Interactions of Haemophilus-Actinobacillus-Pasteurella bacteria with phagocytic cells.

The Haemophilus-Actinobacillus-Pasteurella (HAP) group of bacteria contains a number of important veterinary and human pathogens. Although each species has specific characteristics and host range, most share the general property of being resistant to cellular defense mechanisms. In some cases (e.g. Pasteurella multocida, Pasteurella haemolytica and Actinobacillus pleuropneumoniae) resistance results in part from the presence of an antiphagocytic capsule that protects the bacilli against ingestion by neutrophils and macrophages. In other instances the bacteria aggressively attack mononuclear and polymorphonuclear phagocytes. For example, P. haemolytica, A. pleuro-pneumoniae and Actinobacillus actinomycetemcomitans each produce a leukotoxin that functionally impairs, and ultimately kills, leukocytes from cattle, pigs and human beings, respectively. Components of Pasteurella multocida and Haemophilus somnus have also been reported to adversely affect leukocyte functions. Another important area of research that is just emerging concerns the ability of lipopolysaccharide and other components of HAP bacteria to stimulate or modulate macrophage release of inflammatory mediators such as interleukin-1. In this paper, we provide an overview of the interactions of HAP bacteria with phagocytes and identify some of the common strategies by which they evade cellular defenses.