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Background  Naturally acquired immunity to blood-stage  Plasmodium falciparum  infection develops with age and after repeated infections. In order to identify immune surrogates that can inform vaccine trials conducted in malaria endemic populations and to better understand the basis of naturally acquired immunity it is important to appreciate the temporal stability of cellular and humoral immune responses to malaria antigens.    Methods  Blood samples from 16 adults living in a malaria holoendemic region of western Kenya were obtained at six time points over the course of 9 months. T cell immunity to the 42 kDa C-terminal fragment of Merozoite Surface Protein-1 (MSP-1 42 ) was determined by IFN-γ ELISPOT. Antibodies to the 42 kDa and 19 kDa C-terminal fragments of MSP-1 were determined by serology and by functional assays that measure MSP-1 19  invasion inhibition antibodies (IIA) to the E-TSR (3D7) allele and growth inhibitory activity (GIA). The haplotype of MSP-1 19  alleles circulating in the population was determined by PCR. The kappa test of agreement was used to determine stability of immunity over the specified time intervals of 3 weeks, 6 weeks, 6 months, and 9 months.    Results  MSP-1 IgG antibodies determined by serology were most consistent over time, followed by MSP-1 specific T cell IFN-γ responses and GIA. MSP-1 19  IIA showed the least stability over time. However, the level of MSP-1 19  specific IIA correlated with relatively higher rainfall and higher prevalence of  P. falciparum  infection with the MSP-1 19  E-TSR haplotype.    Conclusion  Variation in the stability of cellular and humoral immune responses to  P. falciparum  blood stage antigens needs to be considered when interpreting the significance of these measurements as immune endpoints in residents of malaria endemic regions.

Temporal stability of naturally acquired immunity to Merozoite Surface Protein-1 in Kenyan Adults