Immunological detection of Plasmodium knowlesi antigens after electrophoretic transfer from SDS-polyacrylamide gels to diazophenylthioether paper

The Identification of Native Epitopes Eliciting a Protective High-Affinity Immunoglobulin Subclass Response to Blood Stages of Plasmodium falciparum: Protocol for Observational Studies

Background

Antibodies to blood stages protective against complications of Plasmodium falciparum infection were found to be of immunoglobulin G 1 (IgG1) and IgG3 subclasses and of high affinity to the target epitopes. These target epitopes cannot be characterized using recombinant antigens because of a lack of appropriate glycosylation, phosphorylation, methylation, and bisulfide bond formation, which determine the structure of conformational and nonlinear epitopes within the tertiary and quaternary structures of native P. falciparum antigens.

Objective

This study aims to develop a method for the comprehensive detection of all P. falciparum schizont antigens, eliciting a protective immune response.

Methods

Purified parasitophorous vacuole membrane–enclosed merozoite structures (PEMSs) containing native schizont antigens are initially generated, separated by two-dimensional (2D) gel electrophoresis and blotted onto nitrocellulose. Antigens eliciting a protective antibody response are visualized by incubation with sera from patients with clinical immunity. This is followed by the elution of low-affinity antibodies with urea and detection of protective antibody responses by incubation with anti-IgG1 and anti-IgG3 antibodies, which were conjugated to horseradish peroxidase. This is followed by visualization with a color reaction. Blot signals are normalized by relating to the intensity of blot staining with a reference antibody and housekeeping antigens. Results are corrected for intensity of exposure by the relation of antibody responses to global P. falciparum antibody titers. Antigens eliciting the protective responses are identified as immunorelevant from the comparison of spot positions, indicating high-affinity IgG1 or IgG3 responses on the western blot, which is unique to or consistently more intensive in clinically immune individuals compared with nonimmune individuals. The results obtained are validated by using affinity chromatography.

Results

Another group previously applied 2D western blotting to analyze antibody responses to P. falciparum. The sera of patients allowed the detection of 42 antigenic spots on the 2D immunoblot. The spots detected were excised and subjected to mass spectrometry for identification. A total of 19 protein spots were successfully identified and corresponded to 13 distinct proteins. Another group used immunoaffinity chromatography to identify antigens bound by IgGs produced by mice with enhanced immunity to Plasmodium yoelii. Immunorelevant antigens were isolated and identified by immobilizing immunoglobulin from immune mice to a Sephadex column and then passing a blood-stage antigen mixture through the column followed by the elution of specific bound antigens with sodium deoxycholate and the identification of those antigens by western blotting with specific antibodies.

Conclusions

2D western blotting using native antigens has the potential to identify antibody responses selective for specific defined isomeric forms of the same protein, including isoforms (protein species) generated by posttranscriptional modifications such as phosphorylation, glycosylation, and methylation. The process involved in 2D western blotting enables highly sensitive detection, high resolution, and preservation of antibody responses during blotting. Validation by immunoaffinity chromatography can compensate for the antigen loss associated with the blotting process. It has the potential for indirect quantification of protective antibody responses by enabling quantification of the amount of eluted antibody bound antigens through mass spectrometry.

International Registered Report Identifier (IRRID)

PRR1-10.2196/15690

Structural studies on a putative protective Plasmodium knowlesi merozoite antigen

Two monoclonal antibodies, which prevent merozoites attaching to or invading erythrocytes, react with the same or closely apposed epitopes on a minor 66 kDa Plasmodium knowlesi antigen. The antigen is processed, at the time of schizont rupture and merozoite release, to 44 and 42 kDa molecules which are present on the merozoite surface [Deans, J. A. et al. (1984) Mol. Biochem. Parasitol. 11, 189-204]. The monoclonal antibody-defined epitope, which is expressed only once on the 66 kDa molecule, is formed by a tertiary folding of the polypeptide chain (minimum size 42 kDa). The conformation of the epitope is maintained by weak intramolecular forces of attraction, rendering the epitope extremely labile; it is completely destroyed by treatment with sodium dodecyl sulphate (SDS). Polyclonal monospecific antiserum raised against SDS-treated antigen did not inhibit parasite proliferation whereas a polyclonal antiserum raised against native antigen was inhibitory. It is postulated that the monoclonal antibody-defined antigenic determinant is crucial for merozoite invasion.

Studies of murine malaria antigens using monoclonal antibodies. Production, selection, and characterization of antibodies

A panel of ten monoclonal antibodies made against Plasmodium chabaudi and Plasmodium yoelii infected mouse erythrocytes were used for characterization of antigens present in murine malaria. Screening of the antibodies in ELISA with different fractions of infected erythrocytes revealed both species-specific and fraction-specific monoclonal antibodies (MAbs), but also MAbs cross-reacting between the species. Two MAbs bound normal erythrocyte components. Subcellular localization of the target antigens was studied by immunofluorescence and their molecular identity by immunoblotting after SDS-PAGE. Of the MAbs to P. yoelii, one reacted with a cytoplasmic granule component of 137 k and two others reacted with vacuole-associated antigens of 26 k and 25/70/73 k, respectively. The latter antibodies cross-reacted with P. chabaudi antigens. Of the MAbs to P. chabaudi, all were species specific, one reacting with parasite surface antigens of 79 and 250 k and two with a vacuole-associated antigen of 70 k.