Antimicrobial Activity of an Abundant Calcium-Binding Protein in the Cytoplasm of Human Neutrophils

Studies of experimental infections in animals indicate that phagocytic cells may sometimes control infective foci without actually ingesting or contacting the invading microorganisms. In the present study, an effective inhibitor of Candida albicans growth, previously detected in neutrophil cytoplasm and found to be released only after lysis of the cells, was identified as an abundant calcium-binding protein originally described in neutrophils as the L1 myelomonocytic antigen or the cystic fibrosis antigen. This substance was demonstrated also to have static activity against several other important human pathogens, including Aspergillusfumigatus, Staphylococcus aureus, and Escherichia coli. Growth of the various microorganisms was inhibited to considerably different degrees by the neutrophil protein, with the effects on S. aureus (the least responsive organism) being significantly enhanced by addition of calcium to the medium. These findings suggest that after its release by the death of neutrophils at sites of tissue infection, this abundant calciumbinding protein could have a host defense function by controlling the growth of pathogenic microorganisms that escape being killed initially and would otherwise be free to proliferate.