Among the four IgG subtypes, namely IgG₁, IgG₂, IgG₃, and IgG₄, IgG4 has the lowest concentration in normal human serum, accounting for only 5% of the total IgG levels (17). Antibodies are immunoglobin (Ig) molecules composed of two heavy (H) and two light (L) chains, both having a constant region (C_H or C_L) identical for all antibodies of the same isotype and a variable region (V_H or V_L) that recognizes and binds a specific antigen. Antibodies comprise two antigen-binding fragments (Fab) that bind to antigens and one constant fragment (Fc) that binds to the cell surface and allows phagocytosis. Most immunoglobin (Ig) G antibodies have these characteristics: (i) they have two identical antigen-binding sites; (ii) they do not change their structure after being secreted by plasma cells. However, Aalberse et al. (17) described IgG₄ as an “odd antibody” because of its unique properties different from other Igs. First, it cannot cross-link identical antigens (“functional monovalency”) (18, 19). Instead, the exchange of half-molecules, also called “Fab-arm exchange” ( Figure 1 ), contributes to bispecific IgG₄ antibodies. Second, unlike other Igs, which are proinflammatory, IgG₄ has a lower affinity for C1q (the q fragment, a part of complement C1, is the site where Igs first bind) and Fc receptor (18). These properties suggest that IgG₄ may have anti-inflammatory activities. Remarkably increased serum IgG₄ levels and multiple IgG₄ ⁺ plasma cell infiltration are important features of IgG₄-RD. Nevertheless, its function is still unknown in the pathogenesis of IgG₄-RD: whether it plays a protective role in IgG₄-RD by participating in the anti-inflammatory process, acts as a pathogenic factor mediating the occurrence of IgG₄-RD, or is merely a manifestation induced by inflammatory stimulation, has not been determined yet. In 2015, Shiokawa et al. (20) reported an interesting finding that pancreatic injury could be induced in neonatal male Balb/c mice by injecting patient IgG₁ or IgG₄, and the injury caused by IgG₁ was more serious. However, the pathogenic activity of IgG₁ and the severity of pancreatic injury were substantially inhibited by simultaneously injecting IgG₄. Moreover, rituximab, a monoclonal antibody, specifically binds to cluster of differentiation 20 (CD20), a biomarker of pre-B and mature B cells, and exerts cytotoxic, anti-proliferative, and apoptotic effects. A prospective, open-label trial by Carruthers et al. (21) discovered that rituximab could relieve the symptoms of IgG₄-RD. However, after rituximab treatment for 12 months, 19 patients with elevated baseline IgG₄ levels showed a marked decrease in IgG₄, but only 42% achieved normal IgG₄ levels. This finding showed that rituximab might not work by decreasing serum IgG₄ levels but by depleting activated B cells; thus, IgG₄ might not have a considerable role in IgG₄-RD. Similarly, Gauiran et al. (22) examined two cases of IgG4 myeloma, of which both showed high serum IgG4 levels, but neither of them manifested typical IgG₄-RD presentations.

IgG₄-RD is characterized by remarkably elevated serum IgG₄ levels and expansion of lymphoid follicles. However, a wide range of IgG₄ concentrations have been observed in patients with IgG₄-RD, with some reaching levels 30 times over the upper limit, but up to 40% of patients show normal IgG₄ levels (23). Next-generation sequencing examinations have led to the identification of a large number of circulating, antigen-specific plasmablasts in the peripheral blood of patients with IgG₄-RD (24). Flow cytometry studies have confirmed a substantial increase in the number of plasmablasts in the blood of IgG₄-RD patients (24). Over 95% of B-lymphocytes (except plasmablasts and plasma cells) express CD20, which makes rituximab, an anti-CD20 drug, effective in eliminating B cells from patients with IgG₄-RD (25). CD19 has a broader expression than CD20 and is found in all B cell lines except plasma cells (26). B cell depletion leads to a decrease in serum IgG₄ concentrations, indicating the preponderance of short-lived plasmablasts and plasma cells in regulating serum IgG₄ levels. However, the depletion of B cells does not cause a complete normalization of IgG₄ concentration, which suggests that long-lived plasma cells might maintain the production of antibodies. Mattoo et al. (24) examined 84 patients diagnosed with IgG₄-RD and found that the number of plasmablasts was higher than the control group even in patients with normal serum IgG₄ concentration. Besides, patients who undergo a relapse after rituximab treatment exhibit an increase in the number of plasmablasts and stronger somatic hypermutation. Therefore, peripheral plasmablasts might be considered as a biomarker for IgG₄-RD (27).

B-lymphocytes are also involved in the fibrosis of diseased tissues. Della-Torre et al. (28) cultured naïve B cells, CD19⁺ B-lymphocytes, memory B cells, or plasmablasts from IgG₄-RD with human fibroblasts and observed that the B cells and plasma cells from IgG₄-RD patients can promote collagen synthesis in fibroblasts by secreting the pro-fibrotic molecules of platelet-derived growth factor B. They also observed those cells could induce the remodeling of the extracellular matrix (ECM) by producing ECM-crosslinking enzymes, such as lysyl oxidase-like 2, and chemotactic factors CCL-4, CCL-5, and CCL-11. This finding suggests that B cells might be directly involved in tissue fibrosis in IgG₄-RD.

T cells are formed in the bone marrow but mature in the thymus. They are involved in cell-mediated immunity and have three major types: helper T cells (Th), cytotoxic T cells (Tc), and regulatory T cells (Treg). Th cells can be subdivided into mutually exclusive Th1 and Th2 subsets. The Th1/Th2 balance has been confirmed to participate in autoimmune and allergic diseases (29, 30).

Macrophages are relatively long-lived phagocytic cells of mammalian tissues derived from blood monocytes. Based on their phenotype and function, activated macrophages can be divided into two main categories, classically activated M1 macrophages and alternatively activated M2 macrophages, which are further divided into pro-allergic M2a, immune-regulatory M2b, and M2c types (86, 87).

When cultured with GM-CSF, monocytes can differentiate into M1 macrophages (88). Some important factors, including bacterial lipopolysaccharide (LPS) (89), monosodium urate monohydrate (90), inflammatory biomarker C-reactive protein (CRP) (91), and Th1 cytokines IFN-γ and tumor necrosis factor-α (TNF-α), promote the production of M1 macrophages. Activated M1 macrophages secrete proinflammatory factors and mediate adaptive immunity, thus eliminating the pathogens that damage normal host tissues. Hong et al. (92) observed markedly increased levels of TNF-α in submandibular glands of patients with IgG₄-related sialadenitis (IgG₄-RS). Besides, TNF-α treatment showed a consistent redistribution of the transcription factor EB in patients with IgG₄-RS. This finding suggests that TNF-α suppresses autophagic flux and lysosomal dysfunction and causes injury of acinar cells through the ERK1/2 pathway. To sum up, M1 macrophages might participate in the formation and progression of IgG₄-RS.

Interestingly, the anti-inflammatory M2 macrophages also participate in the IgG₄-mediated immune response. Usually, M2 macrophages become polarized by the stimulation of Th2-derived IL-4 and IL-13 (93). IL-33, a member of the IL-1 family, can amplify IL-13-induced M2 macrophage polarization (94). However, inflammatory monocytes can differentiate into M2 macrophages via basophil-derived IL-4 during an allergic reaction (95). Watanabe et al. (96) showed that the TLR signaling pathway could enhance immune dysregulation in IgG₄-RD. Besides, Ishiguro et al. (97) observed that TLR-7-, TLR-8-, and TLR-9-related genes were overexpressed in IgG₄-RD. Chang et al. (98) reported that in vitro stimulation with TLR-7 agonist could increase IL-33 production by alveolar macrophages in virus-infected lung tissues. Baenziger et al. (99) proved that, besides being involved in antiviral infection, TLR-7 is involved in acquired immunity. Mice experiments confirmed that the activation of TLR-7-expressing plasmacytoid dendritic cells could lead to arthritis and lupus nephritis (100). These interesting findings suggest that virus infection and/or endogenous RNAs could initiate the formation of IgG₄-RD. Pathogens activate IL-33-secreting M2 macrophages via TLR-7 and thus promote the production of Th2 cytokines, leading to tissue fibrosis and IgG₄ class switching. Bianchini et al. (101) further examined the phenotype of M2 macrophages and found that pro-allergic M2a macrophages could be converted into immune-regulatory M2b macrophages by IgG₄ to maintain a tolerogenic state.

Another crucial macrophage involved in IgG4-RD is CCL-18-producing M2 macrophages. CCL-18, produced from activated M2 macrophages, plays an critical rule in the formation of collagen (102). DNA microarray analysis also proved that CCL-18 was upregulated in IgG4-RD (103). Furukawa et al. (104) examined 7 patients with IgG4-related dacryoadenitis and sialoadenitis and found the level of CD163, one of the markers of M2 macrophage, was significantly higher than that in Sjögren’s syndrome and healthy subjects. Similarly, Takanashi et al. (105) observed massive infiltration of CD163⁺ M2 macrophages in the diseased tissue. CD163 is also co-localized with IL-10 and CCL-18 in the fibrotic region, which indicates CCL-18-secreting M2 macrophages might be involved in the development of fibrosis in IgG4-RD. Thus, CCL-18 might be a useful biomarker for tracing the severity of IgG4-RD (106).

Human IgG antibodies bind to different members of the Fcγ receptors (FcγRs) family, which have a low affinity for IgG and thus bind merely to immune complexes (IC) (107). It is widely believed that the interaction between IgG and FcγRs stimulates the immune system by triggering the phosphorylation of immunoreceptor tyrosine-based activation motif (ITAM) of FcγR (108) and inhibition of signaling pathway by coupling with FcγR IIB (109) and IgG₄ is commonly thought to be not compatible with FcγR (57, 110). However, Bianchini et al. (101) reported that IgG₄ might bind to FcγRIIb on M2a macrophages and cause M2b subtype conversion, leading to IL-10 and CCL1 secretion. IL-10 further contributes to class switching of IgG₄-secreting B cells, whereas CCL1 recruits CCR8⁺Foxp3⁺ Tregs from the periphery. Growing evidence indicates that ITAM-containing FcγR also has an inhibitory intrinsic ability (111). Boekhoudt et al. (112) reported that IFN-γ signaling pathways are inhibited by IC-mediated signaling through FcγR I and M2-like macrophages can be induced by IgG₄ through FcγR I (101).

The fate of organs in inflammation and injury is controlled by the balance of M1/M2 macrophages. When the infection or inflammation caused by those pathogens is severe enough to affect the target organ, macrophages are activated and differentiated to the M1 subtype to antagonize the stimulation by Th1 cytokines. However, if M1 macrophage-mediated immune responses continue, it could cause pathological damage to the tissue. Therefore, M2 macrophages express IL-10 and TGF-β and trigger Th2-mediated reaction to suppress the immune response and promote damaged tissue repair.

During parasitic infections, allergic reactions, and autoimmune diseases, basophil cells are recruited into tissues where they produce Th2 cytokines and participate in the immune response of Th2 cells as antigen-presenting cells (APCs) (113). APCs and T cells secrete Th2 cytokines IL-4, IL-10, and IL-13 to induce IgG₄ production. Several studies show that a combination of basophils and microbial antigens induces the production of Th2 cytokines (114, 115). Therefore, exogenous stimulation may facilitate the occurrence of IgG₄-RD through the activation of TLRs in basophils. Watanabe et al. (96) proved that microbial antigens activate TLRs and NLRs in monocytes to induce IgG₄ secretion by activating of B cell-activated factor (BAFF)-mediated pathways. They also examined IgG₄ and cytokine responses to various NLR and TLR ligands and found that the activation of TLRs in basophils promotes the secretion of IgG₄, BAFF, and IL-13 and thus leads to the progression of IgG₄-RD (116). This finding suggests that TLR-mediated basophil activation could facilitate disease development through the BAFF signaling pathway.

Eosinophils are involved in the pathogenesis of inflammation (117). They secrete various cytokines and affect T cell expansion and Th1/Th2 cell polarization (118). Some reports have shown a connection between eosinophils and IgG₄-RD (119–121). Some researchers also observed that patients with type I AIP, a typical presentation of IgG₄-RD, have a long history of allergies (122, 123). Peripheral eosinophils and serum IgE were elevated in these patients. However, when they tried to determine the allergen by skin prick test or specific IgE quantitation, the allergen sensitization profile failed to reveal the culprit. Furthermore, they tested the mean IgE levels and eosinophil counts and found no connection between those data and the atopic state of patients. This finding suggests that elevated peripheral eosinophil counts and IgE levels are the intrinsic characteristics of IgG₄-RD. TGF-β, one of the major cytokines secreted by eosinophils (118), participates in the formation of tissue fibrosis. Moriyama et al. (123) observed a positive correlation between peripheral eosinophil counts and treatment-free disease duration in IgG₄-RD, which indicated that eosinophils might promote the end-stage development of the disease and participate in the pathogenesis of fibrosis.

Eotaxin-3, or CCL-26, is recently considered to be a potent chemoattractant for eosinophils (124). To date, Eotaxin-3 is thought to have eosinotactic activity both in vitro and in vivo. IL-4 and IL-13 are potent co-inducers of Eotaxins in epithelial and endothelial cells, consistent with Th2 responses in allergic and eosinophilic diseases (125). Eotaxin-3 could also attract eosinophils, basophils, and killer T cells via receptor CCR3 and CX3CR1. Consequently, increased local expression of Eotaxins has been described in various eosinophilic diseases. In 2021, Takanashi et al. (126) analyzed proteins overexpressed in patients with IgG4-RD with lymphadenopathy and discovered that this disease was linked with eosinophilia and Eotaxin-3 could be thought to be a potent biomarker.

Lymphadenopathy in IgG4-RD represents a phenotype associated with high disease activities, eosinophilia and relapsing disease. Eotaxin-3 is a novel biomarker related to IgG4-RD with lymphadenopathy.

The complement system participates in developing IgG₄-RD. It contains a cascade of proteins that lead to the lysis of microorganism-infected cells. The complement system can be activated by three pathways: the classic, alternative, and lectin pathways. These three pathways converge on the production of C3 convertase, an enzyme that triggers the cleavage of C3 into an enzymatically active C3b and an anaphylatoxin C3a that could mediate inflammatory responses. Saeki et al. (127) examined 10 patients with hypocomplementemia of unknown etiology and found six to have high serum IgG₄ levels. Kawano et al. (128) identified 22 patients (53.7%) with hypocomplementemia among 41 IgG₄-related kidney disease (IgG₄-RKD) patients. Besides, 16 showed lower levels of C3, C4, and CH50. IgG₄-RKD is now considered as a complement-triggered inflammatory disease (129). A native renal biopsy showed that ethnic factors might contribute to different incidence rates of IgG₄-RKD ( Table 1 ). Besides, renal interstitial tissue fibrosis is now recognized as an inevitable process of end-stage renal disease. Wang et al. (135) examined serum C3 and C4 levels and found that these levels were negatively correlated with the number of infiltrated IgG₄-secreting plasma cells in the kidney. However, the role of IgG₄ in complement activation remains unclear. In 2006, Muraki et al. (136) found that IgG1 levels were considerably increased, whereas C3 and C4 levels were reduced in AIP, which indicated that IgG1, instead of IgG₄, was involved in the activation of the classic pathway. In 2007, Kolfschoten et al. (18) speculated that IgG₄ might be a protective (anti-inflammatory) antibody because of its unique “Fab-arm exchange” property. In contrast, in 2016, Sugimoto et al. (137) observed that in IgG₄-RD patients with hypocomplementemia, IgG₄ might participate in the activation of the complement system.

Further exploration on the function of the complement system in IgG₄-RD is necessary.