Fine specificity of anti-MSP119 antibodies and multiplicity of Plasmodium falciparum Merozoite Surface Protein 1 types in individuals in Nigeria with sub-microscopic infection

Malaria Journal Fine specificity of anti-MSP1 antibodies and 19 multiplicity of Plasmodium falciparum Merozoite Surface Protein 1 types in individuals in Nigeria with sub-microscopic infection Josiane Ngoundou-Landji 0 3 Roseangela I Nwuba 0 Chiaka I Anumudu 0 Alexander B Odaibo 0 Wenceslas D Matondo Maya 2 7 Henrietta O Awobode 0 Christian M Okafor 0 4 Olajumoke A Morenikeji 0 Adanze Asinobi 5 Mark Nwagwu 0 Anthony A Holder 1 Francine Ntoumi 6 7 0 Cellular Parasitology Programme, Department of Zoology, University of Ibadan , Nigeria 1 Division of Parasitology, MRC National Institute for Medical Research , The Ridgeway, Mill Hill, London NW7 1AA , UK 2 Unite de Recherches Medicales, Hopital Albert Schweitzer , Lambarene , Gabon 3 Universite des Sciences et Techniques de Masuku , Franceville , Gabon 4 College of Arts and Science, Northwest University , Kirkland WA 98033 , USA 5 College of Medicine, University of Ibadan , Nigeria 6 Fondation Congolaise pour la Recherche Medicale/Universite Marien Ngouabi , Republic of Congo 7 Department of Parasitology, Institute for Tropical Medicine, University of Tubingen , Germany Background: The absence of antibodies specific for the 19 kDa C-terminal domain of merozoite surface protein 1 (MSP119) has been associated with high-density malaria parasitaemia in African populations. The hypothesis that a high prevalence and/or level of anti-MSP119 antibodies that may inhibit erythrocyte invasion would be present in apparently healthy individuals who harbour a sub-microscopic malaria infection was tested in this study. Methods: Plasma samples were collected from residents in a region in Nigeria hyperendemic for malaria, who had no detectable parasitaemia by microscopy. Using a competition-based enzyme-linked-immunosorbent assay with two invasion-inhibitory monoclonal antibodies (mAbs) 12.10 and 12.8, the levels and prevalence of specific antibodies were measured. The minimum multiplicity of infection was determined using PCR. The prevalence of anaemia was also measured. Results: Plasma samples from 85% of individuals contained antibodies that bound to MSP119. The inhibition of mAb 12.10 binding was strongly correlated with the prevalence (Spearman correlation test, p < 0.0001) and mean titre of anti-MSP119 antibodies (Spearman correlation test, p < 0.001) in the samples. Comparing samples from individuals with multiple infection (group M) and single infection (Group S), group M contained a higher (p = 0.04) prevalence of anti-MSP119 antibodies that competed with mAb 12.10. Using a logistic regression model, it was found that the presence of antibodies competitive with mAb 12.10 was affected negatively by anaemia (p = 0.0016) and positively by the carriage of multiple parasite genotypes (p = 0.04). Conclusions: In the search for correlates of protection against malaria, which will be essential to evaluate clinical trials of malaria vaccines based on MSP1, this study examines some potential assays and the factors that need to taken into account during their evaluation, using samples from individuals naturally exposed to malaria infection. - Background Among many Plasmodium falciparum merozoite surface antigens, merozoite surface protein (MSP) 1 has been shown to be one of the major targets of antibodies that inhibit the invasion of red blood cells [1-3]. The protein is present on the merozoite surface as a complex of polypeptides that includes a glycosylphosphatidyl inositol (GPI)-anchored 42 kDa C-terminal fragment (MSP142). During merozoite invasion into the red blood cell MSP142 is processed to yield 33- and 19-kDa fragments (MSP133 and MSP119, respectively). Only the GPI anchored MSP119 remains on the merozoite during erythrocytes invasion [4,5]. Some mouse monoclonal antibodies (mAbs) including 12.8 and 12.10 [6] that inhibit the invasion of red blood cells also inhibit the processing of MSP142 [7]. However, it is still not clear that this activity is a significant contribution to protective immunity acquired following exposure to the parasite [8]. Several immuno-epidemiological studies have yielded conflicting results with regards to the association between anti-MSP119 antibodies and protection against clinical malaria [9-15]. At least one study [16] has indicated that the total anti-MSP119 antibody titre is a poor indicator of malaria immunity, suggesting that antibody fine specificity is very important. It has been proposed that functional assays such as growth inhibition assays [17], inhibition of MSP142 processing [2] or Fc-mediated effector mechanisms [18] may provide a more informative readout to identify useful antibodies. The fine specificity of such functional antibodies may be examined using a numbers of methods including direct binding to antigen or modified antigen [19,20] or competition assays using defined mAbs [21-23]. Using these approaches different classes of antibody have been defined and their epitopes partially mapped; for example MSP142 processing and merozoite invasion inhibitory antibodies, blocking antibodies that block the activity of invasion inhibitory antibodies, and neutral antibodies that have no effect on MSP142 processing and merozoite invasion [2,8,20,24]. MSP119-specific invasion inhibitory activity has been associated with resistance to reinfection in Kenya [25]. However, parasite inhibitory activity is limited to a small subset of total anti-MSP119 antibodies. mAbs 12.8 and 12.10 have been used in several sero-epidemiological studies [16,21,23]. In one study in The Gambia [21], individuals with anti-MSP119 antibodies that compete with mAb 12.10 in a specific ELISA, were significantly less likely to have malaria infections with densities of 1,000 parasites/l. In a study in Uganda, competition with mAb 12.10 was highly correlated with resistance to high-density parasitaemia, but there was no such association with mAb 12.8 [23]. The precise epitope mapping of both mAbs has been reported recently [26,27] and although there is considerable overlap of the two epitopes the data above suggest different functions for the corresponding antibodies [26]. A previous study carried out in the rural area of IgboOra, South-western Nigeria [16] showed no correlation between the level of naturally acquired anti-MSP119 antibodies and inhibition of MSP142 processing in plasma samples from P. falciparum infected children and adults. To test the hypothesis that in apparently healthy individuals who harbour sub-microscopic malaria parasite infections there would be a high prevalence and/or high level of anti-MSP119 antibodies that compete with mAbs 12.8 and 12.10, further investigations in the Igbo-Ora population were carried out. To test this hypothesis, apparently healthy subjects without detectable parasites by thick and thin blood smears within all age groups were recruited and both the total antibody response to recombinant MSP119 and the fine specificity of anti-MSP119 antibodies using a competitive ELI (...truncated)