Heavy-chain directed B-cell maturation: continuous clonal selection beginning at the pre-B cell stage

Transcriptional and metabolic pre-B cell receptor-mediated checkpoints: implications for autoimmune diseases

At the pre-B cell stage of lymphocyte development, immunoglobulin light-chains are not yet produced, and heavy-chains are covalently linked to surrogate light-chains composed of VpreB and λ5 to form the pre-B cell receptor (pre-BCR) in a non-covalent association with signal-transducing modules. Even tough the pre-BCR does not have the potential to bind conventional antigens, accumulating evidence indicates that pre-BCR-mediated checkpoints are important both for negative and positive selection of self-reactivity, and that defects in these regulatory nodes may be associated with autoimmune disease. Thus, the transcription factor BACH2, which represents a susceptibility locus for rheumatoid arthritis, has recently emerged as a crucial mediator of negative selection at a pre-BCR checkpoint. The lysosome-associated protein LAPTM5, which is highly expressed in an animal model of Sjögren's syndrome, plays a role in down-modulation of the pre-BCR. Studies of copy number variation in rheumatoid arthritis suggest that a reduced dosage of the VPREB1 gene is involved in disease pathogenesis. Notably, animal models of autoimmune disease exhibit defects in pre-B to naïve B cell checkpoints. Administration of a pre-BCR ligand, which also plays a role in anergy both in human and murine B lymphocytes, ameliorates disease in experimental models of autoimmunity. Further investigation is required to gain a better insight into the molecular mechanisms of pre-BCR-mediated checkpoints and to determine their relevance to autoimmune diseases.

B-cell receptor, clinical course and prognosis in chronic lymphocytic leukaemia: the growing saga of the IGHV3 subgroup gene usage

The immunoglobulin heavy chain variable gene (IGHV) mutational status has been recognized as an important predictor of prognosis in chronic lymphocytic leukaemia (CLL) since 1999. More recently, other features of the B-cell receptor, such as stereotypy, have been identified as capable of refining the prognostic potential of IGHV status in the clinical assessment of CLL patients. In this context, different genes belonging to the IGHV3 subgroup, the most frequently used subgroup in CLL, have been shown to denote disease subsets that either display a bad prognosis (i.e. IGHV3-21, IGHV3-23) or are associated with particularly good clinical outcomes, including a highly stable/indolent clinical course, even prone to spontaneous regression (i.e. IGHV3-72, IGHV3-30). The present review focuses on the molecular and biological features of CLL-expressing specific genes belonging to the IGHV3 subgroup that are known to mark disease subsets with completely different clinical courses, and may be possibly related to CLL pathogenesis via antigen and/or superantigen involvement.

Similarities and differences between the light and heavy chain Ig variable region gene repertoires in chronic lymphocytic leukemia

Analyses of Ig V(H)DJ(H) rearrangements expressed by B-CLL cells have provided insights into the antigen receptor repertoire of B-CLL cells and the maturation stages of B-lymphocytes that give rise to this disease. However, less information is available about the L chain V gene segments utilized by B-CLL cells and to what extent their characteristics resemble those of the H chain. We analyzed the V(L) and J(L) gene segments of 206 B-CLL patients, paying particular attention to frequency of use and association, mutation status, and LCDR3 characteristics. Approximately 40% of B-CLL cases express V(L) genes that differ significantly from their germline counterparts. Certain genes were virtually always mutated and others virtually never. In addition, preferential pairing of specific V(L) and J(L) segments was found. These findings are reminiscent of the expressed VH repertoire in B-CLL. However unlike the V(H) repertoire, V(L) gene use was not significantly different than that of normal B-lymphocytes. In addition, Vkappa genes that lie more upstream on the germline locus were less frequently mutated than those at the 3' end of the locus; this was not the case for Vlambda genes and is not for V(H) genes. These similarities and differences between the IgH and IgL V gene repertoires expressed in B-CLL suggest some novel features while also reinforcing concepts derived from studies of the IgH repertoire.

Similarities and differences between the light and heavy chain Ig variable region gene repertoires in chronic lymphocytic leukemia

Analyses of Ig V(H)DJ(H) rearrangements expressed by B-CLL cells have provided insights into the antigen receptor repertoire of B-CLL cells and the maturation stages of B-lymphocytes that give rise to this disease. However, less information is available about the L chain V gene segments utilized by B-CLL cells and to what extent their characteristics resemble those of the H chain. We analyzed the V(L) and J(L) gene segments of 206 B-CLL patients, paying particular attention to frequency of use and association, mutation status, and LCDR3 characteristics. Approximately 40% of B-CLL cases express V(L) genes that differ significantly from their germline counterparts. Certain genes were virtually always mutated and others virtually never. In addition, preferential pairing of specific V(L) and J(L) segments was found. These findings are reminiscent of the expressed VH repertoire in B-CLL. However unlike the V(H) repertoire, V(L) gene use was not significantly different than that of normal B-lymphocytes. In addition, Vkappa genes that lie more upstream on the germline locus were less frequently mutated than those at the 3' end of the locus; this was not the case for Vlambda genes and is not for V(H) genes. These similarities and differences between the IgH and IgL V gene repertoires expressed in B-CLL suggest some novel features while also reinforcing concepts derived from studies of the IgH repertoire.

“Altered-self” or “near-self” in the positive selection of lymphocyte repertoires?

Positive selection of lymphocyte repertoires is now recognized as applying to both B and T cells. However, much of the early literature on positive selection focussed on cell-mediated immunity (T cells), which biased consideration of its general biological role. The term "altered-self," which initially captured the idea of self (i.e. MHC) altered by the addition of what was later found to be a peptide fragment, has not proven robust and may now be clouding our understanding. It is recommended that the term "near-self" be reintroduced since it captures the essence of the probable underlying adaptive process-sub-threshold self-reactivity to countermand rapid pathogen mutation.

Regulation and chance in the ontogeny of B and T cell antigen receptor repertoires

The adaptive immune system has to economically generate a large array of T and B cell antigen receptors (T cell receptors [TCRs], B cell receptors [BCRs]) that eliminate both longstanding and novel antigens from the host while preventing the production of deleterious (e.g., autoreactive) antigen receptors. Our studies focus on the mechanisms that shape the development of these antigen receptor repertoires during human ontogeny. The key to BCR and TCR diversity is the third complementarity determining region (CDR3) of the variable domain, which in the immunoglobulin heavy chain and TCR beta chain, is created by the junction between the variable, diversity, and joining gene segments. The CDR3 diversity is constrained by overrepresentation of gene segments and lack of N regions during the first trimester of gestation and then increases exponentially during ontogeny until it reaches adult levels months after birth. This process parallels, and may contribute to, the stepwise acquisition of the ability to respond to specific antigens. Recent studies indicate that maturation of the CDR3 repertoire is not accelerated by premature exposition to extrauterine antigen and thus appears to follow a strictly developmentally regulated program whose pacemaker(s) is still unknown.

Emergence and evolution of functional heavy-chain antibodies in Camelidae

Antibodies of jawed-vertebrates are composed of paired heavy (H) and light (L) polypeptide chains. Surprisingly, the sera of camelids, nurse shark and wobbegong shark, and possibly ratfish contain antibodies that lack L-chains. In camelids, these Heavy-chain antibodies (HCAbs) are gamma-isotypes, and are functional in antigen binding. In this review we focus on the dedicated immunoglobulin (Ig) genes that encode the HCAb in Camelidae (camels, dromedaries and llamas), about their origin, and how these camel immunoglobulins evolved and acquire a large and diverse repertoire of antigen binding sites in absence of the H-L combinatorial diversity.

Deletion of immunoglobulin beta in developing B cells leads to cell death

Inducible gene-targeting experiments have shown that Igmu expression is essential for maintaining survival of mature B cells, but the role of Igmu expression in immature B cell survival has not been determined. To assess whether continued B cell receptor (BCR) expression is required for bone marrow B cell precursor development and survival, we developed a method for conditional gene deletion in these cells. Recombination-activating gene regulatory elements were used to express Igbeta cDNA as a transgene to complement Igbeta(-/-) mice. Transgenic Igbeta expression was found in proB and small preB cells and was extinguished in large preB and immature B cells. Igbeta deletion from large preB cells and immature B cells resulted in cell death that could be rescued by transgenic bcl-2 expression. However, transgenic bcl-2 expression was unable to restore B cell development in the absence of Igbeta. We conclude that Igbeta expression is essential to maintain preB cell and immature B cell survival and to mediate B cell differentiation. In addition, complementation of null mutations with cDNAs under the control of heterologous bacterial artificial chromosomes is a useful method for cell-type-specific and developmentally regulated gene ablation in vivo.

The rearranged V(H) domain of a physiologically selected anti-single-stranded DNA antibody as a precursor for formation of IgM and IgG antibodies to diverse antigens

It has been proposed that autoreactivity of modest affinity contributes to positive selection of a preimmunization B cell repertoire, whereas high-affinity autoreactivity leads to negative selection. This hypothesis predicts that a B cell producing a physiologically selected unmutated ssDNA-binding Ab should be a precursor of cells that respond to diverse exogenous Ags. To test this prediction, we prepared transgenic mice bearing the rearranged V(H) domain of an IgM Ab from a nonautoimmune mouse immunized with a DNA-protein complex, poly(dC)-methylated BSA. The Ab, dC1, binds both poly(dC) and ssDNA. It is encoded by V(H) and V(L) gene segments with no mutations, suggesting that the producing cell may have been selected before and activated during immunization. The dC1V(H) transgene was targeted to the IgH locus. In heterozygous mice, on a nonautoimmune C57BL/6 background, the transgene allotype was expressed on B cell surfaces and in serum Ig, but about one-third of B cells expressed the endogenous allele instead. Total serum Ig concentrations were normal and included both transgene- and endogenous gene-coded IgM and IgG. The transgene V(H) D(H)J(H) was expressed in splenic IgM cDNA with few or no mutations, and in IgG cDNA with multiple mutations. The transgene allotype was also expressed in Abs formed on immunization with thyroglobulin, pneumococcal polysaccharide, and ssDNA-methylated BSA. Consistent with the hypothesis, cells with a rearranged autoreactive V(H) domain selected for reactivity with a form of ssDNA did serve as precursors for cells producing IgM and IgG Abs to diverse Ags.

Immunoglobulin variable region structure and B-cell malignancies

The enormous diversity of immunoglobulin (Ig) variable (V) gene sequences encoding the antibody repertoire are formed by the somatic recombination of relatively few genetic elements. In B-lineage malignancies, Ig gene rearrangements have been widely used for determining clonality and cell origin. The recent development of rapid cloning and sequencing techniques has resulted in a substantial accumulation of IgV region sequences at various stages of B-cell development and has revealed stage-specific trends in the use of V, diversity, joining genes, the degree of noncoding nucleotide addition, and the rate of somatic mutations. Furthermore, sequences from B-lineage malignant cells nearly reflect the characteristics of the normal counterpart at each respective stage of development. Alternatively, from the IgV region structure of the malignant cells, it is possible to speculate at which stage of B-cell development the cells were transformed. As the complete nucleotide sequences of the human Ig heavy and Ig light V region loci have now been determined, the study of Ig genetics has entered into the super-information era.

Predominance of a novel splenic B cell population in mice expressing a transgene that encodes multireactive antibodies: support for additional heterogeneity of the B cell compartment

We generated IgHmudelta transgenic mice using a V(H) gene that in A/J mice encodes multireactive BCR in the preimmune B cell compartment and is predominantly expressed by a memory B cell subpopulation. Most primary splenic B cells in these mice have a size, cell-surface phenotype and in vitro response profile distinct from mature follicular (B2), marginal zone (MZ) or B1 B cells, but are long-lived and appear to be slowly cycling. They reside in conventional B cell areas of the spleen and mount robust foreign antigen-driven germinal center responses, but do not efficiently differentiate to secretory phenotype. We propose that these qualities result from ongoing, low-avidity BCR-self-ligand interactions and promote entry into the memory pathway. Given these data, and the enormous diversity and characteristic multireactivity of the preimmune antibody repertoire, we also suggest that it may be more appropriate to view the primary B cell compartment as a continuum of functional and phenotypic 'layers', rather than as a group of discrete B1, B2 and MZ subsets.

Germline diversity of the expressed BALB/c VhJ558 gene family

Although the mouse immunoglobulin heavy chain (Igh) locus contains 15 heavy chain V (Vh) gene families, at least half of the Vh gene segments are members of the VhJ558 family. This large Vh gene family represents the least characterized germline coding regions of any of the mouse antigen receptor loci and the contribution of individual VhJ558 genes to the preimmune repertoire is poorly understood. In fact, relatively few germline VhJ558 sequences have been reported for BALB/c, the foundation strain for mouse immunoglobulin genetics and the prototypic strain of the Igh(a) haplotype. Here we present a database consisting of 66 sequences estimated to represent one-half of the total number of functional BALB/c VhJ558 genes. Our results indicate that a subset of the VhJ558 genes is highly expressed in the preimmune repertoire, with just nine Vh sequences accounting for nearly 50% of the VhJ558 heavy chains expressed by splenic B cells. We show that this disparity in the expressed Vh gene repertoire is not due to the position of the Vh genes relative to the Dh cluster or to multiple germline copies of the highly expressed VhJ558 genes. Together, these data constitute the first detailed analysis of functional BALB/c VhJ558 genes, demonstrate a striking bias in the use of particular VhJ558 genes in the preimmune repertoire, and provide sufficient information to study the regulation of the Dh-distal region of the Igh-V locus at the level of individual genes.

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.

Recognition of auto- and exoantigens by V4-34 gene encoded antibodies

The antigenic specificities of 24 V4-34-encoded monoclonal antibodies were compared with the amino acid sequence. The specificities were divided into three categories, red blood cells, B lymphocytes and auto/exoantigens. Six anti-I monoclonal antibodies, with multiple substitutions in their VH region, did not bind B lymphocytes or auto/exoantigens. Reactivity to these two antigens segregated with the 16 anti-i monoclonal antibodies, which were derived from the near germline V4-34 gene. All anti-i monoclonal antibodies bound B lymphocytes, albeit with varying intensities. B-cell binding correlated with basic amino acids in the VH-CDR3. Reactivity to auto/exoantigens was demonstrated only by a subset anti-i monoclonal antibodies and did not correlate with B-lymphocyte or i-antigen binding. These anti-ssDNA reactive monoclonal antibodies had basic amino acids in the VH-CDR3, strongly supporting the suggested role of arginine in DNA binding. However, an arginine-rich CDR3 was not enough to ensure DNA reactivity, since six other anti-i monoclonal antibodies that fulfilled this criteria did not bind ssDNA. Thus it is possible that the anti-DNA reactivity of V4-34-encoded monoclonal antibodies is mediated by the classic antigen-binding groove generated by the CDRs of the heavy/light chains. In contrast, anti-B-cell/i-antigen reactivity is mediated, unconventionally, by the V4-34 protein with a dominant influence of the VH-CDR3.

CD5 is A potential selecting ligand for B-cell surface immunoglobulin: a possible role in maintenance and selective expansion of normal and malignant B cells

Although the function of CD5 on B cells is unknown, previous studies suggested that CD5 interaction with V(H) framework regions of surface immunoglobulins (Igs) may contribute to survival and expansion of B cells. Here we used B-chronic lymphocytic leukemia (B-CLL) cells and transformed B-cell lines from normal and B-CLL patients to study CD5-Ig interactions. Immobilized Ig binds and permits isolation of CD5 from lysates of CD5-expressing cell lines. Immunoglobulins or Fab fragments of different V(H) families varied in their effectiveness as inhibitors of anti-CD5 staining of CLL cells, appendix and tonsil tissue sections. Human Ig also binds to purified recombinant CD5. We show here for the first time that the unconventional Ig-CD5 interaction maps to the extracellular CD5-D2 domain whereas conventional epitopes recognized by anti-CD5 antibodies are localized in the D1 domain of CD5. We propose that interactions of VH framework regions with CD5 as a ligand may maintain, select or expand normal, autoimmune or transformed B cells and also contribute to skewing of the normal V(H) repertoire.

CDR3 sequences of MALT lymphoma show homology with those of autoreactive B-cell lines

We have examined the CDR3 sequence and adjacent regions of immunoglobulin genes from B-cell lymphoma of mucosa-associated lymphoid tissue (MALT). Twenty-nine sequences (15 sequences from 13 low-grade MALT lymphomas, marginal zone B-cell lymphomas; 7 sequences from 6 high-grade MALT lymphomas; 7 sequences from 7 diffuse large cell lymphomas) were obtained after cloning of the polymerase chain reaction-amplified segments. In the low-grade MALT, high-grade MALT and diffuse large cell lymphomas, the mean length of the CDR3 region was 47.6+/-10.31 (range 21 to 60), 38.71+/-10.37 (range 27 to 57) and 40.86+/-3.34 (range 39 to 48) nucleotides, respectively. The length of the CDR3 region was significantly greater in the low-grade MALT lymphoma group than in the other two groups. CDR3 sequences in lymphoma cell clones of 14 cases showed 60 to 81% homology with autoantibody-associated lymphocyte clones including rheumatoid factor. The incidences of these autoantibody-associated lymphocyte clones were higher in the high-grade MALT (4/6) and diffuse large lymphomas (5/7) than in the low-grade MALT lymphoma (5/13). Cases with more than 70% homology at the nucleotide level were found to have 71 to 82% homology with autoantibodies at the protein level in the low-grade MALT lymphomas (2/13), and 67% homology in the high-grade MALT lymphomas (2/7). These results indicate that MALT lymphomas may be derived from the malignant transformation of autoreactive B-cells.

Germline Structure and Differential Utilization of Igha and Ighb VH10 Genes

Ab heavy chains encoded by mouse VH10 genes have been of particular interest due to their frequent association with DNA binding. We reported previously that VH10 sequences are over-represented in the preimmune repertoire considering the apparent number of germline-encoded VH10 gene segments. In this report, we show that the VH10 family consists of three and two germline genes in the Igha and Ighb haplotypes, respectively. The complete nucleotide sequences of these five genes, including promoters and recombination signal sequences, were determined and allow unambiguous assignment of allelic relationships. The usage of individual VH10 genes varied significantly and ranged from 0.2% to an extraordinary 7.2% of the VH genes expressed by splenic B cells. Since the promoter and recombination signal sequence elements of all five VH10 genes are identical, we suggest that the few amino acid differences encoded by these five germline VH10 genes determine their representation in the preimmune repertoire. Rearrangements of the most frequently used VH10 gene have an apparent bias for histidine at position 95 of complementarity-determining region-3 (CDR3). These CDR3s are also biased for asparagine, an amino acid associated with the CDRs of DNA binding Abs. Together, these results suggest that high VH10 gene use is the result of B cell receptor-mediated selection and may involve DNA and/or ligands that share antigenic features with DNA.

Atypical VH-D-JH Rearrangements in Newborn Autoimmune MRL Mice

Antinuclear Abs are the hallmark of the autoimmune disease systemic lupus erythematosus (SLE). The ability of self reactive autoantibodies to bind to DNA and nucleosomes is partly conferred by an increased number of arginine and asparagine residues in the heavy chain third complementarity determining region. This increased content of cationic residues is primarily the result of unusual VH-D-JH rearrangements, which include D-D fusions and D gene inversions. While self Ag-driven clonal expansion is a major contributor to the production of antinuclear Abs in lupus, we explore in this study the hypothesis that newly emerging B cells from autoimmune mice display more frequently these unusual VH-D-JH rearrangements. To this end, libraries of PCR-generated VH-D-JH junctions from MRL and C3H newborn livers were analyzed. When compared with the C3H controls, D and JH gene usage in MRL junctions suggests a greater frequency of secondary D-JH rearrangements in this strain. Furthermore, B cells from the autoimmune-prone MRL mice have significantly increased numbers of atypical VH-D-JH rearrangements (D-D fusions and D inversions). Therefore, B cells from MRL mice manifest intrinsic defects that could confer an increased propensity to produce unusual VH-D-JH rearrangements early in ontogeny.

Expressed immunoglobulin repertoire of LPS-stimulated splenocytes of unimmunized mice as studied by two-dimensional gel electrophoresis

The repertoire of isolated immunoglobulin polypeptide chains synthesized by LPS-stimulated splenic B cells from unimmunized 6 weeks old mice was studied by two-dimensional gel electrophoresis. These B cells formed mainly mu heavy chains, while only a small amount of gamma chains was detected on two-dimensional electrophoregrams. The number and character of spots corresponding to each class and type of H and L chains were analyzed. Most of the detected 52 spots, which corresponded to L chains, were well resolved with clearly defined round boundaries. Six of them belonged to two isotypes of lambda chains and the rest to the kappa chain. About 25 clusters corresponded to mu chains. They had different appearance from those of L chains and their characteristic elliptic form with prolonged vertical axes indicated the presence of several H chain variants of slightly different length (due probably to the length variations of CDR3 and carbohydrate heterogeneity) in each cluster. The limited number of spots both of H and L chains is explained as being due to restrictions in the expressed repertoire of preimmune splenic B cells, which have no somatic mutations in the immunoglobulin genes. The concept of macrorepertoire (referring to the relatively small number of detected molecular species) and microrepertoire (describing the mutationally altered molecules) is introduced.

Anti-DNA IgA autoantibodies are spontaneously generated in mouse Peyer's patches

IgA antibodies in the mucosal immune system are produced specifically to environmental antigens such as virus and bacteria, and possibly to some food components, which will provide a potential luminal antigen, DNA. To study the immune response to DNA in the gut, we established B-cell hybridomas producing IgA monoclonal antibodies (mAb) from Peyer's patches (PP) of non-immunized, non-autoimmune, specific pathogen-free BALB/c mice, and examined their specificity by enzyme-linked immunosorbent assay (ELISA). Three mAb out of 18 bound strongly to self, bacterial and synthetic DNA, with Kd of about 10-7 m. One of the three mAb also reacted with the histone component and another reacted with some mouse food component. The VH genes of these three mAb have not previously been reported to have anti-DNA specificity, and carry putative somatically mutated sites favouring DNA binding in CDR. The features resemble those of anti-DNA antibodies found in human and murine models of systemic lupus erythmatosus (SLE), and are indicative of an antigen-driven selection process. Our findings suggest that even in normal healthy animals, anti-DNA antibodies of IgA isotype can be produced in certain peripheral environments such as in PP by spontaneous antigenic stimulation.

Immunoglobulin kappa chain receptor editing in systemic lupus erythematosus

To determine whether receptor editing of Vkappa genes was involved in the pathogenesis of systemic lupus erythematosus (SLE), the usage of Vkappa and Jkappa gene elements from individual peripheral CD19(+) B cells obtained from a patient with untreated SLE was examined. No differences in the Vkappa and Jkappa gene usage in the nonproductive gene repertoire of this SLE patient were noted compared with the distribution of genes found in normal adults. However, an increased usage of Jkappa5 segments, and a significant overrepresentation of the Vkappa1 and Vkappa4 families, especially the L15, O14/O4, and B3 genes characterized the productive Vkappa gene repertoire of the SLE patient. Furthermore, Jkappa5-containing Vkappa gene rearrangements in the productive but not the nonproductive repertoire manifested significantly fewer mutations compared with Vkappa genes recombined with Jkappa1-4. These data are consistent with the conclusion that receptor editing of Vkappa is much more apparent in this SLE patient than in normals and suggest that a deficiency in this means to counteract the emergence of autoimmunity is not an essential feature of SLE.

Autoreactivity of Human VH Domains from cDNA Libraries: Analysis with a Bacterial Expression System

Previous studies showed that VH domains of several anti-DNA Abs can bind DNA in the absence of VL. In the current work, we tested the VH autoreactive potential more generally, examining VH domains that did not come from known autoantibodies. Using a bacterial expression system, we produced 11 fusion proteins, each containing a VH domain and a B domain of staphylococcal protein A. The VH domains were coded in cDNA libraries from circulating B cells of healthy young adult humans. Thus, binding properties of the Ig molecules from which they came were unknown. The B cells had not been stimulated in vitro. Seven cDNA clones combined the frequently expressed VH3-23 gene segment with varied DH and JH segments. The other clones contained unmutated VH3-7, VH3-9, VH3-53, and VH4-39 segments. We compared these bacterial expression products with single-chain Fv, VH and VL domains of IgM mAb 18/2, a VH3-23-encoded, DNA-binding autoantibody. Submicromolar concentrations of 5 of the 11 VH domains bound to ssDNA. Those and one more also bound to immobilized poly(dT), and two bound to circular plasmid dsDNA. Soluble poly(dT) was the most potent inhibitor in competitive ELISA. Seven of the VH domains also bound to immobilized nuclear ribonucleoprotein, four to histone and none to thyroglobulin. Two interacted with the matrix of a Sephacryl S-100 column. The polyreactive autoantigen-binding properties of these VH domains raise the question of whether these properties may play a role in the formation of the VH repertoire of circulating B cells.

Similar characteristics of the CDR3 of V(H)1-69/DP-10 rearrangements in normal human peripheral blood and chronic lymphocytic leukaemia B cells

The variable heavy chain (V(H)) gene segment V(H)1-69/DP-10 has been shown to be over-represented in B-cell chronic lymphocytic leukaemia (CLL). Because of certain similar characteristics of their complementarity determining region 3 (CDR3), including preferential utilization of J(H)6 elements and an extended length, it has been suggested that antigenic stimulation might be involved in leukaemogenesis. Utilizing single-cell PCR to amplify and sequence genomic DNA from individual normal human peripheral blood B cells, we have obtained 7/421 productively and 1/69 nonproductively rearranged V(H) genes that used V(H)1-69/DP-10. All productive rearrangements were unmutated, used J(H)6 and had an average CDR3 length similar to that previously found in V(H)1-69/DP-10-expressing CLL cells. These results suggest that CLL may arise from B cells commonly found in the peripheral B-cell repertoire and do not represent expansion of a unique subset of specific antigen-reactive B cells.

B-cell superantigens may play a role in B-cell development and selection in the young rabbit appendix

In order to develop protective antibodies against a wide range of potentially infectious pathogens, the young rabbit must diversify a limited initial repertoire by somatic mechanisms (the high copy number primary repertoire). The majority of rabbit B cells produce heavy chain variable regions by rearranging the VHa allotype-encoding VH1 gene. Thus in normal rabbits the majority of serum immunoglobulins bear VHa allotype (due to VH1 FR1 and FR3 sequences). The young rabbit appendix is a site of diversification of rearranged VH genes by gene-conversion-like and somatic hypermutation mechanisms. The newly generated B cells probably undergo selection processes that involve foreign and self-antigens and superantigens. We find preferential expansion and survival of B cells in normal and VH-mutant ali/ali rabbits based on their heavy chain FR1 and FR3 sequences (VHa allotype). This selection may involve "superantigen"-like interactions with endogenous as well as exogenous ligands.

Evidence That Amyloidogenic Light Chains Undergo Antigen-Driven Selection

AL amyloidosis is characterized by fibrillar tissue deposits (amyloid) composed of monoclonal light chains secreted by small numbers of indolent bone marrow plasma cells whose ontogenesis is unknown. To address this issue and to provide insights into the processes that accompanied pathogenic light chain formation, we isolated the complete variable (V) regions of 14 light (VL) and 3 heavy (VH) chains secreted by amyloid clones at diagnosis (10 Bence Jones and 4 with complete Igs, 9 λ and 5 κ) by using an inverse polymerase chain reaction-based approach free of primer-induced biases. Amyloid V regions were found to be highly mutated compared with the closest germline genes in the databases or those isolated from the patients' DNA, and mutations were not associated with intraclonal diversification. Apparently high usage of the λIII family germline gene V λIII.1 was observed (4 of 9 λ light chains). Analysis of the nature and distribution of somatic mutations in amyloid V regions showed that there was statistical evidence of antigen selection in 8 of 14 clones (7 in VL and 1 in VH). These results indicate that a substantial proportion of the amyloid clones developed from B cells selected for improved antigen binding properties and that pathogenic light chains show evidence of this selection.

B-cell receptor regulation of peripheral B cells

Recent studies indicate that immature B cells compete with recirculating B cells for survival signals. The signals, delivered through the B-cell receptor for antigen, induce immature cells to differentiate into recirculating cells and maintain the survival of recirculating cells. They do not induce proliferation or differentiation to antibody-producing cells.

Rabbit appendix: a site of development and selection of the B cell repertoire

As early as 1963, it was proposed that the rabbit appendix was a homologue of the chicken bursa of Fabricius (ARCHER et al. 1963). The finding that the young rabbit appendix was thymus independent contributed to the concept of central primary lymphoid tissue. Today we know that appendix is a site that generates the high copy number primary repertoire through diversification of rearranged VH genes by gene conversion-like and somatic hypermutation mechanisms. Thus the appendix of young rabbits functions as a mammalian bursal equivalent. In the appendix, newly generated B cells also undergo selection processes involving self and foreign antigens and superantigens. Preferential expansion and survival of B cells in normal and mutant ali rabbits based on FR1 and FR3 expression may involve "superantigen"-like interactions with endogenous and exogenous ligands. One endogenous ligand appears to be CD5. Additional ligands may be produced by gut flora. Further studies in the rabbit model are needed to determine the fates of emigrants from primary GALT, their sites of postulated self-renewal in the periphery, and the nature of secondary diversification in secondary germinal centers where populations of B lymphocyte memory cells may develop. These data may also be helpful in understanding how the repertoire of human B cells is formed and how this repertoire might be manipulated for clinical benefit.

The differences between the structural repertoires of VH germ-line gene segments of mice and humans: implication for the molecular mechanism of the immune response

Although human and murine antibodies are similar when considering their diversification strategies, they differ in the proportion by which kappa and lambda type chains are present in their receptive V, repertoires. It has been shown that this difference implies a divergence in the structural repertoire of the kappa and lambda genes of these species. Nonetheless, the differences in VH have not been systematically studied. In this paper a systematic characterization of the VH structural repertoire of mice is made, so that a comparison with the VH structural repertoire of humans, described in detail elsewhere, could be properly accomplished. Our study shows the structural repertoire of mice to be dominated by canonical structure class 1-2 (approximately 60%), while in humans the dominant one is class 1-3 (approximately 40%). Analysis of the evolutionary relationships between human and mice suggest that this divergence may have a functional meaning. The implications of such findings are discussed.

B cell life span: a review

Debate has surrounded the subject of B cell life span since it was first measured in mice in the early 1970s. In the 25 years which have passed since then, it has become increasingly apparent that the methods employed to measure rates of B cell turnover, such as [3H]-thymidine labelling, cell transfer or cell ablation, brought about significant disruptions to normal physiology which in themselves might have affected B cell turnover. More recently the use of bromodeoxyuridine has overcome many of these methodological difficulties and has allowed rates of B cell renewal to be measured within B cell subpopulations defined by multiparameter flow cytometry. Such studies have largely resolved the issue, concluding that about 85% of peripheral B cells are phenotypically mature and display first-order exponential kinetics defined by a half-life of 5-6 weeks, whilst the remainder are short-lived with a life span of several days. This review examines both traditional and recent methods and discusses the influence of age, self-tolerance and randomness in the overall shaping of a kinetically stable mature B cell population.

In vivo ablation of surface immunoglobulin on mature B cells by inducible gene targeting results in rapid cell death

Gene targeting experiments have demonstrated that the expression of immunoglobulin heavy chain in the pre-B cell receptor (pBCR) and of heavy and light chains in the B cell antigen receptor (BCR) marks checkpoints in early B cell development that the cells have to pass to survive. To investigate whether the persistence of mature B cells in the peripheral immune system also depends on BCR expression, we have generated a transgenic mouse in which the BCR can be inducibly ablated through V region gene deletion. Ablation leads to rapid death of mature B lymphocytes, which is preceded by down-regulation of MHC antigens and up-regulation of CD95 (Fas) and can be delayed by constitutive bcl-2 expression.

Analysis of IgG subclasses of human antitopoisomerase I autoantibodies suggests chronic B cell stimulation

Antitopoisomerase I autoantibodies are highly specific of scleroderma and are mainly IgG. The present study was designed to evaluate the prevalence of each IgG antitopoisomerase I subclass. An ELISA for the detection of IgG antitopoisomerase I subclasses was standardized and used to study the antibodies from 49 antitopoisomerase I-positive patients identified from a total of 541 patients. Correlations and multivariate analysis were performed to determine the frequency of associations between the IgG antitopoisomerase I subclasses. All IgG antitopoisomerase I subclasses were found. Twelve patients (24.5%) had all four IgG antitopoisomerase I subclasses, 13 (26.5%) had three, 16 (32.7%) had two, and 7 (14.3%) had only one antitopoisomerase I subclass. The presence of all four IgG antitopoisomerase I subclasses suggests that this specific B-cell is the target of multiple activation pathways which indicate that there is a complex T-cell-cytokine-driven process. Together with the absence of other autoantibodies in these sera, our results support the concept of a multiple but highly selected and chronic B-cell activation in scleroderma patients with antitopoisomerase I.

Analysis of the human VH gene repertoire. Differential effects of selection and somatic hypermutation on human peripheral CD5(+)/IgM+ and CD5(-)/IgM+ B cells

To analyze the immunoglobulin repertoire of human IgM+ B cells and the CD5(+) and CD5(-) subsets, individual CD19(+)/ IgM+/CD5(+) or CD5(-) B cells were sorted and non-productive as well as productive VH gene rearrangements were amplified from genomic DNA and sequenced. In both subsets, the VH3 family was overrepresented largely as a result of preferential usage of a small number of specific individual family members. In the CD5(+) B cell subset, all other VH families were found at a frequency expected from random usage, whereas in the CD5(-) population, VH4 appeared to be overrepresented in the nonproductive repertoire, and also negatively selected since it was found significantly less often in the productive compared to the nonproductive repertoire; the VH1 family was significantly diminished in the productive rearrangements of CD5(-) B cells. 3-23/DP-47 was the most frequently used VH gene segment and was found significantly more often than expected from random usage in productive rearrangements of both CD5(+) and CD5(-) B cells. Evidence for selection based on the D segment and the JH gene usage was noted in CD5(+) B cells. No differences were found between the B cell subsets in CDR3 length, the number of N-nucleotides or evidence of exonuclease activity. Somatically hypermutated VHDJH rearrangements were significantly more frequent and extensive in CD5(-) compared to CD5(+) IgM+ B cells, indicating that IgM+ memory B cells were more frequent in the CD5(-) B cell population. Of note, the frequency of specific VH genes in the mutated population differed from that in the nonmutated population, suggesting that antigen stimulation imposed additional biases on the repertoire of IgM+ B cells. These results indicate that the expressed repertoire of IgM+ B cell subsets is shaped by recombinational bias, as well as selection before and after antigen exposure. Moreover, the influences on the repertoires of CD5(+) and CD5(-) B cells are significantly different, suggesting that human peripheral blood CD5(+) and CD5(-) B cells represent different B cell lineages, with similarities to murine B-1a and B-2 subsets, respectively.

Influences on the lifespan of B cell subpopulations defined by different phenotypes

The turnover of mature and immature B cells defined by a range of cell surface markers was investigated by feeding normal or bcl-2-transgenic (bcl-2-Tg) mice 5'-bromo-2-deoxyuridine (BrdUrd) for up to 6 weeks. In peripheral lymphoid tissue, B cells accumulated BrdUrd with a 50% labeling time of 4.3 weeks and a pattern of uptake indicative of the presence of both long-lived and short-lived cells. These two kinetic populations could be resolved into immature B220lo/heat-stable antigen (HSA)hi cells which labeled rapidly, and B220hiHSAlo cells which were uniformly long-lived with a half-life of about 6 weeks. During loading and pulse-chase experiments, BrdUrd uptake by cells within the mature B220hiHSAlo population clearly followed an exponential kinetic pattern, suggesting that their loss was governed by stochastic processes. Using other surface markers, the long-lived population could also be defined by high expression of IgD, representing cells in the follicular mantle zone of the spleen, and by the phenotype IgMhiIgDloHSAlo which most likely represented marginal zone memory B cells. CD23 expression on B cells did not differentiate well between long and short-lived cells. Only about half of newly labeled B cells appearing in the spleen progressed to the long-lived compartment, a proportion which was not altered significantly in bcl-2-Tg mice. The most likely explanation was that a combination of both positive and negative selection was operating at this site which was mediated by pathways not regulated by bcl-2. On the other hand, overexpression of bcl-2 did result in a two- to threefold increase in the rate of appearance of newly labeled B cells in the spleen, consistent with a possible role for this protein during early selection events within the bone marrow. Selection processes appeared to be very active in young mice during the shaping of the B cell repertoire, since B cells from 6-week-old non-Ig mice displayed a rapid rate of turnover irrespective of their surface phenotype, and a significant population of long-lived cells did not become evident until the mice had reached about 12 weeks of age.

CD5 is a potential selecting ligand for B cell surface immunoglobulin framework region sequences

In rabbits nearly all B lymphocytes express the glycoprotein CD5, in contrast to mice and humans, where only a small proportion of B cells express this molecule (Raman, C., and K.L. Knight. 1992. J. Immunol. 149:3858-3864). CD5+ B cells appear to develop early in ontogeny and be maintained throughout life by self-renewal. The function of CD5 on B cells is still unknown. We showed earlier that "positive" selection occurs during B lymphocyte development in the rabbit appendix. This selection favors B cell expressing surface immunoglobulins with VHa2 structures in the first and third framework regions (Pospisil, R., G.O. Young-Cooper, and R.G. Mage. 1995. Proc. Natl. Acad. Sci. USA. 92:6961-6965). Here we report that F(ab')2 fragments, especially those bearing VHa2 framework region determinants, specifically interact with the B cell-surface glycoprotein CD5. This interaction can be inhibited by anti-CD5 antibodies. Furthermore, immobilized F(ab')2 fragments selectively bind CD5 molecules in appendix cell lysates. Interactions of VH framework region structures with CD5 may affect maintenance and selective expansion of particular B cells and thus contribute to autostimulatory growth of autoimmune or transformed cells.

Clonal selection and learning in the antibody system

Each antibody-producing B cell makes antibodies of unique specificity, reflecting a series of ordered gene rearrangements which must be successfully performed if the cell is to survive. A second selection process occurs during immune responses in which a new antibody repertoire is generated through somatic hypermutation. Here only mutants binding antigen with high affinity survive to become memory cells. Cells expressing autoreactive receptors are counter-selected at both stages. This stringent positive and negative selection allows the generation and diversification of cells while rigorously controlling their specificity.

Characterization of human variable domain antibody fragments against the U1 RNA-associated A protein, selected from a synthetic and patient-derived combinatorial V gene library

This is the first study describing recombinant human antibody fragments directed to the U1 RNA-associated A protein (U1A). Three anti-U1A antibody fragments (Fab) were isolated from a semi-synthetic human Fab library and one anti-U1A single-chain variable fragment (scFv) was isolated from a library which was derived from the IgG-positive splenic lymphocytes of an autoimmune patient. Competition studies with autoantibodies against the U1 small nuclear ribonucleoprotein (snRNP) particle from patients with systemic lupus erythematosus (SLE) and SLE-overlap syndromes revealed that U1A binding of these antibody fragments can be inhibited by about 40% of the patient sera. All antibody fragments recognized the native U1 snRNP in immunoprecipitation assays. Two of three Fab clones as well as the scFv clone derived from the repertoire of an autoimmune patient use the same heavy chain germ-line gene DP-65. Epitope mapping revealed that these three clones appear to recognize an identical epitope domain present on the C-terminal RNP motif of the U1A protein. The DP-65 heavy chain gene is used in less than 1% of the B cells in healthy individuals, while three out of four anti-U1A antibody fragments use this gene. This points to a restricted VH gene usage in the case of U1A, suggesting that the DP-65 heavy chain has a natural shape complementarity to the U1A protein.

Heavy chain dominance in the binding of DNA by a lupus mouse monoclonal autoantibody

Antibodies H241 and 2C10 are lupus mouse IgG autoantibodies that bind native DNA. In previous experiments, oligonucleotide antigens affinity-labeled both H and L chains of H241 but only the H chain of antibody 2C10. Primary structures of the V regions of the 2C10 H and L chains and the H241 L chain, determined from cDNA, help to explain the previous affinity-labeling experiments. The 2C10 L chain CDRs had several Asp residues and a net negative charge of five, whereas the 2C10 H chain CDRs had four Arg residues and a net positive charge of five. The L chain CDRs of H241 had a net positive charge of one. [The H241 H chain cDNA sequence was published previously by Gangemi et al. (1993) J. Immun. 151, 4660-4671]. Plasmid vectors were used for bacterial expression of H and L chains of 2C10 alone and in combinations in single chain Fv (scFv) molecules. The H chain alone bound native DNA as well as or better than the H-plus-L chain scFv. The H chain alone also bound Z-DNA. Combination of the 2C10 H chain with the L chain of an anti-Z-DNA antibody maintained the selectivity for Z-DNA, whereas its combination with the 2C10 L chain (in the 2C10 Fab) yielded selective B-DNA binding. The results with 2C10 match other examples in which the H chain is sufficient for DNA binding but selectivity is modulated by the L chain. The H chain binding to autoantigen may reflect selective events in early stages of B cell development.

Analysis of immunoglobulin variable region genes of a human IgM anti-myeloperoxidase antibody derived from a patient with vasculitis

Circulating antibodies to myeloperoxidase (MPO) are associated primarily with pauci-immune glomerulonephritis and systemic vasculitis. Anti-MPO antibodies belong to a group of autoantibodies, anti-neutrophil cytoplasmic antibodies, that may play a pathogenic role in vasculitis. We have generated a human monoclonal anti-MPO antibody (E3-MPO) using peripheral blood lymphocytes from a patient with microscopic polyarteritis. Variable region gene analysis of E3-MPO showed that the VH region had 90% homology with the germ line gene VH4-21. E3-MPO was also shown to carry the 9G4 idiotope, which so far has been associated only with human antibodies that utilize the VH4-21 gene. The 9G4 idiotope was also expressed on anti-MPO antibodies in sera from the donor patient and from 4/7 additional patients with active, untreated vasculitis. The nucleotide sequences of both the variable heavy and light chains of E3-MPO showed evidence of an antigen-driven response.

An 'antigenic ligand competition' model for antigen receptor-mediated lymphocyte selection

The 'antigenic ligand competition' model advanced herein represents a principally novel view on lymphocyte selection, postulating a leading role in this process of competition between different determinants expressed on self ligands for binding to antigen receptors. Based on the data indicating that cell viability and cell growth are separately regulated processes, it is speculated that the binding of antigenic receptors with monovalent determinants (MDs) expressed on soluble self ligands may be responsible for lymphocyte survival, whereas the moderate (but not hyper) cross-linking of antigenic receptors with polyvalent determinants (PDs) expressed on other or the same self ligands may provide signalling that is necessary to trigger lymphocyte proliferation. In the light of the model, the lymphocytes whose receptors bind with high affinity to self MDs survive, while not receiving a proliferative stimulus. On the other hand, those lymphocytes whose receptors interact with high affinity with self PDs and consequently undergo hypercross-linking die by apoptosis. Lastly, those lymphocytes whose receptors interact simultaneously with both MDs and PDs in a balanced competitive manner receive both viability and a proliferative stimulus and, as a consequence, it is only they which obtain selective advantage. The balanced competition between self MDs and self PDs for receptor binding seems likely when the receptors have relatively low affinity to such determinants inasmuch as the opposite, namely comparably high specificity (affinity) to structurally distinct determinants, is unlikely. Essentially, the model presented herein also suggests that a balance between distinct antigenic determinants occupying antigen receptors may determine not only self antigen-driven lymphocyte selection, but also immune reactivity of the functionally mature B- and T-cells which have passed through this selection.

Antigen receptor 'capacity' and the sensitivity of self-tolerance

It seems unlikely that the immune system can actively avoid the generation of self-reactive cells in the primary, pre-selected repertoire. On the contrary, by generating a very high proportion of self-reactive B cells in its primary repertoire, the immune system may be pursuing the most efficient strategy for building a repertoire capable of optimally recognizing nonself, as discussed here by David Nemazee.

The BALB/c mouse B-cell response to pigeon cytochrome c initiates as a heteroclitic response specific for the self antigen mouse cytochrome c

Direct evidence is presented in support of the longstanding but unproven hypothesis that B lymphocytes specific for self antigens (Ags) can be used in the immune response to foreign Ags. We show that the B cells in BALB/c mic responding early to pigeon cytochrome c (CYT) produce antibodies that recognize and bind the major antigenic site on mouse CYT with greater affinity than they bind pigeon CYT i.e., they are heteroclitic for the self Ag. Furthermore, these B cells express the same combination of immunoglobulin variable region (V) genes that are known to be used in B-cell recognition of mouse CYT. Over time, the response to pigeon CYT becomes more specific for the foreign Ag through the recruitment of B cells expressing different combinations of V genes and, possibly, somatic mutation of the mouse CYT specific B cells from early in the response. Cross-recognition of pigeon CYT by mouse CYT-specific B cells results from the sharing of critical amino acid residues by the two Ags. Although B-cell recognition of the self Ag, mouse CYT, is very specific, which limits the extent to which foreign Ags can cross-activate the autoreactive B cells, it is possible that polyreactive B cells to other self Ags may be used more frequently in response to foreign Ags.

Natural autoantibodies

Autoantibodies of the IgM, IgG and IgA classes, reactive with a variety of serum proteins, cell surface structures and intracellular structures, are 'naturally' found in all normal individuals. Present in human cord blood and in 'antigen-free' mice, their variable-region repertoire is selected by antigenic structures in the body and remains conserved throughout life. Encoded by germline genes with no, or few, mutations, natural autoantibodies are characteristically 'multireactive' and do not undergo affinity maturation in normal individuals. Natural autoantibodies may participate in a variety of physiological activities, from immune regulation, homeostasis and repertoire selection, to resistance to infections, transport and functional modulation of biologically active molecules.

Mice lacking terminal deoxynucleotidyl transferase: adult mice with a fetal antigen receptor repertoire

TdT knock-out mice have established the role of this enzyme in vivo: TdT mediates the transition from the relatively limited fetal to the highly diverse adult antigen receptor repertoire by adding template independent "N" nucleotides and disrupting homology-directed recombination. Lacking this source of diversity, TdT degree mice harbor essentially fetal antigen receptor repertoires. In alpha beta TCRs, the TdT null mutation affects the length and diversity of the CDR3 loops thought to be important in "directing" MHC/peptide recognition. N- CDR3 loops appear to wield less influence than do their N+ counterparts--positive selection is more efficient in the TdT degree animals and the peripheral repertiore is more polyreactive and less peptide-oriented than is the N+ repertoire. However, this loss of specificity does not markedly diminish the response to specific peptides. Overall, mice harboring essentially fetal repertoires are robust and effectively respond to a wide variety of challenges to the immune system.