The cathelicidin peptides comprise one of several families of antimicrobial peptides that are found in neutrophils and epithelia as components of the early host defenses

The cathelicidin peptides comprise one of several families of antimicrobial peptides that are found in neutrophils and epithelia as components of the early host defenses of mammals against infection. All cathelicidin family members are synthesized and stored in cells as twodomain proteins. These are split on demand to produce a cathelin protein and an antimicrobial peptide. Accumulating evidence indicates that both the cathelin portion and the C-terminal peptide exert biological activities connected with host protection. This review presents an overview of the structure and biology of cathelicidins and discusses recent progress in cathelicidin research with emphasis on the functional properties and role in host defense of the human cathelicidin hCAP18/LL-37. Although investigators initially concentrated their attention on antibiotic activity, it is becoming clear now that LL-37 is a multifunctional molecule that may mediate various host responses, and thus represents an essential component of the innate immune system in humans. Introduction and historical background The cathelicidins comprise a large number of precursors of antimicrobial peptides that typically contain a conserved N-terminal sequence (‘cathelin’ domain) and a C-terminal antimicrobial domain of varied sequence and length (Fig. 1). The C-terminal peptides express antibiotic activity after they have been cleaved from the holoprotein, and current evidence indicates they may contribute to host defense by both direct pathogen inactivation and by other biological activities. The discovery of cathelicidins can be traced back to the isolation of a disulfide-containing cyclic dodecapeptide during studies of the antimicrobial activity of bovine neutrophil lysates (Romeo et al., 1988). The identification of the dodecapeptide was soon followed by the purification of two additional neutrophil antimicrobial peptides that were designated bactenecins (after the latin words ‘bacterium necare’). Bac5 is a C-terminally amidated peptide of 43 amino acid residues with a polycationic sequence characterized by a repeated proline motif; Bac7 is a 60-residue proline-rich peptide with distinct amino acid sequence and proline motif (Gennaro et al., 1989). Cloning of the cDNA of Bac5 from a bovine myeloid cDNA library (Zanetti et al., 1993) was instrumental in showing that despite their diversity, dodecapeptide, bactenecins and yet other structurally unrelated antimicrobial peptides (Agerberth et al, 1991; Selsted et al., 1992; Kokryakov et al., 1993) were in fact closely related members of a single protein family. This became apparent during a molecular cloning step that was intended to amplify a Bac5 cDNA fragment from bovine bone marrow mRNA by pairing an oligo-dT primer with a forward primer based on a cDNA sequence upstream from the peptide coding region (Zanetti et al., 1993). Although the template sequence was thought to be unique to Bac5 mRNA, a variety of additional transcripts were co-amplified using these primers. Each cDNA predicted a distinct peptide sequence that was attached to a propiece showing 75–87% sequence identity to the corresponding Bac5 propiece (Zanetti et al., 1995). The diverse C-terminal peptides turned out to be either previously isolated or novel antimicrobial peptides. A molecular cloning strategy was then devised based on the highly conserved N-terminal propiece, to enable the identification of further members of this novel protein family in other species as deduced from cDNA. Newly discovered cathelicidin family members were designated after the putative C-terminal antimicrobial domain, by using acronyms (e.g., CRAMP for ‘cathelin-related antimicrobial peptide’ or BMAPs for ‘bovine myeloid antimicrobial peptides’), one-letter symbols of key amino acid residues followed by the number of residues of the antimicrobial domain (e.g., LL-37), or referring to other specific peptide features. Sequences corresponding to this domain served as templates for chemical synthesis of peptides that were then functionally characterized. The N-terminal propiece of approximately 11 kDa (99 to 114 residues) is the hallmark of cathelicidins (Zanetti et al., 1995). It generally shares higher than 70% sequence identity to cathelin, a protein that was isolated from porcine neutrophils as an inhibitor of cathepsin L (cathe-l-in is an acronym for cathepsin L inhibitor) (Kopitar et al., 1989) prior to the initial recognition of the cathelicidin family. As will be discussed later, the structural features of porcine cathelin suggested it was a member of the cystatin superfamily of cysteine proteinase inhibitors (Ritonja et al., 1989). Definition and structural diversity The term ‘cathelicidins’ was proposed in 1995 to acknowledge the evolutionary relationship of the novel protein family to cathelin (Zanetti et al., 1995) and it is used to denote holoproteins that contain a cathelin-like sequence and a cationic antimicrobial domain. The mature antimicrobial peptides (Table 1) will be referred to here as cathelicidin peptides. The mammalian cathelicidin peptides display a wide repertoire of The Role of Cathelicidins in the Innate Host