Pneumolysin, the
cholesterol-dependent
cytolysin of Streptococcus pneumoniae, induces inflammatory and apoptotic events in mammalian cells.
Toll-like receptor 4 (TLR4) confers resistance to
pneumococcal infection via its interaction with
pneumolysin, but the underlying mechanisms remain to be identified. In the present study, we found that
pneumolysin-induced apoptosis is also mediated by TLR4 and confers protection against invasive disease. The interaction between TLR4 and
pneumolysin is direct and specific;
ligand-binding studies demonstrated that
pneumolysin binds to TLR4 but not to TLR2. Involvement of TLR4 in
pneumolysin-induced apoptosis was demonstrated in several complementary experiments. First, macrophages from wild-type mice were significantly more prone to
pneumolysin-induced apoptosis than cells from TLR4-defective mice. In gain-of-function experiments, we found that epithelial cells expressing TLR4 and stimulated with
pneumolysin were more likely to undergo apoptosis than cells expressing TLR2. A specific TLR4 antagonist,
B1287, reduced
pneumolysin-mediated apoptosis in wild-type cells. This apoptotic response was also partially
caspase dependent as preincubation of cells with the pan-
caspase inhibitor
zVAD-fmk reduced
pneumolysin-induced apoptosis. Finally, in a mouse model of
pneumococcal infection,
pneumolysin-producing pneumococci elicited significantly more upper respiratory tract cell apoptosis in wild-type mice than in TLR4-defective mice, and blocking apoptosis by administration of
zVAD-fmk to wild-type mice resulted in a significant increase in mortality following nasopharyngeal pneumococcal exposure. Overall, our results strongly suggest that protection against
pneumococcal disease is dependent on the TLR4-mediated enhancement of
pneumolysin-induced apoptosis.