Parasite-specific IgM plays a significant role in the protective immune response to asexual erythrocytic stage Plasmodium chabaudi AS infection

Divergent humoral responses between males and females against 24 kDa excretory-secretory protein of Haemonchus contortus and influence of ovine β-globin polymorphism

Lambs harboring the Hb-AA β-globin haplotype present improved cell-mediated responses and increased resistance against Haemonchus contortus infection. The aim of the present study was to compare the effect of sex and β-globin haplotypes on specific humoral responses and phenotypes of resistance during H. contortus infection in Morada Nova sheep. As expected, females displayed stronger resistance during the first and second experimental challenges. Differential systemic humoral immune responses were observed comparing sex groups, in which higher levels of specific antibodies targeting 24 kDa excretory-secretory (ES24) protein of H. contortus of IgG and IgM antibodies were respectively observed as predominant isotypes in males and females. The IgM levels were significantly correlated with phenotypes of resistance, evaluated by packed cell volume and fecal egg counts. To our knowledge this is the first study reporting divergent humoral responses profiles to H. contortus infection between male and female sheep. The impact of β-globin haplotypes was less pronounced in females compared to males. Notably, only males showed significant weight differences across haplotypes, with Hb-AA lambs being the heaviest. Additionally, Hb-AA males had significantly higher PCV (indicating better red blood cell health) and lower FEC (indicating lower parasite burden). These findings suggest a more pronounced effect of β-globin polymorphisms on H. contortus infection in males, potentially due to their generally weaker resistance compared to females. This study highlights the importance of sex and β-globin haplotypes in shaping immune responses to H. contortus infection. Specifically, IgM antibodies targeting the ES24 protein appear to play a crucial role in host-parasite interactions and may hold promise for therapeutic development.

PLGA-Encapsulated Haemonchus contortus Antigen ES-15 Augments Immune Responses in a Murine Model

Haemonchus contortus is a gastrointestinal parasite that adversely impacts small ruminants, resulting in a notable reduction in animal productivity. In the current investigation, we developed a nanovaccine by encapsulating the recombinant protein rHcES-15, sourced from the excretory/secretory products of H. contortus, within biodegradable poly (D, L-lactide-co-glycolide) (PLGA) nanoparticles (NPs). The development of this nanovaccine involved the formulation of PLGA NPs using a modified double emulsion solvent evaporation technique. Scanning electron microscopy (SEM)verified the successful encapsulation of rHcES-15 within PLGA NPs, exhibiting a size range of 350-400 nm. The encapsulation efficiency (EE) of the antigen in the nanovaccine was determined to be 72%. A total of forty experimental mice were allocated into five groups, with the nanovaccine administered on day 0 and the mice euthanized at the end of the 14-day trial. The stimulation index (SI) from the mice subjected to the nanovaccine indicated heightened lymphocyte proliferation (*** p < 0.001) and a noteworthy increase in anti-inflammatory cytokines (IL-4, IL-10, and IL-17). Additionally, the percentages of T-cells (CD4+, CD8+) and dendritic cell phenotypes (CD83+, CD86+) were significantly elevated (** p < 0.01, *** p < 0.001) in mice inoculated with the nanovaccine compared to control groups and the rHcES-15 group. Correspondingly, higher levels of antigen-specific serum immunoglobulins (IgG1, IgG2a, IgM) were observed in response to the nanovaccine in comparison to both the antigenic (rHcES-15) and control groups (* p < 0.05, ** p < 0.01). In conclusion, the data strongly supports the proposal that the encapsulation of rHcES-15 within PLGA NPs effectively triggers immune cells in vivo, ultimately enhancing the antigen-specific adaptive immune responses against H. contortus. This finding underscores the promising potential of the nanovaccine, justifying further investigations to definitively ascertain its efficacy.

IgM antibody responses against Plasmodium antigens in neotropical primates in the Brazilian Atlantic Forest

Introduction

Zoonotic transmission is a challenge for the control and elimination of malaria. It has been recorded in the Atlantic Forest, outside the Amazon which is the endemic region in Brazil. However, only very few studies have assessed the antibody response, especially of IgM antibodies, in Neotropical primates (NP). Therefore, in order to contribute to a better understanding of the immune response in different hosts and facilitate the identification of potential reservoirs, in this study, naturally acquired IgM antibody responses against Plasmodium antigens were evaluated, for the first time, in NP from the Atlantic Forest.

Methods

The study was carried out using 154 NP samples from three different areas of the Atlantic Forest. IgM antibodies against peptides of the circumsporozoite protein (CSP) from different Plasmodium species and different erythrocytic stage antigens were detected by ELISA.

Results

Fifty-nine percent of NP had IgM antibodies against at least one CSP peptide and 87% against at least one Plasmodium vivax erythrocytic stage antigen. Levels of antibodies against PvAMA-1 were the highest compared to the other antigens. All families of NP showed IgM antibodies against CSP peptides, and, most strikingly, against erythrocytic stage antigens. Generalized linear models demonstrated that IgM positivity against PvCSP and PvAMA-1 was associated with PCR-detectable blood-stage malaria infection and the host being free-living. Interestingly, animals with IgM against both PvCSP and PvAMA-1 were 4.7 times more likely to be PCR positive than animals that did not have IgM for these two antigens simultaneously.

Discussion

IgM antibodies against different Plasmodium spp. antigens are present in NP from the Atlantic Forest. High seroprevalence and antibody levels against blood-stage antigens were observed, which had a significant association with molecular evidence of infection. IgM antibodies against CSP and AMA-1 may be used as a potential marker for the identification of NP infected with Plasmodium, which are reservoirs of malaria in the Brazilian Atlantic Forest.

A DNA vaccine (EG95-PT1/2/3-IL2) encoding multi-epitope antigen and IL-2 provokes efficient and long-term immunity to echinococcosis

Echinococcosis is a highly prevalent global zoonosis, and vaccines are required. The commercial vaccine based on a protein-based subunit (EG95), however, is limited by its insufficient cellular immunity, a short protection period, and limited prevention against novel mutant strains. Herein, we applied bioinformatics to develop a DNA vaccine (pEG95-IL2) expressing both multi-epitope-based antigens (EG95-PT1/2/3) and an IL-2 adjuvant to regulate T cell differentiation and memory cell response. EG95-PT1/2/3 was screened with hierarchical structure prediction from the epitope conformation of B cells with high confidence across various species to guarantee immunogenicity. Importantly, cationic arginine-rich lipid nanoparticles (RNP) were utilized as a delivery vehicle to form lipoplexes that had a transfection efficiency of nearly two orders of magnitude greater than that of commercial reagents (Lipofectamine 2000 and polyethyleneimine) with both immune and nonimmune cells (DC2.4 and L929 cells, respectively). RNP/pEG95-IL2 lipoplexes displayed a robust and long-term antigen expression, as well as adjuvant effects during the immunization. Consequently, intramuscular injection of RNP/pEG95-IL2 elicited similar humoral immune responses and significantly greater cellular responses in mice when compared with those of the commercial vaccine. In addition, the inoculation protocol of RNP/pEG95-IL2 with sequential booster further strengthens cellular immunity in comparison with the homologous booster. Those findings provide a promising strategy for improving plasmid vaccine efficacy.

Effect of black seeds (Nigella sativa) on inflammatory and immunomodulatory markers in Plasmodium berghei-infected mice

Nigella sativa, a core dietary supplement and food additive in folklore is one of the most broadly studied seed plants in the global nutraceutical sector. Malaria infection impairs the ability of principal cells of the immune system to trigger an efficient inflammatory and immune response. Ninety-six mice, weighing 20-25 g, were grouped into 12 consisting of 8 animals each. The mice were infected with standard inoculum of the strain NK65 Plasmodium berghei (chloroquine sensitive) and the percentage parasitemia suppression were evaluated. The individual effect of black seed supplemented diet and its combinatory effect with chloroquine (CQ) were investigated on reactive oxygen species (ROS), glutathione peroxidase (GPx), reduced glutathione (GSH), glutathione-S-transferase (GST), serum immunoglobulins (IgG and IgM), and the hematological parameters (hemoglobin, packed cell volume, and red blood cell count) in P. berghei infected mice. The inflammatory cytokines, tumor necrosis factor (TNF-α), interleukin (IL-6 and IL-10), as well as IgG and IgM were assayed in the serum. The mice temperature and behavioral changes were observed. Infected mice treated with the dietary supplementation of black seed with a percentage inclusion (2.5%, 5%, 10%) showed significantly decreased parasitemia and ROS levels (p < 0.05) compared with the untreated mice. The result demonstrated a significant suppression in the pro-inflammatory cytokines (TNF-α, IL-6) levels and a notable elevation in the anti-inflammatory cytokine (IL-10), antioxidant markers as well as the immunoglobulin levels of the P. berghei-infected mice treated with black seed. The study revealed that black seed enhanced host antioxidant status, modulated inflammatory and immune response by regulating some inflammatory cytokines and immunomodulatory mediators. PRACTICAL APPLICATIONS: Black seed (Nigella sativa) has been a dietary supplement and natural remedy for many centuries. Inflammatory and immune diseases are the most notable cause of mortality in the world and more than 50% of deaths have been attributed to it. However, there is paucity of information on the effect of N. sativa on anti-inflammatory and immunomodulatory ability during malaria infection. The result suggests that N. sativa produced antioxidant, anti-inflammatory, and immunomodulatory effect in Plasmodium berghei-infected mice via the participation of glutathione antioxidant system, serum antibodies, and some inflammatory cytokines.

Mouse Models for Unravelling Immunology of Blood Stage Malaria

Malaria comprises a spectrum of disease syndromes and the immune system is a major participant in malarial disease. This is particularly true in relation to the immune responses elicited against blood stages of Plasmodium-parasites that are responsible for the pathogenesis of infection. Mouse models of malaria are commonly used to dissect the immune mechanisms underlying disease. While no single mouse model of Plasmodium infection completely recapitulates all the features of malaria in humans, collectively the existing models are invaluable for defining the events that lead to the immunopathogenesis of malaria. Here we review the different mouse models of Plasmodium infection that are available, and highlight some of the main contributions these models have made with regards to identifying immune mechanisms of parasite control and the immunopathogenesis of malaria.

Effects of Schyzocotyle acheilognathi (Yamaguti, 1934) infection on the intestinal microbiota, growth and immune reactions of grass carp (Ctenopharyngodon idella)

Our understanding of interactions among intestinal helminths, gut microbiota and host is still in its infancy in fish. In this study, the effects of Schyzocotyle acheilognathi infection on the intestinal microbiota, growth and immune reactions of grass carp were explored under laboratory conditions. 16S rDNA amplification sequencing results showed that S. acheilognathi infection altered the composition of intestinal microbiota only at the genus level, with a significant increase in the relative abundance of Turicibacter and Ruminococcus (P < 0.05) and a significant decrease in the relative abundance of Gordonia, Mycobacterium and Pseudocanthomonas (P < 0.05). Schyzocotyle acheilognathi infection had no significant effect (P > 0.05) on the alpha diversity indices (including Chao1, ACE, Shannon, Simpson index) of intestinal microbiota in grass carp, but PERMANOVA analysis showed that microbial structure significantly (P < 0.01) differed between hindgut and foregut. PICRUST prediction showed that some metabolism-related pathways were significantly changed after S. acheilognathi infection. The relative abundance of Turicibacter was positively correlated with the fresh weight of tapeworm (foregut: r = 0.48, P = 0.044; hindgut: r = 0.63, P = 0.005). There was no significant difference in the body condition of grass carp between the S. acheilognathi infected group and the uninfected group (P > 0.05). Intestinal tissue section with HE staining showed that S. acheilognathi infection severely damaged the intestinal villi, causing serious degeneration, necrosis and shedding of intestinal epithelial cells. The real-time fluorescent quantitative PCR results showed that S. acheilognathi infection upregulated the mRNA expression of the immune-related genes: Gal1−L2, TGF−β1 and IgM.

Influence of α2-Macroglobulin, Anti-Parasite IgM and ABO Blood Group on Rosetting in Plasmodium falciparum Clinical Isolates and Their Associations with Disease Severity in a Ghanaian Population

Purpose

The severity of Plasmodium falciparum infections is associated with the ability of the infected red blood cells to cytoadhere to host vascular endothelial surfaces and to uninfected RBCs. Host blood group antigens and two serum proteins α₂-macroglobulin (α₂M) and IgM have been implicated in rosette formation in laboratory-adapted P. falciparum. However, there is only limited information about these phenotypes in clinical isolates.

Methods

This was a hospital-based study involving children under 12 years-of-age reporting to the Hohoe Municipal Hospital with different clinical presentations of malaria. Parasite isolates were grown and rosette capabilities and characteristics were investigated by fluorescence microscopy. α₂M and IgM were detected by ELISA.

Results

Rosette formation was observed in 46.8% (75/160) of the parasite isolates from all the blood groups tested. Rosettes were more prevalent (55%) among isolates from patients with severe malaria compared to isolates from patients with uncomplicated malaria (45%). Rosette prevalence was highest (30%) among patients with blood group O (30%) and B (29%), while the mean rosette frequency was higher in isolates from patients with blood group A (28.7). Rosette formation correlated negatively with age (r = −0.09, P= 0.008). Participants with severe malaria had a lower IgM concentration (3.683±3.553) than those with uncomplicated malaria (5.256±4.294) and the difference was significant (P= 0.0228). The mean concentrations of anti-parasite IgM measured among the clinical isolates which formed rosettes was lower (4.2 ±3.930 mg/mL), than that in the non rosetting clinical isolates (4.604 ±4.159 mg/mL) but the difference was not significant (P=0.2733). There was no significant difference in plasma α₂M concentration between rosetting and non rosetting isolates (P=0.442).

Conclusion

P. falciparum parasite rosette formation was affected by blood group type and plasma concentration of IgM. A lower IgM concentration was associated with severe malaria whilst a higher α₂M concentration was associated with uncomplicated malaria.

Immunosuppression by UVB radiation exacerbates Leishmania mexicana skin lesions in mice

Cutaneous leishmaniasis is the most common form of leishmaniasis in humans. The disease is caused by several species, such as Leishmania mexicana, a protozoa parasite. Several major risk factors are associated with this disease, including poverty, poor housing, inadequate domestic hygiene, malnutrition, mobility, and occupational exposure. Solar radiation (UVB) has not been considered a risk factor because there is no scientific evidence demonstrating a correlation with increased susceptibility to cutaneous leishmaniasis. In this study, the shaved skin of the back of C57BL/6 mice was irradiated with 24.2 mJ/cm² of UVB. A delayed-type hypersensitivity (DTH) reaction was used to assess UV-induced immune suppression. Skin lesions were quantitated, and parasite burden and the presence of anti-Leishmania mexicana antibodies in serum and germinal centers in draining lymph nodes were determined. We found an increased in the lesion size and parasitic load in UVB-irradiated mice compared to the WT mice and B lymphocyte activation in draining lymph nodes and increased IgG1 production. Our results show an important role of UVB-induced suppression in cutaneous leishmaniasis through local production of IL-10 and systemic IgG1antibodies. This is the first study that demonstrates the effects of UVB radiation on cutaneous leishmaniasis by Leishmania mexicana.

Multimeric antibodies from antigen-specific human IgM+ memory B cells restrict Plasmodium parasites

Multimeric immunoglobulin-like molecules arose early in vertebrate evolution, yet the unique contributions of multimeric IgM antibodies to infection control are not well understood. This is partially due to the difficulty of distinguishing low-affinity IgM, secreted rapidly by plasmablasts, from high-affinity antibodies derived from later-arising memory cells. We developed a pipeline to express B cell receptors (BCRs) from Plasmodium falciparum–specific IgM⁺ and IgG⁺ human memory B cells (MBCs) as both IgM and IgG molecules. BCRs from both subsets were somatically hypermutated and exhibited comparable monomeric affinity. Crystallization of one IgM⁺ MBC-derived antibody complexed with antigen defined a linear epitope within a conserved Plasmodium protein. In its physiological multimeric state, this antibody displayed exponentially higher antigen binding than a clonally identical IgG monomer, and more effectively inhibited P. falciparum invasion. Forced multimerization of this IgG significantly improved both antigen binding and parasite restriction, underscoring how avidity can alter antibody function. This work demonstrates the potential of high-avidity IgM in both therapeutics and vaccines.

Adaptive immune responses mediated age-related Plasmodium yoelii 17XL and 17XNL infections in 4 and 8-week-old BALB/c mice

BACKGROUD

It is important to expound the opposite clinical outcomes between children and adulthood for eradicate malaria. There remains unknown about the correlation between adaptive immune response and age-related in malaria.

METHODS

4 and 8-week-old mice were used to mimic children and adulthood, respectively. Parasitemia and the survival rate were monitored. The proportion and function of Th1 and Th2 cells were detected by FACS. The levels of IFN-γ, IL-4, total IgG, IgG1, IgG2a and Plasmodium yoelii MSP-1-specific IgG were measured by ELISA.

RESULTS

The adult group showed greater resistance to P. yoelii 17XL infection, with lower parasitemia. Compared with 4-week-old mice, the percentage of CD4⁺T-bet⁺IFN-γ⁺ Th1 cells as well as IFN-γ production were significantly increased on day 5 p.i. in the 8-week-old mice after P. yoelii 17XNL infection. The percentage of CD4⁺GATA3⁺IL-4⁺ Th2 cells and CD4⁺CXCR5⁺ Tfh cells, and IL-4 production in the 8-week-old mice significantly increased on day 5 and day 10 after P. yoelii 17XNL infection. Notably, the levels of total IgG, IgG1, IgG2a and P. yoelii MSP-1-specific IgG were also significantly increased in the 8-week-old mice. PD-1, a marker of exhaustion, was up-regulated on CD4⁺ or activated CD4⁺ T cells in the 8-week-old mice as compared to the 4-week-old group.

CONCLUSIONS

Thus, we consider that enhanced cellular and humoral adaptive immunity might contribute to rapid clearance of malaria among adults, likely in a PD-1-dependent manner due to induction of CD4⁺ T cells exhaustion in P. yoelii 17XNL infected 8-week-old mice.

IL-4 Treatment Mitigates Experimental Cerebral Malaria by Reducing Parasitemia, Dampening Inflammation, and Lessening the Cytotoxicity of T Cells

Cytokine responses to malaria play important roles in both protective immunity development and pathogenesis. Although the roles of cytokines such as TNF-α, IL-12, IFN-γ, and IL-10 in immunity and pathogenesis to the blood stage malaria are largely known, the role of IL-4 remains less understood. IL-4 targets many cell types and induces multiple effects, including cell proliferation, gene expression, protection from apoptosis, and immune regulation. Accordingly, IL-4 has been exploited as a therapeutic for several inflammatory diseases. Malaria caused by Plasmodium falciparum manifests in many organ-specific fatal pathologies, including cerebral malaria (CM), driven by a high parasite load, leading to parasite sequestration in organs and consequent excessive inflammatory responses and endothelial damage. We investigated the therapeutic potential of IL-4 against fatal malaria in Plasmodium berghei ANKA-infected C57BL/6J mice, an experimental CM model. IL-4 treatment significantly reduced parasitemia, CM pathology, and mortality. The therapeutic effect of IL-4 is mediated through multiple mechanisms, including enhanced parasite clearance mediated by upregulation of phagocytic receptors and increased IgM production, and decreased brain inflammatory responses, including reduced chemokine (CXCL10) production, reduced chemokine receptor (CXCR3) and adhesion molecule (LFA-1) expression by T cells, and downregulation of cytotoxic T cell lytic potential. IL-4 treatment markedly reduced the infiltration of CD8⁺ T cells and brain pathology. STAT6, PI3K-Akt-NF-κB, and Src signaling mediated the cellular and molecular events that contributed to the IL-4-dependent decrease in parasitemia. Overall, our results provide mechanistic insights into how IL-4 treatment mitigates experimental CM and have implications in developing treatment strategies for organ-specific fatal malaria.

Dynamics of IgM and IgG responses to the next generation of engineered Duffy binding protein II immunogen: Strain-specific and strain-transcending immune responses over a nine-year period

BACKGROUND

A low proportion of P. vivax-exposed individuals acquire protective strain-transcending neutralizing IgG antibodies that are able to block the interaction between the Duffy binding protein II (DBPII) and its erythrocyte-specific invasion receptor. In a recent study, a novel surface-engineered DBPII-based vaccine termed DEKnull-2, whose antibody response target conserved DBPII epitopes, was able to induce broadly binding-inhibitory IgG antibodies (BIAbs) that inhibit P. vivax reticulocyte invasion. Toward the development of DEKnull-2 as an effective P. vivax blood-stage vaccine, we investigate the relationship between naturally acquired DBPII-specific IgM response and the profile of IgG antibodies/BIAbs activity over time.

METHODOLOGY/PRINCIPAL FINDINGS

A nine-year follow-up study was carried-out among long-term P. vivax-exposed Amazonian individuals and included six cross-sectional surveys at periods of high and low malaria transmission. DBPII immune responses associated with either strain-specific (Sal1, natural DBPII variant circulating in the study area) or conserved epitopes (DEKnull-2) were monitored by conventional serology (ELISA-detected IgM and IgG antibodies), with IgG BIAbs activity evaluated by functional assays (in vitro inhibition of DBPII-erythrocyte binding). The results showed a tendency of IgM antibodies toward Sal1-specific response; the profile of Sal1 over DEKnull-2 was not associated with acute malaria and sustained throughout the observation period. The low malaria incidence in two consecutive years allowed us to demonstrate that variant-specific IgG (but not IgM) antibodies waned over time, which resulted in IgG skewed to the DEKnull-2 response. A persistent DBPII-specific IgM response was not associated with the presence (or absence) of broadly neutralizing IgG antibody response.

CONCLUSIONS/SIGNIFICANCE

The current study demonstrates that long-term exposure to low and unstable levels of P. vivax transmission led to a sustained DBPII-specific IgM response against variant-specific epitopes, while sustained IgG responses are skewed to conserved epitopes. Further studies should investigate on the role of a stable and persistent IgM antibody response in the immune response mediated by DBPII.

Using two phases of the CD4 T cell response to blood-stage murine malaria to understand regulation of systemic immunity and placental pathology in Plasmodium falciparum infection

Plasmodium falciparum infection and malaria remain a risk for millions of children and pregnant women. Here, we seek to integrate knowledge of mouse and human T helper cell (Th) responses to blood-stage Plasmodium infection to understand their contribution to protection and pathology. Although there is no complete Th subset differentiation, the adaptive response occurs in two phases in non-lethal rodent Plasmodium infection, coordinated by Th cells. In short, cellular immune responses limit the peak of parasitemia during the first phase; in the second phase, humoral immunity from T cell-dependent germinal centers is critical for complete clearance of rapidly changing parasite. A strong IFN-γ response kills parasite, but an excess of TNF compared with regulatory cytokines (IL-10, TGF-β) can cause immunopathology. This common pathway for pathology is associated with anemia, cerebral malaria, and placental malaria. These two phases can be used to both understand how the host responds to rapidly growing parasite and how it attempts to control immunopathology and variation. This dual nature of T cell immunity to Plasmodium is discussed, with particular reference to the protective nature of the continuous generation of effector T cells, and the unique contribution of effector memory T cells.

The conduit system exports locally secreted IgM from lymph nodes

IgM provides early protection against pathogens. How IgM is exported out of lymph nodes remains unknown. Thierry et al. report that B cells utilize a system of paracortical conduits to rapidly export their IgM to the periphery.

Immunoglobulin M (IgM) is the first type of antibody produced during acute infections and thus provides an early line of specific defense against pathogens. Being produced in secondary lymphoid organs, IgM must rapidly be exported to the blood circulation. However, it is currently unknown how such large pentameric molecules are released from lymph nodes (LNs). Here, we show that upon immunization, IgM transiently gains access to the luminal side of the conduit system, a reticular infrastructure enabling fast delivery of tissue-derived soluble substances to the LN parenchyma. Using microinjections of purified IgM, we demonstrate that conduit-associated IgM is delivered by neither the afferent lymph nor the blood, but is locally conveyed by conduits. Exploiting in vivo models, we further demonstrate that conduit-associated IgM is locally and transiently produced by activated, antigen-specific B cells migrating in the T cell zone. Thus, our study reveals that the conduit system is coopted by B cells to rapidly export secreted IgM out of LNs.

Development and evaluation of the first immunochromatographic test that can detect specific antibodies against Cryptosporidium parvum

Cryptosporidium parvum is a major cause of diarrhea among human and calves, resulting in severe health hazards and drastic economic losses, respectively. Although C. parvum infection leads to high morbidity and mortality in immunocompromised patients and bovine calves, this infection remains a neglected disease. Currently available diagnostic tests for C. parvum are primarily based on detection of oocysts, DNA, or secreted antigens in fecal specimens. Demonstration of specific antibodies with a rapid immunochromatographic test (ICT) will be advantageous not only in providing a simple, rapid, accurate, and affordable tool but also in surveillance because of the ability to recognize recent and past infections. Herein, we developed two ICTs using the diagnostic antigen CpP23 and immunodominant antigen CpGP15 to detect C. parvum-specific antibodies in cattle sera. Because of unavailability of a reference test for antibody detection, evaluation and validation of our developed ICTs were conducted using reference cattle samples and unknown field cattle sera. Serum samples were simultaneously tested by a previously validated enzyme-linked immunosorbent assay (ELISA) using the same antigens (CpGP15 and CpP23). ICTs showed substantial ability to discriminate between positive and negative control cattle sera for both CpGP15 and CpP23. Even against field sera, high sensitivity, specificity, and agreement rates were recorded for ICTs compared with the previously validated ELISA with the same antigens (CpGP15 = 78.78%, 100%, and 85.11%; CpP23 = 80%, 100%, and 80.56%, respectively). Moreover, a high correlation was observed between the test band intensity of ICTs and optical density of ELISA, particularly in the case of CpP23-specific IgM. To our knowledge, this study represents the first development of ICTs that can detect C. parvum-specific antibodies. Our tests will contribute greatly to C. parvum infection control in cattle by providing a method for on-site diagnosis of early and latent infections.

CD28 deficiency leads to accumulation of germinal-center independent IgM+ experienced B cells and to production of protective IgM during experimental malaria

Protective immunity to blood-stage malaria is attributed to Plasmodium-specific IgG and effector-memory T helper 1 (Th1) cells. However, mice lacking the costimulatory receptor CD28 (CD28KO) maintain chronic parasitemia at low levels and do not succumb to infection, suggesting that other immune responses contribute to parasite control. We report here that CD28KO mice develop long-lasting non-sterile immunity and survive lethal parasite challenge. This protection correlated with a progressive increase of anti-parasite IgM serum levels during chronic infection. Serum IgM from chronically infected CD28KO mice recognize erythrocytes infected with mature parasites, and effectively control Plasmodium infection by promoting parasite lysis and uptake. These antibodies also recognize autoantigens and antigens from other pathogens. Chronically infected CD28KO mice have high numbers of IgM⁺ plasmocytes and experienced B cells, exhibiting a germinal-center independent Fas⁺GL7^-CD38⁺CD73^- phenotype. These cells are also present in chronically infected C57BL/6 mice although in lower numbers. Finally, IgM⁺ experienced B cells from cured C57BL/6 and CD28KO mice proliferate and produce anti-parasite IgM in response to infected erythrocytes. This study demonstrates that CD28 deficiency results in the generation of germinal-center independent IgM⁺ experienced B cells and the production of protective IgM during experimental malaria, providing evidence for an additional mechanism by which the immune system controls Plasmodium infection.

sIgM-FcμR Interactions Regulate Early B Cell Activation and Plasma Cell Development after Influenza Virus Infection

Previous studies with mice lacking secreted IgM (sIgM) due to a deletion of the μs splice region (μs^-/- ) had shown sIgM involvement in normal B cell development and in support of maximal Ag-specific IgG responses. Because of the changes to B cell development, it remains unclear to which extent and how sIgM directly affects B cell responses. In this study, we aimed to explore the underlying mechanisms of sIgM-mediated IgG response regulation during influenza virus infection. Generating mice with normally developed μs-deficient B cells, we demonstrate that sIgM supports IgG responses by enhancing early Ag-specific B cell expansion, not by altering B cell development. Lack of FcμR expression on B cells, but not lack of Fcα/μR expression or complement activation, reduced antiviral IgG responses to the same extent as observed in μs^-/- mice. B cell-specific Fcmr^-/- mice lacked robust clonal expansion of influenza hemagglutinin-specific B cells early after infection and developed fewer spleen and bone marrow IgG plasma cells and memory B cells, compared with controls. However, germinal center responses appeared unaffected. Provision of sIgM rescued plasma cell development from μs^-/- but not Fcmr^-/- B cells, as demonstrated with mixed bone marrow chimeric mice. Taken together, the data suggest that sIgM interacts with FcμR on B cells to support early B cell activation and the development of long-lived humoral immunity.

Small molecule-based inhibition of MEK1/2 proteins dampens inflammatory responses to malaria, reduces parasite load, and mitigates pathogenic outcomes

Malaria infections cause several systemic and severe single- or multi-organ pathologies, killing hundreds of thousands of people annually. Considering the existing widespread resistance of malaria parasites to anti-parasitic drugs and their high propensity to develop drug resistance, alternative strategies are required to manage malaria infections. Because malaria is a host immune response-driven disease, one approach is based on gaining a detailed understanding of the molecular and cellular processes that modulate malaria-induced innate and adaptive immune responses. Here, using a mouse cerebral malaria model and small-molecule inhibitors, we demonstrate that inhibiting MEK1/2, the upstream kinases of ERK1/2 signaling, alters multifactorial components of the innate and adaptive immune responses, controls parasitemia, and blocks pathogenesis. Specifically, MEK1/2 inhibitor treatment up-regulated B1 cell expansion, IgM production, phagocytic receptor expression, and phagocytic activity, enhancing parasite clearance by macrophages and neutrophils. Further, the MEK1/2 inhibitor treatment down-regulated pathogenic pro-inflammatory and helper T cell 1 (Th1) responses and up-regulated beneficial anti-inflammatory cytokine responses and Th2 responses. These inhibitor effects resulted in reduced granzyme B expression by T cells, chemokine and intracellular cell adhesion molecule 1 (ICAM-1) expression in the brain, and chemokine receptor expression by both myeloid and T cells. These bimodal effects of the MEK1/2 inhibitor treatment on immune responses contributed to decreased parasite biomass, organ inflammation, and immune cell recruitment, preventing tissue damage and death. In summary, we have identified several previously unrecognized immune regulatory processes through which a MEK1/2 inhibitor approach controls malaria parasitemia and mitigates pathogenic effects on host organs.

Somatically Hypermutated Plasmodium-Specific IgM(+) Memory B Cells Are Rapid, Plastic, Early Responders upon Malaria Rechallenge

Humoral immunity consists of pre-existing antibodies expressed by long-lived plasma cells and rapidly reactive memory B cells (MBC). Recent studies of MBC development and function after protein immunization have uncovered significant MBC heterogeneity. To clarify functional roles for distinct MBC subsets during malaria infection, we generated tetramers that identify Plasmodium-specific MBCs in both humans and mice. Long-lived murine Plasmodium-specific MBCs consisted of three populations: somatically hypermutated immunoglobulin M(+) (IgM(+)) and IgG(+) MBC subsets and an unmutated IgD(+) MBC population. Rechallenge experiments revealed that high affinity, somatically hypermutated Plasmodium-specific IgM(+) MBCs proliferated and gave rise to antibody-secreting cells that dominated the early secondary response to parasite rechallenge. IgM(+) MBCs also gave rise to T cell-dependent IgM(+) and IgG(+)B220(+)CD138(+) plasmablasts or T cell-independent B220(-)CD138(+) IgM(+) plasma cells. Thus, even in competition with IgG(+) MBCs, IgM(+) MBCs are rapid, plastic, early responders to a secondary Plasmodium rechallenge and should be targeted by vaccine strategies.

Immunoglobulin M: Restrainer of Inflammation and Mediator of Immune Evasion by Plasmodium falciparum Malaria

Immunoglobulin M (IgM) is an ancient antibody class that is found in all vertebrates, with the exception of coelacanths, and is indispensable in both innate and adaptive immunity. The equally ancient human malaria parasite, Plasmodium falciparum, formed an intimate relationship with IgM with which it co-evolved. In this article, we discuss the association between IgM and human malaria parasites, building on several recent publications that implicate IgM as a crucial molecule that determines both host and parasite survival. Consequently, a better understanding of this association may lead to the development of improved intervention strategies.

Genetic Resistance to Malaria Is Associated With Greater Enhancement of Immunoglobulin (Ig)M Than IgG Responses to a Broad Array of Plasmodium falciparum Antigens

The breadth and magnitude of P. falciparum-specific IgM and IgG responses are greater in the malaria-resistant Fulani versus the malaria-susceptible Dogon, and P. falciparum-specific IgM responses more strongly distinguish the two ethnic groups.

Background. People of the Fulani ethnic group are more resistant to malaria compared with genetically distinct ethnic groups, such as the Dogon people, in West Africa, and studies suggest that this resistance is mediated by enhanced antibody responses to Plasmodium falciparum antigens. However, prior studies measured antibody responses to <0.1% of P falciparum proteins, so whether the Fulani mount an enhanced and broadly reactive immunoglobulin (Ig)M and IgG response to P falciparum remains unknown. In general, little is known about the extent to which host genetics influence the overall antigen specificity of IgM and IgG responses to natural infections.

Methods. In a cross-sectional study in Mali, we collected plasma from asymptomatic, age-matched Fulani (n = 24) and Dogon (n = 22) adults with or without concurrent P falciparum infection. We probed plasma against a protein microarray containing 1087 P falciparum antigens and compared IgM and IgG profiles by ethnicity.

Results. We found that the breadth and magnitude of P falciparum-specific IgM and IgG responses were significantly higher in the malaria-resistant Fulani versus the malaria-susceptible Dogon, and, unexpectedly, P falciparum-specific IgM responses more strongly distinguished the 2 ethnic groups.

Conclusions. These findings point to an underappreciated role for IgM in protection from malaria, and they suggest that host genetics may influence the antigen specificity of IgM and IgG responses to infection.

Identification of circulating biomarkers in sera of Plasmodium knowlesi-infected malaria patients--comparison against Plasmodium vivax infection

BACKGROUND

Plasmodium knowlesi was identified as the fifth major malaria parasite in humans. It presents severe clinical symptoms and leads to mortality as a result of hyperparasitemia in a short period of time. This study aimed to improve the current understanding of P. knowlesi and identify potential biomarkers for knowlesi malaria.

METHODS

In the present study, we have employed two-dimensional gel electrophoresis-coupled immunoblotting techniques and mass spectrometry to identify novel circulating markers in sera from P. knowlesi-infected patients. Specifically, we have compared serum protein profiles from P. knowlesi-infected patients against those of healthy or P. vivax-infected individuals.

RESULTS

We identified several immunoreactive proteins in malarial-infected subjects, including alpha-2-HS glycoprotein (AHSG), serotransferrin (TF), complement C3c (C3), hemopexin (HPX), zinc-2-alpha glycoprotein (ZAG1), apolipoprotein A1 (Apo-A1), haptoglobin (HAP), and alpha-1-B-glycoprotein (A1BG). However, only TF and HPX displayed enhanced antigenicity and specificity, suggesting that they might represent valid markers for detecting P. knowlesi infection. Additionally, six P. knowlesi-specific antigens were identified (K15, K16, K28, K29, K30, and K38). Moreover, although HAP antigenicity was observed during P. vivax infection, it was undetectable in P. knowlesi-infected subjects.

CONCLUSIONS

We have demonstrated the application of immunoproteomics approach to identify potential candidate biomarkers for knowlesi malaria infection.

Quantifying variation in the potential for antibody-mediated apparent competition among nine genotypes of the rodent malaria parasite Plasmodium chabaudi

Within-host competition among parasite genotypes affects epidemiology as well as the evolution of virulence. In the rodent malaria Plasmodium chabaudi, competition among genotypes, as well as clone-specific and clone-transcending immunity are well documented. However, variation among genotypes in the induction of antibodies is not well understood, despite the important role of antibodies in the clearance of malaria infection. Here, we quantify the potential for antibodies induced by one clone to bind another (i.e., to cause antibody-mediated apparent competition) for nine genetically distinct P. chabaudi clones. We hypothesised that clones would vary in the strength of antibody induction, and that the propensity for clone-transcending immunity between a pair of clones would increase with increasing genetic relatedness at key antigenic loci. Using serum collected from mice 35 days post-infection, we measured titres of antibody to an unrelated antigen, Keyhole Limpet Haemocyanin (KLH), and two malaria antigens: recombinant Apical Membrane Antigen-1 (AMA-1) and Merozoite Surface Protein-119 (MSP-119). Amino acid sequence homology within each antigenic locus was used as a measure of relatedness. We found significant parasite genetic variation for the strength of antibody induction. We also found that relatedness at MSP-119 but not AMA-1 predicted clone-transcending binding. Our results help explain the outcome of chronic-phase mixed infections and generate testable predictions about the pairwise competitive ability of P. chabaudi clones.

IL-27 receptor signaling regulates memory CD4+ T cell populations and suppresses rapid inflammatory responses during secondary malaria infection

Interleukin-27 (IL-27) is known to control primary CD4(+) T cell responses during a variety of different infections, but its role in regulating memory CD4(+) T responses has not been investigated in any model. In this study, we have examined the functional importance of IL-27 receptor (IL-27R) signaling in regulating the formation and maintenance of memory CD4(+) T cells following malaria infection and in controlling their subsequent reactivation during secondary parasite challenge. We demonstrate that although the primary effector/memory CD4(+) T cell response was greater in IL-27R-deficient (WSX-1(-/-)) mice following Plasmodium berghei NK65 infection than in wild-type (WT) mice, there were no significant differences in the size of the maintained memory CD4(+) T population(s) at 20 weeks postinfection in the spleen, liver, or bone marrow of WSX-1(-/-) mice compared with WT mice. However, the composition of the memory CD4(+) T cell pool was slightly altered in WSX-1(-/-) mice following clearance of primary malaria infection, with elevated numbers of late effector memory CD4(+) T cells in the spleen and liver and increased production of IL-2 in the spleen. Crucially, WSX-1(-/-) mice displayed significantly enhanced parasite control compared with WT mice following rechallenge with homologous malaria parasites. Improved parasite control in WSX-1(-/-) mice during secondary infection was associated with elevated systemic production of multiple inflammatory innate and adaptive cytokines and extremely rapid proliferation of antigen-experienced T cells in the liver. These data are the first to demonstrate that IL-27R signaling plays a role in regulating the magnitude and quality of secondary immune responses during rechallenge infections.

Plasmodium chabaudi AS: distinct CD4(+)CD25(+)Foxp3(+) regulatory T cell responses during infection in DBA/2 and BALB/c mice

Malaria infections display variation patterns of clinical course and outcome. Although CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells play an essential role in immune homeostasis, the immune regulatory roles involved in malaria infection remains to be elucidated. Herein, we compared the disparity in Treg cells response during the course of blood stage Plasmodium chabaudi chabaudi AS (P. c chabaudi AS) infection in DBA/2 and BALB/c mice. BALB/c mice initiated a Th1/Th2 profile respond to P. c chabaudi AS infection, but DBA/2 mice failed to control P. c chabaudi AS infection and almost of them died post-peak parasitemia. At the peak parasitemia, we found that higher proportion of Treg cells with elevated Foxp3 expression in DBA/2 than in BALB/c mice. We used anti-CD25 mAb to deplete Treg cells and found that the survival time and rate were prolonged in DBA/2 mice treated with anti-CD25 mAb. Treatment with anti-CD25 mAb in vivo led to enhanced pro-inflammation responses and Foxp3 expression decline on Treg cells. In contrast, after DBA/2 was treatment with anti-IL-10R mAb, IL-10R blockade in vivo caused excessive pro-inflammation responses and Foxp3 expression loss on CD4(+)CD25(+) T cells. Earlier death was found in all of DBA/2 mice with anti-IL-10R mAb. It suggested that IL-2 and IL-10 signal involved in maintaining Foxp3 expression on Treg cells. In all, the moderate suppressive activity of Treg cells may facilitate resistance to P. c chabaudi AS infection.

Low antibodies against Plasmodium falciparum and imbalanced pro-inflammatory cytokines are associated with severe malaria in Mozambican children: a case-control study

Background

The factors involved in the progression from Plasmodium falciparum infection to severe malaria (SM) are still incompletely understood. Altered antibody and cellular immunity against P. falciparum might contribute to increase the risk of developing SM.

Methods

To identify immune responses associated with SM, a sex- and age-matched case–control study was carried out in 134 Mozambican children with SM (cerebral malaria, severe anaemia, acidosis and/or respiratory distress, prostration, hypoglycaemia, multiple seizures) or uncomplicated malaria (UM). IgG and IgM against P. falciparum lysate, merozoite antigens (MSP-1₁₉, AMA-1 and EBA-175), a Duffy binding like (DBL)-α rosetting domain and antigens on the surface of infected erythrocytes were measured by ELISA or flow cytometry. Plasma concentrations of IL-12p70, IL-2, IFN-γ, IL-4, IL-5, IL-10, IL-8, IL-6, IL-1β, TNF, TNF-β and TGF-β1 were measured using fluorescent bead immunoassays. Data was analysed using McNemar’s and Signtest.

Results

Compared to UM, matched children with SM had reduced levels of IgG against DBLα (P < 0.001), IgM against MSP-1₁₉ (P = 0.050) and AMA-1 (P = 0.047), TGF-β1 (P <0.001) and IL-12 (P = 0.039). In addition, levels of IgG against P. falciparum lysate and IL-6 concentrations were increased (P = 0.004 and P = 0.047, respectively). Anti-DBLα IgG was the only antibody response associated to reduced parasite densities in a multivariate regression model (P = 0.026).

Conclusions

The lower levels of antibodies found in children with SM compared to children with UM were not attributable to lower exposure to P. falciparum in the SM group. IgM against P. falciparum and specific IgG against a rosetting PfEMP1 domain may play a role in the control of SM, whereas an imbalanced pro-inflammatory cytokine response may exacerbate the severity of infection. A high overlap in symptoms together with a limited sample size of different SM clinical groups reduced the power to identify immunological correlates for particular forms of SM.

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

B cells and antibodies are essential for the protective immune response against a blood-stage Plasmodium infection. Although extensive research has focused on memory as well as plasma B-cell responses during infection, little is known about how malaria affects B-cell development and splenic maturation into marginal zone B (MZB) and follicular B (FoB) cells. In this study, we show that acute Plasmodium chabaudi AS infection in C57Bl/6 mice causes severe disruption of B lymphopoiesis in the bone marrow, affecting in particular pro-, pre-, and immature B cells as well as the expression of the bone marrow B-cell retention chemokine CXCL12. In addition, elevated apoptosis of transitional T2 and marginal zone (MZ) B cells was observed during and subsequent to the control of the first wave of parasitemia. In contrast, Folllicular (Fo) B cells levels were retained in the spleen throughout the infection, suggesting that these are essential for parasite clearance and proper infection control.

IgM production by bone marrow plasmablasts contributes to long-term protection against intracellular bacterial infection

IgM responses are well known to occur early postinfection and tend to be short-lived, which has suggested that this Ig does not significantly contribute to long-term immunity. In this study, we demonstrate that chronic infection with the intracellular bacterium Ehrlichia muris elicits a protective, long-term IgM response. Moreover, we identified a population of CD138(high)IgM(high) B cells responsible for Ag-specific IgM production in the bone marrow. The IgM-secreting cells, which exhibited characteristics of both plasmablasts and plasma cells, contributed to protection against fatal ehrlichial challenge. Mice deficient in activation-induced cytidine deaminase, which produce only IgM, were protected against fatal ehrlichial challenge infection. The IgM-secreting cells that we have identified were maintained in the bone marrow in the absence of chronic infection, as antibiotic-treated mice remained protected against challenge infection. Our studies identify a cell population that is responsible for the IgM production in the bone marrow, and they highlight a novel role for IgM in the maintenance of long-term immunity during intracellular bacterial infection.

The importance of natural IgM: scavenger, protector and regulator

The existence of IgM has been known for more than a century, but its importance in immunity and autoimmunity continues to emerge. Studies of mice deficient in secreted IgM have provided unexpected insights into its role in several diverse processes, from B cell survival to atherosclerosis, as well as in autoimmunity and protection against infection. Among the various distinct properties that underlie the functions of IgM, two stand out: its polyreactivity and its ability to facilitate the removal of apoptotic cells. In addition, new B cell-targeted therapies for the treatment of autoimmunity have been shown to cause a reduction in serum IgM, potentially disrupting the functions of this immunoregulatory molecule and increasing susceptibility to infection.

Characterization of immune responses to single or mixed infections with P. yoelii 17XL and P. chabaudi AS in different strains of mice

The outcome of Plasmodium yoelii 17XL (P.y17XL)-infected BALB/c and DBA/2 mice, ranging from death to spontaneous cure, depends largely on the establishment of effective Th1 and Th2 responses and a successful switch between Th1 and Th2 responses, as well as appropriate functioning of CD4(+)CD25(+)Foxp3(+)regulatory T cells (Tregs). The infection with another malaria-causing parasite, Plasmodium chabaudi AS (P.cAS), leads to a different outcome in BALB/c and DBA/2 mice compared to mice infected with P.y17XL alone. To understand the consequence of co-infection with P.y17XL and P.cAS, we determined the proliferation curve of parasites, pro-inflammatory/anti-inflammatory cytokine profiles, and the dynamic changes of the number of Tregs in DBA/2 and BALB/c mice with single or mixed-species infections. The infective mode in mixed-species infections was the same as single P.y17XL infections. The multiplication of P.y17XL parasites prevailed in BALB/c and DBA/2 mice with early mixed infections, as detected by RTQ-PCR. Subsequently, the multiplication of P.cAS parasites dominated in DBA/2 mice with mixed infections, while BALB/c mice succumbed to infection. In addition, the dynamic changes in IFN-gamma and IL-4 production in mice with mixed infections, used as a measure of Th1 and Th2 responsiveness, were consistent with P.y17XL-infected mice. Treg activation and the IL-10 level were also closely related to susceptibility to infection. Our findings demonstrate that the characteristics of the immune response during infections with mixed species are dependent on the mode of proliferation of different species of Plasmodium. Indeed, different species of Plasmodium can influence each other in the same host.

Fc-receptors and immunity to malaria: from models to vaccines

The complexity and number of antigens (Ags) seen during an immune response has hampered the development of malaria vaccines. Antibodies (Abs) play an important role in immunity to malaria and their passive administration is effective at controlling the disease. Abs represent approximately 25% of all proteins undergoing clinical trials, and these 'smart biologicals' have undergone a major revival with the realization that Abs lie at the interface between innate and adaptive immunity. At least 18 Abs have FDA approval for clinical use and approximately 150 are in clinical trials, the majority for the treatment of cancer, allograft rejection or autoimmune disease. Despite these triumphs none are in development for malaria, principally because they are perceived as being too expensive for a disease mainly afflicting poor and marginalized populations. Although unlikely, at least in the foreseeable future, that Ab-based prophylaxis will be made available to the millions of people at risk from malaria, they may be incorporated into current vaccine approaches, since Abs act as correlates of protection in studies aimed at defining the best Ags to include in vaccines. Abs may also form the basis for novel vaccination strategies by targeting Ags to appropriate antigen presenting cells. Therefore, to develop the most efficacious vaccines it will be necessary to fully understand which Abs and Fc-receptors (FcRs) are best engaged for a positive outcome.

Effects of CD4+CD25+Foxp3+regulatory T cells on earlyPlasmodium yoelii17XL infection in BALB/c mice

The outcome ofPlasmodium yoelii17XL-infected BALB/c and DBA/2 mice, ranging from death to spontaneous cure, respectively, depends largely on the establishment of effective pro-inflammatory type 1 responses during the early stages of infection and associates with CD4+CD25+Foxp3+regulatory T cells (Tregs). Here, effects of Tregs were analysed on earlyP. yoelii17XL infection in BALB/c and DBA/2 mice.In vivodepletion of Tregs significantly reversed the inhibited establishment of effective pro-inflammatory type 1 responses in BALB/c mice, indicating that this cell population contributed to the suppression of T-cell function in malaria. Moreover, the proportion and absolute numbers of IL-10-secreting Tregs in BALB/c mice were significantly higher than that found in DBA/2 mice by intracytoplasmic staining, and IL-10 production was correlated with the Tregs population. In addition,in vivoTregs depletion decreased the production of IL-10 and the apoptosis of CD4+T cells. Consistently, IL-10R blockade also had the same effect as that of Tregs depletion inP. yoelii17XL-infected BALB/c mice. Our data demonstrate that Tregs perhaps have an important role in regulating pro-inflammatory type 1 responses in an IL-10-dependent manner and induce CD4+T cell apoptosis during the early stage ofP. yoelii17XL infection.

IgM in microbial infections: taken for granted?

Much has been learned about the structure, function, and production of IgM, since the antibody's initial characterization. It is widely accepted that IgM provides a first line of defense during microbial infections, prior to the generation of adaptive, high-affinity IgG responses that are important for long-lived immunity and immunological memory. Although IgM responses are commonly used as a measure of exposure to infectious diseases, it is perhaps surprising that the role of and requirement for IgM in many microbial infections has not been well explored in vivo. This is in part due to the lack of capabilities, until relatively recently, to evaluate the requirement for IgM in the absence of coincident IgG responses. Such evaluations are now possible, using gene-targeted mouse strains that produce only IgM, or isotype-switched IgG. A number of studies have revealed that IgM, produced either innately, or in response to antigen challenge, plays an important and perhaps under appreciated role in many microbial infections. Moreover, the characterization of the roles of various B cell subsets, in the production of IgM, and in host defense, has revealed important and divergent roles for B-1a and B-1b cells. This review will highlight studies in which IgM, in its own right, has been found to play an important role, not only in early immunity, but also in long-term protection, against a variety of microbial pathogens. Observations that long-lived IgM responses can be generated in vivo suggest that it may be feasible to target IgM production as part of vaccination strategies.

Host control of malaria infections: constraints on immune and erythropoeitic response kinetics

The two main agents of human malaria, Plasmodium vivax and Plasmodium falciparum, can induce severe anemia and provoke strong, complex immune reactions. Which dynamical behaviors of host immune and erythropoietic responses would foster control of infection, and which would lead to runaway parasitemia and/or severe anemia? To answer these questions, we developed differential equation models of interacting parasite and red blood cell (RBC) populations modulated by host immune and erythropoietic responses. The model immune responses incorporate both a rapidly responding innate component and a slower-responding, long-term antibody component, with several parasite developmental stages considered as targets for each type of immune response. We found that simulated infections with the highest parasitemia tended to be those with ineffective innate immunity even if antibodies were present. We also compared infections with dyserythropoiesis (reduced RBC production during infection) to those with compensatory erythropoiesis (boosted RBC production) or a fixed basal RBC production rate. Dyserythropoiesis tended to reduce parasitemia slightly but at a cost to the host of aggravating anemia. On the other hand, compensatory erythropoiesis tended to reduce the severity of anemia but with enhanced parasitemia if the innate response was ineffective. For both parasite species, sharp transitions between the schizont and the merozoite stages of development (i.e., with standard deviation in intra-RBC development time <or=2.4 h) were associated with lower parasitemia and less severe anemia. Thus tight synchronization in asexual parasite development might help control parasitemia. Finally, our simulations suggest that P. vivax can induce severe anemia as readily as P. falciparum for the same type of immune response, though P. vivax attacks a much smaller subset of RBCs. Since most P. vivax infections are nonlethal (if debilitating) clinically, this suggests that P. falciparum adaptations for countering or evading immune responses are more effective than those of P. vivax.

Identification of residues in the Cmu4 domain of polymeric IgM essential for interaction with Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1)

The binding of nonspecific human IgM to the surface of infected erythrocytes is important in rosetting, a major virulence factor in the pathogenesis of severe malaria due to Plasmodium falciparum, and IgM binding has also been implicated in placental malaria. Herein we have identified the IgM-binding parasite ligand from a virulent P. falciparum strain as PfEMP1 (TM284var1 variant), and localized the region within this PfEMP1 variant that binds IgM (DBL4beta domain). We have used this parasite IgM-binding protein to investigate the interaction with human IgM. Interaction studies with domain-swapped Abs, IgM mutants, and anti-IgM mAbs showed that PfEMP1 binds to the Fc portion of the human IgM H chain and requires the IgM Cmu4 domain. Polymerization of IgM was shown to be crucial for the interaction because PfEMP1 binding did not occur with mutant monomeric IgM molecules. These results with PfEMP1 protein have physiological relevance because infected erythrocytes from strain TM284 and four other IgM-binding P. falciparum strains showed analogous results to those seen with the DBL4beta domain. Detailed investigation of the PfEMP1 binding site on IgM showed that some of the critical amino acids in the IgM Cmu4 domain are equivalent to those regions of IgG and IgA recognized by Fc-binding proteins from bacteria, suggesting that this region of Ig molecules may be of major functional significance in host-microbe interactions. We have therefore shown that PfEMP1 is an Fc-binding protein of malaria parasites specific for polymeric human IgM, and that it shows functional similarities with Fc-binding proteins from pathogenic bacteria.

Suppression of Plasmodium falciparum by serum collected from a case of Plasmodium vivax infection

BACKGROUND

It has frequently been reported that Plasmodium vivax suppressed Plasmodium falciparum and ameliorated disease severity in patients infected with these two species simultaneously. The authors investigate the hypothesis that immunological responses stimulated by P. vivax may play a role in suppressing co-infecting P. falciparum.

METHODS

Sera, taken sequentially from one of the authors (YN) during experimental infection with P. vivax, were added to in vitro cultures of P. falciparum. Cross-reactive antibodies against P. falciparum antigens, and cytokines were measured in the sera.

RESULTS

Significant growth inhibitory effects upon P. falciparum cultures (maximally 68% inhibition as compared to pre-illness average) were observed in the sera collected during an acute episode. Such inhibitory effects showed a strong positive temporal correlation with cross-reactive antibodies, especially IgM against P. falciparum schizont extract and, to a lesser degree, IgM against Merozoite Surface Protein (MSP)-119. Interleukin (IL)-12 showed the highest temporal correlation with P. vivax parasitaemia and with body temperatures in the volunteer.

CONCLUSION

These results suggest the involvement by cross-reactive antibodies, especially IgM, in the interplay between plasmodial species. IL-12 may be one of direct mediators of fever induction by rupturing P. vivax schizonts, at least in some subjects. Future studies, preferably of epidemiological design, to reveal the association between cross-reactive IgM and cross-plasmodial interaction, are warranted.

A Reappraisal of Humoral Immunity Based on Mechanisms of Antibody‐Mediated Protection Against Intracellular Pathogens

Sometime in the mid to late twentieth century the study of antibody-mediated immunity (AMI) entered the doldrums, as many immunologists believed that the function of AMI was well understood, and was no longer deserving of intensive investigation. However, beginning in the 1990s studies using monoclonal antibodies (mAbs) revealed new functions for antibodies, including direct antimicrobial effects and their ability to modify host inflammatory and cellular responses. Furthermore, the demonstration that mAbs to several intracellular bacterial and fungal pathogens were protective issued a serious challenge to the paradigm that host defense against such microbes was strictly governed by cell-mediated immunity (CMI). Hence, a new view of AMI is emerging. This view is based on the concept that a major function of antibody (Ab) is to amplify or subdue the inflammatory response to a microbe. In this regard, the "damage-response framework" of microbial pathogenesis provides a new conceptual viewpoint for understanding mechanisms of AMI. According to this view, the ability of an Ab to affect the outcome of a host-microbe interaction is a function of its capacity to modify the damage ensuing from such an interaction. In fact, it is increasingly apparent that the efficacy of an Ab cannot be defined either by immunoglobulin or epitope characteristics alone, but rather by a complex function of Ab variables, such as specificity, isotype, and amount, host variables, such as genetic background and immune status, and microbial variables, such as inoculum, mechanisms of avoiding host immune surveillance and pathogenic strategy. Consequently, far from being understood, recent findings in AMI imply a system with unfathomable complexity and the field is poised for a long overdue renaissance.