Bartholomew D. Akanmori

Low plasma concentrations of interleukin 10 in severe malarial anaemia compared with cerebral and uncomplicated malaria

BACKGROUND Severe anaemia is a major complication of malaria but little is known about its pathogenesis. Experimental models have implicated tumour necrosis factor (TNF) in induction of bone-marrow suppression and eythrophagocytosis. Conversely, interleukin 10 (IL-10), which mediates feed-back regulation of TNF, stimulates bone-marrow function in vitro and counteracts anaemia in mice. We investigated the associations of these cytokines with malarial anaemia. METHODS We enrolled 175 African children with malaria into two studies in 1995 and 1996. In the first study, children were classified as having severe anaemia (n=10), uncomplicated malaria (n=26), or cerebral anaemia (n=41). In the second study, patients were classified as having cerebral malaria (n=33) or being fully conscious (n=65), and the two groups were subdivided by measured haemoglobin as normal (>110 g/L), moderate anaemia (60-90 g/L), and severe anaemia (<50 g/L). IL-10 and TNF concentrations were measured by ELISA in plasma samples from all patients. FINDINGS IL-10 concentrations were significantly lower in patients with severe anaemia than in all other groups. In 1995, geometric mean plasma IL-10 in patients with severe anaemia was 270 pg/mL (95% CI 152-482) compared with 725 pg/mL (465-1129) in uncomplicated malaria and 966 pg/mL (612-1526) in cerebral malaria (p<0.03). In 1996, fully conscious patients with severe anaemia also had significantly lower IL-10 concentrations than all other groups, including cerebral-malaria patients with severe anaemia and all patients with moderate anaemia (p<0.001). In both studies, TNF concentrations were significantly higher in cerebral malaria than in fully conscious patients (p<0.01). By contrast, the ratio of TNF to IL-10 was significantly higher in fully conscious patients with severe anaemia than in all other groups (p<0.001). INTERPRETATION Our findings identify severe malarial anaemia as a distinct disorder in which insufficient IL-10 response to high TNF concentrations may have a central role.

Plasma Antibodies from Malaria-Exposed Pregnant Women Recognize Variant Surface Antigens on Plasmodium falciparum-Infected Erythrocytes in a Parity-Dependent Manner and Block Parasite Adhesion to Chondroitin Sulfate A

In areas of intense Plasmodium falciparum transmission, clinical immunity is acquired during childhood, and adults enjoy substantial protection against malaria. An exception to this rule is pregnant women, in whom malaria is both more prevalent and severe than in nonpregnant women. Pregnancy-associated malaria (PAM) in endemic areas is concentrated in the first few pregnancies, indicating that protective immunity to PAM is a function of parity. The placenta is often heavily infected in PAM, and placental parasites show a striking preference for chondroitin sulfate A (CSA) as an adhesion receptor. Plasma Abs from malaria-exposed multiparous women are able to interfere with binding of P. falciparum parasites to CSA in vitro, and acquisition of Abs interfering with CSA-specific parasite sequestration thus appears to be a critical element in acquired protection against PAM. Here we show that adults from an area of hyperendemic P. falciparum transmission generally possessed low levels of Abs specifically recognizing surface Ags expressed by a CSA-adhering parasite isolate, while unselected isolates were well recognized. In marked contrast, most third-trimester pregnant women from that area had very high plasma levels of such Abs. Plasma levels of Abs specifically recognizing the CSA-adhering isolate strongly depended on parity, whereas recognition of CSA-nonadhering isolates did not. Finally, we demonstrate a clear correlation between plasma levels of Abs recognizing the CSA-specific isolate and the ability to interfere with its sequestration to CSA in vitro. Our study supports the hypothesis that Abs inhibiting CSA-specific parasite sequestration are important in acquisition of protection against PAM.

Absolute Levels and Ratios of Proinflammatory and Anti-inflammatory Cytokine Production In Vitro Predict Clinical Immunity to Plasmodium falciparum Malaria

The relationship between malaria-related outcomes and cytokine production in whole blood cultures associated with cellular immune responses and immunity to Plasmodium falciparum malaria was examined in a study in southern Ghana. Production of malaria-specific interferon (IFN)-gamma was associated with reduced risk of fever and clinical malaria. Protective IFN-gamma responses were induced by live schizonts but not by dead parasites. Production of malaria-specific tumor necrosis factor (TNF)-alpha was associated with reduced risk of fever during follow-up. Baseline levels of TNF-alpha and phytohemagglutinin (PHA)-induced interleukin (IL)-10 were positively associated with hemoglobin concentration. IL-12 production was associated with reduced risk of parasitemia. PHA-induced transforming growth factor-beta production was associated with reduced risk of fever during follow-up. High ratios of proinflammatory to anti-inflammatory cytokines were associated with increased risk of fever and higher hemoglobin concentrations. Thus, absolute levels and ratios of proinflammatory and anti-inflammatory cytokines influence susceptibility to infection, clinical disease, and anemia. These data contradict data from cross-sectional clinical studies and indicate a need for detailed analysis of the relationship between cellular immunity to malaria and resistance to disease.

Plasmodium falciparum Variant Surface Antigen Expression Varies Between Isolates Causing Severe and Nonsevere Malaria and Is Modified by Acquired Immunity

In areas of endemic parasite transmission, protective immunity to Plasmodium falciparum malaria is acquired over several years with numerous disease episodes. Acquisition of Abs to parasite-encoded variant surface Ags (VSA) on the infected erythrocyte membrane is important in the development of immunity, as disease-causing parasites appear to be those not controlled by preexisting VSA-specific Abs. In this work we report that VSA expressed by parasites from young Ghanaian children with P. falciparum malaria were commonly and strongly recognized by plasma Abs from healthy children in the same area, whereas recognition of VSA expressed by parasites from older children was weaker and less frequent. Independent of this, parasites isolated from children with severe malaria (cerebral malaria and severe anemia) were better recognized by VSA-specific plasma Abs than parasites obtained from children with nonsevere disease. This was not due to a higher infection multiplicity in younger patients or in patients with severe disease. Our data suggest that acquisition of VSA-specific Ab responses gradually restricts the VSA repertoire that is compatible with parasite survival in the semi-immune host. This appears to limit the risk of severe disease by discriminating against the expression of VSA likely to cause life-threatening complications, such as cerebral malaria and severe anemia. Such VSA seem to be preferred by parasites infecting a nonimmune host, suggesting that VSA expression and switching are not random, and that the VSA expression pattern is modulated by immunity. This opens the possibility of developing morbidity-reducing vaccines targeting a limited subset of common and particularly virulent VSA.

Malaria-Induced Acquisition of Antibodies to Plasmodium falciparum Variant Surface Antigens

ABSTRACT In areas of intense Plasmodium falciparum transmission, protective immunity is acquired during childhood in parallel with acquisition of agglutinating antibodies to parasite-encoded variant surface antigens (VSA) expressed on parasitized red blood cells. In a semi-immune child in such an area, clinical disease is caused mainly by parasites expressing VSA not recognized by preexisting VSA-specific antibodies in that child. Such malaria episodes are known to cause an increase in agglutinating antibodies specifically recognizing VSA expressed by the parasite isolate causing the illness, whereas antibody responses to other parasite isolates are relatively unaffected. However, the detailed kinetics of this VSA antibody acquisition are unknown and hence were the aim of this study. We show that P. falciparum malaria in Ghanaian children generally caused a rapid and sustained increase in variant-specific VSA antibody levels, while more transient and limited increases in levels of antibodies to VSA expressed by other parasite isolates were also seen. Plasma VSA antibody levels were positively correlated with the age of the healthy plasma donors but negatively correlated with the age of the parasite donors (the malaria patient). The data from this first detailed longitudinal study of acquisition of VSA antibodies support the hypothesis that naturally acquired protective immunity to P. falciparum malaria is mediated, at least in part, by VSA-specific antibodies.

Cohort study of the association of antibody levels to AMA1, MSP119, MSP3 and GLURP with protection from clinical malaria in Ghanaian children

BackgroundAntigen-specific antibody-mediated immune responses play an important role in natural protection against clinical malaria, but conflicting estimates of this association have emerged from immuno-epidemiological studies in different geographical settings. This study was aimed at assessing in a standardized manner the relationship between the antibody responses to four malaria vaccine candidate antigens and protection from clinical malaria, in a cohort of Ghanaian children.MethodsStandardized ELISA protocols were used to measure isotype and IgG subclass levels to Apical Membrane Antigen 1 (AMA1), Merozoite Surface Protein 1–19 (MSP119), Merozoite Surface Protein 3 (MSP3) and Glutamate Rich Protein (GLURP) antigens in plasma samples from 352 Ghanaian children, aged three to 10 years with subsequent malaria surveillance for nine months. This is one of a series of studies in different epidemiological settings using the same standardized ELISA protocols to permit comparisons of results from different laboratories.ResultsThe incidence rate of malaria was 0.35 episodes per child per year. Isotype and IgG subclasses for all antigens investigated increased with age, while the risk of malaria decreased with age. After adjusting for age, higher levels of IgG to GLURP, MSP119, MSP3 and IgM to MSP119, MSP3 and AMA1 were associated with decreased malaria incidence. Of the IgG subclasses, only IgG1 to MSP119 was associated with reduced incidence of clinical malaria. A previous study in the same location failed to find an association of antibodies to MSP119 with clinical malaria. The disagreement may be due to differences in reagents, ELISA and analytical procedures used in the two studies. When IgG, IgM and IgG subclass levels for all four antigens were included in a combined model, only IgG1 [(0.80 (0.67–0.97), p = 0.018)] and IgM [(0.48 (0.32–0.72), p < 0.001)] to MSP119 were independently associated with protection from malaria.ConclusionUsing standardized procedures, the study has confirmed the importance of antibodies to MSP119 in reducing the risk of clinical malaria in Ghanaian children, thus substantiating its potential as a malaria vaccine candidate.

Antibodies to Variant Antigens on the Surfaces of Infected Erythrocytes Are Associated with Protection from Malaria in Ghanaian Children

ABSTRACT Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a variant antigen expressed on the surface of infected erythrocytes. Each parasite genome contains about 40 PfEMP1 genes, but only 1 PfEMP1 gene is expressed at a given time. PfEMP1 serves as a parasite-sequestering ligand to endothelial cells and enables the parasites to avoid splenic passage. PfEMP1 antibodies may protect from disease by inhibiting sequestration, thus facilitating the destruction of infected erythrocytes in the spleen. In this study, we have measured antibodies in Ghanaian children to a conserved region of PfEMP1 by enzyme-linked immunosorbent assay and antibodies to variant molecules on erythrocytes infected with field isolates of P. falciparum by flow cytometry. Based on close clinical monitoring, the children were grouped into those who did (susceptible) and those who did not (protected) have malaria during the season. The prevalences of antibodies to both the conserved PfEMP1 peptide and the variant epitopes were greater than 50%, and the levels of immunoglobulin G (IgG) correlated with age. The levels of antibodies to both the conserved peptide and the variant epitopes were higher in protected than in susceptible children. After correcting for the effect of age, the levels of IgG to variant antigens on a Sudanese and a Ghanaian parasite isolate remained significantly higher in protected than in susceptible children. Thus, the levels of IgG to variant antigens expressed on the surface of infected erythrocytes correlated with protection from clinical malaria. In contrast, the levels of IgG to a peptide derived from a conserved part of PfEMP1 did not correlate with protection from malaria.

Do maternally acquired antibodies protect infants from malaria infection

Neonates and infants are relatively protected from clinical malaria, but the mechanism of this protection is not well understood. Maternally derived antibodies are commonly believed to provide protection against many infectious diseases, including malaria, for periods of up to 6–9 months but several recent epidemiological studies have produced conflicting results regarding a protective role of passively acquired antimalarial antibodies. In this article, we review the epidemiological evidence for resistance of young infants to malaria, summarize the data on antimalarial antibody levels and specificity and their association with protection from malaria infection or clinical disease, and explore alternative explanations for resistance to malaria in infants.

Naturally Acquired Antibodies to the Glutamate-Rich Protein Are Associated with Protection against Plasmodium falciparum Malaria

The development of effective malaria vaccines depends on the identification of targets of well-defined protective immune responses. Data and samples from a longitudinal study of a cohort of children from coastal Ghana were used to investigate the role of antibody responses to 3 regions of the Plasmodium falciparum glutamate-rich protein (GLURP). The data show that levels of the GLURP-specific IgG that occurs in the nonrepeat region of the antigen are significantly correlated with clinical protection from P. falciparum malaria, after correction for the confounding effect of age. Furthermore, levels of cytophilic antibodies were found to be of particular importance for protection, lending support to the hypothesis that antibody-dependent cellular inhibition is the important element in GLURP-specific protective immunity.

Lack of association between maternal antibody and protection of African infants from malaria infection.

ABSTRACT Maternally derived antibodies are believed to protect infants against infection, but there is little direct evidence for a protective role of passively acquired antibodies against malaria. A longitudinal study of malaria infection in 143 infants was conducted in a region of southern Ghana where Plasmodium falciparum is endemic. Infants born in the high-transmission season were less likely to become infected in the first 20 weeks of life than children born in the low-transmission season. Plasma, obtained at birth, was tested for immunoglobulin G (IgG) and IgG subclasses to P. falciparumschizonts and recombinant circumsporozoite antigen, MSP-119, MSP-2, AMA-1, and Pf155 (also called ring-infected erytrocyte surface antigen). Antibody levels at birth were not associated with resistance to malaria infection. On the contrary, antibodies at birth were positively associated with infection, indicating that high levels of maternally derived antibodies represent a marker for intensity of exposure to malaria infection in infants. However, all five children who experienced high-density infections (>100 parasites/μl of blood) were seronegative for MSP-119 at the time of infection.