Curiouser and curiouser: The role(s) of AID expression in self-tolerance

Download Curiouser and curiouser: The role(s) of AID expression in self-tolerance

Post on 25-Mar-2017




2 download


Commentary2876 Garnett Kelsoe Eur. J. Immunol. 2014. 44: 28762879DOI: 10.1002/eji.201445102Curiouser and curiouser: The role(s) of AID expressionin self-toleranceGarnett KelsoeDepartment of Immunology and Human Vaccine Institute, Duke University, Durham, NC, USAAicda is crucial for antibody diversification by mediating Ig class-switch recombination,V(D)J hypermutation (SHM) and, in some species, gene conversion. Recently, evidencehas accumulated to show that Aicda is expressed during B-cell development and thatthis expression in some unknown way, mediates tolerance in immature and transitionalB cells. In this issue of the European Journal of Immunology, Umiker et al. [Eur. J. Immunol.2014. 44: 30933108] show that enforced expression of Aicda during early B-cell devel-opment is associated with self-tolerance. Curiously, constitutive Aicda expression thatbegins early in B cells suppresses the generation of autoreactive IgM but promotes theexpression of self-reactive IgG. In contrast, when Aicda is activated later in B-cell develop-ment, self-reactive IgM is abundant but IgG is not. These observations suggest pathwaysfor self-tolerance that have been little explored.Keywords: Autoimmunity Antibodies B cells B cell development ImmunopathologySee accompanying Article by Umiker et al.B lymphocytes exhibit a unique degree of genomic plasticity; notonly are B-cell antigen receptors (BCRs) generated by genomicrearrangements of V, D, and J gene segments, but assembledBCR genes are also modified by class-switch recombination (CSR)and somatic hypermutation (SHM). Whereas V(D)J rearrange-ment occurs during B lymphopoiesis, SHM and CSR are largelyconfined to secondary lymphoid tissues where activated germinalcenter (GC) B cells express high levels of AID, the product of Aicda[1, 2]; however, in amphibians [3], birds [4], and some mam-mals [5, 6], however, AID expression and activity is unequivocallypresent in distinct subsets of developing B cells as well.AID expression, SHM and CSR in mice and humans can also befound in B cells outside the GC, e.g. in autoimmune plasmablasts[7] and human gut B cells [8]. Type I hyper IgM syndrome patientscannot form GC, but nonetheless support populations of B cellscarrying mutated immunoglobulin (Ig) genes [9]. In addition, lowlevels of AID message, and low levels of CSR, and SHM haveCorrespondence: Dr. Garnett Kelsoee-mail: garnett.kelsoe@dm.duke.edubeen recovered from detected in mouse and human immatureand transitional 1 B cells [1014]; this, and evidence for AIDexpression and activity in human fetal liver cells [1416] suggestthat AID expression during B-cell development may be a commonvertebrate trait [46]. Strong, but nonphysiologic, evidence forAID expression early in B-cell development is provided by therescue of B-cell development [17, 18] in some strains of MT mice[19] by early CSR events that restore pre-BCR/BCR signaling.Nonetheless, AID expression in developing B cells remains acontroversial topic, in part because the first AID-reporter mouselines only identified GC B cells but not immature and transitional Bcells, as expressing AID [2022]; however, it should be noted thata later, independent reporter-strain indicated prior AID expres-sion in immature and transitional B cells [23]. There was also ageneral conviction that the low levels of AID expression that hasbeen observed in developing B cells [14, 24] were physiologicallyinsignificant.Direct tests of this conviction in mice [24] and humans[25] showed it to be unfounded. Remarkably, AID expressioncontrols self-tolerance in B cells by a mechanism that is B-cellC 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.euEur. J. Immunol. 2014. 44: 28762879 HIGHLIGHTS 2877intrinsic and acts directly on immature and transitional B cells.In mice, Aicda reduces the fitness of hematopoietic progenitors togenerate B cells but not T lymphocytes or myeloid cells and this reduced fitness becomes manifest in immature andtransitional 1 B cells, the compartments with the highest levels ofintrinsic AID message [12, 14, 24]. The B lymphopoietic advantageof Aicda/ progenitor cells is directly linked to impaired B-cell tol-erance, as the arrested development of autoreactive B cells [26] isrestored in Aicda/ animals [24]. In line with this, the laboratoryof E. Meffre found that recombinant human antibodies made fromthe CD10++CD21loIgMhiCD27 transitional (new emigrant)B cells of AICDnull hyper-IgM syndrome patients exhibited aprimary B-cell repertoire skewed to autoreactivity and rich in VDJrearrangements known to encode self-reactive antibodies [25].In this issue of the European Journal of Immunology, Umikeret al. [27] continue to expand our understanding of the role(s)of AID expression and B-cell autoreactivity in a complex exper-imental model that, unfortunately, may be too clever by half.Essentially, Umiker et al. sought to control the expression of aconditional Aicda transgene (Aicdatg) in 564Igi mice [13] duringB-cell development to discriminate between the effects of AIDexpression early in development versus later expression in matureB cells. On an Aicda/ genetic background, the Aicdatg wasactivated by stage-dependent expression of Cre-recombinase:controlled by the mb1 promoter, Cre was active at all stages ofB-cell development; under the Cd19 promoter, Cre activityincreased with lineage maturation; under the Cd21 promoter Creexpression was confined to mature B cells. Simply put, in thismodel, AID expression ranges from always on, to, with varyingdegrees, on in maturing/mature B cells. A confounding variableof this approach is that each construct pair supports different levelsof AID message in B220+ spleen and bone marrow cells and thatin all cases, AID expression far exceeds physiological levels.The 564Igi knockin mice express a polyreactive BCR/antibodythat binds single-strand (ss) DNA, ssRNA, and to nucleosomesand is pathogenic; this BCR/antibody is imprecisely recognizedby a monoclonal anti-idiotope antibody, B6256 [13]. Curiously,546Igi mice are Janus-like in that whereas B cells bearing the 564BCR are subject to tolerance by deletion, receptor editing, andanergy, 564Igi mice generate substantial titers of serum IgG2a(but not IgM or IgG1) that retains an autoreactive component[13]. The production of autoreactive, 564id+ IgG2a is T-cell inde-pendent but requires TLR-7 [13]. Imanishi-Kari and her colleagueshave proposed [13] that this T-independent, 564 IgG2a is gener-ated when anergic 564 B cells are activated in the periphery bysynergistic BCR and TLR signals [28, 29].To summarize, the experimental model is a series of geneticallymodified mice that express a polyreactive BCR subject to toler-ization by deletion, editing, and anergy and capable of inducingT-independent but TLR-7-dependent IgG2a serum autoantibodyon a genetic background that directs constitutive but variable lev-els of AID expression early (mb1-Cre) or later (Cd19-Cre and Cd21-Cre) during B-cell development (Fig. 1). As is usual in these typesof experiments, one assumes, but cannot know, that T-cells aretolerant to the self-antigens recognized by the 564 BCR.An interesting aspect of this study [27] comes from the con-trols. The authors note that in those mice in which Aicdatg isnot activated, i.e., mice fully deficient in AID activity, significantamounts of 564id+, autoreactive IgM appear in the serum. In addi-tion, the authors also note that receptor editing is diminished inthese control mice, implying that in the absence of AID activity,B-cell tolerance is impaired. This mirrors the findings reported ear-lier by Kuraoka et al. [24] and by Meyers et al. [25] in hyper-IgMsyndrome patients.As might be expected, the results of this study [27] are com-plex and subject to many caveats, however, interestingly there areclear distinctions between the various experimental groups thatare strongly correlated with the timing of AID expression (Fig. 1).Strikingly, 564id+ IgG2a and IgG2b that bound RNA was presentin all mb1-Cre and Cd19-Cre mice, but absent in the great majorityof Cd21-Cre animals. Cd21-Cre mice instead expressed high levelsof 564idRNA IgG antibody. Although the presentation of datamakes quantitative analyses difficult, the genetic data presentedby Umiker et al. [27] provide repeated examples to support theirpoint that SHM and CSR are active in immature B cells from mb1-Cre mice and that in many cases, V(D)J mutations reduce or abol-ish RNA binding. In this way, early AID activity acts as a tolerizingmechanism. The nonautoreactive IgG abundant in Cd21-Cre miceis largely (80%) encoded by VDJ rearrangements that representsecondary recombination events, or mutated 564Igi alleles. It iscurious that the authors observe substantially more H-chain edit-ing than the usually common L-chain replacements. The authorsdo not make much of this unusual reversal but it suggests that thebiology of these 564Igi strains substantially differs from that ofnormal mice.Expression of 564Id+ RNA-binding IgM was reciprocally cor-related with the IgG autoantibody production; that is, lowerlevels were observed in mb1-Cre and Cd19-Cre mice whereas self-reactive IgM was as high in Cd21-Cre animals as in AID deficientmice. Umiker et al. [27] concludes that this must reflect in someway the absence of AID expression early in B-cell development.Given the authors own observations of 564Id+ RNA-binding IgMin Aicda/ controls and the prior work of other groups [24, 25],this seems a likely hypothesis but leaves dangling the problem ofhow these autoreactive B cells are dealt with later. In this exper-imental model, activation by RNA DAMPs must be frequent inthe bone marrow and appears to drive these cells into terminaldifferentiation as short-lived plasmacytes. But what if theencounter with the DAMP/BCR-ligand was less frequent? In thiscase, autoreactive B cells would either become anergic or escapeto maturity in the periphery.In a curious reversal of dogma, Goodnow and colleagues [30]recently showed that anergic B cells can readily be activated inGCs and acquire there, mutations that reduce self-reactivity. Thisprocess of redemption for autoantibodies has also been observedin knockin mice that carry a BCR that neutralizes many strains ofHIV-1 [31, 32] but is subject to tolerization because it cross-reactswith host determinants [33]. In these mice, a T-independent HIV-1vaccine containing TLR4/TLR9 ligands readily elicited HIV-1neutralizing IgG2b antibody. In contrast, T-dependent proteinC 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu2878 Garnett Kelsoe Eur. J. Immunol. 2014. 44: 28762879Figure 1. Synopsis of the methods and critical results of Umiker et al. [27]. (A) To determine the role of early versus later expression of AID inB-cell development and differentiation, the authors controlled expression of a conditional Aicda transgene (Aicdatg) in 564Igi mice that express anautoreactive BCR. On an Aicda/ genetic background, Aicdatg was activated by conditional expression of Cre-recombinase. In mb1-Cre mice, Aicdatgwas active at all stages of B-cell development; under the Cd19 promoter, Cre activity increased with lineage maturation, and; Cd21-Cre animalsexpressed Aicdatg only in mature B cells. Control mice that do not express Cre are negative for AID activity. (B) The autoreactive BCR of 564Igimice binds DNA, RNA and to nucleosomes; it is pathogenic and is subject to tolerization. The 564 BCR is recognized by a monoclonal anti-idiotopeantibody that defines 564id+ B cells. Expression of 564Id+ autoreactive IgM was associated with the absence of AID activity during early B-celldevelopment. Normally, 546Igi mice display the unusual phenotype of substantial titers of autoreactive 564id+ serum IgG2a (and IgG2b) that is T-cellindependent but requires TLR-7. 564id+ RNA-binding, 564id+ IgG2a was present in all mb1-Cre and Cd19-Cre mice, but rare in Cd21-Cre animals;instead, Cd21-Cre mice expressed both 564id+ IgM and high levels of 564idRNA IgG antibody. Umiker et al. [27] conclude that early AID activity(i) suppresses the functional tolerization of IgM+ 564Ig B cells (no Cre and mb-1-Cre) but (ii) is required for the generation of autoreactive, 564id+IgG2a (cd19 and cd21Cre). Finally, AID expression only in mature B cells (iii) constrains the T-independent production of 564id+ RNA-bindingIgG2a but promotes nonautoreactive 564id IgG, in part, by the introduction of redemptive mutations [30] in germinal centers. AID activity actsthen, both early and late in B-cell development to control autoimmunity.antigens elicit non-neutralizing, nonautoreactive IgG responses likely by the redemption pathway. In both of these cases, theknockin BCR is mutated and exhibits high affinity for its autoli-gand. Under such conditions, V(D)J mutations that decrease affin-ity are likely, more so than in lower affinity, germline rearrange-ments. These experiments may be usefully applied to interpretUmiker et al.Imanishi-Kari and her colleagues have made fundamental con-tributions to the growing appreciation for the physiological role ofAID activity during B-cell development [10, 11]. The latest contri-bution of this group to understanding this role, namely the articleby Umiker et al. [27], may be distilled to a single and importantconclusion: early AID activity can tolerize activated, autoreactiveB cells by altering specificity through V(D)J hypermutation, andby driving B cells to terminal differentiation as short-lived plasma-cytes. This model is both interesting and testable. A first test mustbe whether CD154 independent, IgG2b antibody can be observedin normal mice. Obviously, the level of antibody and plasmacytesmust be low, but well within the reach of contemporary single-cellcloning methods. Following the 564Igi model, the BCR recoveredfrom these cells will be specific for DAMPs. These more physiolog-ical tests of Umiker et al. [27] are anxiously awaited.References1 Muramatsu, M., Kinoshita, K., Fagarasan, S., Yamada, S., Shinkai, Y.and Honjo, T., Class switch recombination and hypermutation requireactivation-induced cytidine deaminase (AID), a potential RNA editingenzyme. Cell 2000. 102: 553563.2 Revy, P., Muto, T., Levy, Y., Geissmann, F., Plebani, A., Sanal, O., Catalan,N. et al., Activation-induced cytidine deaminase (AID) deficiency causesthe autosomal recessive form of the Hyper-IgM syndrome (HIGM2). Cell2000. 102: 565575.3 Marr, S., Morales, H., Bottaro, A., Cooper, M., Flajnik, M. and Robert,J., Localization and differential expression of activation-induced cyti-dine deaminase in the amphibian Xenopus upon antigen stimulationand during early development. J. Immunol. 2007. 179: 67836789.4 Reynaud, C. A., Bertocci, B., Dahan, A. and Weill, J. C., Formationof the chicken B-cell repertoire: ontogenesis, regulation of Ig geneC 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.euEur. J. Immunol. 2014. 44: 28762879 HIGHLIGHTS 2879rearrangement, and diversification by gene conversion. Adv. Immunol.1994. 57: 353378.5 Reynaud, C. A., Garcia, C., Hein, W. R. and Weill, J. C., Hypermuta-tion generating the sheep immunoglobulin repertoire is an antigen-independent process. Cell 1995. 80: 115125.6 Mage, R. G., Lanning, D. and Knight, K. L., B cell and antibody repertoiredevelopment in rabbits: the requirement of gut-associated lymphoid tis-sues. Dev. Comp. Immunol. 2006. 30: 137153.7 William, J., Euler, C., Christensen, S. and Shlomchik, M. J., Evolution ofautoantibody responses via somatic hypermutation outside of germinalcenters. Science 2002. 297: 20662070.8 Cerutti, A., The regulation of IgA class switching. Nat. Rev. Immunol. 2008.8: 421434.9 Weller, S., Faili, A., Garcia, C., Braun, M. C., Le Deist, F. F., de SaintBasile, G. G., Hermine, O. Fischer, A. et al., CD40-CD40L independent Iggene hypermutation suggests a second B cell diversification pathway inhumans. Proc. Natl. Acad. Sci. USA 2001. 98: 11661170.10 Mao, C., Jiang, L., Melo-Jorge, M., Puthenveetil, M., Zhang, X., Carroll, M.C. and Imanishi-Kari, T., T cell-independent somatic hypermutation inmurine B cells with an immature phenotype. Immunity 2004. 20: 133144.11 Han, J. H., Akira, S., Calame, K., Beutler, B., Selsing, E. and Imanishi-Kari, T., Class switch recombination and somatic hypermutation in earlymouse B cells are mediated by B cell and Toll-like receptors. Immunity2007. 27: 6475.12 Ueda, Y., Liao, D., Yang, K., Patel, A. and Kelsoe, G., T-independentactivation-induced cytidine deaminase expression, class-switch recom-bination, and antibody production by immature/transitional 1 B cells.J. Immunol. 2007. 178: 35933601.13 Berland, R., Fernandez, L., Kari, E., Han, J. H., Lomakin, I., Akira, S.,Wortis, H. H. et al., Toll-like receptor 7-dependent loss of B cell tolerancein pathogenic autoantibody knockin mice. Immunity 2006. 25: 429440.14 Kuraoka, M., Liao, D., Yang, K., Allgood, S. D., Levesque, M. C., Kelsoe,G. and Ueda, Y., Activation-induced cytidine deaminase expression andactivity in the absence of germinal centers: insights into hyper-IgM syn-drome. J. Immunol. 2009. 183: 32373248.15 Milili, M., Fougereau, M., Guglielmi, P. and Schiff, C., Early occurrenceof immunoglobulin isotype switching in human fetal liver. Mol. Immunol.1991. 28: 753761.16 Scheeren, F. A., Nagasawa, M., Weijer, K., Cupedo, T., Kirberg, J.,Legrand, N. and Spits, H., T cell-independent development and induc-tion of somatic hypermutation in human IgM+IgD+CD27+ B cells. J. Exp.Med. 2008. 205: 20332042.17 Hasan, M., Polic, B., Bralic, M., Jonjic, S. and Rajewsky, K., Incompleteblock of B cell development and immunoglobulin production in micecarrying the muMT mutation on the BALB/c background. Eur. J. Immunol.2002. 32: 34633471.18 Melamed, D., Miri, E., Leider, N. and Nemazee, D., Unexpected autoanti-body production in membrane Ig-mu-deficient/lpr mice. J. Immunol. 2000.165: 43534358.19 Kitamura, D. and Rajewsky, K., Targeted disruption of mu chain mem-brane exon causes loss of heavy- chain allelic exclusion. Nature 1992. 356:154156.20 Crouch, E. E., Li, Z., Takizawa, M., Fichtner-Feigl, S., Gourzi, P., Montano,C., Feigenbaum, L. et al., Regulation of AID expression in the immuneresponse. J. Exp. Med. 2007. 204: 11451156.21 Gourzi, P., Leonova, T. and Papavasiliou, F. N., A role for activation-induced cytidine deaminase in the host response against a transformingretrovirus. Immunity 2006. 24: 779786.22 Gourzi, P., Leonova, T. and Papavasiliou, F. N., Viral induction of AID isindependent of the interferon and the Toll-like receptor signaling path-ways but requires NF-{kappa}B. J. Exp. Med. 2007. 204: 259265.23 Qin, H., Suzuki, K., Nakata, M., Chikuma, S., Izumi, N., Huong le, T.,Maruya, M. et al., Activation-induced cytidine deaminase expression inCD4+ T cells is associated with a unique IL-10-producing subset thatincreases with age. PLoS One 2011. 6: e29141.24 Kuraoka, M., Holl, T. M., Liao, D., Womble, M., Cain, D. W., Reynolds, A. E.and Kelsoe, G., Activation-induced cytidine deaminase mediates centraltolerance in B cells. Proc. Natl. Acad. Sci. USA 2011. 108: 1156011565.25 Meyers, G., Ng, Y. S., Bannock, J. M., Lavoie, A., Walter, J. E., Notarangelo,L. D., Kilic, S. S. et al., Activation-induced cytidine deaminase (AID)is required for B-cell tolerance in humans. Proc. Natl. Acad. Sci. USA2011. 108: 1155411559.26 Chen, C., Nagy, Z., Prak, E. L. and Weigert, M., Immunoglobulin heavychain gene replacement: a mechanism of receptor editing. Immunity 1995.3: 747755.27 Umiker, B. R., McDonald, G., Larbi, A., Medina, C. O., Hobeika, E., Reth, M.and Imanishi-Kari, T., Production of IgG autoantibody requires expres-sion of activation-induced deaminase in early-developing B cells in amouse model of SLE. Eur. J. Immunol. 2014. 44: 30933108.28 Leadbetter, E. A., Rifkin, I. R., Hohlbaum, A. M., Beaudette, B. C., Shlom-chik, M. J. and Marshak-Rothstein, A., Chromatin-IgG complexes activateB cells by dual engagement of IgM and Toll-like receptors. Nature 2002.416: 603607.29 Marshak-Rothstein, A. and Rifkin, I. R., Immunologically active autoanti-gens: the role of toll-like receptors in the development of chronic inflam-matory disease. Annu. Rev. Immunol. 2007. 25: 419441.30 Sabouri, Z., Schofield, P., Horikawa, K., Spierings, E., Kipling, D., Randall,K. L., Langley, D. et al., Redemption of autoantibodies on anergic B cellsby variable-region glycosylation and mutation away from self-reactivity.Proc. Natl. Acad. Sci. USA 2014. 111: E2567E2575.31 Verkoczy, L., Diaz, M., Holl, T. M., Ouyang, Y. B., Bouton-Verville, H.,Alam, S. M., Liao, H. X. et al., Autoreactivity in an HIV-1 broadly reactiveneutralizing antibody variable region heavy chain induces immunologictolerance. Proc. Natl. Acad. Sci. USA 2010. 107: 181186.32 Verkoczy, L., Chen, Y., Bouton-Verville, H., Zhang, J., Diaz, M., Hutchin-son, J., Ouyang, Y. B. et al., Rescue of HIV-1 Broad Neutralizing Antibody-Expressing B Cells in 2F5 VH x VL Knockin Mice Reveals Multiple Toler-ance Controls. J. Immunol. 2011. 187: 37853797.33 Yang, G., Holl, T. M., Liu, Y., Li, Y., Lu, X., Nicely, N. I., Kepler, T. B. etal., Identification of autoantigens recognized by the 2F5 and 4E10 broadlyneutralizing HIV-1 antibodies. J. Exp. Med. 2013. 210: 241256.Abbreviations: CSR: class-switch recombination SHM: somatichypermutationFull correspondence: Dr. Garnett Kelsoe, DUMC 3010, Department ofImmunology, Duke University, Durham, NC 27710, USAFax: +1-919-613-7815e-mail: garnett.kelsoe@dm.duke.eduSee accompanying Article: 10/9/2014Revised: 10/9/2014Accepted: 22/9/2014C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


View more >