SUPERANTIGEN ACTIVATION AND KINETICS OF CYTOKINES IN THE LONG-EVANS RAT

This study was conducted to identify and quantify, over time, selected cytokine responses in Long-Evans rats that were exposed to staphyloco ccus enterotoxin B (SEB). The kinetics of selected cytokines [interleu kin-2 (IL-2), IL-6, interferon-gamma (IFN-gamma) and tumour necrosis f actor (TNF)] and phenotype and cell cycle analysis of T lymphocytes we re determined in Long-Evans rats administered a single intraperitoneal (i.p.) dose of either 50 mu g or 500 mu g of SEB. Rats injected with 50 mu g SEB had significantly elevated levels of IL-2, IL-6 and IFN-ga mma in their serum 2 hr post-injection. IL-2 serum levels were signifi cantly elevated at 2 hr and returned to near control values by 12 hr w hile both IL-6 and IFN-gamma peaked at 6 hr but remained significantly increased at 24 hr post SEB exposure. A 500 mu g dose of SEB did not further enhance these cytokine responses. When spleen cells were colle cted for culture 2 hr after rats were injected i.p. with 50 mu g SEB a nd cocultured with SEB, TNF and IL-6 levels were significantly increas ed after 2 hr incubation, while IL-2 and IL-6 were significantly eleva ted at 6 hr. Production of all these cytokines in spleen cell cultures continued to increase over the 24 hr sampled. Peritoneal cells were c ollected for culture either at 1 hr or 2 hr after injection of either 50 mu g or 500 mu g of SEB. IL-6 was significantly increased after 1 h r in culture while TNF was significantly increased by 2 hr regardless of whether the cells were harvested 1 or 2 hr after SEB injection. The greatest response for both IL-6 and TNF occurred when cells from anim als injected with 50 mu g SEB were restimulated in vitro with SEB. The peak levels for IL-6 were at 12 hr post SEB exposure while TNF peaked at 6 hr. The percentage of CD4(+) cells was significantly increased a t 48 hr and 72 hr post SEB (50 mu g) administration while the percenta ge of CD8(+) cells remained similar to control values for the 168-hr t est period. A similar pattern was observed in cell cycling where the C D4(+) cells proliferated up to 2 days post SEB injection and then were significantly suppressed at day 3. The CD8(+) cells were comparable t o control values. These studies demonstrate that the cytokine response s in Long-Evans rats exposed to a superantigen are somewhat similar to those that occur in mice and humans, e.g. a rapid short increase in t he production of IFN-gamma and TNF that was accompanied by an increase in the production of IL-2. Additional responses noted in this species , however, were a marked increase in IL-6 production, as well as an ea rly increase in the number and cycling of CD4(+) cells followed by a d ownregulation of these events. These activities occurred in the absenc e of notable histopathological alteration of lymphoid organs. The resu lts indicate that the Long-Evans rat is an acceptable animal model to investigate the pathogenesis of superantigen-induced disease and that IL-6 may be an active mediator of this process.