IMMUNE RECRUITMENT AND BACTERICIDAL ACTIVITY OF NEUTROPHILS IN MILK OF COWS VACCINATED WITH STAPHYLOCOCCAL ALPHA-TOXIN

The ability of Staphylococcus aureus alpha-toxin to recruit neutrophil s in the milk of vaccinated cows and the bactericidal efficiency of th ese neutrophils were evaluated. Six lactating Holstein cows that were free of intramammary infection received systemic immunization by subcu taneous injection of Freund's incomplete adjuvant with alpha-toxin (n = 2), alpha-toxin mixed with type 5 capsular polysaccharide (n = 2), o r a conjugate of these two antigens (n = 2). Controls (n = 4) and vacc inated cows (n = 6) received intramammary infusions of alpha-toxin. No increase in somatic cell count was recorded in quarter milk samples f rom unimmunized cows; however, 10 mu g of alpha-toxin induced a local inflammatory reaction in vaccinated cows that was characterized by ear ly and massive cellular recruitment into the mammary gland. More than 90% of the recruited cells were neutrophils. The speed and magnitude o f the cellular recruitment were dose-dependent; the threshold dose was 0.6 mu g. Milk samples showed significant bactericidal activity again st the type 5 S. aureus strain, regardless of the vaccine used, and sh owed a decrease in bacterial count of about 2 log(10) from the initial inoculum. The best efficiency was recorded during the early phase of cellular recruitment with concomitant activation of blood-derived neut rophils. This study demonstrates that a bacterial virulence factor, al pha-toxin, is able to induce immune recruitment of neutrophils for eff icient bactericidal activity in milk when cows are immunized with alph a-toxin that is used either as a nonconjugate vaccine or as a carrier protein in a conjugate vaccine. The study also suggests that neutrophi ls that are recruited from blood are activated during inflammation in response to specific antigens.