Localization of linear cytotoxic and pro-apoptotic epitopes in RTX toxin ApxIII of Actinobacillus pleuropneumoniae.

RTX toxin ApxIII secreted by Actinobacillus pleuropneumoniae affects porcine pulmonary alveolar macrophages and neutrophils. The distribution of ApxIII linear cytotoxic and pro-apoptotic determinants was characterized by neutrophil protection assay, DNA fragmentation and caspase-3 activity using antisera produced against its N-terminus, hydrophobic domain, activation domain, calcium-binding domain and C-terminal half. Neutralization of ApxIII cytotoxicity and pro-apoptotic activities was found in antisera raised against its N- and C-termini whereas ApxIII activation domain-specific antiserum expressed cytotoxic-neutralizing activity. No neutralizing activities were detected in antisera against other structural domains of ApxIII suggesting that the putative linear cytotoxic and pro-apoptotic determinants of ApxIII were mapped to its terminal domains and activation domains.