Pathogenesis of Actinobacillus pleuropneumoniae : role of toxins and fimbriae

Abstract

Actinobacillus pleuropneumoniae causes porcine pleuropneumonia, a disease that occurs world-wide and affects growing pigs of all ages. Infection of pigs with A. pleuropneumoniae can result in high morbidity and mortality. The present work contributes to the understanding of the pathogenesis of A. pleuropneumoniae by providing important novel insights into several major virulence traits of the pathogen. The RTX toxins ApxI, ApxII and ApxIII are essential for the development of clinical symptoms and lesions typical for A. pleuropneumoniae infections. However, most serotypes of A. pleuropneumoniae produce in vivo more than one toxin. To assess the relative contribution of ApxI and ApxII to the infection, the ability of different Apx mutants of A. pleuropneumoniae serotype 1 to induce lesions was investigated. The results indicate that the production of more than one toxin enhances virulence of A. pleuropneumoniae. Adherence to host tissue is regarded as an important first step in the colonisation of the host. In this thesis, we describe an in vitro infection model to study the interaction of A. pleuropneumoniae with primary cultures of porcine lung epithelial cells [LEC]. The efficient binding of A. pleuropneumoniae to LEC indicates that adherence to epithelial cells in the lower respiratory tract may constitute an important step in the pathogenesis. In search for the mechanism of the adherence event, the role of LPS was investigated. On the basis of the results with LEC, LPS seems not to be involved in adherence of A. pleuropneumoniae to cells of the lower respiratory tract. In an attempt to identify factors involved in adherence of A. pleuropneumoniae, we focussed on fimbriae. Fimbriae mediate adherence in a number of species and have been isolated recently from A. pleuropneumoniae. The genome of A. pleuropneumoniae was found to contain at least four genes involved in type IV fimbriae biogenesis. The functionality of the fimbriae genes was demonstrated by expression of these genes from a constitutively active promoter, which resulted in fimbrial subunits and in fimbriae protruding from the bacterial cell surface. The involvement of fimbriae in adherence to LEC could not be demonstrated. Noteworthy is the presence of an alanine in contrast to the consensus glycine at position -1 from the cleavage site of the fimbrial subunit protein ApfA. This appears to be an intrinsic trait of ApfA. The apparent lack of fimbriae expression under routine laboratory growth conditions led us to investigate the conditions necessary for fimbriae promoter activity using a promoter trap vector. The fimbriae promoter appears to be intact but is subject to regulation. Expression of the fimbriae promoter was dependent on the growth phase, was seen only in chemically defined medium and was strongly induced in A. pleuropneumoniae that were adhering to primary cultures of LEC. The latter suggests that contact with epithelial cells may be a trigger for fimbriae production. The importance of a more natural setting for the induction of fimbriae promoter activity was further underlined by our finding that the fimbriae promoter was active in vivo after endobronchial inoculation of pigs.