Neutrophil swarming toward Cryptococcus neoformans is mediated by complement and leukotriene B4.

Swarming behavior of neutrophils has been noticed in both sterile injury and infection models and the mechanisms are being unveiled. So far, no in vitro model has been established to study neutrophil swarming to microbes. In the current study, using live-cell imaging, we observed in vitro neutrophil swarming toward Cryptococcus neoformans, a fungal pathogen causing human meningoencephalitis. Complement C3 and CD11b expression are essential for neutrophils to form cell swarms surrounding C. neoformans. Leukotriene B4 (LTB4) was quickly released by neutrophils during their interactions with C. neoformans. Blockade of LTB4 synthesis inhibited the swarming response to C. neoformans. Importantly, blockade of LTB4 synthesis also significantly reduced neutrophil recruitment in the lung vasculature of mice infected intravenously with C. neoformans, demonstrating a critical role of LTB4 in intravascular neutrophil swarming during infection. Together, this is the first report of neutrophil dynamics of swarming toward a microorganism in vitro, mediated by complement and LTB4.