The effect of Pasteurella haemolytica A1 leukotoxic culture supernate on the in vitro proliferative response of bovine lymphocytes.

The effect of sublethal concentrations of the Pasteurella haemolytica leukotoxic culture supernate on bovine lymphocyte blastogenesis was investigated. Blastogenesis in cultures stimulated with either concanavalin A (Con A) or pokeweed mitogen (PWM) was inhibited in the presence of the supernate, as was the response to purified protein derivative in lymphocytes from BCG-vaccinated cattle. Partially purified leukotoxin had a similar effect. Pre-incubation of the leukotoxic supernate with a polyclonal rabbit antiserum raised to the immunogenic molecule of recombinant leukotoxin (r LktA) abrogated this effect, implicating leukotoxin as the factor responsible for the inhibition. B cell enriched cultures tended to be more sensitive to leukotoxic effects than did T cell enriched cultures. Although only ruminant cells are susceptible to the lethal effects of P. haemolytica leukotoxin, the toxin did inhibit both Con A- and PWM-induced proliferation of human and dog lymphocytes. As well, at high leukotoxin doses, Con A-stimulated pig lymphocyte proliferation was reduced. Rabbit lymphocytes were not affected by leukotoxin in either Con A- or PWM-stimulated cultures.