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Non-immune salivary proteins--including lactoperoxidase, lysozyme, lactoferrin, and histatins--are key components of the innate host defense system in the oral cavity. Many antimicrobial proteins contain multiple functional domains, with the result that one protein may have more than one mechanism of antimicrobial activity. These domains may be separated by proteolytic cleavage, creating smaller proteins with functional antimicrobial activity in saliva as described for lysozyme, lactoferrin, and histatins. These small cationic proteins then exert cytotoxic activity to oral bacteria and fungi. Salivary histatin 5 initiates killing of C. albicans through binding to yeast membrane proteins and non-lytic release of cellular ATP. Extracellular ATP may then activate fungal ATP receptors to induce ultimate cell death. This mechanism for fungal cytotoxicity may be shared by other antimicrobial cationic proteins. Microbicidal domains of salivary and host innate proteins should be considered as potential therapeutic agents in the oral cavity.

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Salivary histatins (Hsts) are antifungal peptides with promise as therapeutic agents against candidiasis. Hst 5 kills the fungal pathogen Candida albicans via a mechanism that involves release of cellular ATP in the absence of cytolysis. Here we demonstrate that released ATP has a further role in Hst 5 killing. Incubation of the cells with ATP analogues induced cell death, and addition of the ATP scavenger apyrase to remove extracellular ATP released during Hst 5 treatment resulted in a reduction in cell killing. Experiments using anaerobically grown C. albicans with decreased susceptibility to Hst 5 confirmed that depletion of cellular ATP as a result of ATP efflux was not sufficient to cause cell death. In contrast to Hst-susceptible aerobic cultures, anaerobically grown cells were not killed by exogenously applied ATP. These findings established that Hst binding, subsequent entry into the cells, and ATP release precede the signal for cytotoxicity, which is mediated by extracellular ATP. In a higher-eukaryote paradigm, released ATP acts as a cytotoxic mediator by binding to membrane nucleotide P2X receptors. Based on a pharmacological profile and detection of a C. albicans 60-kDa membrane protein immunoreactive with antibody to P2X 7 receptor, we propose that released ATP in response to Hst 5 activates candidal P2X 7 -like receptors to cause cell death.Candida albicans is the most prevalent human fungal pathogen causing severe mucosal and systemic infections in hosts with compromised immune systems (5, 28). The toxicity of the currently used polyene antimycotic drugs and emergence of resistant candidal species to the less toxic azole-based agents have initiated a search for innate antibiotics as alternative drug therapies. Innate host defense systems are evolutionarily ancient and are characterized by production of potent antimicrobial molecules that limit infections based on their capacity to selectively discriminate pathogens from self species (21). The first-line host defense of human saliva includes many proteins with potent antibacterial and antifungal activity against resident microflora in the oral cavity.Histatins (Hsts) are 3-to 4-kDa structurally related histidine-rich basic proteins of salivary acinar cell origin that are expressed only in humans and higher subhuman primates (30). Hsts possess in vitro antimicrobial activities, and their efficacy is highest for oral yeasts, particularly C. albicans. Hst 3 (32 amino acids) and Hst 5 (the N-terminal 24 amino acids of Hst 3 generated by proteolytic cleavage) are the most potent candidacidal members of the family in vitro, killing yeast and filamentous forms of Candida species at physiological concentrations (15 to 30 M) (32, 44). Salivary Hsts have potential as therapeutic agents in patients with oral candidiasis, since they are potent antifungal agents while being nontoxic to humans.The physiological activities of the naturally occurring antimicrobial peptides, such as magainins (26), cecropins (2), defensins (20, 22), and bactenecins (33), have been ...