Somatic hypermutation occurs in B cells of terminal deoxynucleotidyl transferase-, CD23-, interleukin-4-, IgD- and CD30-deficient mouse mutants

Two sides of a cellular coin: CD4(+)CD3- cells regulate memory responses and lymph-node organization

We propose that CD4(+)CD3(-) cells have two functions: a well-established role in organizing lymphoid tissue during development, and a newly discovered role in supporting T-cell help for B cells both during affinity maturation in germinal centres and for memory antibody responses. As CD4(+)CD3(-) cells express the HIV co-receptors CD4 and CXC-chemokine receptor 4, we think that infection of these cells by HIV, and their subsequent destruction by the host immune system, could help to explain the loss of memory antibody responses and the destruction of lymphoid architecture that occur during disease progression to AIDS.

Polymerase mu is up-regulated during the T cell-dependent immune response and its deficiency alters developmental dynamics of spleen centroblasts

Mammalian DNA polymerase mu (Polmu), preferentially expressed in secondary lymphoid organs, is shown here to be up-regulated in germinal centers after immunization. Alternative splicing appears to be part of Polmu regulation during an immune response. We generated Polmu-deficient mice that are viable and show no anatomical malformation or serious alteration in lymphoid populations, with the exception of an underrepresentation of the B cell compartment. Young and aged homozygous Polmu(-/-) mice generated similar immune responses after immunization with the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP) coupled to chicken gammaglobulin (CGG), compared with their wild-type littermates. Nonetheless, the kinetics of development of the centroblast population showed significant differences. Hypermutation analysis of the rearranged heavy chain intron region in centroblasts isolated from NP-CGG-immunized Polmu(-/-) mice showed a similar quantitative and qualitative somatic mutation spectrum, but a lower representation of heavily mutated clones. These results suggest that although it is not a critical partner, Polmu modulates the in vivo somatic hypermutation process.

Basal immunoglobulin signaling actively maintains developmental stage in immature B cells

In developing B lymphocytes, a successful V(D)J heavy chain (HC) immunoglobulin (Ig) rearrangement establishes HC allelic exclusion and signals pro-B cells to advance in development to the pre-B stage. A subsequent functional light chain (LC) rearrangement then results in the surface expression of IgM at the immature B cell stage. Here we show that interruption of basal IgM signaling in immature B cells, either by the inducible deletion of surface Ig via Cre-mediated excision or by incubating cells with the tyrosine kinase inhibitor herbimycin A or the phosphatidylinositol 3-kinase inhibitor wortmannin, led to a striking “back-differentiation” of cells to an earlier stage in B cell development, characterized by the expression of pro-B cell genes. Cells undergoing this reversal in development also showed evidence of new LC gene rearrangements, suggesting an important role for basal Ig signaling in the maintenance of LC allelic exclusion. These studies identify a previously unappreciated level of plasticity in the B cell developmental program, and have important implications for our understanding of central tolerance mechanisms.

Gene rearrangement is a hallmark of B cell maturation. By interrupting basal cell signaling through the rearranged IgM receptor, immature B cells "back-differentiate" to an earlier stage in their development

Overexpression of human DNA polymerase mu (Pol mu) in a Burkitt's lymphoma cell line affects the somatic hypermutation rate

DNA polymerase mu (Pol mu) is a DNA-dependent DNA polymerase closely related to terminal deoxynucleotidyl transferase (TdT), and prone to induce template/primer misalignments and misincorporation. In addition to a proposed general role in non-homologous end joining of double-strand breaks, its mutagenic potential and preferential expression in secondary lymphoid tissues support a role in somatic hypermutation (SHM) of immunoglobulin genes. Here, we show that human Pol mu protein is expressed in the nucleus of centroblasts obtained from human tonsils, forming a characteristic foci pattern resembling that of other DNA repair proteins in response to DNA damage. Overexpression of human Pol mu in Ramos cells, in which the SHM process is constitutive, augmented the somatic mutations specifically at the variable (V) region of the immunoglobulin genes. The nature of the mutations introduced, mostly base substitutions, supports the contribution of Pol mu to mutation of G and C residues during SHM. In vitro analysis of Pol mu misincorporation on specific templates, that mimic DNA repair intermediates and correspond to mutational hotspots, indicated that many of the mutations observed in vivo can be explained by the capacity of Pol mu to induce transient template/primer misalignments.

Mice deficient for the type II topoisomerase-like DNA transesterase Spo11 show normal immunoglobulin somatic hypermutation and class switching

Somatic hypermutation in B cells undergoing T cell dependent immune responses generates high affinity antibodies that provide protective immunity. B cells also switch from the expression of immunoglobulin (Ig) M and IgD to that of other Ig classes through somatic DNA recombination. Recent work has implicated DNA strand breaks, possibly DNA double strand breaks (DSB), as the initiating lesions in both class switch recombination and hypermutation, although the etiology of these lesions is not understood. Spo11, a protein structurally related to archaeal type II topoisomerases, generates DSB that initiate meiotic recombination. This characteristic, together with its expression pattern, marks this enzyme as a potential candidate for the initiation of hypermutation, and perhaps also for Ig class switching. To investigate whether Spo11 is involved in these processes, we studied the T cell dependent immune response of Spo11-deficient (Spo11(-/-)) mice against the hapten nitrophenyl (NP). We found that V186.2-bearing IgG1 transcripts had normal levels and patterns of somatic hypermutation. Furthermore, Spo11(-/-) mice showed normal serum levels of all Ig isotypes. These results indicate that Spo11 is not required for Ig hypermutation or class switch recombination.

Emerging links between hypermutation of antibody genes and DNA polymerases

Substantial antibody variability is created when nucleotide substitutions are introduced into immunoglobulin variable genes by a controlled process of hypermutation. Evidence points to a mechanism involving DNA repair events at sites of targeted breaks. In vertebrate cells, there are many recently identified DNA polymerases that inaccurately copy templates. Some of these are candidates for enzymes that introduce base changes during hypermutation. Recent research has focused on possible roles for DNA polymerases zeta (POLZ), eta (POLH), iota (POLI), and mu (POLM) in the process.

Mice reconstituted with DNA polymerase beta-deficient fetal liver cells are able to mount a T cell-dependent immune response and mutate their Ig genes normally

The ubiquitously expressed, error-prone DNA polymerase beta (polbeta) plays a role in base excision repair, and the involvement of this molecule in the nonhomologous end joining (NHEJ) process of DNA repair has recently been demonstrated in yeast. Polbeta-deficient mice are not viable, and studies on conditional mutants revealed a competitive disadvantage of polbeta(-/-) vs. wild-type cells. We show here that polbeta-deficient mice survive up to day 18.5 postcoitum, but die perinatally; a circumstance that allowed the investigation of a potential role of polbeta in lymphocyte development by transfer of fetal liver cells (FLC) derived from polbeta(-/-) embryos into lethally irradiated hosts. FLC transfers using mutant cells lead to an almost normal reconstitution of the lymphocyte compartment, indicating that polbeta-deficiency does not prevent V(D)J recombination, which is known to employ factors of the NHEJ pathway. Mice reconstituted with polbeta(-/-) FLC mount a normal T cell-dependent immune response against the hapten (4-hydroxy-3-nitrophenyl) acetyl (NP). Moreover, germinal center B cells from NP-immunized reconstituted mice show normal levels and patterns of somatic point mutations in their rearranged antibody genes, demonstrating that polbeta is not critically involved in somatic hypermutation.

Induction of Ig Somatic Hypermutation and Class Switching in a Human Monoclonal IgM+ IgD+ B Cell Line In Vitro: Definition of the Requirements and Modalities of Hypermutation

Partly because of the lack of a suitable in vitro model, the trigger(s) and the mechanism(s) of somatic hypermutation in Ig genes are largely unknown. We have analyzed the hypermutation potential of human CL-01 lymphocytes, our monoclonal model of germinal center B cell differentiation. These cells are surface IgM+ IgD+ and, in the absence of T cells, switch to IgG, IgA, and IgE in response to CD40:CD40 ligand engagement and exposure to appropriate cytokines. We show here that CL-01 cells can be induced to effectively mutate the expressed VHDJH-Cμ, VHDJH-Cδ, VHDJH-Cγ, VHDJH-Cα, VHDJH-Cε, and VλJλ-Cλ transcripts before and after Ig class switching in a stepwise fashion. In these cells, induction of somatic mutations required cross-linking of the surface receptor for Ag and T cell contact through CD40:CD40 ligand and CD80:CD28 coengagement. The induced mutations showed intrinsic features of Ig V(D)J hypermutation in that they comprised 110 base substitutions (97 in the heavy chain and 13 in the λ-chain) and only 2 deletions and targeted V(D)J, virtually sparing CH and Cλ. These mutations were more abundant in secondary VHDJH-Cγ than primary VHDJH-Cμ transcripts and in V(D)J-C than VλJλ-Cλ transcripts. These mutations were also associated with coding DNA strand polarity and showed an overall rate of 2.42 × 10−4 base changes/cell division in VHDJH-CH transcripts. Transitions were favored over transversions, and G nucleotides were preferentially targeted, mainly in the context of AG dinucleotides. Thus, in CL-01 cells, Ig somatic hypermutation is readily inducible by stimuli different from those required for class switching and displays discrete base substitution modalities.

Somatic hypermutation and the three R's: repair, replication and recombination

Somatic hypermutation introduces single base changes into the rearranged variable (V) regions of antigen activated B cells at a rate of approximately 1 mutation per kilobase per generation. This is nearly a million-fold higher than the typical mutation rate in a mammalian somatic cell. Rampant mutation at this level could have a devastating effect, but somatic hypermutation is accurately targeted and tightly regulated. Here, we provide an overview of immunoglobulin gene somatic hypermutation; discuss mechanisms of mutation in model organisms that may be relevant to the hypermutation mechanism; and review recent advances toward understanding the possible role(s) of DNA repair, replication, and recombination in this fascinating process.

Hypermutation of immunoglobulin genes in memory B cells of DNA repair-deficient mice

To investigate the possible involvement of DNA repair in the process of somatic hypermutation of rearranged immunoglobulin variable (V) region genes, we have analyzed the occurrence, frequency, distribution, and pattern of mutations in rearranged Vlambda1 light chain genes from naive and memory B cells in DNA repair-deficient mutant mouse strains. Hypermutation was found unaffected in mice carrying mutations in either of the following DNA repair genes: xeroderma pigmentosum complementation group (XP)A and XPD, Cockayne syndrome complementation group B (CSB), mutS homologue 2 (MSH2), radiation sensitivity 54 (RAD54), poly (ADP-ribose) polymerase (PARP), and 3-alkyladenine DNA-glycosylase (AAG). These results indicate that both subpathways of nucleotide excision repair, global genome repair, and transcription-coupled repair are not required for somatic hypermutation. This appears also to be true for mismatch repair, RAD54-dependent double-strand-break repair, and AAG-mediated base excision repair.

Somatic hypermutation of antibody genes: a hot spot warms up

In the course of an immune response, antibodies undergo affinity maturation in order to increase their efficiency in neutralizing foreign invaders. Affinity maturation occurs by the introduction of multiple point mutations in the variable region gene that encodes the antigen binding site. This somatic hypermutation is restricted to immunoglobulin genes and occurs at very high rates. The precise molecular basis of this process remains obscure. However, recent studies using a variety of in vivo and in vitro systems have revealed important regulatory regions, base motifs that are preferred targets of mutation and evidence that transcription may play an active role in hypermutation.

Receptor editing in a transgenic mouse model: site, efficiency, and role in B cell tolerance and antibody diversification

Mice carrying transgenic rearranged V region genes in their IgH and Igkappa loci to encode an autoreactive specificity direct the emerging autoreactive progenitors into a pre-B cell compartment, in which their receptors are edited by secondary Vkappa-Jkappa rearrangements and RS recombination. Editing is an efficient process, because the mutant mice generate normal numbers of B cells. In a similar nonautoreactive transgenic strain, neither a pre-B cell compartment nor receptor editing was seen. Thus, the pre-B cell compartment may have evolved to edit the receptors of autoreactive cells and later been generally exploited for efficient antibody diversification through the invention of the pre-B cell receptor, mimicking an autoreactive antibody to direct the bulk of the progenitors into that compartment.