DIAGNOSIS AND MANAGEMENT OF INHERITABLE DISORDERS OF INTERFERON-γ–MEDIATED IMMUNITY

Clinical immunology review series: an approach to the patient with recurrent superficial abscesses

Patients may be referred to the immunology clinic for investigation of recurrent superficial abscess formation. In the majority of adult patients this clinical presentation does not equate with an underlying primary immune deficiency. Nevertheless, recurrent mucocutaneous abscesses can be associated with significant morbidity and long-term complications, including scarring and fistula formation, and may be associated with underlying immune-mediated disease. This review sets out an approach to the patient with recurrent superficial abscesses, focusing on the differential diagnoses, investigation and management of both the common causes and those associated with specific immune deficiency.

Syndrome de susceptibilite mendélienne aux infections mycobacteriennes : défauts de l’axe Interleukine-12 - Interféron

INTRODUCTION

Environmental non tuberculous mycobacteria and Bacillus Calmette-Guerin vaccines are weakly virulent mycobacteria. Nevertheless they may cause severe diseases in otherwise healthy children with no overt immunodeficiency. Parental consanguinity and familial forms are frequently observed among these patients, therefore this syndrome was named "Mendelian Susceptibility to Mycobacterial Disease".

STATE OF THE ART

In the last nine years, fife genes have been found to be mutated in patients with this syndrome: IFNGR1, IFNGR2, STAT1, IL12B, IL12RB1. Allelic heterogeneity accounts for ten distinct genetic disorders. Clinical phenotype differs between patients. The spectrum of disease extends from early-onset overwhelming mycobacterial infection to adult-onset localized disease and tuberculosis. Impaired IFN-gamma-mediated immunity is the common mechanism of the disease, outlining its major role in mycobacterial immunity.

PERSPECTIVES AND CONCLUSIONS

Better understanding of these disorders reveals an expanding clinical phenotype which justifies studying adult patients with pulmonary non tuberculous mycobacterial infection without known risk factors, severe BCGitis and recurrent tuberculosis. Molecular diagnosis makes it possible to introduce a specific regimen based on physiopathology.

Genetic dissection of immunity to mycobacteria: the human model

Humans are exposed to a variety of environmental mycobacteria (EM), and most children are inoculated with live Bacille Calmette-Guérin (BCG) vaccine. In addition, most of the world's population is occasionally exposed to human-borne mycobacterial species, which are less abundant but more virulent. Although rarely pathogenic, mildly virulent mycobacteria, including BCG and most EM, may cause a variety of clinical diseases. Mycobacterium tuberculosis, M. leprae, and EM M. ulcerans are more virulent, causing tuberculosis, leprosy, and Buruli ulcer, respectively. Remarkably, only a minority of individuals develop clinical disease, even if infected with virulent mycobacteria. The interindividual variability of clinical outcome is thought to result in part from variability in the human genes that control host defense. In this well-defined microbiological and clinical context, the principles of mouse immunology and the methods of human genetics can be combined to facilitate the genetic dissection of immunity to mycobacteria. The natural infections are unique to the human model, not being found in any of the animal models of experimental infection. We review current genetic knowledge concerning the simple and complex inheritance of predisposition to mycobacterial diseases in humans. Rare patients with Mendelian disorders have been found to be vulnerable to BCG, a few EM, and M. tuberculosis. Most cases of presumed Mendelian susceptibility to these and other mycobacterial species remain unexplained. In the general population leprosy and tuberculosis have been shown to be associated with certain human genetic polymorphisms and linked to certain chromosomal regions. The causal vulnerability genes themselves have yet to be identified and their pathogenic alleles immunologically validated. The studies carried out to date have been fruitful, initiating the genetic dissection of protective immunity against a variety of mycobacterial species in natural conditions of infection. The human model has potential uses beyond the study of mycobacterial infections and may well become a model of choice for the investigation of immunity to infectious agents.

Heterogeneity in the granulomatous response to mycobacterial infection in patients with defined genetic mutations in the interleukin 12-dependent interferon-gamma production pathway

Patients with genetic lesions in the Type-1 cytokine/cytokine receptor pathway exhibit a selective susceptibility to severe infections with poorly pathogenic mycobacteria and non-typhi salmonella spp. These experiments of nature demonstrate that IL-12-dependent IFNgamma production is critical for granuloma formation and therefore host immunity against such pathogens. The essential role of granuloma formation for protective immunity to these organisms is emphasized by the differing granuloma forming capabilities and resultant clinical sequelae observed in these patients which seems to reflect their ability to produce or respond to IFNgamma (Fig. 9). At one pole of this spectrum, represented by the complete IFNgammaR1/2 deficient patients, there is a complete absence of mature granuloma formation, whereas with the less severe mutations (i.e. partial IFNgammaR1/2, complete IL-12p40 and complete IL-12Rbeta1 deficiency), granuloma formation is very heterogenous with wide variations in composition being observed. This suggests that in the latter individuals, who produce partial but suboptimal IFNgamma responses, other influences, including pathogen virulence and host genotype may also affect the type and scale of the cellular response elicited.

Impaired interferon gamma-mediated immunity and susceptibility to mycobacterial infection in childhood

Mendelian susceptibility to poorly virulent mycobacteria such as bacillus Calmette-Guerin (BCG) and environmental nontuberculous mycobacteria is a clinically heterogeneous syndrome. The clinical features of affected children cover a continuous spectrum from disseminated lethal bacillus Calmette-Guerin infection to local recurrent nontuberculous mycobacterial infection. Different types of mutations in four genes (IFNGR1, IFNGR2, IL12B, IL12RB1) have revealed both allelic and nonallelic heterogeneity and result in eight different disorders whose common pathogenic pathway is impaired interferon gamma (IFNgamma) mediated immunity. The severity of the clinical phenotype depends on the genotype. Complete IL-12 p40 and IL-12 receptor beta1 deficiencies and partial IFNgamma receptor 1 (IFNgammaR1) and IFNgammaR2 deficiencies generally lead to curable infections at various ages, and antibiotics supplemented with IFNgamma if required are likely to be effective. Complete IFNgammaR1 and IFNgammaR2 deficiencies predispose to overwhelming infection in early childhood, which may respond to antibiotics but relapse when antibiotics are discontinued. Rapid discrimination between complete IFNgammaR1 and IFNgammaR2 deficiency and other defects, therefore, is an important diagnostic step for planning clinical management.

[Primary immunodeficiencies. Clinical features and variant forms]

Periodically the World Health Organization and currently the International Union of Immunology Societies publish a classification of primary immunodeficiency diseases (PID) that includes diagnostic and therapeutic guidelines. The latest of these publications dates from 1999 and includes a new group of PID, the proliferative autoimmune syndromes. Furthermore, new forms of severe combined immunodeficiency (SCID) and of recessive autosomal agammaglobulinemia are described. From the publication of this classification until the end of the year 2000 a minimum of three new PIDs have been described and a further two should probably be added. Progress in the molecular biology of these diseases has given rise not only to more accurate diagnosis but also to greater insight into the clinical spectrum of these diseases. A mutation or deletion in a gene can provoke the complete absence of its product; sometimes expression is partial or normal but functional activity is absent or defective. In certain cases, partial or defective activity causes variant forms of the disease presenting symptomatology or atypical cellular phenotype. In other cases, this is not cause of the variant form, which can appear in interfamilial cases sharing the same mutation. In these cases, these differences can be attributed to environmental factors or to other genes able to modify the affected gene. In this article we provide examples of variant forms in several PIDs. Some are late onset forms, such as X-linked agammaglobulinemias diagnosed in adults, since until diagnosis, clinical symptomatology was minimal. In adenosine-deaminase deficiency, a serious and highly lymphoproliferative form of SCID, patients have been described whose symptomatology began after the age of 20 years. Another SCID, RAG1 and RAG2 recombinase deficiency, may produce a typical form with a characteristic T-B-NK + phenotype, Omenn's syndrome, or forms with an unexpected T-B + NK + phenotype. Deficiency in common gamma chain receptor for IL-2 may produce phenotypical variants that can lead to diagnostic error. X-linked lymphoproliferative syndrome may present as fulminant infectious mononucleosis, as leukemia or lymphoma or as hipo- or agammaglobulinemia. Possibly, some patients diagnosed with common variable immunodeficiency or with x-linked agammaglobulinemia do in fact have this syndrome. Chronic granulomatous disease is usually of early-onset, but late-onset forms have been described. In one case the first clinical manifestation was produced when the patient was 60 years old. The above examples serve to highlight that, even though PIDs are usually suspected by pediatricians, in some cases the diagnosis may be missed by internists or non-pediatricians. Moreover, the clinical and laboratory findings of these variant forms must be determined to carry out an early diagnosis, which is essential for a favorable therapeutic outcome.