Manipulating neutrophil degranulation as a bacterial virulence strategy

Neutrophil degranulation during bacterial infectionArmed with an arsenal of antimicrobial mechanisms, neutrophils are among the first innate immune cells recruited to the site of bacterial infection.Neutrophils utilize both oxidative and non-oxidative strategies to kill invading microorganisms.Components for non-oxidative killing include antimicrobial proteases that are packaged within intracellular vesicles ("granules").Neutrophil granules are preformed vesicles of a defined composition that are released in a regulated manner [1].The process by which neutrophils mobilize granules is called degranulation.Degranulation can occur at the plasma membrane for extracellular release (killing extracellular microorganisms) or to the phagosome for intracellular delivery (killing intracellular microorganisms) [1].Extracellular degranulation is a double-edged sword of neutrophil antimicrobial function: The antimicrobials contained within granules can kill bacteria, but excessive degranulation can damage host tissue [2].Neutrophil granules can be broadly categorized into 4 main types: secretory, tertiary, secondary, and primary.The release of each granule type occurs sequentially, with secretory granules released readily throughout the neutrophil life span to replenish cell surface receptors and primary granules requiring the greatest stimulus for release [1].These granule types are classified based on the specific proteins contained within the lumen or in the membrane of each vesicle [3,4].Secretory granules contain plasma proteins and Fc and complement receptors.The contents of tertiary granules include matrix metalloproteases such as matrix metallopeptidase 9 (MMP9).Secondary granules contain proteins such as lysozyme, pre-cathelicidin, and lactoferrin.Primary granules contain the most pro-inflammatory and antimicrobial proteins, such as myeloperoxidase, defensins, elastase, and azurocidin [5].Additionally, during a normal neutrophil degranulation response, most of primary and secondary granule release is directed to the phagosome as a mechanism for minimizing damage to host tissue [6].The extracellular release of each granule type can be assessed experimentally by detecting amounts of granule proteins present in supernatants by western blotting or ELISA or by quantifying the display of specific membrane-bound proteins (such as CD66b for secondary granules and CD63 for primary granules) on the surface of the neutrophil by flow cytometry or immunofluorescence microscopy.Several bacterial pathogens are known to manipulate neutrophil degranulation as a virulence strategy (Fig 1).By disrupting, dysregulating, or inducing excessive neutrophil degranulation, bacteria can skew the protective effects of neutrophil degranulation in a way that ultimately benefits the pathogen and worsens disease.Understanding the mechanisms by which bacteria alter neutrophil degranulation can provide greater insight into bacterial pathogenesis as well as advance our understanding of neutrophil vesicle trafficking.