TLR4 and infection: Although LPS is widely known to be the PAMP recognized by TLR4 following infection by a range of Gram-negative pathogens [17,20,24], prior experimental assessments of L. pneumophila on this particular topic were murine-based and they had concluded that TLR2 is the receptor responding to L. pneumophila LPS [31,33,34,43,49].