Antimicrobial peptides have established an important role in the
defense against extracellular infections, but the expression of cationic
peptides within macrophages as an antibacterial effector mechanism
against intracellular pathogens has not been demonstrated. Macrophage
expression of the murine cathelicidin-related antimicrobial
peptide (CRAMP) was increased after infection by the intracellular
pathogen Salmonella typhimurium, and this increase required reactive
oxygen intermediates. By using CRAMP-deficient mice or synthetic
CRAMP peptide, we found that CRAMP impaired Salmonella
cell division in vivo and in vitro, resulting in long filamentous bacteria.
This impaired bacterial cell division also depended on intracellular
elastase-like serine protease activity, which can proteolytically activate
cathelicidins. Macrophage serine protease activity induced filamentation
and enhanced the activity of CRAMP in vitro. A peptidesensitive
Salmonella mutant showed enhanced survival within
macrophages derived from CRAMP-deficient mice, indicating that
Salmonella can sense and respond to cationic peptides in the intracellular
environment. Although cationic peptides have been hypothesized
to have activity against pathogens within macrophages, this
work provides experimental evidence that the antimicrobial arsenal
of macrophages includes cathelicidins. These results show that intracellular
reactive oxygen intermediates and proteases regulate macrophage
CRAMP expression and activity to impair the replication of
an intracellular bacterial pathogen, and they highlight the cooperativity
between macrophage antibacterial effectors.