ACTIVATION OF BOVINE LYMPHOCYTE SUBPOPULATIONS BY STAPHYLOCOCCAL-ENTEROTOXIN-C

Staphylococcus aureus is a major mastitis-causing pathogen in cattle, The chronic nature of bovine staphylococcal mastitis suggests that som e products or components of S. aureus may interfere with the developme nt of protective immunity, One class of molecules that could be involv ed are superantigens (SAgs), Although a significant number of mastitis isolates produce SAgs, the effect of these molecules on the bovine im mune system is unresolved. To determine if immunosuppression caused by SAgs could play a role in pathogenesis, we monitored bovine lymphocyt es exposed to staphylococcal enterotoxin C1 (SEC1), Activation of bovi ne lymphocytes by either SEC1 or concanavalin A (ConA) was influenced by the gamma delta/alpha beta T-cell ratio in the culture, Compared to ConA-induced stimulation, cultures stimulated with SEC1 generated sma ll numbers of CD4(+) alpha beta T cells expressing high levels of inte rleukin-2 receptor alpha chain (IL-2R alpha) and major histocompatibil ity complex class II (MHCII), suggesting that SAg exposure does not le ad to full activation of these cells. This state of partial activation was most pronounced in cultures with a high gamma delta/alpha beta ra tio. In contrast, significant numbers of CD8(+) alpha beta T cells exp ressed high levels of IL-2R alpha and MHCII, regardless of the gamma d elta/alpha beta ratio and the stimulant used, CD8(+) blasts in culture s stimulated with SEC1 also expressed another activation marker, ACT3, previously detected predominantly on thymocytes and CD4(+) T cells, A lthough gamma delta CD2(-) and CD2(+) T cells expressed MHCII and IL-2 R alpha following stimulation with SEC1, only a few cells increased to blast size, suggesting that they were only partially activated, The r esults suggest ways in which SAgs might facilitate immunosuppression t hat promotes the persistence of bacteria in cattle and contributes to chronic intramammary infection.