Receptor revision and systemic lupus

Immunoglobulin light chain allelic inclusion in systemic lupus erythematosus

The principles of allelic exclusion state that each B cell expresses a single light and heavy chain pair. Here, we show that B cells with both kappa and lambda light chains (Igκ and Igλ) are enriched in some patients with the systemic autoimmune disease systemic lupus erythematosus (SLE), but not in the systemic autoimmune disease control granulomatosis with polyangiitis. Detection of dual Igκ and Igλ expression by flow cytometry could not be abolished by acid washing or by DNAse treatment to remove any bound polyclonal antibody or complexes, and was retained after two days in culture. Both surface and intracytoplasmic dual light chain expression was evident by flow cytometry and confocal microscopy. We observed reduced frequency of rearrangements of the kappa-deleting element (KDE) in SLE and an inverse correlation between the frequency of KDE rearrangement and the frequency of dual light chain expressing B cells. We propose that dual expression of Igκ and Igλ by a single B cell may occur in some patients with SLE when this may be a consequence of reduced activity of the KDE.

Serum free light chains as biomarkers for systemic lupus erythematosus disease activity

OBJECTIVE

To evaluate serum free light chains (FLC) as a putative biomarker of systemic lupus erythematosus (SLE) activity.

METHODS

Seventy-five SLE patients and 41 age- and sex-matched rheumatoid arthritis (RA) controls were enrolled. Disease activity was assessed using the Safety of Estrogens in Lupus Erythematosus: National Assessment version of the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) definition and physician global assessments for SLE and the Disease Activity Score in 28 joints for RA. Serum FLC levels were compared against other biomarkers (IgG, C3, C4, double-stranded DNA [dsDNA] antibody). Nonparametric tests were used to compare 1) FLC and IgG in SLE versus RA and healthy controls, 2) FLC and IgG among different levels of activity in SLE, and 3) FLC in active versus nonactive RA. Correlation of FLC, C3, C4, dsDNA antibody, and IgG with the SLEDAI and modified SLEDAI (M-SLEDAI) were obtained.

RESULTS

FLC was higher in SLE than in RA; both were higher than referent healthy controls. Total FLC was significantly higher in subjects with greater SLE disease activity than lower/no activity. There were no significant differences in IgG, C4, or dsDNA antibody stratified by disease activity. Total FLC and C3 showed moderate to strong correlation with the SLEDAI and M-SLEDAI. In RA, no differences were seen in FLC levels for different levels of disease activity. Similar results were seen after controlling for renal function, age, and sex. In multiple linear regression, FLC significantly explained 50% variance of the SLEDAI after adjusting for renal function, age, and sex.

CONCLUSION

Serum FLC levels correlate strongly with disease activity in SLE, but not in RA. Serum FLC may be used as a biomarker of SLE disease activity.

Reduced receptor editing in lupus-prone MRL/lpr mice

The initial B cell repertoire contains a considerable proportion of autoreactive specificities. The first major B cell tolerance checkpoint is at the stage of the immature B cell, where receptor editing is the primary mode of eliminating self-reactivity. The cells that emigrate from the bone marrow have a second tolerance checkpoint in the transitional compartment in the spleen. Although it is known that the second checkpoint is defective in lupus, it is not clear whether there is any breakdown in central B cell tolerance in the bone marrow. We demonstrate that receptor editing is less efficient in the lupus-prone strain MRL/lpr. In an in vitro system, when receptor-editing signals are given to bone marrow immature B cells by antiidiotype antibody or after in vivo exposure to membrane-bound self-antigen, MRL/lpr 3-83 transgenic immature B cells undergo less endogenous rearrangement and up-regulate recombination activating gene messenger RNA to a lesser extent than B10 transgenic cells. CD19, along with immunoglobulin M, is down-regulated in the bone marrow upon receptor editing, but the extent of down-regulation is fivefold less in MRL/lpr mice. Less efficient receptor editing could allow some autoreactive cells to escape from the bone marrow in lupus-prone mice, thus predisposing to autoimmunity.

Ig allotypic inclusion does not prevent B cell development or response

B cells expressing two different Ig kappa L chains (allotype included) have been occasionally observed. To determine frequency and function of these cells, we have analyzed gene-targeted mice that carry a human and a mouse Igk C region genes. Using different methodologies, we found that cells expressing two distinct kappa-chains were 1.4-3% of all B cells and that they were present in the follicular, marginal zone, and B1 mature B cell subsets. When stimulated in vitro with anti-IgM, dual kappa surface-positive cells underwent activation that manifested with cell proliferation and/or up-regulation of activation markers and similar to single kappa-expressing B cells. Yet, when activated by divalent reagents that bound only one of the two kappa-chains, dual kappa B cells responded suboptimally in vitro, most likely because of reduced Ag receptor cross-linking. Nonetheless, dual kappa B cells participated in a SRBC-specific immune response in vivo. Finally, we found that Ig allotype-included B cells that coexpress autoreactive and nonautoreactive Ag receptors were also capable of in vitro responses following BCR aggregation. In summary, our studies demonstrate that Ig kappa allotype-included B cells are present in the mouse mature B cell population and are responsive to BCR stimulation both in vitro and in vivo. Moreover, because in vitro activation in response to anti-IgM was also observed in cells coexpressing autoreactive and nonautoreactive Abs, our studies suggest a potential role of allotype-included B cells in both physiological and pathological immune responses.

Interleukin-6 is responsible for aberrant B-cell receptor-mediated regulation of RAG expression in systemic lupus erythematosus

Defective regulation of secondary immunoglobulin V(D)J gene rearrangement promotes the production of autoantibodies in systemic lupus erythematosus (SLE). It remains unclear, however, whether the regulation of the recombination-activating genes RAG1 and RAG2 is effective in SLE. RAG1 and RAG2 messenger RNA expression was analysed before and after in vitro activation of sorted CD19(+) CD5(-) B cells with anti-immunoglobulin M antibodies, in 20 SLE patients and 17 healthy controls. The expression of CDK2 and p27(Kip1) regulators of the RAG2 protein, were examined. The levels of interleukin-6 (IL-6) and its influence on RAG regulation were also evaluated in vitro. SLE patients had increased frequency of RAG-positive B cells. B-cell receptor (BCR) engagement induced a shift in the frequency of kappa- and lambda-positive cells, associated with a persistence of RAG messenger RNA and the maintenance of RAG2 protein within the nucleus. While expression of the RAG2-negative regulator CDK2 was normal, the positive regulator p27(Kip1) was up-regulated and enhanced by BCR engagement. This effect was the result of the aberrant production of IL-6 by SLE B cells. Furthermore, IL-6 receptor blockade led to a reduction in p27(Kip1) expression, and allowed the translocation of RAG2 from the nucleus to the cytoplasm. Our study indicates that aberrant production of IL-6 contributes to the inability of SLE B cells to terminate RAG protein production. Therefore, we hypothesize that because of constitutive IL-6 signalling in association with BCR engagement, SLE B cells would become prone to secondary immunoglobulin gene rearrangements and autoantibody production.

Paucity of V-D-D-J rearrangements and VH replacement events in lupus prone and nonautoimmune TdT-/- and TdT+/+ mice

CDR3 regions containing two D segments, or containing the footprints of V(H) replacement events, have been reported in both mice and humans. However, the 12-23 bp rule for V(D)J recombination predicts that D-D rearrangements, which would occur between 2 recombination signal sequences (RSSs) with 12-bp spacers, should be extremely disfavored, and the cryptic RSS used for V(H) replacement is very inefficient. We have previously shown that newborn mice, which lack TdT due to the late onset of its expression, do not contain any CDR3 with D-D rearrangements. In the present study, we test our hypothesis that most D-D rearrangements are due to fortuitous matching of the second apparent D segment by TdT-introduced N nucleotides. We analyzed 518 sequences from adult MRL/lpr- and C57BL/6 TdT-deficient B cell precursors and found only two examples of CDR3 with D-D rearrangements and one example of a potential V(H) replacement event. We examined rearrangements from pre-B cells, marginal zone B cells, and follicular B cells from mice congenic for the Lbw5 (Sle3/5) lupus susceptibility loci and from other strains of mice and found very few examples of CDR3 with D-D rearrangements. We assayed B progenitor cells, and cells enriched for receptor editing, for DNA breaks at the "cryptic heptamer" but such breaks were rare. We conclude that many examples of apparent D-D rearrangements in the mouse are likely due to N additions that fortuitously match short stretches of D genes and that D-D rearrangements and V(H) replacement are rare occurrences in the mouse.

Consequences of receptor editing at the lambda locus: multireactivity and light chain secretion

To investigate the manner in which B cells with lambda light (L) chains undergo receptor editing, we have studied hybridoma panels from 56R/kappa-deleted (kdel) mice. 56R/kdel mice only produce four L chains (lambda1, lambda2, lambda3, and lambdaX). They also have a simplified heavy (H) chain repertoire: All B cells start out with a 56R anti-DNA H chain. A few frankly autoreactive 56R lambda1 cells appear to escape into the periphery, but the majority of the peripheral B cell repertoire in 56R/kdel is made up of B cells expressing the 56R H chain with the lambdaX L chain. Surprisingly, 56R lambdaX B cells are multireactive, binding to a variety of self and nonself antigens, including dsDNA (albeit at reduced affinity compared with the other lambda L chains). Another significant population in the 56R/kdel mouse consists of allelically included B cells that express lambdaX along with another L chain. The multireactivity of both 56R lambdaX and 56R lambdaX/lambda1 receptors could contribute to autoimmunity if these B cells were to become activated. Also found among 56R/kdel hybridomas are clones that have inactivated the H chain and secrete only L chains. These clones may represent products of exhaustive rearrangement. Multireactivity, allelic inclusion, and L chain secretion are three consequences of editing at the lambda locus that may predispose toward the development of autoimmunity.

Receptor editing for better or for worse

Receptor editing has emerged from its original identification as a minor secondary mechanism of B cell tolerance to be considered as a dominant mechanism by which autoreactive immature B cells are rendered tolerant. Clonal deletion, previously regarded as the major mechanism of central B cell tolerance, has been shown by recent studies to operate secondarily and only when receptor editing is unable to provide a non-autoreactive specificity. Receptor editing has also been shown to operate during the development of wild-type B lymphocytes, and ongoing investigations demonstrate the influence of particular signaling molecules in the induction and/or inhibition of receptor editing. Together, these studies begin to map the signaling pathways that regulate receptor editing in autoreactive and non-autoreactive immature B cells.

V(H) replacement in rearranged immunoglobulin genes

Examples suggesting that all or part of the V(H) segment of a rearranged V(H)DJ(H) may be replaced by all or part of another V(H) have been appearing since the 1980s. Evidence has been presented of two rather different types of replacement. One of these has gained acceptance and has now been clearly demonstrated to occur. The other, proposed more recently, has not yet gained general acceptance because the same effect can be produced by polymerase chain reaction artefact. We review both types of replacement including a critical examination of evidence for the latter. The first type involves RAG proteins and recombination signal sequences (RSS) and occurs in immature B cells. The second was also thought to be brought about by RAG proteins and RSS. However, it has been reported in hypermutating cells which are not thought to express RAG proteins but in which activation-induced cytidine deaminase (AID) has recently been shown to initiate homologous recombination. Re-examination of the published sequences reveals AID target sites in V(H)-V(H) junction regions and examples that resemble gene conversion.

Evidence for selective transformation of autoreactive immature plasma cells in mice deficient in Fasl

Germline mutations in Fas and Fasl induce nonmalignant T cell hyperplasia and systemic autoimmunity and also greatly increase the risk of B cell neoplasms. B lymphomas occurring in Fasl mutant (gld) mice usually are immunoglobulin (Ig) isotype switched, secrete Ig, and are plasmacytoid in appearance but lack Myc translocations characteristic of other plasma cell (PC) neoplasms. Here, we explore the relationship between B cell autoreactivity and transformation and use gene expression profiling to further classify gld plasmacytoid lymphomas (PLs) and to identify genes of potential importance in transformation. We found that the majority of PLs derive from antigen-experienced autoreactive B cells producing antinuclear antibody or rheumatoid factor and exhibit the skewed Ig V gene repertoire and Ig gene rearrangement patterns associated with these specificities. Gene expression profiling revealed that both primary and transplanted PLs share a transcriptional profile that places them at an early stage in PC differentiation and distinguishes them from other B cell neoplasms. In addition, genes were identified whose altered expression might be relevant in lymphomagenesis. Our findings provide a strong case for targeted transformation of autoreactive B cells in gld mice and establish a valuable model for understanding the relationship between systemic autoimmunity and B cell neoplasia.

Insights into the regulation of immunoglobulin light chain gene rearrangements via analysis of the kappa light chain locus in lambda myeloma

Accumulating evidence indicates that B cells may undergo sequential rearrangements at the light chain loci, despite already expressing light chain receptors. This phenomenon may occur in the bone marrow and, perhaps, in germinal centers. As immunoglobulin (Ig)kappa light chains usually rearrange before Iglambda light chains, we analysed, by polymerase chain reaction, the Igkappa locus of bone marrow mononuclear cells from 29 patients with Iglambda myeloma to identify earlier recombinations in marrow plasma cells. The results demonstrated that Igkappa alleles were inactivated via the kappa-deleting element, presumably prior to V(kappa)-J(kappa) rearrangement, in many cases. Eighteen alleles (16 myeloma clones, 55%) showed V(kappa)-J(kappa) rearrangements, with increased utilization of 5' distant V(kappa) and 3' distant Jkappa gene segments (Jkappa4, 56%), an indication of multiple sequential rearrangements. In-frame, potentially functional V(kappa)-J(kappa) rearrangements were found in approximately one-third of available rearrangements (as expected by chance), each one in different myeloma clones: three were germline encoded, while one had several nucleotide substitutions, suggesting inactivation after the onset of somatic hypermutation. Three of four potentially functional V(kappa)-J(kappa)rearrangements involved V(kappa)4-1, a segment considered to be associated with autoimmunity. These findings provide insights into the regulation of light chain rearrangements and support the view that B cells may occasionally undergo sequential light chain rearrangements after the onset of somatic hypermutation.

Characterization of the human Ig heavy chain antigen binding complementarity determining region 3 using a newly developed software algorithm, JOINSOLVER

We analyzed 77 nonproductive and 574 productive human V(H)DJ(H) rearrangements with a newly developed program, JOINSOLVER. In the productive repertoire, the H chain complementarity determining region 3 (CDR3(H)) was significantly shorter (46.7 +/- 0.5 nucleotides) than in the nonproductive repertoire (53.8 +/- 1.9 nucleotides) because of the tendency to select rearrangements with less TdT activity and shorter D segments. Using criteria established by Monte Carlo simulations, D segments could be identified in 71.4% of nonproductive and 64.4% of productive rearrangements, with a mean of 17.6 +/- 0.7 and 14.6 +/- 0.2 retained germline nucleotides, respectively. Eight of 27 D segments were used more frequently than expected in the nonproductive repertoire, whereas 3 D segments were positively selected and 3 were negatively selected, indicating that both molecular mechanisms and selection biased the D segment usage. There was no bias for D segment reading frame (RF) use in the nonproductive repertoire, whereas negative selection of the RFs encoding stop codons and positive selection of RF2 that frequently encodes hydrophilic amino acids were noted in the productive repertoire. Except for serine, there was no consistent selection or expression of hydrophilic amino acids. A bias toward the pairing of 5' D segments with 3' J(H) segments was observed in the nonproductive but not the productive repertoire, whereas V(H) usage was random. Rearrangements using inverted D segments, DIR family segments, chromosome 15 D segments and multiple D segments were found infrequently. Analysis of the human CDR3(H) with JOINSOLVER has provided comprehensive information on the influences that shape this important Ag binding region of V(H) chains.

B lymphocyte selection and survival in systemic lupus

B lymphocytes are an essential element in the body's immune system. Engagement of the B cell receptor is responsible for initiating the signaling events that can activate, inactivate or physically eliminate B cells, depending on the magnitude and duration of the signal. Control of B cell signaling occurs through both positive and negative regulation, as well as through the actions of molecular scaffolds that contribute to the formation of signaling complexes. Inactivation of genes encoding signaling molecules was shown to result in clinical manifestations reminiscent of systemic autoimmunity in experimental animals. Aberrant expression of some signaling molecules was also observed in patients with systemic autoimmune diseases. Understanding the mechanisms that subvert B cell receptor transduction pathways is likely to aid in the development of therapeutic agents to treat autoimmune diseases.

The appearance and disappearance of antiphospholipid autoantibodies subsequent to oxidation–reduction reactions

The mechanisms that cause the appearance of autoantibodies are not understood. Compared to normal antibody production, factors responsible for autoantibody synthesis are more complex; they are thought to disrupt the normal mechanisms proposed to eliminate or down-regulate self-antibodies or to interfere with anti-self-receptor editing. Data presented show that autoantibodies exist in the blood of all normal individuals. The autoantibodies appear after simple oxidation-reduction (redox) reactions and react by ELISA, immunofluorescence, flow cytometry, Western blots, and in lupus anticoagulant (LA) assays. Antiphospholipid antibody (aPL) specificities detected after redox are cardiolipin (aCL), antiphosphatidylserine (aPS), antiphosphatidylethanolamine (aPE), antiphosphatidylcholine (aPC), and LA. These antibody activities were confirmed in several outside laboratories. The aPL isotypes detected in ELISA are plasma protein-dependent and include IgG, IgA, and IgM. Oxidizing agents tested to date include hemin, KMnO4, and NaIO4. Furthermore, aPL appear after exposure to direct current (DC)-mediated electromotive force. Alternating current (AC) is ineffective. Commercial IvIg preparations, also a source of IgG autoantibodies, provide a less complex milieu than plasma or serum for studying the biology of aPL redox-mediated mechanisms. Inhibition of hemin-mediated IvIg aPL conversion can be achieved by the addition of antioxidants, e.g., ascorbic acid, hemopexin, apotransferrin, and by addition of normal plasma or serum. Remarkably, the aPL specificities in the blood of autoimmunity patients disappear subsequent to application of redox reactions. These data document the hitherto unknown existence of redox-reactive autoantibodies in all normal individuals. The evolutionary persistence of these redox-sensitive antibodies raises interesting possibilities about their potentially beneficial role in immunological homeostasis.