Recently, immunoglobulin (Ig)G4-associated cholangitis (IAC) has been described as a new disease entity of the intra- and extrahepatic bile ducts leading to strictures that are difficult to distinguish from primary sclerosing cholangitis (PSC) and bile duct cancer. In contrast to PSC, these strictures disappear after corticosteroid therapy. In these patients, serum IgG4 levels are often, but not always, elevated and IgG4+ B cells and plasma cells are significantly increased in inflamed tissue. IAC is also often associated with autoimmune pancreatitis (AIP). IAC, AIP together with other IgG4-associated syndromes, are currently summarized under the spectrum of so-called IgG4-related disease (IgG4RD) (Table 1).1 IgG4RD is regarded as a systemic disorder with different manifestations, such as IAC and AIP, but also affecting other organs, such as salivary glands, periorbital tissues, kidneys, and protaste (Table 1).2-4 AIP is often not diagnosed until the tissue is analyzed by a pathologist subsequent to pancreatectomy performed because of suspected malignancy. The various syndromes of IgG4RD are characterized by tumefactive lesions, dense lymphoplasmacytic infiltrates rich in IgG4+ plasma cells, storiform fibrosis, often, but not always, elevated IgG4 serum levels, and a usually good response to corticosteroid treatment. AIP, like many organ manifestations of IgG4RD, is associated with a tumor-like swelling of the organ, which may lead to biliary obstruction. Serum IgG4 levels, although very characteristic, have low diagnostic sensitivity and specificity; furthermore, tissue for histological diagnosis is often not available. The so-called histology, imaging, serology, other organ involvement, and response to therapy criteria5 were developed by experts to allow a safer diagnosis because this has enormous consequences for the individual patient. Biliary strictures resulting from IAC can be resolved by corticosteroids within weeks. Corticosteroids, on the other hand, may be harmful in PSC patients, potentially causing bacterial cholangitis. In the differential diagnosis, it is not only important to distinguish the strictures from PSC and biliary malignancy, but also from secondary sclerosing cholangitis (SCC).6 SSC may have different causes leading to progressive destruction of bile ducts, and liver transplantation may become necessary. One of the more common forms of SSC follows long-term intensive care treatment; this type of progressive sclerosing cholangitis is also poorly understood and difficult to treat.6 Therefore, discoveries that help us to further define, diagnose, distinguish, and treat specifically these rare diseases of the biliary tract, such as IAC, PSC, and SSC, are urgently needed.
|IgG4-associated cholangitis (IAC)|
|Autoimmune pancreatitis (AIP)|
|Mikulicz's syndrome (affecting salivary and lacrimal glands)|
|Eosinophilic angiocentric fibrosis (affecting the orbits and upper respiratory tract)|
|Multifocal fibrosclerosis (commonly affecting orbits, thyroid gland, retroperitoneum, mediastinum, and other tissues)|
|Inflammatory pseudotumor (affecting the orbits, lungs, kidneys, and other organs)|
|Retroperitoneal fibrosis (Ormond's disease)|
|Periaortitis and periarteritis|
|Inflammatory aortic aneurysm|
|Idiopathic hypocomplementemic tubulointerstitial nephritis with extensive tubulointerstitial deposits|
The role of IgG4 in IAC and in IgG4RD in general is poorly understood. Is it primary or secondary, chicken or egg? Potential pathogenetic mechanisms are genetic risk factors, infectious agents with or without molecular mimicry with host components, other environmental factors, or autoimmunity. T-helper (Th)2 cells are predominantly activated at affected sites of IAC and IgG4RD. A further characteristic of IgG4RD is the activation of regulatory T cells (Tregs). This finding argues against an autoimmune background of IAC and IgG4RD, because Tregs are usually deficient in autoimmune disorders. In addition, the application of Tregs has been successfully used as a therapeutic approach in some autoimmune disorders. Another argument against IAC and IgG4RD as an autoimmune disorder is a lack of specific autoantibodies and the predominance of males. On the other hand, the striking response to steroids, as shown in the article by Maillette de Buy Wenniger supports the hypothesis of an immune-mediated pathogenesis.7 Activation of Tregs in IgG4RD was demonstrated through the detection of FOXP3 messenger RNA in affected tissue. The physiological and pathophysiological role of the Ig subclass IgG4 itself is poorly understood. IgG4 is the least abundant of all IgG subclasses. In healthy individuals, IgG4 accounts for less than 5% of all IgG molecules. Amino acid differences within the second constant domain lead to negligible binding of IgG4 to C1q and Fc gamma receptors. Thus, in theory, IgG4 does not activate classical complement pathways and has been regarded to play a minor role in immune activation. A unique characteristic of the IgG4 molecule is its fragment antigen-binding arm exchange,8 because disulfite bonds between heavy chains of IgG4 molecules are unstable. Fifty percent of IgG4 molecules consist of heavy chains linked weakly by noncovalent forces. This weak adhesion of IgG4 heavy chains may lead to dissociation of noncovalent bonds and permits chains to separate and recombine randomly. As a consequence asymmetric antibodies with two different antigen-combining sites are formed, they result in bispecific (functionally monovalent) IgG4 molecules that are unable to cross-link antigens and therefore are unable to form immune complexes. IgG4 may bind Fc portions of other IgG molecules. This rheumatoid factor-like function may contribute to the molecule's anti-inflammatory function. Physiologic IgG4 responses are induced by prolonged antigen stimulation and are controlled by Th2 cells. Th2 cytokines, such as interleukin (IL)-4 and IL-13, induce IgG4 and IgE. IgG4 in vivo is induced in a setting of a dominant Th2 cell immune response causing activation of Tregs that produce IL-10. Another important cytokine involved in the disease process is transforming growth factor beta, responsible for fibrosis formation contributing to organ swelling and tumefactive lesions. Nevertheless, all this basic information on IgG4 does not help us to understand the role of IgG4 in IgG4RD, in particular, in the etiology and pathogenesis of IAC.
Mailette de Buy Wenniger et al.7 have therefore applied novel next-generation sequencing (NGS) to screen the B-cell receptor (BCR) repertoires in peripheral blood as well as affected tissues prospectively in 6 patients with IAC. As affected tissues, the researchers used papilla vateri biopsy material. Papilla vateri biopsy material became accessible during routine changes of biliary stents. There is information in the literature that infiltrating immune cells at the papilla vateri, including B cells, resemble the infiltrate in inflamed tissues of the biliary tract. Six healthy controls (HCs) as well as 6 disease controls (DCs) suffering from PSC or pancreaticobiliary malignancy served as controls. Novel NGS technology to screen the BCR heavy-chain repertoire revealed that the most dominant clones for all Ig classes were IgG4 in IAC, but not in healthy (HCs) nor in disease controls (DCs). IgG4+ clones were dominant in the IgG+ clones, but also in the full IgA, IgD, IgM, and IgG repertoire. Comparison of dominant IgG4+ clones in IAC showed distinctly different rearrangements between patients. Mutational analysis of dominant IgG4+ clones showed multiple nonsilent mutations, suggesting affinity maturation.
The dominant IgG4+ clones that are found in peripheral blood are also dominantly present in the biopsy material obtained from the papilla vateri. Duodenal papilla vateri biopsy material was obtained from 2 of the patients, together with paired material from peripheral blood before and 4 and 8 weeks after the start of corticosteroid therapy. In both patients, highly dominant IgG4+ clones were recovered in the BCR repertoire of the biopsy material. The tissue-infiltrating cells seem to be enriched with these IgG4+ clones, suggesting a role in pathogenesis. This important finding argues for IAC as an antigen-driven disease. Further research should focus on the identification of the causative agent(s). Potential candidates are infectious agents or environmental factors, such as chemicals or drugs. These causative agents could trigger the disease process in genetically susceptible individuals. Under steroid treatment, symptoms improved.7 At the same time, elevated serum IgG4 levels fell considerably, but remained elevated, whereas total serum IgG levels stayed unchanged. More important, the dominant IgG4+ clones, as part of the IgG and total BCR repertoire, marginalized; in contrast, the majority of the non-IgG4 B-cell repertoire remained stable during immunosuppressive therapy. Certainly, this is a significant step forward in our understanding of IAC and IgG4RD as an antigen-driven disease based on the application of novel up-to-date and sophisticated methodology. Without any doubt, these investigations have to be extended to (1) more patients with IAC (before and under corticosteroid treatment), (2) more patients with AIP and other organ manifestations of IgG4RD, and (3) analysis of inflamed tissue.
This tissue of affected areas should not only come from the papilla vateri as a surrogate for the stenotic biliary tract lesions. However, tissue from the actual site of the biliary stricture may be difficult to obtain. Therefore, we need more confirmation that the information we obtain from peripheral blood lymphocytes indeed reflects the situation at the affected tissue of the biliary tract or other sites of IgG4RD. It will be interesting to see whether similar data can be collected from pancreatic tissue of patients with AIP obtained by diagnostic endoscopic ultrasound. These investigations of affected tissue are important to further elucidate the pathogenesis, or even etiology, of IAC, AIP, and of IgG4RD.
In the long run, we need more-noninvasive procedures to study the etiology and pathogenesis of IAC and IgG4RD. Recently, antibodies against a peptide with sequence homology to the plasminogen-binding protein of helicobacter pylori and the ubiquitin-protein ligase E3 component n-recognin 2 were reported in the majority of a cohort of patients with AIP.9 Unfortunately, they were not specific because they were also noted in some patients with pancreatic cancer-the disease that primarily has to be excluded in patients with AIP.9 Extramural validation of these findings is still needed. Then, it would be of interest to study these antibodies in IAC and other IgG4RD. Recently, efforts have been undertaken to develop noninvasive procedures to diagnose or exclude biliary malignancies in patients with PSC. Promising results were published on bile and, recently, urine proteomic profiles.10, 11 In the future, such noninvasive procedures, such as proteomic profiles, should prospectively be applied in these rare and poorly understood diseases, including IAC, AIP, IgG4RD, PSC, primary biliary cirrhosis (PBC), SSC, and cholangiocellular carcinoma. Again, we need to understand whether IAC and AIP are two different manifestations of a single disease entity (IgG4RD) and what determines the specific sites for organ manifestation.
Furthermore, we need to study the epidemiology of IgG4+ IAC, AIP, and IgG4RD. PSC, for example, is frequently diagnosed in the Northern Hemisphere (e.g., northern United States, Canada, and Northern Europe), whereas it is rarely reported in other parts of the world, such as China. We will need international networks of basic and clinical scientists to study and unravel the etiology and pathogenesis of IAC, AIP, and IgG4RD and to perform multicenter therapeutic trials that allow us to develop an optimum management for these patients. The genetic background of IgG4RD and IAC also needs to be studied intensively. Such genetic studies could provide surprises. Autoimmune polyendocrine syndrome type 1, for example, is a disease where a single-point mutation of a transcription factor (autoimmune regulator), which normally is expressed in the thymus and in dendritic cells, leads to an autosomal dominant monogenetic disease with a multiorgan involvement affecting various endocrine organs, such as the skin and liver.12 Perhaps IAC, AIP, and the other organ manifestations of IgG4RD are also part of a primarily genetic disorder with different disease phenotypes. Various environmental factors, as disease modifiers, could define time of manifestation and individual disease phenotype. Perhaps environmental agents secreted into the bile lead to lesions of the biliary epithelium. Then, an activation of the local immune system causes Th2 cell-mediated Treg activation. The resulting secretion of Th2 and Treg cytokines, such as IL-4, −5, −13, and −10, causes tissue eosinophilia and high IgE levels, but, in particular, IgG4+ B− and plasma cell infiltrates.1 Currently, we know that specific IgG4+ BCRs dominate the affected tissue infiltrate. Therefore, the next step should be the identification of the antigen(s) driving this IgG4 B-cell response. Hopefully, a causative agent(s) will be identified. This will be a methodological challenge. The researchers themselves indicate that, at present, it is impossible to predict the antigenic specificity of the BCR by its amino acid sequence.
At present, hepatology is dominated by hepatitis C and its novel therapies. Perhaps the next decade will see more focus on research of the various diseases of the biliary tract that still need to be explored. Here, we certainly will need international networks and public funding. At the moment, the pharmaceutical industry, in particular, “Big Pharma,” is reluctant to invest in these rare biliary diseases, including IAC, PBC, PSC, and SSC. The identification of dominant IgG4+ clones within the BCR repertoire of IgG4+-associated cholangitis is a first step and indicates that there is light at the end of the tunnel for this difficult-to-diagnose and difficult-to-manage biliary disease.