Pati, Jos Alejandro Guevara; (1998) Antibody responses to, and the structure of Plasmodium falciparum merozoite surface protein-1: A candidate malaria vaccine antigen. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The malaria merozoite expresses a number of surface proteins, one or more of which are thought to mediate the initial interaction between parasite and host cell. Recent work in this laboratory has focused on the proteolytic processing of the Plasmodium falciparum merozoite surface protein-1 (MSP-1). Initially synthesised as a large (approximately 200 kDa) precursor during intracellular merozoite development, MSP-1 is present on the surface of the released merozoite in the form of a multicomponent protein complex derived via proteolytic processing. At some point between merozoite release and completion of erythrocyte invasion, the membrane-bound component (MSP-142) of this surface complex is fiarther cleaved at a single site to form two fragments, MSP-133 and MSP-119. This results in the majority of the complex being shed from the parasite surface, leaving only MSP-119, representing the extreme end of the MSP-1 precursor and comprised of two epidermal growth factor (EGF)-like motifs, to be taken into the invaded cell on the parasite surface. This so-called secondary processing of MSP-1 is conserved across the genus and invariably goes to completion when a merozoite successfully invades a red blood cell, suggesting that it is a necessary step in the invasion pathway. Certain antibodies directed against MSP-119 can prevent erythrocyte invasion by P. falciparum merozoites. In a previous study of a panel of anti-MSP-li9 monoclonal antibodies (mAbs), it was found that those antibodies which most effectively prevent invasion can, upon binding to MSP-1 on the surface of merozoites, completely prevent secondary processing of the molecule. Of those mAbs which do not affect the processing,some can interfere with the processing-inhibitory activity of the first group of antibodies. This second group of antibodies was referred to as blocking antibodies. In the first part of this project the mechanisms involved in, and the significance of antibody-mediated inhibition of MSP-1 processing, were investigated. It was found that both the processing- inhibitory activity and erythrocyte invasion-inhibitory activity of the anti-MSP-119 mAb 12.10 was completely abrogated following removal of its Fc domain, clearly indicating that processing inhibitory antibodies directly interfere with erythrocyte invasion by inhibiting secondary processing of MSP-1. In the second part of the project, an assay which measures antibody-mediated inhibition of MSP-1 processing was used to evaluate individually the ability of a panel of sera from primates vaccinated with recombinant MSP-1 constructs to inhibit the secondary processing of MSP-1. Significant processing inhibitory activity was detected in some primate sera; it was found that these sera corresponded to those animals that were protected against subsequent blood-stage parasite challenge. The data obtained indicate that vaccination with recombinant MSP-1 may induce protection in vivo by inducing processing-inhibitory antibodies, and that an assay which measures antibody-mediated inhibition of MSP-1 processing can be used to effectively predict protective immunity in vivo following immunisation with candidate MSP-1-based vaccines. The third part of this project focused on the phenomenon of blocking antibodies. It was shown that blocking antibodies act by competing with processing-inhibitory mAbs for binding to the merozoite surface. Naturally-acquired human antibodies specific to the terminal domain of MSP-1 were found to be potent blocking antibodies which can completely abolish the activity of invasion-inhibitory antibodies in vitro. The observations reveal a mechanism by which the parasite can avoid the action of a class of protective antibodies, and have important implications for the optimal design, evaluation and administration of MSP-1-based malaria vaccines. In the fourth part of the study, the post-translational modification of MSP-119 was investigated. Lectin binding studies did not detect any glycosylation of native MSP-119. Attempts were made to determine the molecular mass of merozoite-derived MSP-119 and the identity and position of any post-translational modifications by a combination of peptide-mapping and liquid chromatography-electrospray mass spectrometric analysis.

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