Lymphocyte function-associated antigen 1 is a receptor for Pasteurella haemolytica leukotoxin in bovine leukocytes

Pasteurella (Mannheimia) haemolytica leukotoxin (Lkt) causes cell type- and species-specific effects in ruminant leukocytes. Recent studies indicate t hat P. haemolytica Lkt binds to bovine CD18, the common subunit of all beta 2 integrins. We designed experiments with the following objectives: to ide ntify which member of the beta 2 integrins is a receptor for Lkt; to determ ine whether Lkt binding to the receptor is target cell (bovine leukocytes) specific; to define the relationships between Lkt binding to the receptor, calcium elevation, and cytolysis; and to determine whether a correlation ex ists between Lkt receptor expression and the magnitude of target cell cytol ysis. We compared Lkt-induced cytolysis in neutrophils from control calves and from calves with bovine leukocyte adhesion deficiency (BLAD), because n eutrophils from BLAD-homozygous calves exhibit reduced beta 2 integrin expr ession. The results demonstrate for the first time that Lkt binds to bovine CD11a and CD18 (lymphocyte function-associated antigen 1 [LFA-1]). The bin ding was abolished by anti-CD11a or anti-CD18 monoclonal antibody (MAb). Lk t-induced calcium elevation in bovine alveolar macrophages (BAMs) was inhib ited by anti-CD11a or anti-CD18 MAb (65 to 94% and 37 to 98%, respectively, at 5 and 50 Lkt units per ml; P < 0.05). Lkt-induced cytolysis in neutroph ils and BAMs was also inhibited by anti-CD11a or anti-CD18 MAb in a concent ration-dependent manner. Lkt bound to porcine LFA-1 but did not induce calc ium elevation or cytolysis. In neutrophils from BLAD calves, Lkt-induced cy tolysis was decreased by 44% compared to that of neutrophils from control c alves (P < 0.05). These results indicate that LFA-1 is a Lkt receptor, Lkt binding to LFA-1 is not target cell specific, Lkt binding to bovine LFA-1 c orrelates with calcium elevation and cytolysis, and bovine LFA-1 expression correlates with the magnitude of Lkt-induced target cell cytolysis.