Regulation of immunity to malaria: valuable lessons learned from murine models

Plasma cytokine profiles associated with rhodesiense sleeping sickness and falciparum malaria co-infection in North Eastern Uganda

BACKGROUND

Immunological Human African Trypanosomiasis (HAT) studies often exclude malaria, although both infections overlap in specific endemic areas. During this co-infection, it is not known whether this parasitic interaction induces synergistic or antagonistic cytokine response among humans. This study determined prevalence of Plasmodium falciparum malaria among Trypanosoma brucei rhodesiense HAT and plasma cytokine profile levels associated with HAT and/or malaria infections.

METHODS

Participants were recruited at Lwala hospital in north eastern Uganda: healthy controls (30), malaria (28), HAT (17), HAT and malaria (15) diagnosed by microscopy and PCR was carried out for parasite species identification. Plasma cytokine levels of Interferon-gamma (IFN-γ), Tumour Necrosis Factor-alpha (TNF-α), Interleukin (IL)-6, IL-10 and Transforming Growth Factor-beta (TGF-β) were measured by sandwich Enzyme-Linked Immuno Sorbent Assay and data statistically analysed using Graphpad Prism 6.0.

RESULTS

The prevalence of P. falciparum malaria among T. rhodesiense HAT cases was high (46.8%). Malaria and/or HAT cases presented significant higher plasma cytokine levels of IFN-γ, TNF-α, IL-6, IL-10 and TGF-β than healthy controls (P < 0.05). Levels of IFN-γ, IL-6 and IL-10 were significantly elevated in HAT over malaria (P < 0.05) but no significant difference in TNF-α and TGF-β between HAT and malaria (P > 0.05). Co-infection expressed significantly higher plasma IFN-γ, IL-6, and IL-10 levels than malaria (P < 0.05) but no significant difference with HAT mono-infection (P > 0.05). The TNF-α level was significantly elevated in co-infection over HAT or malaria mono-infections (P < 0.05) unlike TGF-β level. Significant positive correlations were identified between IFN-γ verses TNF-α and IL-6 verses IL-10 in co-infection (Spearman's P < 0.05).

CONCLUSIONS

The T. b. rhodesiense significantly induced the cytokine response more than P. falciparum infections. Co-infection led to synergistic stimulation of pro-inflammatory (IFN-γ, TNF-α), and anti-inflammatory (IL-6, and IL-10) cytokine responses relative to malaria mono-infection. Level of TNF-α partially indicates the effect induced by T. b. rhodesiense and P. falciparum mono-infections or a synergistic interaction of co-infections which may have adverse effects on pathogenesis, prognosis and resolution of the infections.Trial registration VCD-IRC/021, 26/08/2011; HS 1089, 16/01/2012.

A flow cytometric assessment of the lymphocyte immunophenotypes in dogs naturally infected with Babesia rossi

Immunity to Babesia infection requires both innate and acquired responses, including cell mediated- and humoral responses. The aims of this study were to investigate the variation in selected peripheral blood lymphocyte phenotypes in dogs with virulent babesiosis at presentation and over time after treatment, and to determine whether these were correlated with the severity of clinical signs. Forty-four dogs naturally infected with B. rossi were studied and 5 healthy dogs were included as controls. Blood samples were collected from the jugular vein at admission, prior to any treatment, and at 24h and 48-72h. Leukocytes were incubated with canine specific, fluorochrome conjugated anti-CD3, anti-CD4, anti-CD8, and anti-B cell markers. Babesia-infected dogs were divided into complicated or uncomplicated groups on clinical grounds and in-house laboratory assays. The percentage CD3⁺ lymphocytes in the complicated group was lower compared to the controls (P=0.014) and uncomplicated group (P=0.007). The percentage CD4⁺ T lymphocytes in the complicated group was lower compared to the controls (P=0.027) and uncomplicated group (P=0.014). Both the complicated as well as the uncomplicated groups expressed a lower percentage CD8⁺ T lymphocytes compared to the control group (P<0.001 and P=0.005, respectively). The percentage B lymphocytes was higher in the complicated group at 48-72h. These findings could indicate the presence of a functional immune suppression secondary to increased apoptosis or redistribution of effector lymphocytes and/or a combination of other immune modulatory mechanisms induced by B. rossi infection.

Immunophenotyping of blood mononuclear cells from P. berghei (NK-65) infected and immunized BALB/c mice

The present study demonstrates the alterations in the frequencies of helper, cytotoxic and suppressor T cells in blood of P. berghei infected and TPA immunized rodent host towards the induction of protective immune response. Statistically significant (p < 0.005) number of CD4+ T cells was recorded on D9 [Post Challenge (PC)] meanwhile CD4+ Treg cells were found to be declined in immunized mice as compared to infected. This increase in frequencies of T helper cells points towards the stimulation of strong Th2 immune response in immunized mice. Statistically significant (p < 0.005) decline was observed in the frequencies of CD8+ T cells on D9 (PC) in immunized mice. In infected controls cytotoxic T cells were also found to be increased after 24 h post infection. Due to strong Th2 immune response, evident from lower frequencies of CD4+ Treg cells in immunized mice, complete protection was obtained. T reg population was found to be higher in infected controls and all control mice died.

Differential prevalence and diversity of haemosporidian parasites in two sympatric closely related non-migratory passerines

Haemosporidian parasites infecting birds show distinct heterogeneity in their distribution among host species. However, despite numerous studies on the prevalence and diversity of parasite communities across species, very little is known on patterns of differences between them. Such data is lacking because up to date the majority of studies explored the patterns of variation in infections in different years, different time of sampling within a year or a breeding cycle, different study sites or was based on a small sample size, all of which may affect the estimates of prevalence and parasite diversity. Here, the prevalence, richness and diversity of haemosporidian parasites from the genera Plasmodium and Haemoproteus were studied in two closely related non-migratory hole-nesting passerines: Great Tits and Blue Tits. Birds were sampled in sympatrically breeding populations during two seasons at the same stage of their breeding cycle - late nestling care. Great Tits were more prevalently infected with Plasmodium and Haemoproteus parasites (97·1 vs 71·2%), harboured a higher proportion of multiple infections (26·2 vs 3·2%) and had a more diverse parasite community (11 vs 5 parasite lineages) than Blue Tits. Observed differences between two host species are discussed with reference to their breeding densities and immunological and behavioural characteristics.

Experimental Immunization Based on Plasmodium Antigens Isolated by Antibody Affinity

Vaccines blocking malaria parasites in the blood-stage diminish mortality and morbidity caused by the disease. Here, we isolated antigens from total parasite proteins by antibody affinity chromatography to test an immunization against lethal malaria infection in a murine model. We used the sera of malaria self-resistant ICR mice to lethal Plasmodium yoelii yoelii 17XL for purification of their IgGs which were subsequently employed to isolate blood-stage parasite antigens that were inoculated to immunize BALB/c mice. The presence of specific antibodies in vaccinated mice serum was studied by immunoblot analysis at different days after vaccination and showed an intensive immune response to a wide range of antigens with molecular weight ranging between 22 and 250 kDa. The humoral response allowed delay of the infection after the inoculation to high lethal doses of P. yoelii yoelii 17XL resulting in a partial protection against malaria disease, although final survival was managed in a low proportion of challenged mice. This approach shows the potential to prevent malaria disease with a set of antigens isolated from blood-stage parasites.

Reactivity of autoantibodies against not only erythrocytes but also hepatocytes in sera of mice with malaria

In order to further examine the reactivity of autoantibodies, mice were infected with a non-lethal strain of Plasmodium yoelii. Parasitemia appeared between days 10 and 21. During this period, hyperglycemia and hypothermia were serially obeserved and this phenomenon resembled stress-associated responses. In parallel with parasitemia, autoantibodies appeared against nucleus and double-stranded DNA in the sera. To examine further the reactivity of autoantibodies against tissues, immunohistochemical staining using sera from mice with or without malaria was conducted. Autoantibodies contained reactivity to erythrocytes in the spleen, bone marrow and peripheral blood, especially against tissues obtained from mice with malaria. In the liver and intestine, autoantibodies reacted with hepatocytes and intestinal epithelial cells, respectively. These results suggested that the reactivity of autoantibodies against erythrocytes and hepatocytes might be associated with the modulation of the disease course in malaria.

Host susceptibility to malaria in human and mice: compatible approaches to identify potential resistant genes

There is growing evidence for human genetic factors controlling the outcome of malaria infection, while molecular basis of this genetic control is still poorly understood. Case-control and family-based studies have been carried out to identify genes underlying host susceptibility to malarial infection. Parasitemia and mild malaria have been genetically linked to human chromosomes 5q31-q33 and 6p21.3, and several immune genes located within those regions have been associated with malaria-related phenotypes. Association and linkage studies of resistance to malaria are not easy to carry out in human populations, because of the difficulty in surveying a significant number of families. Murine models have proven to be an excellent genetic tool for studying host response to malaria; their use allowed mapping 14 resistance loci, eight of them controlling parasitic levels and six controlling cerebral malaria. Once quantitative trait loci or genes have been identified, the human ortholog may then be identified. Comparative mapping studies showed that a couple of human and mouse might share similar genetically controlled mechanisms of resistance. In this way, char8, which controls parasitemia, was mapped on chromosome 11; char8 corresponds to human chromosome 5q31-q33 and contains immune genes, such as Il3, Il4, Il5, Il12b, Il13, Irf1, and Csf2. Nevertheless, part of the genetic factors controlling malaria traits might differ in both hosts because of specific host-pathogen interactions. Finally, novel genetic tools including animal models were recently developed and will offer new opportunities for identifying genetic factors underlying host phenotypic response to malaria, which will help in better therapeutic strategies including vaccine and drug development.

Plasmodium yoelii blood-stage antigens newly identified by immunoaffinity using purified IgG antibodies from malaria-resistant mice

As the search for an effective human malaria vaccine continues, understanding immune responses to Plasmodium in rodent models is perhaps the key to unlocking new vaccine strategies. The recruitment of parasite-specific antibodies is an important component of natural immunity against infection in blood-stage malaria. Here, we describe the use of sera from naturally surviving ICR mice after infection with lethal doses of Plasmodium yoelii yoelii 17XL to identify highly immunogenic blood-stage antigens. Immobilized protein A/G was used for the affinity-chromatography purification of the IgGs present in pooled sera from surviving mice. These protective IgGs, covalently immobilized on agarose columns, were then used to isolate reactive antigens from whole P. yoelii yoelii 17XL protein extracts obtained from the blood-stage malaria infection. Through proteomics analysis of the recovered parasite antigens, we were able to identify two endoplasmic reticulum lumen proteins: protein disulfide isomerase and a member of the heat shock protein 70 family. Also identified were the digestive protease plasmepsin and the 39 kDa-subunit of eukaryotic translation initiation factor 3, a ribosome associated protein. Of these four proteins, three have not been previously identified as antigenic during blood-stage malaria infection. This procedure of isolating and identifying parasite antigens using serum IgGs from malaria-protected individuals could be a novel strategy for the development of multi-antigen-based vaccine therapies.

Macrophage migration inhibitory factor: a downregulator of early T cell-dependent IFN-gamma responses in Plasmodium chabaudi adami (556 KA)-infected mice

Neutralization of macrophage migration inhibitory factor (MIF) increases anti-tumor cytotoxic T cell responses in vivo and IFN-γ responses in vitro, suggesting a plausible regulatory role for MIF in T cell activation. Considering that IFN-γ production by CD4(+) T cells is pivotal to resolve murine malaria and that secretion of MIF is induced by Plasmodium chabaudi adami parasites, we investigated the effect of MIF deficiency on the infection with this pathogen. Infections with P. c. adami 556 KA parasites were more efficiently controlled in MIF-neutralized and MIF-deficient (knockout [KO]) BALB/c mice. The reduction in parasitemia was associated with reduced production of IL-4 by non-T/non-B cells throughout patent infection. At day 4 postinfection, higher numbers of activated CD4(+) cells were measured in MIF KO mice, which secreted more IFN-γ, less IL-4, and less IL-10 than did CD4(+) T cells from wild-type mice. Enhanced IFN-γ and decreased IL-4 responses also were measured in MIF KO CD4(+) T cells stimulated with or without IL-12 and anti-IL-4 blocking Ab to induce Th1 polarization. However, MIF KO CD4(+) T cells efficiently acquired a Th2 phenotype when stimulated in the presence of IL-4 and anti-IL-12 Ab, indicating normal responsiveness to IL-4/STAT6 signaling. These results suggest that by promoting IL-4 responses in cells other than T/B cells during early P. c. adami infection, MIF decreases IFN-γ secretion in CD4(+) T cells and, additionally, has the intrinsic ability to render CD4(+) T cells less capable of acquiring a robust Th1 phenotype when stimulated in the presence of IL-12.

Immunogenicity and in vitro protective efficacy of recombinant Mycobacterium bovis bacille Calmette Guerin (rBCG) expressing the 19 kDa merozoite surface protein-1 (MSP-1(19)) antigen of Plasmodium falciparum

Vaccine development against the blood-stage malaria parasite is aimed at reducing the pathology of the disease. We constructed a recombinant Mycobacterium bovis bacille Calmette Guerin (rBCG) expressing the 19 kDa C-terminus of Plasmodium falciparum merozoite surface protein-1 (MSP-1(19)) to evaluate its protective ability against merozoite invasion of red blood cells in vitro. A mutated version of MSP-1(19), previously shown to induce the production of inhibitory but not blocking antibodies, was cloned into a suitable shuttle plasmid and transformed into BCG Japan (designated rBCG016). A native version of the molecule was also cloned into BCG (rBCG026). Recombinant BCG expressing the mutated version of MSP-1(19) (rBCG016) elicited enhanced specific immune response against the epitope in BALB/c mice as compared to rBCG expressing the native version of the epitope (rBCG026). Sera from rBCG016-immunized mice contained significant levels of specific IgG, especially of the IgG2a subclass, against MSP-1(19) as determined by enzyme-linked immunosorbent assay. The sera was reactive with fixed P. falciparum merozoites as demonstrated by indirect immunofluorescence assay (IFA) and inhibited merozoite invasion of erythrocytes in vitro. Furthermore, lymphocytes from rBCG016-immunized mice demonstrated higher proliferative response against the MSP-1(19) antigen as compared to those of rBCG026- and BCG-immunized animals. rBCG expressing the mutated version of MSP-1(19) of P. falciparum induced enhanced humoral and cellular responses against the parasites paving the way for the rational use of rBCG as a blood-stage malaria vaccine candidate.

Immune responses of NIH mice infected with avirulent and virulent strains of Plasmodium chabaudi adami single and mixed infections

An understanding of the nature of the immune response to asexual erythrocytic stages of malaria parasites will facilitate vaccine development by identifying which responses the vaccine should preferentially induce. The present study examined and compared the immune responses of NIH mice in either single or mixed infections with avirulent (DK) or virulent (DS) strains of Plasmodium chabaudi adami using the ELISA test for detecting and measurement of cytokines and antibody production. In both single and mixed infections, the study showed that both cell- and antibody-mediated responses were activated. In all experiments, an early relatively high level of IFN-gamma and IgG2a during the acute phase of the infection, and later elevation of IL-4 and IgG1, suggested that there was a sequential Th1/Th2 response. However, in the avirulent DK strain infection a stronger Th1 response was observed compared to the virulent DS strain-infection or in mixed infections. In the virulent DS infection, there was a stronger Th2 response compared to that in the DK and mixed infections. The faster proliferation rate of the virulent DS strain compared to the DK strain was also evident.

Both type-I and type-II responses contribute to murine trypanotolerance

The host immune system has been documented to influence the course and outcome of infection with the phospholipase-C-deficient (PLC(-/-)) Trypanosoma brucei brucei. We addressed the resistant mechanisms during trypanosomosis by comparing the immune response to variant-specific surface glycoprotein (VSG) in relatively susceptible C3H mice and trypanotolerant (C57BL/6 x BALB/c)-F1 (B6B-F1) mice infected with PLC(-/-) parasites. During the early stage of infection, lymphoid cells from both PLC(-/-)-susceptible C3H and -tolerant B6B-F1 mice mainly secreted VSG-specific IFN-gamma. Although C3H mice remained locked in a type-I cytokine environment (IFN-gamma, TNF-alpha) during late stage of infection, B6B-F1 mice switched to production of type-II cytokines (IL-4, IL-10) from late stage of infection onwards. It seems that VSG-specific cytokine responses associated with resistance to murine African trypanosomosis are infection-stage dependent, with type-I cytokine responses being critical during the early stage of infection while type-II cytokine responses appear to be more important during the late and chronic phases of the disease. Because of the striking similarities in the course of the PLC(-/-)infection in B6B-F1 mice with that of the trypanotolerant N'dama cattle naturally-infected with T. congolense, the PLC(-/-)-infected B6B-F1 mice represents a suitable model to study the course of infection and immune responses during bovine trypanosomosis.

Taenia crassiceps infection disrupts estrous cycle and reproductive behavior in BALB/c female mice

Previously, it has been shown that parasitic infections are able to alter the normal mammal physiology, at several extents. Thus, we investigated the effects on estrous cycle and sexual behavior induced by intraperitoneal infection with Taenia crassiceps in female host mice. Along the weeks of infection, parasites were collected from the peritoneal cavity of female mice, showing the maximum parasite load at 16 weeks. No parasites were found outside peritoneal cavity. Vaginal estrous cycle was monitored daily for 4, 8, 12 and 16 weeks of infection, and results compared against age-matched female mice. Female sexual behavior (FSB) tests were performed, one test per week. Immediately after the last behavioral test, blood was collected by cardiac puncture for steroid determinations. First of all, there was a strong tissular damage in the female reproductive tract in all infected females. The phases of the estrous cycle were interrupted at 12 and 16 weeks, with increased leukocytes and the presence of a few cornified epithelial cells and nucleated epithelial cells. The FSB decreased starting 6 weeks post infection. On the 16th week, all infected female mice ceased to exhibit sexual responses, and estradiol levels showed a significant decrease. Control mice continued showing FSB and the different phases of the estrous cycle throughout the observation period. Our results strength the notion that parasites may be considered as an evolutionary force in the reproductive ability of mammals.

Competitive release and facilitation of drug-resistant parasites after therapeutic chemotherapy in a rodent malaria model

Malaria infections frequently consist of mixtures of drug-resistant and drug-sensitive parasites. If crowding occurs, where clonal population densities are suppressed by the presence of coinfecting clones, removal of susceptible clones by drug treatment could allow resistant clones to expand into the newly vacated niche space within a host. Theoretical models show that, if such competitive release occurs, it can be a potent contributor to the strength of selection, greatly accelerating the rate at which resistance spreads in a population. A variety of correlational field data suggest that competitive release could occur in human malaria populations, but direct evidence cannot be ethically obtained from human infections. Here we show competitive release after pyrimethamine curative chemotherapy of acute infections of the rodent malaria Plasmodium chabaudi in laboratory mice. The expansion of resistant parasite numbers after treatment resulted in enhanced transmission-stage densities. After the elimination or near-elimination of sensitive parasites, the number of resistant parasites increased beyond that achieved when a competitor had never been present. Thus, a substantial competitive release occurred, markedly elevating the fitness advantages of drug resistance above those arising from survival alone. This finding may explain the rapid spread of drug resistance and the subsequently brief useful lifespans of some antimalarial drugs. In a second experiment, where subcurative chemotherapy was administered, the resistant clone was only partly released from competitive suppression and experienced a restriction in the size of its expansion after treatment. This finding raises the prospect of harnessing in-host ecology to slow the spread of drug resistance.

Vaccination with a Plasmodium chabaudi adami multivalent DNA vaccine cross-protects A/J mice against challenge with P. c. adami DK and virulent Plasmodium chabaudi chabaudi AS parasites

A current goal of malaria vaccine research is the development of vaccines that will cross-protect against multiple strains of malaria. In the present study, the breadth of cross-reactivity induced by a 30K multivalent DNA vaccine has been evaluated in susceptible A/J mice (H-2a) against infection with the Plasmodium chabaudi adami DK strain and a virulent parasite subspecies, Plasmodium chabaudi chabaudi AS. Immunized A/J mice were significantly protected against infection with both P. c. adami DK (31-40% reduction in cumulative parasitemia) and P. c. chabaudi AS parasites, where a 30-39% reduction in cumulative parasitemia as well as enhanced survival was observed. The 30K vaccine-induced specific IFN-gamma production by splenocytes in response to native antigens from both P. c. chabaudi AS and P. c. adami DK. Specific antibodies reacting with surface antigens expressed on P. c. adami DS and P. c. chabaudi AS infected red blood cells, and with opsonizing properties, were detected. These results suggest that multivalent vaccines encoding conserved antigens can feasibly induce immune cross-reactivity that span Plasmodium strains and subspecies and can protect hosts of distinct major histocompatibility complex haplotypes.

Virulence and resistance in malaria: who drives the outcome of the infection?

Theoretical and experimental studies have established the dynamic nature of virulence and that, like all traits, it has evolved. Understanding parasite evolution offers a conceptual framework for diverse fields and can contribute greatly to decision-making in disease control. Recently, Grech et al. investigated the effects of host genotype-by-parasite genotype interactions on the expression of virulence in an artificial rodent-malaria system. They found that both parasite and host effects explained most of the variance in the virulence, resistance and transmission potential. These findings are a major contribution to the emerging debate on the pros and cons of a coevolutionary approach of virulence evolution; they also hold great potential for more effective control strategies.

Enhanced immunity to Plasmodium falciparum circumsporozoite protein (PfCSP) by using Salmonella enterica serovar Typhi expressing PfCSP and a PfCSP-encoding DNA vaccine in a heterologous prime-boost strategy

Two Salmonella enterica serovar Typhi strains that express and export a truncated version of Plasmodium falciparum circumsporozoite surface protein (tCSP) fused to Salmonella serovar Typhi cytolysin A (ClyA) were constructed as a first step in the development of a preerythrocytic malaria vaccine. Synthetic codon-optimized genes (t-csp1 and t-csp2), containing immunodominant B- and T-cell epitopes present in native P. falciparum circumsporozoite surface protein (PfCSP), were fused in frame to the carboxyl terminus of the ClyA gene (clyA::t-csp) in genetically stabilized expression plasmids. Expression and export of ClyA-tCSP1 and ClyA-tCSP2 by Salmonella serovar Typhi vaccine strain CVD 908-htrA were demonstrated by immunoblotting of whole-cell lysates and culture supernatants. The immunogenicity of these constructs was evaluated using a "heterologous prime-boost" approach consisting of mucosal priming with Salmonella serovar Typhi expressing ClyA-tCSP1 and ClyA-tCSP2, followed by parenteral boosting with PfCSP DNA vaccines pVR2510 and pVR2571. Mice primed intranasally on days 0 and 28 with CVD 908-htrA(pSEC10tcsp2) and boosted intradermally on day 56 with PfCSP DNA vaccine pVR2571 induced high titers of serum NANP immunoglobulin G (IgG) (predominantly IgG2a); no serological responses to DNA vaccination were observed in the absence of Salmonella serovar Typhi-PfCSP priming. Mice primed with Salmonella serovar Typhi expressing tCSP2 and boosted with PfCSP DNA also developed high frequencies of gamma interferon-secreting cells, which surpassed those produced by PfCSP DNA in the absence of priming. A prime-boost regimen consisting of mucosal delivery of PfCSP exported from a Salmonella-based live-vector vaccine followed by a parenteral PfCSP DNA boosting is a promising strategy for the development of a live-vector-based malaria vaccine.

Contribution of T Cells and Neutrophils in Protection of Young Susceptible Rats from Fatal Experimental Malaria

In human malaria, children suffer very high rates of morbidity and mortality. To analyze the mechanisms involved in age-dependent protection against malaria, we developed an experimental model of infection in rats, where young rats are susceptible to Plasmodium berghei and adult rats control blood parasites and survive thereafter. In this study, we showed that protection of young rats could be achievable by adoptive transfer of spleen cells from adult protected rats, among which T cells could transfer partial protection. Transcriptome analysis of spleen cells transferring immunity revealed the overexpression of genes mainly expressed by eosinophils and neutrophils. Evaluation of the role of neutrophils showed that these cells were able to transfer partial protection to young rats. This antiparasitic effect was shown to be mediated, at least in part, through the neutrophil protein-1 defensin. Further adoptive transfer experiments indicated an efficient cooperation between neutrophils and T cells in protecting all young recipients. These observations, together with those from in vitro studies in human malaria, suggest that the failure of children to control infection could be related not only to an immaturity of their adaptive immunity but also to a lack in an adequate innate immune response.

Contrasting adaptive immune defenses and blood parasite prevalence in closely related Passer sparrows

Immune system components differ in their functions and costs, and immune defense profiles are likely to vary among species with differing ecologies. We compared adaptive immune defenses in two closely related species that have contrasting inflammatory immune responses, the widespread and abundant house sparrow (Passer domesticus) and the less abundant tree sparrow (Passer montanus). We found that the house sparrow, which we have previously shown mounts weaker inflammatory responses, exhibits stronger adaptive immune defenses, including antibody responses, natural antibody titers, and specific T-cell memory, than the tree sparrow. Conversely, tree sparrows, which mount strong inflammatory responses, also mount stronger nonspecific inflammatory T-cell responses but weaker specific adaptive responses. Prevalence of avian malaria parasite infections, which are controlled by adaptive immune defenses, was higher in the geographically restricted tree sparrow than in the ubiquitous house sparrow. Together these data describe distinct immune defense profiles between two closely related species that differ greatly in numbers and distributions. We suggest that these immunological differences could affect fitness in ways that contribute to the contrasting abundances of the two species in North American and Western Europe.

Identification of a novel antigen of Schistosoma mansoni shared with Plasmodium falciparum and evaluation of different cross-reactive antibody subclasses induced by human schistosomiasis and malaria

Plasmodium falciparum and Schistosoma mansoni are often found in human coinfections, and cross-reactive antibodies to different components of the two parasites have been detected. In this work, we identified a cross-reactive S. mansoni gene product, referred to as SmLRR, that seems to belong to the leucine-rich repeat protein family. Comparative analysis of SmLRR revealed 57% similarity with a putative gene product encoded in the P. falciparum genome. Antibodies to SmLRR were found in experimental infections and in both S. mansoni- and P. falciparum-infected individuals. Correlative analysis of human anti-SmLRR responses in Kenya and Uganda suggested that malaria and schistosomiasis drive the immunoglobulin G3 (IgG3) and IgG4 isotypes, respectively, against SmLRR, suggesting that there is differential regulation of cross-reactive isotypes depending on the infection. In addition, the levels of anti-SmLRR IgG4, but not the levels of IgG3, correlated positively with the intensity of S. mansoni infection.

IRON CHELATORS: CORRELATION BETWEEN EFFECTS ON PLASMODIUM SPP. AND IMMUNE FUNCTIONS

Iron chelating agents, which permeate through erythrocytic and parasite membranes, are effective against Plasmodium falciparum in vitro. However, the protective effect in humans is transient. We examined the antiplasmodial capacity of several iron chelators in vitro and in vivo. The chelators 3/3hb/2m and 3/2hb/b (together, MoB) were more effective against P. falciparum in vitro than desferrioxamine (DFO) and Salicylaldehyde isonicotinoyl hydrazone (SIH) (together, DoS). Despite similar pharmacokinetics of all iron chelators, mice infected with Plasmodium vinckei and treated with MoB succumbed to malaria, whereas DoS-treated mice survived. However, even in the surviving mice, peak parasitemias were above 30%. These results indicate that the direct effects of the drugs on the parasites were not responsible alone for the complete recovery of the mice. We suggest that the recovery is related to differential effects of the drugs on various immune functions. We concentrated on the effect of the iron chelators on B cell and T cell proliferation and on allogeneic stimulation (MLR), interleukin-10 (IL-10), gamma-interferon (gamma-IFN), tumor necrosis factor-alpha (TNF-alpha), and radical production. All the iron chelators examined inhibited the in vitro proliferation of B cells and T cells, and MLR. This may explain why iron chelators are only slightly efficient in treating human malaria. However, the inhibitory effects of MoB on B cell and T cell proliferation and on MLR were more pronounced than those of DoS. In addition, the release of free radicals by effector cells was inhibited to a greater extent by MoB than by DoS. These results may explain why MoB, which was more efficient in vitro, was not effective in vivo. The DoS effects on the in vitro secretion of cytokines correlate with their in vivo effect; there was a decrease of IL-10 and a parallel increase in gamma-IFN and TNF-alpha production by human mononuclear cells. MoB, which could not rescue the animals from malaria, did not affect IL-10 and TNF-alpha, but reduced gamma-IFN levels. Identical results were obtained when using monocytes instead of mononuclear cells (except for gamma-IFN, which is not produced by monocytes). Our results indicate that an iron chelator, or any antiparasitic drug that kills the parasites in vitro, should also be selected for further evaluation on the basis of its reaction with immune components; it should not interfere with crucial protective immunological processes, but it may still alleviate parasitemia by positive immune modulation.

Complex dynamics and stability of resistance to antimalarial drugs

A succession of antimalarial drugs has been deployed to treat human falciparum malaria but each has, in turn, been nullified by the spread of drug resistance. The consensus view has always been that, once present, resistance will inevitably rapidly increase to 100%. However, recent field evidence has shown this is not inevitable, and that drug resistance may initially spread and then stabilize at relatively low frequencies. It is proposed that intense competition between separate malaria clones co-infecting the same human can generate complex dynamics capable of explaining this observation. Standard population genetic analysis confirms this assertion. The dynamics underlying the evolution of antimalarial resistance may therefore be much more complex than previously realized, and can resolve the apparent paradox between field data and the underlying theory of the evolution of resistance. This explanation is novel and the results are equally applicable to other parasitic species where multiple infections of the same host are common.

Dynamics of Multiple Infection and Within‐Host Competition in Genetically Diverse Malaria Infections

Within-host competition between coinfecting parasite strains shapes the evolution of parasite phenotypes such as virulence and drug resistance. Although this evolution has a strong theoretical basis, within-host competition has rarely been studied experimentally, particularly in medically relevant pathogens with hosts that have pronounced specific and nonspecific immune responses against coinfecting strains. We investigated multiple infection in malaria, using two pairs of genetically distinct clones of the rodent malaria Plasmodium chabaudi in mice. Clones were inoculated into mice simultaneously or 3 or 11 days apart, and population sizes were tracked using immunofluorescence or quantitative polymerase chain reaction. In all experiments, at least one of the two clones suffered strong competitive suppression, probably through both resource- and immune-mediated (apparent) competition. Clones differed in intrinsic competitive ability, but prior residency was also an important determinant of competitive outcome. When clones infected mice first, they did not suffer from competition, but they did when infecting mice at the same time or after their competitor, more so the later they infected their host. Consequently, clones that are competitively inferior in head-to-head competition can be competitively superior if they infect hosts first. These results are discussed in the light of strain-specific immunity, drug resistance, and virulence evolution theory.

Hemozoin-inducible proinflammatory events in vivo: potential role in malaria infection

During malaria infection, high levels of proinflammatory molecules (e.g., cytokines, chemokines) correlate with disease severity. Even if their role as activators of the host immune response has been studied, the direct contribution of hemozoin (HZ), a parasite metabolite, to such a strong induction is not fully understood. Previous in vitro studies demonstrated that both Plasmodium falciparum HZ and synthetic HZ (sHZ), beta-hematin, induce macrophage/monocyte chemokine and proinflammatory cytokine secretion. In the present study, we investigated the proinflammatory properties of sHZ in vivo. To this end, increasing doses of sHZ were injected either i.v. or into an air pouch generated on the dorsum of BALB/c mice over a 24-h period. Our results showed that sHZ is a strong modulator of leukocyte recruitment and more specifically of neutrophil and monocyte populations. In addition, evaluation of chemokine and cytokine mRNA and protein expression revealed that sHZ induces the expression of chemokines, macrophage-inflammatory protein (MIP)-1alpha/CCL3, MIP-1beta/CCL4, MIP-2/CXCL2, and monocyte chemoattractant protein-1/CCL2; chemokine receptors, CCR1, CCR2, CCR5, CXCR2, and CXCR4; cytokines, IL-1beta and IL-6; and myeloid-related proteins, S100A8, S100A9, and S100A8/A9, in the air pouch exudates. Of interest, chemokine and cytokine mRNA up-regulation were also detected in the liver of i.v. sHZ-injected mice. In conclusion, our study demonstrates that sHZ is a potent proinflammatory agent in vivo, which could contribute to the immunopathology related to malaria.

Rodent malaria parasites suffer from the presence of conspecific clones in three-clone Plasmodium chabaudi infections

We studied infection dynamics of Plasmodium chabaudi in mice infected with 3 genetically distinct clones--1 less virulent than the other 2--either on their own or in mixtures. During the acute phase of infection, total numbers of asexual parasites in mixed-clone infections were equal to those produced by the 3 clones alone, suggesting strong in-host competition among clones. During the chronic phase of the infection, mixed-clone infections produced more asexual parasites than single-clone infections, suggesting lower levels of competition than during the acute phase, and indicating that a genetically diverse infection is harder to control by the host immune system. Transmission potential over the whole course of infection was lower from mixed-clone infections than from the average of the 3 single-clone infections. These results suggest that in-host competition reduces both growth rate and probability of transmission for individual parasite clones.

CTLA-4 positive T cells in contrast to procalcitonin plasma levels discriminate between severe and uncomplicated Plasmodium falciparum malaria in Ghanaian children

Procalcitonin (PCT) plasma levels and the fraction of CTLA-4-positive T cells are both elevated in acute Plasmodium falciparum malaria in human adults and the degree of elevation is positively correlated with other markers of disease severity, for example with parasitaemia. However, the clinical manifestations of malaria are strongly age-dependent and children from endemic areas carry the main disease burden. Therefore, we measured PCT plasma levels and CTLA-4 expression by T cells in four groups of children from the Ashanti Region in Ghana: asymptomatic children with or without parasitaemia, children with uncomplicated P. falciparum malaria and children with severe disease. PCT levels were highly elevated in both groups with acute malaria but they did not discriminate between uncomplicated and severe disease. In contrast, CTLA-4-expression by T cells was increased only in severe malaria. The fraction of CTLA-4 positive T cells in the blood of children with severe disease differed significantly from that in uncomplicated malaria, which was not elevated in spite of the high parasite loads observed in these children. This was unexpected, as in adults uncomplicated malaria is associated with a dramatic sixfold increase of the fraction of CTLA-4-positive T cells. The data from this study support the hypothesis that strong T cell activation as measured here by CTLA-4 expression is not just the by-product of a high parasite burden, but that it contributes to the pathogenesis of P. falciparum malaria.

The age-related resistance of rats to Plasmodium berghei infection is associated with differential cellular and humoral immune responses

In this study, we investigated how the age of rats would affect the course of infection of and the immune response to Plasmodium berghei. Both young (4-week-old) and adult rats (8-week-old) can be infected with P. berghei ANKA strain, with significantly higher levels of infected red blood cells in young rats. While 100% of young rats succumbed to infection, adult rats were able to clear blood parasites and no mortality was observed. Analysis of cellular distribution and circulating cytokines demonstrated the persistence of CD4+/CD25+ T cells and high expression of circulating interleukin-10 (IL-10) during the progression of infection in young-susceptible rats, whereas high levels of CD8+ T cells and natural killer T cells are detected in adult-resistant rats. Analysis of antibody isotypes showed that adult rats produced significantly higher levels of interferon-gamma (IFN-gamma)-dependent IgG2c antibodies than young rats during infection. Further evaluation of the role of IL-10, IFN-gamma and of immune cells showed that only the adoptive transfer of spleen cells from adult-resistant rats was able to convert susceptibility of young-susceptible rats to a resistant phenotype. These observations suggest that cell-mediated mechanisms are crucial for the control of a primary infection with P. berghei in young rats.

Immunoglobulin G and subclass responses to Plasmodium falciparum antigens: a study in highly exposed Cameroonians

We conducted a cross-sectional study in Bolifamba village in the South West Province of Cameroon to determine antibody responses to crude Plasmodium falciparum antigens. A total of 347 subjects were examined. Parasite counts were obtained on thick blood films stained with Field's stain. Total immunoglobulin (Ig)G and IgG subclass levels were determined in serum samples from four groups comprising children 1 to 5 years old and adults > or = 18 years with or without falciparum malaria parasites, using enzyme-linked immunosorbent assay with crude blood-stage antigens of Plasmodium falciparum strain F32 as target. Depending on the age group, malaria prevalence varied between 10% and 65% with a mean of 30.8%. Prevalence rate and parasite density declined with increasing age. Total IgG and IgG1-3 levels were significantly higher in adults than in children (p < 0.05). Parasite-bearing individuals in both age groups had higher IgG titres than their non-infected counterparts, while subtype levels were not significantly different (p = 0.05). These findings indicate that Bolifamba village could be a convenient site to study further the protective immunity to malaria.

Plasma and in vitro levels of cytokines during and after a Plasmodium falciparum malaria attack in Gabon

Fifty subjects living in a malaria endemic area were studied at diagnosis of a Plasmodium falciparum attack and 3 weeks later. Absolute numbers of CD3(+), CD4(+) and CD8(+) lymphocytes as well as plasma cytokines and secreted cytokines after in vitro mitogenic stimulation were measured. At enrollment, lymphopenia was observed, lending support to the reallocation hypothesis during the acute phase. A significant elevation of the number of CD8(+) cells was present in the peripheral blood during the recovery phase. During the acute phase, plasma IL-6 levels peaked while in vitro production capacity was high at both phases. Plasma IL-6 concentrations were positively related to blood parasite density at D0, as IL-4 and IFN-gamma, suggesting an early intervention of these cytokines. Plasma IL-2 levels were low at diagnosis although cells retained their ability to produce IL-2, which was found more frequently in plasma after cure. Acquisition of immunity with age was in relation with greater secretion abilities of cells for type 1 and type 2 cytokines during the parasite clearance phase. We conclude to an early implication of type 2 cytokines and IFN-gamma, with particularly high levels of IL-6.

Essential role of extrathymic T cells in protection against malaria

Athymic nude mice carry neither conventional T cells nor NKT cells of thymic origin. However, NK1.1(-)TCR(int) cells are present in the liver and other immune organs of athymic mice, because these lymphocyte subsets are truly of extrathymic origin. In this study, we examined whether extrathymic T cells had the capability to protect mice from malarial infection. Although B6-nu/nu mice were more sensitive to malaria than control B6 mice, these athymic mice were able to survive malaria when a reduced number of parasitized erythrocytes (5 x 10(3) per mouse) were injected. At the fulminant stage, lymphocytosis occurred in the liver and the major expanding lymphocytes were NK1.1(-)TCR(int) cells (IL-2Rbeta(+)TCRalphabeta(+)). Unconventional CD8(+) NKT cells (V(alpha)14(-)) also appeared. Similar to the case of B6 mice, autoantibodies (IgM type) against denatured DNA appeared during malarial infection. Immune lymphocytes isolated from the liver of athymic mice which had recovered from malaria were capable of protecting irradiated euthymic and athymic mice from malaria when cell transfer experiments were conducted. In conjunction with the previous results in euthymic mice, the present results in athymic mice suggest that the major lymphocyte subsets associated with protection against malaria might be extrathymic T cells.

Complex genetic control of susceptibility to malaria in mice

Malaria is a major infectious disease worldwide, with over 1 million deaths in African children every year. The molecular pathways of pathogenesis of the Plasmodium parasite and the host mechanisms of defense against this infection remain poorly understood. Epidemiological studies, together with linkage analyses in endemic areas have clearly pointed at a genetic component of innate susceptibility and severity of disease. In humans, this genetic trait is complex, and has been studied in a mouse experimental model over the past few years. Inbred strains of mice show different degrees of susceptibility to infection with Plasmodium chabaudi, and the genetic component of these inter-strain differences has been studied in standard informative backcross and F2 populations, as well as in recombinant inbred strains and more recently, in recombinant congenic strains. These studies have shown that genetic susceptibility to malaria is also complex in mice, and have led to the mapping of major susceptibility Char (Chabaudi resistance) loci, located on chromosomes 9 (Char1), 8 (Char2), 17 (Char3) and 3 (Char4).

IL-12 is required for antibody-mediated protective immunity against blood-stage Plasmodium chabaudi AS malaria infection in mice

In this study, we investigated the role of endogenous IL-12 in protective immunity against blood-stage P. chabaudi AS malaria using IL-12 p40 gene knockout (KO) and wild-type (WT) C57BL/6 mice. Following infection, KO mice developed significantly higher levels of primary parasitemia than WT mice and were unable to rapidly resolve primary infection and control challenge infection. Infected KO mice had severely impaired IFN-gamma production in vivo and in vitro by NK cells and splenocytes compared with WT mice. Production of TNF-alpha and IL-4 was not compromised in infected KO mice. KO mice produced significantly lower levels of Th1-dependent IgG2a and IgG3 but a higher level of Th2-dependent IgG1 than WT mice during primary and challenge infections. Treatment of KO mice with murine rIL-12 during the early stage of primary infection corrected the altered IgG2a, IgG3, and IgG1 responses and restored the ability to rapidly resolve primary and control challenge infections. Transfer of immune serum from WT mice to P. chabaudi AS-infected susceptible A/J mice completely protected the recipients, whereas immune serum from KO mice did not, as evidenced by high levels of parasitemia and 100% mortality in recipient mice. Furthermore, depletion of IgG2a from WT immune serum significantly reduced the protective effect of the serum while IgG1 depletion had no significant effect. Taken together, these results demonstrate the protective role of a Th1-immune response during both acute and chronic phases of blood-stage malaria and extend the immunoregulatory role of IL-12 to Ab-mediated immunity against Plasmodium parasites.

A model of development of acquired immunity to malaria in humans living under endemic conditions

Malaria remains a significant global health problem. Most morbidity and mortality in an endemic setting is in children less than 5 years old, and increasing resistance to infection and disease with age is thought to reflect a slow, gradual acquisition of protective immunity. It is not clear if the semi-immune status of adults, in which parasites are present at below clinical threshold, is the result of cumulative exposure to Plasmodium falciparum or reflects an underlying difference between adult and infant immunity. Immuno-epidemiological studies of people living in malaria-endemic areas have not produced consistent examples of surrogate markers of protection. This gulf in our understanding of immunity to malaria may be addressed by novel application of an established murine model of immune regulation of blood stage infection. This exploits two examples of loss of immunity, selective immunosuppression in pregnancy, and waning of maternally transferred protection in neonates, to distinguish the immunological determinants involved in the radical transition between susceptible and resistant immune status. It is suggested that application of this unique model should significantly advance knowledge of how acquired immunity to malaria develops and is highly relevant to the pathogenesis of malaria in human pregnancy and the design of antimalarial vaccines for use in children.

Molecular approaches to elucidating innate and acquired immune responses to Babesia bovis, a protozoan parasite that causes persistent infection

For many vector-transmitted protozoal parasites, immunological control of acute infection leads to a state of persistent infection during which parasitemias may cycle unnoticed in infected but otherwise clinically healthy animals. Achieving persistent infection is a strategy that favors parasitism, since both host and, therefore, parasite survive, and endemically infected animal populations provide a reservoir of parasites continually available for subsequent transmission. Examples of the major economically important protozoan pathogens that cause persistent infection in mammals include the related Theileria and Babesia parasites as well as Trypanosoma species. Control of acute infection and maintenance of clinical immunity against subsequent infection are determined by the interplay of innate and acquired immune responses. This review will focus on approaches taken to gain an understanding of the molecular basis for innate and acquired immunity against the hemoprotozoan parasite of cattle, Babesia bovis. Knowledge of mechanisms used by the parasite to survive within infected cattle from acute to persistent infection combined with definition of the correlates of protective immunity in cattle should be applicable to designing effective vaccines.

CD4+ T cells are required for HSP65 expression in host macrophages and for protection of mice infected with Plasmodium yoelii

We have reported that macrophages expressing heat-shock protein 65 play an essential role in protection of mice infected with Plasmodium yoelii. In this study, we investigated the function and expression mechanism of HSP65 in macrophages of mice infected with P. yoelii. C57BL/6 (B6) mice are susceptible to infection with the lethal (L) strain but resistant to infection with the non-lethal (NL) strain of P. yoelii. The percentage of apoptotic macrophages in mice infected with the L strain was higher than that in mice infected with the NL strain. However, the percentage was low in L strain infected mice if they acquired resistance to the infection by primary infection with the NL strain. That apoptosis was reversely correlated with HSP65 expression in splenic macrophages from mice infected with P. yoelii suggests HSP65 may contribute to protective immunity by preventing apoptosis of macrophages in malarial infection. Cell depletion/transfer experiments showed that CD4+ T cells, but not CD8+ T cells, gammadelta T cells, NK cells or NK T cells, were required for HSP65 expression in macrophages as well as for protection of mice infected with P. yoelii. In conclusion, HSP65 may play a role in preventing apoptosis of macrophages in mice infected with P. yoelii. CD4+ T cells are required for HSP65 expression and for protective immunity against P. yoelii infection.

Cytokines and malaria parasitemia

The balance between pro- and antiinflammatory cytokines may be important in malaria presentation and outcome. Malaria tends to be more severe in children than in adults, presumably because partial immunity develops with age. However, the full nature of, and age-related differences in, anti-malarial immunity are unknown. We compared: (1) serum and cell-specific cytokines of patients with acute malaria to those of patients with other acute illnesses and to those of healthy adults and (2) the cytokine responses of parasitemic children and parasitemic adults. Flow cytometry was done on the peripheral blood mononuclear cells of 148 hospitalized children, 161 febrile hospitalized adults, and 20 healthy adults in Malawi, Africa, a malaria-endemic country. Serum cytokines were also assessed for 80 of these patients. Thirty-eight participants were parasitemic with Plasmodium falciparum. Serum interleukin (IL)-10 (an antiinflammatory, immunoregulatory, and type 2 cytokine) levels were higher in malaria patients than in other patients (medians 502 pg/mL vs 16 pg/mL, P = 0.002), and the percentages of various lymphocyte populations making IL-6 (a proinflammatory, type 2 cytokine regulating iron distribution) were lower in malaria patients than in other patients (e.g., for spontaneous production by children's CD8(+) T cells: medians 1.4% vs 33.1%, P = 0.004). For adult patients, the percentages of lymphocytes spontaneously making IL-4 (a type 2 cytokine) were significantly lower in those with malaria than in those without malaria (medians 0.9% vs 2.1%, P = 0.005). The percentages of monocytes spontaneously making IL-8 (a chemotactic, proinflammatory chemokine) were higher in parasitemic children than in parasitemic adults (medians 5.8% vs 1.7%, P = 0.003). A number of cellular proinflammatory, type 1 parameters were significantly higher in all children (with or without malaria) than in all adults; these included the percentages of various lymphocyte populations making IL-6, both IL-6 and interferon-gamma, or IL-8. These data support the importance of IL-10 in malaria parasitemia. Given the lack of an IL-4 (type 2) response, IL-10's primary role may be immunoregulatory rather than type 2 in nature. In this study, the immune response to malaria was more proinflammatory in children than in adults. This difference, if corroborated by other studies, could be related to malaria's greater severity in children.

Resistance to malarial infection is achieved by the cooperation of NK1.1(+) and NK1.1(-) subsets of intermediate TCR cells which are constituents of innate immunity

We previously reported that the major expanding lymphocytes were intermediate TCR (TCR(int)) cells (mainly NK1.1(-)) during malarial infection in mice. Cell transfer experiments of TCR(int) cells indicated that these T cells mediated resistance to malaria. However, TCR(int) cells always contain NK1.1(+)TCR(int) cells (i.e., NKT cells) and controversial results (NKT cells were effective or not for resistance to malaria) have been reported by different investigators. In this study, we used CD1d((-/-)) mice, which almost completely lack NKT cells in the liver and other immune organs. Parasitemia was prolonged in the blood of CD1d((-/-)) mice and the expansion of lymphocytes in the liver of these mice was more prominent after an injection of Plasmodium yoelii-infected erythrocytes. However, these mice finally recovered from malaria. In contrast to B6 mice, CD4(-)8(-) NKT cells as well as NK1.1(-)CD3(int) cells expanded in CD1d((-/-)) mice after malarial infection, instead of CD4(+) (and CD8(+)) NKT cells. These newly generated CD4(-)8(-)NKT cells in CD1d((-/-)) mice did not use an invariant chain of Valpha14Jalpha281 for TCRalpha. Other evidence was that severe thymic atrophy and autoantibody production were accompanied by malarial infection, irrespective of the mice used. These results suggest that both NK1.1(-) and NK1.1(+) subsets of TCR(int) cells (i.e., constituents of innate immunity) are associated with resistance to malaria and that an autoimmune-like state is induced during malarial infection.

Plasmodium chabaudi chabaudi: B-1 cell expansion correlates with semiresistance in BALB/cJ mice

The largest obstacle impeding the development of an effective malaria vaccine is the incomplete understanding of how the immune response is regulated during infection. B-1a cells, a poorly understood subcategory of B lymphocytes, produce nonpathologic autoantibodies of low affinity which have been shown to have distinct immunoregulatory capabilities. What the exact activity of B-1a cells are during the course of malaria has yet to be determined. By use of flow cytometry, it was observed that B-1a cells significantly expand by day 3 postinfection in the spleen and peritoneum of Plasmodium chabaudi chabaudi semiresistant BALB/cJ mice, but not until day 8 postinfection in the spleen of P. chabaudi chabaudi fully susceptible BALB/cByJ mice. The activation of B-1a cells was also demonstrated by the measurement of natural autoantibody IgM production from the serum and cultured peritoneal B-1a cells. Infected semiresistant BALB/cJ mice generated higher levels of anti-ssDNA IgM antibodies than infected fully susceptible BALB/cByJ mice. The preliminary data presented here suggest a possible roll of B-1 cells in contributing to the successful survival of murine malarial infection.