The Levels of CD16/Fcγ Receptor IIIA on CD14+ CD16+ Monocytes Are Higher in Children with Severe Plasmodium falciparum Anemia than in Children with Cerebral or Uncomplicated Malaria▿

Fc gamma receptor IIIA (CD16/FcγRIIIA) on monocytes/macrophages may play an important role in the pathogenesis of severe malarial anemia (SMA) by promoting phagocytosis of IgG-coated uninfected red cells and by allowing the production of tumor necrosis factor alpha (TNF-α) upon cross-linking by immune complexes (ICs). However, not much is known about the differential expression of this receptor on monocytes of children with severe malaria and uncomplicated malaria. Therefore, we investigated the expression of CD16/FcγRIIIA on monocytes of children with SMA, cerebral malaria (CM), and their age-matched uncomplicated malaria controls by flow cytometry. Since CD14low (CD14+) monocytes are considered more mature and macrophage-like than CD14high (CD14++) monocytes, we also compared the level of expression of CD16/FcγRIIIA according to the CD14 level and studied the relationship between CD16/FcγRIIIA expression and intracellular TNF-α production upon stimulation by ICs. CD16/FcγRIIIA expression was the highest overall on CD14+ CD16+ monocytes of children with SMA at enrollment. At convalescence, SMA children were the only ones to show a significant decline in the same parameter. In contrast, there were no significant differences among groups in the expression of CD16/FcγRIIIA on CD14++ CD16+ monocytes. A greater percentage of CD14+ CD16+ monocytes produced TNF-α upon stimulation than any other monocyte subset, and the amount of intracellular TNF-α correlated positively with CD16/FcγRIIIA expression. Furthermore, there was an inverse correlation between hemoglobin levels and CD16/FcγRIIIA expression in children with SMA and their controls. These data suggest that monocytes of children with SMA respond differently to Plasmodium falciparum infection by overexpressing CD16/FcγRIIIA as they mature, which could enhance erythrophagocytosis and TNF-α production

Measurement of Antibody Levels against Region II of the Erythrocyte-Binding Antigen 175 of Plasmodium falciparum in an Area of Malaria Holoendemicity in Western Kenya

Region II of the 175-kDa erythrocyte-binding antigen (EBA-175RII) of Plasmodium falciparum is functionally important in sialic acid-dependent erythrocyte invasion and is considered a prime target for an invasion-blocking vaccine. The objectives of this study were to (i) determine the prevalence of anti-EBA-175RII antibodies in a naturally exposed population, (ii) determine whether naturally acquired antibodies have a functional role by inhibiting binding of EBA-175RII to erythrocytes, and (iii) determine whether antibodies against EBA-175RII correlate with immunity to clinical malaria. We treated 301 lifelong residents of an area of malaria holoendemicity in western Kenya for malaria, monitored them during a high-transmission season, and identified 33 individuals who were asymptomatic despite parasitemia (clinically immune). We also identified 50 clinically susceptible individuals to serve as controls. These 83 individuals were treated and monitored again during the subsequent low-transmission season. Anti-EBA-175RII antibodies were present in 98.7% of the individuals studied. The antibody levels were relatively stable between the beginning and end of the high-transmission season and correlated with the plasma EBA-175RII erythrocyte-binding-inhibitory activity. There was no difference in anti-EBA-175RII levels or plasma EBA-175RII erythrocyte-binding-inhibitory activity between clinically immune and clinically susceptible groups. However, these parameters were higher in nonparasitemic than in parasitemic individuals at enrollment. These results suggest that although antibodies against EBA-175RII may be effective in suppressing some of the wild parasite strains, EBA-175RII is unlikely to be effective as a monovalent vaccine against malaria, perhaps due to allelic heterogeneity and/or presence of sialic acid-independent strains

Comparison of government statistics and demographic surveillance to monitor mortality in children less than five years old in rural western Kenya

Estimates of mortality in children less than five years old using government civil registration statistics (passive surveillance) were compared against statistics generated by active demographic surveillance during a randomized controlled trial of permethrin-treated bed nets (ITNs) in western Kenya. Mortality rates were two-fold lower when estimated through civil registration compared with active prospective surveillance (rate ratio [RR] = 0.51, 95% confidence interval [CI] = 0.44-0.59). While civil registration underestimated deaths, particularly in the neonatal period, the age distribution of deaths in children 1-59 months of age was the same as with active surveillance. Seasonal mortality trends were also similar. There was no agreement between cause of death recorded by active and passive surveillance. Verbal autopsy estimated that half of all deaths were associated with malaria and pneumonia, but civil registration markedly under-reported these illnesses; incidence RR (95% CI) = 0.18 (0.14-0.24), and 0.05 (0.03-0.08), respectively, while over-reporting deaths due to measles (RR = 15.5 [95% CI = 7.3-33.2]). Government statistics under-represent mortality, particularly neonatal mortality, in children less than five years of age in rural areas of Kenya. They can provide accurate information on the age-distribution of deaths among children 1-59 months old, and on seasonal trends, but not on disease-specific mortalit

Association of interferon-gamma responses to pre-erythrocytic stage vaccine candidate antigens of Plasmodium falciparum in young Kenyan children with improved hemoglobin levels: XV. Asembo Bay Cohort Project

Previous studies in animal models have revealed an association between interferon-gamma (IFN-gamma), produced by CD8+ T cells and irradiated sporozoite-induced sterile immunity. To determine whether IFN-gamma can serve as a marker of pre-erythrocytic protective immunity in individuals naturally exposed to malaria, we characterized IFN-gamma and lymphocyte proliferative responses to previously defined CD8+ cytotoxic T lymphocyte (CTL) epitopes from six pre-erythrocytic stage antigens in 107 children six months to two years old from a community-based birth cohort in western Kenya. We found that IFN-gamma positive responders had higher hemoglobin (Hb) levels and significantly reduced prevalence of severe malarial anemia one month after the test compared with IFN-gamma non-responders, suggesting that IFN-gamma immune responses to these pre-erythrocytic antigens were associated with protection against malarial anemia. Children who responded by lymphocyte proliferation had a significantly longer time to first documented malaria parasitemia after birth; however, there was no correlation between the presence of lymphocyte proliferative response and higher Hb levels. We propose that IFN-gamma production could be used as a potential marker of protective immunity against malaria associated anemia in young children living in malaria holoendemic area

Effects of permethrin-treated bed nets on immunity to malaria in western Kenya I. Antibody responses in pregnant women and cord blood in an area of intense malaria transmission

As part of a community-based group-randomized trial on the impact of permethrin-treated bed nets (ITNs) on malaria in pregnancy in a holoendemic area of western Kenya, we assessed their effects on antibody responses to Plasmodium falciparum pre-erythrocytic antigens (recombinant circumsporozoite protein [CSP] and peptides complimentary to the repeat region of the liver stage antigen-1 [LSA-1]) and blood stage antigen (recombinant C-terminal domain of the merozoite surface protein-1 [MSP-1(19) kD]) in paired maternal/cord plasma samples obtained from 296 deliveries (157 from ITN villages and 139 control villages). Levels of total IgG and IgG subclasses 1-3 to LSA-1 and total IgG and IgG3 to MSP-1 were lower, whereas those of total IgG to CSP were significantly higher in women from ITN villages than those from control villages. In cord plasma, levels of total IgG and IgG2 to LSA-1 and IgG3 to MSP-1 were lower in ITN villages than in control villages, but antibody responses to CSP were similar. Our results suggest that the use of ITNs decreases antibody responses to LSA-1 and MSP-1 antigens in pregnant women with associated reductions in levels of the same antibodies in cord blood. In contrast, ITN use was found to be associated with increased antibody responses to CSP in pregnant women, but had no effect on antibody levels to CSP in cord bloo

Effects of permethrin-treated bed nets on immunity to malaria in western Kenya II. Antibody responses in young children in an area of intense malaria transmission

As part of a large community-based trial on the impact of insecticide (permethrin)-treated bed nets (ITNs) on childhood morbidity and mortality in an area of intense perennial malaria transmission in western Kenya, we assessed the effects of ITNs on malaria-specific humoral responses in young children. The IgG responses to Plasmodium falciparum pre-erythrocytic antigens circumsporozoite protein (CSP) and liver stage antigen-1 (LSA-1) and the blood stage antigen merozoite surface protein-1 (MSP-1(19) kD) in children less than three years old were investigated during a series of cross-sectional surveys. At 14 and 22 months after the introduction of ITNs, the frequencies and levels of IgG to CSP and LSA-1 were significantly lower in children from ITN villages than in children from control villages (P < 0.001). In contrast, the prevalence of IgG to MSP-1 was significantly higher in children from ITN villages at 14 months (P = 0.0069), but not at 22 months. Our results show that decreased exposure by ITNs reduces IgG responses to pre-erythrocytic antigens, but there was no evidence that two years of ITN use compromises IgG responses to blood stage antigens in these young children in this malaria holoendemic are