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

Mcl-1 is essential for germinal center formation and B cell memory

Lymphocyte survival during immune responses is controlled by the relative expression of pro- and anti-apoptotic molecules, regulating the magnitude, quality, and duration of the response. We investigated the consequences of deleting genes encoding the anti-apoptotic molecules Mcl1 and Bcl2l1 (Bcl-x(L)) from B cells using an inducible system synchronized with expression of activation-induced cytidine deaminase (Aicda) after immunization. This revealed Mcl1 and not Bcl2l1 to be indispensable for the formation and persistence of germinal centers (GCs). Limiting Mcl1 expression reduced the magnitude of the GC response with an equivalent, but not greater, effect on memory B cell formation and no effect on persistence. Our results identify Mcl1 as the main anti-apoptotic regulator of activated B cell survival and suggest distinct mechanisms controlling survival of GC and memory B cells.

Control systems and decision making for antibody production

This paper synthesizes recent progress toward understanding the integrated control systems and fail-safes that guide the quality and quantity of antibody produced by B cells. We focus on four key decisions: (1) the choice between proliferation or death in perifollicular B cells in the first 3 days after antigen encounter; (2) differentiation of proliferating perifollicular B cells into extrafollicular plasma cells or germinal center B cells; (3) positive selection of B cell antigen receptor (BCR) affinity for foreign antigen versus negative selection of BCR affinity for self antigen in germinal center B cells; and (4) survival versus death of antibody-secreting plasma cells. Understanding the engineering of these control systems represents a challenging future step for treating disorders of antibody production in autoimmunity, allergy and immunodeficiency.

Optimality of mutation and selection in germinal centers

The population dynamics theory of B cells in a typical germinal center could play an important role in revealing how affinity maturation is achieved. However, the existing models encountered some conflicts with experiments. To resolve these conflicts, we present a coarse-grained model to calculate the B cell population development in affinity maturation, which allows a comprehensive analysis of its parameter space to look for optimal values of mutation rate, selection strength, and initial antibody-antigen binding level that maximize the affinity improvement. With these optimized parameters, the model is compatible with the experimental observations such as the ∼100-fold affinity improvements, the number of mutations, the hypermutation rate, and the “all or none” phenomenon. Moreover, we study the reasons behind the optimal parameters. The optimal mutation rate, in agreement with the hypermutation rate in vivo, results from a tradeoff between accumulating enough beneficial mutations and avoiding too many deleterious or lethal mutations. The optimal selection strength evolves as a balance between the need for affinity improvement and the requirement to pass the population bottleneck. These findings point to the conclusion that germinal centers have been optimized by evolution to generate strong affinity antibodies effectively and rapidly. In addition, we study the enhancement of affinity improvement due to B cell migration between germinal centers. These results could enhance our understanding of the functions of germinal centers.

The antibodies in our immune system could efficiently improve their abilities in recognizing new antigens. This is done with the help of proliferation, mutation and selection of B cells which carry antibodies, but we have difficulties in developing a quantitative description of this adaptation process which is consistent with the various aspects of experimental observations. Based on the knowledge from experiments, here we present a theoretical model to calculate the numbers of B cells with different antigen recognizing abilities all the time, and look for the best possible design that improves the antigen recognizing ability most efficiently. We find that the best possible design is consistent with the experimental observations, pointing to the conclusion that the immune system has been optimized in evolution. We then study the trade-offs leading to the optimization of the design. The results will not only improve our understanding of the functions in immune system, but also reveal the design principles behind the details. In addition, the study enhances our understanding of the population dynamics in evolution.

BH3 mimetics antagonizing restricted prosurvival Bcl-2 proteins represent another class of selective immune modulatory drugs

Death by apoptosis shapes tissue homeostasis. Apoptotic mechanisms are so universal that harnessing them for tailored immune intervention would seem challenging; however, the range and different expression levels of pro- and anti-apoptotic molecules among tissues offer hope that targeting only a subset of such molecules may be therapeutically useful. We examined the effects of the drug ABT-737, a mimetic of the killer BH3 domain of the Bcl-2 family of proteins that induces apoptosis by antagonizing Bcl-2, Bcl-X(L), and Bcl-W (but not Mcl-1 and A1), on the mouse immune system. Treatment with ABT-737 reduced the numbers of selected lymphocyte and dendritic cell subpopulations, most markedly in lymph nodes. It inhibited the persistence of memory B cells, the establishment of newly arising bone marrow plasma cells, and the induction of a cytotoxic T cell response. Preexisting plasma cells and germinal centers were unaffected. Notably, ABT-737 was sufficiently immunomodulatory to allow long-term survival of pancreatic allografts, reversing established diabetes in this model. These results provide an insight into the selective mechanisms of immune cell survival and how this selectivity avails a different strategy for immune modulation.

Augmented antibody response with premature germinal center regression in CD40L transgenic mice

Although CD40 signaling is required for activation and differentiation of B cells, including germinal center (GC) formation and generation of memory B cells, in vivo generation of CD40 signaling augments plasma cell differentiation but disrupts GCs. Thus, CD40 signaling is thought to direct B cells to extrafollicular plasma cell fate rather than GC formation. In this study, we analyzed CD40L transgenic (CD40LTg) mice that constitutively express CD40L on B cells. After immunization, activation of B cells, but not dendritic cells, was augmented, although dendritic cells can be activated by CD40 ligation. Bone marrow chimera carrying CD40LTg and nontransgenic B cells showed increased Ab production from transgenic, but not from coexisting nontransgenic, B cells, suggesting that CD40L on a B cell preferentially stimulates the same B cell through an autocrine pathway, thereby augmenting Ab production. Although GCs rapidly regressed after day 5 of immunization and failed to generate late-appearing high-affinity Ab, CD40LTg mice showed normal GC formation up to day 5, as well as normal generation of long-lived plasma cells and memory B cell responses. This observation suggests that CD40 signaling does not block GC formation or differentiation of GC B cells, but it inhibits sustained expansion of GC B cells and augments B cell differentiation.

IL-21 regulates germinal center B cell differentiation and proliferation through a B cell-intrinsic mechanism

Germinal centers (GCs) are sites of B cell proliferation, somatic hypermutation, and selection of variants with improved affinity for antigen. Long-lived memory B cells and plasma cells are also generated in GCs, although how B cell differentiation in GCs is regulated is unclear. IL-21, secreted by T follicular helper cells, is important for adaptive immune responses, although there are conflicting reports on its target cells and mode of action in vivo. We show that the absence of IL-21 signaling profoundly affects the B cell response to protein antigen, reducing splenic and bone marrow plasma cell formation and GC persistence and function, influencing their proliferation, transition into memory B cells, and affinity maturation. Using bone marrow chimeras, we show that these activities are primarily a result of CD3-expressing cells producing IL-21 that acts directly on B cells. Molecularly, IL-21 maintains expression of Bcl-6 in GC B cells. The absence of IL-21 or IL-21 receptor does not abrogate the appearance of T cells in GCs or the appearance of CD4 T cells with a follicular helper phenotype. IL-21 thus controls fate choices of GC B cells directly.

Broad volume distributions indicate nonsynchronized growth and suggest sudden collapses of germinal center B cell populations

Immunization with a T cell-dependent Ag leads to the formation of several hundred germinal centers (GCs) within secondary lymphoid organs, a key process in the maturation of the immune response. Although prevailing perceptions about affinity maturation intuitively assume simultaneous seeding, growth, and decay of GCs, our previous mathematical simulations led us to hypothesize that their growth might be nonsynchronized. To investigate this, we performed computer-aided three-dimensional reconstructions of splenic GCs to measure size distributions at consecutive time points following immunization of BALB/c mice with a conjugate of 2-phenyl-oxazolone and chicken serum albumin. Our analysis reveals a broad volume distribution of GCs, indicating that individual GCs certainly do not obey the average time course of the GC volumes and that their growth is nonsynchronized. To address the cause and implications of this behavior, we compared our empirical data with simulations of a stochastic mathematical model that allows for frequent and sudden collapses of GCs. Strikingly, this model succeeds in reproducing the empirical average kinetics of GC volumes as well as the underlying broad size distributions. Possible causes of GC B cell population collapses are discussed in the context of the affinity-maturation process.

Quantitative analysis of influenza virus-specific B cell memory generated by different routes of inactivated virus vaccination

We consider both Ab-secreting cell (ASC) and memory B cell (B(Mem)) populations in a quantitative analysis of virus-specific B cell memory generated by intramuscular or intranasal vaccination of mice with inactivated influenza virus. After both forms of vaccination, the memory phase was characterized by localization of ASCs in the bone marrow and dispersion of B(Mem) to organized lymphoid tissues. The stronger IgG response to intramuscular vaccination correlated with larger numbers of IgG ASCs in the bone marrow and IgG B(Mem). IgA production was only prominent in the response to intranasal vaccination and was associated with IgA ASC localization in the lung and IgA B(Mem) formation. Notably, few IgG ASCs or B(Mem) localized in the lung after intramuscular vaccination, in contrast to the situation following influenza pneumonia. Our analysis links the nature of immunization to characteristics of the state of B cell memory that may relate to protective immunity.

Multiple layers of B cell memory with different effector functions

Memory B cells are at the center of longstanding controversies regarding the presence of antigen for their survival and their re-engagement in germinal centers after secondary challenge. Using a new mouse model of memory B cell labeling dependent on the cytidine deaminase AID, we show that after immunization with a particulate antigen, B cell memory appeared in several subsets, comprising clusters of immunoglobulin M-positive (IgM(+)) and IgG1(+) B cells in germinal center-like structures that persisted up to 8 months after immunization, as well as IgM(+) and IgG1(+) B cells with a memory phenotype outside of B cell follicles. After challenge, the IgG subset differentiated into plasmocytes, whereas the IgM subset reinitiated a germinal center reaction. This model, in which B cell memory appears in several layers with different functions, reconciles previous conflicting propositions.

Appearance of peripheral blood plasma cells and memory B cells in a primary and secondary immune response in humans

In humans, the kinetics of the appearance of memory B cells and plasma cells during primary immunization are not well defined. In this study, we assessed the primary B-cell response of rabies-antigen naive volunteers during a 3-dose course of rabies vaccine compared with the B-cell response to a booster dose of rabies vaccine given to previously immunized volunteers. After a single dose of vaccine, in the naive group plasma and memory B cells appeared later (peak at day 10) than in the primed group (peak at day 7) and were at lower frequency. The most rapid responses (day 4) were detected after a third immunization in the naive group. This is the first study to document the detailed kinetics of the plasma cell and memory B-cell responses to immunization in adult humans and to demonstrate differences in the responses that relate to the preexisting immune status of the persons.

Omental milky spots develop in the absence of lymphoid tissue-inducer cells and support B and T cell responses to peritoneal antigens

The omentum is a site of B1 cell lymphopoiesis and immune responsiveness to T cell-independent antigens. However, it is unknown whether it supports immune responses independently of conventional lymphoid organs. We showed that the omentum collected antigens and cells from the peritoneal cavity and supported T cell-dependent B cell responses, including isotype switching, somatic hypermutation, and limited affinity maturation, despite the lack of identifiable follicular dendritic cells. The omentum also supported CD4+ and CD8+ T cell responses to peritoneal antigens and recruited effector T cells primed in other locations. Unlike conventional lymphoid organs, milky spots in the omentum developed in the absence of lymphoid tissue-inducer cells, but required the chemokine CXCL13. Although the lymphoid architecture of milky spots was disrupted in lymphotoxin-deficient mice, normal architecture was restored by reconstitution with lymphotoxin-sufficient hematopoietic cells. These results indicate that the milky spots of the omentum function as unique secondary lymphoid organs that promote immunity to peritoneal antigens.

Exploration of factors affecting the onset and maturation course of follicular lymphoma through simulations of the germinal center

Genetic mutations frequently observed in human follicular lymphoma (FL) B-cells result in aberrant expression of the anti-apoptotic protein bcl-2 and surface immunoglobulins (Igs) which display one or more novel variable (V) region N-glycosylation motifs. In the present study, we develop a simulation model of the germinal center (GC) to explore how these mutations might influence the emergence and clonal expansion of key mutants which provoke FL development. The simulations employ a stochastic method for calculating the cellular dynamics, which incorporates actual IgV region sequences and a simplified hypermutation scheme. We first bring our simulations into agreement with experimental data for well-characterized normal and bcl-2(+) anti-hapten GC responses in mice to provide a model for understanding how bcl-2 expression leads to permissive selection and memory cell differentiation of weakly competitive B-cells. However, as bcl-2 expression in the GC alone is thought to be insufficient for FL development, we next monitor simulated IgV region mutations to determine the emergence times of key mutants displaying aberrant N-glycosylation motifs recurrently observed in human FL IgV regions. Simulations of 26 germline V(H) gene segments indicate that particular IgV regions have a dynamical selective advantage by virtue of the speed with which one or more of their key sites can generate N-glycosylation motifs upon hypermutation. Separate calculations attribute the high occurrence frequency of such IgV regions in FL to an ability to produce key mutants at a fast enough rate to overcome stochastic processes in the GC that hinder clonal expansion. Altogether, these simulations characterize three pathways for FL maturation through positively selected N-glycosylations, namely, via one of two key sites within germline V(H) region gene segments, or via a site in the third heavy chain complementarity-determining region (CDR-H3) that is generated from VDJ recombination.

B-cell responses to vaccination at the extremes of age

Infants and the elderly share a high vulnerability to infections and therefore have specific immunization requirements. Inducing potent and sustained B-cell responses is as challenging in infants as it is in older subjects. Several mechanisms to explain the decreased B-cell responses at the extremes of age apply to both infants and the elderly. These include intrinsic B-cell limitations as well as numerous microenvironmental factors in lymphoid organs and the bone marrow. This Review describes the mechanisms that shape B-cell responses at the extremes of age and how they could be taken into account to design more effective immunization strategies for these high-risk age groups.

The tetraspanin protein CD37 regulates IgA responses and anti-fungal immunity

Immunoglobulin A (IgA) secretion by plasma cells in the immune system is critical for protecting the host from environmental and microbial infections. However, the molecular mechanisms underlying the generation of IgA⁺ plasma cells remain poorly understood. Here, we report that the B cell–expressed tetraspanin CD37 inhibits IgA immune responses in vivo. CD37-deficient (CD37−/−) mice exhibit a 15-fold increased level of IgA in serum and significantly elevated numbers of IgA⁺ plasma cells in spleen, mucosal-associated lymphoid tissue, as well as bone marrow. Analyses of bone marrow chimeric mice revealed that CD37–deficiency on B cells was directly responsible for the increased IgA production. We identified high local interleukin-6 (IL-6) production in germinal centers of CD37−/− mice after immunization. Notably, neutralizing IL-6 in vivo reversed the increased IgA response in CD37−/− mice. To demonstrate the importance of CD37—which can associate with the pattern-recognition receptor dectin-1—in immunity to infection, CD37−/− mice were exposed to Candida albicans. We report that CD37−/− mice are evidently better protected from infection than wild-type (WT) mice, which was accompanied by increased IL-6 levels and C. albicans–specific IgA antibodies. Importantly, adoptive transfer of CD37−/− serum mediated protection in WT mice and the underlying mechanism involved direct neutralization of fungal cells by IgA. Taken together, tetraspanin protein CD37 inhibits IgA responses and regulates the anti-fungal immune response.

Antibody, or immunoglobulin (Ig), production by plasma cells in the immune system is important for protecting the host from microbial infections. IgA is the most abundant antibody isotype produced in the body. However, the molecular mechanisms underlying the generation of IgA–producing plasma cells remain poorly understood. We now report that the B cell–expressed protein CD37 regulates IgA immune responses, both in steady-state conditions and during infection. We found highly increased levels of IgA in serum and elevated numbers of IgA⁺ plasma cells in lymphoid tissue of mice that are deficient for CD37 (CD37−/− mice). To demonstrate the importance of CD37 in immunity to infection, CD37−/− mice were exposed to the fungus Candida albicans. C. albicans can cause systemic infection with high mortality in immunocompromised patients. We demonstrate that CD37−/− mice are evidently better protected from infection than wild-type mice, which was dependent on C. albicans–specific IgA antibodies. The underlying mechanism involved direct neutralization of fungal cells by IgA. In summary, the B cell protein CD37 inhibits IgA responses and anti-fungal immunity. This study may contribute to the development of novel immunotherapeutic approaches for invasive fungal disease.

Deriving a germinal center lymphocyte migration model from two-photon data

Recently, two-photon imaging has allowed intravital tracking of lymphocyte migration and cellular interactions during germinal center (GC) reactions. The implications of two-photon measurements obtained by several investigators are currently the subject of controversy. With the help of two mathematical approaches, we reanalyze these data. It is shown that the measured lymphocyte migration frequency between the dark and the light zone is quantitatively explained by persistent random walk of lymphocytes. The cell motility data imply a fast intermixture of cells within the whole GC in approximately 3 h, and this does not allow for maintenance of dark and light zones. The model predicts that chemotaxis is active in GCs to maintain GC zoning and demonstrates that chemotaxis is consistent with two-photon lymphocyte motility data. However, the model also predicts that the chemokine sensitivity is quickly down-regulated. On the basis of these findings, we formulate a novel GC lymphocyte migration model and propose its verification by new two-photon experiments that combine the measurement of B cell migration with that of specific chemokine receptor expression levels. In addition, we discuss some statistical limitations for the interpretation of two-photon cell motility measurements in general.

Maintenance of serological memory

Long-lived plasma cells are key to maintaining long-term humoral immunity after infection or vaccination. Some vaccines and/or infections induce antibody levels that remain stable for the life of the individual. However, the mechanism whereby these long-lived plasma cells are maintained over long periods of time remains an open question. Furthermore, given a finite number of sustainable plasma cells within the bone marrow, it is also unclear how space for newly induced plasma cells is generated without compromising the pre-existing repertoire. Here we review the current understanding of these important issues.

Intraclonal competition inhibits the formation of high-affinity antibody-secreting cells

Protective immunity requires a diverse, polyclonal B cell repertoire. We demonstrate that affinity maturation of the humoral response to a hapten is impaired when preexisting clonally restricted cells recognizing the hapten are dominant in the B cell repertoire. B1-8i(+/-) mice, which feature a high frequency of B cells with nitrophenyl (NP)-binding specificity, respond to NP-haptenated proteins with the production of NP-specific Abs, but affinity maturation is impaired due to insufficient generation of high-affinity Ab-producing cells. We manipulated the frequency of NP-specific B cells by adoptive transfer of B1-8 B cells into naive, wild-type recipients. Remarkably, when 10(4) B1-8 B cells were transferred, these cells supported efficient affinity maturation and plasma cell differentiation. In contrast, when 10(6) B1-8 cells were transferred, affinity maturation did not occur. These data indicate that restricting the frequency of clonally related B cells is required to support affinity maturation.

A quantitative assessment of primary and secondary immune responses in cattle using a B cell ELISPOT assay

The aim of the study was to build a comprehensive picture of the appearance in the blood stream of Ag-specific plasma cells and memory B cells in the bovine model. For this purpose, we have developed a method allowing the detection and quantification of both cell types within individual calves immunised with ovalbumin. During the primary response, we detected a burst of ovalbumin-specific plasma cells at days 6 and 7 post-immunisation, which was followed by the production of specific Ab, whereas a gradual increase of memory B cells was only detected from day 15. As expected, a boost immunisation performed 7 weeks later induced a quicker and stronger secondary response. Indeed, a burst of plasma cells was detected in the blood at days 3 and 4, which was followed by a strong increase in Ab titres. Furthermore, a burst of memory B cells, and not a gradual increase, was detected at days 5 and 6 post-boost immunisation. Importantly, we showed a strong correlation between the anti-ovalbumin-specific IgG titres detected 5 months after secondary immunisation and the plasma cell numbers detected in the blood at the peak response after secondary immunisation. The detection and quantification of plasma cells following an immunisation/vaccination strategy could constitute a very effective means for predicting the magnitude and longevity of an Ab response.

Hyperproliferation of B cells specific for a weakly immunogenic PorA in a meningococcal vaccine model

Highly homologous meningococcal porin A (PorA) proteins induce protective humoral immunity against Neisseria meningitidis group B infection but with large and consistent differences in the levels of serum bactericidal activity achieved. We investigated whether a poor PorA-specific serological outcome is associated with a limited size of the specific B-cell subpopulation involved. The numbers of PorA-specific splenic plasma cells, bone marrow (BM) plasma cells, and splenic memory B cells were compared between mice that received priming and boosting with the weakly immunogenic PorA (P1.7-2,4) protein and those that received priming and boosting with the highly immunogenic PorA (P1.5-1,2-2) protein. Immunoglobulin G (IgG) titers (except at day 42), bactericidal activity, and the avidity of IgG produced against P1.7-2,4 were significantly lower at all time points after priming and boosting than against P1.5-1,2-2. These differences, however, were not associated with a lack of P1.7-2,4-specific plasma cells. Instead, priming with both of the PorAs resulted in the initial expansion of comparable numbers of splenic and BM plasma cells. Moreover, P1.7-2,4-specific BM plasma cells, but not P1.5-1,2-2-specific plasma cells, expanded significantly further after boosting. Likewise, after a relative delay during the priming phase, the splenic P1.7-2,4-specific memory B cells largely outnumbered those specific for P1.5-1,2-2, upon boosting. These trends were observed with different vaccine formulations of the porins. Our results show for the first time that B-cell subpopulations involved in a successfully maturated antibody response against a clinically relevant vaccine antigen are maintained at smaller population sizes than those associated with poor affinity maturation. This bears consequences for the interpretation of immunological memory data in clinical vaccine trials.

The B-cell response to protein antigens in immunity and transplantation

The potential for B cells to play a role in transplantation has been recently increasingly recognized. Given that the field has focused almost exclusively on the T cell, here we review B-cell biology. We first describe the B-cell response to protein antigen, and then examine the antibody-independent effector functions of B cells, in particular antigen presentation to T cells. Finally we use autoimmunity as an analogy to focus on the potential role for B cells in transplantation. The direct evidence for this is then marshalled in the accompanying article by Zarkhin and Sarwal.

Plasma cell differentiation and survival

Humoral immunity depends on the regulated production and maintenance of antibody secreting cells during the course of an immune response. Recent insights into the transcriptional regulation of the initiation of plasma cell differentiation have clarified aspects of this process, particularly with respect to the choice between the memory B cell and plasma cell differentiation pathways. It is now possible to specify the conditions favouring these outcomes and to predict where they might occur within the germinal center. Once formed, plasma cell survival is critically dependent on accessing niches that are formed by stomal elements in both normal and inflamed tissues. The apparent homeostasis of plasma cell numbers means that new specificities can persist only at the expense of existing ones, raising questions on how immunological memory is maintained in the face of new immune responses. The answer appears to be through the reduction of the process to a single cell level, thereby introducing an element of stochasticity.

Distinct cell-specific control of autoimmunity and infection by FcgammaRIIb

FcγRIIb is an inhibitory Fc receptor expressed on B cells and myeloid cells. It is important in controlling responses to infection, and reduced expression or function predisposes to autoimmunity. To determine if increased expression of FcγRIIb can modulate these processes, we created transgenic mice overexpressing FcγRIIb on B cells or macrophages. Overexpression of FcγRIIb on B cells reduced the immunoglobulin G component of T-dependent immune responses, led to early resolution of collagen-induced arthritis (CIA), and reduced spontaneous systemic lupus erythematosus (SLE). In contrast, overexpression on macrophages had no effect on immune responses, CIA, or SLE but increased mortality after Streptococcus pneumoniae infection. These results help define the role of FcγRIIb in immune responses, demonstrate the contrasting roles played by FcγRIIb on B cells and macrophages in the control of infection and autoimmunity, and emphasize the therapeutic potential for modulation of FcγRIIb expression on B cells in inflammatory and autoimmune disease.

CD22 regulates time course of both B cell division and antibody response

Because pathogens induce infectious symptoms in a time-dependent manner, a rapid immune response is beneficial for defending hosts from pathogens, especially those inducing acute infectious diseases. However, it is largely unknown how the time course of immune responses is regulated. In this study, we demonstrate that B cells deficient in the inhibitory coreceptor CD22 undergo accelerated cell division after Ag stimulation, resulting in rapid generation of plasma cells and Ab production. This finding indicates that CD22 regulates the time course of B cell responses and suggests that CD22 is a good target to shorten the time required for Ab production, thereby augmenting host defense against acute infectious diseases as "universal vaccination."

Immune complex-bearing follicular dendritic cells deliver a late antigenic signal that promotes somatic hypermutation

We reasoned that immune complex (IC)-bearing follicular dendritic cells (FDCs) promote somatic hypermutation (SHM). This hypothesis was tested in murine germinal center reactions induced in vitro by coculturing 6-day (4-hydroxy-3-nitrophenyl) acetyl-primed but unmutated lambda+ B cells, chicken gamma-globulin (CGG) memory T cells, FDCs, and ICs (anti-CGG plus NP-CGG). Mutations in primed lambda+ B cells were obtained only when both FDCs and immunogen were present. FDCs alone promoted B cell survival and Ab production but there were no mutations without more immunogen. Moreover, the mutation rate was enhanced when FDCs were activated. Trapped ICs ranged from 200 to 500 A apart on FDC membranes and this correlated with the periodicity known to optimally signal BCRs. FDCs are unique in their ability to retain ICs for months and a second signal mediated by FDC-ICs appeared to be needed a week or more after immunization by immunogen persisting on FDCs. However, the time needed to detect extensive SHM could be reduced to 7 days if ICs were injected together with memory T cells in vivo. In marked contrast, no mutations were apparent after 7 days in vivo if ICs were replaced by free Ag that would not load on FDCs until Ab was produced. The data suggest that specific Ab production leads to the following events: Ab encounters Ag and ICs are formed, ICs are trapped by FDCs, B cells are stimulated by periodically arranged Ag in ICs on FDCs, and this late antigenic signal promotes SHM.

Increased survival is a selective feature of human circulating antigen-induced plasma cells synthesizing high-affinity antibodies

The present study shows that tetanus toxoid (tet) booster releases to the human circulation 2 subsets of specific plasma cells (PCs), as defined by phenotype and morphology, which clearly differed in the staining capacity of their cytoplasmic antibodies (Abs) with fluorescein isothiocyanate (FITC)–labeled tet–fragment C (tetC). These cells, called tetCHIGH and tetCINT PCs according to their either high or intermediate FITC-tetC staining capacity, exhibit similar rapid temporary kinetics in the blood (5-8 days after boost), contain many cycling cells, express equivalent amounts of BLIMP-1 mRNA, and produce similar quantities of IgG. However, Abs synthesized by tetCHIGH PCs show a tetC affinity more than 10 times higher than that exhibited by tetCINT PC Abs, and indicated by IGVH sequence analysis. Chemotaxis to CXCL12, a requisite for bone marrow (BM) PC homing, is similar for both cell types. Circulating nonspecific and tetCINT PCs, but not tetCHIGH PCs, tend to undergo spontaneous apoptosis, as demonstrated by APO2.7 and activated caspase-3 expression, and cell recovery. These results indicate that tet booster generates 2 discrete subsets of specific PCs exhibiting different ranges of Ab affinity for the immunogen, and that only those synthesizing high-affinity Abs show enhanced survival. This inherent property may be essential for determining the BM fate of PCs secreting high-affinity Ab.

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.

Malaria-specific antibody responses and parasite persistence after infection of mice with Plasmodium chabaudi chabaudi

While it is known that antibodies are critical for clearance of malaria infections, it is not clear whether adequate antibody responses are maintained and what effect chronic infection has on this response. Here we show that mice with low-grade chronic primary infections of Plasmodium chabaudi or infections very recently eliminated have reduced second infections when compared with the second infection of parasite-free mice. We also show that parasite-specific antibody responses induced by infection of mice with Plasmodium chabaudi contain both short- and long-lived components as well as memory B cells responsible for a faster antibody response during re-infection. Furthermore, parasite-specific antibodies to the C-terminal fragment of merozoite surface protein-1 (MSP-1) undergo avidity maturation. However, antibodies with both low and high avidity persist throughout infection and after re-infection, suggesting repeated rounds of activation and maturation of memory B cells. Neither the avidity profile of the antibody response, nor its maintenance is affected by persisting live parasites. Therefore, differences in parasitemia in re-infection cannot be explained solely by higher levels of antibody or greater affinity maturation of malaria-specific antibodies. These data suggest that there may be an antibody-independent component to the early control of secondary infections in mice that are chronically infected.

Antibodies and B Cell Memory in Viral Immunity

Humoral immunity, in particular secreted neutralizing antibodies, is of central importance to protect the body against acutely cytopathic viruses, whereas noncytopathic viruses have found ways of balanced coexistence with the immune system to avoid antibody-mediated elimination. There is evidence that polyspecific "natural" antibodies provide early protection, independent of T cell help. If that line of defense is crossed, T cell-dependent immune responses then generate a humoral memory provided by long-lived plasma cells secreting specific antibodies of adapted avidity and function, i.e., isotype, even in the absence of virus. Secreted protective antibodies of humoral memory provide an efficient line of defense against reinfection and are backed up by specific B and T memory cells of reactive memory. Whereas humoral memory has developed effective antiviral protection, some viruses (i.e., HIV) have managed to develop specific evasion strategies to escape it. Thus, coevolution provides us with some insight into just how substantial antiviral antibodies and memory B cell are in protecting the host from virus infection.

Proapoptotic BH3-only protein Bim is essential for developmentally programmed death of germinal center-derived memory B cells and antibody-forming cells

T cell-dependent B-cell immune responses induce germinal centers that are sites for expansion, diversification, and selection of antigen-specific B cells. During the immune response, antigen-specific B cells are removed in a process that favors the retention of cells with improved affinity for antigen, a cell death process inhibited by excess Bcl-2. In this study, we examined the role of the BH3-only protein Bim, an initiator of apoptosis in the Bcl-2-regulated pathway, in the programmed cell death accompanying an immune response. After immunization, Bim-deficient mice showed persistence of both memory B cells lacking affinity-enhancing mutations in their immunoglobulin genes and antibody-forming cells secreting low-affinity antibodies. This was accompanied by enhanced survival of both cell types in culture. We have identified for the first time the physiologic mechanisms for killing low-affinity antibody-expressing B cells in an immune response and have shown this to be dependent on the BH3-only protein Bim.

Initiation of plasma-cell differentiation is independent of the transcription factor Blimp-1

Blimp-1 is considered an essential regulator of the terminal differentiation of B cells into antibody-secreting plasma cells. We show here that Rag1-/- mice reconstituted with fetal liver cells homozygous for a DNA-binding-deficient mutant of Prdm1 (the gene encoding Blimp-1) lack a defined plasma-cell compartment, yet show detectable amounts of all immunoglobulin isotypes. In vitro analysis revealed that Blimp-1 is not required for the initiation of antibody secretion but is essential for subsequent high immunoglobulin production. Blimp-1-independent differentiation was blocked at a preplasmablast stage characterized by decreased Pax5 expression and the activation of plasma-cell genes. Analysis of Blimp-1-sufficient differentiation revealed a phase prior to Blimp-1 expression in which several genes normally repressed by Pax5 are re-expressed, suggesting that plasma-cell differentiation is initiated by the inhibition of Pax5 function. Our results indicate that full plasma-cell differentiation but not commitment to the plasma-cell fate requires the expression of functional Blimp-1.

Salmonella induces a switched antibody response without germinal centers that impedes the extracellular spread of infection

T-dependent Ab responses are characterized by parallel extrafollicular plasmablast growth and germinal center (GC) formation. This study identifies that, in mice, the Ab response against Salmonella is novel in its kinetics and its regulation. It demonstrates that viable, attenuated Salmonella induce a massive early T-dependent extrafollicular response, whereas GC formation is delayed until 1 mo after infection. The extrafollicular Ab response with switching to IgG2c, the IgG2a equivalent in C57BL/6 mice, is well established by day 3 and persists through 5 wk. Switching is strongly T dependent, and the outer membrane proteins are shown to be major targets of the early switched IgG2c response, whereas flagellin and LPS are not. GC responses are associated with affinity maturation of IgG2c, and their induction is associated with bacterial burden because GC could be induced earlier by treating with antibiotics. Clearance of these bacteria is not a consequence of high-affinity Ab production, for clearance occurs equally in CD154-deficient mice, which do not develop GC, and wild-type mice. Nevertheless, transferred low- and high-affinity IgG2c and less efficiently IgM were shown to impede Salmonella colonization of splenic macrophages. Furthermore, Ab induced during the infection markedly reduces bacteremia. Thus, although Ab does not prevent the progress of established splenic infection, it can prevent primary infection and impedes secondary hemogenous spread of the disease. These results may explain why attenuated Salmonella-induced B cell responses are protective in secondary, but not primary infections.

Correction of age-associated deficiency in germinal center response by immunization with immune complexes

In aging, both primary and secondary antibody responses are impaired. One of the most notable changes in age-associated immune deficiency is the diminished germinal center (GC) reaction. This impaired GC response reduces antibody affinity maturation, decreases memory B cell development, and prevents the establishment of long-term antibody-forming cells in the bone marrow. It is of great importance to explore novel strategy in improving GC response in the elderly. In this study, the efficacy of immunization with immune complexes in overcoming age-associated deficiency in GC response was investigated. We show that the depressed GC response in aged mice can be significantly elevated by immunization with immune complexes. Importantly, there is a significant improvement of B cell memory response and long-lived plasma cells. Our results demonstrate that immune complex immunization may represent a novel strategy to elicit functional GC response in aging, and possibly, to overcome age-related immune deficiency in general.

Recirculation of germinal center B cells: a multilevel selection strategy for antibody maturation

Optimization of antibody affinity is a hallmark of the humoral immune response. It takes place in hundreds of transient microstructures called germinal centers (GCs). Their function and time-dependent behavior are subjects of active investigation. According to a generally accepted notion, their individual kinetics follows the average kinetics of all GCs present in the observed lymphatic tissue. In this review, we challenge this view and point out, with the help of mathematical simulations, that inferring the kinetics of individual GCs from cross-sectional evaluation of GC kinetics is virtually impossible. Thus, the time course of individual GCs is open to conjecture. For instance, one possible interpretation is that GCs exist for a time span considerably shorter than that of the observed average kinetics. We explore the implications of different temporal organizations of GCs in the light of the hypothesis that GC B-cell emigrants recolonize GC niches. This assumption leads to a view where GCs work in parallel but are linked by recirculation of B-cell emigrants. In this view, interleaved global and local competition provide for an implementation of multiple levels of B-cell selection in affinity maturation. The concepts of iteration, all-or-none behavior, and phasic mutation schedule are discussed in the light of this hypothesis.

Affinity of antigen encounter and other early B-cell signals determine B-cell fate

Three possible effector fates await the naïve follicular B cell following antigen stimulation in thymus-dependent reactions. Short-lived plasma cells produce an initial burst of germline-encoded protective antibodies, and long-lived plasma cells and memory B cells arise from the germinal center and function to enhance and sustain the humoral immune response. The inherent B-cell receptor affinity of naïve follicular B cells and the contribution of other early B-cell signals pre-determines the pattern of transcription factor expression and the differentiation path taken by these cells. High initial B-cell receptor affinity shunts naïve follicular B-cell clones towards the short-lived plasma cell fate, whereas modest-affinity clones are skewed towards a plasma cell fate and low-affinity clones are recruited into the germinal center and are selected for both long-lived plasma cells and memory B cell pathways. In the germinal center reaction, increased levels of the transcription factor interferon regulatory factor-4 drive the molecular program that dictates differentiation into the long-lived plasma cell phenotype but has no impact on the memory B cell compartment. We hypothesize that graded interferon regulatory factor-4 levels driven by signals to B cells, including B-cell receptor signal strength, are responsible for this branch point in the B-cell terminal differentiation pathway.

FcγRIIb controls bone marrow plasma cell persistence and apoptosis

The survival of long-lived plasma cells, which produce most serum immunoglobulin, is central to humoral immunity. We found here that the inhibitory Fc receptor FcgammaRIIb was expressed on plasma cells and controlled their persistence in the bone marrow. Crosslinking FcgammaRIIb induced apoptosis of plasma cells, which we propose contributes to the control of their homeostasis and suggests a method for therapeutic deletion. Plasma cells from mice prone to systemic lupus erythematosus did not express FcgammaRIIb and were protected from apoptosis. Human plasmablasts expressed FcgammaRIIb and were killed by crosslinking, as were FcgammaRIIb-expressing myeloma cells. Our results suggest that FcgammaRIIb controls bone marrow plasma cell persistence and that defects in it may contribute to autoantibody production.

Imprinting the fate of antigen-reactive B cells through the affinity of the B cell receptor

Long-lived plasma cells (PCs) and memory B cells (B(mem)) constitute the cellular components of enduring humoral immunity, whereas short-lived PCs that rapidly produce Ig correspond to the host's need for immediate protection against pathogens. In this study we show that the innate affinity of the BCR for Ag imprints upon naive B cells their differentiation fate to become short- or long-lived PCs and B(mem). Using BCR transgenic mice with varying affinities for Ag, naive B cells with high affinity lose their capacity to form germinal centers (GCs), develop neither B(mem) nor long-lived PCs, and are destined to a short-lived PC fate. Moderate affinity interactions result in hastened GC responses, and differentiation to long-lived PCs, but B(mem) remain extinct. In contrast, lower affinity interactions show tempered GCs, producing B(mem) and affinity-matured, long-lived PCs. Thus, a continuum of elementary to comprehensive humoral immune responses exists that is controlled by inherent BCR affinity.

Kinetics of the immune response to the (F1+V) vaccine in models of bubonic and pneumonic plague

Protection against aerosol challenge with > 300 MLD of Yersinia pestis was observed 7 days after a single immunisation of mice with the F1+V vaccine. At day 60, mice were protected against injected challenge (10(7)MLD) in a vaccine dose-related manner. Recall responses to rV in splenocytes ex vivo at day 98 correlated significantly (p<0.001) with the immunising dose-level of V antigen; no memory response or anti-V serum IgG was detected in killed whole cell vaccine (KWCV) recipients. This may explain the susceptibility of KWCV recipients to aerosol challenge and the enhanced protection conferred by the F1+V sub-unit vaccine, particularly since the anti-F1 responses induced by either vaccine were similarly IgG1-polarised.

B-cell memory: are subsets necessary?

B-cell memory is provided by populations of quiescent memory B cells and long-lived plasma cells. Whereas it is clear that both of these cell populations arise from germinal centres, the signals and circumstances that trigger germinal-centre B cells to enter and then persist in memory compartments are poorly defined. Here, I propose that germinal centres produce memory B cells and plasma cells throughout the immune response and that memory B cells arise by the emigration of B cells that are chosen at random from the pool available in the germinal centre. The ability of such emigrants to survive as memory B cells depends on their germinal-centre 'history', with the persistence of high-affinity B-cell variants being favoured.

Plasma cell S1P1 expression determines secondary lymphoid organ retention versus bone marrow tropism

After induction in secondary lymphoid organs, a subset of antibody-secreting cells (ASCs) homes to the bone marrow (BM) and contributes to long-term antibody production. The factors determining secondary lymphoid organ residence versus BM tropism have been unclear. Here we demonstrate that in mice treated with FTY720 or that lack sphingosine-1-phosphate (S1P) receptor-1 (S1P1) in B cells, IgG ASCs are induced and localize normally in secondary lymphoid organs but they are reduced in numbers in blood and BM. Many IgG ASCs home to BM on day 3 of the secondary response and day 3 splenic ASCs exhibit S1P responsiveness, whereas the cells remaining at day 5 are unable to respond. S1P1 mRNA abundance is higher in ASCs isolated from blood compared to spleen, whereas CXCR4 expression is lower. Blood ASCs also express higher amounts of Kruppel-like factor (KLF)2, a regulator of S1P1 gene expression. These findings establish an essential role for S1P1 in IgG plasma cell homing and they suggest that differential regulation of S1P1 expression in differentiating plasma cells may determine whether they remain in secondary lymphoid organs or home to BM.

Intrinsic properties of human and murine memory B cells

The central question of how the immune system responds in a qualitatively and quantitatively better way upon re-exposure to a pathogen is largely unanswered. Both the increased frequency of antigen-specific memory cells and the intrinsic properties that memory cells acquire after antigen experience could contribute to the faster and more robust responses seen after repeated exposure to antigen. In the case of the memory B-cell response, it has been difficult to discern the individual contributions of these two effects. However, because of recent advances in identifying memory B cells, there is an increasing understanding of the intrinsic properties of these cells. The current insights into the unique properties of memory B cells and the progress that has been made in understanding how these affect secondary responses in both the human and the mouse systems are discussed. In addition, we compare the various advantages and disadvantages inherent in each of these systems, in terms of studying the intrinsic properties of memory B cells, and introduce the details of the system that we have developed using conventional heavy chain transgenic (Tgic) mice, which addresses some of the drawbacks of traditional memory models.

Long-term comparison of rituximab treatment for refractory systemic lupus erythematosus and vasculitis: Remission, relapse, and re-treatment

OBJECTIVE

Current treatments for systemic lupus erythematosus (SLE) and vasculitis contribute to mortality and incapacity and are only partially effective; thus, newer therapies are clearly needed. Depletion of B cells has led to disease control in patients with autoimmune disorders. We sought to assess the long-term efficacy and safety of a B cell-depleting therapy in patients with SLE and patients with vasculitis.

METHODS

In a prospective study with a median followup of 24 months, 11 patients with active or refractory SLE and 11 patients with active or refractory antineutrophil cytoplasmic antibody-associated vasculitis (AAV) received a course of therapy with rituximab (an anti-CD20 monoclonal antibody) along with a single dose of intravenous cyclophosphamide.

RESULTS

Remission followed rapid B cell depletion, with response rates of 100% among the 11 patients with SLE (6 patients had a complete response, and 5 patients had a partial response) and 91% among the 11 patients with AAV (9 patients had a complete response, and 1 patient had partial remission). A renal response occurred in all 6 patients with lupus nephritis. Clinical improvement was accompanied by significant reductions in the daily dose of prednisolone. Relapse occurred in 64% of the patients with SLE and in 60% of those with AAV. B cell return preceded relapse in the majority of patients, and further treatment with rituximab proved effective. IgG and IgM levels were maintained in the normal range. The incidence of infective complications was low; however, infusion reactions were common, and human antichimeric antibodies developed in 5 of 14 patients.

CONCLUSION

B cell depletion offers the prospect of sustained disease remission and improved disease control combined with low toxicity in patients with active or refractory SLE or AAV. Relapse following treatment is common, but re-treatment is rapidly effective.

High affinity germinal center B cells are actively selected into the plasma cell compartment

A hallmark of T cell–dependent immune responses is the progressive increase in the ability of serum antibodies to bind antigen and provide immune protection. Affinity maturation of the antibody response is thought to be connected with the preferential survival of germinal centre (GC) B cells that have acquired increased affinity for antigen via somatic hypermutation of their immunoglobulin genes. However, the mechanisms that drive affinity maturation remain obscure because of the difficulty in tracking the affinity-based selection of GC B cells and their differentiation into plasma cells. We describe a powerful new model that allows these processes to be followed as they occur in vivo. In contrast to evidence from in vitro systems, responding GC B cells do not undergo plasma cell differentiation stochastically. Rather, only GC B cells that have acquired high affinity for the immunizing antigen form plasma cells. Affinity maturation is therefore driven by a tightly controlled mechanism that ensures only antibodies with the greatest possibility of neutralizing foreign antigen are produced. Because the body can sustain only limited numbers of plasma cells, this “quality control” over plasma cell differentiation is likely critical for establishing effective humoral immunity.

Characterization of (4-hydroxy-3-nitrophenyl)acetyl (NP)-specific germinal center B cells and antigen-binding B220- cells after primary NP challenge in mice

Previous studies examining the primary germinal center (GC) response to SRBC in mice demonstrated a steady ratio of IgM(+) to isotype-switched GC B cells and a persistent population of GC B cells with a founder phenotype. These characteristics held true at the inductive, plateau, and dissociative phases of the GC response, suggesting a steady-state environment. To test whether these characteristics apply to the primary response of other T cell-dependent Ags, the present study examined the GC response after challenge with (4-hydroxy-3-nitrophenyl)acetyl (NP) in C57BL/6 mice. Multiparameter flow cytometric analysis was used to assess the phenotype of splenic NP-reactive cells at multiple time points after immunization. Results of these studies demonstrated the characteristics of the SRBC-induced GC reaction to be fully maintained in the NP response. In particular, there was a steady ratio of nonswitched to switched B cells, with the majority of NP-reactive GC B cells displaying IgM. In addition, a substantial frequency of B220(-) NP-binding cells was observed in the spleen at later time points after NP challenge. Although these cells were IgE(+), they were found to express both kappa and lambda L chains and display the high-affinity IgE Fc (FcepsilonRI) receptor, suggesting that this population is not of B cell origin. Adoptive transfer studies further demonstrated the B220(-) NP-binding subset to be derived from the myeloid lineage.

Re-evaluating the recycling hypothesis in the germinal centre

Mathematical models have been used to study different aspects of the germinal centre reaction, in particular, affinity maturation of antibodies and the hypothesis of recycling. So far, interpretation of several theoretical and experimental results has pointed to the existence of recycling. However, theoretical models have seldom been compared with experimental data from specific immune responses and the potential relevance of recycling in the germinal centre is still an open problem. In this article, we propose a model without recycling that takes into account selection mechanisms that were previously uncovered experimentally. We apply the model to several experimental systems that use different Ag and compare the results with experimental data of affinity maturation whenever available. The results obtained for a primary immune response to the hapten (4-hydroxy-3-nitrophenyl)-acetyl show that recycling is not a necessary mechanism to achieve the level of affinity maturation observed in germinal centre reactions. Similar levels of affinity maturation are obtained for other responses, although for antibodies involving several affinity-enhancing mutations the affinity maturation obtained with the model is much lower. Interpretation of these results and consequences towards the concept of recycling are discussed.

Competence and competition: the challenge of becoming a long-lived plasma cell

Plasma cells provide humoral immunity. They have traditionally been viewed mainly as short-lived end-stage products of B-cell differentiation that deserve little interest. This view is changing, however, because we now know that plasma cells can survive for long periods in the appropriate survival niches and that they are an independent cellular component of immunological memory. Studies of the biology of plasma cells reveal a mechanism of intriguing simplicity and elegance that focuses memory provided by plasma cells on recently encountered pathogens while minimizing the 'fading' of memory for pathogens encountered in the distant past. This mechanism is based on competition for survival niches between newly generated plasmablasts and older plasma cells.