Actinobacillus pleuropneumoniae: molecular aspects of virulence and pulmonary injury.

Contributions made by several laboratories in the area of Actinobacillus pleuropneumoniae virulence and its relationship to pulmonary disease will be reviewed briefly. Lung injury and subsequent disease, after infection with A. pleuropneumoniae, can be related to various bacterial toxins and host factors. Similar to other gram-negative bacteria. A. pleuropneumoniae has cell wall lipopolysaccharides which have been incriminated in a wide variety of toxic and tissue damaging processes. Virulent isolates of A. pleuropneumoniae have been shown to have a thick capsule whereas some avirulent isolates have a thin and easily removed capsule. The capsule of A. pleuropneumoniae is a linear unbranched polysaccharide composed of repeating dissaccharide subunits that bestow antiphagocytic properties to the bacterium but are also immunogenic. In addition, A. pleuropneumoniae has several chemically defined exotoxins. These toxins have generally been shown to be proteinaceous molecules that are hemolytic, cytotoxic, or edemogenic. Some of these toxins are proteolytic and others have the putative activity of being lytic for secretory IgA. Several of these molecules are capable of inducing lesions that are similar to those observed in natural infections and disease. Endogenous host factors have also been implicated in the development of lung lesions after infection by A. pleuropneumoniae Coagulation and inflammatory pathways have been demonstrated to be pivotal in the early phases of lesion development. In addition, the immune status of the animal is clearly related to the severity and ultimate outcome of A. pleuropneumoniae infection. To adequately treat and prevent this disease, we must understand the distinguishable interactions that occur between the host and the various molecular virulence attributes of A. pleuropneumoniae.