Alterations in peripheral blood B cell subsets and dynamics of B cell responses during human schistosomiasis

Schistosome secretomes

Schistosomes are intravascular parasitic platyhelminths (blood flukes) that infect over 200 million people globally. Biomolecules secreted by the worms likely contribute to their ability to survive in the bloodstreams of immunocompetent hosts for many years. Here we review what is known about the protein composition of material released by the worms. Prominent among cercarial excretions/secretions (ES) is a ∼ 30 kDa serine protease called cercarial elastase (SmCE in Schistosoma mansoni), likely important in host invasion. Also prominent is a 117 amino acid non-glycosylated polypeptide (Sm16) that can impact several host cell-types to impinge on immunological outcomes. Similarly, components of the egg secretome (notably the 134 amino acid homodimeric glycoprotein "IL-4 inducing principle of schistosome eggs", IPSE, and the 225-amino acid monomeric T2 ribonuclease - omega-1) are capable of driving Th2-biased immune responses. A ∼36kDa chemokine binding glycoprotein SmCKBP, secreted by eggs, can negate the impact of several cytokines and can impede neutrophil migration. Of special interest is a disparate collection of classically cytosolic proteins that are surprisingly often identified in schistosome ES across life stages. These proteins, perhaps released as components of extracellular vesicles (EVs), include glycolytic enzymes, redox proteins, proteases and protease inhibitors, heat shock proteins, proteins involved in translation/turnover, histones, and others. Some such proteins may display "moonlighting" functions and, for example, impede blood clot formation around the worms. More prosaically, since several are particularly abundant soluble proteins, their appearance in the ES fraction may be indicative of worm damage ex vivo leading to protein leakage. Some bioactive schistosome ES proteins are in development as novel therapeutics against autoimmune, inflammatory, and other, non-parasitic, diseases.

Schistosomiasis in Gabon from 2000 to 2021 - A review

INTRODUCTION

Schistosomiasis is a public health issue of concern in Gabon, with the disease being reported from all regions of the country. The topic has been of interest for the local researchers and physicians for over two decades. The objective of this narrative review was to provide an overview of the research activities in the area from 2000 to early 2021.

METHODS

We performed a narrative literature review. The search strategy was designed to get a broad overview of the different research topics on schistosomiasis and the national control programme, and included grey literature.

RESULTS

A total of 159 articles was screened, and 42 were included into the review in addition to the grey literature. During the past two decades, the work on schistosomiasis originated from five out of the nine provinces of the country, with diverse aspects of the disease investigated; including immunology, epidemiology, diagnosis and treatment. Several studies investigated various aspects of schistosomiasis-related morbidity in the respective study populations. The body of work demonstrates that much effort was made to understand the details of the host immune response to schistosomiasis, and the immune profile changes induced in patients treated with praziquantel. Although some MDA campaigns were conducted in the country; little, however, is known on the epidemiological situation of the disease, particularly of its distribution within the population, as well as co-infections with other parasitic diseases also endemic in the area.

CONCLUSION

Progress has been made over the past two decades in the understanding of schistosomiasis in the country, including disease-related morbidity and its interaction with other parasitic infections, and the immunology and epidemiology of the disease. However, for optimising control of the disease, there is a need to fine-tune these findings with detailed local epidemiological and malacological data. We call for such studies to accomplish the knowledge of schistosomiasis in the country, particularly in areas of moderate or high endemicity, and recommend this approach to comparable schistosomiasis-endemic areas elsewhere.

Schistosoma mansoni infection induces plasmablast and plasma cell death in the bone marrow and accelerates the decline of host vaccine responses

Schistosomiasis is a potentially lethal parasitic disease that profoundly impacts systemic immune function in chronically infected hosts through mechanisms that remain unknown. Given the immunoregulatory dysregulation experienced in infected individuals, this study examined the impact of chronic schistosomiasis on the sustainability of vaccine-induced immunity in both children living in endemic areas and experimental infections in mice. Data show that chronic Schistosoma mansoni infection impaired the persistence of vaccine specific antibody responses in poliovirus-vaccinated humans and mice. Mechanistically, schistosomiasis primarily fostered plasmablast and plasma cell death in the bone marrow and removal of parasites following praziquantel treatment reversed the observed cell death and partially restored vaccine-induced memory responses associated with increased serum anti-polio antibody responses. Our findings strongly suggest a previously unrecognized mechanism to explain how chronic schistosomiasis interferes with an otherwise effective vaccine regimen and further advocates for therapeutic intervention strategies that reduce schistosomiasis burden in endemic areas prior to vaccination.

Schistosoma mansoni (S. mansoni), a schistosomiasis disease-causing parasite species, is most common in sub-Saharan Africa. Schistosoma mansoni can influence immune responses and trigger physiological imbalances in their human and animal hosts, which improve their survival and multiplication in the host. These influences can disrupt the host’s ability to maintain long term protective immunity mounted by vaccines for infectious diseases. Here, we investigated the impact of S. mansoni infection on poliovirus vaccine immunity in school-aged children and mice. We found that the parasite reduced its host’s ability to maintain protective blood antibodies produced by immune responses to poliovirus vaccines. We also found that S. mansoni infection reduces the maintenance of antibody-producing plasma cells in the bone marrow of vaccinated mice. Our data showed that treating S. mansoni infected children and mice with praziquantel mitigated the parasite’s negative influences on vaccine immunity. These findings suggest that in regions where schistosomiasis is endemic, the Schistosoma spp. parasites may be notable causes of suboptimal viral vaccine immunity maintenance by children, leaving them vulnerable to vaccine-preventable diseases.

Schistosome immunomodulators

Schistosomes are long lived, intravascular parasitic platyhelminths that infect >200 million people globally. The molecular mechanisms used by these blood flukes to dampen host immune responses are described in this review. Adult worms express a collection of host-interactive tegumental ectoenzymes that can cleave host signaling molecules such as the "alarmin" ATP (cleaved by SmATPDase1), the platelet activator ADP (SmATPDase1, SmNPP5), and can convert AMP into the anti-inflammatory mediator adenosine (SmAP). SmAP can additionally cleave the lipid immunomodulator sphingosine-1-phosphate and the proinflammatory anionic polymer, polyP. In addition, the worms release a barrage of proteins (e.g., SmCB1, SjHSP70, cyclophilin A) that can impinge on immune cell function. Parasite eggs also release their own immunoregulatory proteins (e.g., IPSE/α1, omega1, SmCKBP) as do invasive cercariae (e.g., Sm16, Sj16). Some schistosome glycans (e.g., LNFPIII, LNnT) and lipids (e.g., Lyso-PS, LPC), produced by several life stages, likewise affect immune cell responses. The parasites not only produce eicosanoids (e.g., PGE2, PGD2-that can be anti-inflammatory) but can also induce host cells to release these metabolites. Finally, the worms release extracellular vesicles (EVs) containing microRNAs, and these too have been shown to skew host cell metabolism. Thus, schistosomes employ an array of biomolecules-protein, lipid, glycan, nucleic acid, and more, to bend host biochemistry to their liking. Many of the listed molecules have been individually shown capable of inducing aspects of the polarized Th2 response seen following infection (with the generation of regulatory T cells (Tregs), regulatory B cells (Bregs) and anti-inflammatory, alternatively activated (M2) macrophages). Precisely how host cells integrate the impact of these myriad parasite products following natural infection is not known. Several of the schistosome immunomodulators described here are in development as novel therapeutics against autoimmune, inflammatory, and other, nonparasitic, diseases.

Vaccine-elicited CD4 T cells prevent the deletion of antiviral B cells in chronic infection

Chronic viral infections subvert protective B cell immunity. An early type I interferon (IFN-I)-driven bias to short-lived plasmablast differentiation leads to clonal deletion, so-called "decimation," of antiviral memory B cells. Therefore, prophylactic countermeasures against decimation remain an unmet need. We show that vaccination-induced CD4 T cells prevented the decimation of naïve and memory B cells in chronically lymphocytic choriomeningitis virus (LCMV)-infected mice. Although these B cell responses were largely T independent when IFN-I was blocked, preexisting T help assured their sustainability under conditions of IFN-I-driven inflammation by instructing a germinal center B cell transcriptional program. Prevention of decimation depended on T cell-intrinsic Bcl6 and Tfh progeny formation. Antigen presentation by B cells, interactions with antigen-specific T helper cells, and costimulation by CD40 and ICOS were also required. Importantly, B cell-mediated virus control averted Th1-driven immunopathology in LCMV-challenged animals with preexisting CD4 T cell immunity. Our findings show that vaccination-induced Tfh cells represent a cornerstone of effective B cell immunity to chronic virus challenge, pointing the way toward more effective B cell-based vaccination against persistent viral diseases.

Immunological and Biochemical Interplay between Cytokines, Oxidative Stress and Schistosomiasis

The host–parasite schistosome relationship relies heavily on the interplay between the strategies imposed by the schistosome worm and the defense mechanisms the host uses to counter the line of attack of the parasite. The ultimate goal of the schistosome parasite entails five important steps: evade elimination tactics, survive within the human host, develop into adult forms, propagate in large numbers, and transmit from one host to the next. The aim of the parasitized host on the other hand is either to cure or limit infection. Therefore, it is a battle between two conflicting aspirations. From the host’s standpoint, infection accompanies a plethora of immunological consequences; some are set in place to defend the host, while most end up promoting chronic disease, which ultimately crosses paths with oxidative stress and cancer. Understanding these networks provides attractive opportunities for anti-schistosome therapeutic development. Hence, this review discusses the mechanisms by which schistosomes modulate the human immune response with ultimate links to oxidative stress and genetic instability.

Transcriptome and IgH Repertoire Analyses Show That CD11chi B Cells Are a Distinct Population With Similarity to B Cells Arising in Autoimmunity and Infection

A distinct B cell population marked by elevated CD11c expression is found in patients with systemic lupus erythematosus (SLE). Cells with a similar phenotype have been described during chronic infection, but variable gating strategies and nomenclature have led to uncertainty of their relationship to each other. We isolated CD11c^hi cells from peripheral blood and characterized them using transcriptome and IgH repertoire analyses. Gene expression data revealed the CD11c^hi IgD⁺ and IgD^- subsets were highly similar to each other, but distinct from naive, memory, and plasma cell subsets. Although CD11c^hi B cells were enriched in some germinal center (GC) transcripts and expressed numerous negative regulators of B cell receptor (BCR) activation, they were distinct from GC B cells. Gene expression patterns from SLE CD11c^hi B cells were shared with other human diseases, but not with mouse age-associated B cells. IgH V-gene sequencing analysis showed IgD⁺ and IgD^- CD11c^hi B cells had somatic hypermutation and were clonally related to each other and to conventional memory and plasma cells. However, the IgH repertoires expressed by the different subsets suggested that defects in negative selection during GC transit could contribute to autoimmunity. The results portray a pervasive B cell population that accumulates during autoimmunity and chronic infection and is refractory to BCR signaling.

Wuchereria bancrofti-infected individuals harbor distinct IL-10-producing regulatory B and T cell subsets which are affected by anti-filarial treatment

Despite worldwide mass drug administration, it is estimated that 68 million individuals are still infected with lymphatic filariasis with 19 million hydrocele and 17 million lymphedema reported cases. Despite the staggering number of pathology cases, the majority of LF-infected individuals do not develop clinical symptoms and present a tightly regulated immune system characterized by higher frequencies of regulatory T cells (Treg), suppressed proliferation and Th2 cytokine responses accompanied with increased secretion of IL-10, TGF-β and infection-specific IgG4. Nevertheless, the filarial-induced modulation of the host`s immune system and especially the role of regulatory immune cells like regulatory B (Breg) and Treg during an ongoing LF infection remains unknown. Thus, we analysed Breg and Treg frequencies in peripheral blood from Ghanaian uninfected endemic normals (EN), lymphedema (LE), asymptomatic patent (CFA+MF+) and latent (CFA+MF-) W. bancrofti-infected individuals as well as individuals who were previously infected with W. bancrofti (PI) but had cleared the infection due to the administration of ivermectin (IVM) and albendazole (ALB). In summary, we observed that IL-10-producing CD19⁺CD24^highCD38d^high Breg were specifically increased in patently infected (CFA+MF+) individuals. In addition, CD19⁺CD24^highCD5⁺CD1d^high and CD19⁺CD5⁺CD1d^highIL-10⁺ Breg as well as CD4⁺CD127^-FOXP3⁺ Treg frequencies were significantly increased in both W. bancrofti-infected cohorts (CFA+MF+ and CFA+MF-). Interestingly, the PI cohort presented frequency levels of all studied regulatory immune cell populations comparable with the EN group. In conclusion, the results from this study show that an ongoing W. bancrofti infection induces distinct Breg and Treg populations in peripheral blood from Ghanaian volunteers. Those regulatory immune cell populations might contribute to the regulated state of the host immune system and are probably important for the survival and fertility (microfilaria release) of the helminth.

Regulation of the host`s immune system by filarial nematodes is crucial for the fertility and survival of the nematode. Indeed, the majority of W. bancrofti-infected individuals are characterized by a regulated state including increased regulatory T cells (Treg), IL-10, TGF-β and filarial-specific IgG4 and suppressed Th2 cytokine responses. However, the functional role of Treg populations and regulatory B cells (Breg) during filarial infection remains unknown. Thus, in this study we investigated whether W. bancrofti-infected individuals from Ghana harbored distinct Breg and Treg populations which might be important for filarial-specific immunomodulation. Overall, this study shows that W. bancrofti induces distinct Breg populations, especially in patently (microfilaremic) infected individuals who presented significantly increased frequencies of IL-10-producing CD19<sup>+</sup>CD24<sup>high</sup>CD38d<sup>high</sup> Breg. Furthermore, clearance of the infection, due to anti-filarial treatment, returned these regulatory immune cells to homeostatic levels showing that an ongoing filarial infection is important for the activation of distinct Breg and Treg subsets. Those regulatory immune cell subsets are a part of a complex system which are induced by filarial nematodes to modulate the host`s immune system and maintain long-term survival.

Are B cells altered in the decidua of women with preterm or term labor?

PROBLEM

The immunophenotype of B cells at the maternal-fetal interface (decidua) in labor at term and preterm labor is poorly understood.

METHOD OF STUDY

Decidual tissues were obtained from women with preterm or term labor and from non-labor gestational age-matched controls. Immunophenotyping of decidual B cells was performed using multicolor flow cytometry.

RESULTS

(a) In the absence of acute or chronic chorioamnionitis, total B cells were more abundant in the decidua parietalis of women who delivered preterm than in those who delivered at term, regardless of the presence of labor; (b) decidual transitional and naïve B cells were the most abundant B-cell subsets; (c) decidual B1 B cells were increased in women with either labor at term or preterm labor and chronic chorioamnionitis compared to those without this placental lesion; (d) decidual transitional B cells were reduced in women with preterm labor compared to those without labor; (e) naïve, class-switched, and non-class-switched B cells in the decidual tissues underwent mild alterations with the process of preterm labor; (f) decidual plasmablasts seemed to increase in women with either labor at term or preterm labor with chronic chorioamnionitis; and (g) decidual B cells expressed high levels of interleukin (IL)-12, IL-6, and/or IL-35.

CONCLUSION

Total B cells are not increased with the presence of preterm or term labor; yet, specific subsets (B1 and plasmablasts) undergo alterations in women with chronic chorioamnionitis. Therefore, B cells are solely implicated in the pathological process of preterm labor in a subset of women with chronic inflammation of the placenta. These findings provide insight into the immunology of the maternal-fetal interface in preterm and term labor.

Worms: Pernicious parasites or allies against allergies?

Type 2 immune responses are most commonly associated with allergy and helminth parasite infections. Since the discovery of Th1 and Th2 immune responses more than 30 years ago, models of both allergic disease and helminth infections have been useful in characterizing the development, effector mechanisms and pathological consequences of type 2 immune responses. The observation that some helminth infections negatively correlate with allergic and inflammatory disease led to a large field of research into parasite immunomodulation. However, it is worth noting that helminth parasites are not always benign infections, and that helminth immunomodulation can have stimulatory as well as suppressive effects on allergic responses. In this review, we will discuss how parasitic infections change host responses, the consequences for bystander immunity and how this interaction influences clinical symptoms of allergy.

Memory B cell dysregulation in HIV-1-infected individuals

OBJECTIVE

To characterize the effect of the HIV-1 infection and antiretroviral treatment (ART) in the human memory B (MEB)-cell compartment.

DESIGN

A cross-sectional study was designed to analyze MEB cells of HIV-1 ART treated and ART-naive study participants, and uninfected individuals.

METHODS

Frequency and absolute counts of MEB cell subsets in blood were determined by multicolor flow cytometry. Spontaneous cell death and B-cell proliferative capacity was evaluated in vitro by cell culture and flow cytometry. Splenic function was determined by pitted erythrocytes quantification in HIV-1 ART-treated study participants.

RESULTS

HIV-1 ART-treated individuals did not show functional hyposplenism despite the lack of recovery IgMIgDCD27 marginal zone-like B cells. Moreover, two germinal center-dependent MEB cells subsets were also dysregulated in HIV-1 individuals: IgMIgDCD27 (IgM only) cells were increased, whereas the switched subset (IgMIgD) was reduced in viremic individuals. Althought ART restored the numbers of these populations; the switched MEB cells were enriched in CD27 cells, which showed the highest susceptibility to spontaneous cell death ex vivo. In addition, B cells from viremic individuals showed a poor response to B-cell receptor and toll-like receptor 9 stimulation that was circumvented when both stimuli were used simultaneously.

CONCLUSION

B cells from HIV-1 study participants show a poor stimulation capacity, that may be bypassed by the proper combination of stimuli, and a dysregulated MEB cell pool that suggest an affectation of the germinal center reaction, only partially normalized by ART. Interestingly, hyposplenism does not explain the lack of recovery of the marginal zone-like B cells in ART-treated HIV-1 individuals.

Schistosome egg antigens, including the glycoprotein IPSE/alpha-1, trigger the development of regulatory B cells

Infection with the helminth Schistosoma (S.) mansoni drives the development of interleukin (IL)-10-producing regulatory B (Breg) cells in mice and man, which have the capacity to reduce experimental allergic airway inflammation and are thus of high therapeutic interest. However, both the involved antigen and cellular mechanisms that drive Breg cell development remain to be elucidated. Therefore, we investigated whether S. mansoni soluble egg antigens (SEA) directly interact with B cells to enhance their regulatory potential, or act indirectly on B cells via SEA-modulated macrophage subsets. Intraperitoneal injections of S. mansoni eggs or SEA significantly upregulated IL-10 and CD86 expression by marginal zone B cells. Both B cells as well as macrophages of the splenic marginal zone efficiently bound SEA in vivo, but macrophages were dispensable for Breg cell induction as shown by macrophage depletion with clodronate liposomes. SEA was internalized into acidic cell compartments of B cells and induced a 3-fold increase of IL-10, which was dependent on endosomal acidification and was further enhanced by CD40 ligation. IPSE/alpha-1, one of the major antigens in SEA, was also capable of inducing IL-10 in naïve B cells, which was reproduced by tobacco plant-derived recombinant IPSE. Other major schistosomal antigens, omega-1 and kappa-5, had no effect. SEA depleted of IPSE/alpha-1 was still able to induce Breg cells indicating that SEA contains more Breg cell-inducing components. Importantly, SEA- and IPSE-induced Breg cells triggered regulatory T cell development in vitro. SEA and recombinant IPSE/alpha-1 also induced IL-10 production in human CD1d⁺ B cells. In conclusion, the mechanism of S. mansoni-induced Breg cell development involves a direct targeting of B cells by SEA components such as the secretory glycoprotein IPSE/alpha-1.

Infection with helminth parasites is known to be inversely associated with hyper-inflammatory disorders. While Schistosoma (S.) mansoni has been described to exert its down-modulatory effects on inflammation by inducing a network of regulatory immune cells such as regulatory B (Breg), the mechanisms of Breg cell induction remain unclear. Here, we use in vivo and in vitro approaches to show that antigens from S. mansoni eggs, among which the major glycoprotein IPSE/alpha-1, directly interact with splenic marginal zone B cells of mice which triggers them to produce the anti-inflammatory cytokine IL-10 and their capacity to induce regulatory T (Treg) cells. We also found that IPSE/alpha-1 induces IL-10 in human CD1d⁺ B cells, and that both natural and recombinant IPSE/alpha-1 are equally effective in driving murine and human Breg cells. Our study thus provides insight into the mechanisms of Breg cell induction by schistosomes, and an important step towards the development of helminth-based treatment strategies against hyper-inflammatory diseases.

Role of CD11c+ T-bet+ B cells in human health and disease

A growing body of evidence suggests that when B cells are chronically stimulated, a phenotypically unique subset expands. Data suggest that this atypical population contains B cell receptor (BCR) specificities capable of binding the antigen, or sets of antigens that initiated the expansion of these cells. These B cells have been given various names, including double negative B cells, atypical memory B cells, tissue-like memory B cells, or age associated B cells (ABCs). However, on close inspection these reports described B cell subsets that closely resemble B cells we refer to as CD11c⁺ B cells that often express T-bet. Here we will review the human studies that describe atypical memory B cells and compare and contrast their phenotype and suggested function in health and disease.

Detection of drug-responsive B lymphocytes and antidrug IgG in patients with β-lactam hypersensitivity

BACKGROUND

Delayed-type β-lactam hypersensitivity develops in subset of patients. The cellular immunological processes that underlie the drug-specific response have been described; however, little is known about involvement of the humoral immune system. Thus, the aim of this study was to utilize piperacillin hypersensitivity as an exemplar to (i) develop cell culture methods for the detection of drug-specific B-cell responses, (ii) characterize drug-specific IgG subtypes and (iii) assess reactivity of IgG antibodies against proteins modified to different levels with piperacillin haptens.

METHODS

IgG secretion and CD19⁺ CD27⁺ expression on B cells were measured using ELISPOT and flow cytometry, respectively. A piperacillin-BSA adduct was used as an antigen in ELISA antibody binding studies. Adducts generated using different ratios of drug to protein were used to determine the degree of conjugation required to detect IgG binding.

RESULTS

B cells from hypersensitive patients, but not controls, were stimulated to secrete IgG and increase CD27 expression when cultured with soluble piperacillin. A piperacillin-BSA adduct with cyclized and hydrolysed forms of the hapten bound to eight lysine residues was used to detect hapten-specific IgG 1-4 subclasses in patient plasma. Hapten inhibition and the use of structurally unrelated hapten-BSA adducts confirmed antigen specificity. Antibody binding was detected with antigens generated at piperacillin/BSA ratios of 10:1 and above, which corresponded to a minimum epitope density of 1 for antibody binding.

CONCLUSION

These data show that antigen-specific B lymphocytes and T lymphocytes are activated in piperacillin-hypersensitive patients. Further work is needed to define the role different IgG subtypes play in regulating the iatrogenic disease.

Patients with Tuberculosis Have a Dysfunctional Circulating B-Cell Compartment, Which Normalizes following Successful Treatment

B-cells not only produce immunoglobulins and present antigens to T-cells, but also additional key roles in the immune system. Current knowledge on the role of B-cells in infections caused by intracellular bacteria is fragmentary and contradictory. We therefore analysed the phenotypical and functional properties of B-cells during infection and disease caused by Mycobacterium tuberculosis (Mtb), the bacillus causing tuberculosis (TB), and included individuals with latent TB infection (LTBI), active TB, individuals treated successfully for TB, and healthy controls. Patients with active or treated TB disease had an increased proportion of antibodies reactive with mycobacteria. Patients with active TB had reduced circulating B-cell frequencies, whereas only minor increases in B-cells were detected in the lungs of individuals deceased from TB. Both active TB patients and individuals with LTBI had increased relative fractions of B-cells with an atypical phenotype. Importantly, these B-cells displayed impaired proliferation, immunoglobulin- and cytokine- production. These defects disappeared upon successful treatment. Moreover, T-cell activity was strongest in individuals successfully treated for TB, compared to active TB patients and LTBI subjects, and was dependent on the presence of functionally competent B-cells as shown by cellular depletion experiments. Thus, our results reveal that general B-cell function is impaired during active TB and LTBI, and that this B-cell dysfunction compromises cellular host immunity during Mtb infection. These new insights may provide novel strategies for correcting Mtb infection-induced immune dysfunction towards restored protective immunity.

In infections with intracellular pathogens like Mycobacterium tuberculosis (Mtb), B-cells have long been ignored as their primary product, immunoglobulins, are unlikely to recognize intracellular bacteria. However, we have analysed here the frequency, phenotype and function of B-cells in tuberculosis (TB) infection and disease. Our data revealed that during active TB disease B-cell numbers are decreased and remaining B-cells are functionally impaired. Surprisingly, also individuals recently infected with Mtb suffered from poorly functional B-cells, but patients cured from the disease recovered with normal B-cell numbers and function. Thus, B-cell dysfunction contributes to impaired immune activation during Mtb infection.

CD23 can negatively regulate B-cell receptor signaling

CD23 has been implicated as a negative regulator of IgE and IgG antibody responses. However, whether CD23 has any role in B-cell activation remains unclear. We examined the expression of CD23 in different subsets of peripheral B cells and the impact of CD23 expression on the early events of B-cell receptor (BCR) activation using CD23 knockout (KO) mice. We found that in addition to marginal zone B cells, mature follicular B cells significantly down regulate the surface expression level of CD23 after undergoing isotype switch and memory B-cell differentiation. Upon stimulation with membrane-associated antigen, CD23 KO causes significant increases in the area of B cells contacting the antigen-presenting membrane and the magnitude of BCR clustering. This enhanced cell spreading and BCR clustering is concurrent with increases in the levels of phosphorylation of tyrosine and Btk, as well as the levels of F-actin and phosphorylated Wiskott Aldrich syndrome protein, an actin nucleation promoting factor, in the contract zone of CD23 KO B cells. These results reveal a role of CD23 in the negative regulation of BCR signaling in the absence of IgE immune complex and suggest that CD23 down-regulates BCR signaling by influencing actin-mediated BCR clustering and B-cell morphological changes.

Subversion of the B-cell compartment during parasitic, bacterial, and viral infections

Recent studies on HIV infection have identified new human B-cell subsets with a potentially important impact on anti-viral immunity. Current work highlights the occurrence of similar B-cell alterations in other viral, bacterial, and parasitic infections, suggesting that common strategies have been developed by pathogens to counteract protective immunity. For this review, we have selected key examples of human infections for which B-cell alterations have been described, to highlight the similarities and differences in the immune responses to a variety of pathogens. We believe that further comparisons between these models will lead to critical progress in the understanding of B-cell mechanisms and will open new target avenues for therapeutic interventions.

B-cell subpopulations in children: National reference values

Peripheral B-lymphocytes undergo a series of changes during the first few years of life. Encounters with foreign antigens lead to maturation and differentiation. Several primary antibody deficiencies (PADs) affecting B-cell development are associated with abnormalities in the composition and/or differentiation of B-cell compartments. The most recent international classifications of primary immunodeficiencies (PIDs) and common variable immunodeficiencies (CVID) have highlighted the importance of B-cell immunophenotyping and age-specific reference intervals for diagnostic purposes. We established national reference values for memory B-cell subpopulations, on the basis of CD27 and surface IgD expression in the peripheral blood of 242 healthy children. We report here the absolute counts and percentages of naive, switched and non-switched memory B-cells for seven age groups, from neonates to adults. We found that the naive B-cells percentage declined between the ages of 6 months and 8 years, after which it remained stable at about 70–80%. Memory B-cells are already present at birth and their numbers increase throughout childhood, stabilizing between the ages of 12 and 18 years. The definition of reference intervals for pediatric B-cell levels should facilitate the screening and diagnosis of various B-cell immunodeficiencies. This multicenter study, providing national reference values, should thus facilitate immunological diagnosis in children.