Toward the molecular dissection of primary biliary cirrhosis # †

Primary biliary cirrhosis (PBC) is the archetypal autoimmune disease of the liver, characteristically associated with highly specific antimitochondrial antibodies directed against the E2 subunits of mitochondrial 2-oxo-acid dehydrogenase enzymes. Primarily affecting women, PBC occurs in about 1 in 1000 women over the age of 40, with other autoimmune diseases frequently developing in these individuals and their families.1 Until recently, relatively little has been known about the genetic and environmental factors underpinning the inflammatory nonsuppurative granulomatous cholangitis and bile duct–infiltrating, autoreactive, T lymphocytes that define PBC.2 However, as for other autoimmune diseases, molecular dissection of PBC is now well underway, with recent data from animal3 and human4 genetic studies identifying the interleukin-12 (IL-12) signaling pathway as a central player in the etiopathogenesis of PBC. IFN, interferon; IL, interleukin; PBC, primary biliary cirrhosis; TGFβRII, transforming growth factor-beta receptor II; TH17, T helper 17 cell. Appreciation of the integral link between IL-12 signaling and emergence of PBC has arisen in the context of major improvements in technologies for animal modeling of human disease and disease gene mapping. PBC-like phenotypes have been reported, for example, in the NODc3.c4 mouse, a recently-derived congenic variant of the diabetes-prone NOD (nonobese diabetic) strain,5 in scurfy mice expressing a mutant form of the Foxp3 (forkhead box p3) transcription factor,6 and in mice deficient for the Cl−/HCO3− anion exchanger 2 transporter.7 Xenobiotic immunization has also been shown to recreate facets of the disease in mice.8 Although they facilitate characterization of PBC pathophysiology, these models have not been extensively investigated as of yet, and do not faithfully replicate the human phenotype, the animals lacking the precise histologic lesions, the female predominance, and/or the late onset and slow progression of disease typical of human PBC. PBC-like disease has also been reported in mice manipulated for T cell–restricted expression of a dominant-negative (dn) form of transforming growth factor-beta receptor II (TGFβRII).9 While again not exactly mimicking the human disease, models such as this one are particularly exciting in view of the integral relationship of TGFβ signaling with development of T helper 17 (TH17) cells, a T helper subset with key roles in driving autoimmune, inflammatory responses.10, 11 Animal model studies have also confirmed a significant influence of the IL-12 cytokine, a heterodimer comprising the p35 and p40 subunits, in expression of autoimmune disease, raising the potential that the IL-12 and TGFβ signaling pathway coordinately contribute to the autoimmune cholangitis seen in dnTGFβRII mice, and the emergence of human PBC. Yoshida and colleagues3 have delved further into this pathway through the analysis of dnTGFβRII mice rendered deficient for either IL-12p40 (IL-12p40−/−) or for IFN-γ (IFN-γ−/−), an IL-12–induced cytokine effector of proinflammatory TH1 responses, as well as a suppressor of TH17 cells. IL-12p40 (encoded by the IL12B gene) covalently associates with the smaller p35 subunit encoded by IL12A to form the heterodimeric cytokine IL-12. It also comprises one of the subunits that form the immunoregulatory and TH17-cell–triggering cytokine IL-23 (p40p19). In this issue of HEPATOLOGY, this group reports that the elevation in intrahepatic proinflammatory cytokine levels and the histopathologic signs of inflammatory cholangitis typically manifested by dnTGFβRII mice are dramatically decreased in the context of concomitant IL-12p40 deficiency, but relatively unchanged by concomitant interferon-gamma (IFN-γ) deficiency. Based on these findings, the authors conclude that IFN-γ may be dispensable for development of autoimmunity, but that IL-12p40 is an essential requirement for development of autoimmune cholangitis. The link between IL-12 signaling and the PBC-like disease manifested by dnTGFβRII mice is particularly exciting in view of our group's recent genetic data implicating the IL-12 pathway in genesis of human PBC.4 Through a genomewide association screen of more than 500 PBC cases and more than 1000 healthy controls, we identified IL12A, IL12 receptor (IL12R)B2, as well as HLA (human leukocyte antigen) gene variants as the loci most strongly associated with risk for PBC in our study cohort (Fig. 1). Not only were the population-attributable risks for all three of these loci very high, but another of the strongest association signals in this screen emanated from a gene locus, STAT4 (signal transducer and activator of transcription 4), encoding a transcription factor essential to the coupling of IL-12/IL-12 receptor engagement to changes in gene transcription. These findings implicate three key components of the IL-12 signaling pathway in PBC (Fig. 2), as is in keeping not only with the findings of Yoshida et al., but also with other genetic data implicating this signaling axis in such autoimmune diseases as psoriasis12 and celiac disease13 and with the recognized importance of IL-12 signaling to the modulation of infectious and autoimmune inflammatory responses.14 Genomewide association analysis in patients with PBC as reported in the New England Journal of Medicine.4 The y-axis represents the level of significance (from the EIGENSTRAT method) for each single-nucleotide polymorphism on each chromosome along the x-axis. The dotted line indicates the threshold for genomewide association significance. HLA-DQB1 denotes the gene encoding major histocompatibility complex class II DQ β chain 1, IL12A denotes the gene encoding interleukin-12, IL12RB2 denotes the gene encoding interleukin-12 receptor β2, and STAT4 denotes the gene encoding signal transducer and activator of transcription 4. Reproduced with permission of the New England Journal of Medicine; © 2009 Massachusetts Medical Society. All rights reserved. Schematic showing possible influence of IL-12 signaling on the T cell–mediated inflammation associated with primary biliary cirrhosis. Proteins highlighted in yellow have been associated with PBC by genomewide association testing, and those in orange have been modulated in animal models of autoimmune cholangitis. IL-12p70 produced by antigen presenting cells interacts with the heterodimeric IL-12 receptor so as to activate STAT4 and thereby promote induction of IFNgamma-producing TH1 cells. The IL-12p40 subunit is also a component of IL-23 and thus contributes to IL-23-mediated induction of TH17 cells. Both IL-12 and IFN-γ can suppress TH17-mediated inflammatory responses that may underlie the bile duct inflammation associated with PBC. The IL-27 driven pathway which also couples TH1 (T-bet/IL-10+) cells to IFNgamma production and regulates TH17 responses is not shown. Although not shown here, the important roles of TGFβ in regulating development of both T regulatory and TH17 cells, is also of relevance to the expression of PBC. TNF, tumor necrosis factor. Although IL-12 has long been recognized as a critical immunomodulatory cytokine, understanding of its biologic roles continues to evolve. Resolution of IL-12 functionality is complicated by the overlap of IL-12 structure with that of IL-23, another immunoregulatory cytokine with a well-established role in promoting development of TH17 cells. In agreement with the findings of Yoshida et al., p40 deficiency induced in mice by genetic manipulation or by antibody treatment suppresses the emergence and, by extension, the inflammatory effects of IL-17–producing TH17 cells. It is in this context that multiple clinical trials have been initiated to test whether monoclonal antibody or transcription inhibitor–mediated p40 suppression is of therapeutic benefit in psoriasis15 and Crohn's disease.16, 17 Interpreting the PBC-modulatory effects of IL-12 and IFN-γ is complicated, confounded by data implicating the putative proinflammatory cytokines IL-12, and the IFN-γ induced by IL-12, in the suppression of TH17 development. From this latter perspective, IFN-γ deficiency would be predicted to increase rather than reduce cholangitis in dnTGFβRII mice. The background model of dnTGFβRII mice is further complicated by TGFβ roles in promoting development of regulatory T cells, another T cell population with high relevance to autoimmunity. Resolving these issues may prove challenging, but is highly germane to the understanding and management of PBC in view of the strong association of this disease with IL12A/IL12RB2/STAT4 alleles, the IL-12p40-PBC link identified by the work of Yoshida et al., and other recent data showing IL-17–producing lymphocytes to be increased in liver and blood from patients with PBC.18-20 Addressing this challenge will require additional genetic association/sequencing studies enabling definition of the specific IL12A and IL12RB2 alleles conferring risk for PBC, molecular analyses that clarify the effector interactions governing the IL-12 and the related IL-23 signaling pathways, and cellular and/or animal experiments that elucidate the relative contributions of TH17, T regulatory, and other immune cellular subpopulations to this disease. Combined with already available strategies for therapeutic manipulation of IL-12 signaling, these investigative approaches should allow the newly-revealed connection between PBC and the IL-12 immunomodulatory axis to be rapidly translated into novel therapy and better outcomes for patients with PBC and other associated autoimmune diseases.