Pneumolysin, a protein toxin of Streptococcus pneumoniae, induces nitric oxide production from macrophages

Nitric oxide (NO) production by inducible NO synthase (iNOS) during inflamm ation is an essential element of antimicrobial immunity but can also contri bute to host-induced tissue damage. Under conditions of bacterial sepsis, l arge amounts of NO are produced, causing hypotension, a critical pathologic al feature of septic shock. In sepsis caused by gram-positive organisms, th e bacterial factors contributing to host NO production are poorly character ized, We show that a soluble toxin of Streptococcus pneumoniae, pneumolysin (Pln), is a key component initiating NO production from macrophages. In co ntrast to wild-type bacteria, a mutant of S. pneumoniae lacking Pin failed to elicit NO production from murine macrophages. Purified recombinant Pin i nduced NO production at low concentrations and independently of exogenous g amma interferon (IFN-gamma) priming of RAW 264.7 macrophages. However, IFN- gamma was essential for Pin-induced NO production, since primary macrophage s from mice lacking the IFN-gamma receptor or interferon regulatory factor 1, a transcription factor essential for iNOS expression, failed to produce NO when stimulated with Pln. In addition, Pln acts as an agonist of tumor n ecrosis factor alpha and interleukin 6 production in macrophages. The prope rties of Pln, previously identified as a pore-forming hemolysin, also inclu de a role as a general inflammatory agonist.