Antimicrobial Peptides In Vertebrates - A Chemical Defense System

Vertebrates must defend themselves against host invasion of occasional and obligate pathogens as well as against uncontrolled proliferation of commensal micro-organisms. In addition to the highly specific cell-mediated immune system, vertebrate animals are endowed for that purpose with an additional chemical defense system involving antimicrobial peptides analogous to those found in invertebrates. These peptides are produced by lymphoid tissues as well as by non-lymphoid cells of the mucous epithelia of the respiratory system, the intestinal system and the skin and cause cell lysis and death of a broad array of micro-organisms including bacteria, fungi and protozoa. Most of the antimicrobial peptides so far isolated from vertebrates are small-sized and positively charged and fall into 3 distinct groups: peptides with amphipathic-alpha-helix structure, disulfide bridged peptides with amphipathic-beta-sheet structure and proline-rich peptides. Whereas the precise mechanism of action of these peptides remains to be defined, their microbicidal effects are very likely due to their capacity to form ion channels which span the microbial membrane and permeabilize the cell. This cell-free defense system provides vertebrates with small-sized peptides that are promptly synthesized upon induction, easily stored in large amounts and readily available for antimicrobial warfare.