Exploring the Mucosal Immune Response in Axial Spondyloarthritis Through Immunoglobulin A-Coated Microbiota

In this review, we focus on the mucosal immune response through Immunoglobulin A (IgA)-coated microbes and their role in gut dysbiosis in axial spondyloarthritis (axSpA) and associated inflammatory bowel disease. IgA-coated microbes contribute significantly to the microbial dysbiosis observed in axSpA, potentially driving gut inflammation and translocating outside of the gut and initiating systemic immune activation, thus contributing to disease pathogenesis. These insights will provide new avenues for understanding and treating axSpA and other immune-mediated inflammatory disorders by targeting specific host immune-microbe interactions.

Key points

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Microbial dysbiosis in axSpA: Multiple studies have revealed gut microbial dysbiosis in axSpA patients, contributing to gut inflammation and disease progression.
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IgA production and mechanisms: IgA, a primary antibody secreted by the gut mucosa through T cell-dependent and independent mechanisms, plays a crucial role in modulating the gut microbiota and maintaining intestinal homeostasis.
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Altered IgA coating in axSpA: axSpA/SpA patients with and without inflammatory bowel disease exhibit distinct IgA-coated microbes compared to healthy individuals, suggesting an altered mucosal immune response and microbial targeting.
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IgA-coated microbes and inflammatory metabolome: IgA-coated microbes in axSpA patients are linked to increased inflammatory metabolic pathways and decreased beneficial metabolic pathways in comparison to IgA-uncoated microbes, highlighting their altered metabolic potential.
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Therapeutic implications: Understanding interactions between IgA-coated microbes and host immune system can provide novel therapeutic interventions aimed at restoring microbial homeostasis and reducing inflammation in multiple immune/inflammatory disorders.

Abbreviations

AIEC	adherent-invasive E. coli
axSpA	axial spondyloarthritis
CD	Crohn’s disease
E. coli	Escherichia coli
ER	endoplasmic reticulum stress
FMT	fecal microbiota transfer
IBD	inflammatory bowel disease
Ig	immunoglobulin
LPS	lipopolysaccharide
sIgA	secretory immunoglobulin A
TH17	T helper 17

Axial spondyloarthritis

Axial spondyloarthritis (axSpA) is an immune mediated inflammatory disease of the spine and the sacroiliac joints, which is associated with inflammation of the gut, eye, and skin. AxSpA typically affects people in their third and fourth decades and can lead to lifelong debilitating disease. Approximately 1% (∼3.4 million adults) of the US population is affected by axSpA (which includes radiographic axSpA—also known as ankylosing spondylitis, and non-radiographic axSpA). There is a strong association of HLA-B27 allele, dysregulated host immune response, as well as altered gut microbiota in axSpA patients in comparison to healthy individuals; however, the exact mechanisms underlying disease pathogenesis are unknown. Together, these findings underscore the complex interplay between host immune response and gut microbiota in the development and progression of axSpA.

Microbial dysbiosis in axial spondyloarthriti

Over the past decade, studies on the gut microbiome have highlighted a crucial role of disease-related alterations in the gut microbial community (microbial dysbiosis) in patients with axSpA compared with healthy individuals. ^, ^, However, whether these microbial alterations are the drivers of the disease or result of an inflamed gut microenvironment is not known. Previous studies on axSpA/SpA focused on identifying microbial dysbiosis, and to identify specific microbes associated with disease severity to determine disease causality. However, various studies found different microbes, such as Ruminococcus gnavus and Dialister as disease biomarkers in SpA/axSpA patients. ^, These distinct microbial biomarkers could be due difference in cohorts, locations of sample collection, and processing, thus highlighting the inherent variability in the gut microbiome in healthy individuals. In addition, different dietary, environmental, and genetic factors further alter the gut microbial community. ^,

This theory was further supported by our study characterizing parallel gut microbiome and host immune response on 3 different rat genetic backgrounds (Lewis, Fischer, and dark Agouti) with different disease penetrance and severity. HLA-B27 transgenic rats on disease susceptible Lewis and Fischer genetic backgrounds exhibited distinct microbial dysbiosis, which was largely non-overlapping. In contrast, the host immune/inflammatory response in HLA-B27 transgenic Lewis and Fischer rats was largely overlapping, thus suggesting a redundancy in the microbial function and an overlap between the microbial metabolic potential. These findings led us to propose an ecological model of microbial dysbiosis, in which the cumulative function of microbial community and their metabolic potential underly SpA pathogenesis.

Further, there has been an effort to determine the mechanisms through which microbes interacts with host immune response and elicit disease pathogenesis. The host immune system interacts with and regulates the composition and metabolic function of the gut microbes through the production of immunoglobulins (Igs). Analysis of the gut microbiota revealed at 10% to 30% of gut microbiota is coated with mucosal IgA. ^, IgA-coated microbes have been shown to be highly inflammatory and the percent IgA coating in the gut microbiota increases with inflammation. Therefore, identifying IgA-coated microbial taxa can uncover important host microbial interactions in healthy individuals and their perturbations in various inflammatory disorders including axSpA.

Mucosal production of immunoglobulin A

The mucosa is the largest lymphoid organ in the body and one of the ways it protects the host is by immobilizing pathogenic microbes in the gut by IgA, leading to their excretion. The gut produces around 40 to 60 mg/kg of IgA, which is the predominant Ig class contributing ∼75% of the total gut mucosal Igs, whereas IgG is present at ∼24%, along with trace amounts of IgM (∼2%). ^, In contrast, the serum IgA contributes to only 15% of the total Igs, with IgG being the most dominant Ig at ∼80%, and IgM at 10%. The secretory IgA (sIgA) in the gut is dimeric and can be produced through T cell-independent and T cell-dependent manner ^, in the lamina propria. Once produced, the IgA dimers binds Ig receptor and gets endocytosed at the basolateral surface of epithelial cells and the dimeric secretory form of IgA is released into the lumen. The T cell-independent IgA production is largely triggered by commensal bacteria, and the plasma cells local to the small intestine make low affinity sIgA, which is polyreactive to gut commensals and promotes mutualist host-microbe interactions. ^, The production and secretion of T cell-dependent IgA involves microbiota dependent activation of B cells in the Peyer’s patches and mesenteric lymph nodes via antigen-primed T follicular helper cells, which upon binding with B cells by CD40 ligand initiates IgM to IgA class switch through recombination and IgA somatic hypermutation. ^, IgA repertoire screening studies have revealed 20% of polyreactive antibodies, whereas most of the antibody clones did not show any reactivity, thus highlighting the complexity and antigenic load of gut microbiota.

Once dimeric sIgA is released in the lumen, it can bind the microbes naturally during homeostatic conditions or may be induced by vaccine response, infection, and inflammation. ^, IgA can bind to various microbes including Firmicutes, Bacteroides, Proteobacteria, and Actinobacteria and their specificity ranges from being cross-species reactive, species specific, and strain specific. IgA can also bind to various microbial antigens broadly such as lipopolysaccharide (LPS), peptidoglycan, flagella or specifically to the fructans, pectin, LPS o-antigen, and capsular polysaccharides underlying their binding patterns. Microbial binding of IgA can block the microbe from invading the lumen and interacting with the host and is targeted to be eventually flushed out of the host. Alternatively, these microbe/microbial antigen IgA complexes are taken up by dendritic cells, which further present these antigens to T cells to elicit T cell-dependent antibody responses to clear pathogens.

IgA antibodies have been shown to have either a beneficial or deleterious effect on IgA-coated commensals. ^, IgA is protective against enteric bacterial pathogens such as Escherichia coli ( E. coli ) and Salmonella as it prevents their association with the intestinal epithelium. While IgA binding with microbes targets them for removal, certain commensal microbes like Bacteroides fragilis form cluster with IgA to remain anchored in the mucus layer, thus avoiding competition from other microbes. In turn, short chain fatty acids produced by Bacteroides fragilis can be absorbed through the intestines and serves as an energy source for the host and promotes intestinal homeostasis (Weis, 2021). Taken together, the molecular function of IgA is complex, as it mediates gut homeostasis, as well as protection against mucosal pathogens.

Altered immunoglobulin A coating in axial spondyloarthritis

Recent advances in next generation sequencing and bacterial flow cytometry methods have helped identify IgA-coated microbes and revealed altered pattern of IgA coating in patients with various inflammatory diseases including axSpA. IgA sequencing is performed by isolating gut microbes from stool, staining them with anti-IgA antibodies to separate IgA-coated and IgA-uncoated microbes using flow cytometry based cell sorting, and then using amplicon sequencing to determine their taxonomy and relative abundance, and calculating their IgA index ( Fig. 1 ). IgA-coated microbes have colitogenic properties, since transfer of IgA-coated microbes from inflammatory bowel disease (IBD) patients to germ free mice resulted in colitis, whereas the mice transferred with IgA-uncoated fraction remained disease free. This study showed the pathogenic nature of IgA-coated microbes and allowed a granular understanding of the microbial drivers of disease.

Since there is a huge overlap between axSpA and IBD, we performed IgA sequencing of the fecal and salivary microbes from axSpA patients. In comparison to age, sex, and body mass index matched healthy individuals, we observed altered IgA index (relative abundance and percent IgA coating) in the salivary and fecal samples from axSpA patients in comparison with heathy individuals highlighted by increased IgA coating of Akkermansia, Ruminococcus, Escherichia/Shigella in feces, and Prevotellaceae in saliva. Increased IgA coating of Clostridiales Family XIII in the fecal samples correlated with disease severity. Using bioinformatics approach (PICRUSt2), we inferred the microbial metabolic potential of IgA-coated and uncoated microbes, which revealed disruptions in inflammation and metabolism-related pathways. Even though only 10% to 30% of the gut microbes are IgA-coated, we observed significant differences in the inferred fecal metabolome in axSpA patients in comparison to the healthy individuals, whereas there were only a few changes in the IgA uncoated fractions. AxSpA patients had increased inferred inflammatory pathways belonging to the bile acid pathways concomitant to the decrease in short chain fatty acid (SCFA) pathways further revealing the potential of IgA to target immunologically active microbes in the gut. The inferred metabolic perturbations were specific to the gut microbes, as we observed only a few differences in the salivary fraction, further solidifying the role of IgA targeting of gut microbes underlying axSpA pathogenesis. These findings suggest that IgA responses significantly impact microbial structure in axSpA, offering new avenues for understanding and treating the disease.

Patients with SpA frequently suffer from IBD, and similar dysbiotic microbes are associated with IBD and SpA. A study explored the link between intestinal inflammation and peripheral SpA in patients with IBD and identified an enrichment of IgA-coated E. coli in patients with Crohn’s disease-associated SpA (CD-SpA). These E. coli isolates resembled the adherent-invasive E. coli ( AIEC ) pathotype. In germ-free mice, CD-SpA E. coli -induced T helper 17 cell (TH17) mucosal immunity, dependent on the enzyme propanediol dehydratase (pduC). Colonization of interleukin-10-deficient or K/BxN mice with CD-SpA E. coli resulted in more severe colitis or inflammatory arthritis, mirroring increased TH17 immunity in CD-SpA patients. This study highlights the utility of IgA-seq in identifying immunoreactive microbes that link mucosal and systemic inflammation, providing potential targets for therapy in CD-SpA. Furthermore, axSpA and CD also overlap in their association to endoplasmic reticulum stress (ER); however, its impact on host-microbe interaction is unclear. CD has been linked with a deficiency in the AGR2 protein, which leads an expansion of IgA-coated mucosal-associated E. coli known as AIEC , resulting in ER stress and ileocolitis in mice and humans. During inflammatory diseases, such as axSpA and IBD, there is an increased diversity and percentage of IgA-coated bacteria especially Akkermansia , Escherichia/Shigella, Clostridiales, and many of these bacteria correlate with disease severity ^, ( Fig. 2 ). These findings highlight the intricate connections between gut microbiota, immune responses, and systemic inflammation in conditions like axSpA and IBD.