Synthesis and characterization of latex-protein complexes from different antigens of Toxoplasma gondii for immunoagglutination assays

The acute phase recombinant protein of Toxoplasma gondii P22Ag was expressed and purified and the homogenate of the parasite was obtained from an infected mouse. These antigens were used to produce latex-protein complexes (LPC) through physical adsorption and chemical coupling onto different latexes, with the aim of producing immunoagglutination (IA) reagents able to detect recently acquired toxoplasmosis. Polystyrene and core-shell latexes where employed, exhibiting varied particle size, functionality (carboxyl or epoxy), and charge density. In sensitization experiments for producing LPC, the recombinant protein showed better coupling efficiency onto the particles surface than the homogenate and this could be explained by the complex mixture of the homogenate, which includes a large number of proteins of different molecular mass, isoelectric points, and hydrophobicity. The synthesized LPC were employed in IA assays. To this effect, the agglutination reaction was followed by measuring the changes in the optical absorbance by turbidimetry. Experiments against control sera were performed to evaluate the performance of various LPC and it was observed that the IA test based on P22Ag and the carboxylated latex of 350 nm of particle diameter allowed a good discrimination between acute sera and chronic/negative ones. The proposed test is cheap, rapid, and easy to implement.