Antimicrobial Synergy of a Ribonuclease and a Peptide Secreted by Human Cells.

LL-37 is a secretory peptide that has antimicrobial activity. Ribonuclease 1 (RNase 1) is a secretory enzyme that is not cytotoxic. We find that human LL-37 and human RNase 1 can act synergistically to kill Gram-negative bacterial cells. In the presence of nontoxic concentrations of LL-37, RNase 1 is toxic to Escherichia coli cells at picomolar levels. Using wild-type RNase 1 and an inactive variant labeled with a fluorophore, we observe the adherence of RNase 1 to E. coli cells and its cellular entry in the presence of LL-37. These data suggest a natural means of modulating the human microbiome via the cooperation of an endogenous peptide (37 residues) and small enzyme (128 residues).