Polyclonal B-cell activation by an arthritogenic Staphylococcus aureus strain: contribution of T-cells and monokines

We have recently described a murine model of Staphylococcus aureus-induced septic arthritis. One of the hallmarks of this disease is a striking hypergammaglobulinemia. In the present study we have used a sensitive ELISPOT technique to assess, at the single cell level, the B-cell differentiation properties of this arthritogenic, toxic shock syndrome toxin-1 (TSST-1)-producing staphylococcal strain. In vivo, inoculation of live S. aureus resulted in lymphoproliferation, early (within 3-4 days) peak of IgM-secreting cells and late (2 weeks after the injection) pronounced increase of IgG-secreting cells. We have documented that this late increase of IgG-secreting cells is a CD4+ T-cell-dependent phenomenon. Furthermore, we have showed that there is a relationship between the hypergammaglobulinemia and the appearance of arthritis, since a nonarthritogenic staphylococcal strain will not give rise to increased frequency of immunoglobulin-secreting cells. To elucidate mechanisms responsible for S. aureus-induced polyclonal B-cell activation, we have assessed in vitro effects of formalin-fixed arthritogenic S. aureus on the release of cytokines. Our results show that the S. aureus LS-1 strain induces in vitro preferentially IgM-secreting cells, many of them displaying autoantibody properties. The magnitude of this response is high and comparable with optimal concentrations of LPS, a potent murine polyclonal B-cell activator. Interleukin-1 alpha (IL-1 alpha), tumor necrosis factor (TNF), and interleukin-6 (IL-6) were all secreted by mouse MNC after in vitro exposure to formalin-killed S. aureus. Inhibition experiments, using neutralizing antibodies to these cytokines, revealed that IL-1 alpha and IL-6 but not TNF-alpha had potent B-cell differentiation properties in S. aureus-stimulated cell cultures.