Submitted to: Veterinary Immunology and Immunopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/3/2006
Publication Date: 9/20/2006
Citation: Lippolis, J.D., Reinhardt, T.A., Goff, J.P., Horst, R.L. 2006. Neutrophil extracellular trap formation by bovine neutrophils is not inhibited by milk. Veterinary Immunology and Immunopathology. 113(1-2):248-255. Interpretive Summary: Infection of the udder (mastitis) is a major economic problem for the dairy industry. White blood cells are what fight infections. This work concentrates on immune cells called neutrophils. Neutrophils fight infections by several mechanisms meant to kill bacteria. However, many of these anti-bacterial mechanisms are not functional when the neutrophils are in milk. Recently a new neutrophil anti-bacterial mechanism has been found in human neutrophils. We show that cattle neutrophils have the same mechanism and importantly this new mechanism is functional in milk. Since this new bacterial killing mechanism is functional in milk, while other neutrophil killing mechanisms are not, this may be a critical means by which the immune system clears an infection in the udder. This information will help us better understand and fight mastitis in the dairy industry.
Technical Abstract: Neutrophils are the first line of defense in a mammary gland infection. However, the process of neutrophil transmigration across a membrane and ingestion of fat and/or casein when incubated in milk have been shown to inhibit bacterial phagocytosis and oxidative burst functions. Recently, a killing mechanism has been described whereby stimulated neutrophils release nuclear and granule material in fibrous webs that physically trap and kill bacteria. We demonstrate that these neutrophil extracellular traps are also produced by bovine neutrophils stimulated with PMA/Ionomycin. Significantly, neutrophil extracellular traps can be formed when neutrophils have been incubated for up to 6 hours in milk prior to stimulation. This contrasts milk’s rapid inhibition of bacterial phagocytosis and oxidative burst functions in the neutrophil. Furthermore, stimulation of neutrophils with bacteria common to mammary gland infections leads to neutrophil extracellular traps being formed in milk. Some bacteria tested stimulated enhanced formation of neutrophil extracellular traps in milk compared to culture media. Therefore, being unaffected by incubation in milk may indicate a central role for neutrophil extracellular traps in defense against mastitis. Neutrophil extracellular traps may be the key innate immune mechanism in mammary infections because of their ability to function in the milk environment in contrast to other neutrophil functions important for bacterial clearance such as phagocytosis and oxidative burst that are inhibited by milk.