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

Andrew N. Rycroft

Department of Veterinary Pathology, University of Glasgow, Glasgow G61 1QH, UK

ABSTRACT

Summary: The ability of pig alveolar macrophages to phagocytose Actinobacillus pleuropneumoniae HK 361, which produces both haemolysin II (Apxlll) and pleurotoxin (Apxlll), has been studied. Macrophages incubated with HK 361 in the presence of normal pig serum were rapidly killed. Incubation of the macrophages with a haemolysin-deficient mutant (HK 361 e), which possesses only cytotoxic activity (Apxlll), also caused gross damage to the macrophages. A mutant (HK 361 h) which produces neither Apxll nor Apxlll 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 Apxll and Apxlll, 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 Apxll and Apxlll, 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 haemolysin-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 Apxll and Apxlll and in the presence of convalescent pig serum A. pleuropneumoniae is readily phagocytosed.

Author for correspondence: Janice M. Cullen. Tel: +44 41 339 8855. Fax: +44 41 330 5602.

Present address: Department of Pathology and Infectious Diseases, The Royal Veterinary College, University of London, London NW1 OTU, UK.

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