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We describe a previously unappreciated role for Bruton's tyrosine kinase (BTK) in fungal immune surveillance against aspergillosis, an unforeseen complication of BTK inhibitors (BTKi) used for treating B cell lymphoid malignancies. We studied BTK-dependent fungal responses in neutrophils from diverse populations, including healthy donors, patients who were treated with BTKi, and X-linked agammaglobulinemia patients. Upon fungal exposure, BTK was activated in human neutrophils in a TLR2-, Dectin-1-, and Fc[gamma]R-dependent manner, triggering the oxidative burst. BTK inhibition selectively impeded neutrophil-mediated damage to Aspergillus hyphae, primary granule release, and the fungus-induced oxidative burst by abrogating NADPH oxidase subunit [p40.sup.phox] and GTPase RAC2 activation. Moreover, neutrophil-specific Btk deletion in mice enhanced aspergillosis susceptibility by impairing neutrophil function, not recruitment or lifespan. Conversely, GM-CSF partially mitigated these deficits by enhancing [p47.sup.phox] activation. Our findings underline the crucial role of BTK signaling in neutrophils for antifungal immunity and provide a rationale for GM-CSF use to offset these deficits in patients who are susceptible., Introduction Invasive aspergillosis (IA), most often caused by the ubiquitous inhaled mold Aspergillus fumigatus, is an opportunistic fungal infection that exploits numeric or functional neutrophil defects (1-3). IA affects over [...]