The front line of research into immunoglobin G4-related disease - Do autoantibodies cause immunoglobin G4-related disease?

Abstract

IgG4-related disease (IgG4-RD) is a novel clinical disease entity characterized by elevated serum IgG4 concentration and tumefaction or tissue infiltration by IgG4-positive plasma cells. Although IgG4-RD is attracting strong attention as a new clinical entity, the pathogenesis of IgG4-RD and the roles of IgG4 are still unknown. Recently, self-antigens including Laminin 511 E8, Galectin-3 and Annexin A11, have been reported from one to the next as autoantigens which may be involved in the pathogenesis of IgG4-RD. In this review, we describe up-to-date information on the research of this emerging disease entity. Moreover, we discuss the pathogenesis of IgG4-RD by focusing on recent reports concerning autoantibodies.

Keywords:

• IgG4-RD
• autoantibody
• autoantigen
• innate immunity
• acquired immunity
• pathogenesis
• autoimmunity

Introduction

IgG4-related disease (IgG4-RD) is a new emerging disease entity with multi-organ involvement characterized in most patients by elevated serum IgG4 concentrations [1]. Other manifestations of IgG-RD include a characteristic histopathological appearance and an elevated number of IgG4-positive plasma cells with a ratio of IgG4-positive to IgG-positive cells of more than 40% [2]. However, it is not clear whether the enhanced production of IgG4 antibody is the cause of IgG4-RD or an epiphenomenon associated with inflammatory reactions.

Characteristics of immunoglobin G4

Among the four subclasses of IgGs, the least abundant is IgG4. Important differences between IgG1 and IgG4 include a few amino acid differences in the CH2 domain, enabling IgG4 in the form of noncovalently linked heavy chains, leading to the appearance of half antibodies [3]. IgG4 antibodies can exchange Fab arms by swapping a heavy chain and form bispecific antibodies as well as functioning as monovalent molecules. Therefore, IgG4 has an anti-inflammatory rather than a proinflammatory role, and may protect against type I allergy by inhibiting IgE functions, and may also prevent type II and III allergies by inhibiting the Fc-mediated effector functions of IgG1 [4].

IgG4-RD occurs comparatively more in the elderly (median age, 58 years old), slowly progresses and shows relatively weak immune responses such as low titer of CRP [1]. One-third of IgG4-RD patients have a history of atopic disease including asthma, allergic rhinitis and atopic dermatitis [5]. It is possible that some antigens or microbes may trigger a break in immunological tolerance in genetically susceptible individuals, generally older men. In this regard, innate immunity may play a pivotal role in the pathogenesis of IgG4-RD through infections and innate systems such as Toll-like receptors, monocytes and basophils [6].

Acquired immunity

Th2 cytokines

Initially, skewed production of Th2 cytokines and an increase in number of regulatory T cells (Treg) have been considered to be the cause of IgG4-RD [7,8]. Indeed, many reports indicated that Th2-dominant immune responses and the production of Th2-type cytokines (IL-4, IL-5, and IL-13), the regulatory cytokine IL-10 and fibroblast stimulating cytokine TGF-β are increased in IgG4-RD [9,10]. In addition, the levels of expression of IL-10 and AID were significantly higher in patients with IgG4-RD than Sjögren’s syndrome (SS) and controls [11]. Since AID and IL-10 as well as the Th2 cytokines can induce IgG4- and IgE-specific class-switch recombinations, overexpression of these factors may be involved in the pathogenesis of IgG4-RD through IgG4-specific class-switch recombination [6].

T cell populations

CD4+ T cells are necessary for development of germinal centers (GCs) where somatic hypermutation of immunoglobin (Ig) variable region genes and selection of high-affinity B cells occur, followed by differentiation into memory B cells and long-lived plasma cells that secrete high-affinity antibodies [12]. T follicular helper (Tfh) cells characterized by positive CD4 and CXCR5 are further divided into three subsets: Tfh1 (CXCR3⁺CCR6⁻), Tfh2 (CXCR3⁻CCR6⁻) and Tfh17 (CXCR3⁻CCR6⁺) [13]. In addition, Tfh2 cells are engaged in promoting the growth, differentiation and class switching of B cells, resulting in immunoglobulin secretion of various isotypes (IgM, IgA, IgG and IgE for Tfh2 cells) by secreting larger amounts of IL-21 in GCs or outside GCs [14]. By this reason, abnormal populations and functions of T cell subsets, especially Tfh2 cells, have attracted attention in studies of the pathogenesis of IgG4-RD. In this regard, it has been reported that the number of circulating Tfh2 cells is higher in patients with IgG4-RD and correlated with disease activity [15–17].

Recently, clonal expansion of a novel T cell population, cytotoxic effector memory CD4+ T cells (CD4+ CTL_EM) have been reported in affected organs from patients with IgG4-RD [18]. CD4+ CTL_EM arises from chronic antigen stimulation and significantly increases and correlates with disease activity in patients with chronic viral infections, including Epstein–Barr virus, cytomegalovirus, and human immunodeficiency virus, and in patients during the course of autoimmune diseases and malignancies [19]. Since CD4+ CTL_EM contributes to chronic inflammation involved in a variety of conditions, it may play an important role in the pathogenesis of IgG4-RD.

Innate immunity

Macrophages composed of two distinct phenotypes of M1 and M2 are tissue-resident monocytes in phagocytosis and initiation of innate immunity. M1 macrophages classically activated by Th1 response play a central role in the host defense against bacterial and viral infections. Whereas, M2 macrophages induced by Th2 cytokines (IL-4 and IL-13) and Treg cytokine (IL-10), function in anti-inflammatory responses, tissue repair and remodeling [20]. Increase of CD163+ M2 macrophages and the amount of fibrosis in biopsy specimens are reported in patients with IgG4-RD. In addition, abundant interleukin-33 (IL-33) produced by macrophages, dendritic cells and damaged epithelial cells was detected in salivary glands, which might activate Th2 immune responses in patients with IgG4-RD [21]. Alternatively, macrophages from patients with IgG4-RD induce IgG4-production by BAFF released from B cells upon stimulation of Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain receptors (NOD)-like receptors [22].

Basophils, major players in allergic inflammatory reactions, are also observed in biopsy specimens from patients with IgG4-RD [23]. Basophils lead to differentiation of inflammatory monocytes into M2 macrophages [24]. In addition, Basophils function as antigen-presenting cells in the Th2 response and are considered a major source of IL-4. In this regard, basophils from patients with IgG4-RD induced IgG4 production by B cell from healthy controls upon activation of TLRs [23,25].

Eosinophilia in tissue from patients is a histological hallmark of IgG-RD and might play a role in IgG4-RD by promoting fibrosis through the production of TGF-β, PDGF and IL-13. Additionally, it has been reported that the levels of expressions of several genes involved in allergy development and innate immunity were lower in PBMCs from IgG4-RD patients than from healthy controls [26]. Altogether, these data suggest that innate immunity may contribute to the pathogenesis of IgG4-RD through crosstalk with acquired immunity.

Do autoantibodies to self-antigens cause immunoglobin G4-related disease?

To date, several autoantibodies including those to pancreatic trypsin inhibitor (PSTI), lactoferrin (LF) and carbonic anhydrase (CA) have been reported in patients with IgG4-related AIP [27]. However, IgG4-type autoantibodies have not been detected in patients with other organ involvement of IgG4-RD. Thus, the idea that autoantibodies may play a major role in IgG4-related disease had been questioned. Nevertheless, several studies involving autoantibodies in IgG4-RD against different antigens have been reported one after another in Europe, US, China and Japan.

Japanese scientists have identified laminin 511-E8, a component of extra-cellular matrix (ECM), as the target antigen in IgG4-related autoimmune pancreatitis (AIP) [28]. Some 51% of patients with IgG4-AIP were found positive for an antibody against laminin 511-E8, compared with 1.6% of healthy controls. Injection of human serum from patients with IgG4-AIP into mice or immunization of mice with human laminin 511-E8 induced pancreatitis similar to IgG4-AIP. Interestingly, four of 25 patients without an antibody to laminin 511-E8 had an antibody to integrin α6β1, a laminin 511-E8 ligand. Thus, laminin was proposed as the candidate autoantigen to produce autoantibodies in IgG4-RD [28].

On the other hand, US scientists have identified galectin-3, a β-galactoside-binding lectin, as an autoantigen for IgG4-RD by immunoaffinity chromatography and mass spectrometry using plasma blast clones from IgG4-RD [29]. Some 28% of 121 patients with IgG4-RD were positive for anti-galectin-3 antibody. They also reported IgE isotype antibody against galectin-3. Since IL-4 and IL-10, which both drive isotype switching of immunoglobins, were increased in patients with IgG4-RD, this finding explains the findings of skewed switching to IgG4 in IgG4-RD [29].

Just before the aforementioned US paper, a Japanese group reported that galectin-3 was 13-fold overexpressed in the pancreas of IgG4-AIP patients compared with in normal pancreas, by proteomic analysis [30]. Indeed, macrophages, dendritic cells and myofibroblasts express galectin-3 in affected organs of IgG4-RD including salivary glands, pancreas, kidney, lung, aorta, retroperitoneum, lymph nodes and bile ducts. Since Galectin-3 is involved in various fibroblast involved proliferative diseases, such as pulmonary fibrosis, chronic kidney disease, and nonalcoholic steatohepatitis [31], autoantibody against galectin-3 may be involved in the tissue fibrosis observed in IgG4-RD. Along with the findings of these reports we can speculate that overexpression of certain antigens may contribute to autoantibody synthesis.

Chinese scientists have identified prohibitin, as a candidate autoantigen of IgG4-RD by affinity purification with patient serum and matrix-assisted laser desorption/ionization time-of-flight tandem (MALDI-TOF/TOF) mass spectrometry [32]. The antibody against prohibitin was found to be 73% positive in patients with IgG4-AIP, 53% in those with IgG4-Mikulizz’s disease, 54% in IgG4-retroperitoneal fibrosis and 89% positive in other patients with probable IgG4-RD, respectively. Whereas, this antibody was detected in only 13% of patients with Sjogren’s syndrome and 1% in healthy controls. Prohibitin is ubiquitously expressed and possesses multiple functions, positively or negatively, in cell proliferation, cell cycle, transcription and signal transduction [33]. In addition, prohibitin binds to DNA and numerous proteins such as complement component C3a and C3b, Annexin A2, protein C, and more [34,35]. Although the function of prohibitin in IgG4-RD is unclear, the antibody against prohibitin may have a role in the mechanism of IgG4-RD.

Finally, European scientists have identified annexin A11 as an autoantigen in IgG4-RD by immunoprecipitation and mass spectrometry and reported that 9 out of 50 patients with IgG4-AIP were positive for the antibody against annexin A11 by western blotting or ELISA assay [36]. Annexins are a family of calcium-dependent phospholipid binding proteins largely present within cells [37]. Since annexin A11 is predominantly located within the nucleus, it should not be exposed to the immune system. However, autoantibodies against annexin A11 have been reported in autoimmune diseases including antiphospholipid syndrome, lupus erythematosus and systemic sclerosis. Upon cellular injury, annexin A11 may be recognized as an autoantigen by the immune system in a similar manner to that in autoimmune diseases.

The European authors also identified two epitopes of annexin A11 shared by IgG1- and IgG4-isotype antibodies. Interestingly, IgG4 antibody against annexin A11 inhibited the binding of IgG1 antibody to annexin A11, which could be part of regulatory function of IgG4.

Lessons and the future perspective

Reports of autoantibodies against autoantigens such as laminin 511-E8, galectin-3, prohibitin and annexin A11 in IgG4-RD have been accumulating recently. All of these autoantigens are ubiquitous proteins expressed within cells or extra cellular matrix. Are autoantibodies to autoantigens involved in the pathogenesis of IgG4-RD? Perhaps, the answer is yes. However, the positivity of each antibody reported is not very high with 51% for laminin 511-E8 [28], 28% for galectin-3 [29], 73% for prohibitin [32] and 18% for annexin A11 [36]. In addition, antibodies against components of some organs have not detected in IgG4-RD patients with another organ involvement. Therefore, it is speculated that it is not just one particular autoantigen that causes IgG4-RD.

Two study groups from different countries independently identified galectin-3 as a prospective involved autoantigen [29,30]. Galectin-3 is overexpressed in patients with IgG4-RD, and plasma blast clones from patients with multiple organ involvement produced anti-galectin-3 antibody [30]. This suggests that ubiquitously or overexpressed proteins might be candidates for autoantigens of IgG4-RD. In this regard, the Japanese group reported that autoantibodies against laminin 511-E8 and its ligand, integrin α6β1 were both detected in patients with IgG4-RD [28].

One important feature of IgG4-RD is that hyperproduction of IgG4 is frequently concomitant with allergy [1]. In this regard, the US group reported both IgG4- and IgE-isotype antibodies against galectin-3 produced by plasmablasts clone from patients with IgG4-RD [29]. Although further investigation is required, the mechanism underlying isotype switching of immunoglobins to IgG4/IgE may be important in the pathogenesis of IgG4-RD (Figure 1).

Figure 1. Players in pathogenesis of IgG4-RD.

A basic question is, why is IgG4 so abundant in IgG4-RD? In classical autoimmune diseases such as systemic erythematosus (SLE), autoantibodies caused severe damage to organs through type II (antibody-dependent cytotoxicity) and type III (immunocomplex-mediated cytotoxicity) allergies. In contrast, IgG4-RD occurs comparatively more in the elderly, slowly progresses and shows relatively weak immune responses [1]. The European group reported IgG4-isotype antibody to annexin A11 as well as IgG1-isotype, and that IgG4 antibody can inhibit the binding of IgG1 antibody to annexin A11 [36]. Due to its characteristic structure, IgG4 functions as an antiinflammatory rather than proinflammatory agent. Therefore, IgG4 may prevent immune responses by inhibiting the activities of IgG1 and the formation of immune complexes that may lead to damage of vascular organs [6].

Several antigens from various components, including bacteria, virus, tumor, as well as autoantigens might be involved in the pathogenesis of IgG4-RD, which may vary by the condition and affected organs of patients. Although clinical symptoms of patients were varied depending on the affected organs and several autoantibodies to self-antigens have been reported frequently in IgG4-RD, the common features are IgG4 hyper-production and increase of IgG4-positive plasma cells in affected organs. Therefore, to clarify the mechanisms of skewed class switching to IgG4 production seems to be important for further understanding of the pathogenesis of IgG4-RD.

Conflict of interest

None

Acknowledgments

We sincerely thank the many contributing researchers and collaborators who participated in the all Japan IgG4-RD Research Group.

Additional information

Funding

This work was supported by Intractable Diseases and Health and Labor Sciences Research Grants from the Ministry of Health, Labor and Welfare of Japan.