Preparation and characteristic of typhoid Vi conjugate vaccine
Objective To prepare a typhoid Vi conjugate vaccine and investigate its characteristics. Methods Recombinant exoprotein A( rEPA) of Pseudomonas aeruginosa was derived with adipic acid dihydrazide (ADH) and bound to the capsular polysaccharide of Salmonella typhi (Vi). The conjugates were analyzed with proper chemical and biological methods. Immunogenicity of the conjugates was tested in particular in mice model. Results The conjugates were much larger in size than polysaccharide and carrier protein alone. The Vi/protein ratio (wt/wt) was 1.2 and more than 70% of Vi stayed in conjugate form. Analysis of HPLC(TSK5000) showed that the conjugates with Kd(partition coefficient)≤0.2 occupied 94% . Endotoxin content, anaphylactoid test, abnormal toxicity test in guinea pigs and mice gave evidence of the safety of the conjugates. Double-immunodiffusion demonstrated that both Vi and rEPA in the conjugates kept intact immunogenicity. Immunogenicity test in mice showed that the conjugates could induce significant responses, and elicit boosted responses while Vi not. Conclusion Vi conjugate vaccine was successfully developed. It was safe and got strong immunogenicity in mice.