Acute Disruption of Bone Marrow B Lymphopoiesis and Apoptosis of Transitional and Marginal Zone B Cells in the Spleen following a Blood-Stage Plasmodium chabaudi Infection in Mice

Differential differentiation of B cell lymphopoiesis in lethal and non-lethal murine malaria models

B cells in protection against malaria and need of experiencing many episodes in humans to achieve a state of immunity is largely unknown. The cellular basis of such defects in terms of B cell generation, maturation and trafficking was studied by taking Plasmodium chabaudi, a non-lethal and Plasmodium berghei, a lethal murine model. A flow cytometry (FCF) based evaluation was used to study alterations in generation and maintenance of B cells in patients with Plasmodium falciparum malaria as well as in murine malaria models. A significant accumulation of mature B cells in bone marrow and immature B cells in circulation was a feature observed only in lethal malaria. At peak parasitaemia, both the models induce a significant decrease in T2 (transitional) B cells with expansion of T1B cells. Studies in patients with acute Pf malaria showed a significant expansion of memory B cells and TB cells with a concomitant decrease in naive2 B cells as compared with healthy controls. This study clearly demonstrates that acute malarial infection induces major disturbances in B cell development in lymphoid organs and trafficking in periphery.

Effect of Leishmania infantum infection on B cell lymphopoiesis and memory in the bone marrow and spleen

Visceral leishmaniasis (VL) is characterized by an uncontrolled infection of internal organs such as the spleen, liver and bone marrow (BM) and can be lethal when left untreated. No effective vaccination is currently available for humans. The importance of B cells in infection and VL protective immunity has been controversial, with both detrimental and protective effects described. VL infection was found in this study to increase not only all analyzed B cell subsets in the spleen but also the B cell progenitors in the BM. The enhanced B lymphopoiesis aligns with the clinical manifestation of polyclonal hypergammaglobulinemia and the occurrence of autoantibodies. In line with earlier reports, flow cytometric and microscopic examination identified parasite attachment to B cells of the BM and spleen without internalization, and transformation of promastigotes into amastigote morphotypes. The interaction appears independent of IgM expression and is associated with an increased detection of activated lysosomes. Furthermore, the extracellularly attached amastigotes could be efficiently transferred to infect macrophages. The observed interaction underscores the potentially crucial role of B cells during VL infection. Additionally, using immunization against a fluorescent heterologous antigen, it was shown that the infection does not impair immune memory, which is reassuring for vaccination campaigns in VL endemic areas.

Antiplasmodial action of 4-nitrobenzenesulfonamide chalcones: Design, synthesis, characterisation, in vitro and in silico evaluation against blood stages of Plasmodium falciparum 3D7

Malaria is an intracellular protozoan parasitic disease caused by Plasmodium species with significant morbidity and mortality in endemic regions. The complex lifecycle of the parasite and the emergence of drug-resistant Plasmodium falciparum have hampered the efficacy of current anti-malarial agents. To circumvent this situation, the present study attempts to demonstrate the blood-stage anti-plasmodial action of 26 hybrid compounds containing the three privileged bioactive scaffolds (sulfonamide, chalcone, and nitro group) with synergistic and multitarget action. These three parent scaffolds exhibit divergent activities, such as antibacterial, anti-malarial, anti-fungal, anti-inflammatory, and anticancer. All the synthesised compounds were characterised using various spectroscopic techniques. The in vitro blood-stage inhibitory activity of 26 hybrid compounds was evaluated against mixed-stage culture (asynchronize) of human malarial parasite P. falciparum, Pf 3D7 at different concentrations ranging from 25.0 µg/mL to 0.78 µg/mL using SYBR 1 green assay, with IC50 values determined after 48 h of treatment based on the drug-response curves. Two potent compounds (11 and 10), with 2-Br and 2,6-diCl substitutions, showed pronounced activity with IC50 values of 5.4 µg/mL and 5.6 µg/mL, whereas others displayed varied activity with IC50 values ranging from 7.0 µg/mL to 22.0 µg/mL. Both 11 and 10 showed greater susceptibility towards mature-stage trophozoites than ring-stage parasites. The hemolytic and in vitro cytotoxicity assays revealed that compounds 11 and 10 did not cause any toxic effects on host red blood cells (uninfected), human-derived Mo7e cells, and murine-derived BA/F3 cells. The in vitro observations are consistent with the in silico studies using P. falciparum-dihydrofolate reductase, where 11 and 10 showed a binding affinity of -10.4 Kcal/mol. This is the first report of the hybrid scaffold, 4-nitrobenzenesulfonamide chalcones, demonstrating its potential as an anti-plasmodial agent.

Loss of B1 and marginal zone B cells during ovarian cancer

Recent advances in immunotherapy have not addressed the challenge presented by ovarian cancer. Although the peritoneum is an "accessible" locus for this disease there has been limited characterization of the immunobiology therein. We investigated the ID8-C57BL/6J ovarian cancer model and found marked depletion of B1 cells from the ascites of the peritoneal cavity. There was also selective loss of the B1 and marginal zone B cell subsets from the spleen. Immunity to antigens that activate these subsets validated their loss rather than relocation. A marked influx of myeloid-derived suppressor cells correlated with B cell subset depletion. These observations are discussed in the context of the housekeeping burden placed on innate B cells during ovarian cancer and to foster consideration of B cell biology in therapeutic strategies to address this challenge.

Current understanding of the immune potential of B-cell subsets in malarial pathogenesis

In the past several decades, our understanding of how B cells are generated and what function they perform has continued to advance. It is widely accepted that B-cell subsets play a critical role in mediating immune response. Surprisingly, human and murine malarial infections cause major alterations in the composition of B-cell subsets in both the spleen and periphery. Multiple B-cell subsets are well characterized in murine models following primary and secondary infection, although in human malarial infection, these subsets are not well defined. Furthermore, a rare known function of B cells includes the potential role of regulating the activities of other cells in the body as regulatory cells. Plasmodium infection strongly alters the frequency of these regulatory B cells indicating the immunoregulatory function of B cells in malarial. It is important to note that these subsets, taken together, form the cellular basis of humoral immune responses, allowing protection against a wide array of Plasmodium antigens to be achieved. However, it remains a challenge and an important area of investigation to understand how these B-cell subsets work together to provide protection against Plasmodium infection.

BCG-mediated protection against M. tuberculosis is sustained post-malaria infection independent of parasite virulence

Tuberculosis (TB) and malaria remain serious threats to global health. Bacillus Calmette-Guerin (BCG), the only licensed vaccine against TB protects against severe disseminated forms of TB in infants but shows poor efficacy against pulmonary TB in adults. Co-infections have been reported as one of the factors implicated in vaccine inefficacy. Given the geographical overlap of malaria and TB in areas where BCG vaccination is routinely administered, we hypothesized that virulence-dependent co-infection with Plasmodium species could alter the BCG-specific immune responses thus resulting in failure to protect against Mycobacterium tuberculosis. We compared virulent Plasmodium berghei and non-virulent Plasmodium chabaudi, their effects on B cells, effector and memory T cells, and the outcome on BCG-induced efficacy against M. tuberculosis infection. We demonstrate that malaria co-infection modulates both B- and T-cell immune responses but does not significantly alter the ability of the BCG vaccine to inhibit the growth of M. tuberculosis irrespective of parasite virulence. This malaria-driven immune regulation may have serious consequences in the early clinical trials of novel vaccines, which rely on vaccine-specific T-cell responses to screen novel vaccines for progression to the more costly vaccine efficacy trials.

The spleen: "epicenter" in malaria infection and immunity

The spleen is a complex secondary lymphoid organ that plays a crucial role in controlling blood‐stage infection with Plasmodium parasites. It is tasked with sensing and removing parasitized RBCs, erythropoiesis, the activation and differentiation of adaptive immune cells, and the development of protective immunity, all in the face of an intense inflammatory environment. This paper describes how these processes are regulated following infection and recognizes the gaps in our current knowledge, highlighting recent insights from human infections and mouse models.

The pathology of the spleen in lethal canine babesiosis caused by Babesia rossi

To provide useful information based on the macropathology, histopathology and immunohistochemical investigation in the spleens of dogs with Babesia rossi infection. Control spleens were collected from four healthy dogs euthanized for welfare reasons. Nine dogs that died naturally because of a mono-infection with Babesia rossi were selected for the diseased group. One haematoxylin-and-eosin-stained section of splenic tissue from each of the infected and control dogs was examined under the light microscope. Immunohistochemical markers were applied to characterize different immunocyte populations. The application of analytic software enabled semi-quantitative comparison of leucocyte subpopulations. Routine splenic histopathology revealed diffuse intermingling of white and red pulp from infected dogs with a clear loss of distinction between these zones. Immunohistochemistry revealed an increase in the proportion of tissue resident and bone marrow origin macrophages in the infected spleens. Apart from a few remnant lymphocytes within the peri-arteriolar lymphatic sheaths and follicles, the majority of the immunocytes redistributed to the red pulp, supporting the observation of white and red pulp intermingling. The majority of our findings are in agreement with histomorphological descriptions of the spleen in a variety of noncanid mammalian hosts with lethal malaria or babesiosis.

Plasmodium asexual growth and sexual development in the haematopoietic niche of the host

Plasmodium spp. parasites are the causative agents of malaria in humans and animals, and they are exceptionally diverse in their morphology and life cycles. They grow and develop in a wide range of host environments, both within blood-feeding mosquitoes, their definitive hosts, and in vertebrates, which are intermediate hosts. This diversity is testament to their exceptional adaptability and poses a major challenge for developing effective strategies to reduce the disease burden and transmission. Following one asexual amplification cycle in the liver, parasites reach high burdens by rounds of asexual replication within red blood cells. A few of these blood-stage parasites make a developmental switch into the sexual stage (or gametocyte), which is essential for transmission. The bone marrow, in particular the haematopoietic niche (in rodents, also the spleen), is a major site of parasite growth and sexual development. This Review focuses on our current understanding of blood-stage parasite development and vascular and tissue sequestration, which is responsible for disease symptoms and complications, and when involving the bone marrow, provides a niche for asexual replication and gametocyte development. Understanding these processes provides an opportunity for novel therapies and interventions.

Humoral Immunity vs. Salmonella

In primary infection with Salmonella, it has been reported—without consideration of Salmonella's functions—that humoral immunity plays no role in the clearance of bacteria. In fact, Salmonella targets and suppresses several aspects of humoral immunity, including B cell lymphopoiesis, B cell activation, and IgG production. In particular, the suppression of IgG-secreting plasma cell maintenance allows the persistence of Salmonella in tissues. Therefore, the critical role(s) of humoral immunity in the response to Salmonella infection, especially at the late phase, should be re-investigated. The suppression of IgG plasma cell memory strongly hinders vaccine development against non-typhoidal Salmonella (NTS) because Salmonella can also reduce humoral immune memory against other bacteria and viruses, obtained from previous vaccination or infection. We propose a new vaccine against Salmonella that would not impair humoral immunity, and which could also be used as a treatment for antibody-dependent autoimmune diseases to deplete pathogenic long-lived plasma cells, by utilizing the Salmonella's own suppression mechanism of humoral immunity.

B-cell memory in malaria: Myths and realities

B-cell and antibody responses to Plasmodium spp., the parasite that causes malaria, are critical for control of parasitemia and associated immunopathology. Antibodies also provide protection to reinfection. Long-lasting B-cell memory has been shown to occur in response to Plasmodium spp. in experimental model infections, and in human malaria. However, there are reports that antibody responses to several malaria antigens in young children living with malaria are not similarly long-lived, suggesting a dysfunction in the maintenance of circulating antibodies. Some studies attribute this to the expansion of atypical memory B cells (AMB), which express multiple inhibitory receptors and activation markers, and are hyporesponsive to B-cell receptor (BCR) restimulation in vitro. AMB are also expanded in other chronic infections such as tuberculosis, hepatitis B and C, and HIV, as well as in autoimmunity and old age, highlighting the importance of understanding their role in immunity. Whether AMB are dysfunctional remains controversial, as there are also studies in other infections showing that AMB can produce isotype-switched antibodies and in mouse can contribute to protection against infection. In light of these controversies, we review the most recent literature on either side of the debate and challenge some of the currently held views regarding B-cell responses to Plasmodium infections.

Intravital microscopy: Imaging host-parasite interactions in lymphoid organs

Intravital microscopy allows imaging of biological phenomena within living animals, including host-parasite interactions. This has advanced our understanding of both, the function of lymphoid organs during parasitic infections, and the effect of parasites on such organs to allow their survival. In parasitic research, recent developments in this technique have been crucial for the direct study of host-parasite interactions within organs at depths, speeds and resolution previously difficult to achieve. Lymphoid organs have gained more attention as we start to understand their function during parasitic infections and the effect of parasites on them. In this review, we summarise technical and biological findings achieved by intravital microscopy with respect to the interaction of various parasites with host lymphoid organs, namely the bone marrow, thymus, lymph nodes, spleen and the mucosa-associated lymphoid tissue, and present a view into possible future applications.

To B or Not to B: Understanding B Cell Responses in the Development of Malaria Infection

Malaria is a widespread disease caused mainly by the Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) protozoan parasites. Depending on the parasite responsible for the infection, high morbidity and mortality can be triggered. To escape the host immune responses, Plasmodium parasites disturb the functionality of B cell subsets among other cell types. However, some antibodies elicited during a malaria infection have the potential to block pathogen invasion and dissemination into the host. Thus, the question remains, why is protection not developed and maintained after the primary parasite exposure? In this review, we discuss different aspects of B cell responses against Plasmodium antigens during malaria infection. Since most studies have focused on the quantification of serum antibody titers, those B cell responses have not been fully characterized. However, to secrete antibodies, a complex cellular response is set up, including not only the activation and differentiation of B cells into antibody-secreting cells, but also the participation of other cell subsets in the germinal center reactions. Therefore, a better understanding of how B cell subsets are stimulated during malaria infection will provide essential insights toward the design of potent interventions.

Painless Nerve Growth Factor: A TrkA biased agonist mediating a broad neuroprotection via its actions on microglia cells

Nerve Growth Factor (NGF) is a therapeutic candidate for Alzheimer's disease, based on its well known actions on basal forebrain cholinergic neurons. However, because of its pro-nociceptive activity, in current clinical trials NGF has to be administered intraparenchymally into the brain by neurosurgery via cell or gene therapy approaches. To prevent the NGF pain-inducing collateral effects, thus avoiding the necessity for local brain injection, we developed painless NGF (hNGFp), based on the human genetic disease Hereditary Sensory and Autonomic Neuropathy type V (HSAN V). hNGFp has similar neurotrophic activity as wild type human NGF, but its pain sensitizing activity is tenfold lower. Pharmacologically, hNGFp is a biased receptor agonist of NGF TrkA receptor. The results of recent studies shed new light on the neuroprotective mechanism by hNGFp and are highly relevant for the planning of NGF-based clinical trials. The intraparenchymal delivery of hNGFp, as used in clinical trials, was simulated in the 5xFAD mouse model and found to be inefficacious in reducing Aβ plaque load. On the contrary, the same dose of hNGFp administered intranasally, which was rather widely biodistributed in the brain and did not induce pain sensitization, blocked APP processing into amyloid and restored synaptic plasticity and memory in this aggressive neurodegeneration model. This potent and broad neuroprotection by hNGFp was found to be mediated by hNGFp actions on glial cells. hNGFp increases inflammatory proteins such as the soluble TNFα receptor II and the chemokine CXCL12. Independent work has shown that NGF has a potent anti-inflammatory action on microglia and steers them towards a neuroprotective phenotype. These studies demonstrate that microglia cells are a new target cell of NGF in the brain and have therapeutic significance: i) they establish that the neuroprotective actions of hNGFp relies on a widespread exposure of the brain, ii) they identify a new anti-neurodegenerative pathway, linking hNGFp to inflammatory chemokines and cytokines via microglia, a common target for new therapeutic opportunities for neurodegenerative diseases, iii) they extend the neuroprotective potential of hNGFp beyond its classical cholinergic target, thereby widening the range of neurological diseases for which this neurotrophic factor might be used therapeutically, iv) they help interpreting the results of current NGF clinical trials in AD and the design of future trials with this new potent therapeutic candidate.

Vivax infection alters peripheral B-cell profile and induces persistent serum IgM

B cell-mediated humoral responses are essential for controlling malarial infection. Studies have addressed the effects of Plasmodium falciparum infection on peripheral B-cell subsets but not much is known for P. vivax infection. Furthermore, majority of the studies investigate changes during acute infection, but not after parasite clearance. In this prospective study, we analysed peripheral B-cell profiles and antibody responses during acute P. vivax infection and upon recovery (30 days post-treatment) in a low-transmission area in India. Dengue patients were included as febrile-condition controls. Both dengue and malaria patients showed a transient increase in atypical memory B cells during acute infection. However, transient B cell-activating factor (BAFF)-independent increase in the percentage of total and activated immature B cells was observed in malaria patients. Naïve B cells from malaria patients also showed increased TLR4 expression. Total IgM levels remained unchanged during acute infection but increased significantly at recovery. Serum antibody profiling showed a parasite-specific IgM response that persisted at recovery. A persistent IgM autoantibody response was also observed in malaria but not dengue patients. Our data suggest that in hypoendemic regions acute P. vivax infection skews peripheral B-cell subsets and results in a persistent parasite-specific and autoreactive IgM response.

Antibody responses to α-Gal in African children vary with age and site and are associated with malaria protection

Naturally-acquired antibody responses to malaria parasites are not only directed to protein antigens but also to carbohydrates on the surface of Plasmodium protozoa. Immunoglobulin M responses to α-galactose (α-Gal) (Galα1-3Galβ1-4GlcNAc-R)-containing glycoconjugates have been associated with protection from P. falciparum infection and, as a result, these molecules are under consideration as vaccine targets; however there are limited field studies in endemic populations. We assessed a wide breadth of isotype and subclass antibody response to α-Gal in children from Mozambique (South East Africa) and Ghana (West Africa) by quantitative suspension array technology. We showed that anti-α-Gal IgM, IgG and IgG_1–4 levels vary mainly depending on the age of the child, and also differ in magnitude in the two sites. At an individual level, the intensity of malaria exposure to P. falciparum and maternally-transferred antibodies affected the magnitude of α-Gal responses. There was evidence for a possible protective role of anti-α-Gal IgG3 and IgG4 antibodies. However, the most consistent findings were that the magnitude of IgM responses to α-Gal was associated with protection against clinical malaria over a one-year follow up period, especially in the first months of life, while IgG levels correlated with malaria risk.

Immunogenicity of the Plasmodium vivax merozoite surface protein 1 paralog in the induction of naturally acquired antibody and memory B cell responses

Background

The Plasmodium vivax merozoite surface protein 1 paralog (PvMSP1P-19) is a glycosylphosphatidylinositol (GPI)-anchored blood-stage protein that is expressed on the merozoite surface. It is proposed as a blood-stage vaccine candidate against P. vivax because of its ability to induce immune responses upon natural P. vivax exposure and in immunized animals. This study aimed to demonstrate the presence of inhibitory antibodies and memory B cell responses to the PvMSP1P-19 antigen during acute P. vivax infection and after recovery from infection.

Methods

To evaluate the antibody responses to PvMSP1P-19 during and after recovery from P. vivax infection, heparinized blood was collected from P. vivax-infected patients and recovered subjects to detect the total IgG response. The seropositive samples were defined into high and low responders, according to their optical density (OD) values obtained from ELISA. High responders were the subjects who had OD values above the OD of antisera from non-exposed controls plus 4× standard deviations, whereas low responders were the subjects who had OD values less than OD of antisera from non-exposed controls plus 4× standard deviations. The plasma from high and low responders were taken for testing the inhibitory activity against PvMSP1P-19-erythrocyte binding by in vitro EBIA. The sustainability of PvMSP1P-19-specific memory B cell responses after recovery from infection was analysed by ELISPOT.

Results

The anti-PvMSP1P-19 antibody levels were significantly higher in acutely infected P. vivax patients compared to healthy controls (P <  0.0001). Monitoring of the anti-PvMSP1P-19 antibody titre showed that the antibody was maintained for up to 9 months after recovery. Almost all high-responder groups strongly inhibited PvMSP1P-19 binding to erythrocytes, whereas no inhibition was shown in most low-responder samples. Interestingly, the inhibitory activity of the antibodies in some individuals from high-responder samples were stable for at least 12 months. The longevity of the antibody response was associated with the presence of PvMSP1P-19-specific memory B cells at 9 months after recovery from infection.

Conclusions

The PvMSP1P-19 antigen has immunogenicity during the induction of the antibody response, in which both the levels and inhibitory activity are maintained after the patient recovered from P. vivax infection. The maintenance of the antibody response was associated with the response of PvMSP1P-19-specific memory B cells. Therefore, the PvMSP1P-19 antigen should also be considered as a reliable vaccine candidate to develop a blood-stage vaccine against P. vivax.

IL-17 Promotes Differentiation of Splenic LSK- Lymphoid Progenitors into B Cells following Plasmodium yoelii Infection

Lineage^-Sca-1⁺c-Kit^- (LSK^-) cells are a lymphoid progenitor population that expands in the spleen and preferentially differentiates into mature B cells in response to Plasmodium yoelii infection in mice. Furthermore, LSK^- derived B cells can subsequently contribute to the ongoing immune response through the generation of parasite-specific Ab-secreting cells, as well as germinal center and memory B cells. However, the factors that promote their differentiation into B cells in the spleen postinfection are not defined. In this article, we show that LSK^- cells produce the cytokine IL-17 in response to Plasmodium infection. Using Il-17ra^-/- mice, IL-17R signaling in cells other than LSK^- cells was found to support their differentiation into B cells. Moreover, primary splenic stromal cells grown in the presence of IL-17 enhanced the production of CXCL12, a chemokine associated with B cell development in the bone marrow, by a population of IL-17RA-expressing podoplanin⁺CD31^- stromal cells, a profile associated with fibroblastic reticular cells. Subsequent blockade of CXCL12 in vitro reduced differentiation of LSK^- cells into B cells, supporting a direct role for this chemokine in this process. Immunofluorescence indicated that podoplanin⁺ stromal cells in the red pulp were the primary producers of CXCL12 after P. yoelii infection. Furthermore, podoplanin staining on stromal cells was more diffuse, and CXCL12 staining was dramatically reduced in Il-17ra^-/- mice postinfection. Together, these results identify a distinct pathway that supports lymphoid development in the spleen during acute Plasmodium infection.

The chemokine CXCL12 mediates the anti-amyloidogenic action of painless human nerve growth factor

Nerve growth factor is a therapeutic candidate for Alzheimer’s disease. Due to its pain-inducing activity, in current clinical trials nerve growth factor is delivered locally into the brain by neurosurgery, but data on the efficacy of local nerve growth factor delivery in decreasing amyloid-β deposition are not available. To reduce the nerve growth factor pain-inducing side effects, thus avoiding the need for local brain injection, we developed human painless nerve growth factor (hNGFp), inspired by the human genetic disease hereditary sensory and autonomic neuropathy type V. hNGFp has identical neurotrophic potency as wild-type human nerve growth factor, but a 10-fold lower pain sensitizing activity. In this study we first mimicked, in the 5xFAD mouse model, the intraparenchymal delivery of hNGFp used in clinical trials and found it to be ineffective in decreasing amyloid-β plaque load. On the contrary, the same dose of hNGFp delivered intranasally, which was widely biodistributed in the brain and did not induce pain, showed a potent anti-amyloidogenic action and rescued synaptic plasticity and memory deficits. We found that hNGFp acts on glial cells, modulating inflammatory proteins such as the soluble TNFα receptor II and the chemokine CXCL12. We further established that the rescuing effect by hNGFp is mediated by CXCL12, as pharmacological inhibition of CXCL12 receptor CXCR4 occludes most of hNGFp effects. These findings have significant therapeutic implications: (i) we established that a widespread exposure of the brain is required for nerve growth factor to fully exert its neuroprotective actions; and (ii) we have identified a new anti-neurodegenerative pathway as a broad target for new therapeutic opportunities for neurodegenerative diseases.

African Trypanosomes Undermine Humoral Responses and Vaccine Development: Link with Inflammatory Responses?

African trypanosomosis is a debilitating disease of great medical and socioeconomical importance. It is caused by strictly extracellular protozoan parasites capable of infecting all vertebrate classes including human, livestock, and game animals. To survive within their mammalian host, trypanosomes have evolved efficient immune escape mechanisms and manipulate the entire host immune response, including the humoral response. This report provides an overview of how trypanosomes initially trigger and subsequently undermine the development of an effective host antibody response. Indeed, results available to date obtained in both natural and experimental infection models show that trypanosomes impair homeostatic B-cell lymphopoiesis, B-cell maturation and survival and B-cell memory development. Data on B-cell dysfunctioning in correlation with parasite virulence and trypanosome-mediated inflammation will be discussed, as well as the impact of trypanosomosis on heterologous vaccine efficacy and diagnosis. Therefore, new strategies aiming at enhancing vaccination efficacy could benefit from a combination of (i) early parasite diagnosis, (ii) anti-trypanosome (drugs) treatment, and (iii) anti-inflammatory treatment that collectively might allow B-cell recovery and improve vaccination.

Plasmodium chabaudi infection induces AID expression in transitional and marginal zone B cells

Introduction

Endemic Burkitt's lymphoma (eBL) is associated with Epstein–Barr virus and repeated malaria infections. A defining feature of eBL is the translocation of the c‐myc oncogene to the control of the immunoglobulin promoter. Activation‐induced cytidine deaminase (AID) has been shown to be critical for this translocation. Malaria infection induces AID in germinal center B cells, but whether malaria infection more broadly affects AID activation in extrafollicular B cells is unknown.

Methods

We either stimulated purified B cells from AID‐green fluorescence protein (GFP) reporter mice or infected AID‐GFP mice with Plasmodium chabaudi, AID fluorescence was monitored in B cell subsets by flow cytometry.

Results

In vitro analysis of B cells from these mice revealed that CpG (a Toll‐like receptor 9 ligand) was a potent inducer of AID in both mature and immature B cell subsets. Infection of AID‐GFP mice with Plasmodium chabaudi demonstrated that AID expression occurs in transitional and marginal zone B cells during acute malaria infection. Transitional B cells were also capable of differentiating into antibody secreting cells when stimulated in vitro with CpG when isolated from a P. chabaudi‐infected mouse.

Conclusions

These data suggest that P. chabaudi is capable of inducing AID expression in B cell subsets that do not participate in the germinal center reaction, suggesting an alternative role for malaria in the etiology of eBL.

Maintenance of B cells during chronic murine Trypanosoma brucei gambiense infection

African trypanosomosis is a debilitating parasitic disease occurring in large parts of sub-Saharan Africa. Trypanosoma brucei gambiense accounts for 98% of the reported HAT infections and causes a chronic, gradually progressing disease. Multiple experimental murine models for trypanosomosis have demonstrated inflammation-dependent apoptosis of splenic follicular B (FoB) cells and the destruction of B-cell memory against previously encountered pathogens. Here, we report that during murine infection with a chronic T. b. gambiense field isolate, FoB cells are retained. This coincided with reduced levels of IFN-γ and TNF-α during the acute phase of the infection. This result suggests that in chronic infections with low virulent parasites, less inflammation is elicited and consequently no FoB cell destruction occurs.

An Atypical Splenic B Cell Progenitor Population Supports Antibody Production during Plasmodium Infection in Mice

Hematopoietic stem and progenitor cells (HSPCs) function to replenish the immune cell repertoire under steady-state conditions and in response to inflammation due to infection or stress. Whereas the bone marrow serves as the primary niche for hematopoiesis, extramedullary mobilization and differentiation of HSPCs occur in the spleen during acute Plasmodium infection, a critical step in the host immune response. In this study, we identified an atypical HSPC population in the spleen of C57BL/6 mice, with a lineage(-)Sca-1(+)c-Kit(-) (LSK(-)) phenotype that proliferates in response to infection with nonlethal Plasmodium yoelii 17X. Infection-derived LSK(-) cells upon transfer into naive congenic mice were found to differentiate predominantly into mature follicular B cells. However, when transferred into infection-matched hosts, infection-derived LSK(-) cells gave rise to B cells capable of entering into a germinal center reaction, and they developed into memory B cells and Ab-secreting cells that were capable of producing parasite-specific Abs. Differentiation of LSK(-) cells into B cells in vitro was enhanced in the presence of parasitized RBC lysate, suggesting that LSK(-) cells expand and differentiate in direct response to the parasite. However, the ability of LSK(-) cells to differentiate into B cells was not dependent on MyD88, as myd88(-/-) LSK(-) cell expansion and differentiation remained unaffected after Plasmodium infection. Collectively, these data identify a population of atypical lymphoid progenitors that differentiate into B lymphocytes in the spleen and are capable of contributing to the ongoing humoral immune response against Plasmodium infection.

B cell sub-types following acute malaria and associations with clinical immunity

BACKGROUND

Repeated exposure to Plasmodium falciparum is associated with perturbations in B cell sub-set homeostasis, including expansion atypical memory B cells. However, B cell perturbations immediately following acute malaria infection have been poorly characterized, especially with regard to their relationship with immunity to malaria.

METHODS

To better understand the kinetics of B cell sub-sets following malaria, the proportions of six B cell sub-sets were assessed at five time points following acute malaria in four to 5 years old children living in a high transmission region of Uganda. B cell sub-set kinetics were compared with measures of clinical immunity to malaria-lower parasite density at the time of malaria diagnosis and recent asymptomatic parasitaemia.

RESULTS

Atypical memory B cell and transitional B cell proportions increased following malaria. In contrast, plasmablast proportions were highest at the time of malaria diagnosis and rapidly declined following treatment. Increased proportions of atypical memory B cells were associated with greater immunity to malaria, whereas increased proportions of transitional B cells were associated with evidence of less immunity to malaria.

CONCLUSIONS

These findings highlight the dynamic changes in multiple B cell sub-sets following acute, uncomplicated malaria, and how these sub-sets are associated with developing immunity to malaria.

IFN-γ mediates early B-cell loss in experimental African trypanosomosis

African trypanosomes infect humans and animals throughout the African continent. These parasites maintain chronic infections by various immune evasion strategies. While antigenic variation of their surface coat is the most studied strategy linked to evading the host humoral response, African trypanosomes also induce impaired B-cell lymphopoiesis, the destruction of the splenic B-cell compartment and abrogation of protective memory responses. Here we investigate the mechanism of follicular B-cell destruction. We show that during infection follicular B cells undergo apoptosis, correlating to enhanced Fas death receptor surface expression. Investigation of various type 1 cytokine knockout mice indicates a crucial role of IFN-γ in the early onset of FoB cell destruction. Indeed, both IFN-γ(-/-) and IFN-γR(-/-) mice are protected from trypanosomosis-associated FoB cell depletion, exhibiting an inhibition of B-cell apoptosis as well as a reduced activation of FoB cells during the first week post-infection. The data presented herein offer new insights into B-cell dysfunctioning during experimental African trypanosome infections.

Pathogen manipulation of B cells: the best defence is a good offence

B cells have long been regarded as simple antibody production units, but are now becoming known as key players in both adaptive and innate immune responses. However, several bacteria, viruses and parasites have evolved the ability to manipulate B cell functions to modulate immune responses. Pathogens can affect B cells indirectly, by attacking innate immune cells and altering the cytokine environment, and can also target B cells directly, impairing B cell-mediated immune responses. In this Review, we provide a summary of recent advances in elucidating direct B cell-pathogen interactions and highlight how targeting this specific cell population benefits different pathogens.

Escape mechanisms of African trypanosomes: why trypanosomosis is keeping us awake

African trypanosomes have been around for more than 100 million years, and have adapted to survival in a very wide host range. While various indigenous African mammalian host species display a tolerant phenotype towards this parasitic infection, and hence serve as perpetual reservoirs, many commercially important livestock species are highly disease susceptible. When considering humans, they too display a highly sensitive disease progression phenotype for infections with Trypanosoma brucei rhodesiense or Trypanosoma brucei gambiense, while being intrinsically resistant to infections with other trypanosome species. As extracellular trypanosomes proliferate and live freely in the bloodstream and lymphatics, they are constantly exposed to the immune system. Due to co-evolution, this environment however no longer poses a hostile threat, but has become the niche environment where trypanosomes thrive and obligatory await transmission through the bites of tsetse flies or other haematophagic vectors, ideally without causing severe side infection-associated pathology to their host. Hence, African trypanosomes have acquired various mechanisms to manipulate and control the host immune response, evading effective elimination. Despite the extensive research into trypanosomosis over the past 40 years, many aspects of the anti-parasite immune response remain to be solved and no vaccine is currently available. Here we review the recent work on the different escape mechanisms employed by African Trypanosomes to ensure infection chronicity and transmission potential.

Plasmodium falciparum malaria and invasive bacterial co-infection in young African children: the dysfunctional spleen hypothesis

Children with recent or acute malaria episodes are at increased risk of invasive bacterial infections (IBI). However, the exact nature of the malaria-IBI association is still unclear. Young children have an age-related spleen immunologic immaturity, mainly due to the still ongoing development of the marginal zone (MZ) B cell subset. By mounting a rapid antibody response against encapsulated bacteria, these cells are critical for the defence against highly pathogenic microorganisms that do not elicit classical T cell-dependent responses. There is increasing evidence that the anatomy of the spleen becomes disorganized during malaria infection, with complete dissolution of the MZ and apoptosis of MZ B cells. Correspondingly, a reduction in the frequency of the peripheral equivalent of the MZ B cells has been found in malaria endemic areas. A remarkable similarity exists in IBI susceptibility between African children with malaria and hyposplenic or splenectomized patients. However, studies specifically assessing the immune function of the spleen in controlling bacterial infections in young children with malaria are scarce.

Here, it is hypothesized that Plasmodium falciparum malaria infection constitutes a detrimental factor in the still immature spleen function of young children, resulting in a factually hyposplenic state during malaria episodes, putting children with malaria at a high risk to develop life-threatening bacterial infections. Studies to confirm or reject this hypothesis are greatly needed, as well as the development of affordable and feasible tools to assess the immune spleen function against encapsulated bacteria in children with malaria.

Chronic Trypanosoma congolense infections in mice cause a sustained disruption of the B-cell homeostasis in the bone marrow and spleen

Trypanosoma congolense is one of the main species responsible for Animal African Trypanosomosis (AAT). As preventive vaccination strategies for AAT have been unsuccessful so far, investigating the mechanisms underlying vaccine failure has to be prioritized. In T. brucei and T. vivax infections, recent studies revealed a rapid onset of destruction of the host B-cell compartment, resulting in the loss of memory recall capacity. To assess such effect in experimental T. congolense trypanosomosis, we performed infections with both the cloned Tc13 parasite, which is considered as a standard model system for T. congolense rodent infections and the noncloned TRT55 field isolate. These infections differ in their virulence level in the C57BL/6 mouse model for trypanosomosis. We show that early on, an irreversible depletion of all developmental B cells stages occur. Subsequently, in the spleen, a detrimental decrease in immature B cells is followed by a significant and permanent depletion of Marginal zone B cells and Follicular B cells. The severity of these events later on in infection correlated with the virulence level of the parasite stock. In line with this, it was observed that later-stage infection-induced IgGs were largely nonspecific, in particular in the more virulent TRT55 infection model.

BAFF and BAFF receptor levels correlate with B cell subset activation and redistribution in controlled human malaria infection

Characteristic features of Plasmodium falciparum malaria are polyclonal B cell activation and an altered composition of the blood B cell compartment, including expansion of CD21(-)CD27(-) atypical memory B cells. BAFF is a key cytokine in B cell homeostasis, but its potential contribution to the modulation of the blood B cell pool during malaria remains elusive. In the controlled human malaria model (CHMI) in malaria-naive Dutch volunteers, we therefore examined the dynamics of BAFF induction and B cell subset activation and composition, to investigate whether these changes are linked to malaria-induced immune activation and, in particular, induction of BAFF. Alterations in B cell composition after CHMI closely resembled those observed in endemic areas. We further found distinct kinetics of proliferation for individual B cell subsets across all developmental stages. Proliferation peaked either immediately after blood-stage infection or at convalescence, and for most subsets was directly associated with the peak parasitemia. Concomitantly, plasma BAFF levels during CHMI were increased and correlated with membrane-expressed BAFF on monocytes and dendritic cells, as well as blood-stage parasitemia and parasite-induced IFN-γ. Correlating with increased plasma BAFF and IFN-γ levels, IgD(-)CD38(low)CD21(-)CD27(-) atypical B cells showed the strongest proliferative response of all memory B cell subsets. This provides unique evidence for a link between malaria-induced immune activation and temporary expansion of this B cell subset. Finally, baseline BAFF-R levels before CHMI were predictive of subsequent changes in proportions of individual B cell subsets. These findings suggest an important role of BAFF in facilitating B cell subset proliferation and redistribution as a consequence of malaria-induced immune activation.

Dysfunctional adaptive immunity during parasitic infections

Parasite-driven dysfunctional adaptive immunity represents an emerging hypothesis to explain the chronic or persistent nature of parasitic infections, as well as the observation that repeated exposure to most parasitic organisms fails to engender sterilizing immunity. This review discusses recent examples from clinical studies and experimental models of parasitic infection that substantiate the role for immune dysfunction in the inefficient generation and maintenance of potent anti-parasitic immunity. Better understanding of the complex interplay between parasites, host adaptive immunity, and relevant negative regulatory circuits will inform efforts to enhance resistance to chronic parasitic infections through vaccination or immunotherapy.

Recovery of an antiviral antibody response following attrition caused by unrelated infection

The homeostatic mechanisms that regulate the maintenance of immunological memory to the multiple pathogen encounters over time are unknown. We found that a single malaria episode caused significant dysregulation of pre-established Influenza A virus-specific long-lived plasma cells (LLPCs) resulting in the loss of Influenza A virus-specific Abs and increased susceptibility to Influenza A virus re-infection. This loss of LLPCs involved an FcγRIIB-dependent mechanism, leading to their apoptosis. However, given enough time following malaria, the LLPC pool and humoral immunity to Influenza A virus were eventually restored. Supporting a role for continuous conversion of Influenza A virus-specific B into LLPCs in the restoration of Influenza A virus immunity, B cell depletion experiments also demonstrated a similar requirement for the long-term maintenance of serum Influenza A virus-specific Abs in an intact LLPC compartment. These findings show that, in addition to their established role in the anamnestic response to reinfection, the B cell pool continues to be a major contributor to the maintenance of long-term humoral immunity following primary Influenza A virus infection, and to the recovery from attrition following heterologous infection. These data have implications for understanding the longevity of protective efficacy of vaccinations in countries where continuous infections are endemic.

Antibody responses to infectious pathogens are critical in host survival, recovery and protection from reinfection; they also correlate with the success of vaccination. It is currently thought that antibody serum titers are maintained at protective levels over long periods of time by specialized long-lived antibody-secreting plasma cells residing in the bone marrow. Indeed, antibodies against the original virus can still be found in survivors of the 1918 Spanish Flu, more than 90 years ago. However, it is also becoming clear that subsequent infection with heterologous pathogens may cause attrition of previously established immunological memory, in order to accommodate new lymphocyte specificities in the finite space of the host. This phenomenon is seemingly at odds with long-term maintenance of immunological memory. We also show that a single episode of malaria, caused by infection by Plasmodium chabaudi, leads to the loss of preexisting plasma cells, serum antibodies and protective immunity against Influenza A virus. However, Influenza A virus-specific immunity does eventually recover in these animals with the replenishment of plasma cells by B cells over the course of several weeks. Thus, the reported mechanism reconciles attrition of immunological memory by heterologous infection and long-term stability, and places B cells, instead of their descendant plasma cells, at the center of humoral memory.

Changes in B Cell Populations and Merozoite Surface Protein-1-Specific Memory B Cell Responses after Prolonged Absence of Detectable P. falciparum Infection

Clinical immunity to malaria declines in the absence of repeated parasite exposure. However, little is known about how B cell populations and antigen-specific memory B cells change in the absence of P. falciparum infection. A successful indoor residual insecticide spraying campaign in a highland area of western Kenya, led to an absence of blood-stage P. falciparum infection between March 2007 and April 2008. We assessed memory B cell responses in 45 adults at the beginning (April 2008) and end (April 2009) of a subsequent 12-month period during which none of the adults had evidence of asymptomatic parasitemia or clinical disease. Antibodies and memory B cells to the 42-kDa portion of the merozoite surface protein-1 (MSP-1₄₂) were measured using ELISA and ELISPOT assays, respectively. B cell populations were characterized by flow cytometry. From 2008 to 2009, the prevalence of MSP-1₄₂-specific memory B cells (45% vs. 55%, respectively, P = 0.32) or antibodies (91% vs. 82%, respectively, P = 0.32) did not differ significantly, although specific individuals did change from positive to negative and vice versa, particularly for memory B cells, suggesting possible low-level undetected parasitemia may have occurred in some individuals. The magnitude of MSP-1₄₂-specific memory B cells and levels of antibodies to MSP-1₄₂ also did not differ from 2008 to 2009 (P>0.10 for both). However, from 2008 to 2009 the proportions of both class-switched atypical (CD19+IgD-CD27-CD21-IgM-) and class-switched activated (CD19+IgD-CD27+CD21-IgM-) memory B cells decreased (both P<0.001). In contrast, class-switched resting classical memory B cells (CD19+IgD-CD27+CD21+IgM-) increased (P<0.001). In this area of seasonal malaria transmission, a one- year absence of detectable P. falciparum infection was not associated with changes in the prevalence or level of MSP-1₄₂ specific memory B cells, but was associated with major changes in overall memory B cell subsets.

B-Cell Response during Protozoan Parasite Infections

In this review, we discuss how protozoan parasites alter immature and mature B cell compartment. B1 and marginal zone (MZ) B cells, considered innate like B cells, are activated during protozoan parasite infections, and they generate short lived plasma cells providing a prompt antibody source. In addition, protozoan infections induce massive B cell response with polyclonal activation that leads to hypergammaglobulnemia with serum antibodies specific for the parasites and self and/or non related antigens. To protect themselves, the parasites have evolved unique ways to evade B cell immune responses inducing apoptosis of MZ and conventional mature B cells. As a consequence of the parasite induced-apoptosis, the early IgM response and an already establish humoral immunity are affected during the protozoan parasite infection. Moreover, some trypanosomatides trigger bone marrow immature B cell apoptosis, influencing the generation of new mature B cells. Simultaneously with their ability to release antibodies, B cells produce cytokines/quemokines that influence the characteristic of cellular immune response and consequently the progression of parasite infections.

Suppression of circulating IgD+CD27+ memory B cells in infants living in a malaria-endemic region of Kenya

BACKGROUND

Plasmodium falciparum infection leads to alterations in B cell subset distribution. During infancy, development of peripheral B cell subsets is also occurring. However, it is unknown if infants living a malaria endemic region have alterations in B cell subsets that is independent of an age effect.

METHODS

To evaluate the impact of exposure to P. falciparum on B cell development in infants, flow cytometry was used to analyse the distribution and phenotypic characteristic of B cell subsets in infant cohorts prospectively followed at 12, 18 and 24 months from two geographically proximate regions in western Kenya with divergent malaria exposure i.e. Kisumu (malaria-endemic, n = 24) and Nandi (unstable malaria transmission, n = 21).

RESULTS

There was significantly higher frequency and absolute cell numbers of CD19+ B cells in Kisumu relative to Nandi at 12(p = 0.0440), 18(p = 0.0210) and 24 months (p = 0.0493). No differences were observed between the infants from the two sites in frequencies of naïve B cells (IgD+CD27-) or classical memory B cells (IgD-CD27+). However, immature transitional B cells (CD19+CD10+CD34-) were higher in Kisumu relative to Nandi at all three ages. In contrast, the levels of non-class switched memory B cells (CD19+IgD+CD27+) were significantly lower overall in Kisumu relative to Nandi at significantly at 12 (p = 0.0144), 18 (p = 0.0013) and 24 months (p = 0.0129).

CONCLUSIONS

These data suggest that infants living in malaria endemic regions have altered B cell subset distribution. Further studies are needed to understand the functional significance of these changes and long-term impact on ability of these infants to develop antibody responses to P. falciparum and heterologous infections.