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

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-142) 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-119 invasion inhibition antibodies (IIA) to the E-TSR (3D7) allele and growth inhibitory activity (GIA). The haplotype of MSP-119 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.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-119 IIA showed the least stability over time. However, the level of MSP-119 specific IIA correlated with relatively higher rainfall and higher prevalence of P. falciparum infection with the MSP-119 E-TSR haplotype.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.Individuals living in areas where transmission of Plasmodium falciparum is intense and stable develop naturally acquired immunity that is characterized by a high degree of protection against high-density parasitaemia and clinical illness. This immunity develops as a consequence of experiencing multiple episodes of blood stage infection throughout infancy and childhood, and may be lost, or markedly diminished, in the absence of periodic boosting by clinically asymptomatic blood stage infections during adulthood [1]. Adaptive cellular and humoral immune responses to blood stage malaria antigens may be influenced by the intensity and temporal patte