High frequency of somatically mutated IgM molecules in the human adult blood B cell repertoire

Tracking Immunoglobulin Repertoire and Transcriptomic Changes in Germinal Center B Cells by Single-Cell Analysis

In response to T-cell-dependent antigens, mature B cells in the secondary lymphoid organs are stimulated to form germinal centers (GCs), which are histological structures deputed to antibody affinity maturation, a process associated with immunoglobulin gene editing by somatic hypermutation (SHM) and class switch recombination (CSR). GC B cells are heterogeneous and transition across multiple stages before being eliminated by apoptosis or committing to post-GC differentiation as memory B cells or plasma cells. In order to explore the dynamics of SHM and CSR during the GC reaction, we identified GC subpopulations by single-cell (sc) transcriptomics and analyzed the load of immunoglobulin variable (V) region mutations as well as the isotype class distribution in each subpopulation. The results showed that the large majority of GC B cells display a quantitatively similar mutational load in the V regions and analogous IGH isotype class distribution, except for the precursors of memory B cells (PreM) and plasma cells (PBL). PreM showed a bimodal pattern with about half of the cells displaying high V region germline identity and enrichment for unswitched IGH, while the rest of the cells carried a mutational load similar to the bulk of GC B cells and showed a switched isotype. PBL displayed a bias toward expression of IGHG and higher V region germline identity compared to the bulk of GC B cells. Genes implicated in SHM and CSR were significantly induced in specific GC subpopulations, consistent with the occurrence of SHM in dark zone cells and suggesting that CSR can occur within the GC.

Memory B cells and plasma cells: The differentiative continuum of humoral immunity

Immunological memory is a composite of lasting antibody titers maintained by plasma cells in conjunction with memory T and B cells. Memory B cells are a critical reservoir for plasma cell generation in the secondary response. Identification of memory B cells requires that they be distinguished from naïve, activated, and germinal center precursors and from plasma cells. Memory B cells are heterogeneous in isotype usage, immunoglobulin mutational content, and phenotypic marker expression. Phenotypic subsets of memory B cells are defined by PD-L2, CD80, and CD73 expression in mice, by CD27 and FCRL4 expression in humans and by T-bet in both mice and humans. These subsets display marked functional heterogeneity, including the ability to rapidly differentiate into plasma cells versus seed germinal centers in the secondary response. Memory B cells are located in the spleen, blood, other lymphoid organs, and barrier tissues, and recent evidence indicates that some memory B cells may be dedicated tissue-resident populations. Open questions about memory B cell longevity, renewal and progenitor-successor relationships with plasma cells are discussed.

Role of Polymeric Immunoglobulin Receptor in IgA and IgM Transcytosis

Transcytosis of polymeric IgA and IgM from the basolateral surface to the apical side of the epithelium and subsequent secretion into mucosal fluids are mediated by the polymeric immunoglobulin receptor (pIgR). Secreted IgA and IgM have vital roles in mucosal immunity in response to pathogenic infections. Binding and recognition of polymeric IgA and IgM by pIgR require the joining chain (J chain), a small protein essential in the formation and stabilization of polymeric Ig structures. Recent studies have identified marginal zone B and B1 cell-specific protein (MZB1) as a novel regulator of polymeric IgA and IgM formation. MZB1 might facilitate IgA and IgM transcytosis by promoting the binding of J chain to Ig. In this review, we discuss the roles of pIgR in transcytosis of IgA and IgM, the roles of J chain in the formation of polymeric IgA and IgM and recognition by pIgR, and focus particularly on recent progress in understanding the roles of MZB1, a molecular chaperone protein.

Structure, Function, and Therapeutic Use of IgM Antibodies

Natural immunoglobulin M (IgM) antibodies are pentameric or hexameric macro-immunoglobulins and have been highly conserved during evolution. IgMs are initially expressed during B cell ontogeny and are the first antibodies secreted following exposure to foreign antigens. The IgM multimer has either 10 (pentamer) or 12 (hexamer) antigen binding domains consisting of paired µ heavy chains with four constant domains, each with a single variable domain, paired with a corresponding light chain. Although the antigen binding affinities of natural IgM antibodies are typically lower than IgG, their polyvalency allows for high avidity binding and efficient engagement of complement to induce complement-dependent cell lysis. The high avidity of IgM antibodies renders them particularly efficient at binding antigens present at low levels, and non-protein antigens, for example, carbohydrates or lipids present on microbial surfaces. Pentameric IgM antibodies also contain a joining (J) chain that stabilizes the pentameric structure and enables binding to several receptors. One such receptor, the polymeric immunoglobulin receptor (pIgR), is responsible for transcytosis from the vasculature to the mucosal surfaces of the lung and gastrointestinal tract. Several naturally occurring IgM antibodies have been explored as therapeutics in clinical trials, and a new class of molecules, engineered IgM antibodies with enhanced binding and/or additional functional properties are being evaluated in humans. Here, we review the considerable progress that has been made regarding the understanding of biology, structure, function, manufacturing, and therapeutic potential of IgM antibodies since their discovery more than 80 years ago.

Related Mechanisms of Antibody Somatic Hypermutation and Class Switch Recombination

The primary antibody repertoire is generated by mechanisms involving the assembly of the exons that encode the antigen-binding variable regions of immunoglobulin heavy (IgH) and light (IgL) chains during the early development of B lymphocytes. After antigen-dependent activation, mature B lymphocytes can further alter their IgH and IgL variable region exons by the process of somatic hypermutation (SHM), which allows the selection of B cells in which SHMs resulted in the production of antibodies with increased antigen affinity. In addition, during antigen-dependent activation, B cells can also change the constant region of their IgH chain through a DNA double-strand-break (DSB) dependent process referred to as IgH class switch recombination (CSR), which generates B cell progeny that produce antibodies with different IgH constant region effector functions that are best suited for a elimination of a particular pathogen or in a particular setting. Both the mutations that underlie SHM and the DSBs that underlie CSR are initiated in target genes by activation-induced cytidine deaminase (AID). This review describes in depth the processes of SHM and CSR with a focus on mechanisms that direct AID cytidine deamination in activated B cells and mechanisms that promote the differential outcomes of such cytidine deamination.

Complexity of the human memory B-cell compartment is determined by the versatility of clonal diversification in germinal centers

Our knowledge about the clonal composition and intraclonal diversity of the human memory B-cell compartment and the relationship between memory B-cell subsets is still limited, although these are central issues for our understanding of adaptive immunity. We performed a deep sequencing analysis of rearranged immunoglobulin (Ig) heavy chain genes from biological replicates, covering more than 100,000 memory B lymphocytes from two healthy adults. We reveal a highly similar B-cell receptor repertoire among the four main human IgM(+) and IgG(+) memory B-cell subsets. Strikingly, in both donors, 45% of sequences could be assigned to expanded clones, demonstrating that the human memory B-cell compartment is characterized by many, often very large, B-cell clones. Twenty percent of the clones consisted of class switched and IgM(+)(IgD(+)) members, a feature that correlated significantly with clone size. Hence, we provide strong evidence that the vast majority of Ig mutated B cells--including IgM(+)IgD(+)CD27(+) B cells--are post-germinal center (GC) memory B cells. Clone members showed high intraclonal sequence diversity and high intraclonal versatility in Ig class and IgG subclass composition, with particular patterns of memory B-cell clone generation in GC reactions. In conclusion, GC produce amazingly large, complex, and diverse memory B-cell clones, equipping the human immune system with a versatile and highly diverse compartment of IgM(+)(IgD(+)) and class-switched memory B cells.

Translating transitions - how to decipher peripheral human B cell development

During the last two decades our understanding of human B cell differentiation has developed considerably. Our understanding of the human B cell compartment has advanced from a point where essentially all assays were based on the presence or not of class-switched antibodies to a level where a substantial diversity is appreciated among the cells involved. Several consecutive transitional stages that newly formed IgM expressing B cells go through after they leave the bone marrow, but before they are fully mature, have been described, and a significant complexity is also acknowledged within the IgM expressing and class-switched memory B cell compartments. It is possible to isolate plasma blasts in blood to follow the formation of plasma cells during immune responses, and the importance and uniqueness of the mucosal IgA system is now much more appreciated. Current data suggest the presence of at least one lineage of human innate-like B cells akin to B1 and/or marginal zone B cells in mice. In addition, regulatory B cells with the ability to produce IL-10 have been identified. Clinically, B cell depletion therapy is used for a broad range of conditions. The ability to define different human B cell subtypes using flow cytometry has therefore started to come into clinical use, but as our understanding of human B cell development further progresses, B cell subtype analysis will be of increasing importance in diagnosis, to measure the effect of immune therapy and to understand the underlying causes for diseases. In this review the diversity of human B cells will be discussed, with special focus on current data regarding their phenotypes and functions.

Epstein-Barr virus infection of naïve B cells in vitro frequently selects clones with mutated immunoglobulin genotypes: implications for virus biology

Epstein-Barr virus (EBV), a lymphomagenic human herpesvirus, colonises the host through polyclonal B cell-growth-transforming infections yet establishes persistence only in IgD⁺ CD27⁺ non-switched memory (NSM) and IgD⁻ CD27⁺ switched memory (SM) B cells, not in IgD⁺ CD27⁻ naïve (N) cells. How this selectivity is achieved remains poorly understood. Here we show that purified N, NSM and SM cell preparations are equally transformable in vitro to lymphoblastoid cells lines (LCLs) that, despite upregulating the activation-induced cytidine deaminase (AID) enzyme necessary for Ig isotype switching and Ig gene hypermutation, still retain the surface Ig phenotype of their parental cells. However, both N- and NSM-derived lines remain inducible to Ig isotype switching by surrogate T cell signals. More importantly, IgH gene analysis of N cell infections revealed two features quite distinct from parallel mitogen-activated cultures. Firstly, following 4 weeks of EBV-driven polyclonal proliferation, individual clonotypes then become increasingly dominant; secondly, in around 35% cases these clonotypes carry Ig gene mutations which both resemble AID products and, when analysed in prospectively-harvested cultures, appear to have arisen by sequence diversification in vitro. Thus EBV infection per se can drive at least some naïve B cells to acquire Ig memory genotypes; furthermore, such cells are often favoured during an LCL's evolution to monoclonality. Extrapolating to viral infections in vivo, these findings could help to explain how EBV-infected cells become restricted to memory B cell subsets and why EBV-driven lymphoproliferative lesions, in primary infection and/or immunocompromised settings, so frequently involve clones with memory genotypes.

Epstein-Barr virus (EBV), a growth-transforming virus linked to several B cell lymphomas in man, is usually carried as an asymptomatic latent infection in B lymphocytes. Such virus carriage selectively involves memory, but not naive, B cells. How this selectivity is achieved is poorly understood since we find that naive and memory cell types are equally susceptible to infection and growth transformation to lymphoblastoid cell lines in vitro. Here we ask if EBV-transformation of purified naïve B cells can induce key features of memory cells, namely immunoglobulin (Ig) class switching and Ig gene mutation. We find that EBV does not induce Ig class switching (though the infected cells remain responsive to exogenous switch signals) but can induce Ig gene mutation. Thus, within 4 weeks of infecting naive B cell preparations, one can often detect cells carrying Ig mutations which appear to have arisen by somatic hypermutation in vitro. Furthermore, in many cases such cells become dominant during clonal evolution of the emergent EBV-transformed cell line. Overall these findings suggest a possible explanation as to why EBV is selectively found in memory B cell populations in vivo and why EBV-positive lymphoproliferative lesions/lymphomas so frequently involve clones with mutated Ig genotypes.

Glycosylation of surface Ig creates a functional bridge between human follicular lymphoma and microenvironmental lectins

Surface Ig (sIg) of follicular lymphoma (FL) is vital for tumor cell survival. We found previously that the Ig in FL is unusual, because the variable region genes carry sequence motifs for N-glycan addition. These are introduced by somatic mutation and are tumor specific. Unexpectedly, added glycans terminate at high mannose, suggesting a potentially important interaction of FL cells with mannose-binding lectins of the innate immune system. We have now identified mannosylated IgM at the surface of primary lymphoma cells. Recombinant lectin domains of the mannose receptor (MR) or DC-SIGN bind mannosylated Igs in vitro and bind to FL cells, signaling sIgM-associated increases in intracellular Ca(2+). Lectins also bind to normal B cells but fail to signal. In contrast, anti-Ig signaled similarly in both FL and normal B cells. Mannosylation patterns were mimicked by FL Ig-derived single-chain Fvs (scFv), providing probes for potential receptors. Mannosylated scFv bound specifically to the lectin domains of the MR and DC-SIGN and blocked signaling. Mannosylated scFv also bound to DC-SIGN on the surface of dendritic cells. This unique lymphoma-specific interaction of sIg with lectins of innate immunity reveals a potential route for microenvironmental support of tumor cells, mediated via the key B-cell receptor.

How do natural killer T cells help B cells?

CD1d-restricted natural killer (NKT) cells are important contributors to antigen-specific antibody responses. There is, therefore, considerable interest in the design and implementation of strategies to appropriately activate NKT cells and boost vaccine-induced protective antibody responses. In order to achieve these goals, investigators are examining the mechanisms by which NKT cells enhance antibody responses. Although information is limited, it is now appreciated that both cognate and noncognate interactions between CD1d-expressing B cells and NKT cells drive enhanced antibody responses. NKT cells may provide B-cell help in the form of direct receptor-mediated interactions as well as by secretion of soluble effectors, including cytokines. In this article, we review the evidence in support of these mechanisms and discuss how they likely take place in the context of interactions of NKT cells with other cell types, such as dendritic cells and helper T cells. We also discuss the evidence that NKT cells affect discrete differentiation events in the multistep process by which a naive B cell experiences antigen and develops into a memory B cell or an antibody-secreting plasma cell. Since most information on NKT cells and humoral immunity has been derived from murine studies, we discuss what is known about human NKT cells and humoral immunity. We offer thoughts on whether the findings in murine systems will translate to humans.

Human marginal zone B cells

Human marginal zone (MZ) B cells are, in a sense, a new entity. Although they share many properties with their mouse counterpart, they also display striking differences, such as the capacity to recirculate and the presence of somatic mutations in their B cell receptor. These differences are the reason they are often not considered a separate, rodent-like B cell lineage, but rather are considered IgM memory B cells. We review here our present knowledge concerning this subset and the arguments in favor of the proposition that humans have evolved for their MZ B cell compartment a separate B cell population that develops and diversifies its Ig receptor during ontogeny outside T-dependent or T-independent immune responses.

T cell-independent development and induction of somatic hypermutation in human IgM+ IgD+ CD27+ B cells

IgM⁺IgD⁺CD27⁺ B cells from peripheral blood have been described as circulating marginal zone B cells. It is still unknown when and where these cells develop. These IgM⁺IgD⁺CD27⁺ B cells exhibit somatic hypermutations (SHMs) in their B cell receptors, but the exact nature of the signals leading to induction of these SHMs remains elusive. Here, we show that IgM⁺IgD⁺CD27⁺ B cells carrying SHMs are observed during human fetal development. To examine the role of T cells in human IgM⁺IgD⁺CD27⁺ B cell development we used an in vivo model in which Rag2^−/−γ_C^−/− mice were repopulated with human hematopoietic stem cells. Using Rag2^−/−γ_C^−/− mice on a Nude background, we demonstrated that development and induction of SHMs of human IgM⁺IgD⁺CD27⁺ B cells can occur in a T cell–independent manner.

Systematic comparison of gene expression between murine memory and naive B cells demonstrates that memory B cells have unique signaling capabilities

Memory B cells play essential roles in the maintenance of long-term immunity and may be important in the pathogenesis of autoimmune disease, but how these cells are distinguished from their naive precursors is poorly understood. To address this, it would be important to understand how gene expression differs between memory and naive B cells to elucidate memory-specific functions. Using model systems that help overcome the lack of murine memory-specific markers and the low frequency of Ag-specific memory and naive cells, we undertook a global comparison of gene expression between memory B cells and their naive precursors. We identified genes with differential expression and confirmed the differential expression of many of these by quantitative RT-PCR and of some of these at the protein level. Our initial analysis revealed differential expression patterns of genes that regulate signaling. Memory B cells have increased expression of genes important in regulating adenosine signaling and in modulating cAMP responses. Furthermore, memory B cells up-regulate receptors that are essential for embryonic stem cell self-renewal. We further demonstrate that one of these, leukemia inhibitory factor receptor, can initiate functional signaling in memory B cells whereas it does not in naive B cells. Thus, memory and naive B cells are intrinsically wired to signal differently from one another and express a functional signaling pathway that is known to maintain stem cells in other lineages.

Human IgM+CD27+ B cells: memory B cells or "memory" B cells?

Memory B cells are generated in germinal centers (GC) and contribute to serological immunity by rapidly differentiating into plasma cells. Human memory B cells can be identified by the expression of CD27. These cells exhibit more rapid responses than naive (CD27-) B cells following stimulation in vitro, consistent with the heightened kinetics of secondary responses in vivo. CD27+ B cells express mutated Ig V region genes; however a significant proportion continue to express IgM, suggesting the existence of IgM+ memory B cells. The observation that mutated IgM+CD27+ B cells are generated in humans who cannot form GC led to the conclusions that these cells are generated independently of GC and thus are not memory cells and that they mediate responses to T cell-independent Ag. Although some studies support the idea that IgM+CD27+ B cells participate in T cell-independent responses, many others do not. In this review we will provide alternate interpretations of the biology of IgM+CD27+ B cells and propose that they are indeed memory cells.

Antibody repertoire development in fetal and neonatal piglets. XIII. Hybrid VH genes and the preimmune repertoire revisited

The expressed porcine VH genes belong to the VH3 family (clan), four of which, VHA, VHB, VHC, and VHE, alone comprise approximately 80% of the preimmune repertoire. However, so-called "hybrid" VH genes that use CDR1 of one VH gene and the CDR2 of another are frequently encountered. We studied > 3000 cloned VDJs and found that such hybrids can contribute up to 10% of the preimmune repertoire. Based on the 1) recovery of hybrid VH genes from bacterial artificial chromosome clones, 2) frequency of occurrence of certain hybrids in the preimmune repertoire, and 3) failure to recover equal numbers of reciprocal hybrids, we concluded that some chimeric genes are present in the genome and are not PCR artifacts. Two chimeric germline genes (VHZ and VHY), together with VHF and the four genes mentioned above, constitute the major VH genes and these account for > 95% of the preimmune repertoire. Diversification of the preimmune IgG and IgM repertoires after environmental exposure was mainly due to somatic hypermutation of major VH genes with no evidence of gene conversion. Somatic hypermutation was 3- to 10-fold higher in CDRs than in framework regions, most were R mutations and transversions and transitions equally contributed. Data were used to 1) develop an index to quantify the degree of VH repertoire diversification and 2) establish a library of 29 putative porcine VH genes. One-third of these genes are chimeric genes and their sequences suggest that the porcine VH genome developed by duplication and splicing from a small number of prototypic genes.

Rotavirus-specific CD5+ B cells in young children exhibit a distinct antibody repertoire compared with CD5- B cells

Antiviral antibody responses in infants are limited in quality. One reason for this finding could be that the majority of B cells in infants are CD5+ cells, a subset of B cells that is thought to contain cells expressing polyreactive, low-affinity B cell receptors. We analyzed the rotavirus (RV)-specific antibody heavy chain variable region (VH) repertoire in CD5+ and CD5- B cells of four RV-infected children between 10 and 19 months of age. We found that the RV-specific B cell repertoire in CD5+ cells was VH3 family biased, in contrast to the VH1/VH4 dominance seen in CD5- B cells. The immunodominant RV-specific gene segment in CD5- B cells was VH1-46, which is the dominant segment used in RV-specific peripheral blood B cells from infants and adults. In contrast, the immunodominant gene segment was VH3-23 in RV-specific CD5+ B cells, which is the dominant gene segment in randomly selected B cells. Both RV-specific CD5+ and RV-specific CD5- B cells from all children studied demonstrated very low frequencies of somatic mutations. In conclusion, CD5+ B cells in infants responding to RV use an antibody gene repertoire that differs from the virus-specific repertoire of CD5- B cells, and both CD5+ and CD5- RV-specific B cells exhibit a low frequency of somatic mutations.

Human blood IgM "memory" B cells are circulating splenic marginal zone B cells harboring a prediversified immunoglobulin repertoire

The human peripheral B-cell compartment displays a large population of immunoglobulin M-positive, immunoglobulin D-positive CD27(+) (IgM(+)IgD(+)CD27(+)) "memory" B cells carrying a mutated immunoglobulin receptor. By means of phenotypic analysis, complementarity-determining region 3 (CDR3) spectratyping during a T-independent response, and gene-expression profiling of the different blood and splenic B-cell subsets, we show here that blood IgM(+)IgD(+)CD27(+) cells correspond to circulating splenic marginal zone B cells. Furthermore, analysis of this peripheral subset in healthy children younger than 2 years shows that these B cells develop and mutate their immunoglobulin receptor during ontogeny, prior to their differentiation into T-independent antigen-responsive cells. It is therefore proposed that these IgM(+)IgD(+)CD27(+) B cells provide the splenic marginal zone with a diversified and protective preimmune repertoire in charge of the responses against encapsulated bacteria.

Visualization of the genesis and fate of isotype-switched B cells during a primary immune response

The life history of isotype-switched B cells is unclear, in part, because of an inability to detect rare antigen-specific B cells at early times during the immune response. To address this issue, a small population of B cells carrying targeted antibody transgenes capable of class switching was monitored in immunized mice. After contacting helper T cells, the first switched B cells appeared in follicles rather than in the red pulp, as was expected. Later, some of the switched B cells transiently occupied the red pulp and marginal zone, whereas others persisted in germinal centers (GCs). Antigen-experienced IgM B cells were rarely found in GCs, indicating that these cells switched rapidly after entering GCs or did not persist in this environment.

Diversification of Ig heavy chain genes in human preterm neonates prematurely exposed to environmental antigens

Preterm neonates are exposed to extrauterine environmental Ags during the time period that corresponds to the last trimester of normal intrauterine development. To study whether this precocious exposure to Ags accelerates the Ig repertoire diversification, we compared IgH chain genes of preterm neonates (gestational age, 25-29 wk) during their first postnatal months with those of term neonates. Preterm infants approaching their expected date of delivery after 8-13 wk of extrauterine life used a similar V(H), D(H), and J(H) gene segment repertoire as term neonates born after intrauterine development. Furthermore, the length increase of the NDN region between V(H) and J(H) by 0.25 nt per gestational week (r = 0.556, p < 0.0001) was not accelerated. Thus, the generation of the V(H) region gene repertoire is developmentally controlled and independent of environmental influences. However, exposure to extrauterine Ags induced class switch and somatic mutations in IgH chain genes within 2 wk after premature birth and IgG transcript diversity and mutational frequency increased with the duration of extrauterine life. Three-month-old preterm infants expressed a heterogeneous IgG repertoire at their expected date of delivery with V(H) region genes carrying significant numbers of somatic mutations with evidence for Ag selection. Term neonates, however, had no such IgG repertoire. We conclude that restrictions in the neonatal Ig V(H) region gene repertoire persist until term despite exposure to environmental Ags. Yet, many weeks before term the immune system of the preterm neonate can already support germinal center reactions in response to environmental Ags.

Analysis of microsatellite instability and hypermutation of immunoglobulin variable genes in Werner syndrome

Werner syndrome (WS) is a human premature aging syndrome, which is associated with high frequencies of neoplasia and genetic instability. We have examined the occurrence of microsatellite instability, which may result from defective mismatch repair, in lymphoblastoid cell lines derived from nine WS patients. Instability was measured at the D2S123 locus by gel analysis of PCR products. Three WS cell lines had 4-13% altered alleles, compared with 0% in the other six lines. The increased frequency of microsatellite instability could not readily be associated with overt cancer or any other known clinical condition in the three patients. To examine whether the WS defect affected the humoral immune system, we measured the hypermutation of immunoglobulin variable genes in peripheral blood cells from the WS patient who donated the cell line with the highest frequency of microsatellite instability. The frequency and pattern of mutation was similar to that from normal individuals, suggesting that the Werner protein is not involved in generating hypermutation.

Third complementarity-determining region of mutated VH immunoglobulin genes contains shorter V, D, J, P, and N components than non-mutated genes

The third complementarity-determining region (CDR3) of immunoglobulin variable genes for the heavy chain (VH) has been shown to be shorter in length in hypermutated antibodies than in non-hypermutated antibodies. To determine which components of CDR3 contribute to the shorter length, and if there is an effect of age on the length, we analysed 235 cDNA clones from human peripheral blood of VH6 genes rearranged to immunoglobulin M (IgM) constant genes. There was similar use of diversity (D) and joining (JH) gene segments between clones from young and old donors, and there was similar use of D segments among the mutated and non-mutated heavy chains. However, in the mutated heavy chains, there was increased use of shorter JH4 segments and decreased use of longer JH6 segments compared to the non-mutated proteins. The overall length of CDR3 did not change with age within the mutated and non-mutated categories, but was significantly shorter by three amino acids in the mutated clones compared to the non-mutated clones. Analyses of the individual components that comprise CDR3 indicated that they were all shorter in the mutated clones. Thus, there were more nucleotides deleted from the ends of VH, D, and JH gene segments, and fewer P and N nucleotides added. The results suggest that B cells bearing immunoglobulin receptors with shorter CDR3s have been selected for binding to antigen. A smaller CDR3 may allow room in the antibody binding pocket for antigen to interact with CDRs 1 and 2 as well, so that as the VDJ gene undergoes hypermutation, substitutions in all three CDRs can further contribute to the binding energy.

Impact of age on hypermutation of immunoglobulin variable genes in humans

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

CD40-CD40L independent Ig gene hypermutation suggests a second B cell diversification pathway in humans

Somatically mutated IgM(+)-only and IgM(+)IgD(+)CD27(+) B lymphocytes comprise approximately 25% of the human peripheral B cell pool. These cells phenotypically resemble class-switched B cells and have therefore been classified as postgerminal center memory B cells. X-linked hyper IgM patients have a genetic defect characterized by a mutation of the CD40L gene. These patients, who do not express a functional CD40 ligand, cannot switch Ig isotypes and do not form germinal centers and memory B cells. We report here that an IgM(+)IgD(+)CD27(+) B cell subset with somatically mutated Ig receptors is generated in these patients, implying that these cells expand and diversify their Ig receptors in the absence of classical cognate T-B collaboration. The presence of this sole subset in the absence of IgM(+)-only and switched CD27(+) memory B cells suggests that it belongs to a separate diversification pathway.

Analysis of immunoglobulin (Ig) isotype diversity and IgM/D memory in the response to phenyl-oxazolone

The distribution of immunoglobulin (Ig) isotypes within specific B cell clones in vivo after immunization is not well defined. Using an IgV(H)/CDR3- and isotype-specific reverse transcription polymerase chain reaction method, we have carried out a survey of the diversification of the isotype in a splenic response to phenyl-oxazolone (phOx) on a chicken serum albumin carrier. The phOx-specific V(H) (V(H)Ox-1 with specific CDR3 motif) is associated with all of the heavy chains (mu, delta, alpha, gamma, and epsilon) after simple immunization with antigen in alum. The kinetics of expression of each isotype are distinct and reproducible. Focusing mainly on the expression of secretory Ig transcripts, IgM, IgG1, and IgE are found after priming, whereas IgD and IgA appear after boosting. Secretory IgD transcripts are found reproducibly at moderate levels and may, therefore, contribute significantly to the secreted Ig response in mice. Most crucially, we find enhanced levels of secretory IgM/V(H)Ox-1 transcripts (with 'phOx-specific' CDR3) after boosting, strongly indicating the existence of IgM memory cells that give rise to an enhanced specific IgM secretion in the secondary response.

Identification of murine germinal center B cell subsets defined by the expression of surface isotypes and differentiation antigens

Germinal centers (GCs) are inducible lymphoid microenvironments that support the generation of memory B cells, affinity maturation, and isotype switching. Previously, phenotypic transitions following in vivo B cell activation have been exploited to discriminate GC from non-GC B cells in the mouse and to delineate as many as seven distinct human peripheral B cell subsets. To better understand the differentiative processes occurring within murine GCs, we sought to identify subpopulations of GC B cells corresponding to discrete stages of GC B cell ontogeny. We performed multiparameter flow-cytometric analyses of GC B cells at consecutive time points following immunization of BALB/c mice with SRBC. We resolved the murine GC compartment into subsets based on the differential expression of activation markers, surface Ig isotypes, and differentiation Ags. Class-switched and nonswitched GC B cells emerged contemporaneously, and their relative frequencies remained nearly constant throughout the GC reaction, perhaps reflecting the establishment of a steady state. A significant percentage of the nonswitched B cells with a GC phenotype exhibited surface markers associated with naive B cells, including CD23, surface IgD, and high levels of CD38 consistent with either prolonged recruitment into the GC reaction or protracted expression of these markers during differentiation within the GC. Expression of the activation marker BLA-1 was dynamic over time, with all GC B cells being positive early after immunization, followed by progressive loss as the GC reaction matured into the second and third week. Implications of these results concerning GC evolution are discussed.

The influence of CD40-CD154 interactions on the expressed human V(H) repertoire: analysis of V(H) genes expressed by individual B cells of a patient with X-linked hyper-IgM syndrome

Analysis of the V(H)DJ(H) repertoire of peripheral blood IgM(+) B cells from a patient with X-linked hyper-IgM syndrome (X-HIgM) was undertaken to determine whether the distribution of V(H) families in the productive repertoire might be regulated by in vivo CD40-CD154 interactions. The distribution of V(H) genes in the non-productive repertoire of IgM(+) B cells was comparable in X-HIgM and normals. Unlike the normal productive V(H) repertoire, however, in the X-HIgM patient the V(H)4 family was found at almost the same frequency as the V(H)3 family. This reflected a diminution in the positive selection of the V(H)3 family observed in normals and the imposition of positive selection of the V(H)4 family in the X-HIgM patient. Unique among the V(H)3 genes, V(H)3-23/DP-47 was positively selected in both normals and the X-HIgM patient. No major differences in the usage of J(H) or D segments or the complementarity-determining region (CDR) 3 were noted, although the foreshortening of the CDR3 noted in the mutated V(H) rearrangements of normals was absent in the X-HIgM patient. Finally, a minor degree of somatic hypermutation was noted in the X-HIgM patient. These results have suggested that specific influences on the composition of the V(H) repertoire in normals require CD40-CD154 interactions.

Characteristics of IgVH genes used by human intestinal plasma cells from childhood

Plasma cells secreting immunoglobulin M (IgM) and IgA in human intestinal mucosa are the largest antibody-producing population in the human body. Despite this there have been relatively few studies of the characteristics and maturation of the genes which encode the mucosal immunoglobulins. We have previously demonstrated that intestinal plasma cells use highly mutated IgVH genes, likely to reflect germinal centre origin. Here we show that IgVH genes used by intestinal lamina propria plasma cells secreting IgM and IgA are highly mutated from childhood, with no change in the frequency of mutation through to adulthood, though IgVH genes used by IgM are significantly less mutated than those used by IgA. There was no difference between the IgA subclasses in either the frequency or distribution of mutations. The frequency of mutation in IgVH4-34 genes used by IgG was also studied in the adult biopsies, and was found to be of the same order as that observed in IgA and was significantly higher than that observed in IgM. We have identified IgM and IgA sequences which share identical CDR3 and distribution of mutations. Isotype switching may therefore occur after extensive mutation of IgM sequences, and IgM- and IgA-secreting plasma cells with the same specificity may occur within the same microenvironment. IgM should therefore be considered to be a component of secondary immune responses in the gut.

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.

The spleen in the Wiskott-Aldrich syndrome: histopathologic abnormalities of the white pulp correlate with the clinical phenotype of the disease

The Wiskott-Aldrich syndrome (WAS) is a X-linked hematologic disorder characterized by thrombocytopenia, eczema, and immunodeficiency of variable severity. Reported here are the results of a morphologic, morphometric, and immunophenotypic analysis of splenic lymphoid tissue in 12 WAS patients with documented molecular defect and with different disease severity. Spleens from 29 age-matched patients with different diseases were used as controls. Paraffin-embedded tissue (from all cases) and fresh-frozen samples (from 5 WAS patients and 4 control subjects) were used to study the different white pulp compartments by classic morphologic, immunophenotyping, and image analysis techniques. Data were statistically analyzed by both parametric and nonparametric tests. Spleens from WAS patients showed a significant depletion of the total white pulp (p = 0.0008), T cell (p < 0.05), and B cell (p = 0.0002) areas and marginal zone (MZ) thickness (p < 0.0001). Among WAS patients, a negative correlation was found between the score of severity of the disease and all variables considered (Spearman's rank correlation coefficient, r = -0.79, r = -0.73, r = -0.68, and r = -0.56, respectively). In conclusion, this study shows that in WAS a general depletion of the splenic white pulp occurs, supporting the evidence that WAS is characterized by a combined immune defect. The significant reduction of the MZ may explain the inability of WAS patients to mount a response to T-independent antigens.

Marginal-Zone B Cells in the Human Lymph Node and Spleen Show Somatic Hypermutations and Display Clonal Expansion

Splenic marginal-zone B cells, marginal-zone B cells of Peyer’s patches in the gut, and nodal marginal-zone B cells (also identified as monocytoid B cells) share a similar morphology and immunophenotype. These cells likely represent a distinct subset of B cells in humans and rodents, but their precise ontogenetic relationship as well as their origin from B cells of the germinal center is still debated. To study this, we performed a mutation analysis of the rearranged immunoglobulin variable genes (VH) of microdissected single nodal and splenic marginal-zone cells. In addition, we investigated the presence of proliferating cells and B-cell clones in the human splenic and nodal marginal zone as well as adjacent germinal centers. This was performed by immunohistochemical staining for the Ki-67 antigen and denaturing gradient gel analysis of amplified immunoglobulin heavy chain genes’ complementarity determining region 3 of microdissected cell clusters. A variable subset of nodal and splenic marginal-zone B cells showed somatic mutations in their rearranged VH genes, indicating that both virgin and memory B cells are present in the nodal and splenic marginal zone. Nodal and splenic marginal-zone B cells preferentially rearranged VH3 family genes such as DP47, DP49, DP54, and DP58. A preferential rearrangement of the same VH genes has been shown by others in the peripheral CD5− IgM+ B cells. These data suggest that the splenic and nodal marginal-zone B cells are closely related B-cell subsets. We also showed that marginal-zone B cells may cycle and that clones of B cells are frequently detected in the nodal as well as the splenic marginal zone. These clones are not related to those present in adjacent germinal centers. These data favor the hypothesis that clonal expansion occurs in the marginal zone. Whether the somatic hypermutation mechanism is activated during the clonal expansion in the marginal zone and which type of immune response triggers the clonal expansion need to be elucidated.

Marginal-Zone B Cells in the Human Lymph Node and Spleen Show Somatic Hypermutations and Display Clonal Expansion

Splenic marginal-zone B cells, marginal-zone B cells of Peyer’s patches in the gut, and nodal marginal-zone B cells (also identified as monocytoid B cells) share a similar morphology and immunophenotype. These cells likely represent a distinct subset of B cells in humans and rodents, but their precise ontogenetic relationship as well as their origin from B cells of the germinal center is still debated. To study this, we performed a mutation analysis of the rearranged immunoglobulin variable genes (VH) of microdissected single nodal and splenic marginal-zone cells. In addition, we investigated the presence of proliferating cells and B-cell clones in the human splenic and nodal marginal zone as well as adjacent germinal centers. This was performed by immunohistochemical staining for the Ki-67 antigen and denaturing gradient gel analysis of amplified immunoglobulin heavy chain genes’ complementarity determining region 3 of microdissected cell clusters. A variable subset of nodal and splenic marginal-zone B cells showed somatic mutations in their rearranged VH genes, indicating that both virgin and memory B cells are present in the nodal and splenic marginal zone. Nodal and splenic marginal-zone B cells preferentially rearranged VH3 family genes such as DP47, DP49, DP54, and DP58. A preferential rearrangement of the same VH genes has been shown by others in the peripheral CD5− IgM+ B cells. These data suggest that the splenic and nodal marginal-zone B cells are closely related B-cell subsets. We also showed that marginal-zone B cells may cycle and that clones of B cells are frequently detected in the nodal as well as the splenic marginal zone. These clones are not related to those present in adjacent germinal centers. These data favor the hypothesis that clonal expansion occurs in the marginal zone. Whether the somatic hypermutation mechanism is activated during the clonal expansion in the marginal zone and which type of immune response triggers the clonal expansion need to be elucidated.

Human immunoglobulin (Ig)M+IgD+ peripheral blood B cells expressing the CD27 cell surface antigen carry somatically mutated variable region genes: CD27 as a general marker for somatically mutated (memory) B cells

Immunoglobulin (Ig)M+IgD+ B cells are generally assumed to represent antigen-inexperienced, naive B cells expressing variable (V) region genes without somatic mutations. We report here that human IgM+IgD+ peripheral blood (PB) B cells expressing the CD27 cell surface antigen carry mutated V genes, in contrast to CD27-negative IgM+IgD+ B cells. IgM+IgD+CD27(+) B cells resemble class-switched and IgM-only memory cells in terms of cell phenotype, and comprise approximately 15% of PB B lymphocytes in healthy adults. Moreover, a very small population (<1% of PB B cells) of highly mutated IgD-only B cells was detected, which likely represent the PB counterpart of IgD-only tonsillar germinal center and plasma cells. Overall, the B cell pool in the PB of adults consists of approximately 40% mutated memory B cells and 60% unmutated, naive IgD+CD27(-) B cells (including CD5(+) B cells). In the somatically mutated B cells, VH region genes carry a two- to threefold higher load of somatic mutation than rearranged Vkappa genes. This might be due to an intrinsically lower mutation rate in kappa light chain genes compared with heavy chain genes and/or result from kappa light chain gene rearrangements in GC B cells. A common feature of the somatically mutated B cell subsets is the expression of the CD27 cell surface antigen which therefore may represent a general marker for memory B cells in humans.

Insight into the origin and clonal history of B-cell tumors as revealed by analysis of immunoglobulin variable region genes

Recombination of VH, DH and JH genes is a unique first step in normal B-cell development. Subsequent differentiation to a mature plasma cell is accompanied by further events in the Ig genes, including VL-JL joining, somatic hypermutation and isotype switching. Chromosomal changes leading to B-cell tumors can occur at many points in this sequence, and may be partly a consequence of the genetic mobility and mutability permitted in order to generate a diverse antibody repertoire. V genes of neoplastic B cells may reflect the point of maturation reached by the B cell of origin, prior to transformation. Analysis of tumors therefore provides useful information on V-gene patterns in normal B cells, and may add another dimension to classification of B-cell tumors. Transformation may also preserve cell populations normally destined to die by apoptosis. Tumor cells arrested in the site where somatic hypermutation and isotype switch are occurring can still be subject to these processes, and could be influenced by persisting antigen. However, mutation is silenced at the point of exit to the periphery, leading to fixed mutational patterns in tumors of mature B cells. V-gene analysis provides an invaluable tool for understanding the genesis of neoplastic change. It also has a clear clinical relevance in tracking tumor cells, measuring residual disease, and finally in offering the opportunity of developing vaccines for treatment.

Delineation of Selective Influences Shaping the Mutated Expressed Human Ig Heavy Chain Repertoire

After Ag exposure, somatic hypermutation and subsequent selection play significant roles in shaping the peripheral B cell repertoire. However, the disparate impact of each process has not been completely delineated. To address this, the mutational patterns of a large panel of productive VHDJH rearrangements of individual human B cells were analyzed and compared with those of a previously reported panel of nonproductive VHDJH rearrangements. The productive VH rearrangements exhibited a significantly lower mutational frequency and a significantly smaller number of replacement mutations than the nonproductively rearranged genes, suggesting that structural constraints of the Ig molecule and selective influences both impacted the repertoire, militating against replacement mutations. Positive selection favored a mean of four to six replacements in complementarity-determining region 1 (CDR1) and CDR2, and less than two replacements in the framework regions (FRs). In contrast, the negative impact of replacement mutations generated an increased number of silent mutations within both the CDRs and FRs of the productive repertoire accompanied by a net increase in the ratio of replacement to silent mutations in the CDRs compared with that in the FRs. Moreover, there was a negative influence on the distribution of amino acid changes resulting from mutations of highly mutable codons, such as AGY, TAY, GTA, and GCT, preferentially leading to conservative changes in the expressed Ig repertoire. The results are consistent with the conclusion that the expressed repertoire is limited, compared with the potential generated by the mutational machinery, by the dual requirements of avoiding autoreactivity and satisfying structural constraints of an intact Ig molecule.

Comparable impact of mutational and selective influences in shaping the expressed repertoire of peripheral IgM+/CD5- and IgM+/CD5+ B cells

Somatic hypermutation and subsequent selection play a significant role in shaping the peripheral B cell repertoire. This repertoire is composed of CD5+ (5%) and CD5- B cells (95%) which are known to traffic through different lymphoid compartments. Previous studies have shown that V(H) gene usage by CD5+ and CD5- B cells is similar, although mutations are more frequent in the latter. However, the effect of mutation and subsequent selection on the expressed V(H) repertoire of CD5+ and CD5- B cells has not been delineated in detail. This study, therefore, analyzed the mutational pattern of individual IgM+/CD5+ and IgM+/CD5- B cells. In both populations, mutations can occur without heavy chain isotype switching. Despite the differences in mutational frequency, the patterns of mutation and subsequent selection were comparable in CD5+ and CD5- B cells. These results imply that although mutations are more frequent in CD5- B cells, the overall mechanisms governing somatic hypermutation and subsequent positive and negative selection are similar in CD5+ and CD5- B cells.

Marginal zone B-cell lymphomas including mucosa-associated lymphoid tissue type lymphoma (MALT), monocytoid B-cell lymphoma and splenic marginal zone cell lymphoma and their relation to the reactive marginal zone

The marginal zone of the B follicle represents a well-defined compartment of the B area. Its cellular composition is distinct from that of the follicle centre, from which it also differs in its functional role in the immune response. Several newly identified lymphoma entities, e.g. extranodal MALT type lymphoma, nodal monocytoid B-cell lymphoma and splenic marginal zone B-cell lymphoma, display in common a very peculiar organoid growth pattern reminiscent of the marginal zone. Moreover, their neoplastic components share morphologic and phenotypic similarities to the cellular components of the marginal zone. The clinical characteristics of these various marginal zone cell lymphomas may differ depending of the organ which is involved. Nevertheless, they all share common cytogenetic abnormalities suggesting a common pathogenesis.

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.

Molecular Characterization of IgA- and/or IgG-Switched Chronic Lymphocytic Leukemia B Cells

The immunoglobulin (Ig) variable region (V) genes expressed by IgM chronic lymphocytic leukemia (CLL) B cells display little or no somatic mutations. However, preliminary findings have shown that Ig V genes of IgA and IgG CLLs may be somatically mutated, suggesting that isotype-switched CLLs may represent a “subtype” of the disease. To investigate the degree and nature of somatic mutations and the role of antigen (Ag) in the clonal selection and expansion of isotype-switched CLLs, and to determine whether specific oncogene or tumor suppressor gene mutations are associated with isotype-switched CLLs, we analyzed the expressed Ig VH gene, bcl-1 and bcl-2 proto-oncogene, and p53 tumor suppressor gene configurations of 3 IgA-, 1 IgG-, and 1 IgA/IgG-expressing CLLs. These isotype-switched CLL B cells expressed surface HLA-DR, CD19, CD23, and CD5, and displayed no alterations of the bcl-1 and bcl-2 oncogenes and the p53 tumor-suppressor gene. The cDNA VH -D-JH gene sequence was joined with that of the Cα gene in the B cells of the three IgA CLLs, and with that of the Cγ gene in the IgG CLL B cells. In the IgA/IgG-coexpressing CLL B cells, identical VH -D-JH cDNA sequences were spliced to either Cα or Cγ genes. In all five CLLs, the pattern of Cμ DNA probe hybridization to the digested genomic DNAs was consistent with deletion of the Cμ exon from the rearranged Ig gene locus, suggesting that these CLL B cells had undergone DNA switch recombination. In one IgA CLL, the expressed VH gene was unmutated. In all other class-switched CLLs, the Ig VH segment gene was mutated, but the point mutations were not associated with intraclonal diversification. In one IgA and in the IgA/IgG-coexpressing CLL, the nature and distribution of the mutations were consistent with Ag selection. These findings suggest that IgA- and/or IgG-expressing CLLs represent, in their VH gene structure, transformants of B cells at different stages of ontogeny. They also suggest that Ag may play a role in the clonal selection of some of these isotype-switched leukemic cells, but bcl-1 and bcl-2 oncogene rearrangements and p53 tumor suppressor gene mutation are not associated with the pathogenesis of isotype-switched CLLs.

Molecular Characterization of IgA- and/or IgG-Switched Chronic Lymphocytic Leukemia B Cells

The immunoglobulin (Ig) variable region (V) genes expressed by IgM chronic lymphocytic leukemia (CLL) B cells display little or no somatic mutations. However, preliminary findings have shown that Ig V genes of IgA and IgG CLLs may be somatically mutated, suggesting that isotype-switched CLLs may represent a “subtype” of the disease. To investigate the degree and nature of somatic mutations and the role of antigen (Ag) in the clonal selection and expansion of isotype-switched CLLs, and to determine whether specific oncogene or tumor suppressor gene mutations are associated with isotype-switched CLLs, we analyzed the expressed Ig VH gene, bcl-1 and bcl-2 proto-oncogene, and p53 tumor suppressor gene configurations of 3 IgA-, 1 IgG-, and 1 IgA/IgG-expressing CLLs. These isotype-switched CLL B cells expressed surface HLA-DR, CD19, CD23, and CD5, and displayed no alterations of the bcl-1 and bcl-2 oncogenes and the p53 tumor-suppressor gene. The cDNA VH -D-JH gene sequence was joined with that of the Cα gene in the B cells of the three IgA CLLs, and with that of the Cγ gene in the IgG CLL B cells. In the IgA/IgG-coexpressing CLL B cells, identical VH -D-JH cDNA sequences were spliced to either Cα or Cγ genes. In all five CLLs, the pattern of Cμ DNA probe hybridization to the digested genomic DNAs was consistent with deletion of the Cμ exon from the rearranged Ig gene locus, suggesting that these CLL B cells had undergone DNA switch recombination. In one IgA CLL, the expressed VH gene was unmutated. In all other class-switched CLLs, the Ig VH segment gene was mutated, but the point mutations were not associated with intraclonal diversification. In one IgA and in the IgA/IgG-coexpressing CLL, the nature and distribution of the mutations were consistent with Ag selection. These findings suggest that IgA- and/or IgG-expressing CLLs represent, in their VH gene structure, transformants of B cells at different stages of ontogeny. They also suggest that Ag may play a role in the clonal selection of some of these isotype-switched leukemic cells, but bcl-1 and bcl-2 oncogene rearrangements and p53 tumor suppressor gene mutation are not associated with the pathogenesis of isotype-switched CLLs.

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

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

Memory B lymphocytes migrate to bone marrow in humans

IgM-bearing B lymphocytes with mature phenotype (CD10- CD24(lo) IgD+) are acquired after birth in the bone marrow of humans. These B cells are defined here as relatively large, nondividing lymphocytes, variable proportions of which express cell surface molecules indicative of relatively recent activation. Analysis of V(H)5(2) (heavy chain variable region) gene transcripts indicated point mutations throughout the Ig variable region from the mature IgM+ B population but not from the immature B cells in the bone marrow. The mutations were concentrated in the complementarity determining regions, and amino acid substitutions were favored over silent mutations, findings indicative of antigen selection within germinal centers in peripheral lymphoid tissues. The V(H) sequence analysis also revealed the existence of clonal relatives in individual bone marrow samples. These antigen-experienced lymphocytes did not secrete Ig spontaneously but could be induced to do so in vitro. The data suggest that a subpopulation of memory B lymphocytes generated during antigen responses recirculates to the bone marrow in humans.

Immunoglobulin switch transcript production in vivo related to the site and time of antigen-specific B cell activation

Immunoglobulin (Ig) class switch recombination is associated with the production and splicing of germline IgCH messenger RNA transcripts. Levels of gamma 1 transcripts in mouse spleen sections were assessed by semiquantitative analysis of reverse transcriptase polymerase chain reaction (PCR) products during primary and secondary antibody responses to chicken gamma globulin (CGG). This was correlated with the appearance of CGG-specific B cells and their growth and differentiation to plasma cells. After primary immunization with CGG, gamma 1 switch transcripts appeared after 4 d, peaked at a median of six times starting levels between 10 and 18 d after immunization, and returned to background levels before secondary immunization at 5 wk. By contrast, after secondary challenge with CGG, a sevenfold increase in transcripts occurs during the first d. The level again doubles by day 3, when it is six times that which is seen at the peak of the primary response. After day 4, there was a gradual decline over the next 2-3 wk. Within 12 h of secondary immunization, antigen-specific memory B cells appeared in the outer I zone and by 24 h entered S phase, presumably as a result of cognate interaction with primed T cells. Over the next few hours, they migrated to the edge of the red pulp, where they grew exponentially until the fourth day, when they synchronously differentiated to become plasma cells. The same pattern was seen for the migration, growth, and differentiation of virgin hapten-specific B cells when CGG-primed mice were challenged with hapten protein. The continued production of transcripts after day 3 indicates that switching also occurs in germinal centers, but in a relatively small proportion of their B cells. The impressive early production of switch transcripts during T cell-dependent antibody responses occurs in cells that are about to undergo massive clonal expansion. It is argued that Ig class switching at this time, which is associated with cognate T cell-B cell interaction in the T zone, has a major impact on the class and subclasses of Ig produced during the response.

Spectrotype analysis of human ABO antibodies: evidence for different clonal heterogeneity of IgM, IgG, and IgA antibody populations

Clonal characteristics of ABO antibodies in 18 paired samples of serum and breast milk were analyzed by isoelectric focusing and affinity immunoblotting. Anti-A and/or -B (anti-A/B) IgM showed a uniform, polyclonal spectrotype with more than 20 bands and only minimal interindividual differences. In contrast, IgG spectrotypes were oligoclonal ( < 12 bands) and individually distinct. IgA of serum as well as of breast milk showed oligoclonal motifs of up to 15 bands, but with more interindividual variance in intensity than IgM. The bands did not appear at an identical pH in milk and serum samples. The uniformity of IgM spectrotypes could be due to an absence of somatic mutations and thus reflect the use of unmutated germline genes. The greater clonal heterogeneity of IgG as compared to IgA indicates a difference in isotype-switch regulation. Somatic hypermutation may be less active during the switch from IgM to IgA than during the switch to IgG, or the latter switch may be accompanied by a repertoire shift. Alternatively, the anti-A/B IgA and IgG antibody populations could be derived from different clonal precursors.

Analysis of mutations in immunoglobulin heavy chain variable region genes of microdissected marginal zone (MGZ) B cells suggests that the MGZ of human spleen is a reservoir of memory B cells

The splenic marginal zone (MGZ), which surrounds the mantle zone (MTZ) in human splenic white pulp, contains a phenotypically and morphologically distinct population of B cells. The origin of MGZ B cells is uncertain. Whereas some experiments in rodents have suggested that they are a distinct cell lineage responsible for the immune response to T-independent type 2 antigens, others have suggested that they are memory B cells derived from a germinal center (GC) response. The progeny of a GC reaction is expected to have rearranged immunoglobulin (Ig) genes that are mutated. The distribution of mutations would be expected to reflect the selection of Ig by its affinity for antigen. We have analyzed rearranged Ig heavy chain variable region (VH) 6 and VH 4.21 genes in MGZ and MTZ B cells microdissected from frozen sections of human spleen to determine whether these genes have the properties of an affinity-selected memory B cell population. MTZ B cells contained germline Ig VH genes, confirming previous reports and providing an internal control for mutational analysis. MGZ B cells contained Ig VH genes that were mutated, showing that these cells had been subjected to a mutational mechanism characteristically active in the GC. The rearranged VH 6 genes showed patterns of mutation indicative of an antigen selection process, whereas the distribution of mutations in VH 4.21 genes was not characteristic of gene selection by conventional T-dependent antigen. Our studies provide the first evidence that the human splenic MGZ is a reservoir of memory B cells.

The clonogenic precursor cell in multiple myeloma

Multiple myeloma is characterized by the monoclonal expansion of plasma cells in the bone marrow. Although the predominant cell type is the plasma cell, the initial oncogenic transformation is considered to take place in a more immature B cell. There is still much controversy about this precursor cell type. Phenotypic analysis of bone marrow and peripheral blood revealed that in multiple myeloma a great diversity exists in the phenotype of the cells considered to be involved. Because of the lack of a myeloma specific genetic lesion it is very difficult to trace back the cell in which the transforming event, leading to multiple myeloma, took place. The only real clonal marker is the idiotype of the immunoglobulin molecule expressed by the myeloma cells. With recombinant DNA technology it is now possible to produce clonal markers for each individual myeloma patient which recognize only the immunoglobulin genes expressed by the myeloma cell and its precursors. The sequences of these myeloma immunoglobulin genes do reveal a lot of information about the stage in the B-cell differentiation pathway in which the oncogenic event might have taken place. The presence of somatic mutations in a non-random fashion without intraclonal variation leads to the conclusion that the precursor myeloma cell could not possibly be a pre-B cell or stem cell but has to be a mature B cell that has been in contact with antigen and has past through the phase of somatic mutation, like a memory B cell or plasmablast.(ABSTRACT TRUNCATED AT 250 WORDS)

Superantigen properties of a human sialoprotein involved in gut-associated immunity

Protein Fv (pFv) is a recently described 175-kD gut-associated sialoprotein with a potent capacity for augmentation of antibody-dependent immune functions. To investigate the molecular basis for Fab-mediated binding of pFv, we evaluated a panel of 52 monoclonal IgM and found that approximately 40% bound pFv. Whereas the majority (> or = 75%) of V H3 and V H6 IgM strongly bound pFv, only a small minority (< 20%) of IgM from other V H families bound pFv, and these antibodies had weaker binding interactions. Inhibition studies suggested that all binding occurred at the same (or overlapping) site(s) on pFv. Surface plasmon resonance studies demonstrated binding affinity constants up to 6.7 x 10(8) M-1 for pFv. Biopanning of IgM and IgG Fab phage-display libraries with pFv preferentially selected for V H3 and V H6 antibodies, but also obtained certain V H4 IgM. V H sequence analyses of 36 pFv-binding antibodies revealed that binding did not correlate with CDR sequence, JH, or L chain usage. However, there was preferential selection of pFv binders with V H CDR3 of small size. These studies demonstrate that a protein which enhances immune defense in the gut has structural and functional properties similar to known superantigens.

Pattern of usage of the VH4-21 gene by B lymphocytes in a patient with EBV infection indicates ongoing mutation and class switching

Following infection with EBV, patients have selectively raised serum levels of immunoglobulins encoded by the VH4-21 gene. In order to follow the detailed pattern of usage of the VH4-21 gene by blood B lymphocytes of a typical patient during infection, EBV lines were established, and transformed B cells were hybridized and cloned. In addition, to widen the genetic analysis, cDNA preparations from the EBV transformants using the gene were also analysed by polymerase chain reaction, cloning and sequencing. The majority (12/15) of the clonally distinct sequences derived from IgM utilized the VH4-21 gene in germ line configuration; however, 3/15 showed replacement mutations. For one of these, a heterogeneous pattern of mutation within the clone indicated ongoing mutation, and one sequence contained a stop codon. Three distinct clones which had rearranged to C gamma were obtained, and all were extensively mutated, with some evidence for a role for antigen in selection. Following resolution of the infection, no VH4-21-encoded products were detectable by this approach. It appears therefore that infection with EBV leads to selective activation, mutation and class switching of the VH4-21 gene, with the unusual feature that B cells harbouring deleterious mutations in the functional gene are able to survive in the circulation.