The role of the recruited neutrophil in the innate response to Aspergillus fumigatus
Bonnett, Colin Russell
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Strong clinical and experimental evidence links qualitative and quantitative neutrophil deficiencies to fatal infections caused by A. fumigatus. Yet the role of the neutrophil in mediating the protection observed in normal hosts remains largely unknown. Recent studies indicate neutrophils from CXCR2-/- mice are unable to migrate toward chemokine gradients of KC and MIP-2, rendering these animals susceptible to fatal aspergillosis. Mice with a mutation in the gene encoding for the gp91phox component of the NADPH oxidase lack the ability to generate the reactive oxygen metabolites used by phagocytes in killing microbial pathogens, and these mice are also susceptible to invasive pulmonary aspergillosis. In this investigation of the innate response to A. fumigatus, CXCR2-/- and gp91-/- mice were used to mimic the qualitative and quantitative neutrophil defects that are known to predispose to invasive pulmonary aspergillosis, the most lethal form of Aspergillus diseases. By comparing the nature of the predisposition of these mice with the robust immunity observed in normal and immunocompetent mice, insight was gained on the involvement of this key phagocyte in the innate response. Several important parameters of the innate response in the lung were analyzed in this investigation, including (1) leukocyte recruitment and organism engagement, (2) organism killing mechanisms, (3) cytokine levels of bronchoalveolar lavage fluid, and (4) in vitro organism killing by leukocytes. Following intratracheal challenge of A. fumigatus conidia a delay of 3 hours in neutrophil recruitment to lungs of CXCR2-/- animals was observed, allowing significant conidial germination and hyphal formation not seen in normal animals. In contrast, the gp91phox-/- mice recruited neutrophils normally but failed to inhibit conidial germination and hyphal proliferation, apparently as a result of their inability to generate the appropriate conidiacidal mediators. In normal mice inoculated conidia were rapidly sequestered within neutrophil aggregates involving a response by the phagocyte NADPH oxidase detected by formazan deposition. These results suggest a previously undescribed role for neutrophils that conduct early inflammatory events following exposure to A. fumigatus conidia, involving first sequestration of ungerminated conidia within neutrophil complexes, and subsequently oxidant generation that prevents hyphal outgrowth.