Maintaining a sterile urinary tract: the role of antimicrobial peptides

The normally sterile urinary tract is constantly challenged by microbial invasion leading to a high prevalence of isolated, recurrent and catheter associated urinary tract infection. The continuous emergence of bacterial resistance following overuse of traditional antibiotics requires the urgent development of alternative treatment strategies. The involvement of innate immune mechanisms in host defense is an emerging field of microbiological research with recent work focusing on the urinary tract. We performed a comprehensive literature review to establish the current level of knowledge concerning the role of innate immunity and specifically antimicrobial peptides within the human urinary tract.

A systematic review of the literature was performed by searching PubMed from January 1988 to September 2008. Electronic searches were limited to the English language using the key words antimicrobial, peptide and urinary. Reference lists from relevant reviews were hand searched and appropriate articles were retrieved. The proceedings of conferences held in the last 2 years by the American Urological Association, European Association of Urology and British Association of Urological Surgeons were also searched.

Several defensive mechanisms have evolved in response to the threat of urinary infection, comprising physical factors and innate immune responses characterized by the expression of antimicrobial peptides. Antimicrobial peptides are small (less than 10 kDa), cationic and amphipathic peptides of variable length, sequence and structure with broad spectrum killing activity against a wide range of microorganisms including gram-positive and gram-negative bacteria. Several antimicrobial peptides have been identified in the urinary tract, and the amount and type of antimicrobial peptides expressed vary according to tissue source and disease state. These differences may reflect altered levels of innate response and, hence, susceptibility to infection. Antimicrobial peptides are already being exploited therapeutically for skin and endovascular catheter infection, and prospects for useful application in the urinary tract are emerging.

Although investigation of antimicrobial peptide function in the human urinary tract is at an early stage, it is clear that there is considerable potential for the future design of novel therapeutic strategies. More knowledge is needed concerning the pathway of involvement of antimicrobial peptides in the maintenance of urinary tract sterility and the ways in which this is altered during active infection.