Our case report highlights several peculiar aspects of late-onset X-linked CGD in female patients, underscoring the need for high clinical suspicion, but also the possibility of curative HSCT even in patients over 50 years of age. In our case, an immunodeficiency was suspected due to the occurrence of a severe and unusual infection in a woman with normal white blood cell counts and normal serum immunoglobulins without a significant previous infection history. She did have a history of discoid lupus, a finding frequently reported in female carriers of X-linked CGD [21], yet this association may go unnoticed. After secondary causes of immunodeficiency were ruled out, CGD was suspected and eventually confirmed by the finding of abnormal results at DHR testing. The presence of two functional populations of neutrophils, together with the detection of a recessive mutation in the CYBB gene confirmed the diagnosis of X-linked recessive CGD in a woman with skewed X chromosome lyonization. It is already known that female carriers can present with clinical manifestations later in life due to the aging of their hematopoietic cell lineage and to progressive skewing of X chromosome inactivation [11–18]. The factors leading to skewing of X chromosome inactivation in some female carriers are not completely known. X chromosome inactivation is usually a random event, yet the presence of micro-deletions or other kinds of deleterious damages on one of the two X chromosomes may lead to its preferential inactivation, leading to a skewed X inactivation [22–25]. Moreover, differential DNA methylation has been described in hematopoietic cell lineage with aging, possibly resulting in greater functional skewing [26–29]. These processes may ultimately result in the selection of mutated alleles in other genes, as in the case of X-linked CGD female carriers, and thus to clinical disease when the number of functional neutrophils falls under a critical level [11–18].
Current treatment of CGD is based on prevention of severe infections, prompt and intensive antibiotic therapy, and proper treatment of granulomas and inflammatory complications. In our patient, the first episode of life-threatening pneumonia resolved only after rescue treatment with IFNγ was started. IFNγ enhances phagocyte activity in healthy subjects through NADPH oxidase and inducible isoform of Nitric Oxide Synthase (iNOS) transcription [30]. Even in CGD patients, IFNγ increases the oxidative burst in defective neutrophils both in vitro and in vivo, although though the mechanism is not fully understood [31–34]. Although IFNγ role and effectiveness for infection prophylaxis and treatment in CGD patients is still debated [35–37], our experience may suggest that it could have a greater role in heterozygous females as compared to male patients, as it could potentiate oxidase activity of the residual part of neutrophils expressing the wild type allele [38]. In fact, DHR testing in our patient showed a decreased oxidative burst also in the residual oxidase-positive neutrophils, possibly due to the concomitant sepsis, which can induce immunoparalysis of both innate and adaptive immunity [39–41]. Indeed, immunoparalysis may be a critical issue in subjects starting with already compromised immune functions.
HSCT remains the only definitive treatment for X-linked CGD to date, with a higher success rate in younger patients. Older patients, on the other hand, are more likely to develop transplant complications and have higher transplant-related mortality due to active infections, inflammation, and organ damage at the time of transplantation [42-44]. Gene therapy is also under development, with promising outcomes recently, but availability is still limited [45]. Our patient underwent HSCT because of her worsening condition despite standard prophylactic treatments (antibiotics, anti-fungal, steroids, and IFNγ). She was considered at high risk for transplant-related morbidity and mortality due to her age and the presence of active infectious and inflammatory complications; therefore, a non-myeloablative conditioning regimen followed by high dose PBSC graft was used. Reduced-intensity or reduced-toxicity regimens with low-dose busulfan (AUC 45-65 mg/L*h) or treosulfan have been shown to be associated with a higher over-survival (>90%) and event-free-survival (>85%) compared to myeloablative regimens, [46,47] but can lead to mixed chimerism as a long-term outcome as occurred in this patient. Her donor neutrophil chimerism remains stable at 60% which appears to be sufficient to maintain her current state of good health. The NIH regimen incorporates low dose busulfan (30-40mg/L*h) and alemtuzumab (1 mg/kg) for patients with related donors and even lower dose busulfan (median AUC of 20-30mg/L*h) combined with low dose total body irradiation (TBI) and alemtuzumab followed by sirolimus as single agent GvHD prophylaxis [48]. For this patient, enrolled on a new protocol, she also received post-transplant cyclophosphamide along with a high dose of unmanipulated PBSC (10 million CD34 per kg) to ensure better engraftment but prevent GVHD. This HSCT protocol then resulted in stable donor chimerism with minimal transplant morbidity despite the patient’s risk factors and age. With follow up of more than three years, this suggests that transplantation using this regimen is viable even in older patients with high risk factors.
This case demonstrates a number of points. First of all, it underscores an emerging cornerstone of clinical immunology, i.e. that even a single severe or unusual infection should raise the suspicion of a primary immune deficiency disorder, even in previously healthy adults. Secondly, IFNγ may have a role as a rescue target therapy in severe infections, especially in females with X-linked CGD, as these patients still have a fraction of normal neutrophils. Lastly, HSCT with non-myeloablative conditioning with low dose busulfan and alemtuzumab, combined with an increased stem cell dose and post-transplant cyclophosphamide may represent a curative option even in high-risk patients at an advanced age.