Evidence that the V kappa III gene usage is nonstochastic in both adult and newborn peripheral B cells and that peripheral CD5+ adult B cells are oligoclonal

Somatic Hypermutation Defects in Common Variable Immune Deficiency

Common variable immunodeficiency (CVID) is a heterogeneous disorder characterized by impaired antibody production and recurrent infections. In the last 20 years, several groups have reported that B cells from CVID patients have an impaired somatic hypermutation (SHM). The reported frequency of this defect among CVID patient cohorts is highly variable and so is the methodology used to evaluate this process. Interestingly, the low level of SHM on B cells from CVID patients has been correlated with the presence of infectious and non-infectious complications. In this review, an overview of the studies regarding SHM in CVID patients is presented. We highlight the importance of SHM studies in CVID patients as a clinical tool due to the reported association with clinical complications by several groups. We also considered SHM measurement useful to guide future investigations in order to identify genetic defects involved in the development of the disease.

The shape of the lymphocyte receptor repertoire: lessons from the B cell receptor

Both the B cell receptor (BCR) and the T cell receptor (TCR) repertoires are generated through essentially identical processes of V(D)J recombination, exonuclease trimming of germline genes, and the random addition of non-template encoded nucleotides. The naïve TCR repertoire is constrained by thymic selection, and TCR repertoire studies have therefore focused strongly on the diversity of MHC-binding complementarity determining region (CDR) CDR3. The process of somatic point mutations has given B cell studies a major focus on variable (IGHV, IGLV, and IGKV) genes. This in turn has influenced how both the naïve and memory BCR repertoires have been studied. Diversity (D) genes are also more easily identified in BCR VDJ rearrangements than in TCR VDJ rearrangements, and this has allowed the processes and elements that contribute to the incredible diversity of the immunoglobulin heavy chain CDR3 to be analyzed in detail. This diversity can be contrasted with that of the light chain where a small number of polypeptide sequences dominate the repertoire. Biases in the use of different germline genes, in gene processing, and in the addition of non-template encoded nucleotides appear to be intrinsic to the recombination process, imparting "shape" to the repertoire of rearranged genes as a result of differences spanning many orders of magnitude in the probabilities that different BCRs will be generated. This may function to increase the precursor frequency of naïve B cells with important specificities, and the likely emergence of such B cell lineages upon antigen exposure is discussed with reference to public and private T cell clonotypes.

Divergent human populations show extensive shared IGK rearrangements in peripheral blood B cells

We have analysed the transcribed immunoglobulin kappa (IGK) repertoire of peripheral blood B cells from four individuals from two genetically distinct populations, Papua New Guinean and Australian, using high-throughput DNA sequencing. The depth of sequencing data for each individual averaged 5,548 high-quality IGK reads, and permitted genotyping of the inferred IGKV and IGKJ germline gene segments for each individual. All individuals were homozygous at each IGKJ locus and had highly similar inferred IGKV genotypes. Preferential gene usage was seen at both the IGKV and IGKJ loci, but only IGKV segment usage varied significantly between individuals. Despite the differences in IGKV gene utilisation, the rearranged IGK repertoires showed extensive identity at the amino acid level. Public rearrangements (those shared by two or more individuals) made up 60.2% of the total sequenced IGK rearrangements. The total diversity of IGK rearrangements of each individual was estimated to range from just 340 to 549 unique amino acid sequences. Thus, the repertoire of unique expressed IGK rearrangements is dramatically less than previous theoretical estimates of IGK diversity, and the majority of expressed IGK rearrangements are likely to be extensively shared in individual human beings.

Deficiency of somatic hypermutation of the antibody light chain is associated with increased frequency of severe respiratory tract infection in common variable immunodeficiency

Reduced levels of somatic hypermutation (SHM) have recently been described in IgG-switched immunoglobulin genes in a minority of patients with common variable immunodeficiency (CVID), demonstrating a disruption of the normal linkage between isotype switch and SHM. To see if, irrespective of isotype, there is a tendency to use unmutated immunoglobulin genes in CVID, we studied SHM in kappa light-chain transcripts using a VkappaA27-specific restriction enzyme-based hot-spot mutation assay (IgkappaREHMA). Hot-spot mutations were found in 48% (median; reference interval, 28%-62%) of transcripts from 53 healthy controls. Values were significantly lower in 31 patients (median, 7.5%; range, 0%-73%; P < .0000001) of whom 24 (77%) had levels below the reference interval. Low levels of SHM correlated with increased frequency of severe respiratory tract infection (SRTI; P < .005), but not with diarrhea (P = .8). Mannose-binding lectin (MBL) deficiency also correlated with SRTI score (P = .009). However, the correlation of SHM and SRTI was also seen when only patients with normal MBL genotypes were analyzed (n = 18, P = .006). A slight decline of mutated fractions over years was noted (P = .01). This suggests that most patients with CVID fail to recruit affinity-maturated B cells, adding a qualitative deficiency to the quantitative deficiency characterizing these patients.

The kappa gene repertoire of human neonatal B cells

The kappa chain repertoire of individual IgD(+) human neonatal B cells was analyzed using a single cell PCR technique. A total of 104 productive and 90 non-productive VkappaJkappa rearrangements from three cord blood B cell samples were sequenced and compared to the adult IgM(+) peripheral B cell VkappaJkappa repertoire. All six Vkappa families were present in neonatal B cells, but the distribution was not random. In the non-productive repertoire Vkappa2 and Vkappa6 families were less frequent, Vkappa1 and Vkappa3 families were as frequent, and Vkappa4 and Vkappa5 families were more frequent than expected from random chance. Notably, the Vkappa2 family was negatively selected into the productive repertoire. In contrast, the Vkappa1 family was positively selected because of positive selection of three specific genes, O12/O2, L12a and L9. B3 (Vkappa4) and B2 (Vkappa5) were over-represented in the non-productive repertoire and then were expressed less frequently in the productive repertoire. In contrast, the Vkappa3 family gene, A27, was also over-represented in the non-productive repertoire but not further selected into the productive repertoire. Compared to the adult repertoire, junctional diversity was less marked because of a diminished influence of TdT activity, whereas the mean CDR3 length was comparable to that of normal adult B cells. Comparison of the distribution of Vkappa and Jkappa genes with those found in normal adult subjects suggested that there was less receptor editing in neonatal B cells. When neonatal CD5(+) B cells were compared with CD5(-) IgD(+) B cells, it was noted that the Vkappa gene A30 was used only in CD5(+) B cells in both the productive and non-productive repertoires. The results indicate that the usage of Vkappa genes by neonatal B cells is biased by both intrinsic molecular processes and selection. The evidence of selection indicates that the Vkappa repertoire is shaped by self antigens, since exposure to exogenous antigens is limited at the time of birth.

Pathogenesis of rheumatoid arthritis. Role of B lymphocytes

Despite many years of investigation, there remain many unanswered fundamental questions on the role of B cells in RA. Why is RF found in the sera of 80% of patients with RA and often in other chronic inflammatory diseases? What signals lead B lymphocytes to migrate into the subsynovial lining of joints? Does receptor revision in synovium play a role in the generation of autoantibodies in RA? What is the relative contribution of B-cell inhibition on the salutary effect of medications for RA? Can targeting autoreactive B cells, in conjunction with other therapies, provide therapeutic benefit in RA? We are hopeful that through continued basic, clinical, and translational research, these questions can be answered.

The V lambda J lambda repertoire in human fetal spleen: evidence for positive selection and extensive receptor editing

VlambdaJlambda rearrangements obtained from genomic DNA of individual IgM(+) B cells from human fetal spleen were analyzed. A nonrandom pattern of lambda gene rearrangements that differed from the adult Vlambda repertoire was found. The Vlambda distal genes 8A and 4B were absent from the nonproductive fetal repertoire, whereas 2E and 3L were overrepresented and 1B was underrepresented in the productive fetal repertoire. Positive selection of the Vlambda gene, 2E, along with Vlambda rearrangements employing homologous VlambdaJlambda joins were observed in the fetal, but not in the adult Vlambda repertoire. Overrepresentation of Jlambda distal cluster C genes rearranging to the Vlambda distal J segment, Jlambda7, in both productive and nonproductive fetal repertoires suggested that receptor editing/replacement was more active in the fetus than in adults. Numerous identical VlambdaJlambda junctions were observed in both the productive and nonproductive repertoire of the fetus and adult, but were significantly more frequent in the productive repertoire of the fetus, suggesting expansion of B cells expressing particular lambda-light chains in both stages of development, with more profound expansion in the fetal repertoire. Notably, B cells expressing identical lambda-light chains expressed diverse heavy chains. These data demonstrate that three mechanisms strongly influence the shaping of the human fetal lambda-chain repertoire that are less evident in the adult: positive selection, receptor editing, and expansion of B cells expressing specific lambda-light chains. These events imply that the expressed fetal repertoire is shaped by exposure to self Ags.

CD5-positive B cells in healthy elderly humans are a polyclonal B cell population

B cell chronic lymphocytic leukemia (B-CLL) is a disease of the elderly and is characterized by a malignant clone of CD5+ B cells. In old mice, clonal expansions of CD5+ B cells are a common feature, and these animals frequently develop B-CLL. To investigate whether clonal expansion of CD5+ B cells also occurs in elderly humans, predisposing for the development of B-CLL, we analyzed VH gene rearrangements of CD5+ B cells from blood samples of four healthy, 65-82-years-old volunteers as markers of clonality. CD5+ and CD5-B cells were obtained by cell sorting, CDRIII of rearranged VH genes were amplified by polymerase chain reaction, and fragment length analysis was performed. All samples demonstrated a polyclonal pattern of VH gene length distribution. In addition, VH gene rearrangements were amplified and sequenced from sorted single cells of two of the donors. No clonally related CD5+ or CD5- B cells were observed. Thus, development of dominant clones of CD5+ peripheral blood B cells is unlikely to be a common trait of elderly individuals.

The diversity of antigen-specific antibodies in humans and in two xenochimeric SCID mouse models

We applied two-dimensional gel electrophoresis (2-DE) to study the repertoire of tetanus toxoid (TT)-specific antibodies produced after TT immunization in healthy humans and in severe combined immunodeficient mice xenotransplanted with either human peripheral blood leukocytes (PBLe) or with human adult tonsil (hu-ton) pieces. Specific anti-TT antibodies, as well as total immunoglobulins (Ig), were purified by affinity chromatography on TT-Sepharose or Protein G-Sepharose, respectively. 2-DE unambiguously allowed us to differentiate between the specific humoral responses produced either by humans or by the two xenochimeric mouse models. Anti-TT antibodies produced by humans were polyclonal with a superimposed oligoclonality that was donor-dependent and that did not change upon time. By contrast, immunized hu-PBLe-SCID mice exhibited an evident clonal restriction of the Ig, which increased with time after boosting. Hu-ton-SCID mice showed a clonal diversity which was intermediate between those observed in humans and in hu-PBLe-SCID mice, and which was stable over time. In addition, information was gained by 2-DE, correlating with data obtained by enzyme-linked immunosorbent assay (ELISA), on the isotype composition of the anti-TT IgM response. Altogether, our results clearly demonstrated that the clonal diversity of monospecific antibodies can be appreciated by 2-DE, and that the largest diversity was found in humans when compared to that in xenochimeric models. In addition, mice implanted with pieces of lymphoid organs had the broadest anti-TT Ig diversity, an observation supporting the use of this model for the generation of antibodies with restricted specificity.

E2A and EBF act in synergy with the V(D)J recombinase to generate a diverse immunoglobulin repertoire in nonlymphoid cells

Immunoglobulin (Ig) and T cell receptor (TCR) genes are assembled during lymphocyte maturation through site-specific V(D)J recombination events. Here we show that E2A proteins act in concert with RAG1 and RAG2 to activate Ig VK1J but not Iglambda VlambdaIII-Jlambda1 rearrangement in an embryonic kidney cell line. In contrast, EBF, but not E2A, promotes VlambdaIII-Jlambda1 recombination. Either E2A or EBF activate IgH DH4J recombination but not V(D)J rearrangement. The Ig coding joints are diverse, contain nucleotide deletions, and lack N nucleotide additions. IgK VJ recombination requires the presence of the E2A transactivation domains. These observations indicate that in nonlymphoid cells a diverse Ig repertoire can be generated by the mere expression of the V(D)J recombinase and a transcriptional regulator.

Dominance of intrinsic genetic factors in shaping the human immunoglobulin Vlambda repertoire

The expressed human immunoglobulin Vlambda repertoire demonstrates a strong bias in the use of individual Vlambda segments. Mechanisms that underlie such biases can be divided into two categories: intrinsic genetic processes that lead to the preferential rearrangement and/or expression of certain segments; and selection following light chain expression. Here, we have used two approaches to investigate the factors that shape the human Vlambda repertoire. Firstly, we characterised 136 Vlambda rearrangements (59 productive and 77 non-productive) amplified from the human genomic DNA of peripheral blood cells. Secondly, we analysed Vlambda segment use in a library of 2000 cDNA clones from a transgenic mouse containing a 380 kb region (including 15 functional Vlambda segments) from the human immunoglobulin lambda locus. By hybridisation and sequencing we found that the patterns of use of human Vlambda segments in the transgenic mouse were similar to those found in the expressed human peripheral blood repertoire and in productive and non-productive genomic DNA rearrangements. These data indicate the importance of intrinsic genetic factors in shaping the human Vlambda repertoire and highlight the remarkable conservation of the molecular mechanisms involved in the production of the antibody repertoire in mouse and man. Therefore, transgenic mice represent a good model for analysis of the human antibody repertoire and for the production of human antibodies.

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.

Clonal Analysis of a Human Antibody Response. III. Nucleotide Sequences of Monoclonal IgM, IgG, and IgA to Rabies Virus Reveal Restricted Vκ Gene Utilization, Junctional VκJκ and VλJλ Diversity, and Somatic Hypermutation

In previous work, we generated four IgM, five IgG1, and one IgA1 mAbs to rabies virus using B cells from four subjects vaccinated with inactivated rabies virus, a thymus-dependent (TD) mosaic Ag, and sequenced the mAb VHDJH genes. Here, we have cloned the VκJκ and VλJλ genes to complete the primary structure of the Ag-binding site of these mAbs. While the anti-rabies virus mAb selection of Vλ genes (2e.2.2 twice, DPL11, and DPL23) reflected the representation of the Vλ genes in the human haploid genome (stochastic utilization), that of Vκ genes (O2/O12 twice, O8/O18, A3/A19, A27, and L2) did not (p = 0.0018) (nonstochastic utilization). Furthermore, the selection of both Vκ and Vλ genes by the anti-rabies virus mAbs vastly overlapped with that of 557 assorted VκJκ rearrangements, that of 253 VλJλ rearrangements in λ-type gammopathies, and that of other Abs to thymus-dependent Ags, including 23 anti-HIV mAbs and 51 rheumatoid factors, but differed from that of 43 Abs to Haemophilus influenzae type b polysaccharide, a prototypic thymus-independent (TI) Ag. The anti-rabies virus mAb VκJκ and VλJλ segments displayed variable numbers of somatic mutations, which, in mAb58 and the virus-neutralizing mAb57, entailed a significant concentration of amino acid replacements in the complementarity-determining regions (p = 0.0028 and p = 0.0023, respectively), suggesting a selection by Ag. This Ag-dependent somatic selection process was superimposed on a somatic diversification process that occurred at the stage of B cell receptor for Ag rearrangement, and that entailed V gene 3′ truncation and N nucleotide additions to yield heterogeneous CDR3s.

Frequent N addition and clonal relatedness among immunoglobulin lambda light chains expressed in rheumatoid arthritis synovia and PBL, and the influence of V lambda gene segment utilization on CDR3 length

BACKGROUND

In rheumatoid arthritis (RA), B-lineage cells in the synovial membrane secrete large amounts of immunoglobulin that contribute to tissue destruction. The CDR3 of an immunoglobulin light chain is formed by rearrangements of VL and JL gene segments. Addition of non-germline-encoded (N) nucleotides at V(D)J joins by the enzyme terminal deoxynucleotidyl transferase (TdT) enhances antibody diversity. TdT was previously thought to be active in B cells only during heavy chain rearrangement, but we and others reported unexpectedly high levels of N addition in kappa light chains. We also found clonally related kappa chains bearing unusually long CDR3 intervals in RA synovium, suggesting oligoclonal expansion of a set of atypical B lymphocytes. In this study, we analyzed lambda light chain expression to determine if N addition occurs throughout immunoglobulin gene rearrangement and to compare CDR3 lengths of lambda and kappa light chains in RA patients and normal individuals.

MATERIALS AND METHODS

Reverse transcription-polymerase chain reaction (RT-PCR) amplification of V lambda III transcripts was performed on RA synovia and peripheral blood lymphocytes (PBL) and normal PBL for which kappa repertoires were previously analyzed. Representative lambda + PCR products were cloned and sequenced.

RESULTS

Analysis of 161 cDNA clones revealed that N addition occurs in lambda light chains of RA patients and normal controls. The lambda light chain repertoires in RA were enriched for long CDR3 intervals. In both RA and controls, CDR3 lengths were strongly influenced by which V lambda gene segment was present in the rearrangement. Five sets of clonally related sequences were found in RA synovia and PBL; one set was found in normal PBL.

CONCLUSIONS

In humans, unlike mice, N addition enhances antibody diversity at all stages of immunoglobulin assembly, and the structural diversity of lambda CDR3 intervals is greater than that of kappa light chains. Clonally related V lambda gene segments in RA support an antigen-driven B-cell response.

The frequent expansion of a subpopulation of B cells that express RF-associated cross-reactive idiotypes: evidence from analysis of a panel autoreactive monoclonal antibodies

Preferential expression of VH gene segments is evident within the adult human primary B-cell repertoire. The repertoire may be influenced by genetic factors, e.g. VH gene segment polymorphisms, or in a temporal manner due to the exposure to environmental antigens. The molecular characteristics of 15 autoreactive human monoclonal antibodies (MoAbs) are reported. All antibodies were of the IgM isotype, and 12 of the 15 were polyreactive and included rheumatoid factor type specificity, i.e. reactivity with IgG. Nine of the 15 MoAbs are products of VH3 gene segments, as evidenced by staphylococcal protein A binding; four of these express the cross-reactive idiotype recognized by the mouse MoAb 3H7 and are thus products of the VH26 gene segment. One of the five remaining VH3 gene products expresses the cross-reactive idiotypes recognized by the mouse MoAbs B6 and D12. V-gene family usage, determined by polymerase chain reaction (PCR) amplification of cDNA and further hybridization with family-specific oligonucleotide probes, confirmed the cross-reactive idiotype studies and showed that only VH3-gene-encoded proteins bound staphylococcal protein A. Five of the six non-VH3 gene segment products express the cross-reactive idiotype recognized by the mouse MoAb LC1 and could be assumed to be products of the VH4.21 gene segment; however, one human MoAb is shown to be the product of a VH2 gene segment. This is interesting because it turns LC1 from being an anti-cross-reactive idiotype antibody into an anticlan reagent.

Sequence of the spacer in the recombination signal sequence affects V(D)J rearrangement frequency and correlates with nonrandom Vkappa usage in vivo

Functional variable (V), diversity (D), and joining (J) gene segments contribute unequally to the primary repertoire. One factor contributing to this nonrandom usage is the relative frequency with which the different gene segments rearrange. Variation from the consensus sequence in the heptamer and nonamer of the recombination signal sequence (RSS) is therefore considered a major factor affecting the relative representation of gene segments in the primary repertoire. In this study, we show that the sequence of the spacer is also a determinant factor contributing to the frequency of rearrangement. Moreover, the effect of the spacer on recombination rates of various human Vkappa gene segments in vitro correlates with their frequency of rearrangement in vivo in pre-B cells and with their representation in the peripheral repertoire.

Myosin-reactive autoantibodies in rheumatic carditis and normal fetus

EBV-transformed B cells from a 20-week human fetal spleen and from blood of patients with poststreptococcal rheumatic carditis were studied. Most antibodies from nine fetal and six patient myosin-reactive B cell clones were multireactive (reacting with cardiac myosin, Streptococcus pyogenes, and rat cardiac myocytes) which supports a role for molecular mimicry in stimulation of these autoantibodies. Sequence analysis revealed that fetal and patient anti-myosin repertoires were composed of unrelated clones with diverse V gene usages. Fetal and patient antibodies had reduced VH CDR3 length on average and reduced light chain N region addition with a low rate of somatic mutation in the variable region genes, characteristics generally associated with fetal B cells but also with some adult B cells. Five of six myosin-reactive patient clones used VH3, whereas only two of nine fetal clones used VH3, suggesting skewing from the average 50-60% VH3 gene usage found in randomly selected adult and fetal antibodies.

Somatic hypermutation in normal and transformed human B cells

In the human, most IgM+IgD+ as well as CD5+ peripheral blood B cells express unmutated V genes and thus can be assigned to a pre-germinal centre (GC) stage of development. The memory B-cell compartment generated in the GC reaction and characterized by cells bearing somatically mutated V-region genes consists not only of class-switched cells, but also of IgM-only B cells and perhaps a subset of IgM+IgD+B cells expressing the CD27 antigen. Comparison of the rearranged V-region genes of human B-cell lymphomas with those of the normal B-cell subsets allows the identification of the progenitor cells of these tumours in terms of their stage of maturation. On this basis, most B-cell non-Hodgkin lymphomas, and in addition Hodgkin and Reed-Sternberg (HRS) cells in Hodgkin's disease (HD), are derived from B cells at a GC or post-GC stage of development. The mutation pattern indicates that the precursors of the tumour clones have been stringently selected for expression of a functional antigen receptor with one notable exception: HRS cells in classical (but not lymphocyte-predominant) HD appear to be derived from "crippled" GC B cells. Sequence analysis of rearranged V genes amplified from single tonsillar GC B cells revealed that the somatic hypermutation process introduces deletions and/or insertions into V-region genes more frequently than indicated by previous investigations. Presumably, this feature of the hypermutation mechanism is often responsible for the generation of heavy chain disease, and also several types of chromosomal translocations of oncogenes into immunoglobulin loci in human B-cell lymphomas.

Evidence for selection of 11 amino acid CDR3 domains in V kappa III-derived immunoglobulin light chains in Kawasaki disease

Kawasaki disease (KD) is a rheumatic disease that occurs during childhood. Although T cells have been implicated as having an important role in the pathogenesis of KD, the role of B cells is unclear. To detect preferential expression of 11 amino acid complementarity determining region (CDR)3 domains, we used two-stage PCR (polymerase chain reaction) to analyze the CDR3 lengths of VkIII-derived immunoglobulin kappa light chains expressed in peripheral blood B cells during the acute, subacute, and convalescent phase of this disease. As controls, the peripheral blood B cells of age-matched normal and children with acute febrile diseases other than KD were tested. In 5 of 7 KD patients, expression of kappa light chains containing 11 amino acid codon CDR3 intervals was increased during the acute and subacute phase of KD but decreased during the convalescent phase. Two of the 7 KD patients showed the same pattern during the subacute and convalescent phase, but not during the acute phase. Two of the 5 patients with acute febrile diseases other than KD showed increased expression of kappa chains with 11 amino acid codon CDR3 intervals, but it was not a major fraction. Three of the 5 patients with acute febrile diseases other than KD and all normal control subjects showed only 9 and 10 amino acid CDR3 domains. These results strongly suggest that B cells expressing kappa light chains with the 11 amino acid CDR3 domains might be involved in the pathogenesis of KD.

Molecular single-cell analysis reveals that CD5-positive peripheral blood B cells in healthy humans are characterized by rearranged Vkappa genes lacking somatic mutation

B cells expressing the CD5 cell surface antigen are involved in certain B cell malignancies and autoimmune diseases. From studies in the mouse, it emerged that CD5+ B cells represent a separate lineage of B lymphocytes that, in contrast to conventional (CD5-) B cells, are not driven into T cell-dependent immune responses in which rearranged variable (V) region genes are diversified by somatic hypermutation. Against this background it came as a surprise that human disease-involved CD5-positive autoreactive B cells as well as B cell chronic lymphocytic leukemias can harbor somatically mutated V region genes. Recent V gene analyses on CD5+ B cells in healthy adults did not give rise to a clear picture about the fraction of somatically mutated among all CD5+ B cells. In this work we used a molecular single-cell analysis to determine reliably the frequency of mutated CD5+ B cells in healthy humans: single, kappa light chain-expressing CD5+ peripheral blood B cells were isolated by flow cytometry, and rearranged Vkappa genes were amplified by PCR. From one donor, CD5+CD19+ B cells were analyzed. Since CD5+ B cells were found among IgM+IgD+ and IgM+IgD- cells (but almost not among class-switched cells) from two other donors, individual cells corresponding to these IgM-expressing subsets were investigated separately. The sequence analysis of rearranged Vkappa genes revealed that most if not all CD5+ B cells in healthy humans carry unmutated V region genes. From one of the donors, a novel polymorphic Jkappa2 gene segment was identified. To explain the discrepancy between the frequent occurrence of disease-associated somatically mutated CD5+ B cells and the low incidence or absence of somatic mutation in normal CD5+ B cells, we speculate that CD5+ B cells usually do not participate in germinal center reactions, but if they occasionally do so, they may be at an increased risk to become involved in autoimmune diseases or B cell malignancies.

Molecular mechanisms and selective influences that shape the kappa gene repertoire of IgM+ B cells

To analyze the human kappa chain repertoire and the influences that shape it, a single cell PCR technique was used that amplified Vkappa Jkappa rearrangements from genomic DNA of individual human B cells. More than 350 productive and 250 nonproductive Vkappa Jkappa rearrangements were sequenced. Nearly every functional Vkappa gene segment was used in rearrangements, although six Vkappa gene segments, A27, L2, L6, L12a, A17, and O12/O2 were used preferentially. Of these, A27, L2, L6, and L12a showed evidence of positive selection based on the variable region and not CDR3, whereas A17 was overrepresented because of a rearrangement bias based on molecular mechanisms. Utilization of Jkappa segments was also nonrandom, with Jkappa1 and Jkappa2 being overrepresented and Jkappa3 and Jkappa5 underrepresented in the nonproductive repertoire, implying a molecular basis for the bias. In B cells with two Vkappa Jkappa rearrangements, marked differences were noted in the Vkappa segments used for the initial and subsequent rearrangements, whereas Jkappa segments were used comparably. Junctional diversity was generated by n-nucleotide addition in 60% and by exonuclease trimming in 75% of the Vkappa Jkappa rearrangements analyzed. Despite this large degree of diversity, a strict CDR3 length was maintained in both productive and nonproductive rearrangements. More than 23% of the productive rearrangements, but only 7% of the nonproductive rearrangements contained somatic hypermutations. Mutations were significantly more frequent in Vkappa sequences derived from CD5- as compared with CD5+ B cells. These results document that the gene segment utilization within the Vkappa repertoire is biased by both intrinsic molecular processes as well as selection after light chain expression. Moreover, IgM+ memory cells with highly mutated kappa genes reside within the CD5- but not the CD5+ B cell compartment.

VHDJH gene sequences and antigen reactivity of monoclonal antibodies produced by human B-1 cells: evidence for somatic selection

To understand whether the distinct VHDJH gene utilization by natural polyreactive Abs reflects the developmentally restricted Ig VHDJH rearrangements putatively expressed by B-1 cells, we generated 11 (8 IgM, 1 IgG3, 2 IgA1), 7 (6 IgM, 1 IgG1), and 7 (2 IgM, 3 IgG1, 2 IgG3) mAb-producing lines using B-1a (surface CD5+, CD45RAlow), B-1b (surface CD5-, CD45RAlow, CD5 mRNA+), and B-2 (surface CD5-, CD45RAhigh, CD5 mRNA-) cells, respectively, sorted from adult human peripheral blood. Most B-1a and B-1b, but no B-2, cell-derived mAbs were polyreactive; i.e., they bound different self and foreign Ags with different affinities. B-1a and B-2 mAbs preferentially utilized VH4 (p = 0.003) and VH3 (p = 0.010) genes, respectively. All three mAb populations utilized DXP, DLR, DN DH genes, and JH6, but no mAb utilized DHQ52. There were fewer unencoded nucleotide (N) additions in the VHDJH junctions of B-1b (3.00 +/- 2.52, mean +/- SD) than of B-1a (12.45 +/- 3.93, p = 1.23 x 10(-5)) or B-2 (8.29 +/- 4.75, p = 0.020) mAbs. Partly due to the fewer N additions and a paucity of D-D fusions, the B-1b mAb CDR3s were significantly shorter than the B-1a mAb CDR3s (p = 0.013), which contained a nonrandom Tyr distribution (p = 0.003). Finally, all but two B-1 cell-derived mAbs were mutated, in a fashion similar to that of the Ag-selected B-2 mAbs. Thus, in the human adult, B-1 cells that make natural polyreactive Abs may not be representative of the predominantly B-1 developmental waves of colonization of the fetal and neonatal B cell repertoires, and are somatically selected.

Evidence for a Large Compartment of IgM-Expressing Memory B Cells in Humans

The recent finding of somatically mutated μ heavy chain transcripts in human peripheral blood (PB) B lymphocytes suggests that T-dependent B-cell memory might not be restricted to class-switched cells. We provide here evidence that IgM-only PB B cells are likely to be the IgM-expressing counterpart of classical (IgM−IgD−) memory B cells in humans. As shown by molecular single cell analysis, most IgM-only cells carry mutated V region genes, like class-switched cells. Although both subsets represent populations of nonactivated, resting cells, they express higher levels of Ig mRNA than naive (IgM+IgD+) B cells. IgM-only and class-switched cells are CD38−CD77−, and mostly CD23−, thus neither resembling germinal center nor naive B cells. Because many IgM-expressing B cells located in secondary lymphoid tissues resemble IgM-only PB B cells in terms of cell phenotype, we propose that the human lymphoid system contains a large compartment of IgM-expressing memory cells. Moreover, these cells seem to represent the nonmalignant counterparts of IgM-expressing tumor cells in sporadic Burkitt's lymphoma, MALT lymphoma, monocytoid B-cell lymphoma, and diffuse large-cell lymphoma that were found to harbor somatically mutated V genes.

The B lymphocyte in rheumatoid arthritis: analysis of rearranged V kappa genes from B cells infiltrating the synovial membrane

The participation of the humoral immune system in rheumatoid arthritis (RA) is characterized by the production of rheumatoid factors (RF). RF are autoantibodies against the Fc part of IgG which are encoded by diverse germ-line genes. Most of the RF-encoding genes are unmutated, but in RA, a substantial quantity is encoded by somatically mutated genes. In addition, the synovial membranes (SM) of the diseased joints of RA patients are infiltrated by B lymphocytes which form germinal center-like aggregates. To analyze the local immune response, B cell foci from two RA SM were isolated by micromanipulation. From DNA of these foci, the rearranged kappa light chain variable region (V kappa) genes were amplified by polymerase chain reaction (PCR), cloned and sequenced. The amplification of different V kappa-J kappa combinations of different foci suggested oligoclonal expansion of B lymphocytes, which was confirmed by sequence analysis: each PCR product contained members of a single B cell clone. The sequence analysis of 29 different clones revealed rearrangements of diverse V kappa genes. Both frequent representatives of the V kappa 3 and the V kappa 1 family, as well as rarely used genes such as the L10 and B2 genes of the V kappa 2 and V kappa 5 families were found. Of the eleven potentially functional gene rearrangements, eight were significantly mutated, indicating their derivation from antigen-selected B cells. Intraclonal diversity in one of these clones may suggest ongoing mutation in the diseased synovial membrane of patients with RA.

Somatic mutation and CDR3 lengths of immunoglobulin kappa light chains expressed in patients with rheumatoid arthritis and in normal individuals

Immunoglobulin secretion by plasma cells infiltrating synovial membranes is a prominent feature of RA. Previous analyses of a cDNA library generated from synovium of RA patient BC revealed immunoglobulin kappa light chain transcripts with extensive somatic mutation, frequent N region addition, and unexpected variation in the lengths of CDR3 regions which form the center of the antigen binding site. To determine if these characteristics are present in other individuals, we performed reverse transcription-polymerase chain reaction amplification and sequenced > or = 10 V kappa-containing amplicons from nine tissue samples: synovia of three individuals with long-standing RA (including patient BC), PBLs of two of these individuals, and PBLs or splenocytes of four normal individuals. Increased levels of somatic mutation in PBLs appeared to correlate with increased age, which may reflect accumulation of circulating memory cells and/or decreased bone marrow production of naive B lymphocytes. Two of three RA synovial samples and both RA PBL samples exhibited increased proportions of clones with unusual CDR3 lengths. Enrichment for these antibody binding sites could be due to abnormal regulation of the emerging repertoire or to selection for B lymphocytes bearing antibodies of unusual specificity, and may play a role in the pathogenesis of RA.

Burkitt's lymphoma is a malignancy of mature B cells expressing somatically mutated V region genes

BACKGROUND

The developmental stage from which stems the malignant B cell population in Burkitt's lymphoma (BL) is unclear. An approach to answering this question is provided by the sequence analysis of rear-ranged immunoglobulin (Ig) variable region (V) genes from BL for evidence of somatic mutations, together with a phenotypic characterization. As somatic hypermutation of Ig V region genes occurs in germinal center B cells, somatically mutated Ig genes are found in germinal center B cells and their descendents.

MATERIALS AND METHODS

Rearranged V kappa region genes from 10 kappa-expressing sporadic and endemic BL-derived cell lines (9 IgM and 1 IgG positive) and three kappa-expressing endemic BL biopsy specimens were amplified by polymerase chain reaction and sequenced. In addition, VH region gene sequences from these cell lines were determined.

RESULTS

All BL cell lines and the three biopsy specimens carried somatically mutated V region genes. The average mutation frequency of rearranged V kappa genes from eight BL cell lines established from sporadic BL was 1.8%. A higher frequency (6%) was found in five endemic cases (three biopsy specimens and two BL cell lines).

CONCLUSIONS

The detection of somatic mutations in the rearranged V region genes suggests that both sporadic and endemic BL represent a B-cell malignancy originating from germinal center B cells or their descendants. Interestingly, the mutation frequency detected in sporadic BL is in a range similar to that characteristic for IgM-expressing B cells in the human peripheral blood and for mu chain-expressing germinal center B cells, whereas the mutation frequency found in endemic BL is significantly higher.