V(D)J hypermutation and receptor revision: coloring outside the lines

CryoEM Structure of an Influenza Virus Receptor-Binding Site Antibody-Antigen Interface

Structure-based vaccine design depends on extensive structural analyses of antigen-antibody complexes.Single-particle electron cryomicroscopy (cryoEM) can circumvent some of the problems of x-ray crystallography as a pipeline for obtaining the required structures. We have examined the potential of single-particle cryoEM for determining the structure of influenza-virus hemagglutinin (HA):single-chain variable-domain fragment complexes, by studying a complex we failed to crystallize in pursuing an extended project on the human immune response to influenza vaccines.The result shows that a combination of cryoEM and molecular modeling can yield details of the antigen-antibody interface, although small variation in the twist of the rod-likeHA trimer limited the overall resolution to about 4.5Å.Comparison of principal 3D classes suggests ways to modify the HA trimer to overcome this limitation. A closely related antibody from the same donor did yield crystals when bound with the same HA, giving us an independent validation of the cryoEM results.The two structures also augment our understanding of receptor-binding site recognition by antibodies that neutralize a wide range of influenza-virus variants.

Hepatitis C virus (HCV)-induced immunoglobulin hypermutation reduces the affinity and neutralizing activities of antibodies against HCV envelope protein

Hepatitis C virus (HCV) often causes persistent infection despite the presence of neutralizing antibodies against the virus in the sera of hepatitis C patients. HCV infects both hepatocytes and B cells through the binding of its envelope glycoprotein E2 to CD81, the putative viral receptor. Previously, we have shown that E2-CD81 interaction induces hypermutation of heavy-chain immunoglobulin (V(H)) in B cells. We hypothesize that if HCV infects antibody-producing B cells, the resultant hypermutation of V(H) may lower the affinity and specificity of the HCV-specific antibodies, enabling HCV to escape from immune surveillance. To test this hypothesis, we infected human hybridoma clones producing either neutralizing or non-neutralizing anti-E2 or anti-E1 antibodies with a lymphotropic HCV (SB strain). All of the hybridoma clones, except for a neutralizing antibody-producing hybridoma, could be infected with HCV and support virus replication for at least 8 weeks after infection. The V(H) sequences in the infected hybridomas had a significantly higher mutation frequency than those in the uninfected hybridomas, with mutations concentrating in complementarity-determining region 3. These mutations lowered the antibody affinity against the targeting protein and also lowered the virus-neutralizing activity of anti-E2 antibodies. Furthermore, antibody-mediated complement-dependent cytotoxicity with the antibodies secreted from the HCV-infected hybridomas was impaired. These results suggest that HCV infection could cause some anti-HCV-antibody-producing hybridoma B cells to make less-protective antibodies.

Impaired elimination of DNA double-strand break-containing lymphocytes in ataxia telangiectasia and Nijmegen breakage syndrome

The repair of DNA double-strand breaks is critical for genome integrity and tumor suppression. Here we show that following treatment with the DNA-intercalating agent actinomycin D (ActD), normal quiescent T cells accumulate double-strand breaks and die, whereas T cells from ataxia telangiectasia (AT) and Nijmegen breakage syndrome (NBS) patients are resistant to this death pathway despite a comparable amount of DNA damage. We demonstrate that the ActD-induced death pathway in quiescent T lymphocytes follows DNA damage and H2AX phosphorylation, is ATM- and NBS1-dependent and due to p53-mediated cellular apoptosis. In response to genotoxic 2-Gy gamma-irradiation, on the other hand, quiescent T cells from normal donors survive following complete resolution of the damage thus induced. T cells from AT and NBS patients also survive, but retain foci of phosphorylated H2AX due to a subtle double-strand break (DSB) repair defect. A common consequence of these two genetic defects in the DSB response is the apparent tolerance of cells containing DNA breaks. We suggest that this tolerance makes a major contribution to the oncogenic risk of patients with chromosome instability syndromes.

Expression of RAGs in Peripheral B Cells outside Germinal Centers Is Associated with the Expression of CD5

Previous studies have indicated that mature B cells reactivate secondary V(D)J recombination inside and outside the germinal center (GC) of peripheral lymphoid organs. The nature of the B cells undergoing Ig rearrangement before they enter GC is unknown. In this study, we present evidence that activated mature CD5-positive human tonsil B cells coexpress both RAG1 and RAG2 mRNA and protein, and display DNA cleavage resulting from their recombinase activity. Furthermore, in vitro activation of CD5-negative naive mature B cells by IgR and CD40 cross-linking induces expression of CD5 on a subset of cells, and leads to the up-regulation of RAG1 and RAG2 only in cells turned positive for CD5. Thus, RAG gene expression is closely related to CD5 expression outside GCs. These data suggest that CD5 is associated with receptor revision in activated mature B cells and likely to promote expression of suitable IgR capable of initiating the GC reaction.

Insights into the regulation of immunoglobulin light chain gene rearrangements via analysis of the kappa light chain locus in lambda myeloma

Accumulating evidence indicates that B cells may undergo sequential rearrangements at the light chain loci, despite already expressing light chain receptors. This phenomenon may occur in the bone marrow and, perhaps, in germinal centers. As immunoglobulin (Ig)kappa light chains usually rearrange before Iglambda light chains, we analysed, by polymerase chain reaction, the Igkappa locus of bone marrow mononuclear cells from 29 patients with Iglambda myeloma to identify earlier recombinations in marrow plasma cells. The results demonstrated that Igkappa alleles were inactivated via the kappa-deleting element, presumably prior to V(kappa)-J(kappa) rearrangement, in many cases. Eighteen alleles (16 myeloma clones, 55%) showed V(kappa)-J(kappa) rearrangements, with increased utilization of 5' distant V(kappa) and 3' distant Jkappa gene segments (Jkappa4, 56%), an indication of multiple sequential rearrangements. In-frame, potentially functional V(kappa)-J(kappa) rearrangements were found in approximately one-third of available rearrangements (as expected by chance), each one in different myeloma clones: three were germline encoded, while one had several nucleotide substitutions, suggesting inactivation after the onset of somatic hypermutation. Three of four potentially functional V(kappa)-J(kappa)rearrangements involved V(kappa)4-1, a segment considered to be associated with autoimmunity. These findings provide insights into the regulation of light chain rearrangements and support the view that B cells may occasionally undergo sequential light chain rearrangements after the onset of somatic hypermutation.

Recombination activating genes (RAG) induce secondary Ig gene rearrangement in and subsequent apoptosis of human peripheral blood circulating B lymphocytes

Recombination activating gene (RAG) re-expression and secondary Ig gene rearrangement in mature B lymphocytes have been reported. Here, we have studied RAG expression of peripheral blood B lymphocytes in humans. Normal B cells did not express RAG1 and RAG2 spontaneously. More than a half of circulating B cells expressed RAG proteins, when activated with Staphylococcus aureus Cowan I (SAC) + IL-2. DNA binding activity of the RAG complex has been verified by a gel shift assay employing the recombination signal sequence (RSS). Secondary Ig light chain rearrangement in the RAG-expressing B cells was confirmed by linker-mediated (LM)-PCR. Highly purified surface kappa+ B cells activated by SAC + IL-2 became RAG+, and thereafter they started to express lambda chain mRNA. 2 colour immunofluorescence analysis disclosed that a part of the RAG+ cells derived from the purified kappa+ B cells activated by SAC + IL-2 turned to lambda+ phenotype in vitro. Similarly, apoptosis induction was observed in a part of the RAG+ B cells. Our study suggests that a majority of peripheral blood B cells re-expresses RAG and the RAG+ B lymphocytes could be eliminated from the B cell repertoire either by changing Ag receptor specificity due to secondary rearrangement or by apoptosis induction. Thus, RAG expression of mature B cells in peripheral blood would contribute to not only receptor revision for further diversification of B cell repertoire but in some cases (or in some B cell subsets) to prevention or induction of autoAb responses at this differentiation stage in humans.

Differential effect of IL-4 and IL-13 on the expression of recombination-activating genes in mature B cells from human peripheral blood

We examined the expression of recombination-activating genes (RAG-1 and RAG-2) and activation-induced cytidine deaminase (AID) by mature human blood B cells stimulated with anti-CD40 in the presence of IL-4 or IL-13. IL-4 was an effective cofactor for RAG-1 and RAG-2 expression, whereas IL-13 was not. In addition, IL-4-dependent RAG expression combined with AID and IgE expression allowed predominant expression of newly rearranged lambda light chains on IgE+ cells generated from kappa+ cells. Although the magnitudes of IL-4- and IL-13-dependent AID and IgE expression were related to expression levels of binding subunits of the IL-4 and IL-13 receptors, IL-13 was ineffective for light chain replacement in the induced IgE+ cells due to the failure in RAG expression. Our studies using mature blood B cells indicate that IL-4-responsive cells, unlike IL-13-responsive cells, undergo lambda gene rearrangement leading to replacement in parallel with RAG expression and suggest that this replacement may contribute to the regulation of affinity maturation of IgE antibodies.

A spatial model of germinal center reactions: cellular adhesion based sorting of B cells results in efficient affinity maturation

Affinity maturation of humoral responses to T-cell-dependent antigens occurs in germinal centers (GC). In GCs antigen-specific B cells undergo rounds of somatic mutations that alter their affinity. High-affinity mutants take over GCs very soon after they appear; the replacement rate is as high as 4 per day (Radmacher et al., Immunol. Cell Biol. 76 (1998) 373). To gain more insight into this selection process, we present a spatial model of GC reactions, where B cells compete for survival signals from follicular dendritic cells (FDC). Assuming that high-affinity B cells have increased cellular adhesion to FDCs, we obtain an affinity-based sorting of B cells on the FDC. This sorting imposes a very strong selection and therefore results in a winner-takes-all behavior. By comparing our sorting model with "affinity-proportional selection models", we show that this winner-takes-all selection is in fact required to account for the fast rates at which high affinity mutants take over GCs. Another important feature of in vivo GC reactions is that they are non-mixed, i.e. GCs contain either no high-affinity cells at all or they are dominated by high-affinity cells. We here show that this all-or-none behavior can be obtained if B cells are sorted based on their affinity on the FDC surface. Affinity-proportional selection models, in contrast, always produce mixed GCs.

Complete analysis of the B-cell response to a protein antigen, from in vivo germinal centre formation to 3-D modelling of affinity maturation

Somatic hypermutation of immunoglobulin variable region genes occurs within germinal centres (GCs) and is the process responsible for affinity maturation of antibodies during an immune response. Previous studies have focused almost exclusively on the immune response to haptens, which may be unrepresentative of epitopes on protein antigens. In this study, we have exploited a model system that uses transgenic B and CD4+ T cells specific for hen egg lysozyme (HEL) and a chicken ovalbumin peptide, respectively, to investigate a tightly synchronized immune response to protein antigens of widely differing affinities, thus allowing us to track many facets of the development of an antibody response at the antigen-specific B cell level in an integrated system in vivo. Somatic hypermutation of immunoglobulin variable genes was analysed in clones of transgenic B cells proliferating in individual GCs in response to HEL or the cross-reactive low-affinity antigen, duck egg lysozyme (DEL). Molecular modelling of the antibody-antigen interface demonstrates that recurring mutations in the antigen-binding site, selected in GCs, enhance interactions of the antibody with DEL. The effects of these mutations on affinity maturation are demonstrated by a shift of transgenic serum antibodies towards higher affinity for DEL in DEL-cOVA immunized mice. The results show that B cells with high affinity antigen receptors can revise their specificity by somatic hypermutation and antigen selection in response to a low-affinity, cross-reactive antigen. These observations shed further light on the nature of the immune response to pathogens and autoimmunity and demonstrate the utility of this novel model for studies of the mechanisms of somatic hypermutation.

Switch recombination in fetal porcine thymus is uncoupled from somatic mutation

Since fetal serum Ig isotype profiles suggested that IgG and IgA could be of de novo origin, we studied their transcription and secretion. IgM transcripts were present at 50 days of gestation in major fetal lymphoid tissues, IgG and IgA transcription was pronounced at 60 days in fetal thymus and both transcription and secretion in this organ increased in late fetal life. The CDR3 spectratype of thymic IgG and IgA transcripts was as polyclonal as that of IgM already at 70 days in utero indicating a broad repertoire of switched B-cells. However, VDJs transcribed with the switched isotypes were not hypermutated as were those from immunized fetuses, indicating that switch recombination and somatic mutation are not coupled in utero in piglets. This finding and the fact that the oligoclonal IgA and IgM repertoires in a non-inductive site of the mucosal immune system (parotid gland) becomes polyclonal in piglets reared germ-free, suggest that initial expansion of switched B-cells in fetal and neonatal piglets is not driven by environmental antigen. Our findings collectively suggest that all IgA and IgM may result from de novo synthesis while some IgG probably results from selective transport. The latter is consistent with the gradual decline in serum IgG concentration in germ-free isolator piglets and the expression of FcRn in the porcine placenta.

Role for complement receptors (CD21/CD35) in the regulation of recombination activating gene expression in murine peripheral B cells

A population of peripheral B cells have been shown to express recombination activating gene products, RAG-1 and RAG-2, which are considered to be involved in revising the B cell antigen receptor (BCR) in the periphery. BCR engagement has been reported to turn off RAG expression in peripheral B cells, whereas the same treatment has an opposite effect on immature B cells in the bone marrow. In contrast to receptor editing that is involved in the removal of autoreactivity in immature B cells, it has been shown that secondary V(D)J rearrangement in peripheral B cells, termed receptor revision, contributes to affinity maturation of antibodies. Here, we show that RAG-2 expression in murine splenic B cells was abrogated by the coligation of BCR with complement receptors (CD21/CD35) much more efficiently than by the engagement of BCR alone. On the other hand, the same coligation augmented proliferation of anti-CD40-stimulated B cells. These findings suggest a crucial role for CD21/CD35 in directing the conservation or the revision of BCRs in peripheral B cells.

Combinatorial regulation of the murine RAG-2 promoter by Sp1 and distinct lymphocyte-specific transcription factors

The recombination activation genes, RAG-1 and RAG-2, encode the critical components of the recombinase complex responsible for the generation of functional antigen receptor genes. In order to gain an insight into the transcription factors and cis-acting elements that regulate the lymphocyte-specific expression of RAG-2, the promoter-region of this gene was isolated and characterized. This analysis demonstrated that a relatively small promoter fragment could confer lymphocyte-restricted expression to a reporter construct. Our work and that of others subsequently revealed that RAG-2 promoter expression is positively regulated by BSAP (PAX-5) and c-Myb transcription factors in B- and T-lineage cells, respectively. Although BSAP and c-Myb were deemed necessary for lymphocyte-specific expression, our analysis also uncovered a G-rich region at the 5'-end of the core promoter that was essential for full activity in lymphocyte cell lines. Site-directed mutagenesis revealed that a GA-box within the G-rich region was required for full promoter activity and subsequent DNA binding assays demonstrated that this element was specifically recognized by Sp1. Apart from showing that Sp1 interacts within the RAG-2 promoter, we also demonstrate that the Sp1-binding site is necessary for the high-level activation of this promoter.

Contribution of light chain rearrangement in peripheral B cells to the generation of high-affinity antibodies

Recently, peripheral B cells have been shown to undergo secondary V(D)J rearrangement of immunoglobulin genes, but the physiological role of this event has not been fully elucidated. To investigate whether rearrangement of L chain genes in the periphery is involved in the generation of high-affinity antibodies (Ab), we used the 17.2.25 rearranged VHDJH gene (VHT)-knockin mouse whose B cell diversity is limited due to the expression of the site-directed transgene. Immunization of the mouse with p-nitrophenylacetyl (pNP)-conjugated chicken gamma-globulin preferentially led to the production of anti-pNP IgG Ab comprised of non-VHT-encoded H chains and lambda chains. lambda(+) IgG constituted a majority of high-affinity Ab to this hapten. RAG-2 mRNA and the recombination signal sequence break of the lambda1 gene increased in the draining lymph node of immunized mice, but not of nonimmunized animals. There was a close correlation between the levels of these parameters implicating lambda gene rearrangement and the production of lambda(+ )high-affinity anti-pNP IgG. These observations were reproduced in RAG-1-deficient mice that were reconstituted with the spleen cells ofthe knockin mouse. Thus, our findings suggest that L chain rearrangement that occurs in the periphery can contribute to affinity maturation of Ab.

Immunoglobulin gene rearrangement analysis in composite hodgkin disease and large B-cell lymphoma: evidence for receptor revision of immunoglobulin heavy chain variable region genes in Hodgkin-Reed-Sternberg cells?

Immunoglobulin heavy chain gene (IgH) rearrangement was studied in a patient showing the occurrence of classical Hodgkin disease and large B-cell lymphoma (LBCL) in the same lymph node. The VHDHJH region was amplified by polymerase chain reaction, the template being the DNA extracted from single Hodgkin and Reed-Sternberg and LBCL cells, microdissected on hematoxylin-eosin-stained sections by laser capture. A repeated VH4DH3JH4 segment was found in Reed-Sternberg cells, whereas a repeated VH3DH3JH4 segment was observed in LBCL cells. Rearranged VH genes carried somatic mutations in both populations, indicating a common germinal center cell origin. The IgH rearrangement found in clonally related Reed-Sternberg cells differed from the one of LBCL cells in the VH region but showed the same JH and DH segments with no variation from the respective germline sequence. The DH-JH junction is the first immunoglobulin gene segment rearranged in precursor B cells. Because the possibility of secondary Ig gene rearrangement in peripheral lymphoid organs has recently been reported, in the patient described here Reed-Sternberg and LBCL cells might originate from a common precursor in which secondary VH replacement took place during the germinal center reaction, giving rise to two different clonally related lymphomas.

Gene conversion-like sequence transfers in a mouse antibody transgene: antigen selection allows sensitive detection of V region interactions based on homology

Gene conversion is important for antibody diversification in chickens, rabbits and cows. In mice, however, conversion events appear to be infrequent among endogenous antibody genes. DNA sequence transfer events that resemble gene conversions have been reported for a mouse H chain transgene (VVC(mu)) that contains two closely spaced homologous VDJ segments. Surprisingly, these reported VVC(mu) sequence transfers were found frequently among mouse B cells responding to immunization. Transgene sequence transfers could be occurring at high frequency in responding VVC(mu) B cells or could be occurring at lower frequency with subsequent amplification by preferential antigen selection. To distinguish these possibilities, we have analyzed a second transgene (InVVC(mu)) that is identical to VVC(mu) except that the two VDJ regions have been exchanged in position. We find that transgene sequence transfers are much less frequent among responding B cells in InVVC(mu) mice, demonstrating the importance of selection in the frequent transgene conversions observed in VVC(mu) mice. These results suggest that mice, like other species, can use gene conversion to diversify antibodies. Such diversification events are apparently infrequent, however, and might only be detected among endogenous Ig genes with a favorable arrangement of V genes and an antigenic stimulation that selects cells with conversions. For both VVC(mu) and InVVC(mu) mice, conversion-like sequence transfers are strongly correlated with somatic hypermutation. Based on these results, we hypothesize that, in mice, gene conversions represent infrequent alternative reactions of a homology-based DNA repair process that is central in the somatic hypermutational mechanism.

Somatic hypermutation targeting to intrinsic hotspots of immunoglobulin genes in follicular lymphoma and multiple myeloma

In this study, we analyzed the targeting of the somatic hypermutation (SHM) mechanism at specific hotspot sequence motifs in the V(H) and Vkappa genes of 10 follicular lymphoma (FL) cases and the Vkappa and Vlambda genes of 11 kappa- and six lamda-light chain expressing multiple myeloma (MM) cases. These sequences were analyzed for targeting of specific motifs, ie certain highly mutable trinucleotides (3-NTPs), the tetranucleotide (4-NTP) RGYW and its complementary, WRCY (where R = purine, Y = pyrimidine and W = A or T). Comparisons were carried out between mutation frequencies in RGYW vs WRCY and the incidence of mutations in complementarity determining region (CDR)-1 vs CDR2 vs CDR3. Statistically significant differences were obtained when comparing: (1) the ratio of mutations in 4-NTPs (RGYW, WRCY, RGYW+WRCY)/mutations in the whole V sequence in MM-Vkappa vs MM-Vlamda; (2) the total number of mutated 4-NTPs in MM-Vkappa vs FL-Vkappa; (3) the number of mutated RGYW 4-NTPs in MM-Vkappa vs FL-Vkappa and FL-V(H) vs FL-Vkappa; (4) the number of mutated WRCY 4-NTPs in MM-Vkappa vs FL-Vkappa (P= 0.006) and FL-V(H) vs FL-Vkappa; (5) the targeting of RGYW vs WRCY in the CDRs of FL-V(H) genes. Similar results (regarding statistical significance) were obtained when undertaking intergroup comparisons for 3-NTPs. These findings conform well with relevant data derived from normal peripheral B cells. The differences observed in favor of 4-NTP (RGYW and WRCY) targeting in FL-V(H) vs FL-Vkappa and MM-Vkappa vs FL-Vkappa may implicate differences in the evolution of SHM coupled with selection in different stages of B cell ontogeny. Several explanations can be offered for the fact that hotspot sequences were not always targeted by SHM in FL and MM: (1) other unrecognized motifs may be targets of SHM; (2) 'inappropriately' introduced mutations were fixed and propagated by the neoplastic process; (3) certain FL and MM cases might have lost their ability to correct mutations introduced in classic hotspots due to deficient mismatch-repair (MMR) mechanisms; conversely, in other cases with intact MMR function, the hotspot to non-hotspot targeting of somatic hypermutation is balanced.

Agricultural risk factors for t(14;18) subtypes of non-Hodgkin's lymphoma

The t(14;18) translocation is a common somatic mutation in non-Hodgkin's lymphoma (NHL) that is associated with bcl-2 activation and inhibition of apoptosis. We hypothesized that some risk factors might act specifically along t(14;18)-dependent pathways, leading to stronger associations with t(14;18)-positive than t(14;18)-negative non-Hodgkin's lymphoma. Archival biopsies from 182 non-Hodgkin's lymphoma cases included in a case-control study of men in Iowa and Minnesota (the Factors Affecting Rural Men, or FARM study) were assayed for t(14;18) using polymerase chain reaction amplification; 68 (37%) were t(14;18)-positive. We estimated adjusted odds ratios (OR) and 95% confidence intervals (CI) for various agricultural risk factors and t(14;18)-positive and -negative cases of non-Hodgkin's lymphoma, based on polytomous logistic regression models fit using the expectation-maximization (EM) algorithm. T(14;18)-positive non-Hodgkin's lymphoma was associated with farming (OR 1.4, 95% CI = 0.9-2.3), dieldrin (OR 3.7, 95% CI = 1.9-7.0), toxaphene (OR 3.0, 95% CI = 1.5-6.1), lindane (OR 2.3, 95% CI = 1.3-3.9), atrazine (OR 1.7, 95% CI = 1.0-2.8), and fungicides (OR 1.8, 95% CI = 0.9-3.6), in marked contrast to null or negative associations for the same self-reported exposures and t(14;18)-negative non-Hodgkin's lymphoma. Causal relations between agricultural exposures and t(14;18)-positive non-Hodgkin's lymphoma are plausible, but associations should be confirmed in a larger study. Results suggest that non-Hodgkin's lymphoma classification based on the t(14;18) translocation is of value in etiologic research.

Antibody repertoire development in fetal and neonatal piglets. IV. Switch recombination, primarily in fetal thymus, occurs independent of environmental antigen and is only weakly associated with repertoire diversification

The epitheliochorial placenta of swine is considered a barrier to Ag and selective transport of IgG, so this species should be an excellent model with which to determine whether switch recombination is Ag dependent. Analysis of Ig levels and Ig isotype profiles in >150 normal and virus-infected fetuses from 38-110 days of gestation (DG) suggested that IgG, IgA, and IgM were most likely the result of de novo fetal synthesis. Although transcripts for IgM could be recovered at DG 50 (114 DG is full gestation) in all major fetal lymphoid tissues, those for IgG and IgA first became prominent at 60 DG in thymus, and transcription and spontaneous secretion became especially pronounced in this organ in older fetuses. Data on transcription, secretion, and serum isotype profiles suggest that although all fetal IgA and IgM may result from de novo synthesis, some IgG may result from low-level selective transport. The complementarity-determining region 3 spectratypes of thymic IgA and IgG transcripts at 70 and 90 days, respectively, were as polyclonal as that of IgM, indicating a broad repertoire of switched B cells although the VDJs transcribed with these switched isotypes in normal fetuses were not diversified in comparison to those from animals exposed to environmental Ags such as age-matched, virus-infected fetuses, colonized isolator piglets, and conventional adults. However, VDJs expressed with switched isotypes were more diversified than those expressed with IgM. Thus, switch recombination in fetal life does not appear to be driven by environmental Ag and is only weakly coupled to VDJ diversification. These findings, and the fact that the oligoclonal IgA and IgM repertoires in a noninductive site of the mucosal immune system (parotid gland) become polyclonal in piglets reared germfree, suggest that initial expansion of the switched cells in the B cell compartment of fetal and neonatal piglets is not driven by environmental Ag.

DNA quadruplexes and dynamical genetics

In a recent paper, we have put forward the hypothesis that there exist smart purposive mechanisms - tandem repeat length managers - which regulate the length of some tandem repeat, or cause rearrangements, and are almost always driven by some variable number tandem repeat. We have called the framework in which such mechanisms act 'dynamical genetics'. The purpose of this paper is to contribute to lay the foundations of a molecular study of the above mechanisms, by proposing a hypothesis, based on various kinds of supporting evidence and plausibility arguments, about the special importance of DNA quadruplexes for dynamical genetics, and by considering the involved enzymes. This hypothesis states that a tandem repeat length manager acts almost always by monitoring a DNA tract that has the characteristics of being a variable number tandem repeat and/or forming a DNA quadruplex, and that it is almost always driven by at least one of them.

Telomerase activity and clinical progression in chronic lymphoproliferative disorders of B-cell lineage

The activation of telomerase, which specifically occurs in neoplastic cells to avoid telomere attrition at each cell division, is a necessary event in tumorigenesis. The evidence that telomerase is also present in normal B cells at different levels according to their differentiation and activation state makes the study of telomerase activity in B cell tumors particularly interesting. This review summarizes data concerning telomerase activity in chronic lymphoproliferative disorders of B-cell lineage (B-CLD), making suggestions regarding B-cell development and B-cell tumor histogenesis. The role of telomerase activity as a potential prognostic marker, as well as a target of new antineoplastic strategies is discussed.

Generation of heterogeneous rabbit anti-DNP antibodies by gene conversion and hypermutation of rearranged VL and VH genes during clonal expansion of B cells in splenic germinal centers

The mechanisms described here account for development of the heterogeneous high-affinity anti-DNP antibodies that rabbits can produce. Rearranged immunoglobulin light and heavy chain genes from single DNP-specific splenic germinal center B cells were amplified by PCR. We found that in clonal lineages, rearranged V[kappa] and V[H] are further diversified by gene conversion and somatic hypermutation. The positive and negative selection of amino acids in complementarity-determining regions observed allows emergence of a variety of different combining site structures. A by-product of the germinal center reaction may be cells with sequences altered by gene conversion that no longer react with the immunizing antigen but are a source of new repertoire. The splenic germinal center would thus play an additional role in adults similar to that of the appendix and other gut-associated lymphoid tissues of young rabbits.

Lymphocytes rearrange, edit and revise their antigen receptors to be useful yet safe

Pamela Fink and Catherine McMahan discuss how B and T cells test for useful antigen receptors and weed out potentially harmful ones, with special attention paid to T-cell receptor revision, a newly described mechanism by which mature T cells can maintain self tolerance.

Accumulation of clonally related B lymphocytes in the cerebrospinal fluid of multiple sclerosis patients

The accumulation of B lymphocyte clones in the cerebrospinal fluid (CSF) of patients with multiple sclerosis (MS) and patients with other neurological disorders was investigated using PCR technologies. Oligoclonal B cell accumulations were detected in 10 of 10 MS patients, but only in 3 of 10 of the patients with other neurological disorders. Analyses of the Ig V(D)J sequences on the CSF from MS patients disclosed that VH3 and VH4 genes were extensively mutated compared with germline sequences. Moreover, a substantial proportion of the molecular clones analyzed shared the same third CDR of the H chain variable region gene (HCDR3) and the same VH genes, albeit with different numbers and locations of point mutations, thus indicating an ongoing process of intraclonal diversification. A larger number of clonally related VH sequences could be obtained by using a VH3 gene-specific PCR so that genealogical trees depicting the process of diversification could be drawn. Analyses of the Ig V(D)J from the CSF of a patient with viral meningitis and oligoclonal B cell accumulations revealed that VH3 genes were extensively mutated. However, no intraclonal diversification could be observed even using VH3 gene-specific PCR methodologies. Clone-specific PCR and sequencing was used to detect the V(D)J found in the CSF of one MS patient in the PBL of the same patient. Only 1/3 of the V(D)J sequences investigated could be demonstrated in the PBL, indicating that the V(D)J genes utilized by B cells in the CSF are much less represented in the PBL. Collectively, the data suggest that in MS there is a compartmentalized clonal expansion.

RAG2:GFP knockin mice reveal novel aspects of RAG2 expression in primary and peripheral lymphoid tissues

We generated mice in which a functional RAG2:GFP fusion gene is knocked in to the endogenous RAG2 locus. In bone marrow and thymus, RAG2:GFP expression occurs in appropriate stages of developing B and T cells as well as in immature bone marrow IgM+ B cells. RAG2:GFP also is expressed in IgD+ B cells following cross-linking of IgM on immature IgM+ IgD+ B cells generated in vitro. RAG2:GFP expression is undetectable in most immature splenic B cells; however, in young RAG2:GFP mice, there are substantial numbers of splenic RAG2:GFP+ cells that mostly resemble pre-B cells. The latter population decreases in size with age but reappears following immunization of older RAG2:GFP mice. We discuss the implications of these findings for current models of receptor assembly and diversification.