Tracing the development of single memory-lineage B cells in a highly defined immune response

Know Your ABCs: Discovery, Differentiation, and Targeting of T-Bet+ B Cells

Since their first description in 2008, T-bet+ B cells have emerged as a clinically important B cell subset. Now commonly known as ABCs (Age-associated B Cells), they are uniquely characterized by their expression of the transcription factor T-bet. Indeed, this singular factor defines this B cell subset. This review will describe the discovery of T-bet+ B cells, their role in bacterial infection as T cell-independent (TI) plasmablasts, as well as long-term follicular helper T cell-dependent (TD) IgM+ and switched memory cells (i.e., T-bet+ ABCs), and later discoveries of their role(s) in diverse immunological responses. These studies highlight a critical, although limited, role of T-bet in IgG2a class switching, a function central to the cells' role in immunity and autoimmunity. Given their association with autoimmunity, pharmacological targeting is an attractive strategy for reducing or eliminating the B cells. T-bet+ ABCs express a number of characteristic cell surface markers, including CD11c, CD11b, CD73, and the adenosine 2a receptor (A2aR). Accordingly, A2aR agonist administration effectively targeted T-bet+ ABCs in vivo. Moreover, agonist treatment of lupus-prone mice reduced autoantibodies and disease symptoms. This latter work highlights the potential therapeutic use of adenosine agonists for treating autoimmune diseases involving T-bet+ ABCs.

Memory B-cell diversity: From early generation to tissue residency and reactivation

Memory B cells (MBCs) have a crucial function in providing an enhanced response to repeated infections. Upon antigen encounter, MBC can either rapidly differentiate to antibody secreting cells or enter germinal centers (GC) to further diversify and affinity mature. Understanding how and when MBC are formed, where they reside and how they select their fate upon reactivation has profound implications for designing strategies to improve targeted, next-generation vaccines. Recent studies have crystallized much of our knowledge on MBC but also reported several surprising discoveries and gaps in our current understanding. Here, we review the latest advancements in the field and highlight current unknowns. In particular, we focus on timing and cues leading to MBC generation before and during the GC reaction, discuss how MBC become resident in mucosal tissues, and finally, provide an overview of factors shaping MBC fate-decision upon reactivation in mucosal and lymphoid tissues.

Antibody and complement levels in patients with hypersplenism associated with cirrhotic portal hypertension and therapeutic principles

BACKGROUND

Hypersplenism associated with cirrhotic portal hypertension is a common condition often resulting from hepatitis B-related cirrhosis. However, the levels of immunoglobulin (Ig) and complement in patients with hypersplenism associated with cirrhotic portal hypertension remain unclear. This study was undertaken to determine the levels of Ig and complement in these patients, the relationship between these levels and Child-Pugh class and their clinical significance.

AIM

To investigate the antibody (Ig) and complement levels in patients with hypersplenism associated with cirrhotic portal hypertension and their clinical significance.

METHODS

Clinical data of 119 patients with hypersplenism associated with cirrhotic portal hypertension were statistically analyzed and compared with those of 128 control patients.

RESULTS

IgA and IgG levels in patients with hypersplenism were significantly higher than controls (P < 0.001). There was no significant difference in IgM between the two groups (P = 0.109). C3 and C4 levels in patients with hypersplenism were significantly lower than controls (P < 0.001). As liver function decreased, IgA and IgG levels increased (P < 0.001), and C3 and C4 levels decreased (P < 0.001).

CONCLUSION

Patients with hypersplenism associated with cirrhotic portal hypertension have significantly higher antibody (IgA and IgG) levels and significantly lower complement (C3 and C4) levels, which are both related to liver damage. Clinically, the administration of anti-hepatitis virus agents and protection of liver function should be strengthened.

Restricted Clonality and Limited Germinal Center Reentry Characterize Memory B Cell Reactivation by Boosting

Repeated exposure to pathogens or their antigens triggers anamnestic antibody responses that are higher in magnitude and affinity than the primary response. These involve reengagement of memory B cell (MBC) clones, the diversity and specificity of which determine the breadth and effectiveness of the ensuing antibody response. Using prime-boost models in mice, we find that secondary responses are characterized by a clonality bottleneck that restricts the engagement of the large diversity of MBC clones generated by priming. Rediversification of mutated MBCs is infrequent within secondary germinal centers (GCs), which instead consist predominantly of B cells without prior GC experience or detectable clonal expansion. Few MBC clones, generally derived from higher-affinity germline precursors, account for the majority of secondary antibody responses, while most primary-derived clonal diversity is not reengaged detectably by boosting. Understanding how to counter this bottleneck may improve our ability to elicit antibodies to non-immunodominant epitopes by vaccination.

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Memory B cell reentry into germinal centers is rare under typical boost regimens

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Most (>90%) B cells in secondary GCs have no prior GC experience

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A clonality bottleneck restricts the diversity of recall antibody-producing cells

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Most primary diversity is found in an MBC compartment not accessed by boosting

An optimized method for enumerating CNS derived memory B cells during viral-induced inflammation

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Memory B cell markers characterizing peripheral B cell phenotypes show more diverse expression patterns in the infected central nervous system (CNS).

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TLR7/8 stimulation for 2 days prior to ELISPOT analysis achieves optimal conversion of CNS-derived memory B cells to ASC while minimizing cell loss.

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In vitro stimulation allows simultaneous assessment of antibody secreting cell and memory B cell isotype, antigen specificity, and temporal alterations during CNS inflammation.

Background

CNS inflammation resulting from infection, injury, or neurodegeneration leads to accumulation of diverse B cell subsets. Although antibody secreting cells (ASC) within the inflamed CNS have been extensively examined, memory B cell (Bmem) characterization has been limited as they do not secrete antibody without stimulation. Moreover, unlike human Bmem, reliable surface markers for murine Bmem remain elusive.

New method

Using a viral encephalomyelitis model we developed a modified limiting dilution in vitro stimulation assay to convert CNS-derived virus specific Bmem into ASC.

Comparison with existing methods

Stimulation methods established for lymphoid tissue cells using prolonged stimulation with viral lysate resulted in substantial ASC loss and minimal Bmem to ASC conversion of CNS-derived cells. By varying stimulation duration, TLR activators, and culture supplements, we achieved optimal conversion by culturing cells with TLR7/8 agonist R848 in the presence of feeder cells for 2 days.

Results

Flow cytometry markers CD38 and CD73 characterizing murine Bmem from lymphoid tissue showed more diverse expression patterns on corresponding CNS-derived B cell subsets. Using the optimized TLR7/8 stimulation protocol, we compared virus-specific IgG Bmem versus pre-existing ASC within the brain and spinal cord. Increasing Bmem frequencies during chronic infection mirrored kinetics of ASC. However, despite initially similar Bmem and ASC accumulation, Bmem prevailed in the brain, but were lower than ASC in the spinal cord during persistence.

Conclusion

Simultaneous enumeration of antigen-specific Bmem and ASC using the Bmem assay optimized for CNS-derived cells enables characterization of temporal changes during microbial or auto-antigen induced neuroinflammation.

CD80 and PD-L2 define functionally distinct memory B cell subsets that are independent of antibody isotype

Memory B cells (MBCs) are long-lived sources of rapid, isotype-switched secondary antibody-forming cell (AFC) responses. Whether MBCs homogeneously retain the ability to self-renew and terminally differentiate or if these functions are compartmentalized into MBC subsets has remained unclear. It has been suggested that antibody isotype controls MBC differentiation upon restimulation. Here we demonstrate that subcategorizing MBCs on the basis of their expression of CD80 and PD-L2, independently of isotype, identified MBC subsets with distinct functions upon rechallenge. CD80(+)PD-L2(+) MBCs differentiated rapidly into AFCs but did not generate germinal centers (GCs); conversely, CD80(-)PD-L2(-) MBCs generated few early AFCs but robustly seeded GCs. The gene-expression patterns of the subsets supported both the identity and function of these distinct MBC types. Hence, the differentiation and regeneration of MBCs are compartmentalized.

Memory B cells in mouse models

One of the principles behind vaccination, as shown by Edward Jenner in 1796, and host protection is immunological memory, and one of the cells central to this is the antigen-experienced memory B cell that responds rapidly upon re-exposure to the initiating antigen. Classically, memory B cells have been defined as progenies of germinal centre (GC) B cells expressing isotype-switched and substantially mutated B cell receptors (BCRs), that is, membrane-bound antibodies. However, it has become apparent over the last decade that this is not the only pathway to B cell memory. Here, we will discuss memory B cells in mice, as defined by (1) cell surface markers; (2) multiple layers; (3) formation in a T cell-dependent and either GC-dependent or GC-independent manner; (4) formation in a T cell-independent fashion. Lastly, we will touch upon memory B cells in; (5) mouse models of autoimmune diseases.

Epitope-specific human influenza antibody repertoires diversify by B cell intraclonal sequence divergence and interclonal convergence

We generated from a single blood sample five independent human mAbs that recognized the Sa antigenic site on the head of influenza hemagglutinin and exhibited inhibitory activity against a broad panel of H1N1 strains. All five Abs used the V(H)3-7 and J(H)6 gene segments, but at least four independent clones were identified by junctional analysis. High-throughput sequence analysis of circulating B cells revealed that each of the independent clones were members of complex phylogenetic lineages that had diversified widely using a pattern of progressive diversification through somatic mutation. Unexpectedly, B cells encoding multiple diverging lineages of these clones, including many containing very few mutations in the Ab genes, persisted in the circulation. Conversely, we noted frequent instances of amino acid sequence convergence in the Ag combining sites exhibited by members of independent clones, suggesting a strong selection for optimal binding sites. We suggest that maintenance in circulation of a wide diversity of somatic variants of dominant clones may facilitate recognition of drift variant virus epitopes that occur in rapidly mutating virus Ags, such as influenza hemagglutinin. In fact, these Ab clones recognize an epitope that acquired three glycosylation sites mediating escape from previously isolated human Abs.

Memory B and memory plasma cells

Vaccination provides a powerful means to control infections. It exploits and exemplifies the ability of the immune system to preserve the information that a specific pathogen has been encountered in the past. The cells and molecular mechanisms of immunological memory are still being discussed controversially. Here, we review the current concepts of memory B cells, the signals involved in their maintenance, and their role in enhanced secondary reactions. Memory plasma cells, secreting protective antibodies over lifetime, have been recognized only recently. Their characterization as cells resting in terms of proliferation and migration, and surviving in dedicated stromal niches, in the absence of antigen, has generated new concepts of how memory cells in general are organized by stroma cells, the 'resting memory'. In autoimmunity and chronic inflammation, memory B cells and memory plasma cells can be essential players, and they require special attention, as they do not respond to most conventional therapies. Their selective targeting will depend on a molecular understanding of their lifestyle.

Taking advantage: high-affinity B cells in the germinal center have lower death rates, but similar rates of division, compared to low-affinity cells

B lymphocytes producing high-affinity Abs are critical for protection from extracellular pathogens, such as bacteria and parasites. The process by which high-affinity B cells are selected during the immune response has never been elucidated. Although it has been shown that high-affinity cells directly outcompete low-affinity cells in the germinal center (GC), whether there are also intrinsic differences between these cells has not been addressed. It could be that higher affinity cells proliferate more rapidly or are more likely to enter cell cycle, thereby outgrowing lower affinity cells. Alternatively, higher affinity cells could be relatively more resistant to cell death in the GC. By comparing high- and low-affinity B cells for the same Ag, we show here that low-affinity cells have an intrinsically higher death rate than do cells of higher affinity, even in the absence of competition. This suggests that selection in the GC reaction is due at least in part to the control of survival of higher affinity B cells and not by a proliferative advantage conferred upon these cells compared with lower affinity B cells. Control over survival rather than proliferation of low- and high-affinity B cells in the GC allows greater diversity not only in the primary response but also in the memory response.

Memory B cells in common variable immunodeficiency: clinical associations and sex differences

Common variable immunodeficiency (CVID) is a heterogeneous syndrome characterized by impaired antibody responses, recurrent infections, inflammatory, autoimmune and malignancy-related conditions. We evaluated the relationship between memory B cell phenotype, sex, age at diagnosis, immunologic and clinical conditions in 105 CVID subjects from one medical center. Reduced numbers of switched memory B cells (cutoff <or=0.55% of B cells) were an independent risk factor of granulomas, autoimmune diseases and splenomegaly (p<0.001). Not previously noted, CVID females had significantly more switched memory cells (p=0.007) than males. Splenectomized subjects did not have fewer IgM memory B cells and these numbers were not related to the development of lung disease, as previously proposed. Lower baseline serum IgG was an independent predictor of pneumonia (p=0.007) and severe infections (p=0.001). We conclude that outcomes in CVID depend on an interplay of factors including sex, numbers of switched memory B cells, and baseline serum IgG and IgA levels.

Silent development of memory progenitor B cells

T cell-dependent immune responses generate long-lived plasma cells and memory B cells, both of which express hypermutated Ab genes. The relationship between these cell types is not entirely understood. Both appear to emanate from the germinal center reaction, but it is unclear whether memory cells evolve while obligatorily generating plasma cells by siblings under all circumstances. In the experiments we report, plasma cell development was functionally segregated from memory cell development by a series of closely spaced injections of Ag delivered during the period of germinal center development. The injection series elevated serum Ab of low affinity, supporting the idea that a strong Ag signal drives plasma cell development. At the same time, the injection series produced a distinct population of affinity/specificity matured memory B cells that were functionally silent, as manifested by an absence of corresponding serum Ab. These cells could be driven by a final booster injection to develop into Ab-forming cells. This recall response required that a rest period precede the final booster injection, but a pause of only 4 days was sufficient. Our results support a model of memory B cell development in which extensive affinity/specificity maturation can take place within a B cell clone under some circumstances in which a concomitant generation of Ab-forming cells by siblings does not take place.

New markers for murine memory B cells that define mutated and unmutated subsets

The study of murine memory B cells has been limited by small cell numbers and the lack of a definitive marker. We have addressed some of these difficulties with hapten-specific transgenic (Tg) mouse models that yield relatively large numbers of antigen-specific memory B cells upon immunization. Using these models, along with a 5-bromo-2′-deoxyuridine (BrdU) pulse-label strategy, we compared memory cells to their naive precursors in a comprehensive flow cytometric survey, thus revealing several new murine memory B cell markers. Most interestingly, memory cells were phenotypically heterogeneous. Particularly surprising was the finding of an unmutated memory B cell subset identified by the expression of CD80 and CD35. We confirmed these findings in an analogous V region knock-in mouse and/or in non-Tg mice. There also was anatomic heterogeneity, with BrdU⁺ memory cells residing not just in the marginal zone, as had been thought, but also in splenic follicles. These studies impact the current understanding of murine memory B cells by identifying new phenotypes and by challenging assumptions about the location and V region mutation status of memory cells. The apparent heterogeneity in the memory compartment implies either different origins and/or different functions, which we discuss.

T cell tolerance to germline-encoded antibody sequences in a lupus-prone mouse

The BCR V region has been implicated as a potential avenue of T cell help for autoreactive B cells in systemic lupus erythematosus. In principle, either germline-encoded or somatically generated sequences could function as targets of such help. Preceding studies have indicated that class II MHC-restricted T cells in normal mice attain a state tolerance to germline-encoded Ab diversity. In this study, we tested whether this tolerance is intact in systemic lupus erythematosus-prone (New Zealand Black x SWR)F1 mice (SNF1). Using a hybridoma sampling approach, we found that SNF1 T cells were tolerant to germline-encoded Ab sequences. Specifically, they were tolerant to germline-encoded sequences derived from a lupus anti-chromatin Ab that arose spontaneously in this strain. This was true both for diseased and prediseased mice. Thus, there does not appear to be a global defect in T cell tolerance to Ab V regions in this autoimmune-prone strain either before or during autoimmune disease.

Expression of the immunoregulatory molecule FcRH4 defines a distinctive tissue-based population of memory B cells

The FcRH4 transmembrane molecule, a member of the Fc receptor homologue family, can potently inhibit B cell receptor (BCR) signaling. We show that cell surface expression of this immunoregulatory molecule is restricted to a subpopulation of memory B cells, most of which lack the classical CD27 marker for memory B cells in humans. The FcRH4⁺ and FcRH4⁻ memory B cells have undergone comparable levels of immunoglobulin isotype switching and somatic hypermutation, while neither subpopulation expresses the transcription factors involved in plasma cell differentiation. The FcRH4⁺ memory cells are morphologically distinctive large lymphocytes that express the CD69, CD80, and CD86 cell activation markers. They are also shown to be poised to secrete high levels of immunoglobulins in response to stimulation with T cell cytokines, but they fail to proliferate in response either to BCR ligation or Staphylococcus aureus stimulation. A heightened expression of the CCR1 and CCR5 chemokine receptors may facilitate their preferential localization in lymphoid tissues near epithelial surfaces. Cell surface FcRH4 expression thus marks a unique population of memory B cells with distinctive morphology, functional capabilities, and tissue localization.

Ligation of CD27 on B cells in vivo during primary immunization enhances commitment to memory B cell responses

Ligation of CD27 on B cells has been shown to inhibit terminal differentiation of activated murine B cells into plasma cells. We show in this study that this inhibition is accompanied by an enhanced movement of activated B cells toward differentiation into memory cells. Treatment of mice with anti-CD27 during immunization leads to the generation of greater numbers of Ag-binding B cells in draining lymph nodes that persist for longer periods of time, and they contain a greater proportion of cells of a postgerminal center phenotype. Limiting dilution analyses reveal that they contain a higher frequency of cells that can be stimulated to secrete specific IgG, and adoptive transfer experiments confirm that they can generate higher secondary responses in carrier-primed recipients. Remarkably, significant secondary responses are also seen following primary immunization with a T-independent Ag in the presence of anti-CD27, confirming that ligation of CD27 on B cells during priming induces differentiation into the memory lineage. Treatment with anti-CD27 during priming also increases the average affinity of the secondary response, suggesting that high affinity clones generated early in a primary response may normally differentiate preferentially into plasma cells and are rescued from this fate by CD27 ligation. Anti-CD40 treatment shows similar effects in vivo. However, unlike CD27, CD40 coligation also enhances proliferation, survival, and isotype switching of LPS-stimulated B cells, suggesting that the two receptors may enhance commitment to B cell memory by different mechanisms, or that a common mechanism is used through both receptors that does not involve cell cycle control or survival.

Somatic translocation and differential expression of Ig mu transgene copies implicate a role for the Igh locus in memory B cell development

Memory B cells of mice with Ig mu transgenes often carry transgene copies that have moved into the Igh locus via somatic translocation. This phenomenon has been attributed to a selection pressure for somatic hypermutations, which generally are observed at much higher frequencies in translocated copies than in ectopic copies. We tested this idea by immunizing Ig-mu transgenic mice in a manner designed to select B cells that required only one V(H) mutation for a switch in antigenic specificity and recruitment into the memory pool. Despite the minimal mutation requirement, hybridomas carrying somatic translocations to the Igh locus were obtained. Importantly, this occurred despite the fact that translocated and untranslocated mu-transgenes were mutated comparably. Evidently, a strong selection advantage was conferred upon B cells by the somatic translocations. Among the hybridomas, translocated mu-transgenes were active, while ectopic mu-transgenes were uniformly silent. The translocated copy that had conferred an affinity-based selection advantage was expressed at the highest level. Moreover, translocated copies were differentially expressed among hybridoma members, which belonged to a common post-mutational lineage. This suggests that adjustments in transgene expression levels had occurred during memory cell development. These results indicate that, apart from their potential influences on somatic hypermutagenesis and class switch recombination, elements in the Igh locus promote the selection of memory B cells in another way, possibly by regulating the level of Ig expression at various stages of antigen-driven differentiation.

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

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

Distinct clonal Ig diversification patterns in young appendix compared to antigen-specific splenic clones

The young rabbit appendix is a dynamic site for primary B cell repertoire development. To study diversification patterns during clonal expansion, we collected single appendix B cells from 3- to 9-wk-old rabbits and sequenced rearranged H and L chain genes. Single cells obtained by hydraulic micromanipulation or laser capture microdissection were lysed, PCR amplified, and products directly sequenced. Gene conversion-like changes occurred in rearranged H and L chain sequences by 3-4 wk of age. Somatic mutations were found in the D regions that lack known conversion donors and probably also occurred in the V genes. A few small sets of clonally related appendix B cells were found at 3-5 wk; by 5.5 wk, some larger clones were recovered. The diversification patterns in the clones from appendix were strikingly different from those found previously in splenic germinal centers where an immunizing Ag was driving the expansion and selection process toward high affinity. Clonally related appendix B cells developed different amino acid sequences in each complementarity-determining region (CDR) including CDR3, whereas dominant clones from spleen underwent few changes in CDR3. The variety of combining sites generated by diversification within individual clones suggests that at least some clonal expansion and selection, known to require normal gut flora, may be driven through indirect effects of microbial components rather than solely by their recognition as specific foreign Ags. This diversity of combining sites within B cell clones supports the proposed role of appendix in generating the preimmune repertoire.

Structural analysis of mutants of high-affinity and low-affinity p-azophenylarsonate-specific antibodies generated by alanine scanning of heavy chain complementarity-determining region 2

Alanine scanning was used to determine the affinity contributions of 10 side chain amino acids (residues at position 50-60 inclusive) of H chain complementarity-determining region 2 (HCDR2) of the somatically mutated high-affinity anti-p-azophenylarsonate Ab, 36-71. Each mutated H chain gene was expressed in the context of mutated (36-71L) and the unmutated (36-65L) L chains to also assess the contribution of L chain mutations to affinity. Combined data from fluorescence quenching, direct binding, inhibition, and capture assays indicated that mutating H:Tyr(50) and H:Tyr(57) to Ala in the 36-71 H chain results in significant loss of binding with both mutated (36-71L) or unmutated (36-65L) L chain, although the decrease was more pronounced when unmutated L chain was used. All other HCDR2 mutations in 36-71 had minimal effect on Ab affinity when expressed with 36-71 L chain. However, in the context of unmutated L chain, of H:Gly(54) to Ala resulted in significant loss of binding, while Abs containing Asn(52) to Ala, Pro(53) to Ala, or Ile(58) to Ala mutation exhibited 4.3- to 7.1-fold reduced affinities. When alanine scanning was performed instead on certain HCDR2 residues of the germline-encoded (unmutated) 36-65 Ab and expressed with unmutated L chain as Fab in bacteria, these mutants exhibited affinities similar to or slightly higher than the wild-type 36-65. These findings indicate an important role of certain HCDR2 side chain residues on Ab affinity and the constraints imposed by L chain mutations in maintaining Ag binding.

Inappropriate expression of IgD from a transgene inhibits the function of antigen-specific memory B cells

IgD expression has been shown to be downmodulated upon mitogenic or antigenic activation of B cells. To investigate whether this decrease is of functional significance we studied a mouse strain that expresses transgenic IgD on all B cells. The rearranged gene encoding the heavy chain of this IgD requires endogenous gene rearrangement before it can be expressed; therefore, normal B cell development is not affected. As a result, both transgenic IgD and endogenous IgM and IgD are expressed on all peripheral B cells. We show that the presence of extraneous IgD does not affect normal B cell activation by polyclonal stimulators, nor does it affect the primary IgM or IgG responses to TI or TD antigens. However, the secondary memory response is significantly diminished. The decrease is not attributable to a defective generation of memory B cells; instead the activation of memory cells appears to be compromised. Since the depressed response can be overcome by prior aggregation of the transgenic IgD with allotype-specific anti-IgD antibodies, it appears that persistence of the transgenic IgD on memory cells may influence their ability to be activated. Thus, the decrease in IgD expression on normal B cells after activation may be necessary for optimal activation of memory cells.

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.

A receptor presentation hypothesis for T cell help that recruits autoreactive B cells

To uncover mechanisms that drive spontaneous expansions of autoreactive B cells in systemic lupus erythematosus, we analyzed somatic mutations in variable region genes expressed by a panel of (NZB x SWR)F(1) hybridomas representing a large, spontaneously arising clone with specificity for chromatin. A single mutation within the Jkappa intron that was shared by all members of the lineage indicated that the clone emanated from a single mutated precursor cell and led to the prediction that a somatic mutation producing a functionally decisive amino acid change in the coding region would also be universally shared. Upon cloning and sequencing the corresponding germline V(H) gene, we found that two replacement somatic mutations in FR1 and CDR2 were indeed shared by all seven clone members. Surprisingly, neither mutation influenced Ab binding to chromatin; however, one of them produced a nonconservative amino acid replacement in a mutationally "cold" region of FR1 and created an immunodominant epitope for class II MHC-restricted T cells. The epitope was restricted by IA(q) (SWR), and the SWR MHC locus is associated with systemic lupus erythematosus in (NZB x SWR)F(1) mice. These, and related findings, provoke the hypothesis that autoreactive B cells may be recruited by a "receptor presentation" mechanism involving cognate interactions between T cells and somatically generated V region peptides that are self-presented by B cells.

Activation and anergy in bone marrow B cells of a novel immunoglobulin transgenic mouse that is both hapten specific and autoreactive

Available evidence indicates that B cell tolerance is attained by receptor editing, anergy, or clonal deletion. Here, we describe a p-azophenylarsonate (Ars)-specific immunoglobulin transgenic mouse in which B cells become anergic as a consequence of cross-reaction with autoantigen in the bone marrow. Developing bone marrow B cells show no evidence of receptor editing but transiently upregulate activation markers and appear to undergo accelerated development. Mature B cells are present in normal numbers but are refractory to BCR-mediated induction of calcium mobilization, tyrosine phosphorylation, and antibody responses. Activation marker expression and acquisition of the anergic phenotype is prevented in bone marrow cultures by monovalent hapten. In this model, it appears that induction of anergy in B cells can be prevented by monovalent hapten competing with autoantigen for the binding site.

Antigen-specific B cell memory: expression and replenishment of a novel b220(-) memory b cell compartment

The mechanisms that regulate B cell memory and the rapid recall response to antigen remain poorly defined. This study focuses on the rapid expression of B cell memory upon antigen recall in vivo, and the replenishment of quiescent B cell memory that follows. Based on expression of CD138 and B220, we reveal a unique and major subtype of antigen-specific memory B cells (B220(-)CD138(-)) that are distinct from antibody-secreting B cells (B220(+/)-CD138(+)) and B220(+)CD138(-) memory B cells. These nonsecreting somatically mutated B220(-) memory responders rapidly dominate the splenic response and comprise >95% of antigen-specific memory B cells that migrate to the bone marrow. By day 42 after recall, the predominant quiescent memory B cell population in the spleen (75-85%) and the bone marrow (>95%) expresses the B220(-) phenotype. Upon adoptive transfer, B220(-) memory B cells proliferate to a lesser degree but produce greater amounts of antibody than their B220(+) counterparts. The pattern of cellular differentiation after transfer indicates that B220(-) memory B cells act as stable self-replenishing intermediates that arise from B220(+) memory B cells and produce antibody-secreting cells on rechallenge with antigen. Cell surface phenotype and Ig isotype expression divide the B220(-) compartment into two main subsets with distinct patterns of integrin and coreceptor expression. Thus, we identify new cellular components of B cell memory and propose a model for long-term protective immunity that is regulated by a complex balance of committed memory B cells with subspecialized immune function.

Contrasting the In Situ Behavior of a Memory B Cell Clone During Primary and Secondary Immune Responses

Whether memory B cells possess altered differentiative potentials and respond in a qualitatively distinct fashion to extrinsic signals as compared with their naive precursors is a current subject of debate. We have investigated this issue by examining the participation of a predominant anti-arsonate clonotype in the primary and secondary responses in the spleens of A/J mice. While this clonotype gives rise to few Ab-forming cells (AFC) in the primary response, shortly after secondary immunization its memory cell progeny produce a massive splenic IgG AFC response, largely in the red pulp. Extensive clonal expansion and migration take place during the secondary AFC response but Ab V region somatic hypermutation is not reinduced. The primary and secondary germinal center (GC) responses of this clonotype are both characterized by ongoing V gene hypermutation and phenotypic selection, little or no inter-GC migration, and derivation of multiple, spatially distinct GCs from a single progenitor. However, the kinetics of these responses differ, with V genes containing a high frequency of total as well as affinity-enhancing mutations appearing rapidly in secondary GCs, suggesting either recruitment of memory cells into this response, or accelerated rates of hypermutation and selection. In contrast, the frequency of mutation observed per V gene does not increase monotonically during the primary GC response of this clonotype, suggesting ongoing emigration of B cells that have sustained affinity- and specificity-enhancing mutations.

Gene Conversion and Hypermutation During Diversification of VH Sequences in Developing Splenic Germinal Centers of Immunized Rabbits

The young rabbit appendix and the chicken bursa of Fabricius are primary lymphoid organs where the B cell Ab repertoire develops in germinal centers (GCs) mainly by a gene conversion-like process. In human and mouse, V-gene diversification by somatic hypermutation in GCs of secondary lymphoid organs leads to affinity maturation. We asked whether gene conversion, somatic hypermutation, or both occur in rabbit splenic GCs during responses to the hapten DNP. We determined DNA sequences of rearranged heavy and light chain V region gene segments in single cells from developing DNP-specific GCs after immunization with DNP-bovine γ-globulin and conclude that the changes at the DNA level that may lead to affinity maturation occur by both gene conversion and hypermutation. Selection was suggested by finding some recurrent amino acid replacements that may contribute increased affinity for antigen in the complementarity-determining region sequences of independently evolved clones, and a narrower range of complementarity-determining region 3 lengths at day 15. Some of the alterations of sequence may also lead to new members of the B cell repertoire in adult rabbits comparable with those produced in gut associated lymphoid tissues of young rabbits.

Analyses of Single B Cells by Polymerase Chain Reaction Reveal Rearranged VH with Germline Sequences in Spleens of Immunized Adult Rabbits: Implications for B Cell Repertoire Maintenance and Renewal

We used PCR to amplify rearranged VHDJHgenes in single cells collected by micromanipulation from splenic germinal centers of immunized adult rabbits. In the course of the study, the objective of which was to analyze diversification of rearranged VHDJH sequences, we were surprised to find cells 7 and 10 days after immunization with rearranged VH1a2 as well as a-negative (y33 and x32) sequences that were identical or close to germline (10 or fewer changes). About 58% (82/140) of the sequences had unique CDR3 regions and were unrelated. In seven different germinal centers, we found one to four different clones with two to seven members. Clonally related cells underwent diversification by hypermutation and gene conversion. We found that contrary to published reports, adult rabbits indeed have newly diversifying B cell receptors in splenic germinal centers. The attractive idea that the rabbit, like the chicken, develops its B cell repertoire early in life and depends upon self-renewing cells in the periphery to maintain its B lymphocyte pool throughout life, is challenged by the current finding. Although a major population of B lymphocytes may be generated early in life, diversified extensively, and maintained by self-renewal in the periphery, some sources of cells with sequences close to germline do exist in adult rabbits and appear in the developing germinal centers. Although considerable repertoire diversity is generated in young rabbits, mechanisms for continued generation of B cell receptor diversity are retained in adult life, where they may confer survival advantage.

Kinetics of Establishing the Memory B Cell Population as Revealed by CD38 Expression

In this report, we detail changes in the expression of CD38 on murine B cells during the course of a T cell-dependent immune response. CD38 is expressed on all naive B cells but is down-regulated on isotype-switched B cells from both the germinal centers (GCs) and the foci of Ab-forming cells which arise during the first weeks of the response. The down-regulation on GC B cells, however, is reversible since Ag-specific IgG1 B cells with high levels of CD38 are apparent by 2 wk postimmunization. These cells have characteristics that resemble recirculating memory B cells, in that they are small and bind low levels of peanut agglutinin. Such characteristics indicate that the restoration of CD38 levels is coincidental with the transition from GC to memory B cell. Using this observation, we plotted the development of the memory population and the demise of the GC reaction as a function of time after immunization. Our results indicate that the GC reaction ceases gradually over many weeks rather than suddenly, which corresponds with the formation of the memory B cell population. Furthermore, by segregating memory B cells and GC B cells, it was possible to assess the in vitro survival characteristics of each compared with naive B cells. These experiments demonstrated that memory B cell survival in vitro was comparable with naive B cell survival but less than the survival seen for bcl-2-transgenic B cells, whereas GC B cell survival, as expected, was very poor. Hence, by resolving murine Ag-specific memory B cells and GC B cells, we have been able to quantify the development of the memory B cell population.

Harmful somatic mutations: lessons from the dark side

The ability of somatic mutation to modify the course of an immune response is well documented. However, emphasis has been placed almost exclusively on the ability of somatic mutation to improve the functional characteristics of representative antibodies. The harmful effects of somatic mutation, its dark side, have been far less well characterized. Yet evidence suggests that the number of B cells directed to wastage pathways as a result of harmful somatic mutation probably far exceeds the number of cells whose antibodies have been improved. Here we review our recent findings in understanding the structural and functional consequences of V-region mutation.

The role of germinal centers for antiviral B cell responses

Germinal centers (GCs) are crucially involved in T cell-dependent B cell responses. B cells rapidly proliferate within GCs and their Ig variable region genes undergo hypermutation. Cognate T helper cells and antigen presented in native form on follicular dendritic cells (FDCs) select B cells expressing high-affinity Igs, leading to affinity maturation and the generation of memory B cells. In addition to these well-established functions of GCs, this article presents evidence that they also play a crucial role for the maintenance of specific memory Ig titers and for the prevention of viral antibody escape mutants.

The extent of affinity maturation differs between the memory and antibody-forming cell compartments in the primary immune response

Immunization with protein-containing antigens results in two types of antigen-specific B cell: antibody forming cells (AFCs) producing antibody of progressively higher affinity and memory lymphocytes capable of producing high affinity antibody upon re-exposure to antigen. The issue of the inter-relationship between affinity maturation of memory B cells and AFCs was addressed through analysis of single, antigen-specific B cells from the memory and AFC compartments during the primary response to a model antigen. Only 65% of splenic memory B cells were found capable of producing high affinity antibody, meaning that low affinity cells persist into this compartment. In contrast, by 28 days after immunization all AFCs produced high affinity antibody. We identified a unique, persistent sub-population of bone marrow AFCs containing few somatic mutations, suggesting they arose early in the response, yet highly enriched for an identical affinity-enhancing amino acid exchange, suggesting strong selection. Our results imply that affinity maturation of a primary immune response occurs by the early selective differentiation of high affinity variants into AFCs which subsequently persist in the bone marrow. In contrast, the memory B-cell population contains few, if any, cells from the early response and is less stringently selected.