Killing of Actinobacillus actinomycetemcomitans and Haemophilus aphrophilus by human polymorphonuclear leukocytes in serum and saliva.

The ability of polymorphonuclear leukocytes from human peripheral blood to kill Actinobacillus actinomycetemcomitans and Haemophilus aphrophilus was examined with fresh isolates and laboratory strains from each species (5 strains within each group) under different conditions. Bacterial cells were mixed with a polymorphonuclear leukocyte suspension in the presence of either active serum or heat-inactivated serum or active serum together with sterile-filtered saliva. Surviving bacteria were determined by counting the number of bacterial colony-forming units in the mixtures after a 60-min incubation at 37 degrees C. Mixtures without polymorphonuclear leukocytes served as controls for the evaluation of the degree of killing of the bacteria. In general, A. actinomycetemcomitans resisted phagocytic killing to a greater extent than H. aphrophilus, and the killing of the former species mainly depended on the presence of heat-labile serum components, probably complement factors. Laboratory strains of A. actinomycetemcomitans were more easily killed than fresh isolates. The presence of saliva in the reaction mixtures decreased the degree of killing. However, strain-dependent variations in the killing were found under either condition. The leukotoxic activity of A. actinomycetemcomitans strains, determined by a [51Cr]-release assay, was not correlated with the resistance of these strains to the phagocytic killing. The results point out a strain-dependent difference in the ability of A. actinomycetemcomitans to evade the inflammatory response associated with polymorphonuclear leukocytes. This difference may constitute a potential virulence factor for this periodontopathogen. Furthermore, the leukotoxicity of the strains is not the main determinant that modifies the interaction of A. actinomycetemcomitans with human neutrophils.