Protein A of Staphylococcus aureus evokes Th1 type response in mice

Protein A (PA) of Staphylococcus aureus is known to elicit several cytokines such as IFNγ, TNFα and IL1. However, it has not been delineated yet as to which differentiation pathway lymphocytes follow after stimulation by PA. In this report, we attempted to collect such evidences. Cytokines, such as IFNγ, IL2, IL4, IL6, IL10, TNFα, IL1α and IL1β were measured in serum by ELISA. Our results show that 1 μg dose of PA stimulates the production of IFNγ (115±5 pg/ml), TNFα (250±8 pg/ml) and IL1α (100±5 pg/ml) as compared to control levels of, 22±2, 20±2 and 35±3 pg/ml respectively whereas IL2 and IL1β secretion were less (beyond the lower detection limit of the kit and 25±1 pg/ml, respectively) as compared to control (28±2 and 52±4 pg/ml, respectively). Larger dose of PA (10 μg) increases the expression of IL2 (75±3 pg/ml), TNFα (1380±120 pg/ml), IL1 α (495±10 pg/ml) and IL1 β (110±7 pg/ml) as compared to controls described above. We also observed that 1 μg dose of PA decreases IL4, IL6 and IL10 secretion to 9±1, 10±1 and 10±2 pg/ml, respectively, whereas 10 μg dose also decreased them to 11±1, 12±2 and 30±4 pg/ml, respectively as compared to the background controls, i.e. 50±5, 50±2 and 215±9 pg/ml respectively. The ratio of IFNγ to IL4 increased and the peak value at 4 h, came to 13±1 and 9.6±0.5 with 1 μg and 10 μg PA, respectively, which is an established parameter indicating a Th1 type response. Flow cytometry analysis of CD4+/CD8+ cells, and c-myc protein expression by splenocytes indicate that 1 μg dose of PA causes 2-fold increase of CD4+ cells with no change in CD8+ cells, and 10-fold increase in c-myc protein, whereas 10 μg dose increases CD4+ cells 4-fold, CD8+ cells 3-fold and c-myc protein 100-fold. The cell cycle data shows an induction of apoptosis in thymocytes and splenocytes with the large dose (10 μg), whereas the 1 μg dose does not show any apoptosis. This report indicates that a Th1 response is induced in mice, after PA inoculation at a dose of 1 μg/animal. Thus, cytokine mediated therapeutic strategies should consider the fact that an induction of large concentration of some cytokines might become detrimental to the host.