Assessment of candidate antigens of Actinobacillus pleuropneumoniae for the serodiagnosis of porcine pleuropneumonia

A 39 kDa outer membrane protein (OMP) was extracted and purified from Actinobacillus pleuropneumoniae serovar 1 (HS54), 7 (WF83) and 12 (HS143). The 39 kDa OMP of all three serovars showed strong immunoreactivity against sera from pigs challenged with A. pleuropneumoniae serovars 1, 7 and 12. Furthermore, the 39 kDa OMP from serovar 1, when used as an ELISA antigen showed strong immunoreactivity against sera from pigs that were infected with various A. pleuropneumoniae serotypes. This result suggests that the 39 kDa OMP may be used as an ELISA antigen to detect A. pleuropneumoniae regardless of the infecting serotype. Actinobacillus pleuropneumoniae is known to produce three members of the repeat in toxin (RTX) family of toxins, which included, Apxl, Apxll and Apxlll. Recently, a fourth RTX toxin has been identified (ApxIV), cloned, and the N-termainal (N-apxIV) and C-terminal (C-apxIV) domains expressed. We were able to purify and assess the immunoreativity of the recombinant ApxIV domains. Recombinant C-apxIV was observed to be more immunoreative than recombinant N-apxIV when used as ELISA antigent. In an ELISA assay involving hemolysin (Apxl) toxin along with the recombinant C-apxIV and N-apxIV, sera from pigs infected with A. pleuropneumoniae recognized all three toxins. However, the immunoreactivity was significantly different for the recombinant ApxIV toxin domains compared to that of Apxl. Apxl showed significantly stronger reactivity at all stages of infection. This result suggests that further work is needed to confirm whether the recombinant ApxIV toxin domains can be used as ELISA reagents in the detection of A. pleuropneumoniae during the course of an infection.