Cross-protective efficacy of recombinant transferrin-binding protein A of Haemophilus parasuis in guinea pigs.

The causative agent of Glasser's disease in swine is Haemophilus parasuis. Commercial bacterins are widely used for protection of the swine population. However, cross protection is limited because H. parasuis has more than 15 serovars. Transferrin-binding protein A has shown potential as a broad-spectrum vaccine candidate against homologous and heterologous strains. Here we amplified the full-length tbpA gene from an H. parasuis serovar 13 isolate and cloned it into a pET-SUMO expression vector. We then expressed and purified the TbpA protein by Ni affinity chromatography. First, the immunogenicity and protective efficacy of the protein were evaluated in guinea pigs by two subcutaneous immunizations with different doses of Montanide IMS 206 VG adjuvant. The immunized guinea pigs were, respectively, challenged on week 3 after a booster immunization with homologous strain LJ3 (serovar 13) and heterologous strain FX1 (serovar 4), and vaccine-inoculated groups were compared with nonvaccinated controls. All immunized groups showed serum antibody titers higher than those of negative-control groups. Furthermore, the cytokine and chemokine levels were evaluated at the transcriptional level by the real-time PCR analysis of six cytokines and chemokines. Gamma interferon and interleukin-5 in groups immunized with 100 μg were elevated more than 15-fold over those in negative-control groups. The protection rates were 80 and 60% after a challenge with strains LJ3 and FX1, respectively, in the groups vaccinated with 100 μg of recombinant TbpA protein. Subsequently, the data showed that guinea pigs immunized with a single dose (100 μg) were protected at levels of 80, 80, and 60% against LJ3, FX1, and another heterologous strain, SZ (serovar 14), respectively. The results indicate for the first time that TbpA protein cross protects guinea pigs against serovars 13, 4, and 14 of H. parasuis. Taken together, these results suggest that the recombinant TbpA protein is a promising vaccine candidate that needs to be confirmed in a swine population.