nave B cells express self-reactive Abs that are biased to
OpinionIn the bone marrow, tolerance to self-antigens in the Bcell lineage is maintained by three important mechanismsDuring development, recombination and somatic mutationof immunoglobulin (Ig) genes generates heavy (H) and light(L) chains that combine to form B cell receptors (BcR)capable of recognizing a virtually unlimited number ofantigenic determinants. These temporally ordered pro-cesses can also give rise to potentially threatening poly-reactive BcRs able to recognizemore than one epitope, or toBcRs endowed with self-reactivity. The immune systemmust therefore deploy cellular and molecular strategies tokeep autoreactive B cells in check.
analysis of B cell clones obtained from patients with idio-pathic lupus and producing pathogenic autoAbs suggestedthat receptor editing might be defective in human SLE. In patients with lupus nephritis, lack of receptorediting in B cells producing anti-double-stranded (dsDNA)Abs has also been suggested to occur , leading to theproposal that impaired receptor editing could play a role inthe persistence of threatening autoreactivity .
However, these early studies did not take into accountdisease activity, a hallmark of human lupus. Investigationof SLE patients with active disease revealed that matureReceptor editing: a key mechanism of B cell tolerance
eases is increasing, the molecular basis underlying theirinduction and persistence remains enigmatic. Initially,Receptor editing anin rheumatic autoimMoncef Zouali
Inserm U606 and University of Paris Diderot-Paris 7, Centre Vig75475 Paris Cedex 10, France
Receptor editing is a key mechanism of B cell tolerancethat modifies the B cell receptor (BcR) specificity ofself-reactive lymphocytes. It acts through initiation ofsecondary immunoglobulin rearrangements, throughgeneration of newly rearranged endogenous l chainsthat displace k chains, or through isotypic and allelicinclusion of dual BcRs (k+/l+ or k+/k+ B cells). Mountingevidence indicates that receptor editing is eitherimpaired or accelerated in patients suffering from rheu-matic autoimmune diseases. Remarkably, both altera-tions can promote the pathogenesis of autoimmunedisorders by favoring the uncontrolled emergenceand/or persistence of autoreactivity. Whereas impairedsecondary rearrangements might result in ineffectivesilencing of B cells, exacerbation of receptor editingcan give rise to autoreactive receptors from clones thatwere initially devoid of autoreactivity.
Accumulating evidence indicates that B cells are keyplayers in the innate and adaptive branches of immunity,and that impairment of some of their functions can lead toa variety of disorders in humans. In autoimmune diseases,such as rheumatoid arthritis (RA) and systemic lupuserythematosus (SLE), several B cell alterations have beenrecognized . This relatively recent insight has led tonovel immunointervention strategies based on specific Bcell targeting, with beneficial effects in a variety of humanautoimmune diseases . Despite this significant thera-peutic progress, the precise mechanisms that lead to loss ofB cell tolerance to self-antigens in autoimmune diseaseremain under scrutiny.Corresponding author: Zouali, M. (firstname.lastname@example.org).
1471-4906/$ see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.it.2007.12receptor revisionmune diseases
Petersen, Hopital Lariboisie`re, 2 rue Ambroise Pare,
(Figure 1). First, clonal deletion purges self-reactive B cellsfrom the repertoire by apoptosis . Second, clonal anergysilences autoreactive B cells and renders them unrespon-sive to BcR crosslinking . Third, receptor editingmodifies the BcR specificity of self-reactive cells throughthe initiation of secondary Ig gene rearrangements [5,6].Studies with transgenic and knockin mice revealed thatediting is an important mechanism of B cell tolerance thatcan function in the apparent absence of major cell loss.Compared to mature B cells, immature B cells are highlysensitive to receptor editing . Even encounters withultra-low-affinity, membrane-bound autoantigens canreinduce expression of sufficient recombination-activatinggene (RAG)-1 and RAG-2 activities and can extinguishautoreactivity by successive gene rearrangements thatdisplace productively rearranged k chain genes . Re-ceptor editing can also result from the generation of newlyrearranged endogenous l chains that displace k chains,and they can form edited BcRs with no, or reduced, auto-reactivity (Table 1). As a result, 47% of l+ lymphocyteshave evidence for rearrangement in normal mice, andalmost all l+ lymphocytes have undergone k gene assemblyin humans . Another strategy for dealing with autoreac-tive B cells is the expression of two different L chains (k+/l+
and k+/k+ B cells). Such isotypically and allelically includeddual receptor B cells are part of the peripheral repertoire inhumans and mice .
Inefficient receptor editing in human autoimmunerheumatic diseasesAlthough knowledge about the autoantibodies (autoAbs)produced by patients with rheumatic autoimmune dis-contain 30 Vk genes in association with 50 Jk elements .Patients in remission, however, exhibited a normalized,
.004 Available online 6 February 2008 103
Opinionunskewed Ig Vk gene repertoire as well as an overalldecrease in autoreactivity. In lupus-prone mice, in whichimmature B cells in the bone marrow can be accessedand B cell transit followed, this crucial self-tolerancecheckpoint was also found to be reduced . It is there-fore tempting to conclude that active SLE is associatedwith increased expression of unedited self-reactiveB cells.
Consistent with the view that impaired receptor editingmight play a role in the etiology of SLE, defects in second-ary Ig gene recombination have also been reported in other
Figure 1. Negative selection of self-reactive B cells. Shown here are three mechani
reactive B cells become functionally unresponsive to BcR engagement, a state that c
antigen results in apoptosis and physical elimination of the clonotype. In receptor ed
extinguishing their potential to recognize self-antigens, and acquire a new specificit
allows them to persist in the immune repertoire.rheumatic diseases. L chain Ab sequences from threepatients with RA showed an increase in downstream Vkgenes associated with the most upstream Jk element, Jk1,suggesting inefficient secondary recombination and, there-fore, a potential defect in receptor editing . In oligoar-ticular juvenile idiopathic arthritis, there is a decrease inthe frequency of RAG-2-expressing, CD19+CD27+, memoryB cells as well as a lack of secondary rearrangements inperipheral CD27+CD5 B-2 cells when compared tohealthy individuals . This reduced RAG expressionsuggests that impaired editing could contribute to auto-immune pathogenesis in oligoarticular juvenile idiopathicarthritis.
Table 1. Molecular mechanisms of receptor editing that canmodify B cell reactivity
Molecular mechanism Refs
Initiation of secondary Ig L chain rearrangements that leadto expression of new L chains.
Generation of newly rearranged endogenous l chains thatdisplace k chains and form edited BcRs with no, or reduced,autoreactivity.
Secondary rearrangements of VH genes that give rise tononself-reactive receptors.
Isotypic and allelic inclusion leading to the emergence ofdual receptor B cells expressing two different L chains(k+/l+ and k+/k+ B cells).
104Receptor revision: a diversity-driven process in theperipheryInitially, receptor editing was thought to take place essen-tially in the bone marrow to extinguish self-reactivity.With further investigation, additional facets of thismechanism were uncovered, and in experimental animals,receptor editing did not always completely extinguish self-reactivity and could even generate autoreactivity .
In addition to secondary rearrangements, isotypic andallelic inclusion represent another potential mechanismthat can give rise to autoreactivity. Specifically, editing of
that maintain tolerance to self-antigens in the B cell lineage. In clonal anergy, self-
e reversed. In clonal deletion, the encounter between autoreactive B cells and self-
, autoreactive B cells undergo Ig gene replacements to modify their BcR specificity,
ence, extinction of autoreactivity by receptor editing changes the fate of B cells and
Trends in Immunology Vol.29 No.3autoreactive BcRs could give rise to allelically included Bcells that carry two L chains, one imparting autoreactivityand the other not . The emergence of such multi-reractive B cells expressing bispecific receptors, includingpartial autoreactivity, might contribute to autoimmunity.
In immature B cells, receptor editing seems to be drivenby the need to maintain tolerance to self-antigens. Inperipheralmature B cells, however, editing, called receptorrevision, might be driven by the need for receptor diversity,and it acts as a mechanism for expanding the B cell reper-toire . Examples of receptor revision in the peripheryhave involved RAG expression in germinal centers (GCs) ofnormal mice, suggesting a physiological role of this mech-anism . Key to the view that secondary rearrangementscan contribute to repertoire diversification is a set ofexperiments performed with mice bearing a transgene thatrenders their primary B cell repertoire theoretically mono-specific . When infected with different viruses, the miceexhibited V gene replacement, together with hypermuta-tion, and generated protective antiviral antibody specifici-ties, suggesting that both mechanisms could contribute tothediversification of theB cell repertoire of normalmice andthat the expanded repertoire is functional .
In humans, the function of receptor revision in theperiphery is not settled. Early studies indicated that
human B cells that have lost surface Ig re-express RAG-1and RAG-2, suggesting that secondary Ig gene rearrange-ments leading to replacement might occur in human GCand might contribute to the peripheral B cell repertoire. It has also been suggested that secondary recombina-tion in the periphery might participate in repertoire diver-sification by improving very low-affinity receptors . InGCs from human tonsils, RAG expression was upregulatedin activated nave B cells, but not in centroblasts , andin activated mature CD5+ B cells . Further support forthe contribution of receptor revision to the secondaryimmune response comes from the detection of recombina-tion-generated hybrid V genes in normal human tonsils.
In addition to diversifying the humoral repertoire andto generating useful pathogen-specific Abs , receptorrevision represents a potential source of autoreactive Bcells. For example, anergic B cells from autoimmune-pronemice could be activated to produce pathogenic autoAbsafter somatic mutation and receptor revision ,suggesting that receptor revision can lead to BcR changesthat overcome anergy. Support for this view also comesfrom the K/BxN mouse model of RA . In this model,animals carry a transgene encoding a self-reactive T cellreceptor that recognizes cells presenting a peptide from theglycolytic enzyme glucose-6-phosphate isomerase (GPI).Knockin targeting of the rearranged H and L chain vari-able gene segments of an anti-GPI hybridoma into thecorresponding germline JH and JL loci revealed that nega-tive selection of B cells expressing high-affinity anti-GPIspecificities operates mainly at the transitional B cellstages in the spleen. Receptor editing allowed significantnumbers of autoreactive cells to escape negative selection,and gave rise to cell precursors that produce, upon T cellhelp, pathogenic autoAbs .
Accelerated receptor revision in human autoimmunityThese experimental studies suggested that ongoing B cellstimulation could overwhelm the capacity of receptor edit-ing to prevent autoimmunity. This hypothesis was testedin experimental models of lupus. In NZB mice, B-1 cells,like GC B cells, were found to express RAG-1 and RAG-2and to undergo secondary Ig V(D)J recombination . Inaddition, B cells from autoimmune-prone mice showedhigh levels of RAG mRNA and recombination. Based onaltered somatic VHmutation profiles,Rag gene expression,and circular VHDH gene excision products in mice bearingthe Sle3/5 locus , and on a preferred usage ofVH-proximal DH genes and distal JH genes , it hasbeen proposed that IgH chain receptor revision mightcontribute to the development of autoreactive antibodies.
In humans, profiling the distribution of Vk and Jk geneelements in individual B cells of a patient with early,untreated SLE led to the conclusion that receptor editingappeared to be greatly enhanced in the SLE patient .The authors also found that SLE patients exhibiteda significant increase in the frequency of RAG-1 andRAG-2 mRNA+ B cells in the peripheral blood .Analysis of the frequency of RAG-expressing cells in the
+ + +
OpinionCD19 IgD CD5 and CD5 peripheral blood B cell popu-lations of three juvenile SLE patients and two age-matchedhealthy controls also suggested that RAG expression isupregulated in the CD5+ B cell population of certain SLEpatients .
Extensive secondary Ig gene recombination has alsobeen reported in B cells from patients with RA, suggestingthat it can be an abnormal mechanism shared by differentrheumatic diseases. In one study of an RA patient, k chainsfrom new emigrant B cells showed an increase in upstreamVk genes associated with downstream Jks, probably reflect-ing extensive secondary recombination in those cells . Arelated investigation identified a group of human B cellsthat coexpress surrogate and conventional L chains(V-preB+L+). Whereas conventional B cells preferentiallyuse Jk1 and Jk2 segments, V-preB
+L+ B cells derived fromnormal donors showed decreased Jk1 and Jk2 usage andincreased downstream Jk4 and Jk5 segment usage .The V-preB+L+ B cells also exhibited a higher proportion ofunusually long Igk CDR3s (complementarity-determiningregion), a persistent RAG expression, and an overrepre-sentation of the Vk4 family member, B3, suggestingincreased secondary V(D)J recombination and receptorrevision. In addition, the B cells had an unusual Ab reper-toire associated with self-reactivity and were enriched inthe joints of RA patients .
Whereas little is known regarding the function ofhuman V-preB+L+ B cells, their murine counterpartsappear to be cells with low-affinity anti-self Abs that failto be edited or deleted, and that survive to enter theperipheral circulation . Under normal circum-stances, these B cells are probably anergic ; how-ever, in autoimmune-prone backgrounds they mightescape tolerance mechanisms and produce autoAbs .
SLE and RA are characterized by the expansion ofautoAb-producing B cells that often increase in affinityas the disease progresses and that often correlate withdisease se...