Towards common denominators in primary biliary cirrhosis: The role of IL-12
Authors: Ana Lleo, M. Eric Gershwin, Alberto Mantovani, Pietro Invernizzi
Journal: Journal of Hepatology (2011)

There have been significant advances in our understanding of the immunobiology of primary biliary cirrhosis (PBC) and, in particular, a rigorous dissection of not only the serologic abnormalities, including antimitochondrial autoantibodies (AMA), but also the definition of autoreactive CD4+ and CD8+ T cells [[1]Invernizzi P. Selmi C. Gershwin M.E. Update on primary biliary cirrhosis.Dig Liver Dis. 2010; 42: 401-408Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar]. Further, there is increasing evidence for the interplay of genetic and environmental factors in individual host susceptibility. One paradox has been the selected destruction of small bile ducts in PBC despite the presence of mitochondrial antigens in virtually all nucleated cells. This enigma is being addressed with the critical observation that during apoptosis, biliary epithelial cells (BECs) translocate immunologically intact PDC-E2 into apoptotic bodies, constituting an apotope, which is able to induce pro-inflammatory cytokine secretion from mature monocyte derived macrophages (MDM) from patients with PBC in the presence of AMA, including high levels of interleukin-12 (IL-12) [2Lleo A. Bowlus C.L. Yang G.X. Invernizzi P. Podda M. Van de Water J. et al.Biliary apotopes and anti-mitochondrial antibodies activate innate immune responses in primary biliary cirrhosis.Hepatology. 2010; 52: 987-998Crossref PubMed Scopus (170) Google Scholar, 3Lleo A. Selmi C. Invernizzi P. Podda M. Coppel R.L. Mackay I.R. et al.Apotopes and the biliary specificity of primary biliary cirrhosis.Hepatology. 2009; 49: 871-879Crossref PubMed Scopus (167) Google Scholar]. Importantly, data from genetic studies of humans with PBC, murine models, and in vitro experiments have identified the IL-12 signaling pathway as a key player in the effector mechanisms that lead to biliary destruction. In fact, genome wide association studies from three different populations have identified at least three IL-12 related genes strongly associated to PBC: IL12A, IL12RB2, and STAT4 [4Hirschfield G.M. Liu X. Xu C. Lu Y. Xie G. Gu X. et al.Primary biliary cirrhosis associated with HLA, IL12A, and IL12RB2 variants.N Engl J Med. 2009; 360: 2544-2555Crossref PubMed Scopus (503) Google Scholar, 5Liu X. Invernizzi P. Lu Y. Kosoy R. Bianchi I. Podda M. et al.Genome-wide meta-analyses identify three loci associated with primary biliary cirrhosis.Nat Genet. 2010; 42: 658-660Crossref PubMed Scopus (326) Google Scholar, 6Mells G.F. Floyd J.A. Morley K.I. Cordell H.J. Franklin C.S. Shin S.Y. et al.Genome-wide association study identifies 12 new susceptibility loci for primary biliary cirrhosis.Nat Genet. 2011; 43: 329-332Crossref PubMed Scopus (378) Google Scholar]. In addition, the deletion of IL-12p40 on the transforming growth factor β receptor II dominant negative (dnTGF-βRII) murine model of PBC has established that the IL-12p40 subunit is essential for the development of autoimmune cholangitis [[7]Yoshida K. Yang G.X. Zhang W. Tsuda M. Tsuneyama K. Moritoki Y. et al.Deletion of interleukin-12p40 suppresses autoimmune cholangitis in dominant negative transforming growth factor beta receptor type II mice.Hepatology. 2009; 50: 1494-1500Crossref PubMed Scopus (106) Google Scholar]. IL-12 is a heterodimer comprised of two subunits: a 35-kDa light chain or p35 (encoded by IL12A gene) and a 40-kDa heavy chain known as p40 (encoded by IL12B gene), that are secreted by activated antigen presenting cells (APCs). IL-12 is a major cytokine for the differentiation of T helper 1 (Th1) cells and plays an essential pro-inflammatory role in both innate and adaptive immune responses [[8]Trinchieri G. Interleukin-12 and the regulation of innate resistance and adaptive immunity.Nat Rev Immunol. 2003; 3: 133-146Crossref PubMed Scopus (2934) Google Scholar]. Two major pathways seem to be involved in the IL-12 driven Th1 differentiation; first, the specific cellular effects of IL-12 are due mainly to its ability to induce activation of the transcription factor STAT4, which promotes interferon-γ (IFN-γ) production, and, together with the triggering of Th1 cell responses, contributes to loss of tolerance in several models of autoimmunity [[9]Becher B. Durell B.G. Noelle R.J. Experimental autoimmune encephalitis and inflammation in the absence of interleukin-12.J Clin Invest. 2002; 110: 493-497Crossref PubMed Scopus (365) Google Scholar]. Of note, the p40 subunit is also a component of the dimeric cytokine IL-23, essential for the differentiation of Th17 cells. The most important target cells of IL-12 are: T cells, NK cells, and NKT cells, for which IL-12 induces proliferation, differentiation, enhancement of cytotoxicity, and the production of cytokines, particularly IFN-γ; and B cells, for which IL-12, directly or through the effects of IFN-γ, enhances the activation and production of Th1-associated classes of immunoglobulin. Further, IL-12 through STAT4 activation stabilizes t-bet, which itself drives Th1 differentiation [[10]Usui T. Preiss J.C. Kanno Y. Yao Z.J. Bream J.H. O'Shea J.J. et al.T-bet regulates Th1 responses through essential effects on GATA-3 function rather than on IFNG gene acetylation and transcription.J Exp Med. 2006; 203: 755-766Crossref PubMed Scopus (249) Google Scholar]. The IL-12 heterodimer signals through the cell surface IL-12 receptor, which is composed of two chains, IL-12Rβ1 (encoded by IL12RB1 gene) and IL-12Rβ2 (encoded by IL12RB2 gene). IL-12R is expressed mainly by activated T cells and NK cells, but has been shown also on other cell types, such as DCs and B-cell lines. The above observations allow one to propose a schematic pathogenesis of PBC (Fig. 1). In apoptosis of biliary epithelial cells, PDC-E2 remains intact [Fig. 1, step 1] and can provide an autoantigenic stimulus as unmodified lipoylated PDC-E2, as xenobiotic/hapten-modified PDC-E2, or as a microbial mimic of PDC-E2 [step 2]. PDC-E2 is endocytosed by an APC and leads to its maturation [step 3] and IL-12 secretion [step 4]. If the APC is activated via stimulation of TLR, self peptides will be presented in immunogenic mode via MHC class II to autoreactive CD4+ T cells and via MHC class I to CD8+ T cells (cross-priming) (not shown). The IL-12 heterodimer binds then its receptor on the surface of naïve T cells, NK and NKT cells which leads to Th1 differentiation [step 5], NK cytotoxicity and ultimately to IFN-γ secretion [step 6]. Cytotoxic CD8+ T cells get activated [step 7] which potentiates the destruction of small apoptotic BECs expressing PDC-E2 [step 8]. INF-γ determines also macrophage activation and possibly M1 polarization [2Lleo A. Bowlus C.L. Yang G.X. Invernizzi P. Podda M. Van de Water J. et al.Biliary apotopes and anti-mitochondrial antibodies activate innate immune responses in primary biliary cirrhosis.Hepatology. 2010; 52: 987-998Crossref PubMed Scopus (170) Google Scholar, 11Mantovani A. Locati M. Orchestration of macrophage polarization.Blood. 2009; 114: 3135-3136Crossref PubMed Scopus (94) Google Scholar]. CD4+ T cells provide help to autoreactive B cells that produce AMA [step 9]. AMA can form complexes with PDC-E2 that are phagocyted by APCs via Fc receptors as another source for antigen presentation [step 10]. Thus, a multilineage anti-PDC-E2 response is generated. The BEC is vulnerable because of expression of intact PDC-E2 [see step 1], and expression of MHC molecules. Intact PDC-E2 released from damaged BEC maintains self-perpetuating disease. Interpreting the effects of IL-12 and IFN-γ in PBC is complicated, and addressing this issue will require additional studies that clarify the effector mechanisms involved in the IL-12 and the related IL-23 signaling pathways. First, based on the lack of PBC-like disease on the dnTGF-βRII animal model with deletion of IL-12p40 [[7]Yoshida K. Yang G.X. Zhang W. Tsuda M. Tsuneyama K. Moritoki Y. et al.Deletion of interleukin-12p40 suppresses autoimmune cholangitis in dominant negative transforming growth factor beta receptor type II mice.Hepatology. 2009; 50: 1494-1500Crossref PubMed Scopus (106) Google Scholar], and the increase secretion of IL-12 from human macrophages co-cultured with apoptotic BECs [[2]Lleo A. Bowlus C.L. Yang G.X. Invernizzi P. Podda M. Van de Water J. et al.Biliary apotopes and anti-mitochondrial antibodies activate innate immune responses in primary biliary cirrhosis.Hepatology. 2010; 52: 987-998Crossref PubMed Scopus (170) Google Scholar], we have herein assumed that IL-12 production is increased in PBC; however, low levels of IL-12 could also be deleterious if there is loss of control of Th17 cells; at present, experiments correlating genes and biologic change have not been done. Second, IL-12 associations could also be consistent with an infectious etiology, however, no data linking IL-12 production to damage of a specific cell type i.e. BECs in PBC, are available. Third, other important genes e.g. IRF5, CXCR5, NFKB1 have been shown to be genetically associated with PBC, although functional data are still missing. Importantly, high expression of IRF5 is characteristic of M1 macrophages, in which it directly activated transcription of the genes encoding IL-12 [[12]Krausgruber T. Blazek K. Smallie T. Alzabin S. Lockstone H. Sahgal N. et al.IRF5 promotes inflammatory macrophage polarization and TH1-TH17 responses.Nat Immunol. 2011; 12: 231-238Crossref PubMed Scopus (845) Google Scholar]. Finally, the IL-12 pathway highlights the interplay between innate and acquired immunity, and dissection of this relationship may provide opportunities for therapeutic intervention. The authors declared that they do not have anything to disclose regarding funding of conflict of interest with respect to this manuscript.