Phagocytosis by pig alveolar macrophages of Actinobacillus pleuropneumoniae serotype 2 mutant strains defective in haemolysin II (ApxII) and pleurotoxin (ApxIII)

The ability of pig alveolar macrophages to phagocytose Actinobacillus pleuropneunioniae HK 361, which produces both hemolysin II (ApxII) and pleurotoxin (ApxIII), has been studied. Macrophages incubated with HK 361 in the presence of normal pig serum were rapidly killed. Incubation of the macrophages with a hemolysin-deficient mutant (HK 361 e), which possesses only cytotoxic activity (ApxIII), also caused gross damage to the macrophages. A mutant (HK 361 h) which produces neither ApxII nor ApxIII in its culture supernatant allowed longer survival of the macrophages than did either the parent strain or mutant e when incubated with normal pig serum. Prolonged incubation with mutant h resulted in an increase in the number of damaged macrophages, but not to the same extent as with either HK 361 or mutant e. The number of mutant h cells phagocytosed in the presence of normal pig serum was low. The addition of either hyperimmune rabbit serum, raised against whole formalin-treated HK 361 cells, or convalescent pig serum from a pig recovering from a serotype 3 infection, which contained antibody against both ApxII and ApxIII, did not increase the survival of macrophages incubated with either HK 361 or mutant e. However, incubation of mutant h with convalescent pig serum did result in damage-free macrophages. This serum, which possessed neutralizing capabilities against the toxic activities of ApxII and ApxIII enhanced the number of mutant h cells phagocytosed compared to the numbers phagocytosed in normal pig serum. Killed bacteria were rapidly phagocytosed and did not damage macrophages. The number of phagocytosed killed bacteria appeared to be similar to that seen with live mutant h cells when incubated in the presence of convalescent pig serum. The presence of a toxic activity associated with the hemolysin- and cytotoxin-negative mutant may represent an additional cell-associated toxin which is not present in its culture supernatant. It appears therefore, that in the absence of extracellular ApxII and ApxIII and in the presence of convalescent pig serum A. pleuropneumoniae is readily phagocytosed.