Abstract |
Neutrophils serve as the first line of defense in our immune system against microbial
pathogens, especially fungi. When compared to other phagocytes, neutrophils employ
an impressive array of specialized effector mechanisms, including phagocytosis,
degranulation and extracellular vehicle release, neutrophil extracellular trap (NET)
formation, and coordinated clustering towards a target (swarming), to combat invading
pathogens. The dynamic changes in size, shape, and surface composition of cell wall
of airborne filamentous fungi (molds) during physiological interaction with phagocytes
triggers unique immune responses, making them model pathogens to understand
neutrophil biology. Accordingly, molds such as Aspergillus spp. and the Mucorales are
emerging causes of life-threatening respiratory infections in an expanding population
of immunocompromised patients with incompletely understood defects in neutrophil
function. In this study, we aimed to gain molecular insights on the physiological
mechanisms of neutrophil decision-making during interaction with Mucorales conidia.
Of interest, Mucorales, in stark contrast to other fungal pathogens, are inhibited via
neutrophil swarming.
We found that neutrophils form specifically clusters (swarm) around the head of
germinating conidia of Mucorales, a response that is mediated by β-glucan exposure
on this region of the fungal cell wall. Recognition of this immunostimulatory
polysaccharide occurs via activation of Complement Receptor 3 (CR3), rather than
Dectin-1, the specialized receptor for β-glucan in macrophages. This recognition
induces a robust and prolonged Ca2+ influx. Neutrophil activation during swarming is
characterized by excessive expansion of the plasma membrane associated with
profound changes in actin cytoskeleton organization. Enzymatic degradation of βglucan abolishes the swarming phenotype, rendering the fungus immunologically
inert. Importantly, we additionally found that the mechanosensitive enzyme cPLA2,
which is physiologically regulated by cytosolic Ca2+ and changes in membrane
tension, orchestrated initiation of neutrophil swarming towards Mucorales through its
translocation to the nuclear membrane for subsequent LTB4 release. Collectively, our
findings set the stage for a new understanding of cell-cell interactions using the fungal
cell wall as a model.
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