding specificity in other individuals. They were constantly found to be restricted to either IgM or IgG in all individuals tested so far. We discuss several alternatives to explain this finding and conclude that switching from IgM to IgG is either a very rare event in human B cells of this specificity, or leads to loss and gain of idiotopes and/or specificity due to frequent somatic mutations. The phenomenon of idiotope restriction to certain isotypes might be important if anti-idiotopic monoclonal antibodies are considered for use in therapeutical approaches aimed at the manipulation of antibody production.     

The Antibody Repertoire of Early Human B Cells

Our previous studies have shown that a high frequency of Epstein-Barr virus (EBV)-immortalized cord blood (CB) and fetal liver (FL) clones produce IgM antibodies which display extensive autoreactivity for IgG Fc (rheumatoid factor, RF). To investigate further the repertoire of these early B cells, we have examined the expression of CRI associated with RF paraproteins in relation to antibody specificity and polyreactivity. CRI were detected by ELISA and/or flow cytometry using a panel of well-characterized monoclonal antibodies defining idiotopes associated with particular V kappa and VH gene family products and raised against Fc-specific paraproteins. Many of the CRI were expressed by these clones, suggesting that they may be markers of early B cells. The presence of the CRI was not always associated with Fc specificity. Three of eight CB/FL clones expressed the V kappa III subgroup of light chains, and two of these expressed the V kappa III sub-subgroup associated CRI, 17-109. These two clones reacted with IgG Fc, and one also bound to single-stranded DNA. The VHIII-associated idiotope D12 was expressed on IgM from 4 out of 9 FL and 5 out of 12 CB clones. D12 and B6 (also a VHIII-associated CRI) were coexpressed in 4 out of 5 CB clones but not in the four FL clones. Seven out of nine clones expressing these idiotopes were polyreactive, and five had Fc-binding activity. Three of the 12 CB clones expressed the VHI-associated conformational idiotope G8. One of 20 CLL clones expressed both B6 and D12, and another expressed both 17-109 and the VHI-associated G6 and G8 idiotopes. Taken together, these data provide evidence for the frequent usage, in early B cells, of V kappa subgroups and VH-associated idiotopes of RF paraproteins. The expression of these CRI was not a prerequisite for binding to IgG Fc, but there was a frequent association of these idiotopes with it. Differences in expression of CRI between CLL and early B-cell clones may suggest differences in the pattern of VH usage between these subsets of B cells.     

Incomplete expression of the MOPC 460 idiotype in the SERA of BaLB/c mice immunized either with DNP antigen or with anti-idiotypic

The MOPC 460 idiotype, as defined by polyclonal probes, has been described as a recurrent marker among the anti-DNP antibodies synthetized by IgHC^a mice. In this paper, we demonstrate, using syngeneic monoclonal anti-idiotypic probes, that only a part of the idiotopes of this idiotype are indeed ercurrently expressed in BALB/c mice (IgHC^a) after immunization with DNP antigen. We will also show that the immunization of BALB/c mice with monoclonal anti-idiotypic antibody specific for the recurrent determinants results firstly in the synthesis of anti-DNP antibodies and secondly in the expression of the same recurrent M460 idiotope present on a part of induced anti-DNP molecules. Contrary to this, the immunization with the monoclonal anti-idiotypic antibody specific for the private idiotope never resulted in the synthesis of anti-DNP antibodies. These results clearly suggest that, after DNP or anti-idiotypic immunization, the M460 idiotype is not expressed in its entirety.     

A high incidence of cross-reactive idiotypes among murine natural autoantibodies

Anti-idiotypic (anti-Id) antibodies were produced in rabbits against two natural monoclonal IgM autoantibodies (NmAb), D23 and E7, which exhibited a broad reactivity and were derived from fusions of spleen cells from adult unprimed BALB/c mice and nonsecreting myeloma cell lines. They were used to test the reactivities of 12 NmAb obtained from adult and newborn unprimed mice. Both anti-Id recognized cross-reactive idiotopes frequently shared by NmAb; 8 out of the 12 NmAb reacted with anti-IdD23, while 5 of them also reacted with anti-IdE7. All of the Id-bearing antibodies possessed widespread reactivity with structurally dissimilar self and nonself antigens. In most cases, their cross-reactive Id determinants seemed to be located outside of their antigen-binding sites. Furthermore, the presence in normal mouse sera of significant levels of D23 and E7 idiotopes correlated with the presence of natural antibody activity and was mainly associated with IgM and IgG2b fractions. Finally, D23 idiotope(s) were also found on induced murine anti-myosin antibodies. The high incidence of cross-reactive idiotopes found among NmAb produced by clones derived from different mice and their presence in normal BALB/c mouse serum Ig fractions suggest that families of germ-line genes may encode for at least a part of them.     

Immunoregulatory role for a public IgM idiotype in the induction of autoimmune diseases in Mycoplasma pneumoniae infection

Mycoplasma pneumoniae (MP) infection is associated with the emergence of various autoimmune disorders and autoantibody production. The most common autoantibodies induced are of anti-I specificity and express cold agglutinin (CA) activity. However, the mechanisms by which the microbial infection triggers the appearance of these autoantibodies are still unknown. To investigate these mechanisms, we used BALB/c mice as experimental models. In this paper, we show that BALB/c mice polyclonal antisera to MP react with human CA IgMs, and reciprocally, that BALB/c mice polyclonal antisera to human IgM CA react with MP. However, antibodies directed against MP and against CA IgM triggered by both immunizations represent two separate sets of antibodies. This was also confirmed using monoclonal antibodies derived from the immunized mice. Among these MAb we selected a monoclonal antibody MAb1D3 which reveals a cross-reactive idiotope (CRI) shared by human CA and other MIgMs with various autoantibody activities (anti-MAG and anti-IF). The CRI defined by MAb1D3 is a recurrent interspecies idiotope that is expressed by post infectious IgM antibodies to MP. Hence, we present in this study new data showing that the concomitant appearance of CAs and anti-MP IgM antibodies during acute MP infection is the consequence of a common idiotypic regulation of antibodies to infectious and to self antigens.     

The immune response against anti-idiotope antibodies. I. Induction of idiotope-bearing antibodies and analysis of the idiotope repertoire

In the present analysis we dissect the idiotype repertoire, independently of haptenbinding specificity, by immunizing different strains of mice with cross-linked monoclonal anti-idiotopet antibodies against antibody B1-8. B1-8 is a monoclonal antibody with specificity for the hapten (4-hydroxy-3-nitro-phenyl)acetyl (NP) and carries a germ line gene-encoded variable region. The results demonstrate that the expression of B1-8 idiotopes and their association with eachother and with NP-binding specificity are strain-specific. Certain idiotopes are expressed on antibodies differing in antigen-binding specificity, whereas one of the idiotopes appears strictly associated with NP-binding antibodies. The genetic analysis provides strong evidence that the strain specificity of the idiotope repertoire is a result of V region polymorphism in the mouse.     

Isogeneic monoclonal antibodies against anti-alpha(1----3)dextran idiotypes. II. Neonatally induced idiotope-specific suppression: a comparative analysis

From a panel of isogeneic monoclonal anti-idiotope antibodies several were used as agents in neonatal idiotope suppression. They differed from one another in isotype, and in idiotope specificity, as described in the preceding report (Eur. J. Immunol. 1987. 17: 255). In their effects they were compared with respect to the following variables: (a) minimum dose required for suppression; (b) duration of suppression, and its relationship to the dose applied neonatally; (c) half-life of anti-idiotope in the immune system of the young mice; (d) specificity of suppression as achieved by a given anti-idiotope: in how far does it affect idiotopes defined by alternate anti-idiotopes? The following results were obtained: (a) the minimum effective dose varied widely between anti-idiotopes. One, belonging to the IgM class, was completely ineffective; others varied from ≈ 10 μg/mouse, required for complete suppression, to ≈ 100 μg/mouse. (b) The dose-response characteristic was independent of whether the state of suppression was tested (by immunization against α(1 → 3)dextran) 26 days or 70 days after neonatal anti-idiotope treatment. We take this as an indication that the anti-idiotope effect occurs during an early postnatal period. (c) There appeared to be a relationship between the rate of decay of anti-idiotope in the system and the dose required for complete suppression: the faster the decay, the more is needed initially. The persistence of effective molecules in the animals appears to depend on their isotype (as has been noted by others before): IgM decays fastest, and was ineffective in our experiments; IgG₁ stays longest, and the smallest dose was required for suppression. IgG_2b was intermediate. (d) The specificity of neonatal suppression was clearly correlated with the serological specificity of the anti-idiotope monoclonal antibodies, as well as with the representation of the corresponding idiotopes in physiological anti-dextran sera, as described in the preceding report: private anti-idiotopes suppressed their counterpart idiotopes only, while the public anti-idiotope suppressed all other idiotopes in concert.     

A highly conserved determinant on human rheumatoid factor idiotypes defined by a mouse monoclonal antibody

Different human IgM rheumatoid factor (IgM RF) idiotypes have been described defined by polyclonal rabbit anti-idiotypic antibodies. These antisera do not allow clear genetic analysis of the idiotypic determinants, be they cross-reactive or private. Therefore, we tried to obtain a set of monoclonal anti-idiotypic antibodies directed against RF idiotypes. Purified IgM RF serum from a patient with classical rheumatoid arthritis was used to immunize BALB/c mice. The spleen cells were fused with Sp 2/0 Ag 14, a nonsecreting mouse myeloma cell line, and a hybrid producing monoclonal anti-idiotypic antibody was selected. The mouse antibody, an IgG1 kappa, reacts with an identical or similar determinant located on (or close to) the binding site of all tested monoclonal or polyclonal IgM RF from totally unrelated patients with Waldenstr?ms's macroglobulinemias or rheumatoid arthritis. The monoclonal antibody also reacts with 2 rheumatoid arthritis patients' IgG RF and with a low proportion of normal polyclonal IgM without detectable RF activity. An hypothesis is proposed to explain the existence of a such highly conserved determinant on RF idiotypes.     

Analysis of the repertoire of anti-(4-hydroxy-3-nitrophenyl)acetyl (NP) antibodies in C 57 BL/6 mice by cell fusion. II. Characterization of idiotopes by monoclonal anti-idiotope antibodies.

CBA and SJL mice were immunized against the hybrid cell antibody Bl-8 (μ,λ₁) which we have previously isolated from the primary response of C57BL/6 mice against the hapten (4-hydroxy-3-nitro-phenyl)acetyl (NP) (Eur. J. Immunol. 1978. 8: 393). Cell lines secreting monoclonal antibodies against B1–8 idiotopes were isolated, and the anti-idiotope antibodies were used to characterize the variable portion of B1–8 and of other anti-NP antibodies all belonging to an antibody family which carries a major idiotypic marker, namely the NP^b idiotype. Three groups of idiotopes were defined by the monoclonal anti-idiotope antibodies on the B1–8 protein; a first, in the NP binding site so that the corresponding anti- idiotopes compete with the NP hapten for binding to B1–8 (group 1); a second, presumably further apart from the binding site so that there is competition of the corresponding anti-idiotopes with a polyvalent NP-protein conjugate but not the free hapten for binding to B1–8 (group 2); and a third (group 3), specific for a determinant constructed by B1–8 μ chains and MOPC 21 χ chains which are still present in the B1–8 preparation as the product of the X63-Ag8 parent cell line. Group 1 idiotopes were expressed on 1–10% of χ₁-bearing primary anti-NP antibodies of C57BL/6 origin and group 2 idiotopes on 10–100% of these antibodies. Group 3 idiotopes were not detected in anti-NP antisera. The analysis of a set of monoclonal hybrid cell anti-NP antibodies from C57BL/6 confirmed that group 2 idiotopes are more abundantly expressed in the primary anti-NP response than any given group 1 idiotope. Group 2 idiotopes can thus be expressed independently of group 1 idiotopes. In addition, group 1 idiotopes appear to exhibit a higher degree of diversity than idiotopes of group 2. The present analysis dissects the NP^b idiotype into a large set of idiotopes which can be grouped in terms of localization on the antibody molecule and of variability and which are combined in various ways on a large number of χ chain-bearing antibody species. At least some of the idiotopes recognized by the monoclonal antibodies are under the control of the heavy chain linkage group, as shown by the idiotope analysis of serum antibodies from Igh-congenic mice. In the course of the fusion experiments, also cell lines secreting antibodies of SJL origin with specificity for the constant region of χ₁ chains were isolated, and we also describe the preparation of an Fv fragment from B1–8, an IgM antibody.     

Search for cross-reactive idiotypes on monoclonal and myasthenia gravis acetylcholine receptor antibodies.

Myasthenia gravis patients have serum anti-acetylcholine receptor antibodies that compete with monoclonal antibodies for binding to epitopes on the human acetylcholine receptor. To investigate the presence of shared idiotypes we immunised syngeneic mice with each of ten well-characterised monoclonal antibodies, previously raised against purified human acetylcholine receptor, and tested the polyclonal antisera and seven monoclonal anti-idiotype antibodies, for binding to the antigen-combining site, to framework idiotopes, and by ELISA. The polyclonal sera were mostly directed against antigen-combining site idiotopes and cross-reacted only with monoclonal anti-acetylcholine receptor antibodies that bound to the same region on the acetylcholine receptor. In contrast, five of the seven IgM monoclonal anti-idiotypic antibodies raised, none of which demonstrated antigen-combining site specificity in solution, cross-reacted with mAbs binding to more than one region. None of the antisera showing reactivity with the antigen-combining site inhibited the binding of MG anti-acetylcholine receptor antibody.     

Idiotypic relationships among anti-p-azophenylarsonate antibodies: shared public idiotopes among A-strain minor CRI and BALB/c CRIc

Four monoclonal antibodies, each previously categorized as a member of the minor cross-reactive idiotypes of the A/J anti-p-azophenylarsonate (Ar) response, have been subjected to a detailed study of their idiotopes. All four were found to share at least one common public idiotope, with other public idiotopes expressed on some and not others of these antibodies. These CRIm were compared with 7 monoclonal anti-Ar derived from BALB/c mice that have been previously found to bear idiotopes of the BALB/c CRIc. Considerable homology of public idiotopes was demonstrated between the 4 A/J CRIm and 5 of the BALB/c CRIc(+) anti-Ar referred to as the 5AF6 family. The implications of these findings with respect to the structural basis and regulation of these idiotypes are discussed.     

Allotypic restriction of the expression of MOPC460 idiotope after immunization with either anti-2,4-dinitrophenyl (DNP) or anti-idiotypic antibodies

The MOPC460 idiotype is expressed in mice with the IghCa allotypic haplotype after anti-2,4-dinitrophenyl (DNP) immunization. We have previously shown that two monoclonal syngeneic anti-idiotypic antibodies (IDM 92-13 and IDM 41-27) define two distinct idiotopes (the 460.92 and the 460.41) on the M460 idiotype. The current study demonstrates that only one idiotope (460.92) is recurrently expressed after antigen immunization in IghCa positive mice and also that, immunization against the monoclonal anti-idiotypic molecules induces the synthesis of 460.92 idiotope positive anti-DNP antibodies. However, the detection of such molecules is only possible when animals with the IghCa allotypic haplotype are immunized with the IDM 92-13 molecules. Immunization of mice with either of the two monoclonal anti-idiotypic antibodies never results in the synthesis 460.41 positive molecules. Therefore, whatever protocol of immunization used, the expression of 460.92 was allotypic restricted.     

Anti-idiotypes against anti-H-2 antibodies VI. Detection of shared idiotypes among monoclonal anti-H-2 antibodies

Anti-idiotypes produced against monoclonal anti-H-2 antibodies have been used to examine idiotope sharing among a panel of anti-major histocompatibility complex (MHC) antibodies detecting the same and different specificities. Both xenogeneic anti-100-30 and anti-3-83, which did not react with predominant idiotypes in conventional alloantisera, detected idiotopes on 3-83 and 100-30 as well as on 2 other monoclonal anti-H-2Kk antibodies. All 4 monoclonal antibodies recognized the same epitope cluster on the Kk molecule and detected the same serological specificity. H-2.5. In contrast, syngeneic and allogeneic anti-idiotype produced against the same monoclonal antibodies did not detect these cross-reactive determinants. In several instances, xenogeneic anti-idiotype reacted with anti-MHC antibodies which recognized distinct H-2 determinants, suggesting that anti-MHC antibodies detecting the same or different specificities may share idiotypic determinants. These reagents may be useful probes of the anti-MHC immune repertoire.     

Monoclonal gammopathies in Japanese patients with Sjögren's syndrome

We report 10 Japanese patients with Sjögren's syndrome (SS) who developed monoclonal gammopathies (MG). One was of the IgG class, five of IgA, three of IgM, and one of IgG/IgM. The monoclonality of 7 of 10 M proteins was studied using antiidiotypic (Id) antibodies against M proteins. Four (three IgA and one IgM) of 10 M proteins had rheumatoid factor (RF) activity. Hemagglutination inhibition tests and enzyme-linked immunosorbent assays (ELISA) showed that the RF activity was inhibited by anti-Id antibodies in all four monoclonal RFs. In two patients examined, many cells infiltrating into the salivary glands were stained with anti-Id antibodies. Our review of 19 Japanese SS patients with MG revealed that the non-IgM class predominated (13/19). This contrasts with 19 reported non-Japanese SS patients, among whom 14 were IgM. In both Japanese and non-Japanese patients there was a higher incidence of MG in primary than in secondary SS. The difference in the dominant heavy-chain class may reflect a difference in the genetic factors affecting B cell differentiation in immunologically disordered states.