A report published in Nature Immunology presents the first detailed characterization of a new family of antimicrobial peptides in the mouse intestine.

Antimicrobial peptides are small, endogenous peptides containing <100 residues and are an important component of innate immunity. So far, many different classes of antimicrobial peptides have been identified in mammals, including α-, β- and θ-defensins and cathelicidins. For more information on defensins in primates, see the review by Robert Lehrer on page 727 of this issue.

Hornef et al. were interested in a family of mouse-intestine-specific CRS (cryptdin-related sequence) peptides. CRS mRNAs have been characterized previously but the peptide products have never been characterized before. Using mRNA from intestinal tissue of healthy mice, Hornef et al. identified 15 new cDNAs encoding CRS peptides, bringing the total number of CRS peptide coding sequences to 23, comparable to the number of mouse α-defensin genes. Each CRS gene encodes a peptide containing a pro-region that is cleaved off during processing, leaving a mature cationic peptide containing nine conserved cysteines. Of the 15 new sequences identified, most of the sequence variation observed was in the pro-region, and so a total of 7 mature peptides were predicted by the coding sequences.

To screen for mature CRS peptides in vivo, whole mouse intestinal tissue was fractionated by reverse-phase HPLC. Analysis of the mass of fractions in the presence and absence of a reducing agent indicated the presence of dimers. To exclude the possibility that dimer formation was an experimental artefact, the extraction process was repeated in the presence of an alkylating agent that inhibits the formation of disulphide bonds. This confirmed that CRS peptides form covalent dimers in vivo. Six mature peptides were identified in a variety of homo- and heterodimers and, additionally, four peptides were also found in truncated form. Examination of the antibacterial activity of synthetic CRS dimers showed these agents had potent activity against bacterial species including Salmonella enterica serovar Typhimurium and Streptococcus pyogenes, with no cytotoxicity.

The complement of antimicrobial peptides varies from species to species. This study has revealed that in mice, the complement of CRS peptides matches that of the α-defensins in number, and perhaps also in potency. The ability to form hetero- and homodimers is a neat trick to expand the repertoire of intestinal peptide diversity and strengthen antimicrobial defence.