MALARIA PARASITAEMIA AND HUMORAL IMMUNE RESPONSES TO SOME DEFINED PLASMODIUM FALCIPARUM ANTIGENS IN NEWBORNS, INFANTS AND ADULT NIGERIANS
A cohort of mothers and then newborns at Igbo-Ora, Oyo State was studied longitudinally for 12 months to determine the incidence of malaria parasitaemia, episodes of clinical malaria and their humoral immune response to malaria infection. Cross-sectional studies were also performed on adults at the Government Technical College, Igbo-Ora and blood donors at the University College Hospital, Ibadan during the rainy and dry seasons. Peripheral and cord blood samples were collected from 116 women at delivery and maternal-newborn malariometric indices were recorded. Infants were monitored fortnightly to detect episodes of clinical malaria and serial blood samples were collected at bi-monthly clinics. Blood samples were collected from 100 volunteers at the G.T.C. Igbo-Ora in July, 1991 and 33 of these volunteers in February, 1992; 224 blood donors at the U.C.H., Ibadan between October and November, 1991 and in 192 donors in March, 1992. Immunological assays included single radial immunodiffusion assay for IgG, IgM and IgA; immunofluorescence assay for antibodies to total blood stage antigens; erythrocyte membrane immunofluorescence (EMIF) assay to detect antibodies to the Pfl55/RESA; and an enzyme-linked immunosorbent assay (ELISA) for antibodies to four synthetic peptides. Malaria parasites were detected in 2.5% of cord blood samples and in 22.4% of the parturient women. The malaria parasite rates and densities of the study infants increased significantly with increasing age. Parasite rates at the July and February surveys at the G.T.C. were similar (P>0.50) while parasite density was higher (P<0.01) at the July survey. The parasite rate of blood donors at the October-November survey was higher (P<0.001) than at the March survey while parasite density in March was higher (P<0.001) than at the October-November survey. Cord blood IgG was significantly lower than maternal IgG levels and a correlation was observed between cord and maternal IgG but not IgM levels. During the first year of life, IgM but not IgG and IgA was significantly higher in malaria positive infants compared with negative infants. Antibodies to total blood stage antigens were detected in all sera tested. Malaria-specific IgM was detected in 5.8% of cord blood samples. There was a correlation between maternal and cord blood antibody titres to the Pfl55/RESA (P<0.001) antigen. In addition a correlation was obtained between maternal and cord blood ELISA (OD405) values to the (EENV)6, LJ5 and MAP2 peptides but not (NANP)6 peptide. There was no correlation between cord blood IgG, IgM, anti- Pfl55 antibody titres, ELISA (OD405) values to the (EENV)6, (NANP)6, U5 and MAP2 peptides and duration of onset of malaria in the infant. Cord blood seropositivity for antibodies to the Pfl55/RESA and (NANP)6 antigens or (EENV)6 and (NANP)6 peptides did not influence age of onset of clinical malaria. However, infants with haemoglobin AS whose cord blood was seropositive for antibodies to the Pfl55/RESA and (NANP)6 antigens or (EENV)6 and (NANP)6 peptides showed delayed onset of clinical malaria compared with AA infants. In adults, anti-Pfl55 antibody titres and ELISA seroreactivities to the (EENV)6, LJ5 and MAP2 peptides showed a wide variation and individual levels were similar on consecutive surveys. Seroreactivity to the (NANP)6, was higher at the end of the rainy season than at the end of the dry season. The presence and level of antibodies to the Pfl55/RESA, (EENV)6, (NANP)6, U5 and MAP2 antigens did not influence the presence and density of malaria parasites. Parasitological data in infants suggest some relative protection within the first 2-3 months of life. However, maternally acquired antibodies alone may not be responsible for this observation. The presence of malaria-specific IgM in cord blood suggest intrauterine sensitization of the foetus by malarial antigens. Although no relationship was observed between malarial antibody levels and parasite rates/densities in the adult subjects, these antibodies may still play a role in immune protection against malaria.