Components of bacterial peptidoglycans have potent
biological activities, including adjuvant effects, cytotoxicity, and induction of sleep. Mixtures of
peptidoglycan components also induce
inflammation in the lung, subarachnoid space, and joint, but the structural requirements for activity are unknown. Using a rabbit model for
meningitis, we determined the
biological activities of 14 individual muramyl
peptides constituting > 90% of the
peptidoglycan of the gram-negative pediatric pathogen Haemophilus influenzae. Upon intracisternal inoculation, most of the muropeptides induced
leukocytosis in cerebrospinal fluid (CSF), influx of
protein into CSF, or
brain edema, alone or in combination. The
disaccharide-tetrapeptide, the major component of all gram-negative peptidoglycans, induced CSF
leukocytosis and
protein influx at doses as low as 0.4 microgram (0.42 nM). Modification of the N-acetyl
muramic acid or substitution of the
alanine at position four in the
peptide side chain decreased
leukocytosis but enhanced
brain edema. As the size of the muropeptide increased, the inflammatory activity decreased. Muropeptide carrying the diaminopimelyl-
diaminopimelic acid cross-link specifically induced
cytotoxic brain edema. These findings significantly expand the spectrum of
biological activities of natural muramyl
peptides and provide the basis for a structure-activity relationship for the inflammatory properties of bacterial muropeptides.