Impaired neutrophil function in 24p3 null mice contributes to enhanced susceptibility to bacterial infections

Lipocalin 24p3 (24p3) is a neutrophil secondary granule protein. 24p3 is also a siderocalin, which binds several bacterial siderophores. It was therefore proposed that synthesis and secretion of 24p3 by stimulated macrophages or release of 24p3 upon neutrophil degranulation sequesters iron-laden siderophores to attenuate bacterial growth. Accordingly, 24p3-deficient mice are susceptible to bacterial pathogens for which siderophores would normally be chelated by 24p3. Specific granule deficiency (SGD) is a rare congenital disorder characterized by complete absence of proteins in secondary granules. Neutrophils from SGD patients, who are prone to bacterial infections, lack normal functions, but the potential role of 24p3 in neutrophil dysfunction in SGD is not known. In this study, we show that neutrophils from mice genetically deficient for lipocalin 24p3 (24p3(-/-)) are defective in many neutrophil functions. Specifically, neutrophils in 24p3(-/-) mice do not extravasate to sites of infection and are defective for chemotaxis. A transcriptome analysis revealed that genes that control cytoskeletal reorganization are selectively suppressed in 24p3(-/-) neutrophils. Additionally, small regulatory RNAs (microRNAs) that control upstream regulators of cytoskeletal proteins are also increased in 24p3(-/-) neutrophils. Further, 24p3(-/-) neutrophils failed to phagocytose bacteria, which may account for the enhanced sensitivity of 24p3(-/-) mice to both intracellular (Listeria monocytogenes) and extracellular (Candida albicans and Staphylococcus aureus) pathogens. Listeria does not secrete siderophores, and additionally, the siderophore secreted by Candida is not sequestered by 24p3. Therefore, the heightened sensitivity of 24p3(-/-) mice to these pathogens is not due to sequestration of siderophores limiting iron availability, but is a consequence of impaired neutrophil function.