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In vitro Antimicrobial Activity of the Human Neutrophil Cytosolic S-100 Protein Complex, Calprotectin, Against Capnocytophaga sputigena

Calprotectin is a complex of two anionic proteins found in abundance in the cytosol of neutrophils, certain macrophages, and oral epithelial keratinocytes. Bacteria of the genus Capnocytophaga are pathogens of periodontal origin which can cause systemic infection in neutropenic subjects. Recently, it has been observed that Capnocytophaga may be internalized by neutrophils within the cytosol rather than within a membrane-delimited phagosome. The purpose of this study was to test the in vitro antibacterial effect of the cytosolic complex, calprotectin, against Capnocytophaga sputigena. Calprotectin was purified from the cytosol of human neutrophils by gel filtration and anion exchange FPLC, and it exerted potent in vitro antimicrobial effects against C. sputigena. Net bacteriostatic activity was exerted up to 18 h, after which bactericidal effects were observed. Both net bacteriostatic and bactericidal activity occurred at concentrations above 20 μg/mL and exhibited identical dose-response characteristics. Particle counts increased in the presence of calprotectin, despite net bacteriostasis as assessed by changes in colony-forming units (CFU). Dose-response characteristics and direct particle counts suggested that net bacteriostatic effects were the result of balanced cell division and death, rather than suspension of cell division. We conclude that calprotectin can be a significant contributor to host defense against infection by Capnocytophaga.