Immunoglobulin G4-related disease (IgG4-RD) is the name applied to a corticosteroid
and/or B-cell depletion responsive illness, in which patients present with the consequences
of usually multiorgan, relapsing and remitting, fibroinflammation.1 The disease is
histologically characterised by obliterative phlebitis, storiform fibrosis and a dense
lymphoplasmacytic infiltrate.2 IgG4-RD is not a new disease, but is benefiting from
the application of new technologies in the pursuit of better biological understanding.
The histologic enrichment of IgG4-expressing plasma cells is a diagnostic hallmark
of disease that additionally serves as a biological phenomenon driving scientific
evaluation.3 Key disease themes have evolved to include a large clonal expansion of
activated plasmablasts and CD4+ cytotoxic, inflammatory and profibrotic lymphocytes.
Therapeutically, a reduced frequency of CD4+ cytotoxic lymphocytes are seen after
B-cell depletion; such therapy may consequently impact on antigen presentation.4–6
To date, the activity of IgG4-RD is not readily tracked by disease biomarkers with
even serum IgG4 concentration remaining an imperfect diagnostic and prognostic tool
for many patients.7
IgG4 molecules have structural and functional characteristics suggesting anti-inflammatory
and tolerance-inducing effects,8 9 and in IgG4-RD a reported oligoclonal reactivity
to multiple antigens.4 10 In Gut, Hubers et al describe a body of work that identifies
the first IgG4 autoantibody against an antigen which appears to be specific to IgG4-RD
(IgG4-associated cholangitis (IAC) and autoimmune pancreatitis (AIP)), at the exclusion
of its major differential diagnoses: primary sclerosing cholangitis and cholangiocarcinoma.11
The authors demonstrate that patients with IgG4-RD raise IgG1 and IgG4 in their sera
that recognise a 56 kDa cytosolic protein in an immortalised cell line lysate (H69
cholangiocytes) and in human liver lysate. Both IgG1 and IgG4 antibodies recognise
the same 56 kDa protein, and subsequent label-free quantitative liquid chromatography-tandem
mass spectrometry analysis and immunoprecipitation identifies the cytosolic protein,
Annexin A11, as the target. Annexin A11 IgG4 antibodies were also found in sera from
patients with IgG4-sialadenitis, in the absence of IAC or AIP, suggesting the target
antigen to not be site specific.
This work is in the context of next-generation sequencing studies—a technique that
has enabled identification of immunoglobulin clones within a restricted repertoire—yielding
strong evidence to support an antigen-driven process driving the pathology of IgG4-RD.
An examination of circulating plasmablasts in those with active IgG4-RD has found
the expanded pool of cells to have undergone class switching to IgG4, to be oligoclonally
restricted4 10 12 13 and to be subject to extensive somatic hypermutation.10 Flow
cytometric evaluation of circulating IgG4+ B cells confirms increased numbers of blood
IgG4+ memory B cells with reduced expression of CD27 and CXCR5 and increased signs
of antibody maturation.14 In affected tissue, CD4+ T cells constitute the most abundant
cell type, and an analysis of the nature of these cells in disease revealed prominent
clonal expansion of CD4+ T cells with a cytolytic phenotype.5 These findings in concert
strongly suggest an antigen-driven process that requires critical T cell and B cell
interaction.15
Nevertheless, the number and nature (foreign or self) of the antigens that drive the
disease remains a subject for ongoing study, collaboration and cross-validation. Dutch
and UK patient questionnaires revealed an association with chronic exposure to industrial
dusts, gases, oils, solvents and pesticides in ‘blue-collar’ professionals—though
further work to elucidate candidate antigens and causality is important.16 A series
of prior studies implicated molecular mimicry between antigens on Helicobacter pylori
and self (eg, α-carbonic anhydrase of H. pylori and human carbonic anhydrase II) to
drive disease.17–19 However, an association between exposure to H. pylori infection
and IgG4-RD has since been strongly disputed.20 Similarly, lactoferrin, pancreatic
secretory trypsin inhibitors21 22 and pancreatic trypsinogens22 have been reported
to be associated with AIP, though all of these have lacked specificity or sensitivity
to IgG4-RD, and the nature of the autoantibody has not been further examined.
Notwithstanding these shortcomings, there is compelling evidence of extant autoantigens
in disease. The passive transfer of purified human immunoglobulins (IgG1 and IgG4)
from people with active IgG4-RD into neonatal mice led to binding, and subsequent
damage to exocrine organs (salivary gland and pancreas).23 Using cloned immunoglobulins
from IgG4-RD patients’ dominantly expanded plasmablasts in single cell sorted plasmablasts,
investigators were able to demonstrate the secreted monoclonal antibodies to be self-reactive
versus a cytosolic cellular component.10 However, the identity of the cytosolic antigen
in this study was not determined.
Thus, the identification of specific antibodies against a cytosolic target by Hubers
is consistent with findings from others.10 However, why Annexin A11 would be targeted
still demands explanation. It is an intracellular protein, so it would follow that
the antigen would only be presented to the antibody in the event of cellular damage.
Furthermore, there is no obvious clue as to how binding to Annexin A11 would influence
pathology. Annexins are a family of calcium-dependent phospholipid-binding proteins—their
role in a fibroinflammatory disease is unclear, though, as the authors point out,
autoantibodies against Annexin A11 have also been demonstrated in systemic lupus erythematosus,
systemic sclerosis and primary antiphospholipid syndrome.24
Some caution must remain about the observations because there is a lack of validation
in an external cohort, and of the 50 patients with IgG4-RD, only 9 had sera that reacted
to Annexin A11—the authors have rightly not presented this as a diagnostic test. Validating
this selective finding and understanding whether and how it relates to disease pathogenesis
is key to appreciating the long-term impact of the work. The same group have published
elegant work demonstrating dominant IgG4+ B-cell receptor clones accurately distinguish
patients with IAC and AIP from primary sclerosing cholangitis and cholangiocarcinoma.13
IgG4+ B-cell receptor clones constituted a greater proportion of the total IgG+ repertoire
in patients with IgG4-RD—and there were multiple clones, suggesting that there may
be multiple antigens driving the observed response. Furthermore, the longitudinal
examination of plasmablast clones in patients who have relapsing disease after successful
initial treatment with B-cell depletion therapy (rituximab) has shown that the circulating
plasmablasts that re-emerge are clonally distinct and exhibit enhanced somatic mutation
compared with the initial circulating plasmablasts in the same patients.10 It is unclear
whether the same antigens are recognised during the initial disease process and at
the time of relapse. This raises the question as to how to measure and understand
the significance of specific antigens in the disease process.
The causal relationship between the observed immunoglobulin response and pathology
remains another hole in our knowledge. Hubers demonstrates that IgG4 from sera of
patients diminishes IgG1 binding to Annexin A11.11 The authors speculate that IgG4
may act to dampen the IgG1-mediated pathogenesis in response to Annexin A11 binding—supporting
an anti-inflammatory role for IgG4 in IgG4-RD. This follows published work in 2016,
where the investigators demonstrated the pathogenicity of circulating IgG in patients
with IgG4-RD by the passive transfer IgG1 and IgG4 into neonatal mice by subcutaneous
injection.23 Both IgG1 and IgG4 bound to murine pancreas and salivary glands and led
to subsequent damage, yet the effect was more pronounced in mice injected with patient
IgG1. However, the potent pathogenic effects of patient IgG1 were significantly blunted
by simultaneous injection of patient IgG4. It seems as though IgG4, though pathogenic,
can competitively bind to target organs in preference to IgG1 and dampen its exaggerated
effects.
The tolerogenic effects of IgG4 in IgG4-RD remain speculative, although they are well
established in other disease settings.25 Peculiarities of the structure of IgG4 subclass
lend itself to an anti-inflammatory role. Weaknesses between the heavy chains allow
it to dissociate as two half molecules and associate with another IgG4 half molecule—a
phenomenon known as ‘Fab arm exchange’.8 This results in a functionally monovalent
IgG with bispecificity—thereby restricting the formation of immune complexes. Moreover,
IgG4 has poor affinity to Fc-gamma receptors on effector cells, and to C1q—rendering
them unable at activating the classical complement pathway.9 The classic example of
IgG-mediated immune tolerance is seen in beekeepers, which are naturally exposed to
high levels of bee venom allergen. Tolerant individuals secrete high concentrations
of venom-specific IgG4 as opposed to other IgG subclasses and IgE.26 27 IgG4 is thought
to competitively bind to the allergen in preference to IgE, thereby inhibiting IgE-mediated
immune complex formation and mast-cell activation. Conversely, the immune-dampening
effects of IgG4 can interfere with beneficial humoral responses. Melanoma cells secrete
interleukin (IL)-4 and IL-10 to direct a modified T helper cell-2 response.28 Secreted
IgG4 can block the effects of melanoma-specific IgG1, which are potent activators
of macrophages and thus capable of initiating tumour cell death. Consequently, tumour-specific
serum IgG4 concentrations correlate to mortality.29 The relevance of IgG4 in the pathogenesis
of IgG4-RD remains confusing. Though serum levels do not faithfully correlate to disease
activity, the excess of circulating IgG4 in active disease intuitively argues against
a protective role. We cannot yet extrapolate whether IgG4 antibodies are primarily
pathogenic, protective or neither.
Seemingly, IgG4-RD is, in part, antigen driven, and Hubers’ article in this issue
of Gut lends a significant boost to the evidence base. How this reflects host risk
continues to evolve and of note are conference reports now exploring host genetic
risk, in robustly collected populations. A genome-wide association study of IgG4-RD,
for example, has been performed in a Japanese population (Terao et al. International
Symposium of IgG4-RD and Fibrosis, Feb 2017), and this reported three susceptibility
loci consistent with antigen-driven disease: HLA-DRB1, HLA-A and FCGR2B, the latter
encoding a low affinity receptor for IgG.
IgG4-RD is as much related to IgG4, as it is to clonally expanded B-cell populations,
and an array of T cell subsets, although it is not classically preneoplastic, with
plasmablast expansion being oligoclonal, not polyclonal. New technologies have increased
our understanding of the changes in B-cell populations in different stages of the
disease, but focus is now shifting additionally to delineating the role of T cells,
in particular T follicular helper (Tfh) cells. Tfh cells help B cells and augment
germinal centre development. They play a critical role in immunoglobulin somatic hypermutation
and class switching of antibodies.30 In IgG4-RD, they are increased in numbers both
in circulation and at sites of active disease, with increased expression of effector
cytokines and regulators.31 In particular, the Tfh2 subset is associated with disease
activity, the number of affected organs, B-cell differentiation and serum IgG4 levels,
and responds to glucocorticoid treatment to parallel clinical improvements.32 33 As
with the previously mentioned clonally expanded cytolytic CD4+ T cells, Tfh cell and
B cell interactions are critical to the disease process. Type 2 Tfh cells seemingly
activate B cells, which become memory B cells or plasmablasts. Activated B cells and
plasmablasts can present antigen to CD4+ cytotoxic T cells at sites of disease.34
35 Supporting this, of course, is the apparently very positive impact of rituximab
(anti-CD20) as a therapy.36
IgG4-RD, while very rare, remains an informative disease to study. Therapeutically,
it portrays an immune-mediated disease with treatment options beyond corticosteroids,
thanks to a greater understanding of the underlying pathophysiology (figure 1). Scientifically,
it describes an evolving immunobiologic process, the unravelling of which will aid
the understanding of all autoimmune disease.
Figure 1
Immunoglobulin G4-related disease (IgG4-RD) and immune pathways to therapy. Naïve
B cells are activated by exposure to antigens. In tertiary lymph nodes or in tertiary
lymphoid tissue within an affected tissue, T follicular helper (Tfh) cells help B
cells differentiate into antibody secreting cells. Interleukins (IL) 4, 10 and 21
are critical to B-cell affinity maturation, class switching and clonal expansion.
At the site of disease, B cells are thought to interact with cytolytic T cells by
the mutual expression of signalling lymphocytic activation molecule 7 (SLAM7). These
effector T cells secrete profibrogenic cytokines that may be critical to subsequent
storiform fibrosis, and cytolytic enzymes. The exact nature of the B cell to cytotoxic
T cell interaction is still unclear. Therapy targeting CD20 (rituximab) leads to a
reduction of plasmablasts as a consequence of killing their parent cells; plasmablasts
do not express CD20. XmAb587 1 is a monoclonal antibody therapy that targets CD19
and enhances FcγRIIb-mediated inhibition—a receptor that inhibits B-cell function.
A phase II trial examining the effect of XmAb5871 in IgG4-RD has completed enrolment.
Elotuzumab leads to SLAM7-induced antibody directed cellular cytotoxicity in multiple
myeloma. The utility of elotuzumab in IgG4-RD is currently only theoretical. Other
therapies that may interfere with the pathogenic process are beyond the scope of this article,
but could include therapy targeting the BAFF APRIL pathway (belimumab, atacicept);
BAFF is critical for B-cell survival. Ag, antigen; APRIL, a proliferation-inducing
ligand; BAFF, B-cell activating factor; BCR, B-cell receptor; CXCR5, chemokine receptor
type 5; MHC, major histocompatibility complex; TCR, T cell receptor.