TLR4 and Recurrent lower respiratory tract infections: The interaction between pneumolysin and TLR4 was found to contribute to a protective immune response to S. pneumoniae, in particular in a model of upper airway colonization (Malley et al., 2003; Srivastava et al., 2005) and to a lesser extent during experimental lower respiratory tract infection (Branger et al., 2004).