Reconstitution of protection against Aspergillus infection in chronic granulomatous disease (CGD)

To the editor:

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 gp91^phox 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 referred to in the Bianchi paper. It might be possible that reconstitution of other superoxide-dependent steps, such as direct effects or indirectly via IDO activity, rather than the restoration of the capability of NET formation, will protect CGD patients from Aspergillus infection.