Introduction

Chronic granulomatous disease (CGD) was initially called “fatal granulomatous disease of childhood” in the 1950s [1]. It is a rare primary immunodeficiency disease characterized by recurrent life-threatening bacterial, fungal infections, and tissue granuloma formation, which is caused by defects in genes encoding the subunits of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex [2]. The incidence of CGD is estimated at approximately 1/200,000 live births in the United States, and the highest incidence estimated to be 1.5/100,000 in the Israel Arab population [3,4,5].

Pathogenesis

The NADH oxidase is a multicomponent enzyme consisting of a membrane-bound, heterodimeric b-type cytochrome comprising the subunits gp91phox (also known as Nox2) and p22phox, together with cytoplasmic subunits p47phox, p40phox, and p67phox. Upon activation, the cytosolic components translocate to the membrane and associate with a cofactor Rac2 GTPase. In a recent study, essential for reactive oxygen species (Eros) transmembrane protein acts as a chaperone and is required for the gp91phox and p22phox heterodimer expression [6, 7] (Fig. 1).

Fig. 1
figure 1

Structure and function of the NADPH oxidase. NADPH oxidase consists of gp91phox, p22phox in the plasma membrane and intracellular granules. After phagocytes activation, the cytosolic components p47phox, p40phox, p67phox and the GTPase protein Rac2 are translocated to the membrane of phagosome, where they assemble with p22phox and gp91phox to form a functional NADHP oxidase. Eos, a chaperone protein, is required for the gp91phox and p22phox heterodimer expression. The NADPH oxidase catalyze the formation of superoxide free radical (O2−) by transferring an electron from NADPH to molecular oxygen (O2). The superoxide is converted to hydrogen peroxide (H2O2) spontaneously or by superoxide dismutase (SOD). Hydrogen peroxide is converted to more potent reactive oxidants, such as OH˙ or HOCl by myeloperoxidase (MPO) for bacterial killing. The activation of NADPH oxidase suppresses NF-ĸB activity, the following nuclear transcription of proinflammatory cytokines, and the inflammasome-mediated IL-1β production in macrophages. NADPH oxidase enhances autophagy and neutrophil extracellular traps activation and release (NETosis) in stimulated neutrophils

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The NADPH oxidase transfers electrons to molecular oxygen to generate superoxide anion (O2−), which is dismutated to hydrogen peroxide (H2O2). These two primary reactive oxygen species (ROS) can be further transformed into more reactive metabolites, such as hydroxyl radical (OH˙) or hypochlorous acid (HOCl). HOCl is highly microbicidal, and its formation requires neutrophil myeloperoxidase (MPO) [8]. ROS are chemically reactive metabolites of oxygen. The phagocyte NADPH oxidase generates higher levels of ROS than other cellular oxidases such as mitochondria or nitric oxide synthases.

Phagocytes NADPH oxidase can be assembled in the plasma membrane and membranes of intracellular vesicles. ROS production in phagocytes, which is also known as a “respiratory burst” reaction, contributes to microbial killing and triggering the formation of neutrophil extracellular traps (NETs) [9]. NETs are decondensed chromatin fibrils coated with granular proteases and histones, which can trap and kill extracellular pathogens. Neutrophils from patients with CGD are deficient in NETosis in previous reports [10, 11].

Mice defective for ROS and patients with CGD have a proinflammatory phenotype. Nox2 expressed in neutrophils and macrophages limits NF-ĸB activation and pro-inflammatory cytokines production [12]. NADPH oxidase deficient phagocytes from patients with CGD display a defect in autophagy and activation of inflammasome [13,14,15], which leads to long-lasting production of proinflammatory cytokines (TNF-α, IL-1β, IL-6), interferons, chronic hyper-inflammation, and autoimmunity [14, 16]. The increased L-1β levels by activated macrophages provide a signal for the massive influx of neutrophils in the granulomatous lesions. Rescue of a functional NADPH oxidase complex Ncf1/p47phox in mononuclear phagocytes (macrophages and dendritic cells), not in neutrophils, leads to the loss of the hyper-inflammation in a transgenic mice model of CGD [16]. Recent studies also demonstrated the crucial roles of NADPH oxidase in limiting systemic inflammation and immunomodulation in innate and adaptive immune system [12].

The genetic basis for CGD is summarized in Table 1. Germline null mutations in CYBB, the gene encoding the gp91 subunit of the NADPH oxidase, impair the respiratory burst of all types of phagocytes and result in X-linked CGD. Over 90% of patients with NCF1 gene mutations in the p47phox subunit of the oxidase complex carry the deletion c.75_76delGT (ΔGT) [2, 17, 18]. Patients with Eros deficiency due to CYBC1 homozygous mutation presented as infectious and inflammatory symptoms due to loss of respiratory burst and gp91phox expression in phagocytes [6].

Table 1 Genetic defects of chronic granulomatous disease (CGD)
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Diagnosis

According to European Society for Immunodeficiencies (ESID) working definitions for the clinical diagnosis of inborn errors of immunity, patients with at least one of the following need work-up for CGD: (1) deep-seated infection due to bacteria and/or fungi (abscesses, osteomyelitis, lymphadenitis), (2) recurrent pneumonia, (3) lymphadenopathy and/or hepatomegaly and/or splenomegaly, (4) obstructing/diffuse granulomata in gastrointestinal or urogenital tract, (5) chronic inflammatory manifestations (colitis, liver abscess, and fistula formation); (6) failure to thrive, (7) affected family member [19].

The nitroblue tetrazolium (NBT) test has been used historically to measure superoxide generation in phagocytes after phorbol myristate acetate (PMA) stimulation. The yellow-colored dye is reduced by the NADPH oxidase complex in neutrophils to formazan, which is a dark-blue precipitate in the stimulated phagocytes [20]. Since the late 1990s, the activity of oxidase activation can be measured by flow cytometry using fluorescent probes 2′7’-dichlorofluorescein diacetate (DCF) or dihydrorhodamine (DHR)-123. In stimulated neutrophils, DHR-123 is oxidized by hydrogen peroxide (produced in the presence of normal NADPH oxidase and myeloperoxidase) to rhodamine-123 that emits fluorescence [21]. Frequently used stimuli are PMA, serum-treated zymosan and the bacterial peptide formyl-methionyl-leucyl-phenylalanine (fMLP) [22]. The diagnosis of CGD can be made by the absent or significantly decreased respiratory burst in stimulated neutrophils by NBT, DHR, or DCF test at least twice [19, 21].

The mean fluorescence intensity of DHR test has been shown to be correlated with reactive oxygen intermediate production and survival in CGD patients. Compared with absent superoxide production levels in gp91phox deficient patients, p47phox deficient patients were associated with decreased (not absent) ROS production and had better survival [23]. The PMA-induced DHR oxidation assay can differentiate between X-linked CGD, autosomal recessive (AR)-CGD, and X-linked carrier status [21]. However, routine DHR assay could be normal or mildly impaired in patients with NCF4/p40phox deficiency. Using serum-opsonized E.coli as a stimulus in DHR tests to diagnose NCF4/p40phox deficiency is recommended [24].

Abnormal DHR neutrophil assay results should be confirmed with gene mutation analysis, which is also mandatory for genetic counseling. Genetic and prenatal testing can be useful for screening for other family member carriers. Besides, large deletions in Xp21.1 have been reported in some patients with X-linked CGD. A chromosome microarray analysis is indicated for patients suspected as X-linked CGD but with clinical manifestations of other genetic defects that lie in close proximity to CYBB at Xp21.1, such as McLeod syndrome (Duchenne muscular dystrophy, retinitis pigmentosa, or ornithine transcarbamylase deficiency) [25].

Differential Diagnosis

Hypomorphic (p.Thr178Pro, p.Gln231Pro) missense mutations of CYBB gene cause defective respiratory bursts in macrophages and B cells (not in neutrophils or monocytes) and X-linked recessive Mendelian susceptibility to mycobacterial disease (MSMD) [26]. RAC2 deficiency results in decreased neutrophil chemotaxis, ROS generation, and neutrophil immunodeficiency syndrome [27]. Myeloperoxidase (MPO) deficiency has defects in the oxidation of hydrogen peroxide to hypohalous acid in phagocytes. Patients frequently are asymptomatic or otherwise are susceptible to Candida infections. Complete MPO deficiency exhibit low DHR signals and normal NBT test results [28]. A rare, severe form of glucose-6-phosphate dehydrogenase (G6PD) deficiency lacks sufficient NADPH generation for the NADPH oxidase in the leukocytes, thus similar to a mild type of CGD [29].

Clinical Presentations

Most patients with CGD present initially with infections early in life and are diagnosed before five years old. Patients with X-linked CGD tend to present with earlier onset and more severe disease than patients with AR-CGD [30]. AR-CGD can present in adulthood in which residual production of superoxide can be seen [21]. ROS production mainly mediates the effect of genotype on phenotype and has been directly associated with survival [23]. NCF4/p40phox deficiency is considered as a mild, atypical form of CGD that patients suffer from hyper-inflammation and peripheral infections (mostly Staphylococcal), but do not have invasive bacterial or fungal infections commonly seen in X-linked CGD [24].

Infection

Patients have increased susceptibility to infections by catalase-positive organisms, including Staphylococcus aureus, Burkholderia cepecia complex, Serratia marcescens, Nocardia species, and Aspergillus species in the lungs, lymph nodes, skin, liver, and bones [30, 31]. Invasive fungal infections remain the most common cause of mortality in CGD, including Aspergillus species, Phaeoacremonium parasiticum, and Candida species [30]. Aspergillus spp. (most commonly Aspergillus fumigatus or A. nidulans) account for 35% of all deaths attributable to infections in patients with CGD, which often presented as pneumonia or pneumonia with dissemination to the liver or brain [3]. Tuberculosis, Mycobacterium bovis Bacille Calmette-Guérin (BCG), or Salmonella species are also important causes of infections in regions of the world where these organisms are prevalent, or BCG vaccine is routinely administered at birth [32]. Rarely, Actinomyces species (catalase-negative organisms) infection, or methylotrophs (Granulibacter bethesdensis, Acidomonas methanolica, Methylobacterium lusitanum) necrotizing lymphadenitis or fatal meningitis also have been reported [33, 34]. Common and uncommon organisms found in patients with CGD are summarized in Table 2 and Supplementary Table 1 [30, 32, 33, 35,36,37]. The application of genomic or 16S rRNA sequencing and molecular probes to target tissues can identify the microorganisms that are difficult to culture, such as Phellinus or methylotroph infections.

Table 2 Common organisms and treatment of infections in CGD
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Inflammation and Autoimmunity

Dysregulation of pro-inflammatory cascades and defective apoptosis predispose patients to granulomatous inflammation, typically in the lungs, gastrointestinal and genitourinary tracts [17, 38]. Nearly 50% of patients encounter inflammatory bowel disease (IBD), which resembles Crohn’s disease [39]. Chronic complications, including growth failure, decreased pulmonary function, poor quality of life are common in patients who have not undergone a transplant [40, 41].

A high incidence of infections, accompanied by an inappropriate hyperinflammatory response may render these patients with CGD at risk of developing hemophagocytic lymphohistiocytosis (HLH)/macrophage activation syndrome (MAS). There were 29 CGD patients with CYBB, CYBA, NCF1, and CYBC1 mutations developed HLH/MAS in the previous literature [6, 32, 42,43,44].

Treatment

Acute Infections

Symptoms of infections are initially vague. A significant and sustained rise of laboratory markers of inflammation, such as the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), often indicates an occult infection that requires imaging (computed tomography or magnetic resonance imaging), microbiologic diagnosis, or tissue biopsy. Prolonged use of antibiotics/anti-fungal treatment is needed even for common bacterial or fungal infections. Voriconazole or posaconazole may empirically use if mold infection is suspected [31]. Granulocyte transfusions is safe to use in the setting of invasive fungal infections, but they could lead to alloimmunization and complicate future stem cell transplantation [45, 46]. The overall incidence and severity of infections decreased in the 2nd decade of life, but the increased risk of infections is lifelong [30].

Prophylactic Antibiotic and Anti-Fungal Therapy

Trimethoprim-sulfamethoxazole (TMP/SMX) and itraconazole prophylaxis is the cornerstone of prevention of bacterial and fungal infections [30, 47]. Prophylaxis with TMP/SMX prolonged life-threatening infection-free intervals from 1 episode every ten months to 1 episode every 40 months in patients with CGD [48]. TMP/SMX prophylaxis also decreased the incidence of bacterial infections by 56% (from 15.8 to 6.9 infections per 100 patient-months) in patients with X-linked CGD and 66% (from 7.1 to 2.4 infections per 100 patient-months) in patients with AR-CGD [49]. In patients with sulfa allergy, TMP alone can be used as an alternative treatment. In patients with TMP/SMX intolerance, other options, including fluoroquinolone, second- or third-generation cephalosporin or clindamycin can be considered [47].

Prophylaxis with itraconazole has shown to be effective in reducing the frequency of fungal infections and well-tolerated in patients with CGD [50]. It also prolonged the median time to invasive fungal infection from 4 years to 10 years in patients with CGD and decreased mortality rate [51]. Because of the emergence of itraconazole resistant organisms and occasional medication intolerance, the use of posaconazole or voriconazole as secondary prophylaxis has increased [47]. In recent studies in patients with X-linked CGD under azole prophylaxis, Phellinus species emerge as causes of indolent but refractory fungal infections. Sending tissue specimens for molecular diagnosis is essential in managing non-Aspergillus invasive fungal infections [37].

Interferon-Gamma (IFN-γ) Prophylaxis

Immunomodulatory therapy with IFN-γ is part of the prophylactic regimen in many centers, while some experts use IFN-γ only in the setting of acute infection, rather than as primary prophylaxis [35]. A randomized, double-blind placebo-controlled study showed that IFN-γ 50 μm/m2 3 times per week significantly reduced the number of hospitalizations and serious infections (22% treatment vs. 46% placebo group) after a follow-up period of 8.9 months [52]. However, a prospective study comparing prophylaxis with TMP/SMX and itraconazole versus addition of IFN-γ showed no significant difference in the rates of infections [53].

IFN-γ therapy can increase nitric oxide (NO) production by neutrophils and circulating NO levels in patients with CGD. NO acts as a bactericidal agent and plays a role in host defense [54]. In a recent study, IFN-γ has been documented to restore autophagy for Aspergillus through death-associated protein kinase 1 (DAPK1) pathway and inhibit inflammasome activation in mouse models and patients with CGD [55].

Common adverse effects of IFN-γ are flu-like symptoms (fever, fatigue, myalgia), rash, and local injection-site reactions (erythema or tenderness). Many experts consider the risk/benefit ratio to be favorable for the use of IFN-γ to prevent invasive fungal infections, particularly in young patients with X-linked CGD and those with a history of invasive fungal infections [35].

Inflammatory Complications

Corticosteroids

Corticosteroids may be useful for antral and urethral obstruction, severe granulomatous colitis, lung granuloma, and liver abscess. Liver abscesses occur in about one-third of patients and are commonly caused by S. aureus. Liver abscesses in CGD are frequently multi-loculated with a thickened pseudo-capsule. Under antibiotics treatment, surgical resection of liver abscesses is considered to be safe and associated with less recurrence and shortened hospitalization in patients with CGD [56]. Treatment with CGD-associated liver abscesses with corticosteroids (median dose 1 mg/kg/day and median duration of five months) and targeted antimicrobial therapy was associated with improved outcome and fewer subsequent hepatic interventions, compared to invasive treatments such as interventional radiology therapy or open liver surgery [57]. Steroids might reduce systemic inflammation, restore local immunity in the liver, and provide better tissue penetration of the antibiotics in a less inflamed environment [57].

Immunomodulators and Biological Therapies

The treatment of hyper-inflammation remains a challenge because immunomodulators can be associated with a higher risk of infections in CGD. The use of TNF-α antagonist infliximab has shown to be effective in treating CGD associated colitis, but it predisposes patients to severe infections, including deaths [58]. Extreme caution should be taken, such as aggressive antimicrobial prophylaxis and aggressive surveillance for infections under anti-TNF treatment. Clinical trials of IL-1β receptor antagonist anakinra in patients with CGD colitis have mixed results [59, 60]. Inhibition of mammalian target of rapamycin complex 1 (mTORC1) by rapamycin has been shown to inhibit inflammasome activation, IL-1β and TNF-α production of phagocytes in vitro through autophagy induction and immunomodulatory effect in phagocytes from patients with CGD [61]. However, the clinical effectiveness of rapamycin in the management of CGD related inflammatory complications remains to be assessed by clinical trials.

Pioglitazone or rosiglitazone, a peroxisome proliferator-activated receptor gamma (PPAR-γ) agonist, has been approved for the treatment of type 2 diabetes. It has been shown that pioglitazone restores phagocyte (monocytes and neutrophils) mitochondrial superoxide production, suppresses acute sterile inflammation (peritonitis), and enhances bactericidal capacity in vitro and in vivo in mice models with X-CGD [62,63,64]. The clinical improvement by partially restored inflammatory status and host defense was observed in a 5-month-old infant with X-linked CGD with multiple severe infections who received pioglitazone. After titration of pioglitazone to 3 mg/kg, a DHR assay showed oxidative activity of 12.9% of granulocytes (versus total absence respiratory burst before therapy) [65].

Pioglitazone and rosiglitazone can also induce significant NET formation in CGD patients [63]. Although the exact mechanism for induction of NETosis through mitochondrial ROS enhancement is unclear, it has been shown to depend on calcium-activated potassium channel and signaling cascades, including AKT and p38 [66]. In a recent study, Tamoxifen, an FDA-approved drug in use for breast cancer treatment, has been shown to restore NETosis and NET-mediated bacterial killing in a ROS-independent manner by activating autophagy in neutrophils from patients with CGD [11].

Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) can be curative for CGD. However, high-risk patients with intractable infection or autoinflammation as well as adolescents and young adults have traditionally been difficult cases for HSCT due to higher rates of organ dysfunction, graft failure, graft-versus-host disease (GVHD) and transplant-related mortality. Since levels of residual superoxide production have correlated well with overall survival, HSCT has been recommended in patients with X-linked or autosomal recessive CGD if a matched donor is identified, and if no residual oxidase activity or if severe disease complications develop [67]. The use of non-myeloablative conditioning regimens has greatly decreased the regimen related toxicity and improved survival (Table 3) [40].

Table 3 Summary of the outcomes of hematopoietic stem cell transplantations in patients with CGD
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It is now expected that about 90% of patients with CGD will survive into young adulthood with or without HSCT given an improved diagnosis, prophylaxis with interferon-γ, itraconazole, and antimicrobials agents [40]. However, older children and adolescents are especially noncompliant with prophylaxis therapy, increasing their risk of complications. The overall quality of life is reduced in patients with CGD, whereas patients with CGD who have undergone HSCT report quality of life comparable to healthy children [68]. The ESID and the European Blood and Marrow Transplantation (EBMT) indications for HSCT in patients with X-linked or AR-CGD include patients with a matched donor or mismatched unrelated donor plus one clinical or social complications (Table 4).

Table 4 ESID/EBMT guidelines 2017 for hematopoietic stem cell transplantations in patients with X-linked or AR-chronic granulomatous disease
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Finding a suitable donor is one of the most important steps for patients who undergo allogeneic HSCT. The ideal donor for HSCT is HLA-matched sibling donors (MSD) with the highest overall survival. But based on average family size, the availability of such a donor is less than 30% of patients [69, 70]. HLA-matched unrelated donors (MUD) have been preferred in the absence of MSD. Alternative donors such as HLA-mismatched unrelated donors (MMURD with one or more HLA mismatch at the HLA-A, HLA-B, HLA-C, or HLA-DRB1 loci), HLA-haploidentical donors, and umbilical cord blood (UCB) are considered tertiary options. Greater risk of graft failure, graft versus host disease (GVHD), and transplant-related mortality with grafts from genetically distant donors drives the donor hierarchy [71, 72]. However, advances in HLA typing technology, GVHD prophylaxis, and management of complications have substantially reduced the risk of mortality, and HSCT from an alternative donor has been increasingly safe and feasible [69, 72]. The outcomes of haploidentical transplantation can differ largely according to GVHD prophylaxis methods which include T cell depletion, post-transplant cyclophosphamide, and anti-thymocyte globulin [72,73,74]. The average success rate of recent studies of HSCT for CGD was 40/42 (95.2%) for MSD, 97/107 (90.6%) for MUD, 20/23 (87.0%) for MMUD, 3/3 (100%) for haploidentical donors, and 4/5 (80%) for UCB. The complication rate of HSCT was 50.0% for MSD, 57.9% for MUD, 78.3% for MMUD, 33.3% for haploidentical donor, and 20% for UCB [75,76,77,78].

From the studies of X-linked carrier females, it suggests that the level of myeloid chimerism that protects patients from infection may be lower than that to protect from autoinflammation [79, 80]. In recent studies, reduced-intensity/reduced toxicity or myeloablative conditioning regimens are safe and effective in CGD patients, including patients with IBD. The results achieved more than 90% myeloid donor chimerism and resolution of inflammatory colitis by two years following allogeneic HSCT [76, 81]. The study by Güngör et al. in 2014 also demonstrated that improvement in pulmonary function after HSCT in adults [78].

A recent study of 507 CGD patients by Yonkof et al. and the United States Immunodeficiency Network reported that transplant-related survival improved in patients who underwent allogeneic HSCT at age 14 years or younger (93% vs. 82% at five years post-transplant). Overall survival (OS) was negatively associated with CYBB mutation (HR 6.25, p = 0.034). HSCT was associated with reduced infection incidence and improved functional performance. The survival was estimated to be 65% 30 years after diagnosis [77].

Gene Therapy

The clinical trials of hematopoietic stem and progenitor cell gene therapy were initiated with gammaretroviral vectors. After the occurrence of insertional oncogenesis following gammaretroviral-based gene therapy, the development of gene therapy shifted away from gamma-retrovirus to lentiviral vectors since 2000 [82, 83]. From the previous studies of female carriers of X-CGD, it is estimated that more than 10–20% oxidase-positive circulating myeloid cells corrected by gene therapy may reach clinical benefit with normal resistance to infection [84].

To retain the efficacy, minimize the mutagenic risk, and enhance safety, a self-inactivating lentiviral vector was designed with a chimeric internal promotor that preferentially drives gp91phox expression in phagocytes. The gene therapy for X-linked CGD using autologous CD34+ enriched cell population that contains hematopoietic stem and progenitor cells (HSPCs) transduced ex vivo by this lentiviral vector encoding the human gp91phox gene demonstrated stable vector copy numbers (0.4 to 1.8 copies per neutrophil) and the persistence of 16% to 46% oxidase-positive neutrophils in six of the nine patients at 12 months follow-up [84]. Based on this promising result, the U.S. Food and Drug Administration (FDA) has granted this gene therapy OTL-102 orphan drug designation for the treatment of X-linked CGD in January 2020.

Despite the advances in the vector design, gene therapy based on the integration of an additional copy of a wild-type gene carries unavoidable risks of insertional mutagenesis. New genome editing technologies can mediate gene addition, gene ablation, gene correction, and other highly targeted genome modifications on cells ex vivo or in vivo. Gene editing via site-specific endonucleases such as zinc finger nucleases (ZFN), transcription activator-like effector nucleases (TALEN), or CRISPR (clustered regularly interspaced short palindromic repeat)/Cas9 nucleases has great potential as a therapeutic approach for monogenic hematological diseases because of the availability of autologous HSPCs. A targeted DNA alteration is initiated by the creation of a nuclease-induced double-stranded break (DSB). ZFN or TALEN approaches require the design of a specific pair of nucleases for each new DNA target [85, 86]. CRISPR/Cas9 nucleases, guided by a designed single-guide RNA complementary to the target site of interest, can achieve efficient DSB, which undergo repair by non-homologous end joining (NHEJ) causing insertions or deletions or by homology-directed repair (HDR) allowing donor DNA incorporation [87]. Correction of mutations at a single-base level without DSB via Cas9-based targeting of “base editors” has been reported [88].

Preclinical studies using targeted gene-editing technology for gene targeting with HDR in induced pluripotent stem cells (iPSCs) or HSPCs from patients with CGD have achieved a functional restoration of expression of NADPH oxidase genes and NADPH oxidase activity in differentiated neutrophils (Table 5). The major advantages of iPSCs are that they represent an unlimited source of stem cells. The corrected iPSCs-derived granulocytes and macrophages provide sufficient ROS levels, induction of NET formation, and the ability to kill bacteria [89]. There are many challenges of the field to overcome, including genotoxicity from off-target genome editing, increasing gene editing efficiency to levels necessary for effective treatment, immune responses to the nucleases, and the high price. These will require further studies to reach the goal of clinical applications.

Table 5 Target repairs using gene editing technology for patients with CGD
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X-Linked Carriers of CGD

X-linked CGD carrier females usually show random X chromosome inactivation by lyonization that one X chromosome is silenced epigenetically. X-linked carrier status can be demonstrated by DHR tests of the two populations of neutrophils. Those neutrophils with inactivation of the CYBB mutated X chromosome will have a normal superoxide production, whereas those neutrophils with inactivation of the normal X chromosome will have a defective respiratory burst [90].

The female carriers of X-linked CGD usually do no develop significant infections. However, infection risk is increased in those carrier females who have a normal neutrophil population of less than 20% by the study by Marciano BE et al. [79]. Skewing (non-random) of X chromosome inactivation occurs in a minority (22%) of female carriers. The X-linked CGD carrier females with CGD-type infection had median %DHR+ values of 8% (range 0.06% to 48%), and those carriers with autoimmune or inflammation had median %DHR+ values of 39% (range 7.4% to 74%) [79]. About half of X-linked female carriers have various symptoms of autoimmunity, such as Raynaud phenomenon, arthritis, discoid lupus, aphthous ulcers, granulomatous colitis, systemic lupus erythematosus (SLE), and hypothyroidism/hyperthyroidism [79, 80, 91].

X-linked carrier females need therapy for infections or autoimmunity when required. Antimicrobial and antifungal prophylaxis should be considered for cases with recurrent infections and low DHR responses and cases under immunosuppression or biologic treatments for autoimmunity [90]. Allogeneic HSCT showed successful treatment for refractory IBD in two X-linked female carriers [92].

Summary

The development of effective infection prophylaxis regimens and aggressive interventions have much improved the prognosis in the past decade. It is important to optimize conditioning regimens of HSCT to achieve safety and a higher level of donor chimerism necessary to protect against infections and inflammatory complications. Gene therapy is a promising approach for patients with CGD. More studies are needed to evaluate long-term outcomes of contemporary and experimental CGD management, including gene therapy and other potential targeted therapy.