Susceptibility to Mycobacterial Infection and Cytokine Receptor Abnormalities
July 1, 1998 9 minutes read
Susceptibility to Mycobacterial Infection and Cytokine Receptor Abnormalities
ABSTRACTS & COMMENTARY
Synopsis: Mutations in the gene encoding for the interleukin-12 receptor subunit, IL-12Rb1, like those involving the interferon-gamma receptor, markedly impair the immune response to mycobacterial infection.
Sources: Altarte F, et al. Impairment of mycobacterial immunity in human interleukin-12 receptor deficiency. Science 1998;280:1432-1435; de Jong R, et al. Severe mycobacterial and Salmonella infections in interleukin-12 receptor-deficient patients. Science 1998;280:1435-1438.
Two overlapping groups of investigators have deciphered a novel genetic abnormality causing susceptibility to severe mycobacterial infection.
Altarte and colleagues evaluated four patients from three unrelated kindreds in Morocco, Turkey, and Cyprus with disseminated mycobacteria and/or Salmonella infection. Two patients had disseminated BCG infection, one of whom also had S. enteritidis infection. Two others, both from Cyprus, had Mycobacterium avium infection and one of these also had S. enteritidis infection. Extensive conventional evaluation of underlying immunodeficiency was unrevealing in all, and none had evidence of mutations involving interferon-g (IFN-g), IFN-g receptor, IFN-g receptor-associated molecules, or IL-12 subunit. However, mutations in the IL-12 receptor (IL-12R) gene were found in three of the four subjects.
Histological examination of sites of mycobacterial infection revealed mature granulomas. Activated natural killer cells and T cells obtained from the patients, however, exhibited markedly reduced secretion of IFN-g. In one of the patients, M. avium infection, which had failed to improve during antibiotic therapy, appeared to respond to the administration of IFN-g.
de Jong and colleagues evaluated three unrelated individuals, two Dutch and one Turkish, with severe mycobacterial (M. avium in two, BCG in one) infection; all three also had Salmonella spp. infection. Their peripheral blood mononuclear cells evidenced deficient IFN-g production, but displayed normal IL-12 production, IFN-g receptor expression, and IFN-g receptor function. Their T lymphocytes, however, did not express detectable IL-12 receptor beta-1 chain (IL-12Rb1) and did not respond to exogenous IL-12. All three subjects proved to be homozygous for individual mutations in the IL-12 receptor gene leading to impaired function of IL-12Rb1. Each mutation uncovered caused a premature stop codon in the extracellular domain-encoding region of the gene, resulting in the lack of its expression. All the parents of the patients were heterozygous for these mutations.
COMMENT BY STAN DERESINSKI, MD, FACP
TH1/TH2 Differentiation and Control of Intracellular Infection
Control of infection by intracellular pathogens, such as mycobacteria and Salmonella spp., requires an immune response of the TH1 type characteristic of cellular immunity. The development of such a response is highly dependent upon the immunomodulatory activity of several cytokines, especially IL-12 and IFN-g.
During antigen priming and activation, previously naive CD4+ lymphocytes are driven to differentiation into phenotypically dissimilar subtypes depending upon a number of factors, including their cytokine exposure. These differentiated CD4+ cell types are termed TH1 and TH2 lymphocytes and are characterized by their cytokine production patterns which, in turn, determine their effector function. Although there is great overlap between types, especially in humans, TH1 cells principally produce IFN-g, lymphotoxin and IL-2, as well as TNF-b and promote cellular immunity, while TH1 principally cells produce IL-4, and IL-5, and IL-10 and promote humoral immunity.
The key cytokines which determine the direction of development of naive lymphocytes, are IL-12 and IL-4, which can be considered, respectively, TH1 and TH2 differentiation factors. IL-12 has been called an "initiation cytokine for cellular immunity." When macrophages ingest microbes, IL-12 production results. Secreted IL-12 then interacts, via specific receptors, with, among other cell types, naive lymphocytes, resulting in their development toward a TH1 phenotype. These TH1 cells in turn produce IFN-g, which interacts with receptors on macrophages and activates them, leading to the killing of intracellular microbes. While IL-12 is critical in driving T lymphocyte differentiation toward a TH1 phenotype, primarily by priming them for IFN-g production, other factors, such as manner of antigen presentation and glutathione levels within antigen presenting cells, also have an effect.1
The Role of IL-12
The cytokines which are critical to the killing of
intracellular pathogens are IL-12 and IFN-g. The IL-12 molecule is a 70 kD disulfide-linked heterodimer with subunits of 35 kD and 40 kD molecular mass that is produced by monocytes, macrophages, dendritic cells, B lymphocytes, and mast cells. IL-12 secreted by these cells interacts with cognate receptors expressed by T cells, NK cells, and subsets of B lymphocytes. The IL-12 receptor (IL-12R) consists of two subunits, each a member of the "b type" cytokine receptor gp130 subgroup of the cytokine receptor superfamily (See Figure 1). High affinity binding of IL-12 requires the association of both subunits. IL-12Rb2 is 100 amino acids longer than IL-12Rb1 because the latter lacks the N-terminal IgG family motif present in IL-12Rb2. The subunits also differ in their cytoplasmic portions; IL-12Rb1 has a short cytoplasmic tail and lacks tyrosine residues, of which IL-12Rb2 has three.2 The IL-12 p40 subunit binds to IL-12Rb1, while both subunits interact with IL-12Rb2.
Binding of IL-12 to its receptor triggers intracellular signaling via the Janus kinase/signal transduction and activator of transcription (Jak/STAT) pathway, with use of Jak-2, Tyk-2, STAT3, and STAT4. IL-12 and the interferon-a are the only cytokines known to use STAT4.3 In response to receptor signaling, NK and T cells secrete IFN-g while, at the same time, the cytolytic activity of NK cells and CD8+ T cells is enhanced. Finally, secreted IL-12 acts upon naive CD4+ T cells to cause differentiation into a TH1 phenotype.
IL-12 receptor expression plays an important role in this phenotypic differentiation. IL-12Rb2 is expressed on human TH1 lymphocyte clones, but not on TH2 clones.4 IL-12Rb2 is not expressed by naive resting CD4+ T cells; antigen activation via the T cell receptor causes induction of this receptor subunit, as well as of IL-12Rb1.5 In the mouse, TH2 differentiation is accompanied by down-regulation of high-affinity IL-12 receptor expression and resultant loss of IL-12-induced signal activation, and by an inability to produce IFN-g in response to IL-12. Receptor function is enhanced by IFN-g.6 IFN-g also acts in a positive feedback loop by enhancing IL-12 production.7
The near uniqueness among the cytokines of IL-12 in its use of STAT4 in its signaling pathway may account for its ability to cause differentiation along the TH1 pathway. Studies in knock-out mice lacking in STAT4 indicate that this molecule induces the transcriptional machinery necessary for TH1 differentiation and it directly regulates IFN-g production.8 Similarly, IL-12Rb1(-/-) mice have impaired ability to produce IFN-g in response to endotoxin.9 Thus, any disruption in the steps from IL-12 binding onward may lead to, among other things, impairment of TH1 differentiation and IFN-g production, with resultant inability to control intracellular pathogens such as mycobacteria.
The Role of IFN-g
IFN-g is a homodimeric protein, although its subunits differ somewhat in molecular mass as a result of different degrees of glycosylation. IFN-g is produced by CD4+ and CD8+ lymphocytes and NK cells. Its transcription by these cells is initiated by antigen activation and enhanced by IL-2 and IL-12.
Receptors for IFN-g are expressed on all types of human cells with the exception of mature erythrocytes. The IFN-g receptor (IFN-gR), a member of the class II cytokine receptor complex, consists of two distinct chains. The ligand-binding unit, IFN-gR1, and IFN-gR2, whose intracellular domain is necessary for signaling, undergo oligomerization upon binding of IFN-g. Binding initiates signal transduction, which involves a series of events with activation of Jak1 and Jak2 receptor -associated protein tyrosine kinases, phosphorylation of Tyr440 of the IFN-gR1 intracellular domain, and phosphorylation and activation of STAT1a, the latent transcription factor. These events lead to transcription of regulatory genes called interferon-stimulated response elements that function as binding sites for transcription factors, including interferon consensus sequence-binding protein.10
IFN-g has critical immunomodulatory properties. Knock-out mice lacking either the gene for production of IFN-g or its receptor, the gene encoding for the interferon consensus sequence-binding protein (ICBSP), or of the signal transduction factor STAT1, have increased susceptibility to intracellular infections. IFN-g is the principal macrophage-activating factor, stimulating these cells to kill phagocytosed microorganisms, although other cytokines, especially GM-CSF, have similar activity. It is thus the key effector molecule produced by TH1 lymphocytes, cells whose primary effector function is the promotion of phagocyte-mediated killing of pathogenic microorganisms. At the same time, IFN-g impairs development of TH2 lymphocytes. IFN-g also enhances antigen presentation by increasing class I and class II MHC expression. It stimulates the cytolytic activity of NK cells, activates neutrophils, and also activates vascular endothelial cells so that CD4+ lymphocyte adhesion is increased as well as lymphocyte extravasation.
Deficient IFNg and IL-12 Receptor Function and Intracellular Infection
Mutations in the IFNgR1 gene, which introduce a premature stop codon, lead to truncation of the receptor and complete lack of function.11,12 Other mutations leading to partial impairment of receptor function have also been described.13 While complete IFNgR1 deficiency in humans is associated with severe disseminated mycobacterial infections (e.g., disseminated BCG infection) and associated lack of mature granuloma formation, incomplete deficiency is associated with less severe, more easily treatable infections with normal granuloma formation. The patients with IL-12Rb1 gene mutations discussed above exhibit a phenotype resembling that of patients with partial IFNgR1 deficiency.
The final pathway for patients with deficient IL-12R and IFN-gR is the same (See Figure 2). In IL-12Rb1-deficient subjects, IFN-g production by NK and T cells is severely diminished. The residual IFN-g production, by non-IL-12 dependent pathways, observed in these patients is sufficient to lessen the severity of the immunologic defect, relative to that seen in individuals with deficient IFN-gR1. These residual pathways are obscure, but may include a role for IL-18, which has been called IFN-g inducing factor. The apparent response of one patient with IL-12Rb1 deficiency to exogenous IFN-g also strongly suggests that the problem, in such patients, is inadequate production of the latter cytokine. Thus, mycobacterial infections in IL-12Rb1-deficient patients appears to be of lesser severity than similar infections in those with complete IFN-gR1 deficiency and is closer in severity to those with incomplete deficiency of the latter.
The findings reviewed here represent an exciting advance in our understanding of the host response to intracellular infections. Other work, investigating the potential role of genetic differences in the NRAMP1 gene, the human counterpart of the mouse Bcg gene, which is critical in the murine response to intracellular infection, may open additional vistas.14
References
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2. Persky DH, et al. A functional interleukin 12 receptor complex is composed of two beta-type cytokine receptor subunits. PNAS 1996;93:14002-14007.
3. Stern AS, et al. Structural and functional aspects of the IL-12 receptor complex. Chem Immunol 1997;68:23-37.
4. Rogge L, et al. Selective expression of an interleukin-12 receptor component by human T helper 1 cells. J Exp Med 1997;185:825-831.
5. Szabo SJ, et al. Regulation of the interleukin (IL)-12R beta 2 subunit expression in developing T helper (TH1) and Th2 cells. J Exp Med 1997;185:817-824.
6. Gollob JA, et al. Interferon-gamma and interleukin-4 regulate T cell interleukin-12 responsiveness through the differential moculation of high-affinity interleukin-12 receptor expression. Eur J Immunol 1997;27: 647-652.
7. Wu C, et al. Regulation of interleukin-12 receptor beta1 chain expression and interleukin-12 binding by human peripheral blood mononuclear cells. Eur J Immunol 1997;27:147-154.
8. Jacobson NG. Molecular mechanism of interleukin-12 signaling in T helper phenotype development. PHD Thesis, Washington Univ; DAI-B 58/09, March 1998;4711.
9. Wu C, et al. Characterization of IL-12 receptor beta1 chain (IL-12Rbeta)-deficient mice: IL12Rbeta1 is an essential component of the functional mouse IL-12 receptor. J Immunol 1997;159:1658-1665.
10. Giese NA, et al. Interferon (IFN) consensus sequence-binding protein, a transcription factor of the IFN regulatory factor family, regulates immune responses in vivo through control of interleukin-12 expression. J Exp Med 1997;186:1535-1546.
11. Newport MJ, et al. A mutation in the interferon-g-receptor gene and susceptibility to mycobacterial infection. N Eng J Med 1996;335:1941-1949.
12. Jouangay E, et al. Interferon-g-receptor deficiency in an infant with fatal Bacille Calmette-Guerin infection. N Engl J Med 1996;335:1956-1961.
13. Jouangay E, et al. Partial interferon-gamma receptor 1 deficiency in a child with tuberculoid bacillus Calmette-Guerin infection and a sibling with clinical tuberculosis. J Clin Invest 1997;1:2658-2564.
14. Skamene E, et al. Infection genomics: Nramp1 as a major determinant of natural resistance to intracellular infections. Ann Rev Med 1998;49:275-287.
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