A human follicular lymphoma B cell line hypermutates its functional immunoglobulin genes in vitro

Responses to rituximab vary among follicular lymphoma B cells of different maturation stages

The chimeric anti-CD20 monoclonal antibody rituximab has been used for the treatment of non-Hodgkin's lymphomas with varying responses. Rituximab has been demonstrated to act by direct complement-dependent cytotoxity (CDC) and by inducing apoptosis, complement-, and antibody-dependent cellular cytotoxity. In the present study, we determined whether rituximab's effector mechanisms differed between two human follicular lymphoma cell lines that originate from different maturation stages of B cell germinal centre (GC) development. The tested HF-1 and HF-4b lymphoma cells represent GC centrocytes and centroblasts, respectively. Both cell lines responded to rituximab treatment by undergoing apoptosis yet the HF-1 cells were more sensitive. A major difference was seen in the proliferation response as only the proliferation of HF-1 cells was inhibited by rituximab. In the presence of normal human serum (NHS) rituximab almost completely inhibited DNA synthesis and induced necrosis of both cell lines because of CDC. Our results show that the CD20-positive HF-1 and HF-4b cells respond differentially to rituximab-induced apoptosis and inhibition of proliferation but similarly to complement-mediated killing. The increased sensitivity of the HF-1 cell line to apoptosis and inhibition of proliferation may reflect a tendency of centrocytic cells for negative selection and a role for CD20 in this process.

Somatic mutations and activation-induced cytidine deaminase (AID) expression in established rheumatoid factor-producing lymphoblastoid cell line

Epstein-Barr virus (EBV) transforms human peripheral B cells into lymphoblastoid cell lines (LCLs), allowing the production of specific antibody-secreting cell lines. We and others have previously found that in contrast to peripheral blood B cells, EBV-transformed lymphoblastoid cell lines express the activation-induced cytidine deaminase (AID) gene. The opposite is true for the germinal center-specific BCL6 gene: it is expressed in adult peripheral blood B cells and is no longer expressed in LCLs. The present work extends our findings and shows that whereas AID expression is rapidly induced following EBV infection, BCL6 expression is gradually down-regulated and is fully extinguished in already established LCLs. The question of whether AID activation induces the process of somatic hypermutation (SHM) was investigated in adult-derived LCLs. It was found that the VH gene from the rheumatoid factor-producing RF LCL (derived from a rheumatoid arthritis patient), accumulated somatic point mutations in culture. Overall, nine unique mutations have accumulated in the rearranged VH gene since the generation of the RF cell line. Four additional intraclonal mutations were found among 10 cellular clones of the RF cells. One out of the four was in CDR1 and could be correlated with loss of antigen-binding activity in three out of the 10 clones. Altogether, these 13 mutations were preferentially targeted to the DGYW motifs and showed preference for CG nucleotides, indicating that they were AID-mediated. By contrast, mutations were not detected among 3700-4000 nucleotides each of the Vlambda, Cmu and GAPDH genes derived from the same RF cell cultures and the cellular clones. Our results thus show that AID may generate point mutations in the rearranged Ig VH during in vitro cell culture of adult-LCLs and that these mutations may be responsible, at least in part, for the known instability and occasional loss of antigen-binding activity of antibody-secreting LCLs.

Activation-induced cytidine deaminase expression in follicular lymphoma: association between AID expression and ongoing mutation in FL

Activation-induced cytidine deaminase (AID) is required for somatic hypermutation (SHM) and class switch recombination (CSR) of the immunoglobulin (Ig) gene. AID has been reported to be specifically expressed in the germinal center (GC). Follicular lymphoma (FL) cells are known to be exposed to GC reaction, as characterized by a high degree of SHM with some heterogeneity in terms of intraclonal microheterogeneity and antigen selection. The heterogeneity of SHM pattern in FL intrigued us to investigate the AID expression. AID expression was investigated in 19 FL materials consisting of 15 cases of FL fresh cells and four cell lines. In all, 10 fresh cells and three cell lines expressed AID, but the others did not. SHM was investigated in 12 fresh cells and four cell lines. The ongoing mutation was significantly different between AID-positive and AID-negative FL fresh cells (unpaired Student's t-test, P=0.047). Ongoing mutation was not seen in any of the cell lines. AID expression was associated with the ongoing mutation in FL fresh cells (two-tailed Pearson's coefficient correlation, r=0.899, P=0.01). The switch off of AID expression may start in the B-lineage differentiation stage counterpart of FL after optimizing SHM, indicated by the cessation of the ongoing mutation in AID-negative FL fresh cells.

Quantification of human Aiolos splice variants by real-time PCR

Aiolos is a transcriptional regulator of B cell development and belongs to the Ikaros family of chromatin remodelling transcription factors. All the members of Ikaros family produce multiple isoforms via alternative mRNA splicing. Altered expression of Ikaros isoforms has been found in patients with acute lymphoblastic leukemia but it is not studied whether the altered expression of Aiolos isoforms also has a role in the development of leukemias or lymphomas. We developed a quantitative real-time PCR application to detect the relative expression of Aiolos splice variants. The method is based on fluorescence resonance energy transfer (FRET)-labelled isoform specific hybridisation probes used with the LightCycler instrument. The isoform specificity is obtained by targeting the probes at the edges of chosen exons. The probes are here shown to represent a rapid, high throughput, specific and reproducible quantification method. We designed and optimised the analysis for a dominant negative Aiolos isoform, but the described method is applicable to any isoform-forming gene. This study shows that the real-time PCR with exon edge spanning probe pairs can be applied generally to reveal the importance of alternative splicing and the role of isoforms in normal development and diseases.

Molecular mechanism of class switch recombination: linkage with somatic hypermutation

Class switch recombination (CSR) and somatic hypermutation (SHM) have been considered to be mediated by different molecular mechanisms because both target DNAs and DNA modification products are quite distinct. However, involvement of activation-induced cytidine deaminase (AID) in both CSR and SHM has revealed that the two genetic alteration mechanisms are surprisingly similar. Accumulating data led us to propose the following scenario: AID is likely to be an RNA editing enzyme that modifies an unknown pre-mRNA to generate mRNA encoding a nicking endonuclease specific to the stem-loop structure. Transcription of the S and V regions, which contain palindromic sequences, leads to transient denaturation, forming the stem-loop structure that is cleaved by the AID-regulated endonuclease. Cleaved single-strand tails will be processed by error-prone DNA polymerase-mediated gap-filling or exonuclease-mediated resection. Mismatched bases will be corrected or fixed by mismatch repair enzymes. CSR ends are then ligated by the NHEJ system while SHM nicks are repaired by another ligation system.

Both normal and leukemic B lymphocytes express multiple isoforms of the human Aiolos gene

Aiolos is a chromatin remodeling transcription regulator that plays an antiproliferative role in B lymphocyte function. In contrast to the related Ikaros factors, mammalian Aiolos has not been reported to generate splice variants. In addition, although human leukemic lymphoblasts express non-DNA-binding Ikaros isoforms with potential dominant negative effect on other interacting factors,the role of Aiolos in human lymphoid disorders has remained obscure. To address the question, why Aiolos should delineate from Ikaros in such a marked way, we have here analyzed whether also human Aiolos could generate alternate isoforms. According to the results obtained, both normal and neoplastic B lineage cells were found to express at least five novel Aiolos variants. Also structurally dominant negative variants with less than three DNA-binding domains were identified. In conclusion, given the multiplicity of also human Aiolos isoforms and thereby the evidently more intricate contribution of Aiolos to the chromatin remodeling machinery, it is suggested, that not only Ikaros, but also Aiolos could participate in a more versatile manner in the regulation of B lymphocyte function.

Impact of age on hypermutation of immunoglobulin variable genes in humans

Chronological aging is associated with an accumulation of DNA mutations that results in cancer formation. The effect of aging on spontaneous mutations in humans is difficult to study because mutations are infrequent in the overall genome and tumors are relatively rare. In contrast, somatic mutations in immunoglobulin variable genes are abundant and can be studied in peripheral blood lymphocytes. To determine if aging alters the frequency and pattern of hypermutation, we sequenced 331 cDNA clones with rearranged V(H)6 genes and compared 452 mutations from young humans to 570 mutations from old humans. There were more mutated clones in the young population compared to the old population. Among the mutated clones, the frequency, location, and types of substitutions were similar between the young and the old groups. However, the ratio of replacement-to-silent mutations was much higher in the complementarity-determining regions of heavy chains from old people, which indicates that their B cells had been selected by antigen. Among individuals, there was variability in the frequency of tandem mutations, which we have observed in mice defective for the PMS2 mismatch repair protein. Microsatellite variability in DNA, which is caused by impaired mismatch repair, was then measured, and there was a strong correlation between the frequency of tandem mutations and microsatellite alterations. The data suggest that individuals vary in their mismatch repair capacity, which can affect the mutational spectra in their antibodies.

Signals sustaining human immunoglobulin V gene hypermutation in isolated germinal centre B cells

Affinity maturation of antibody responses depends on somatic hypermutation of the immunoglobulin V genes. Hypermutation is initiated specifically in proliferating B cells in lymphoid germinal centres but the signals driving this process remain unknown. This study identifies signals that promote V gene mutation in human germinal centre (GC) B cells in vitro. Single GC B cells were cultured by limiting dilution to allow detection of mutations arising during proliferation in vitro. Cells were first cultured in the presence of CD32L cell transfectants and CD40 antibody (the 'CD40 system') supplemented with combinations of cytokines capable of supporting similar levels of CD40-dependent GC B-cell growth [interleukin (IL)-10 + IL-1beta + IL-2 and IL-10 + IL-7 + IL-4]. Components of the 'EL4 system' were then added to drive differentiation, providing sufficient immunoglobulin mRNA for analysis. Analysis of VH3 genes from cultured cells by reverse transcription-polymerase chain reaction (RT-PCR)-based single-strand conformation polymorphism indicated that the combination IL-10 + IL-1beta + IL-2 promoted active V gene mutation whereas IL-10 + IL-7 + IL-4 was ineffective. This was confirmed by sequencing which also revealed that the de novo generated mutations were located in framework and complementarity-determining regions and shared characteristics with those arising in vivo. Somatic mutation in the target GC B-cell population may therefore be actively cytokine driven and not simply a consequence of continued proliferation. The experimental approach we describe should facilitate further studies of the mechanisms underlying V gene hypermutation.

Somatic mutations and intraclonal variations in the rearranged Vkappa genes of B-non-Hodgkin's lymphoma cell lines

Three established Burkitt's lymphoma (BL) cell lines (Daudi, Raji and DG-75) and three B-non-Hodgkin's lymphoma (B-NHL) of other types (Pfeiffer, Farage and Toledo) were analyzed with respect to the presence of somatic point mutations in their rearranged immunoglobulin Vkappa genes. Two of the Vkappa sequences of BL and two of those of the B-NHL were heavily mutated (up to 11%), when compared with their closest germline variable region counterparts ("clonal mutations"). Only one of the six cell lines contained an unmutated germline Vkappa sequence. The clonal mutations have features characteristic of the mutation machinery operating in the course of the T-dependent immune response, such as a preference of mutations in purine bases, more transitions than transversions and targeting to CDR and to known "hotspot" motifs. Sequence variations among different Vkappa PCR clones isolated from each of the cell lines ("intraclonal mutations") showed that the Vkappa of Toledo exhibited about 5-fold higher mutation frequency (MF) than the background level of Taq polymerase error (approximately 0.12% mut/bp). Similarly, the MF of Vkappa of two of the BL cell lines was 3-4-fold higher than the Taq polymerase misincorporation rate. In contrast, the mutation frequencies of the Vkappa of DG-75, Farage and Pfeiffer did not significantly exceed the level of Taq polymerase error. Our combined results show that 5 out of the 6 B-cell lines studied originated from B-cells that have already somatically mutated in vivo their rearranged Vkappa genes. Moreover, two of the Burkitt's and one of the B-NHL cell lines exhibit intraclonal variation indicating that the process of somatic hypermutation continued following the neoplastic event, either in vivo or in culture. These results are in accord with the presumed origin of the majority of the BL and some types of the B-NHL, from centrocytes or centroblasts of the germinal centers in which the process of somatic hypermutation is taking place.

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.

Alternative Splicing and Hypermutation of a Nonproductively Rearranged TCR α-Chain in a T Cell Hybridoma

Like Ig genes, TCR genes are formed by somatic rearrangements of noncontiguous genomic V, J, and C regions. Unlike Ig genes, somatic hypermutation of TCR V regions is an infrequent event. We describe the occurrence of spontaneous hypermutation in a nonproductively rearranged TCR α-chain gene in a clonal T cell hybridoma that had lost its productively rearranged α-chain. The mutating hybridoma was eventually supplanted in culture by a nonmutating variant that had restored an open reading frame in the nonproductively rearranged TCR α-chain through the use of cryptic splice sites in the Vα region. Evidence is presented for the presence of cDNA reverse transcripts of the TCR α-chain within the hybridoma, suggesting a role for reverse transcriptase in the generation of mutations.

TdT-accessible breaks are scattered over the immunoglobulin V domain in a constitutively hypermutating B cell line

Searching for an in vitro model for somatic hypermutation, we have identified an IgM-expressing Burkitt lymphoma line that constitutively diversifies its immunoglobulin V domain at high rate during culture. As in in vivo, the mutations are largely nucleotide substitutions with the pattern of substitutions revealing a component of the human hypermutation program that is preferentially targeted to G/C residues. The substitutions frequently create stop codons with IgM-loss variants also being generated by V domain-specific deletions and duplications. However, in transfectants expressing terminal deoxynucleotidyl transferase, many IgM-loss variants additionally arise through short nontemplated nucleotide insertions into the V (but not C) domain. Thus, antibody hypermutation is likely accompanied by DNA strand breaks scattered within the mutation domain.

Critical test of hot spot motifs for immunoglobulin hypermutation

In hypermutation at the immunoglobulin loci, some bases are much more mutable than others. The increased mutability of the hot spots has been attributed to their being embedded in short sequence motifs. Among the suggested motifs are palindromes, TAA and RGYW (i.e. A/G G C/T A/T). We have tested these proposed motifs in a transfection system in vitro, which ordinarily uses the hypermutable stop codon TAG. The stop codon TAA is not hypermutable in our system, even when embedded in the pentamer and hexamer palindromes TAATA and ATTAAT; in fact, the revertants isolated were due to deletions. Single or double base changes in an RGYW motif containing a hypermutable stop codon result in a reduction of one order of magnitude or more in point mutation frequency. When the nonamer GACTAGTAT, which includes the same RGYW motif, was moved over hundred base pairs upstream, hypermutability was reduced by an order of magnitude. Thus, while RGYW apparently is a hypermutability motif, it cannot be the sole determinant of mutability.

Induction of somatic mutation in a human B cell line in vitro

Both the B cell-surface trigger(s) and the intracellular molecular mechanism(s) of somatic hypermutation in immunoglobulin (Ig) variable region genes remain unknown, partly because of the lack of a simple and reproducible in vitro model. Here, we show that upon surface immunoglobulin cross-linking followed by co-culture with activated cloned T cells, the Burkitt's lymphoma cell line BL2 is induced to mutate its IgV(H) gene. Repeated activation of BL2 cells increased the frequency of mutation. The in vitro-induced mutations, which do not affect the IgM constant region, are point mutations distributed over the entire V(H)DJ(H) gene segment and do not show evidence of antigen-driven selection.