Dibutyryl cyclic AMP and inflammatory cytokines mediate C3 expression in Schwann cells

Schwann cells (SchC), the myelinating glia of the peripheral nervous system, are immunocompetent cells and secrete a variety of immune and inflammatory mediators. In this report, we show that rat SchC in vitro express both C3 mRNA and protein in response to dibutyryl cyclic AMP (dbcAMP) and the cytokines IFN-gamma, TNF-alpha, and IL-1beta. SchC in culture constitutively expressed low levels of C3 which were significantly upregulated upon stimulation with 1mM dbcAMP by 24 hours, and persisted up to 120 hours. This response was minimally enhanced by costimulation with 100 U/ml IFN-gamma, whereas costimulation with 100 U/ml IFN-gamma together with 150-450 ng/ml TNF-alpha induced a greatly increased C3 response. TNF-alpha alone did not induce C3 expression in SchC. Cycloheximide inhibited this dbcAMP-dependent delayed C3 production, thus implying an intermediary signal in the induction pathway requiring protein synthesis. Treatment with 0.1-10 ng/ml IL-1beta for 0-72 hours induced C3 mRNA and protein in a dose-dependent manner. C3 mRNA was detectable at 1 hour and mRNA and protein peaked by 6-12 hours on stimulation with 10 ng/ml IL-1beta, or at 48 hours with 1.0 ng/ml IL-1beta. Furthermore, IL-1beta mRNA was detected at 6 hours in dbcAMP-treated SchC, preceding the dbcAMP-induced C3 expression by 18 hours. Induction of C3 mRNA and protein by dbcAMP at 24 hours was inhibited >85% by a neutralizing anti-IL-1beta antibody and 76% with an IL-1 receptor antagonist. This suggests that dbcAMP-induced synthesis of IL-1beta mediates the C3 production by SchC in an autocrine/paracrine fashion by binding to a functional IL-1 receptor expressed on the surface of SchC. Endoneurial IL-1 and C3 production by SchC may therefore contribute to the inflammatory events associated with peripheral nerve demyelination.