Complement-dependent cytotoxicity against hepatoma cells mediated by IgM antibodies in serum from tumor-bearing rats

Complement-dependent cytotoxic antibodies in syngeneic serum from rats carrying transplanted aminoazo dye-induced hepatomas (D23 and D33) and from rats immunized with irradiated tumor cell or homogenates were studied by a short-term 51Cr release assay. The tumor-bearer sera (TBS) were subjected to chromatography on unsolubilized protein A and Sepharose 4B. The cytolytic activity of D23 TBS was recovered in the IgM molecular weight region, whereas no such activity was obtained in the IgG fraction. As judged from immunodiffusion experiments, the IgM molecular weight fraction did not contain any aggregated or complexed IgG. Moreover, in immunofluorescence tests against viable hepatoma cells, using a specific anti-rat IgG conjugate, the TBS were negative. Cross-testing of D23 and D33 TBS against the two hepatomas and cross-absorption of the sera with tumor plasma membranes revealed no tumor specificity in these cytotoxicity reactions. Furthermore, neither fetal nor early postnatal liver cells could absorb out the activity. Absorptions with adult liver plasma membranes, however, abrogated the cytotoxic activity, and even more effective in this respect were homogenates from kidney and small intestine, thus indicating that the cytotoxic antibodies are directed against 'normal' adult antigen(s) present also in tissues other than liver.

Cytokine mRNA expression and iron status in children living in a malaria endemic area

Iron deficiency has been reported to affect both malaria pathogenesis and cell-mediated immune responses; however, it is unclear whether the protection afforded by iron deficiency is mediated through direct effects on the parasite, through immune effector functions or through both. We have determined cytokine mRNA expression levels in 59 children living in a malaria endemic area on the coast of Kenya who we selected on the basis of their biochemical iron status. Real-time quantitative reverse transcriptase polymerase chain reaction analysis of cytokine mRNA levels of peripheral blood mononuclear cells (PBMC) obtained from these children showed an association between interleukin-4 (IL-4) mRNA levels and all the biochemical indices of iron that we measured. Furthermore, IL-10 mRNA was higher in parasite blood smear-positive children than in blood smear-negative children irrespective of their iron status. This study suggests that IL-4 expression by PBMC may be affected by iron status.

Why do Plasmodium falciparumm-infected erythrocytes form spontaneous erythrocyte rosettes?

Plasmodium falciparum malaria is one o f the most widespread o f human parasitic diseases and is responsible for the deaths of several million people in subtropical and tropical regions o f the world. The interaction o f malarial merozoites with erythrocytes and the adherence o f infected erythrocytes to the endothelium are among the cellular interactions extensively studied to define candidate antigens for a blood stage vaccine. However, the exact mechanisms underlying the invasion o f erythrocytes by P. falciparum merozoites and their subsequent binding to endothelium are not yet understood. Here Mats Wahlgren, Johan Carlson, Rachonee Udomsangpetch and Peter Perlmonn discuss a novel cytoodherence phenomenon which may be o f great importance in this context, that is, the spontaneous binding o f uninfected erythrocytes to those infected with late-stage parasites (trophozoites/schizonts).

Immunoglobulin E (IgE) containing complexes induce IL-4 production in human basophils: effect on Th1-Th2 balance in malaria

Human basophils are potent producers of IL-4 following cross-linking of the high affinity receptor for IgE (Fc epsilon R1). Elevated levels of both total- and malaria-specific IgE have been demonstrated in sera from people living in malaria-endemic regions. Whether or not these IgE antibodies are pathogenic is unclear. Serum containing high IgE levels obtained from malaria individuals was used to establish whether IgE-immune complexes could induce IL-4 production in human basophils. The basophils, obtained from healthy donors, were primed with 10 ng/ml of IL-3 before being transferred to wells containing goat anti-human IgE or human antimalarial IgE-immune complexes. IL-4 was induced upon stimulation of human basophils by plate bound IgE-containing immune complexes. Basophils treated similarly but with goat anti-IgG/human antimalarial- IgG-immune complexes did not secrete IL-4. Similarly mononuclear cells depleted of basophils in parallel culture did not secrete IL-4. Thus, human basophils may contribute to the polarization of T-helper type 2 in the (Th2) responses in malaria hosts via IgE-induced IL-4 production.

Malaria blood-stage infection and its control by the immune system

Malaria is caused by the protozoon Plasmodium, transmitted to humans by Anopheles mosquitoes. The most dangerous of the plasmodia infecting humans is Plasmodium falciparum. The disease is caused by those parasite stages which multiply asexually in red blood cells. In non-immune individuals, P. falciparum may cause severe and life-threatening disease. Another risk group is constituted by pregnant women, particularly during their first pregnancies. Immunity to malaria usually requires repeated exposure to the parasite to become long lasting. One reason for this is the capacity of the parasite to vary the antigens which are major targets for protective antibodies. Antibody-dependent protection is primarily mediated by cytophilic IgG antibodies activating cytotoxic and phagocytic effector functions of neutrophils and monocytes. Malaria infection also involves elevated production of IgE antibodies. However, IgE-containing immune complexes are pathogenic rather than protective by crosslinking IgE receptors (CD23) on monocytes, leading to local overproduction of TNF, a major pathogenic factor in this disease. T cells are essential for both acquisition and regulation of malaria immunity. The major T cells controlling blood stage infections are CD4+ of both the Thl and Th2 subsets. However, T cells carrying the gamma6 receptor also contribute to this control. The balance between the cytokines produced by different cell types is critical for the course of infection, with IFN-gamma having a key role in anti-malaria defence. Blood-stage infections are also under complex genetic control. Among the regulatory genes, those involved in elevated production of TNF are associated with increased risk of severe disease and death due to P. falciparum infection.

IgE deposition in brain microvessels and on parasitized erythrocytes from cerebral malaria patients

Postmortem brain tissues of 21 cerebral malaria cases were obtained in Myanmar and Vietnam. The tissues were examined by light microscopy and by an immunohistochemical method. Brain microvessels (capillaries and venules) were examined for the presence of immunoglobulins IgE and IgG, Plasmodium falciparum antigen, and parasitized erythrocytes (PRBC). Deposition of IgE, IgG, and P. falciparum antigen was observed in the microvessels from all specimens examined. Sequestered PRBC in the microvessels were positive for IgG in all 21 cases and for IgE in six cases. In the latter cases, the percentage of microvessels with sequestered PRBC was > 50%, with the frequency of IgE-positive cells ranging from 42% to 52%. In contrast, in five cases that were only weakly positive for IgE, the percentage of microvessels with sequestered PRBC was remarkably low (< 1%). These data indicate that the degree of deposition of IgE in microvessels and on PRBC from cerebral malaria patients correlated with that of PRBC sequestration. As IgE-containing immune complexes are known to induce local overproduction of tumor necrosis factor-alpha (TNF-alpha), a major pathogenic factor in cerebral malaria, IgE may contribute to the pathogenesis of this severe disease.

A preliminary analysis of Wuchereria bancrofti microfilarial antigens for potential use in diagnosis

Several antigens from the microfilarial stage of Wuchereria bancrofti have been identified using immunoblots of microfilarial antigens and screening with immune sera and tropical pulmonary eosinophilia (TPE) sera. This analysis revealed an array of antigens with apparent molecular weights of 14kDa, 35kDa, 42kDa, 63kDa, 88kDa, 97kDa and 200kDa. Among these only the 14kDa and 42kDa antigens were consistently recognized by most of the immune sera. A 132kDa antigen was recognized only by TPE sera. Analysis of rabbit immune sera revealed that the 42kDa antigen was shared by two developmental stages of W. bancrofti, namely L3 and mF. This antigen could become a potential vaccine candidate. The 14kDa antigen seems specific for the microfilarial stage and therefore could be a diagnostic marker for active infection. The 132kDa antigen could aid in the diagnosis of TPE.

Contrasting functions of IgG and IgE antimalarial antibodies in uncomplicated and severe Plasmodium falciparum malaria

Plasmodial infection results in a significant elevation of the blood concentrations of immunoglobulins including IgE. Two well-characterized groups of adult Thai patients with either uncomplicated or severe Plasmodium falciparum malaria were studied over a period of four weeks. The mean parasitemias were approximately three-fold higher in patients with severe malaria than in those with uncomplicated disease. The mean concentrations of both total IgG and IgG antiplasmodial antibodies tended to be highest in the group with uncomplicated disease while total IgE and IgE antibodies were higher in the group with severe disease. The IgE antibodies detected in approximately 65% of the patients were positively correlated to parasitemia. These results suggest that antiplasmodial IgG antibodies are involved in reducing the severity of P. falciparum malaria, while IgE antibodies may contribute to the pathogenesis of this infection.

Immune regulation of protection and pathogenesis in Plasmodium falciparum malaria

The immune mechanisms whereby malaria parasites are eliminated by the human host or how they may avoid the immune response are poorly understood. Individuals living in malaria-endemic areas gradually acquire immunity. It is well established that this immunity involves both cell-mediated and humoral mechanisms and that T cells are the major regulators in both these events. The existence of functionally distinct P. falciparum-specific CD4+ T-cell subsets in humans has been shown in several studies. However, in contrast to what is the case in murine models there is no definitive link between the activation of various T cells and the course of human P. falciparum blood-stage infection. In the present paper we will review recent findings which illustrate how the balance between functionally different T-cell subsets affects the development of malaria immunity but also may contribute to its pathogenicity. An example of the latter is the deposition of IgE-containing immune complexes in small vessels, probably leading to local overproduction of tumor-necrosis factor (TNF), a pathogenic factor in malaria.

Human immune responses to the highly repetitive Plasmodium falciparum antigen Pf332

The B and T cell responses to EB200, a repetitive part of the Plasmodium falciparum antigen Pf332, were examined in malaria-exposed Senegalese adults. Most donors had high levels of antibodies to recombinant EB200 and 17 overlapping peptides spanning EB200. Taking proliferation and/or cytokine (interferon-gamma and interleukin-4) production as a measure of T cell activation, eight of the EB200-derived peptides induced responses in > 40% of the donors tested. There was no general association between the different types of T cell responses measured, emphasizing the importance of including multiple parameters when analyzing T cell responses and suggesting that EB200 induces functionally distinct T cell responses. The most efficient peptide for induction of proliferative responses was one previously shown to induce T cell responses in five different H-2 congenic mouse strains primed with EB200, suggesting that this is a universal T cell epitope. The presence of multiple B and T cell epitopes in EB200, widely recognized by humans, is important since EB200 has been shown to elicit protective antibody responses in monkeys and may be considered for inclusion in malaria subunit vaccines.

Inhibition of in vitro growth of plasmodium falciparum field isolates mediated by human antibodies to Pf155/RESA and Pf332

The capacity of antibodies to interfere with Plasmodium falciparum growth in in vitro cultures is considered to reflect some of their potential protective effects in vivo. Almost all previous analyses of antibody mediated inhibition of parasite growth in vitro were performed with different laboratory strains of P. falciparum. This study was performed to investigate if the long-term culturing of parasites has any effect on their susceptibility to such growth inhibition. The growth inhibitory effects of human antibodies to the vaccine candidate antigens Pf155/RESA and Pf332 on fresh field isolates from children in Burkina Faso were analysed and compared with their effect on an established laboratory strain of the parasite. Although there was variation in the inhibition titres between different isolates tested against one antibody preparation, the differences in inhibition capacity for the three different antibodies were highly significant. No correlation was found between serum levels of anti-Pf155/RESA or -Pf332 antibodies and sensitivity of the corresponding parasite isolates to antibody mediated growth inhibition.

Antibodies to a non-repeat region of Plasmodium falciparum antigen Pf155/RESA in individuals from malaria-endemic areas

Human antibodies to the repeat regions of the Plasmodium falciparum asexual blood stage antigen Pf155/RESA interfere with parasite growth in vitro, but the significance in this respect of antibodies to non-repetitive epitopes is less clear. In this study the levels of antibodies to a non-repetitive part of Pf155/RESA (residue 199-221) in malaria-exposed individuals were analysed, as was the parasite-inhibitory capacity of such antibodies. Residue 199-221 is of particular interest since it includes a sequence homologous to a cytoadherence-related motif from band 3. Sera from donors in Liberia and Tanzania were analysed for reactivity in ELISA with synthetic peptides together overlapping this part of Pf155/RESA. High antibody reactivity was observed in most of the sera with two peptides including residues 199-211 and 202-214, respectively. Specific antibodies were affinity-purified from selected sera using these peptide sequences and were shown to react with Pf155/RESA by immunofluorescence and Western blotting. The purified antibodies were furthermore shown to inhibit parasite growth in vitro. The results suggest that both repeat and non-repeat epitopes in Pf155/RESA elicit antibodies with potential to protect against malaria infection.

Characterization of antibody responses to a Plasmodium falciparum blood-stage antigen induced by a DNA prime/protein boost immunization protocol

The humoral immune responses elicited by priming with a DNA plasmid and boosting with either the plasmid or the corresponding recombinant protein in alum adjuvant were compared. The plasmid DNA encoded a sequence (M3) derived from the Plasmodium falciparum antigen Pf155/RESA, and the recombinant protein consisted of the identical malarial sequence fused to an albumin-binding region (BB) of streptococcal protein G. Mice of different genetic backgrounds (CBA, Balb/c and C57Bl/6) were primed with plasmid DNA and boosted with either plasmid or recombinant protein. In all strains of mice, boosting with protein elicited higher anti-M3 antibody levels than obtained by boosting with plasmid, yet the kinetics and longevity of the secondary responses were comparable. Antiserum obtained after protein boosting displayed an immunoglobulin (Ig)G subclass profile skewed to the IgG1 isotype, regardless of the mouse strain. In contrast, mice receiving a second injection with plasmid responded with a more mixed IgG subclass profile. Inclusion of a P. falciparum circumsporozoite protein-derived T-helper epitope (CS.T3) in the immunization plasmid as well as in the fusion protein, did not significantly change the humoral responses to M3. The results show the potential of DNA vaccination for the purpose of priming an antibody response against the malarial blood-stage antigen Pf155/RESA. When combined with a protein boost, this DNA priming results in high-titred and long-lasting anamnestic responses.

IgE and tumor necrosis factor in malaria infection

IgE, the immunoglobulin instrumental in atopic diseases is also elevated in many infections. This paper reports on the occurrence and possible pathogenic role of IgE in human Plasmodium falciparum malaria, one of the most widely spread and severe infectious diseases world wide. Plasmodial infections induce IgE elevation in the blood of the majority of people living in malaria endemic areas and up to 5% of this IgE constitutes anti-malaria antibodies. Production of IgE is controlled by T cells and elevated IgE concentrations in the blood of malaria patients are the result of an increased ratio of T-helper 2 (Th2) over T-helper 1 (Th1) cells. The underlying Th1 to Th2 switch is controlled by a variety of environmental and genetic factors. The importance of the latter is demonstrated by the IgE levels occurring in monozygotic or dizygotic twins originating from malarious areas of Africa. While these levels were indistinguishable within monozygotic twin pairs, they were different within the dizygotic pairs. Comparison of the levels of total IgE or IgE anti-malaria antibodies in patients with uncomplicated malaria with those in patients with the severe form of the disease (cerebral malaria or severe malaria without cerebral involvement) indicate that these levels are significantly higher in the cases with severe disease. This is the reverse with IgG and suggests that IgE plays a role in malaria pathogenesis. An important pathogenic mediator causing malaria fever and tissue lesions is tumor necrosis factor (TNF), generally believed to be induced by toxins released from the parasite. However, sera from malaria patients can also cause TNF release from monocytes in a reaction dependent on the presence of IgE containing immune complexes or aggregates. This results in induction and cross-linking of Fcepsilon receptor II (CD23) and by binding to and activating these cells, IgE will contribute to a local over-production of TNF in capillaries and post-capillary venules where P. falciparum parasites or their products accumulate in the severe forms of this disease.

Antibodies to sequences in a non-repeat region of Plasmodium falciparum antigen Pf155/RESA inhibit either cytoadherence or parasite growth in vitro

Antibodies to a non-repeat region of the Plasmodium falciparum antigen Pf155/RESA were investigated for their capacity to inhibit parasite cytoadherence to melanoma cells and parasite growth in vitro. The activities of these antibodies were studied since the target region in Pf155/RESA includes a cytoadherence-related motif also found in loop 3 and 7 of human erythrocyte band 3 protein. Overlapping multiple antigen peptides (MAPs) together spanning residues 199-220 of Pf155/RESA were used to raise antibodies in rabbits. Analysis of the fine specificity of these antibodies revealed that antibodies raised against largely overlapping sequences displayed highly different specificity patterns. Similarly, striking differences were seen when analysing the biological effect of antibodies to these MAPs. Antibodies to the cytoadherence-related motif of Pf155/RESA, as well as antibodies raised against a MAP based on a corresponding band 3 motif, inhibited cytoadherence but not parasite growth. In contrast, antibodies to sequences adjacent to the Pf155/RESA cytoadherence motif inhibited parasite growth in vitro but had no effect on cytoadherence.

Antibodies to nonrepeat sequences of antigen Pf155/RESA of Plasmodium falciparum inhibit parasite growth in vitro

Immune responses to the repeat regions of the Plasmodium falciparum antigen Pf155/RESA have been extensively studied, and antibodies to the repeats are known to interfere with parasite growth both in vitro and in vivo. Less is known with regard to the effect on parasites of antibodies to the nonrepeat regions of the antigen. In the present study, rabbits were immunized with synthetic peptides corresponding to three different nonrepeated sequences of antigen Pf155/RESA. The reactivity of the antibodies with the particular peptides was analyzed by enzyme-linked immunosorbent assay (ELISA) and that with the parasite antigen, by immunoblotting and immunofluorescence. Although all antisera reacted strongly with the corresponding synthetic peptides, they reacted only weakly with full-length Pf155/RESA in either of the methods used. The specificity of the antibodies for Pf155/RESA was confirmed by their failure to stain Pf155/RESA-deficient parasites in erythrocyte membrane immunofluorescence, a method mainly detecting this antigen. Antibodies to the nonrepeat sequences also efficiently inhibited the merozoite invasion in vitro of Pf155/RESA+ parasites. However, these antibodies also inhibited Pf155/RESA-deficient parasites, indicating the presence of an antigen exhibiting a high degree of homology with Pf155/RESA. The results indicate that nonrepeat sequences of Pf155/RESA are immunogenic and may serve as targets for parasite-neutralizing antibodies, and, thus, the potential of the antigen as a vaccine candidate is emphasized.

Effect of Fc-receptor modulation on mumps-virus-dependent lymphocyte-mediated cytotoxicity in vitro

The natural cytotoxicity of peripheral blood lymphocytes (PBL) from normal human donors to a variety of tissue culture target cells increases upon brief exposure of lymphocytes to mumps virus. The effector cells operative in this system have Fc receptors for IgG (FcR), since cytotoxicity was abolished when FcR+ cells were removed by passage of the lymphocyte over immune-complex columns. When PBL were treated with immune complexes for 16 h at 37 degrees C, their FcR activity was sharply decreased (modulation), as indicated by a significantly reduced capacity of the treated cells to display antibody-dependent cytotoxicity (ADCC). Modulation had variable effects on natural cytotoxicity. In contrast, the virus-dependent cytotoxicity above the natural cytotoxicity remained essentially unchanged, indicating that a functionally intact FcR is not required in this system for carrying out cytolysis.

Determination of the immunoglobulin class of complement-dependent cytotoxic antibodies in serum of D23 hepatoma-bearing rats

The immunoglobulin class of the complement-dependent cytotoxic antibodies in serum from D23 hepatoma-bearing rats (D23 TBS) for D23 hepatoma cells was analysed. When studied by affinity chromatography with concanavalin A, protamine, or staphylococcal protein A conjugated to Sepharose, the cytotoxic activity bound to the former two but not protein A. The binding fractions were further characterized by column chromatography on Sepharose CL-4B. The cytotoxic activity was recovered exclusively in the high molecular weight fractions corresponding to human IgM. Monitoring with IgG- or IgM-specific rabbit antibodies indicated that these high molecular weight cytotoxic fractions contained both IgG and IgM. However, fractionation of D23 TBS at low pH suggested that cytotoxicity was due to IgM antibodies rather than to immune-complexed IgG antibodies. This was supported by the findings that rabbit antirat IgM antibodies inhibited the cytotoxicity of TBS completely when added at high dilutions.

Selected problems of malaria blood stage immunity

Both antibody dependent and cell mediated mechanisms contribute to immunity in malaria. The parasites vary in sensitivity to antibody mediated inhibition due to underlying antigenic variation. When Plasmodium falciparum isolates are tested with antibodies from the donor originally harbouring the parasites or with those from other donors, growth inhibition is usually lowest in the autologous combinations. Parasites with decreased sensitivity are also generated in vitro by culturing them for prolonged periods in the presence of certain anti-plasmodial antibodies. When the antibodies are removed, a successive return of sensitivity develops. The decrease in sensitivity to inhibition may either be due to down-regulation of synthesis of the antigen or a selection of parasites with low antigen expression from the heterogeneous original populations. Both T lymphocytes carrying alpha/beta and gamma/delta antigen-receptors play a role in malaria immunity. However, although gamma/delta T cells may expand 40-fold or more in the peripheral immune system in acutely infected humans and also inhibit parasite growth in vitro and in vivo, their relative importance for protection or pathogenicity is presently unclear. Of the two major T cell subsets (CD4+, CD8+) carrying alpha/beta T cell receptors, the role of CD8+ T cells in blood stage infections appears to be limited. Instead, CD4+ T cells are of major importance. These cells comprise at least two functionally different subsets (Th1, Th2), distinguished on the basis of lymphokine secretion. In some rodent malaria models, Th1 cells producing primarily IL2 and IFN gamma give rise to protection in early infection while Th2 cells producing IL4 are essential for parasite clearance in late infection. In other mouse strains, the same parasites induce a strong Th2 response in early infection, resulting in a lethal course. CD4+ T cells of either Th1 or Th2 type also have regulatory functions in human P. falciparum malaria. Most humans living in areas of high endemicity have significantly elevated blood levels of IgE, reflecting a skewing of the underlying T helper cell ratio in favour of Th2, responsible for the switch in immunoglobulin isotypes. Less than 5% of the IgE in malaria represents antibodies to P. falciparum. IgE elevation is highest in patients with severe and particularly cerebral malaria and is frequently associated with an elevation of tumour necrosis factor alpha (TNF). The release of this cytokine from monocytes/macrophages may reflect crosslinking of their low affinity receptors for IgE (CD23) by IgE containing immune complexes from malarial sera. Local overproduction of TNF is considered a major pathogenic mechanism, responsible for fever and tissue lesions in severe malaria. Although TNF overproduction in malaria is generally assumed to be due to direct stimulation of effector cells by certain parasite derived toxins, the present results suggest that IgE elevation constitutes yet another mechanism contributing to the pathogenicity of P. falciparum in human malaria.

Differential induction of immunoglobulin G subclasses by immunization with DNA vectors containing or lacking a signal sequence

The route and method used to immunize mice with antigen-expressing DNA plasmids have an impact on the resulting T-helper cell response and IgG subclass distribution. Previous findings further indicate that the intracellular targeting of expressed antigens influences the differentiation of naive T-cells into either a Th1 or a Th2 type of response. In the present study, we analyzed the levels of IgG1 and IgG2a antibodies, as correlates of Th2 and Th1 responses, respectively, after intramuscular injection of mice with plasmids encoding a chimeric protein containing a Plasmodium falciparum blood stage antigen expressed in two different forms. One plasmid expresses the antigen in a secreted form as it is preceded by a signal sequence while expression from the other plasmid, lacking this sequence, results in cytoplasmic localization of the antigen. Mice immunized with the plasmid encoding secreted antigen responded with predominantly IgG1 antibodies. In contrast, sera from mice immunized with the plasmid expressing cytosolic protein displayed a mixed IgG1/IgG2a profile. In line with previous findings, our results suggest that the intracellular targeting of proteins expressed by DNA plasmids is an important factor for the differentiation of Th cells and the resulting subclass pattern of IgG responses.

Immunogenicity of chimeric multiple antigen peptides based on Plasmodium falciparum antigens: impact of epitope orientation

Assembly of B and T epitopes in multiple antigen peptides (MAP) can bypass genetically predisposed unresponsiveness to B epitopes. Although the underlying mechanisms are unknown, B-cell responses to such diepitope MAP are influenced by intramolecular epitope orientation. In this study, MAP constructs were synthesized, encompassing two epitopes derived from the Plasmodium falciparum antigens circumsporozoite protein (CS) and Pf332. In addition to B epitopes, the sequences comprised T epitopes restricted to mouse H-2b (CS) or to H-2d and H-2k (Pf332) haplotypes. Congenic H-2b, H-2d and H-2k Balb mice were immunized with MAP in which the two epitopes were arranged either tandemly or in parallel. Tandemly arranged (B-T)4 MAP, in which the relevant T epitope was positioned adjacent to the lysine core [(Pf332-CS)4-core for H-2b mice and (CS-Pf332)4-core for H-2d and H-2k mice], elicited the most potent antibody responses in terms of reactivity to both epitopes. Additionally, the (B-T)4 constructs were generally most efficient in recalling proliferative T-cell responses in vitro, irrespective of the MAP used for in vivo priming. As high antibody titers were generated to both epitopes, the position of B epitopes in the constructs does not appear to be critical for an efficient B-cell response. Rather, the association of strong B- and T-cell responses to the (B-T)4 MAP constructs suggests that the intramolecular position of the relevant T epitope determines the magnitude of specific antibody production.

A malariometric survey in a rural community in the Muheza district, Tanzania: age profiles in the development of humoral immune responses

A malariometric survey was carried out in a rural community situated in a malaria holoendemic endemic area of Tanzania. A random sample (n = 228) of different age groups was taken to elucidate the association between anti-Pf155/RESA and anti-Pf332 antibody responses and classical malaria indices. Parasitaemia, fever, splenomegaly, haematocrit and antimalarial consumption were assessed. Antibody responses against Pf155/RESA and Pf332 peptides were determined by ELISA. The age profiles of parasite density, splenomegaly, fever, haematocrit values and prevalence of antibody responses indicated intensive malaria exposure and the highest impact of malaria in small children. Forty-five percent of the study population had detectable chloroquine and desethyl-chloroquine blood levels, and the highest frequency and concentrations were recorded in the 12-23 months old. There was no significant association between the presence of drug and parasite density in the different age groups, although in the < 15 years old there was lower parasite prevalence among the children positive for drug in their blood (P < 0.05). High prevalence of antibody responses to all antigens was observed already at an early age, but the mean anti-Pf155/RESA and anti-Pf332 antibody levels increased significantly only in the adult group (P < 0.01). Significantly lower mean parasite densities were observed in high responders to Pf155/RESA and Pf332 peptides for the > or = 10 years old. For the 1-9 years, a similar difference was only observed in the high responders to Pf332. For the whole material, anti-Pf155/RESA and anti-Pf332 antibody levels correlated positively with age. When the effect of age was allowed for in analysing the relationship between parasite density and antibody level against the different antigens, a significant negative correlation was found only with regard to Pf332 in the > = 10 years age group. These results suggest that anti-Pf332 antibodies appear to be a better indicator for antiparasitic immunity, but both antigens are important for immune protection.

T- and B-cell responses of malaria immune individuals to synthetic peptides corresponding to non-repeat sequences in the N-terminal region of the Plasmodium falciparum antigen Pf155/RESA

While the C-terminal repeat region of Pf155/RESA, a Plasmodium falciparum vaccine candidate has been extensively studied for B- and T-cell reactivities, little is so far known about the non-repeat region in this respect. The present study aimed at investigating the non-repeat sequence 171-227 of Pf155/RESA for T- and B-cell epitopes. Eight overlapping peptides were synthesised and assayed for their ability to stimulate peripheral blood mononuclear cells obtained from P. falciparum-immune donors to proliferate and to induce secretion of interferon-gamma (IFN-gamma) and/or interleukin 4 (IL-4) using the ELISPOT assay. The plasmas of the corresponding donors were tested for antibody reactivity with the same peptides in ELISA. The individual cellular responses to the different peptides varied and in general they were not correlated, emphasising the importance of including several parameters for T-cell activation. The most frequent T-cell responses (proliferation, IFN-gamma and/or IL-4) were seen with two partially overlapping peptides corresponding to the sequences 171-185 and 181-195 that induced responses in 71 and 62% of the donors, respectively. Although, the frequency of responders was high, the magnitude of the responses was generally low. Two overlapping peptides corresponding to the sequence 186-206 bound antibodies from a large number of plasma samples. IL-4 producing cells were frequently found in donors whose sera contained antibodies to the corresponding peptide. However, there was no absolute correlation and many donors having anti-peptide antibodies could also be induced to produce IFN-gamma. In conclusion, the non-repeat region of Pf155/RESA contains several epitopes inducing functionally distinct T-cell responses. The sequence 171-206 was found to contain both B- and T-cell epitopes recognised by almost all individuals naturally primed to malaria. Thus, this sequence should be a useful tool in future immuno-epidemiological studies and/or for inclusion into a subunit vaccine against the asexual blood stages of the P. falciparum parasite.

Elevated plasma levels of IgE in Plasmodium falciparum-primed individuals reflect an increased ratio of IL-4 to interferon-gamma (IFN-gamma)-producing cells

People living in Plasmodium falciparum-endemic areas frequently have elevated levels of total as well as P. falciparum-specific serum IgE. This study aimed at investigating whether the elevated serum IgE levels reflect a shift in the balance between CD4+ T helper 1 (Th1) and T helper 2 (Th2) cells in individuals naturally exposed to the P. falciparum parasite. To investigate the role of Th1 and Th2 cells in the human P. falciparum system we used the ELISPOT assay to determine the ratio of IFN-gamma- and IL-4-producing cells after specific antigen or mitogen activation in vitro. The donors were individuals who had acquired immunity through natural exposure to the parasite. In response to the specific malaria antigens, very few IL-4-producing cells were seen. However, in the response of individual donors to the polyclonal T cell activator, leucoagglutinin (La), the anti-malarial IgE levels in plasma were correlated with an increased ratio of IL-4/IFN-gamma producing cells. Thus, donors with ratios of IL-4/IFN-gamma > 1 exhibited mean plasma anti-malarial IgE levels significantly greater than those with ratios < 1. In individuals not living in P. falciparum-endemic areas the ratio of IL-4/IFN-gamma was always < 1. Taken together, our data suggest a shift in the balance between Th1 and Th2 cells in naturally P. falciparum-primed individuals, associated with elevated anti-P. falciparum plasma IgE levels. The role and biological significance of IgE (Th2-type immune response) for protection against P. falciparum and/or pathogenesis of malaria require further study.

Comparative study of DNA-based immunization vectors: effect of secretion signals on the antibody responses in mice

The presence of a signal sequence preceding the gene encoding a target antigen in a DNA vaccine should facilitate secretion of the in vivo translated antigen. The immune responses elicited upon injection with such a vector could differ from those induced by the same vector lacking a signal sequence. In the present study, the humoral responses elicited in mice immunized with two plasmids, either containing or lacking the human tissue plasminogen activator signal sequence, were compared. Both plasmids encode the chimeric antigen ZZN4, containing a malaria antigen Pf332-derived sequence (N4) linked to a bacterial fusion partner (ZZ). In vitro transfection of COS cells with each plasmid and treatment of the transfectants with brefeldin A confirmed that secretion of ZZN4 via the endoplasmic reticulum and Golgi pathway only occurred in cells transfected with the signal peptide-encoding plasmid. Repeated intramuscular injections of mice with either of the plasmids elicited comparable antibody responses to ZZN4 with regard to kinetics, specific IgG levels and persistence. These results indicate that in vivo transfection of muscle cells by either of these two plasmids generated comparable levels of antigen available for B-cell recognition and for uptake by antigen-presenting cells, despite the differential intracellular targeting of the encoded antigen. The relevance of these findings for the design of DNA vaccine vectors is discussed.

Predominance of H-2d- and H-2k-restricted T-cell epitopes in the highly repetitive Plasmodium falciparum antigen Pf332

Genetic restriction of immune responses to malaria antigens is an important issue for a better comprehension of malaria immunity as well as for development of subunit vaccines. To experimentally define the major histocompatibility complex restriction of immune responses to the highly repetitive Plasmodium falciparum high-molecular-weight antigen Pf332, H-2-congenic mice were immunized with EB200, a recombinant fragment of Pf332 consisting of degenerate repeat motifs. Strong B- and T-cell responses were elicited in H-2d and H-2k mice whereas responses in H-2b, H-2q and H-2s mice were of lower magnitude. The T-cell specificity elicited by EB200 was defined by in vitro proliferative responses to a panel of overlapping peptides spanning EB200. Dominant epitopes were identified for H-2d and H-2k mice, respectively, and an additional epitope was recognized by all five mouse strains. Selected EB200-derived peptides were further investigated for their ability to elicit T-cell help when injected as multiple antigen peptides. Defined H-2d- and H-2k-restricted T-cell epitopes generated high antibody levels in the respective mouse strains, as did several peptides lacking defined epitopes indicating the presence of additional H-2d- and H-2k-restricted, cryptic or subdominant T-cell epitopes in EB200. The biased H-2 restriction pattern of T-cell epitopes in Pf332 and, as previously reported, in structurally related repeats in the malaria antigens Pf11.1 and Pf155/RESA may be explained by a shared motif for H-2d and H-2k class II-restricted T-cell epitopes, as revealed by alignment of these sequences.

Plasmodium falciparum: selection of parasite subpopulations with decreased sensitivity for antibody-mediated growth inhibition in vitro

The chronic, recrudescent nature of malaria has been linked to antigenic diversity of the parasite in which protective immunity against Plasmodium falciparum may be obtained after repeated exposure to infection during a long time. In this study we show that parasite populations with decreased sensitivity to antibody-mediated growth inhibition are readily generated in vitro. A laboratory strain, F32, was cultured for long periods (10-12 weeks) in the presence of suboptimal inhibitory antibody concentrations. The antibodies used were the human monoclonal antibody 33G2 reacting with a linear epitope of the P. falciparum blood-stage antigen 332 and rabbit antibodies to repeat sequences of the blood-stage antigen Pf155/RESA. Our data indicate that the P. falciparum parasites adapt to antibody pressure as reflected by their specifically decreased sensitivity to growth inhibition. A relative resistance of the parasite to growth inhibition mediated by the antibodies used in the culture developed successively, while the parasite remained sensitive to growth inhibition by other antibodies. When the antibody pressure was removed a successive return of sensitivity to growth inhibition developed. Immunoflourescence did not reveal any significant difference in antigen expression between the parasite populations. However, PCR analysis showed that a new population appeared in the parasites grown in the presence of mAb 33G2, while no such change was detected in those grown in the presence of the rabbit antibodies. Our data suggest that the specific decrease in sensitivity to growth inhibition may either be due to down-regulation of antigen synthesis or expression by antibody pressure or, that antibody pressure selects for parasites with low expression of a specific antigen from a heterogeneous parasite population.

The serum albumin-binding region of streptococcal protein G: a bacterial fusion partner with carrier-related properties

In this study, we have explored the use of the serum albumin-binding region (BB) from streptococcal protein G (SpG) as a bacterial fusion partner for production of peptide immunogens. The fusion protein BB-M3, containing BB and repeated structures from the Plasmodium falciparum malaria antigen Pf155/RESA, was efficiently purified from Escherichia coli culture supernatants by affinity chromatography using BB as an affinity tag. Rabbits immunized with BB-M3 in Freund's adjuvant produced high levels of antibodies which reacted with both M3 and BB in ELISA and stained intact Pf155/RESA in the membrane of infected erythrocytes. These antibody levels were sustained for more than 30 weeks. BB-M3 also induced antibody responses to M3, BB and intact Pf155/RESA in a number of mouse strains, including several strains which are non-responders to the malaria sequences. In the latter mice, however, BB-M3 only activated BB-specific T cells, suggesting that BB has ability to provide carrier-related T cell help for antibody production. Moreover, the minimal albumin-binding motif of SpG, containing only 46 amino acids, was immunogenic in both B10.BR, B10.D2 and C57BL/6 mice (H-2k, H-2d and H-2b, respectively). These results indicate that BB has both affinity tag and carrier-related properties and suggest that fusion proteins containing BB can be efficient tools for the generation of antibody responses to peptides which are weak immunogens.

Wild isolates of Plasmodium falciparum malaria show decreased sensitivity to in vitro inhibition of parasite growth mediated by autologous host antibodies

Antigenic diversity in field populations of Plasmodium falciparum parasites may delay the acquisition of protective immunity to malaria, the development of which may thus require repeated exposure to infection over a prolonged period of time. In this study we show that P. falciparum parasites may vary in their sensitivity to antibody-mediated invasion/growth inhibition in vitro. Wild isolates of P. falciparum from children living in an endemic area of Burkina Faso were tested for their sensitivity to the growth inhibitory effects of antibodies originating from the same (autologous) and from other donors (heterologous). A significantly lower invasion inhibition activity was obtained when the isolates and antibodies were tested in autologous compared with heterologous combinations. The lower sensitivity to growth inhibition by autologous antibodies may be due to immune pressure in vivo, selecting from a heterogeneous parasite population those with a low expression of the antigens recognized by the host's antibodies. Alternatively, the parasites cultured from each child might represent expanding parasite populations, mainly constituting strains not earlier seen by the immune system of that specific host. The results reinforce the concern about Plasmodium antigenic diversity as a major obstacle towards the development of an effective malaria vaccine.

Immunoglobulin E, a pathogenic factor in Plasmodium falciparum malaria

Most children and adults living in areas where the endemicity of Plasmodium falciparum malaria is high have significantly elevated levels of both total immunoglobulin E (IgE) and IgE antimalarial antibodies in blood. This elevation is highest in patients with cerebral malaria, suggesting a pathogenic role for this immunoglobulin isotype. In this study, we show that IgE elevation may also be seen in severe malaria without cerebral involvement and parallels an elevation of tumor necrosis factor alpha (TNF). IgE-containing serum from malaria immune donors was added to tissue culture plates coated with rabbit anti-human IgE antibodies or with P. falciparum antigen. IgE-anti-IgE complexes as well as antigen-binding IgE antibodies induced TNF release from peripheral blood mononuclear cells (PBMC). Nonmalaria control sera with no IgE elevation induced significantly less of this cytokine, and the TNF-inducing capacity of malaria sera was also strongly reduced by passing them over anti-IgE Sepharose columns. The cells giving rise to TNF were adherent PBMC. The release of this cytokine probably reflects cross-linking of their low-affinity receptors for IgE (CD23) by IgE-containing immune complexes known to give rise to monocyte activation via the NO transduction pathway. In line with this, adherent monocytic cells exposed to IgE complexes displayed increased expression of CD23. As the malaria sera contained IgG anti-IgE antibodies, such complexes probably also play a role in the induction of TNF in vivo. Overproduction of TNF is considered a major pathogenic mechanism responsible for fever and tissue lesions in P. falciparum malaria. This overproduction is generally assumed to reflect a direct stimulation of effector cells by certain parasite-derived toxins. Our results suggest that IgE elevation constitutes yet another important mechanism involved in excessive TNF induction in this disease.

Lack of association between levels of transplacentally acquired Plasmodium falciparum-specific antibodies and age of onset of clinical malaria in infants in a malaria endemic area of Nigeria

A cohort of 117 newborns was followed longitudinally for 12 months to determine the age of onset of clinical malaria and the subsequent episodes of malaria, and to investigate the possible existence of a correlation between level of transplacentally acquired Plasmodium falciparum-specific antibodies and age of onset of malaria in the infant. The mean age of onset of malaria in 49 infants was 4.48 +/- 1.54 months. Mean (+/- S.D.) age of onset of clinical malaria in haemoglobin AA infants (4.38 +/- 1.14) was significantly (P < 0.05) lower compared with haemoglobin AS (5.58 +/- 2.43) infants. No correlation was obtained between the age of onset of malaria and the level of cord serum total IgG, IgM and antibodies to P. falciparum antigens. Cord blood seropositivity for antibodies to the blood stage antigen Pf155/RESA and its C-terminal repeat sequence (EENV)6 or to the (NANP)6 peptide representing repeats of the circumsporozoite protein (CSP) did not influence the age of onset of clinical malaria. However, infants with haemoglobin AS whose cord blood was seropositive for antibodies to the (EENV)6 or (NANP)6 peptide showed delayed onset (P < 0.001) of malaria compared with AA seropositive infants. Although our results indicate that transplacentally acquired antibodies to the studied antigens alone offer no significant protection against malaria during the first few months of life, antibodies in concert with other factors such as haemoglobin genotype may contribute to the protection of the newborn.

Immune responses in congenic mice to multiple antigen peptides based on defined epitopes from the malaria antigen Pf332

Repeat sequences from the Plasmodium falciparum blood stage antigen Pf332 frequently comprise the pentapeptide VTEEI, an epitope recognized by certain parasite neutralizing antibodies. This B-cell epitope was assembled in an octavalent multiple antigen peptide (MAP) system either as trimers (VTEEI)3 (MAP1) or as an integral part of a naturally occurring Pf332 undecamer repeat sequence SVTEEIAEEDK (MAP2). Characteristics of the immunogenicity of these subunit constructs were evaluated in H-2 congenic mice. MAP1 generated antibody responses in mice of the H-2d, H-2k and H-2q haplotypes, but not in H-2b or H-2s mice, whereas MAP2 only induced antibodies in mice of H-2k haplotype. When analysing T-cell responses induced by the MAP, lymph node cells from responder strains primed in vivo with MAP1 proliferated in response to restimulation with both MAP1 and the peptide (VTEEI)3. MAP2, however, did not induce a detectable T-cell proliferation. Additionally, the lack of antibody response to MAP1 in H-2b mice could be circumvented by combining the MAP1 peptide and a H-2b-restricted T-cell epitope in a diepitope MAP construct. Despite the fact that the motif VTEEI has not been identified in Pf332 sequences in the form of a trimer, MAP1 did induce Pf332 protein-reactive antibodies. Assembly of multimers of short defined epitopes in MAP constitutes an interesting approach for the design of polyvalent subunit immunogens.

Generation of in vitro natural cytotoxicity of horse lymphocytes against sarcoid-derived tumor cells not expressing major histocompatibility complex antigens

OBJECTIVE

To analyze in vitro lymphocyte-mediated immune responses of horses with sarcoids against allogeneic sarcoid cells containing endogenous retrovirus but not expressing major histocompatibility complex antigens.

DESIGN

Lymphocyte-mediated immune reactions were assessed by means of proliferative responses in mixed lymphocyte tumor cell culture (MLTC) assay and lymphocyte-mediated cytotoxicity against various equine target cells.

ANIMALS

12 horses with sarcoid tumors and 15 control horses.

PROCEDURE

Blood lymphocytes were cocultured in MLTC with allogeneic sarcoid cells (Mc-1, BayMc-1), equine testis cells, or normal equine dermal fibroblasts. Lymphocytes were assayed for proliferative responses by [3H]thymidine uptake and for cytotoxicity against the same targets by 51Cr release assay. The lymphocyte populations were analyzed for some common surface markers.

RESULTS

Lymphocytes from horses with sarcoids exerted an anamnestic proliferative response in MLTC against Mc-1 cells, but this procedure never generated cytotoxic lymphocytes. However, lymphocytes from all horses cultured in medium with 10% allogeneic serum only had selective. natural cytotoxicity against Mc-1 that was generated without DNA synthesis. Approximately 80% of the lymphocytes disappeared during culture; however the remaining population of small, viable lymphocytes indicated a decrease of CD4+ T lymphocytes, but numbers of T cells with receptors for Helix pomatia A hemagglutinin were unaffected. Few lymphocytes had Fc-receptors for IgG, were complement-reactive positive cells or were B cells expressing surface immunoglobulin.

CONCLUSIONS

Results may indicate a natural defense system, which preferentially recognizes and lyses tumor cells that are deficient in surface expression of major histocompatibility complex antigens, without intervention of conventional T-cell receptors or antibodies.

Immunoglobulin E elevation in Plasmodium chabaudi malaria

In order to investigate the mechanism of immunoglobulin E (IgE) elevation in malaria we studies mice infected with asexual blood stages of the rodent malaria parasite Plasmodium chabaudi chabaudi for total IgE and IgE antimalarial antibodies. Multiply infected mice had elevated levels of total as well as malaria-specific IgE in their sera. Sera taken from mice 3 weeks after one infection with P. chabaudi showed no IgE elevation, indicated that prolonged or repeated exposure to the parasite is necessary for the induction of an IgE response, which also is induced independently of previous or simultaneous infection with other pathogens such as helminths.

Plasmodium falciparum: differential parasite growth inhibition mediated by antibodies to the antigens Pf332 and Pf155/RESA

Plasmodium falciparum-reactive antibodies can inhibit growth of parasite blood stages in vitro by interfering at different stages of parasite development. Antibodies reactive with repeat sequences in Pf155/ring-infected surface antigen (Pf155/RESA) or in antigen Pf332 inhibit parasite growth as determined by a reduction of newly infected erythrocytes. Antibodies to Pf155/RESA have been implicated in merozoite invasion inhibition. Since it was considered unlikely that antibodies to the late stage antigen Pf332 act on merozoite invasion, we investigated at what developmental stage this growth inhibition takes place. Parasites were cultured in the presence of antibodies to repeat sequences of either Pf332 or Pf155/RESA and were examined with regard to the distribution of different stages and their morphology. As expected, antibodies to both antigens decreased the number of ring stages after reinfection. They did not affect the intraerythrocytic development of ring stages or trophozoites. However, antibodies to Pf332 induced high levels of schizonts displaying an abnormal morphology while antibodies to Pf155/RESA induced considerably lower levels of degenerated schizonts. Thus, Pf332-reactive antibodies seem to interfere with the schizont development by blocking the rupture of mature schizonts or, alternatively, by interfering intraerythrocytically with late stage parasites.

T cell-derived IL-3 induces the production of IL-4 by non-B, non-T cells to amplify the Th2-cytokine response to a non-parasite antigen in Schistosoma mansoni-infected mice

We describe a novel amplification mechanism underlying the increased early IL-4 production observed in Schistosoma mansoni-infected mice in response to a non-parasite Ag, sperm whale myoglobin (SwMb). Earlier studies have shown that splenic Fc epsilon R+ non-B, non-T (NBNT) cells from schistosome-infected mice secrete IL-4 after stimulation with parasite Ag. We now demonstrate that purified NBNT cells from SwMb-immunized S. mansoni-infected mice do not respond directly to SwMb, but produce IL-4 in response to IL-3. Accordingly, we show that the early SwMb-specific IL-4 response of spleen cells (SC) from immunized infected mice is dependent on IL-3 and on CD4+ T cells. Thus, most of the early SwMb-induced IL-4 from SC of infected mice appears to be produced by NBNT cells triggered by IL-3 synthesized by SwMb-specific CD4+ T cells. IL-3-induced IL-4 production was also observed in purified NBNT cells from immunized uninfected mice, but the frequency and/or IL-4-producing capacity of splenic IL-3-responsive cells was found to be 8 to 16 times higher in immunized infected animals. IL-4 production by purified CD4+ cells from immunized infected mice was also seen after SwMb stimulation, but this response showed slower kinetics than those of total SC, was IL-3-independent, and on average threefold greater than that by CD4+ cells from immunized uninfected controls. Thus, increased SwMb-induced IL-4 production in immunized S. mansoni-infected mice results from direct synthesis by CD4+ T cells, as well as their stimulation via IL-3 of an expanded population of NBNT cells. The latter pathway may serve as an amplification loop for Th2-cytokine responses.

T cell-derived IL-3 induces the production of IL-4 by non-B, non-T cells to amplify the Th2-cytokine response to a non-parasite antigen in Schistosoma mansoni-infected mice

We describe a novel amplification mechanism underlying the increased early IL-4 production observed in Schistosoma mansoni-infected mice in response to a non-parasite Ag, sperm whale myoglobin (SwMb). Earlier studies have shown that splenic Fc epsilon R+ non-B, non-T (NBNT) cells from schistosome-infected mice secrete IL-4 after stimulation with parasite Ag. We now demonstrate that purified NBNT cells from SwMb-immunized S. mansoni-infected mice do not respond directly to SwMb, but produce IL-4 in response to IL-3. Accordingly, we show that the early SwMb-specific IL-4 response of spleen cells (SC) from immunized infected mice is dependent on IL-3 and on CD4+ T cells. Thus, most of the early SwMb-induced IL-4 from SC of infected mice appears to be produced by NBNT cells triggered by IL-3 synthesized by SwMb-specific CD4+ T cells. IL-3-induced IL-4 production was also observed in purified NBNT cells from immunized uninfected mice, but the frequency and/or IL-4-producing capacity of splenic IL-3-responsive cells was found to be 8 to 16 times higher in immunized infected animals. IL-4 production by purified CD4+ cells from immunized infected mice was also seen after SwMb stimulation, but this response showed slower kinetics than those of total SC, was IL-3-independent, and on average threefold greater than that by CD4+ cells from immunized uninfected controls. Thus, increased SwMb-induced IL-4 production in immunized S. mansoni-infected mice results from direct synthesis by CD4+ T cells, as well as their stimulation via IL-3 of an expanded population of NBNT cells. The latter pathway may serve as an amplification loop for Th2-cytokine responses.

Surface display compared to periplasmic expression of a malarial antigen in Salmonella typhimurium and its implications for immunogenicity

Two different expression systems were investigated for the production of an 80 amino acid polypeptide, M3, from the C-terminus of the Plasmodium falciparum blood stage antigen Pf155/RESA in an attenuated Salmonella typhimurium vaccine strain. Upon expression, the malarial polypeptide was targeted either to the periplasm as a soluble fusion protein containing two IgG-binding domains (ZZ) from the staphylococcal protein A or, to the bacterial surface as an insert within a chimeric outer membrane protein A (OmpA) derived from Escherichia coli and Shigella dysenteriae. Both the ZZM3 and the OmpAM3 proteins were stably expressed in the periplasm or on the surface of Salmonella, respectively. The ZZ expression system yielded 10-100 times more malarial immunogen than did the OmpA system. Live recombinant Salmonella expressing ZZM3 or OmpAM3 were used to immunize mice intraperitoneally. Both the ZZM3 and OmpAM3 genes persisted for up to three weeks in bacteria isolated from different lymphoid organs. Bacteria expressing ZZM3 induced antibodies to M3, ZZ and to the Pf155/RESA antigen whereas, bacteria producing OmpAM3 induced similar levels of antibodies reactive with M3 but not with Pf155/RESA. Both recombinants induced a memory response of antibodies reactive with both M3 and Pf155/RESA. The high levels of M3 produced by the ZZ expression system make it suitable for the expression of heterologous antigens in Salmonella. Nevertheless, in spite of the quantitative difference in M3 expression, the ZZ and OmpA constructs elicited comparable immune responses to M3.

Cellular mechanisms in the immune response to malaria in Plasmodium vinckei-infected mice

Infection of mice with the malaria parasite Plasmodium vinckei vinckei is 100% lethal. However, after two infections followed by drug cure, BALB/c mice develop a solid immunity which is antibody independent but mediated by CD4+ T cells. To elucidate the mechanisms of this immunity, spleen cells from immune mice were challenged in vitro with lysates of P. vinckei-infected or uninfected erythrocytes. The parasite antigen induced proliferation of T cells from immune mice but not from nonimmune mice. When gamma interferon production by cells from immune mice was assayed at the single-cell level, 1 to 3 cells per 1,000 cells were found to release this cytokine when exposed to antigen. In contrast, the numbers of interleukin 4 (IL-4)-producing cells from both immune and control mice were < or = 4 per 10(6) cells, regardless of antigen exposure. Investigation in a bioassay showed that P. vinckei antigen induced the release of IL-4 from spleen cells of immune mice but not from those of control mice. Nevertheless, that IL-4 is of minor significance in this system is also suggested by the absence of elevation of immunoglobulin E levels in blood samples from these mice, in contrast to what is seen with P. chabaudi infection, in which IL-4-producing Th2 cells are of major importance for immunity during later phases of infection. Taken together, the present results indicate that immunity to P. vinckei is a Th1 response, with gamma interferon being an important protective factor. Whether or not the Th1 response, through overproduction of tumor necrosis factor alpha, is also responsible for pathology and death in this infection remains to be clarified.

Antibodies to Pf155/RESA and circumsporozoite protein of Plasmodium falciparum in paired maternal-cord sera from Nigeria

Paired maternal-cord serum samples were analysed for antibodies to the Pf155/RESA and circumsporozoite protein (CSP) antigens of Plasmodium falciparum. Malaria parasites were found in 2.6% (3/117) of cord blood and 22.4% (26/116) of maternal samples. Immunofluorescence assays detected P. falciparum-specific IgG antibodies in all paired samples while P. falciparum-specific IgM was detected in 5.8% (7/121) of cord samples. The positivity rates for antibodies to Pf155/RESA and (NANP)6 but not (EENV)6, a C-terminal repeat sequence of Pf155/RESA, were significantly higher in maternal as compared with cord samples. Seropositivity rates to Pf155/RESA and (EENV)6 were not related to maternal parity group while positivity rates to the (NANP)6 peptide were higher in primiparae and multiparae of > or = 4 parity. These data confirm the transplacental transfer of P. falciparum-specific antibodies and the higher incidence of malaria parasitaemia in primiparae. The presence of P. falciparum-specific IgM in some cord samples suggests intrauterine sensitization of the foetus to malarial antigens.

Immunogens containing sequences from antigen Pf332 induce Plasmodium falciparum-reactive antibodies which inhibit parasite growth but not cytoadherence

Immunogens based upon sequences from the P. falciparum asexual blood stage antigen Pf332 were assessed for their capacity to induce antibodies inhibiting parasite growth or cytoadherence of infected erythrocytes in vitro. Selection of the Pf332 sequences was based on their reactivity with the human monoclonal antibody (MoAb) 33G2 which inhibits parasite growth as well as cytoadherence in vitro. Octameric multiple antigen peptides (MAP) were assembled based upon either a trimer of the minimal epitope recognized by the MoAb, VTEEI, or a Pf332 sequence including that motif, SVTEEIAEEDK. A dimer of SVTEEIAEEDK was also expressed in Escherichia coli, genetically fused to ZZ, two IgG-binding domains of staphylococcal protein A. Rabbit antibodies elicited by the immunogens reacted with Pf332 in immunofluorescence and in ELISA with Pf332 peptides which were also recognized by MoAb 33G2. The MAP with branched (VTEEI)3 peptide induced the highest titres of P. falciparum-reactive antibodies. In contrast to MoAB 33G2, none of the polyclonal Pf332 reactive sera cross-reacted with repeat sequences of the malaria antigen Pf155/RESA. The polyclonal Pf332-reactive antibodies inhibited parasite growth efficiently but had no or very low inhibitory effect in a cytoadherence assay. Thus, while Pf332 may be an important target for parasite neutralizing antibodies its involvement in cytoadherence is unclear.

Cytoadherence-related homologous motifs in Plasmodium falciparum antigen Pf155/RESA and erythrocyte band 3 protein

Cytoadherence of Plasmodium falciparum-infected erythrocytes plays an important role in the pathogenesis of cerebral malaria. The identity of cell surface molecules on parasitized erythrocytes involved in cytoadherence is of great interest to understand the molecular basis of this mechanism. Peptide sequences derived from exofacial loops of the erythrocyte antigen band 3 from parasitized erythrocytes have previously been shown to inhibit cytoadherence. We now report that a non-repeated region of Pf155/RESA (residues 213-218) contains a hexapeptide motif being highly homologous to cytoadherence inhibitory sequences from band 3. Synthetic peptides containing the hexapeptide motif of Pf155/RESA inhibited the binding of P. falciparum-infected erythrocytes to melanoma cells in vitro. Furthermore, individuals residing in malaria-endemic areas have antibodies reactive with epitopes involving these motifs in band 3 and in Pf155/RESA.

A longitudinal study of seroreactivities to Plasmodium falciparum antigens in Nigerian infants during their first year of life

The kinetics of passively transferred maternal antibodies to antigens of Plasmodium falciparum and the dynamics of acquisition of these antibodies during the first year of life was investigated in infants born in a malaria endemic area of south-western Nigeria. Blood samples were collected from the infants at bi-monthly follow-up visits for the analysis of total serum immunoglobulin G, IgM, IgA and antibodies to the antigen Pf155/RESA and against synthetic peptides representing antigenic sequences of the blood stage antigen Pf155/RESA and Ag332 or the circumsporozoite protein (CSP). IgG levels fell from birth till 4 months and a steady rise was observed thereafter till ten months of life. On the contrary mean IgM and IgA levels increased throughout the first year of life. Generally the number of infants positive for antibodies to the antigens under investigation fell from birth and between 4-6 months of age was either low or absent. None of the infants were positive for antibodies to the peptide representing Ag332 during the first year of life. The earliest seroconversion was detected at 6 months of age involving the Pf155/RESA and (NANP)6 antigens. The results indicate a high level of exposure in this study area to malaria infection early in life. The finding of an active antibody response to malarial antigens in infancy encourages the hope that a malaria vaccine administered early in life may accelerate the development of naturally acquired immunity and thus protect the population most at risk.

Genetic restriction and specificity of the immune response in mice to fusion proteins containing repeated sequences of the Plasmodium falciparum antigen Pf155/RESA

The genetic restriction and specificity of the immune response in mice to two fusion proteins, ZZ-M3 and ZZ-M5, were studied. These proteins contain two IgG-binding domains (ZZ) from staphylococcal protein A, and repeated sequences from the C-terminal [(VEHDAEEN)5 (VEEN)10] (M3) or central [(VEEPTVADDEH)3(VEEPTVAEEH)2] (M5) regions of the Plasmodium falciparum malaria blood stage antigen Pf155/RESA. Strong antibody and T-cell responses to M3 and M5 were linked to expression of the I-Ak allele, and T-cell responses to the bacterial fusion partner ZZ were restricted to mice of the H-2k haplotype. The response to M5 was less restricted than that to M3, giving intermediate responses in mice of H-2d haplotypes as well. However, ZZ-M5-primed lymph node (LN) cells from these mice were primarily induced to proliferate in vitro by the complete ZZ-M5 construct and not by synthetic peptides representing the repeated subunits in M5. The reactivity with intact Pf155/RESA in erythrocyte membrane immunofluorescence was weak of antisera from mice immunized with ZZ-M5, whereas the reactivity of antisera from mice immunized with ZZ-M3 roughly paralleled their reactivity with M3 in an enzyme-linked immunosorbent assay (ELISA). The antibody responses induced by immunization with ZZ-M3 or ZZ-M5 were specific for M3 or M5, respectively, while activated T cells displayed cross-reactivity between M3 and M5 in an in vitro proliferation assay. The results indicate that the assembly of repeated sequences in fusion proteins affects both the MHC class II restriction and the specificity of the induced antibody and T-cell responses.