We would like to comment on the recent paper by Bianchi and coworkers. 1 Patients diagnosed with chronic granulomatous disease (CGD) suffer from recurrent and life-threatening infections. Analysis of clinical data from 429 European CGD patients recently confirmed that Aspergillus is a major threat, causing mostly pneumonia but also brain abscesses. 2 Reconstitution of an intact X-linked gene encoding gp91phox is a remarkable accomplishment and a promising new therapy for X-CGD. In their Blood paper, Bianchi et al correlate the reconstituted nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity in this patient with protection against Aspergillus infection, as a result of reconstituted neutrophil extracellular trap (NET) formation. NETs are composed of chromatin decorated with granular proteins and bind Gram-positive and-negative bacteria, as well as fungi, in a nonspecific way. Recently, ex vivo NET formation was shown to depend on superoxide production by activated NADPH oxidase. 3 The only empirical data provided by Bianchi and coauthors to underscore the crucial role of NET formation against Aspergillus infection are ex vivo observations showing a microbicidal effect of NETs against Aspergillus species. Treating the ex vivo findings as proof of a causal link between (reconstituted) NET formation and protection against Aspergillus may be a step too far and may not be essential. NETs do not have any specificity in microbicidal activity, whereas CGD patients show a selective susceptibility only to certain microbes. Moreover, Aspergillus infection has recently been studied in a CGD mouse model, and activity of indoleamine 2, 3-dioxygenase (IDO) was shown to be crucial for the survival of Aspergillus infection. 4 IDO converts L-tryptophan into L-kynurenine but requires superoxide as a cofactor for its activity. Secreted L-kynurenine subsequently acts as an anti-inflammatory agent by mechanisms that are incompletely understood but have been shown to induce cell death in proinflammatory γδ T-cell subsets producing interleukin-17. 4 Romani et al thus concluded that hyperinflammation caused a lethal outcome for CGD mice upon challenge with Aspergillus, rather than a defective clearance itself, as was previously suggested in CGD patients with an overwhelming pulmonary aspergillosis. 5 This seems plausible in view of a large body of evidence showing that CGD patients often display exaggerated immune responses against immunologic challenges, 6 and granulomas have been shown to develop in the absence of any detectable pathogen, even after wound sterilization or after injection of heat-inactivated pathogens. 7

Romani and coworkers underscore their conclusion by demonstrating that CGD mice, which all died upon Aspergillus challenge, survive this infection when treated with the IDO product L-kynurenine in combination with interferon-γ. In turn, wild-type mice, which normally survive this challenge, no longer overcome Aspergillus infection when treated with the IDO inhibitor 1-methyl tryptophan. 8 Although the specificity of the challenge and the background of the CGD mouse strain are also subject for debate, the abovementioned relevant findings were not discussed or