FUNCTIONAL AND STRUCTURAL-CHANGES OF PORCINE ALVEOLAR MACROPHAGES INDUCED BY SUBLYTIC DOSES OF A HEAT-LABILE, HEMOLYTIC, CYTOTOXIC SUBSTANCE PRODUCED BY ACTINOBACILLUS-PLEUROPNEUMONIAE

Alterations in the size and functions of porcine alveolar macrophages exposed to sublytic amounts of heat-labile, hemolytic cytotoxin produc ed by Actinobacillus pleuropneumoniae (App) serotype 1, strain HS54 in to the culture medium were studied in vitro. Alveolar macrophages were sensitive to the cytotoxin; treatment of the macrophages with low con centrations of cytotoxin (0.016 hemolytic unit) resulted in severe, ir reversible cell swelling. However, high doses of cytotoxin (2.0 hemoly tic units) were required to cause substantial cell death, as indicated by the influx of propidium iodide into and release of lactate dehydro genase from cells. Macrophages exposed to low, sublytic doses of cytot oxin failed to migrate toward chemoattractant, were unable to attach t o glass, and failed to phagocytize optimally opsonized erythrocytes. M acrophages already attached to glass surfaces detached when exposed to sublytic doses of cytotoxin. The swelling and impairment of functions of alveolar macrophages observed in this study could not be attribute d to endotoxic effects, because heat treatment of the cytotoxin prepar ation for 60 minutes at 60 C resulted in complete loss of cytotoxicity . We conclude that sublytic doses of heat-labile, hemolytic cytotoxic substances produced by App depress alveolar macrophage function at con centrations likely to develop in association with acute pulmonary infe ction with App. The Apx (A pleuropneumoniae Rtx toxins) exotoxins secr eted by the bacteria into culture medium were considered responsible f or the toxic activity of the cytotoxin preparation. The Apx of the App field strain used in this study were likely to be similar to those of serotype-1 reference strain (S4707). Analysis by use of DNA-DNA hybri dization indicated that genomic DNA of the field strain contained sequ ences similar to those encoding structural protein of ApxI (apxIA) and ApxII (apxIIA) of the serotype-1 reference strain. Therefore, Apr pro duced by the field strain of App used in this study are likely to be o f similar pathogenic importance worldwide.