Immune responses to Plasmodium falciparum antigens during a malaria vaccine trial in Tanzanian children

Among Tanzanian children living in an area of intense and perennial malaria transmission, prevalence of naturally acquired IgG antibodies that recogni ze SPf66, NRNP, p190 and a 29kDa fragment of the merozoite surface protein- 1 (MSP-1) is high and increases with age. This possibly reflects the high l evel of natural exposure of the children to P. falciparum. The prevalences of IgG antibodies that recognize the three putative merozoite derived seque nces contained in the malaria vaccine SPf66 (83.1, 55.1 and 35.1) is low bu t also show some age dependence. Three doses of the SPf66 vaccine induce a strong IgG antibody response against both the SPf66 construct, NANP and the three individual peptides. Vaccination with SPf66 did not result in an inc rease of anti19 kDa fragment antibodies. This reflects the specificity of t he humoral immune response induced by the SPf66 construct. Among vaccinated children, antibody titres against SPf66 decreased over time following the third dose. However, 18 months after the third dose, SPf66 recipients still had significantly higher IgG titres and stimulation indices of peripheral blood mononuclear cells (PBMC) than placebo recipients. Within the vaccine group, there is a trend for increasing anti-SPf66 Ige titre to be associate d with decreasing risk of clinical malaria but this was not statistically s ignificant. Results also show the difficulties of establishing whether anti body responses are related to protection infield trials in endemic areas.