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BCG vaccines and environmental NTM are known to cause severe diseases in immunocompromised children. It is less well known that otherwise healthy children may also be affected (1–4). Unlike children with classic immunodeficiency, who suffer from other clinical diseases caused by various viruses, bacteria, fungi, and protozoa, children with severe idiopathic BCG and NTM infection generally do not have other associated symptomatic infections apart from salmonellosis, which occurs in less than half the cases. Clinical diseases caused by cytomegalovirus, herpes simplex virus, Listeria monocytogenes, and Histoplasma capsulatum have each been found in only one patient (5–9).

Although these children are not vulnerable to a wide range of infectious agents, the syndrome was thought to be due to impaired immunity specifically altering host defenses against mycobacteria. Parental consanguinity and familial forms are frequent, and the syndrome is often described as Mendelian susceptibility to mycobacterial infection (MIM 209950) (10). The clinical syndrome is rare, and its inheritance may differ between kindreds. Although autosomal recessive in most cases, autosomal dominant (8, 9) and X-linked recessive inheritance have been reported (3, 11).

Cases from various ethnic groups and geographic regions have been reported. The prevalence of the syndrome is difficult to determine, as it includes a continuous spectrum from disseminated lethal BCG infection to local recurrent NTM infection. Disseminated forms themselves are heterogeneous, and clinical features correlate with the type of histopathologic lesions present (12), so that children with lepromatous-like BCG granulomas generally die of overwhelming infection, whereas children with tuberculoid granulomas have a favorable prognosis. The prevalence of idiopathic disseminated BCG infection in France has been estimated to be at least 0.59 cases per million children vaccinated (1).

Mutations have been found in four genes (IFNGR1, IFNGR2, IL12B, IL12RB1). Different types of mutations result in eight different disorders whose common pathogenic mechanism is impaired IFNγ-mediated immunity. However, these defects account for only a minority of patients. IL-12, a heterodimeric cytokine, is secreted by phagocytes and dendritic cells upon infection with mycobacteria. IL-12 is composed of two subunits, p40 and p35, that together form the biologically active p70 heterodimer. IL-12R consist of two chains, IL-12Rβ1 and IL-12Rβ2, that are selectively expressed on natural killer and activated T cells. Binding of IL-12 to these receptors leads to STAT4 activation, followed by the induction of IFNγ (Figs. 1 and 2) (13, 14). IL-12p40 and IL-12Rβ1 deficiencies, thus, result in impaired IFNγ production.

Figure 1
figure 1

Pathways of IL-12-dependent IFNγ-mediated immunity. The heterodimer IL-12 (p35 and p40), secreted by mononuclear and dendritic cells, binds to a receptor consisting of a β1 and a β2 chain, which is specifically expressed on natural killer (NK) and T cells. It thereby stimulates IFNγ secretion in NK and T lymphocytes. IFNγ, in turn, binds to a ubiquitous receptor, IFNγR, which consists of a ligand-binding (IFNγR1) and a signaling (IFNγR2) chain. This leads to phosphorylation of STAT1, which, after translocation as a homodimer to the nucleus, activates IFNγ-inducible genes. The products of the genes in which disease-defining mutations have been described (IFNGR1, IFNGR2, IL12B, IL12RB1) are shown in black.

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Figure 2
figure 2

IL-12 signaling pathway. Binding of IL-12 to the IL-12R induces phosphorylation of Tyk2 and JAK2, thus activating the signal-transducing protein STAT4. This protein dislocates from the receptor and translocates as homodimer to the nucleus where it acts as a transcriptional activator, binding to specific DNA response elements in the promoter region of IL-12-inducible genes.

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IFNγ, a homodimer with pleiotropic effects, is one of the principal macrophage-activating cytokines. IFNγ acts through a ubiquitous cell-surface receptor composed of two chains, IFNγR1, the ligand-binding chain, and IFNγR2, which is required for signal transduction. Upon ligand binding, STAT1, an essential component of the IFNγ signaling pathway, is phosphorylated and translocates to the nucleus as a homodimer where it acts as a transcriptional activator, interacting with the promoter region of IFNγ-inducible genes (Fig. 3) (15, 16). Various types of IFNγR1 and IFNγR2 defects impair cellular responses to IFNγ (see Figs. 4 and 5 and text below).

Figure 3
figure 3

IFNγ signaling pathway. IFNγ is a homodimer that, upon binding to the IFNγR, initiates a complex formation that brings together IFNγR1- and IFNγR2-associated JAK1 and JAK2, respectively. Two STAT1 molecules are phosphorylated by the receptor complex and are translocated as a homodimer to the nucleus where they bind specific DNA response elements and activate gene transcription.

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Figure 4
figure 4

Allelic heterogeneity of IFNγR1 deficiency. A wild-type IFNγR1 molecule with its extracellular (EC), transmembrane (TM), and intracellular (IC) domain is shown. The horizontal bars in the IC region indicate the JAK1 and STAT1 binding motifs and the receptor recycling motif. Four types of mutant IFNγR1 molecules are shown (right; see text for more details). The first (from left to right) mutant lacks most of the IC domain, the second probably binds IFNγ with low affinity, the third does not bind IFNγ at all, and the fourth is not expressed at the surface due to a stop codon upstream from the TM domain. These mutations, therefore, define four types of IFNγR1 deficiency with various inheritance patterns (AD indicates autosomal dominant;AR, autosomal recessive), IFNγR1 cell-surface expression (+++ indicates overexpression; +, normal expression; −, lack of expression), 125I-IFNγ binding to cells (+, normal; ±, impaired; −, abolished), and their IFNγ signaling defect (partial, cellular responses to IFNγ impaired;complete, cellular responses to IFNγ abolished).

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Figure 5
figure 5

Allelic heterogeneity of IFNγR2 deficiency. A wild-type IFNγR2 molecule with its EC, TM, and IC domain is shown. The horizontal bar in the IC region indicates the JAK2 binding motif. Two types of mutant IFNγR2 molecules are shown. The first (left) mutant either binds IFNγR1 with low affinity or impedes the associated signaling machinery; the second (right) is not expressed at the cell surface due to a stop codon upstream from the TM domain. 125I-IFNγ binding to the cells was shown (+) or is expected to be normal (+?). The mutations, therefore, define two types of AR IFNγR2 deficiency that differ in IFNγR2 cell-surface expression (+, normal; −, lack of expression) and in the type of signaling defect (partial or complete).

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These disorders generally manifest in childhood, although they may become apparent during adulthood. The rarity and heterogeneity of the syndrome make accurate diagnosis and treatment difficult for pediatricians, especially as the clinical boundaries are poorly defined and most patients lack a clear genetic etiology. In this article, we briefly review the known inheritable disorders, emphasizing their molecular pathogenesis, clinical features, diagnosis, and management. Reviews of other aspects of the syndrome can be found elsewhere (17–19).

GENETICS AND IMMUNOLOGY

A child with a homozygous recessive deletion in the p40 subunit of IL-12, which leads to complete IL-12p40 deficiency, has been reported (20). Recently, we identified other families with IL-12p40 deficiency, showing that this disorder is not limited to a single kindred (Picard C, in preparation). Patients with other IL-12 regulation defects and undefined X-recessive genetic defects in trans have been reported (3). Due to a lack of stimulation through IL-12, these patients produce less IFNγ, which may be partially corrected in a dose-dependent manner with exogenous recombinant IL-12. This confirms that the impairment of IFNγ production is not a primary event but a consequence of inherited IL-12 deficiency. In other patients, recessive mutations preclude expression of the IL-12Rβ1 chain on the surface of natural killer and T cells (21–26). To date, no partial defect of IL-12 or IL-12R has been identified.

Complete IFNγR1 deficiency (Fig. 4) may be due to recessive mutations that either preclude surface expression of the receptor ligand-binding chain (6, 7, 27–30) or result in normal surface expression of IFNγR1 chains that do not bind IFNγ(31, 32). One child has been reported to have complete IFNγR2 deficiency (Fig. 5) due to a homozygous recessive deletion in the coding region of the gene, resulting in a premature stop codon in the extracellular domain (33). Cell-surface expression of IFNγR1 was normal, but no IFNγR2 expression was detected. Other patients have been diagnosed with complete IFNγR2 deficiency, showing that this disorder is not limited to a single kindred (34).

A homozygous recessive mutation, which probably reduces the affinity of IFNγR1 for its ligand, has been identified in two siblings (35). Cell-surface expression of the receptor ligand-binding chain was normal. Another patient had a homozygous recessive mutation in the IFNγR2 chain with impaired responses to IFNγ and normal surface expression (36). Both recessive defects lead to partial, as opposed to complete, IFNγR deficiency (Figs. 4 and 5).

An autosomal dominant form of partial IFNγR1 deficiency was found in 18 patients of 12 unrelated kindreds (8, 9). The mutant allele encodes an intracellular truncated receptor that binds IFNγ with normal affinity but cannot transduce IFNγ-triggered signals and accumulates on the cell surface. The combination of normal IFNγ binding, impaired signaling, and the accumulation of the receptor on the cell surface (five to 10 times the normal number of receptors) accounts for the dominant negative effect of the truncated mutant IFNγR1 molecules. Most IFNγR1 molecules are nonfunctional, but the few wild-type dimers that do form cause the defect to be partial rather than complete (Figs. 4 and 5).

HISTOLOGIC AND CLINICAL FEATURES

A multicenter survey is underway to define precisely the clinical and histologic phenotypes of patients with the syndrome and each of the underlying genetic defects. In all patients known to date, mycobacterial infection is the principal clinical presentation, and none has clinical atopy or serologic evidence of sensitization to common aeroallergens or signs of autoimmunity [Wood P, in preparation and (37)].

Generally, patients with IL-12 and IL-12R deficiencies have mild symptoms with delayed but good granuloma formation in response to BCG vaccination (20) and impaired granuloma formation after NTM infection. The child with complete IL-12p40 deficiency presented with curable BCG and Salmonella enteritidis infections (20), and the patients with IL-12 regulation deficiency had Mycobacterium avium infections (3). The patients with complete IL-12Rβ1 deficiencies presented with curable BCG infection upon vaccination and NTM infections after the age of 3 y, with these infections not occurring until adulthood in two. In one patient, the infection was fatal. The milder clinical phenotype in comparison to children with complete IFNγR deficiencies is probably due to residual, albeit low, IL-12-independent IFNγ-mediated immunity.

Surprisingly, the affected sister of one of the IL-12Rβ1-deficient patients with BCG infection was resistant to three inoculations of BCG but developed abdominal tuberculosis at the age of 18 y. The two siblings, thus, had the same genotype but different clinical phenotypes (25). This demonstrates phenotypic heterogeneity for a given genotype in IL-12Rβ1 deficiency. Moreover, it raises the question as to whether other children with severe tuberculosis may have IL-12Rβ1 deficiencies.

The clinical phenotype of patients with partial IFNγR deficiency is generally mild like that in IL-12R deficiency. One patient with partial recessive IFNγR1 deficiency presented with clinical BCG and Salmonella enteritidis infections, and the other patient who was not vaccinated had symptomatic tuberculosis. In all tissue biopsies, mature granulomas were seen. The patient with partial recessive IFNγR2 deficiency had a history of BCG and Mycobacterium abscessus infections. In the dominant forms of partial IFNγR deficiency, the clinical phenotype is mild even though the course of infection appears to be somewhat more severe than in partial recessive IFNγR deficiency (8, 9).

Patients with complete IFNγR1 or IFNγR2 deficiency suffer from a severe form of the syndrome, with BCG infection after immunization and early onset NTM infection (often before 3 y of age). Lethal infection due to Mycobacterium smegmatis, one of the least virulent mycobacteria that had never been previously identified as a cause of disseminated clinical disease, has occurred once (30). Other rapidly growing mycobacterial species such as M. fortuitum but also M. avium, Salmonella, and Listeria monocytogene infections have been reported. Severe diseases caused by viruses, such as CMV and HSV, have been diagnosed each in one child (5). No other opportunistic infections were observed, and common childhood infections followed a normal course. Lepromatous-like lesions, particularly in response to BCG vaccination, were observed and are suggestive of complete IFNγR1 or IFNγR2 deficiency.

There is a correlation between genotype and cellular and clinical phenotype for patients with IFNγR deficiencies. Thus, IFNγ-mediated cell activation is a genetically controlled continuous quantitative trait that determines the outcome of mycobacterial infection in men (38). In contrast, the clinical phenotype of patients with IL-12 or IL-12R gene defect is more heterogeneous between and within families.

DIAGNOSIS

Rapid diagnosis of complete IFNγR deficiency is essential for the planning of clinical management and can be made by determining serum IFNγ levels by ELISA (34). High IFNγ levels suggest complete IFNγR deficiency, whereas low or undetectable levels indicate IL-12, IL-12R, partial IFNγR, or undetermined defects (6). More specialized assays are necessary to determine the exact molecular etiology of the disease in each patient.

IL-12p40 deficiency can be diagnosed by ELISA, with the levels of IL-12p40, IL-12p70, and IFNγ secretion of stimulated peripheral blood mononuclear cells being low. IL-12Rβ1 deficiency results in low levels of IFNγ production, and FACS analysis cannot detect the IL-12Rβ1 chain on activated T cells. Gene sequencing and, depending on the mutation, in vitro gene transfer are needed to confirm the diagnosis.

Several functional studies have assessed the cellular responses to recombinant IFNγ. ELISA can be used to quantify tumor necrosis factor-α (TNF-α) production by blood cells in response to lipopolysaccharide alone and lipopolysaccharide plus IFNγ(4, 28). STAT1 nuclear translocation can be determined by electrophoretic mobility shift assay, using cultured EBV-transformed B cells (35). Alternatively, it is possible to detect intracellular phosphorylated STAT1 by flow cytometry (39). HLA-DR surface induction can be detected in SV40-transformed fibroblasts by flow cytometry with specific antibodies (27).

Cells from patients with complete IFNγR deficiency do not respond to IFNγ even at high concentrations (105 IU/mL), whereas cells from patients with partial deficiencies respond to high but not to low concentrations of IFNγ. In addition, over-expression of the IFNγR1 chain in the autosomal dominant form facilitates rapid diagnosis of this form of partial IFNγR1 deficiency by FACS analysis (8, 9). Here, too, gene sequencing and, depending on the mutation, in vitro gene transfer provide the molecular diagnosis of the patients.

TREATMENT AND PROGNOSIS

For all patients, appropriate antibiotic therapy based on the sensitivity of the mycobacterial species identified is crucial. However, for initial empirical therapy, a history of BCG vaccination is important, and unvaccinated children should be considered affected by NTM, although tuberculosis should be considered in some cases. Antimycobacterial therapy may have to be continued for extended periods, and supplementary measures such as the drainage of pus and attention to nutrition and growth are important. Occasionally, splenectomy (40) or surgical resection of refractory infectious sites such as abdominal lymph nodes may be required. BCG immunization is contraindicated, and it is prudent to avoid other live vaccines.

Patients with IL-12, IL-12R, and partial IFNγR defects usually respond well to antibiotic treatment, and, in those who do not respond well, additional IFNγ therapy has been shown to be effective. Experience shows that this option should be considered individually for each patient. An initial dose of 30 to 50 μg/m2 given s.c. three times a week should be adapted according to clinical and in vitro responses. Doses as high as 500 μg/m2 may be necessary (18).

The molecular diagnosis of complete IFNγR1 and IFNγR2 deficiency has important therapeutic implications, as mycobacterial disease in patients with such deficiencies is generally overwhelming and may be refractory to antibiotic treatment. Although some patients respond initially to antibiotics, a full remission is rarely achieved and relapses invariably occur if antibiotics are discontinued. Bone marrow transplantation probably remains the treatment of choice, because IFNγ is ineffective in the absence of specific functional receptors. However, transplantation has proved difficult in several patients (unpublished observations). Gene therapy should be developed in the future for the treatment of complete IFNγR deficiencies, but further research is required to make this possibility a reality.

CONCLUSIONS

Mendelian susceptibility to mycobacterial infection is a rare and heterogeneous syndrome of childhood that is probably underdiagnosed. The clinical features of affected children range from disseminated lethal BCG infection to local and recurrent NTM infection. Various mutations in IFNGR1, IFNGR2, IL12B, and IL12RB1 define eight genetically different disorders. These disorders are immunologically related, as impaired IFNγ-mediated immunity is the common pathogenic mechanism underlying mycobacterial infection in all patients. The severity of the clinical phenotype depends on the genotype. Complete IL-12p40 and IL-12Rβ1 deficiencies and partial IFNγR1 and IFNγR2 deficiencies generally predispose the patient to curable infections at various ages. Antibiotics supplemented with IFNγ if required are likely to be effective, and bone marrow transplantation is, therefore, not indicated. Complete IFNγR1 and IFNγR2 deficiencies predispose the patient to overwhelming infection in early childhood. The response to antibiotics is poor and IFNγ treatment ineffective, which probably makes bone marrow transplantation the treatment of choice. There is a genotype-phenotype correlation for IFNγR defects unlike for IL-12 and IL-12R defects. Rapid discrimination between complete IFNγR deficiency and other defects by ELISA determination of serum IFNγ levels is an important diagnostic step for planning clinical management. However, diagnosis of the molecular defect remains difficult, as the syndrome is heterogeneous and few standardized assays are available. Tests to detect IFNγ and IL-12 cytokines and their receptors, followed by functional assays, gene sequencing, and in vitro gene transfer, provide a definite diagnosis in only a minority of cases. The treatment of other patients with unknown genetic defects remains empirical. Future research will focus on the search for other underlying genetic defects to provide a rational basis for the management of patients with mycobacterial disease.