Selecting B cells and plasma cells to memory

A mathematical model to describe antibody-dependent enhancement and assess the effect of limiting cloning for plasma cells in heterologous secondary dengue infection

We propose a mathematical model to study the antibody-dependent enhancement (ADE) phenomenon. Here, we explore the interaction between macrophages, dengue virus and plasma cells, especially the effect of a limitation on plasma cell proliferation, which occurs due to immunological memory. The model has up to three equilibrium points: one virus-free equilibrium and two virus-presence equilibrium, depending on the value of two thresholds. We determine the existence regions for the model equilibrium points and their stability, a sensitivity analysis was performed in the model thresholds. Numerical simulations illustrate that ADE can occur even when the basic reproduction number is less than one.

Adaptive immune responses in channel catfish exposed to Edwardsiella ictaluri live attenuated vaccine and wild type strains through the specific gene expression profiles

We extend the previous findings on the differential activity of immune-related genes in the lymphoid organs of channel catfish in the 7 days post-challenge (dpc) with E. ictaluri live attenuated vaccines (LAVs) and wild type (WT) strains by assessing the expression of these genes in the 21 dpc. The expression of T and B cell-specific genes were significantly elevated in the spleen at 14 dpc and in the AK at 21 dpc in catfish treated with E. ictaluri WT and LAV strains compared to a non-treated control group. The gene expression of IFN-γ correlated with adaptive immunity genes in the lymphoid tissues of catfish. These data indicate that two novel LAVs were able to trigger the activation of T helper1 polarization cytokine IFN-γ gene and specific lymphocyte genes in the spleen followed by their activation in the AK of catfish without causing inflammation, thus providing protective immunity in E. ictaluri infection.

Generation of HBsAg-reactive T- and B-cells following HBV vaccination in serological non-responders under hemodialysis treatment

HBV vaccination is recommend for hemodialysis patients, but only 50-60% of the patients show seroconversion. HBV vaccine-induced generation of HBV reactive T and B cells could be detected regardless of their capacity to mount a serological response, indicating that patients without seroconversion are potentially protected by their HBV-reactive T cell pool.

A simple mathematical model to describe antibody-dependent enhancement in heterologous secondary infection in dengue

We develop a mathematical model to describe the role of antibody-dependent enhancement (ADE) in heterologous secondary infections, assuming that antibodies specific to primary dengue virus (DENV) infection are being produced by immunological memory. The model has a virus-free equilibrium (VFE) and a unique virus-presence equilibrium (VPE). VFE is asymptotically stable when VPE is unstable; and unstable, otherwise. Additionally, there is an asymptotic attractor (not a fixed point) due to the fact that the model assumes unbounded increase in memory cells. In the analysis of the model, ADE must be accounted in the initial stage of infection (a window of time of few days), period of time elapsed from the heterologous infection until the immune system mounting an effective response against the secondary infection. We apply the results yielded by model to evaluate ADE phenomonon in heterologous DENV infection. We also associate the possible occurrence of severe dengue with huge viremia mediated by ADE phenomenon.

Peripheral and synovial mechanisms of humoral autoimmunity in rheumatoid arthritis

One of the hallmarks of rheumatoid arthritis (RA) is the development of humoral autoimmunity resulting in circulating autoantibodies. The clinical efficacy of B cell-depleting biologic treatments highlighted a key role for autoreactive B cell activation in the pathogenesis of RA. In this review, we discuss the key mechanisms leading to breach of B cell self-tolerance in the peripheral compartment. We also highlight the contribution of synovial ectopic lymphoid structures (ELS) in the development of functional niches of autoreactive B cells promoting humoral autoimmunity in the inflamed RA joints over and above secondary lymphoid organs (SLO).

Differences in mouse and human nonmemory B cell pools

Identifying cross-species similarities and differences in immune development and function is critical for maximizing the translational potential of animal models. Coexpression of CD21 and CD24 distinguishes transitional and mature B cell subsets in mice. In this study, we validate these markers for identifying analogous subsets in humans and use them to compare the nonmemory B cell pools in mice and humans, across tissues, and during fetal/neonatal and adult life. Among human CD19(+)IgM(+) B cells, the CD21/CD24 schema identifies distinct populations that correspond to transitional 1 (T1), transitional 2 (T2), follicular mature, and marginal zone subsets identified in mice. Markers specific to human B cell development validate the identity of marginal zone cells and the maturation status of human CD21/CD24 nonmemory B cell subsets. A comparison of the nonmemory B cell pools in bone marrow, blood, and spleen in mice and humans shows that transitional B cells comprise a much smaller fraction in adult humans than mice. T1 cells are a major contributor to the nonmemory B cell pool in mouse bone marrow, in which their frequency is more than twice that in humans. Conversely, in spleen, the T1:T2 ratio shows that T2 cells are proportionally ∼ 8-fold higher in humans than in mice. Despite the relatively small contribution of transitional B cells to the human nonmemory pool, the number of naive follicular mature cells produced per transitional B cell is 3- to 6-fold higher across tissues than in mice. These data suggest differing dynamics or mechanisms produce the nonmemory B cell compartments in mice and humans.

Role of lymphoid chemokines in the development of functional ectopic lymphoid structures in rheumatic autoimmune diseases

A sizeable subset of patients with the two most common organ-specific rheumatic autoimmune diseases, rheumatoid arthritis (RA) and Sjögren's syndrome (SS) develop ectopic lymphoid structures (ELS) in the synovial tissue and salivary glands, respectively. These structures are characterized by perivascular (RA) and periductal (SS) clusters of T and B lymphocytes, differentiation of high endothelial venules and networks of stromal follicular dendritic cells (FDC). Accumulated evidence from other and our group demonstrated that the formation and maintenance of ELS in these chronic inflammatory conditions is critically dependent on the ectopic expression of lymphotoxins (LT) and lymphoid chemokines CXCL13, CCL19, CCL21 and CXCL12. In this review we discuss recent advances highlighting the cellular and molecular mechanisms, which regulate the formation of ELS in RA and SS, with particular emphasis on the role of lymphoid chemokines. In particular, we shall focus on the evidence that in the inflammatory microenvironment of the RA synovium and SS salivary glands, several cell types, including resident epithelial, stromal and endothelial cells as well as different subsets of infiltrating immune cells, have been shown to be capable of producing lymphoid chemokines. Finally, we summarize accumulating data supporting the conclusion that ELS in RA and SS represent functional niches for B cells to undergo affinity maturation, clonal selection and differentiation into plasma cells autoreactive against disease-specific antigens, thus contributing to humoral autoimmunity over and above that of secondary lymphoid organs.

Inducible tertiary lymphoid structures, autoimmunity, and exocrine dysfunction in a novel model of salivary gland inflammation in C57BL/6 mice

Salivary glands in patients with Sjögren's syndrome (SS) develop ectopic lymphoid structures (ELS) characterized by B/T cell compartmentalization, the formation of high endothelial venules, follicular dendritic cell networks, functional B cell activation with expression of activation-induced cytidine deaminase, as well as local differentiation of autoreactive plasma cells. The mechanisms that trigger ELS formation, autoimmunity, and exocrine dysfunction in SS are largely unknown. In this article, we present a novel model of inducible ectopic lymphoid tissue formation, breach of humoral self-tolerance, and salivary hypofunction after delivery of a replication-deficient adenovirus-5 in submandibular glands of C57BL/6 mice through retrograde excretory duct cannulation. In this model, inflammation rapidly and consistently evolves from diffuse infiltration toward the development of SS-like periductal lymphoid aggregates within 2 wk from AdV delivery. These infiltrates progressively acquire ELS features and support functional GL7(+)/activation-induced cytidine deaminase(+) germinal centers. Formation of ELS is preceded by ectopic expression of lymphoid chemokines CXCL13, CCL19, and lymphotoxin-β, and is associated with development of anti-nuclear Abs in up to 75% of mice. Finally, reduction in salivary flow was observed over 3 wk post-AdV infection, consistent with exocrine gland dysfunction as a consequence of the inflammatory response. This novel model has the potential to unravel the cellular and molecular mechanisms that regulate ELS formation and their role in exocrine dysfunction and autoimmunity in SS.

Differential regulation of self-reactivity discriminates between IgG+ human circulating memory B cells and bone marrow plasma cells

Long-term humoral immunity is maintained by the formation of high-affinity class-switched memory B cells and long-lived antibody-secreting plasma cells. In healthy humans, a substantial fraction of IgG-positive memory B cells express self-reactive and polyreactive IgG antibodies that frequently develop by somatic mutations. Whether self- and polyreactive IgG-secreting B cells are also tolerated in the long-lived plasma cell pool is not known. To address this question, we cloned and expressed the Ig genes from 177 IgG-producing bone marrow plasma cells of four healthy donors. All antibodies were highly mutated but the frequency of self- and polyreactive IgG antibodies was significantly lower than that found in circulating memory B cells. The data suggest that in contrast to the development of memory B cells, entry into the bone marrow plasma cell compartment requires previously unappreciated selective regulation by mechanisms that limit the production of self- and polyreactive serum IgG antibodies.

B cells and immunosenescence: a focus on IgG+IgD-CD27- (DN) B cells in aged humans

Immunosenescence contributes to the decreased ability of the elderly to control infectious diseases, which is also reflected in their generally poor response to new antigens and vaccination. It is known that the T cell branch of the immune system is impaired in the elderly mainly due to expansion of memory/effector cells that renders the immune system less able to respond to new antigens. B lymphocytes are also impaired in the elderly in terms of their response to new antigens. In this paper we review recent work on B cell immunosenescence focusing our attention on memory B cells and a subset of memory B cells (namely IgG(+)IgD(-)CD27(-)) that we have demonstrated is increased in healthy elderly.

Factor VIII-specific B cell responses in haemophilia A patients with inhibitors

Development of inhibitory antibodies to factor VIII (FVIII) provides a major complication of replacement therapy in patients with haemophilia A. The risk of inhibitor formation is influenced by the underlying FVIII gene defect. Moreover, genetic determinants in the promoter region of IL-10 and TNFalpha have been linked to an increased risk of inhibitor development. Recent cohort-studies have provided evidence that the risk of inhibitor formation is linked to intensity of treatment. Eradication of FVIII inhibitors can be achieved by frequent infusion of high dosages of FVIII, so-called immune tolerance induction (ITI). Until now, the mechanisms involved in downmodulation of the immune response to FVIII during ITI have not been unraveled. Studies performed in an animal model for haemophilia A have suggested that elimination of FVIII-specific memory B cells by high dosages of FVIII contributes to the decline in FVIII inhibitor levels during ITI. Limited knowledge is available with respect to the development and persistence of FVIII-specific memory B cells in patients with haemophilia A. Two recent studies suggest that the frequency of peripheral FVIII-specific memory B cells in haemophilia A patients with inhibitors range from <0.01 to 0.40% of that of total IgG(+) B cells. No or very low frequencies of FVIII-specific memory B cells are observed in haemophilia A patients without inhibitors and in patients treated successfully by ITI. Possible implications of these findings are discussed in the context of currently available information on the role of antigen-specific memory B cells and long-living antibody producing plasma cells in humoral immunity.

A double-edged sword in B-cell-targeted therapy for inflammatory diseases

Cells of the immune system, including B cells, perform inflammatory functions against microbial invasion, accompanied by anti-inflammatory responses to avoid host damage. B-cell-depletion therapy using anti-CD20 monoclonal antibodies against inflammatory diseases has beneficial or adverse effects depending on the timing and/or microenvironment in which they are used. To achieve effective B-cell-targeted therapy, it is necessary to identify and understand the modes of action of pathogenic and regulatory B cells, which include antibody production, formation of immune complexes, cytokine and chemokine production, cytotoxic killing, lymphoid neogenesis and antigen presentation. B cells interact with multiple cells, including dendritic cells, T cells and natural killer T cells, creating a complex regulatory network. Specific targeting of B-cell subsets and/or their interaction partners might lead to clinical benefits with minimal host damage.

Studies of the anti-platelet factor 4/heparin immune response: adapting the enzyme-linked immunosorbent spot assay for detection of memory B cells against complex antigens

BACKGROUND

The anti-platelet factor 4 (PF4)/heparin immune response, which underlies heparin-induced thrombocytopenia (HIT), has several atypical features: relatively rapid onset even without previous heparin exposure, lack of immune anamnesis, and transience of antibody production.

STUDY DESIGN AND METHODS

We modified the enzyme-linked immunosorbent spot (ELISPOT) assay to investigate for PF4/heparin-specific memory B cells in cardiac surgery patients, in whom the high anti-PF4/heparin immunization rate made a prospective study feasible. The PF4-containing antigen complexes were attached to microtiter plates via a spacer, rather than using nitrocellulose, and the final reaction enzyme substrate was added in melted agarose which, after rapid hardening, localized color development of enzyme-tagged anti-immunoglobulin G (IgG) probes to single PF4/heparin-specific B cells. This modified ELISPOT assay was applied to 58 consecutive patients (testing blood from preoperative baseline and Postoperative Days 6 and 10), in which we compared detectability of PF4/heparin-specific B cells to tetanus toxin-specific B cells (comparator group with presumed vaccination).

RESULTS

No patient had detectable PF4/heparin-specific memory B cells at baseline. In 2 of 30 patients (6.7%) who formed anti-PF4/heparin IgG, PF4/heparin-specific memory B cells (three to four spots/well) were detected by Postoperative Day 10, whereas tetanus toxin-specific memory B cells were found in 12 of 24 (50.0%) patients tested (3-25 spots/well; p < 0.001).

CONCLUSIONS

HIT lacks a strong memory B-cell response, perhaps explaining transience and lack of anamnesis of the anti-PF4/heparin immune response. The technical modifications we describe for the ELISPOT assay, which permit detection of B-cell reactions to complex antigens, could be useful for studying other immunohematologic disorders, for example, drug-dependent thrombocytopenia and acquired hemophilia.

Ectopic lymphoid structures support ongoing production of class-switched autoantibodies in rheumatoid synovium

BACKGROUND

Follicular structures resembling germinal centres (GCs) that are characterized by follicular dendritic cell (FDC) networks have long been recognized in chronically inflamed tissues in autoimmune diseases, including the synovium of rheumatoid arthritis (RA). However, it is debated whether these ectopic structures promote autoimmunity and chronic inflammation driving the production of pathogenic autoantibodies. Anti-citrullinated protein/peptide antibodies (ACPA) are highly specific markers of RA, predict a poor prognosis, and have been suggested to be pathogenic. Therefore, the main study objectives were to determine whether ectopic lymphoid structures in RA synovium: (i) express activation-induced cytidine deaminase (AID), the enzyme required for somatic hypermutation and class-switch recombination (CSR) of Ig genes; (ii) support ongoing CSR and ACPA production; and (iii) remain functional in a RA/severe combined immunodeficiency (SCID) chimera model devoid of new immune cell influx into the synovium.

METHODS AND FINDINGS

Using immunohistochemistry (IHC) and quantitative Taqman real-time PCR (QT-PCR) in synovial tissue from 55 patients with RA, we demonstrated that FDC+ structures invariably expressed AID with a distribution resembling secondary lymphoid organs. Further, AID+/CD21+ follicular structures were surrounded by ACPA+/CD138+ plasma cells, as demonstrated by immune reactivity to citrullinated fibrinogen. Moreover, we identified a novel subset of synovial AID+/CD20+ B cells outside GCs resembling interfollicular large B cells. In order to gain direct functional evidence that AID+ structures support CSR and in situ manufacturing of class-switched ACPA, 34 SCID mice were transplanted with RA synovium and humanely killed at 4 wk for harvesting of transplants and sera. Persistent expression of AID and Igamma-Cmu circular transcripts (identifying ongoing IgM-IgG class-switching) was observed in synovial grafts expressing FDCs/CD21L. Furthermore, synovial mRNA levels of AID were closely associated with circulating human IgG ACPA in mouse sera. Finally, the survival and proliferation of functional B cell niches was associated with persistent overexpression of genes regulating ectopic lymphoneogenesis.

CONCLUSIONS

Our demonstration that FDC+ follicular units invariably express AID and are surrounded by ACPA-producing plasma cells provides strong evidence that ectopic lymphoid structures in the RA synovium are functional and support autoantibody production. This concept is further confirmed by evidence of sustained AID expression, B cell proliferation, ongoing CSR, and production of human IgG ACPA from GC+ synovial tissue transplanted into SCID mice, independently of new B cell influx from the systemic circulation. These data identify AID as a potential therapeutic target in RA and suggest that survival of functional synovial B cell niches may profoundly influence chronic inflammation, autoimmunity, and response to B cell-depleting therapies.

Ectopic lymphoid structures support ongoing production of class-switched autoantibodies in rheumatoid synovium

BACKGROUND

Follicular structures resembling germinal centres (GCs) that are characterized by follicular dendritic cell (FDC) networks have long been recognized in chronically inflamed tissues in autoimmune diseases, including the synovium of rheumatoid arthritis (RA). However, it is debated whether these ectopic structures promote autoimmunity and chronic inflammation driving the production of pathogenic autoantibodies. Anti-citrullinated protein/peptide antibodies (ACPA) are highly specific markers of RA, predict a poor prognosis, and have been suggested to be pathogenic. Therefore, the main study objectives were to determine whether ectopic lymphoid structures in RA synovium: (i) express activation-induced cytidine deaminase (AID), the enzyme required for somatic hypermutation and class-switch recombination (CSR) of Ig genes; (ii) support ongoing CSR and ACPA production; and (iii) remain functional in a RA/severe combined immunodeficiency (SCID) chimera model devoid of new immune cell influx into the synovium.

METHODS AND FINDINGS

Using immunohistochemistry (IHC) and quantitative Taqman real-time PCR (QT-PCR) in synovial tissue from 55 patients with RA, we demonstrated that FDC+ structures invariably expressed AID with a distribution resembling secondary lymphoid organs. Further, AID+/CD21+ follicular structures were surrounded by ACPA+/CD138+ plasma cells, as demonstrated by immune reactivity to citrullinated fibrinogen. Moreover, we identified a novel subset of synovial AID+/CD20+ B cells outside GCs resembling interfollicular large B cells. In order to gain direct functional evidence that AID+ structures support CSR and in situ manufacturing of class-switched ACPA, 34 SCID mice were transplanted with RA synovium and humanely killed at 4 wk for harvesting of transplants and sera. Persistent expression of AID and Igamma-Cmu circular transcripts (identifying ongoing IgM-IgG class-switching) was observed in synovial grafts expressing FDCs/CD21L. Furthermore, synovial mRNA levels of AID were closely associated with circulating human IgG ACPA in mouse sera. Finally, the survival and proliferation of functional B cell niches was associated with persistent overexpression of genes regulating ectopic lymphoneogenesis.

CONCLUSIONS

Our demonstration that FDC+ follicular units invariably express AID and are surrounded by ACPA-producing plasma cells provides strong evidence that ectopic lymphoid structures in the RA synovium are functional and support autoantibody production. This concept is further confirmed by evidence of sustained AID expression, B cell proliferation, ongoing CSR, and production of human IgG ACPA from GC+ synovial tissue transplanted into SCID mice, independently of new B cell influx from the systemic circulation. These data identify AID as a potential therapeutic target in RA and suggest that survival of functional synovial B cell niches may profoundly influence chronic inflammation, autoimmunity, and response to B cell-depleting therapies.

Manipulating B cell homeostasis: a key component in the advancement of targeted strategies

Understanding the homeostatic mechanisms governing lymphocyte pools achieves critical importance as lymphocyte-targeted therapies expand in use and scope. The primacy of B lymphocyte stimulator (BLyS) family ligands and receptors in governing B lymphocyte homeostasis has become increasingly clear in recent years, affording insight into novel opportunities and potential pitfalls for targeted B cell therapeutics. Interclonal competition for BLyS-BR3 interactions determines the size of naïve B cell pools and can regulate the stringency of selection applied as cells complete maturation. Thus one of the predicted consequences of ablative therapies targeting primary pools is relaxed negative selection. This suggests that BLyS levels and B cell reconstitution rates may serve useful prognostic roles and that BLyS itself might be targeted to circumvent relapse. Alternatively, manipulations that allow rare, minimally autoreactive specificities to survive and mature may lead to opportunities in cases where antibody-based vaccine development has heretofore been unsuccessful. BLyS family ligands and receptors also play a role in activated and memory B cell pools, suggesting they might likewise be targeted to promote or delete particular antigen-experienced subpopulations in a similar way.

Phenotypic and functional heterogeneity of human memory B cells

Memory B cells are more heterogeneous than previously thought. Given that B cells play powerful antibody-independent effector functions, it seems reasonable to assume division of labor between distinct memory B cells subpopulations in both protective and pathogenic immune responses. Here we review the information emerging regarding the heterogeneity of human memory B cells. A better understanding of this topic should greatly improve our ability to target specific B cell subsets either in vaccine responses or in autoimmune diseases and organ rejection among other pathological conditions where B cells play central pathogenic roles.

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.

The kinetics and phenotype of the human B-cell response following immunization with a heptavalent pneumococcal-CRM conjugate vaccine

Primary immunization of infants with protein-polysaccharide conjugate vaccines induces antipolysaccharide antibody and is highly effective in preventing invasive disease caused by encapsulated bacteria. However, recent experience from the UK indicates that this immunity is not sustained in the absence of booster doses of vaccine. This study aimed to establish the kinetics and phenotype of B-cell subpopulations responding to booster immunization with a heptavalent pneumococcal conjugate vaccine (Pnc7), which is to be introduced into the primary immunization schedule in the UK during 2006. Six adult volunteers received a booster dose of Pnc7 12-18 months after primary immunization. CD27hi CD38hi CD20(+/-) IgG antibody-forming cells were detected in peripheral blood with maximum frequency at days 6-7 after immunization. This was accompanied by a more prolonged rise in memory B cells that required in vitro stimulation with Staphylococcus aureus Cowan strain and interleukin-2 to induce antibody secretion. These data provide evidence for at least two subsets of antibody-forming cells involved in the secondary humoral response to a glycoconjugate vaccine in primed individuals. A briefly circulating subset of B cells that spontaneously secrete immunoglobulin G may be responsible for early defence against re-encountered encapsulated bacteria. However, the kinetics of the appearance of these cells may indicate that the humoral immune response is too slow in defence against an organism that invades within days of acquisition. The more sustained presence of a memory population may provide persistence of antipolysaccharide antibody after a booster dose of vaccine and may also include re-circulatory populations responsible for further anamnestic responses.

Role of B Cells in Pathogenesis of Multiple Sclerosis

Despite the current limited understanding of the etiology of multiple sclerosis (MS), genetic susceptibility and environmental influences are known driving factors. MS is considered a T-cell-mediated disease given the prevalence of T cells in plaques. Plaque formation is characteristic of this disease attributable to immune mechanisms, triggered by an autoimmune attack aimed at antigens in the myelin sheath or oligodendrocyte proteins. The attack consists of the following: The role of the B cells is twofold: first, as autoreactive B cells they produce autoantibodies, secrete cytokines, clonally replicate memory B cells, and long-living plasma cells which serve to advance the diseased state by their constant production of autoantibodies. Second, as antigen-presenting cells they activate the autoreactive T cells. For this reason, the stipulation that T cell is the cornerstone of MS must be reevaluated. Various studies on pathogenesis of MS have indicated that B cells, as the humoral component of the adaptive immune system, are active participants in pathogenesis and lesion maintenance throughout the disease process. The active role of B cells and autoantibodies makes them an encouraging therapeutic target. Advances in the understanding of B-cell development and activity would allow for an enhanced strategy in the design of autoimmune treatment. For this reason, further investigation is necessary to determine whether depletion of B cells or antibodies may restore immune function.

Adaptation of humoral memory

Immunological memory, as provided by antibodies, depends on the continued presence of antibody-secreting cells, such as long-lived plasma cells of the bone marrow. Survival niches for these memory plasma cells are limited in number. In an established immune system, acquisition of new plasma cells, generated in response to recent pathogenic challenges, requires elimination of old memory plasma cells. Here, we review the adaptation of plasma cell memory to new pathogens. This adaptation is dependent upon the influx of plasmablasts, generated in a secondary systemic immune reaction, into the pool of memory plasma cells, the efficiency of competition of new plasmablasts with old plasma cells, and the frequency of infection with novel pathogens. To maintain old plasma cells at frequencies high enough to provide protection and to accommodate as many specificities as possible, an optimal influx rate per infection exists. This optimal rate is approximately three times higher than the minimal number of plasma cells providing protection. Influx rates of plasmablasts generated by vaccination approximately match this optimum level. Furthermore, the observed stability of serum concentrations of vaccine-specific antibodies implies that the influxing plasmablasts mobilize a similar number of plasma cells and that competitive infectious challenges are not more frequent than once per month.

A new memory CD27-IgG+ B cell population in peripheral blood expressing VH genes with low frequency of somatic mutation

In humans, up to 40% of peripheral B cells express CD27 and have hypermutated variable regions in their Ig genes. The CD27+ B cells are considered to be derived from germinal center following specific antigenic stimulation. Actually, somatic hypermutation in Ig genes and CD27 expression are hallmarks of memory B cells. However, the blood IgM+ IgD+ CD27+ B cells were recently associated to splenic marginal zone B cells and proposed to be a subset distinct from germinal center-derived memory B cells showing premutated Igs. The results presented herein further weaken this bona fide association because B cells expressing surface IgG, but not CD27, were found in human blood. Representing 1-4% of all peripheral B cells and approximately 25% of the IgG+ blood B cells, this population expressed mutated IgG genes showing antigenic selection characteristics but with lower mutation frequencies than that of CD27+ IgG+ B cells. However, their morphology and phenotype were similar to that of CD27+ IgG+ cells. Interestingly, the proportion of IgG2 over IgG3 transcripts was opposite in CD27- IgG+ and CD27+ IgG+ cells, suggesting distinct functions or origins. Overall, these findings extend the memory B cell reservoir beyond the CD27+ compartment and could provide further insights into B cell disorders of unknown etiology.

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.

Initial clinical trial of epratuzumab (humanized anti-CD22 antibody) for immunotherapy of systemic lupus erythematosus

B cells play an important role in the pathogenesis of systemic lupus erythematosus (SLE), so the safety and activity of anti-B cell immunotherapy with the humanized anti-CD22 antibody epratuzumab was evaluated in SLE patients. An open-label, single-center study of 14 patients with moderately active SLE (total British Isles Lupus Assessment Group (BILAG) score 6 to 12) was conducted. Patients received 360 mg/m2 epratuzumab intravenously every 2 weeks for 4 doses with analgesic/antihistamine premedication (but no steroids) prior to each dose. Evaluations at 6, 10, 18 and 32 weeks (6 months post-treatment) follow-up included safety, SLE activity (BILAG score), blood levels of epratuzumab, B and T cells, immunoglobulins, and human anti-epratuzumab antibody (HAHA) titers. Total BILAG scores decreased by > or = 50% in all 14 patients at some point during the study (including 77% with a > or = 50% decrease at 6 weeks), with 92% having decreases of various amounts continuing to at least 18 weeks (where 38% showed a >/= 50% decrease). Almost all patients (93%) experienced improvements in at least one BILAG B- or C-level disease activity at 6, 10 and 18 weeks. Additionally, 3 patients with multiple BILAG B involvement at baseline had completely resolved all B-level disease activities by 18 weeks. Epratuzumab was well tolerated, with a median infusion time of 32 minutes. Drug serum levels were measurable for at least 4 weeks post-treatment and detectable in most samples at 18 weeks. B cell levels decreased by an average of 35% at 18 weeks and remained depressed at 6 months post-treatment. Changes in routine safety laboratory tests were infrequent and without any consistent pattern, and there was no evidence of immunogenicity or significant changes in T cells, immunoglobulins, or autoantibody levels. In patients with mild to moderate active lupus, 360 mg/m2 epratuzumab was well tolerated, with evidence of clinical improvement after the first infusion and durable clinical benefit across most body systems. As such, multicenter controlled studies are being conducted in broader patient populations.

B CELL IMMUNOBIOLOGY IN DISEASE: Evolving Concepts from the Clinic

The pathogenic roles of B cells in autoimmune diseases occur through several mechanistic pathways that include autoantibodies, immune complexes, dendritic and T cell activation, cytokine synthesis, chemokine-mediated functions, and ectopic neolymphogenesis. Each of these pathways participate to different degrees in autoimmune diseases. The use of B cell-targeted and B cell subset-targeted therapies in humans is illuminating the mechanisms at work in a variety of human autoimmune diseases. In this review, we highlight some of these recent findings that provide insights into both murine models of autoimmunity and human autoimmune diseases.

Blimp-1 is required for maintenance of long-lived plasma cells in the bone marrow

Long-lived plasma cells, residing primarily in the bone marrow, continuously secrete antibody and provide an important component of humoral memory. However, when such cells secrete autoantibodies or become transformed, they can be pathogenic. We have shown recently that the transcriptional repressor B lymphocyte–induced maturation protein 1 (Blimp-1) is required for the formation of plasma cells. To determine what role Blimp-1 might play in maintenance of plasma cells, we generated mice in which the gene encoding Blimp-1 could be deleted in an inducible manner. Deletion of Blimp-1 either in vitro or in vivo leads to loss of previously formed B220^LOCD138^HI plasma cells. Using BrdU incorporation, we confirmed that Blimp-1 is required for the maintenance of nondividing, long-lived plasma cells in the bone marrow. Blimp-1 is also required for long-term maintenance of antigen-specific immunoglobulin in serum. Thus Blimp-1 is required not only for the formation but also for the maintenance of long-lived plasma cells. This finding provides the possibility of new drug design strategies for autoimmunity and multiple myeloma focused on blocking Blimp-1 expression or activity.

B-cell differentiation in the CNS of patients with multiple sclerosis

Clonally expanded populations of Ig variable gene-mutated B cells are found in the central nervous system (CNS) of subjects with multiple sclerosis (MS), suggesting the occurrence of a germinal center-like reaction. Recent studies have demonstrated that the cerebrospinal fluid (CSF) of MS patients is enriched with centroblasts and B cells with a memory phenotype compared to peripheral blood. In the same individuals, antibody-secreting cells (ASC) are detected in the CSF and appear to correlate with CNS inflammation. These B-cell subsets are the output of a germinal center reaction, which is likely to occur in the CNS. Recent findings suggest that the inflamed brain can become a favorable niche for B-cell survival and proliferation and, under some circumstances, sustain the formation of ectopic lymphoid structures. Thus, B cells are likely to expand and mature inside the CNS, giving rise to ASC, which may play an effector role in the pathogenesis of MS.