E.M. Riley

Naturally acquired cellular and humoral immune responses to the major merozoite surface antigen (PfMSP1) of Plasmodium falciparum are associated with reduced malaria morbidity

Summary We have investigated the pattern of acquired immune responses to the major surface protein of Plasmodium falciparum merozoites (gp 190, Pf MSP1) in a malaria endemic population in West Africa. A prospective longitudinal study in 3‐ to 8‐year‐old children was conducted to examine the relationship between naturally acquired immune responses to Pf MSP1 and subsequent susceptibility to malaria infection and clinical disease. A population cross‐sectional survey was performed to investigate changes in immune response with age. The prevalence and concentration of antibodies to all regions of the molecule increased with age with the highest prevalence of antibodies being detected against regions of the molecule which are highly conserved between parasite isolates. In vitro lymphoproliferation and interferon‐gamma production in response to recombinant proteins representing polymorphic regions of the molecule also increased with age. Interestingly, proliferative responses to some regions of the molecule, including some highly conserved sequences, were highest in young children and decreased markedly with increasing age. Significant associations were observed between antibody and lymphoproliferative responses to proteins from the C terminus of the molecule and resistance to episodes of fever associated with high parasitaemia in partially immune children. In addition, high concentrations of antibodies to a conserved region close to the N terminus of Pf MSP1 were also significantly associated with protection.

Primary structure and localization of a conserved immunogenicPlasmodium falciparum glutamate rich protein (GLURP) expressed in both the preerythrocytic and erythrocytic stages of the vertebrate life cycle

A gene coding for a 220-kDa glutamate rich protein (GLURP), an exoantigen of Plasmodium falciparum, was isolated and its nucleotide sequence was determined. The deduced amino acid sequence contains 2 repeat regions. The sequence of one of these was shown to be conserved among geographically dispersed isolates, and a fusion protein containing that sequence was able to stimulate B- and T-cells. Antibodies against GLURP stained erythrocytic stages of the parasite as well as the hepatic stage as detected by electron microscopy.

Aflatoxin exposure, malaria and hepatitis B infection in rural Gambian children

Aflatoxin-albumin adduct levels were measured in serum samples obtained from a group of Gambian children. The relationships between exposure to aflatoxin and the prevalence of malaria, between exposure and humoral and cellular responses in vitro to defined malaria antigens and, amongst children with evidence of exposure to hepatitis B infection, between aflatoxin and carriage of the hepatitis B surface antigen (HBsAg), were assessed. Aflatoxin-albumin adduct was found in nearly all serum samples collected during a survey performed at the end of the dry season and levels of adduct were generally high (up to 720 pg aflatoxin-lysine equivalent/mg albumin). Higher levels of aflatoxin-albumin adduct were detected in Wollof children than in children of other ethnic groups and marked variation in mean adduct levels between villages was observed. Aflatoxin-albumin adduct levels were higher in children who were HbsAg positive and in children with Plasmodium falciparum parasitaemia than in controls. However, levels of adduct had no consistent effect on either malaria-specific antibody responses, lymphoproliferative responses in vitro, or morbidity from malaria during the subsequent rainy season. Much lower levels of aflatoxin-albumin adduct were detected in repeat samples obtained at the end of the rainy season. There was poor correlation between dry and rainy season levels of adduct in individual children. We have shown that Gambian children are exposed to high levels of aflatoxin. The seasonal variation of aflatoxin-albumin adduct and marked fluctuation of adduct with time in individual children need to be considered in the future planning of epidemiological studies using this marker of exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Recognition of dominant T cell-stimulating epitopes from the circumsporozoite protein of Plasmodium falciparum and relationship to malaria morbidity in Gambian children

Cellular immune responses to the Plasmodium falciparum circumsporozoite (CS) protein were measured by proliferation and interferon-gamma production in a cohort of children aged 3 to 8 years, living in The Gambia. Anti-CS antibody titres, malariometric indices and sickle cell status were also determined. Malaria morbidity in the ensuing malaria transmission season was monitored by weekly health questionnaire, axillary temperature measurements and examination of blood films. Exposure to malaria was inferred from entomological data collected during the transmission season. Immunological and parasitological measurements were repeated at the end of the rainy season. Immunological findings were compared between children who experienced clinical malaria or asymptomatic infection and children who had no evidence of infection. No association was found between cellular immune responses to the CS protein at the beginning of the transmission season and subsequent susceptibility to infection except among children with high titres of antibody to (NANP)40. Seropositive children who did not become infected had a higher mean proliferative response to the Th3R epitope than seropositive children who did become infected. High titres of anti-(NANP)40 antibodies alone were not protective. Responses to the Th2R epitope were significantly higher at the end of the rainy season than at the beginning in children who experienced an asymptomatic infection. Responses to variant sequences of the 2 epitopes were highly correlated at an individual level but there was no correlation between proliferative and interferon responses to a particular epitope.

Morbidity from malaria and immune responses to defined Plasmodium falciparum antigens in children with sickle cell trait in The Gambia.

Morbidity from Plasmodium falciparum malaria and humoral and in vitro cellular immune responses to defined malaria antigens were measured in rural Gambian children with haemoglobin phenotype AS (HbAS) and in those with a normal haemoglobin (HbAA). In a survey undertaken during the dry season, HbAS children had a higher parasite rate than HbAA children but a lower prevalence of parasitaemia at a level of 500/microliters or greater. Malariometric indices measured during a rainy season survey were similar in the 2 groups of children. During the rainy season, the incidence of infection with P. falciparum did not vary with haemoglobin phenotype. However, in children aged 6 years or less, a significantly smaller proportion of HbAS children who acquired infection developed clinical symptoms than did HbAA children. During both the dry season and rainy season surveys, humoral and in vitro cellular immune responses to defined antigens from the sporozoite and merozoite stages of P. falciparum were similar in the 2 groups of children. Thus, despite the differences in parasite indices and morbidity from malaria between the 2 groups of children, we found no evidence of an enhanced immune response to malaria infection amongst HbAS children compared with normal children.

Association between immune recognition of the malaria vaccine candidate antigen Pf155/RESA and resistance to clinical disease: a prospective study in a malaria-endemic region of West Africa

We have measured cellular and humoral immune responses to short synthetic peptides representing epitopes of the malaria vaccine candidate antigen Pf155/RESA in a longitudinal, prospective study of clinical immunity to Plasmodium falciparum malaria in a cohort of 354 Gambian children aged 3-8 years. A significant association was observed between presence of antibodies to the 3 repeat region peptide (EENV)6 and resistance to clinical malaria. The prevalence of protective antipeptide antibodies varied significantly between different ethnic groups, suggesting that immune recognition of some Pf155/RESA epitopes may be genetically regulated. There was no obvious association between proliferative or interferon gamma responses to T cell epitopes of Pf155/RESA and resistance to malaria infection or disease. At an individual level, the presence of peptide-binding antibodies was associated with the induction of interleukin 4 messenger ribonucleic acid expression in T cells activated with the overlapping T cell epitope EENVEHDA(EENV)2. This suggests that measurement of interleukin 4 production by T cells may represent a functional assay for T helper activity.

Cellular immune responses to Plasmodium falciparum antigens in children receiving long term anti-malarial chemoprophylaxis

Fifty-two Gambian children who had received fortnightly chemoprophylaxis with maloprim, (pyrimethamine and dapsone), and 45 receiving placebo, were studied. Cellular immune responses to malaria antigens, measured by lymphoproliferative responses and interferon production, were higher in children who had received prophylaxis than in controls, although the anti-malarial antibody levels were lower. During a one-year period after termination of prophylaxis, there was no increase in the frequency of clinical episodes of malaria in the children who had received Maloprim. These results suggest that chemoprophylaxis for 3 years may lower malaria antibody levels, but does not interfere with the development of protective immunity, perhaps by enhancing cell-mediated immune responses to malaria in protected children.

Soluble pIasma IL‐2 receptors and maIaria

PIasma levels of soluble IL‐2 receptor (sIL‐2R) were measured by immunoassay in 180 individuals, aged 1–70 years, living in a maIaria‐endemic community in West Africa. slL‐2R levels were compared with age. maIaria parasitacmia. maIaria‐associated morbidity and cellular immune responses to PIasmodium falciparum antigens. PIasma levels ofsIL‐2R were independently associated with both age and patent maIaria parasitacmia. No significant association was observed between IL‐2R levels and concurrent maIaria morbidity (i.e. Fever associated with maIaria), but the number of individuals with clinical maIaria at the time of sampling was small. Although there was no association between pIasma sIL‐2R levels and in vitro proliferativc responses of peripheral blood mononuclear cells (PBMC) to a number of defined maIaria antigens, we did find a significant negative association between sIL‐2R and in vitro proliferation of unslimulatcd PBMC. High levels of sIL‐2R (up to 5500 U ml) were detected in the pIasma of maIaria‐infected individuals: this is indicative of a vigorous cellular immune response to malaria antigens in vivo and does not support the notion that maIaria infections are generally immunosuppressive. Indeed, we found that, at the low levels of parasitacmia present in study subjects, there was no significant difference in the mean prolifcrative response to maIaria antigens in infected subjects when compared with uninfected subjects.

Human leucocyte antigen (HLA) and malaria morbidity in a Gambian community.

Human leucocyte antigen (HLA) class I and class II typing was performed on 177 children in a rural area of The Gambia who were followed for 2 years in a longitudinal study of malaria morbidity. A comparison was made between those who experienced an episode of clinical malaria in one or both years and those who showed no evidence of infection in either year. No convincing association was found between morbidity and class I phenotype. An overall association of morbidity with the distribution of class II haplotypes was seen, but association with individual DR-DQ haplotypes were not conclusive.

Suppression of in–vitro lymphoproliferative responses in acute malaria patients can be partially reversed by indomethacin

Summary In–vitro lymphoproliferative responses to malaria antigens are suppressed in patients with acute Plasmodium falciparum infection. Studies with other parasitic diseases have suggested that monocyte/macrophage–derived prostaglandins may be responsible for immunosuppression. Since acute malaria infection is characteristically associated with fever it is likely that prostaglandin E production will also be enhanced in these patients. In this study, indomethacin, a cyclo–oxygenase inhibitor which blocks the synthesis of prostaglandins, was added to the culture medium during assays of lymphoproliferative responses to malaria antigens and other soluble proteins. Responses to several antigens were enhanced in the presence of indomethacin, indicating that prostaglandins may have a generalized immunosuppressive role in malaria–infected individuals. However, responses to malaria antigens were particularly enhanced by indomethacin, suggesting that malaria–specific T–cells are especially sensitive to the effects of prostaglandin, possibly due to prior activation in vivo by circulating malaria antigens.