Max Planck Institute for Infection Biology, Berlin, Germany.
A microbiologist wonders how antimicrobial peptides beat infection.
My group is interested in why, although people often pick up infections and sometimes become ill, they almost always recover. Recovery is the result of a fantastically efficient immune system that relies in part on proteins and peptides that kill microbes.
Antimicrobial peptides were discovered in systematic searches for potential drugs, and there are several types. Most are cationic and bind to the anionic surface of microbes. Recently, Roberto Lande at the University of Texas in Houston and his colleagues convincingly showed that one antimicrobial peptide, LL-37, can also bind DNA and serve as an activator for other immune cells (R. Lande et al. Nature 449, 564–569; 2007).
LL-37 is one of several antimicrobial peptides that do more than kill microbes and activate immune cells. Its other functions include chemoattraction and wound repair. But, as its name indicates, LL-37 has only 37 amino acids. It is plausible that its multiple effects on the host are due to its interaction with specific receptors that, in combination with other signals, result in diverse biological functions. More intriguing, however, is the fact that LL-37 kills microbes in the first place.
The question is whether LL-37 and other antimicrobial peptides truly function as bacterial killing agents in the host. Their antimicrobial activity has so far been demonstrated only in vitro, where it might be a reflection of their cationic character. Another more attractive possibility is that infections are such an important threat to the host that, during evolution, many cationic proteins with diverse function were co-opted to serve as antimicrobial agents. If we do have many ways to kill microbes, maybe it is not so surprising that we often recover from infections.