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Mechanism and Regulation Igh Chain Class Switch Recombination IgH Chain Class Switch Recombination: Mechanism and Regulation Janet Stavnezer and Carol E. Schrader This information is current as J Immunol 2014; 193:5370-5378; ; of September 26, 2021. doi: 10.4049/jimmunol.1401849 http://www.jimmunol.org/content/193/11/5370 Downloaded from References This article cites 133 articles, 66 of which you can access for free at: http://www.jimmunol.org/content/193/11/5370.full#ref-list-1 Why The JI? Submit online. http://www.jimmunol.org/ • Rapid Reviews! 30 days* from submission to initial decision • No Triage! Every submission reviewed by practicing scientists • Fast Publication! 4 weeks from acceptance to publication *average by guest on September 26, 2021 Subscription Information about subscribing to The Journal of Immunology is online at: http://jimmunol.org/subscription Permissions Submit copyright permission requests at: http://www.aai.org/About/Publications/JI/copyright.html Email Alerts Receive free email-alerts when new articles cite this article. Sign up at: http://jimmunol.org/alerts The Journal of Immunology is published twice each month by The American Association of Immunologists, Inc., 1451 Rockville Pike, Suite 650, Rockville, MD 20852 Copyright © 2014 by The American Association of Immunologists, Inc. All rights reserved. Print ISSN: 0022-1767 Online ISSN: 1550-6606. Th eJournal of Brief Reviews Immunology IgH Chain Class Switch Recombination: Mechanism and Regulation Janet Stavnezer and Carol E. Schrader IgH class switching occurs rapidly after activation of with an S region farther downstream. S regions are G rich and mature naive B cells, resulting in a switch from expres- have a high density of WGCW (A/T-G-C-A/T) motifs, the sion of IgM and IgD to expression of IgG, IgE, or IgA; preferred target for activation-induced cytidine deaminase this switch improves the ability of Abs to remove the (AID), the enzyme that initiates CSR by deaminating cytosines pathogen that induces the humoral immune response. within S-region DNA, converting deoxycytidine to dU (2, 3). Class switching occurs by a deletional recombination Enzymes of the base excision repair (BER) and mismatch repair between two switch regions, each of which is associated (MMR) pathways then convert the dUs to DNA DSBs, which with a H chain constant region gene. Class switch re- are required for CSR (4, 5) (Fig. 2). The DSBs are subse- Downloaded from combination (CSR) is instigated by activation-induced quently recombined by an end-joining type of DNA recom- cytidine deaminase, which converts cytosines in switch bination, predominantly by nonhomologous end joining regions to uracils. The uracils are subsequently removed (NHEJ). The use of NHEJ, rather than homologous recom- by two DNA-repair pathways, resulting in mutations, bination, is consistent with the facts that S-region DSBs are single-strand DNA breaks, and the double-strand breaks induced and recombined during the G1 phase (6–9) and that http://www.jimmunol.org/ required for CSR. We discuss several aspects of CSR, different S regions do not share long stretches of identity (1), which are required for homologous recombination. including how CSR is induced, CSR in B cell progen- CSR occurs very rapidly postinfection or immunization, itors, the roles of transcription and chromosomal loop- prior to formation of germinal centers, which generally form ing in CSR, and the roles of certain DNA-repair enzymes 7–10 d after exposure to Ag. For example, using mice in CSR. The Journal of Immunology, 2014, 193: expressing a transgenic BCR, both IgM+ and IgG2a+ cells 5370–5378. were detected in B cell follicles from days 2 to 4 after im- munization, but only IgG2a+ cells were detected in germinal fter immunization or infection, activated naive centers, indicating that CSR occurred prior to germinal center by guest on September 26, 2021 BcellscanswitchfromexpressingIgMandIgDon formation (10). Also, CSR was detected in nontransgenic A their surface to expressing IgG, IgE, or IgA. This mice 4 d postinfection with Salmonella (11). However, CSR is isotype/class switch changes the effector function of the Ab also detected in germinal center B cells from human tonsils and improves its ability to eliminate the pathogen that in- (12), and IgA CSR occurs in Peyer’s patch germinal centers duced the response. Isotype switching involves replacement (13–15), in which B cells are constantly stimulated by the gut microbiota. CSR also occurs during T-independent responses, of the m and d HchainC(CH) regions of the expressed Ig which do not induce germinal centers (16). Thus, CSR starts with g, «,ora CH regions, and it occurs by a DNA re- combination event termed “class switch recombination” prior to somatic hypermutation (SHM) of V region [V(D)J] genes, an AID-dependent process that occurs mainly in ger- (CSR). Fig. 1 is a diagram (not to scale) of the CH genes and minal centers, and which, after selection, can result in Abs CSRinmice;humanCH genes are similarly arranged al- though not identical. with increased affinity for Ag. The data suggest that CSR may CSR is a deletional DNA recombination occurring between continue as long as B cells are undergoing activation. switch (S) regions, which are located upstream of all of the Induction of CSR CH genes, with the exception of Cd, and are 1–10 kb in length (1). Recombination occurs between dsDNA breaks (DSBs) Many studies of CSR have been performed using cultures of introduced into the donor m S(Sm) region and a downstream/ mouse splenic B cells, because these cells can be induced in acceptor S region located from ∼65–160 kb downstream; oc- culture to undergo robust CSR within ∼3 d by treatment with casionally, downstream S regions can subsequently recombine the B cell mitogen LPS, acting through the innate receptor Department of Microbiology and Physiological Systems, University of Massachusetts Abbreviations used in this article: AID, activation-induced cytidine deaminase; AP, Medical School, Worcester, MA 01605 apyrimidinic/apurinic; APE, AP endonuclease; ATM, ataxia-telangiectasia mutant; BER, base excision repair; 3C, chromosome conformation capture; Cm, m C region; Received for publication July 21, 2014. Accepted for publication September 2, 2014. CH, H chain C; CSR, class switch recombination; DSB, dsDNA break; Em,Em intron This work was supported by National Institutes of Health Grants R01-AI23283 and enhancer; 39Ea,39 Ca enhancer; GLT, germline transcript; HEL, hen egg lysozyme; R21-AI99988 (both to J.S. and C.E.S.). MMR, mismatch repair; MRN, Mre11-Nbs1-Rad50; NHEJ, nonhomologous end join- ing; Pol, polymerase; Sm, m S; S, switch; SHM, somatic hypermutation; ss, single Address correspondence and reprint requests to Dr. Janet Stavnezer, Department of stranded; SSB, ssDNA break; UNG, uracil DNA glycosylase; WT, wild-type. Microbiology and Physiological Systems, University of Massachusetts Medical School, AS8-1053, 368 Plantation Street, Worcester, MA 01605. E-mail address: janet. Ó [email protected] Copyright 2014 by The American Association of Immunologists, Inc. 0022-1767/14/$16.00 www.jimmunol.org/cgi/doi/10.4049/jimmunol.1401849 The Journal of Immunology 5371 FIGURE 1. Diagram of the mouse IgH genes in naive mature B cells expressing IgM and IgD as well as CSR to IgG2b. During CSR to IgG2b, AID deaminates the Sm and Sg2b regions, insti- gating DSB formation. The Sm and Sg2b regions recombine by an intrachromosomal deletional recombination, which causes the expressed VDJ segment to become associated with the Cg2b gene. Splicing diagrams of the m and d mRNAs, the g2b GLTs, and g2b mRNA are indicated beneath the genes. Em and 39Ea are the two major enhancers that regulate expression of Ig H genes and CSR. Downloaded from TLR4, or by signaling through CD40, the most important Likewise, when HEL is added to HEL-specific B cells acti- receptor for T cell help. LPS alone induces AID expression, vated with TLR7 or TLR9, CSR to IgG1 is also greatly in- but a cytokine, such as IL-4, must be added to induce AID creased (22). Unlike mouse B cells, human peripheral blood http://www.jimmunol.org/ when Ab to CD40 (anti-CD40) is used. These ligands also or tonsillar B cells switch poorly in culture to either CD40 or induce cell proliferation, another requirement for CSR (17, TLR signaling (29); perhaps activation through BCR signal- 18). Surprisingly, splenic B cells do not require a signal ing would help. provided by the BCR to switch in culture, because neither In conclusion, it is likely that CSR in vivo depends upon LPS nor anti-CD40 triggers signaling via the BCR. This activation of B cells via their BCR, in addition to secondary might be explained by the large amounts of LPS (usually 10– signals from CD40 and TLR signaling. It is unclear whether 50 mg/ml) used in these cultures, which would not normally the BCR signaling is required for inducing CSR or is only be provided to B cells in vivo. Also, much more continuous required for initial activation of the B cells, and the T cell by guest on September 26, 2021 CD40 signaling is provided in these cultures than normally signals are responsible for inducing CSR. In addition, it would be available in vivo. In vivo, CD40 signaling would be appears likely that TLR signaling is important for both T- delivered to Ag-specific B cells during contacts with Ag- independent and T-dependent responses in vivo. It is possible specific Th cells. CD40 signaling is very important for CSR that primary and multiple secondary signals given together in vivo, as demonstrated by the lack of B cell proliferation and increase the robustness of the CSR response. CSR in response to T-dependent Ags in mice and humans deficient in CD40 signaling (19, 20). Also, TLR signaling is Regulation of isotype specificity by transcription of unrearranged CH important for in vivo immune responses and CSR in response genes to viruses (21).
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