Analysis on Intestinal Microbiota in Rheumatoid Arthritis

Ruining Peng

School of Engineering and Applied Sciences, University of Pennsylvania, Philadelphia, PA 19104, U.S.A.

Keywords: Rheumatoid Arthritis, Intestinal Microbes, RA Pathogenesis, Immune System, Probiotics.

Abstract: Rheumatoid Arthritis (RA) is a prevalent symmetrical facet joint involved systemic autoimmune disease,

which is mainly characterized by swelling and deformity of the joints, causing mobility problems and

affecting living quality. Factors such as heredity, environment, and microbial infection often lead to the onset

of RA, which is also related to immune dysfunctions, but the specific pathogenesis is still unclear. Tens of

billions of microbes in the gastrointestinal tract, the biggest immune organ, participate in immunological

regulation of the human body, changes in the composition and function of which are closely related to

rheumatoid diseases such as RA. Hence, this review focuses on the relationship between intestinal microbes

and RA. Based on the analysis and summary of existing materials, it is found that the composition of microbes

in the gastrointestinal tract in RA patients is different from that in healthy group. Reductions in Bacteroides

and increases in Prevotella are strongly correlated with disease in new-onset untreated rheumatoid arthritis

(NORA) subjects. In addition, intestinal microbes and their metabolites affect the production and

differentiation of regulatory T cells (Treg cells). Besides, these substances also disrupt the balance between

Treg and helper T cells, induce the release of pro-inflammatory factors, interfere with the host immune system,

and get involved in the occurrence of autoimmune diseases. The study of the intestinal-articular axis provides

a new perspective of RA pathogenesis.

1 INTRODUCTION

RA is a chronic systemic immune disease with

synovitis and pannus as the main pathological

manifestations. The global incidence of RA is about

1%. Previous studies believe that RA susceptibility

genes are the most common cause of the disease.

However, recent epidemiological studies have

reported that the proband-wise concordance rate of

RA in monozygotic twins is lower than that in

dizygotic twins (Svendsen, Anders et al 2002),

indicating that the environment should be the

environment instead of genetic factor play a role in

such autoimmune diseases. Environmental changes,

such as humid climate, imbalance the immune

homeostasis through epigenetic modification, leading

to RA and other rheumatoid disorders (Calabresi,

Emanuele et al. 2018).

With in-depth research on microorganisms and

new understandings of their functions in the human

body, the relationship between microbes and RA has

gradually been revealed. The early hypothesis was

that Europeans were exposed to a certain version of

the pathogen in America that led to the RA on-set;

recent studies have found that RA's pathogenesis may

involve the colonization of specific microorganisms

in the human body. It has been reported that RA

might be caused by intestinal flora participating in

cell, especially immunological cells, interactions,

changing immune homeostasis, triggering

inflammatory reactions.

Changes in the composition of intestinal microbes

in RA patients, pathogenesis involved

microorganisms and their role in immune regulation,

and the application prospects of probiotics as new

treatments are current research hotspots. This review

collects, analyzes the existing research results,

discusses the genetic and environmental

susceptibility factors, changes in the microbial

composition in RA patients, the participation of

intestinal flora in human immunity, and the possible

role of RA pathogenesis. As the specific pathogenic

cause of RA is still unclear, the relationship between

gut microbes and RA will provide new research

perspectives and new ideas for adjuvant therapeutic

methods.

238

Peng, R.

Analysis on Intestinal Microbiota in Rheumatoid Arthritis.

DOI: 10.5220/0011197100003443

In Proceedings of the 4th International Conference on Biomedical Engineering and Bioinformatics (ICBEB 2022), pages 238-243

ISBN: 978-989-758-595-1

2 RA PATHOGENESIS

RA is a systemic immunological disease, which

pathogenesis includes innate immune dysfunction

and acquired immune responses, involving antigen-

presenting cells, self-reactive T cells, and antibodies.

Thus immunological dysfunction should be the

leading cause of joint damages and RA.

Cytokines and inflammatory mediators, for

example, tumor necrosis factor α (TNF-α),

interleukin-17 (IL-17), and granulocyte-macrophage

colony-stimulating factor (GM-CSF) could induce a

strong inflammatory response, play a key role in RA

pathogenesis, which is one of the critical floating

activity indicators for RA diagnosis and prognosis.

Besides, cytokines and their signaling pathways work

as effective therapeutic targets applied in RA clinical

treatment.

The high level of serum TNF-α, one of the

inflammatory factors involved in the pathogenesis of

RA, indicates the active stage. TNF is mainly

produced by activated mononuclear macrophages in

the synovial membrane, and its various effector

functions, such as inflammatory effects, are related to

RA pathogenesis. TNF triggers the activation of

several immune cells (leukocytes, endothelial cells,

etc.) as well as the production of a series of cytokines

and chemokines (IL-1, IL-6, IL-8, GM-CSF, etc.)

(McInnes, Iain B, and Georg Schett. 2007) TNF also

drives the differentiation of osteoclasts. Meanwhile,

it inhibits the formation and function of osteoblasts,

which disrupts the balance between bone formation

and destruction, causing damages to tissues in the

patient's joints. Study shows that THF antagonist can

clinically reduce the production of several cytokines,

slow down inflammatory responses, and relieve joint

damages.

The production of self-reactive antibodies is

another decisive factor in the progression of RA

disease. B cells, induced by the antigen-antibody

recognition process, stimulate T cell activation,

initiate immune responses, and trigger the production

of autoantibodies (for example, rheumatoid factors),

which will then precipitate with IgG leading to the

onset and progression of RA. Regulatory B cells

(Breg cells) direct Th cells to develop into the

memory T cell and reduce inflammatory T cells'

proliferation, inhibiting RA progression. Since it is

well-known that the imbalance between pro- and anti-

inflammatory T cells is closely correlated with the

RA disease. Th 17 cell, a new subtype of CD4+ T cell,

could induce autoantibody through the secretion of

IL-17, which is one of the pro-inflammatory factors

that amplify inflammatory responses. It has been

reported that the large amount of IL-17 positively

correlated with RA severity. Sun J, et al. found that

metallothionein-1 (MT-1) can correct the relationship

between pro- and anti-inflammatory T cells, alleviate

the pathological symptoms (for example, synovitis)

of RA by inhibiting Th17 cell while inducing the

proliferation of Treg cells (Sun, Li, Li, Ding, Liu,

Chen, Zhang, Qi, Du, Huang 2018).

3 RA SUSCEPTIBILITY

FACTORS

RA is closely related to the genetic background while

also involves several other susceptibility factors.

Among them, the body's immunity and microbial

infection are at the central position; endocrine and

environmental factors also increase RA's

susceptibility (Figure 1) (Deane, Kevin D et al. 2017).

Certain microorganisms, such as Epstein-Barr virus

and parvovirus B19, infect the human body and then

mediate autoimmune responses via polypeptide

fragments; or work as an initiating factor, first cause

local inflammatory responses (for example,

pharyngitis and sinusitis), causing immunity to start

the regulation process to fight against inflammation

but might trigger systemic immune disease including

RA as well (Mathew, Ashish Jacob, and Vinod

Ravindran. 2015). In addition, microbial infection

may change the micro-ecological environment in the

oral cavity and gastrointestinal tracts, which disrupts

the normal composition of microorganisms,

interferes with immune regulation, leading to the

enhanced susceptibility of RA. Except for microbial

infections, the incidence of RA in women is

significantly higher than that in men at the same age,

indicating endocrine and gender might participate in

RA progression. Finally, cold and humid climates,

smoking, obesity, etc. can also aggravate disease

conditions.

Analysis on Intestinal Microbiota in Rheumatoid Arthritis

239

Figure 1: A general model of RA susceptibility factors.

4 RA AND GASTROINTESTINAL

MICROBES

Joint deformities and dysfunction are common

clinical manifestations of RA. While patients in the

earlier stage only have elevated serum autoantibodies

level with no obvious clinical synovitis. And research

suggests that synovial self-reactive immunity in oral

mucosa, lungs, intestines, etc., may cause systemic

autoimmune diseases, including RA (Mankia,

Kulveer, and Paul Emery. 2015). Wu et al. found that

mice under sterile conditions had alleviated RA

symptoms with the reduced number of Th17 cells and

serum self-reactive antibody titers. When using

intestinal segmented filamentous bacteria (SFB)

infected these mice, Th17 cells in lamina propria and

self-reactive antibody recovered. It indicated the

critical functions of intestinal microbes in regulating

pro- and anti-inflammatory T cells and the

progression of autoimmune diseases. Still, it should

not be ignored that the difference between human and

mouse intestinal microbiota. Analysis with RA

patient’s saliva and stool samples using sub-genome

shotgun sequencing and whole-genome association

studies (MGWAS) demonstrates the imbalance of

microbiota in both oral cavity and gastrointestinal

tract. Specifically, Haemophilus spp. decrease and

Lactobacillus salivarius increase significantly

compared to the control, indicating the probable role

of such microorganisms in autoantibody production

and disease activity.

When the environment changes, the mutual

relationship in microbial flora will be disrupted;

conditioned pathogens gain an ability to cause

disease. Prevotella is a commensal bacterium locates

ubiquitously in mucous membranes in healthy

subjects and rarely causes inflammation.

Epidemiological studies have found Prevotella can

lead to periodontitis which associates with an

increased risk of systemic immune diseases. Research

by Maeda Y, et al. showed that patients in RA early-

stage carry Prevotella dominated intestinal flora

(Maeda, Kurakawa, Umemoto, Motooka, et al. 2016),

which correlated with the decreased population of

Bacteroides and other beneficial bacteria. Thus,

intestinal microbe plays an essential role in RA

pathogenesis. In addition, infection of Prevotella goes

along with the increased number of Th17 cells in

mice, causing severe arthritis symptoms (Maeda,

Kurakawa, Umemoto, Motooka, et al. 2016). And

Prevotella can induce secretion of IL-6 and IL-23,

stimulate the proliferation of bone marrow-derived

dendritic cells, and promote five times higher IL-17.

It can be seen that changes in intestinal flora

correspond to RA disease activity.

5 THE ROLE OF

GASTROINTESTINAL

MICROBES IN RA

PATHOGENESIS

The onset and progression of RA are related to

dysfunctional immunity, while the specific

mechanism is still unclear. It is believed that the

imbalance of Treg cells and Th cells, which can cause

immune system disorders, might get involved in RA

development and disease activity. The typical healthy

human digestive tract is planted with a larger

population of Treg cells as well as 1014

microorganisms, which is ten times the number of

human cells. These large, various microbes have

significantly enriched the diversity of the host

genome, encoding 3.5 million genes, which is about

150 times more complex than the host self-genes

(Figure 2) (Cresci, Gail A, and Emmy Bawden.

2015). It has been reported that intestinal microbes

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and these genes can encode proteases that host cannot

possess, which play an important role in regulating

host metabolic functions (Kho, Zhi, and Sunil K Lal.

2018). Intestinal microbes and their metabolites

affect the production and proliferation of Treg cells,

that is, by interfering with immune cells

differentiation to disrupt the normal capacity of

immune system and trigger the development of

systemic immune disease. It is also agreed that

intestinal microbes participate in cell

communications, induce inflammatory responses via

mucosal barrier devastation caused by mutual

recognition of immunological molecules. In

summary, intestine-colonized microbes can affect the

production of Treg cells, induce releases of pro-

inflammatory mediators using molecular simulation

mechanisms, interfere with the body’s immune

regulation functions, then initiate systemic

autoimmune diseases, including RA.

Figure 2: Typical microorganisms colonized human gastrointestinal tracts.

The host immune system is functioned by various

immune mediators, cells, and complicated biological

processes to defend against pathogen invasions and

diseases. When biological, pharmaceutical,

environmental factors simulate the host,

inflammatory responses will immediately be initiated

to fight against infected tissues, and repair processes

will then start. Breg cells differentiate in response to

early inflammation and restrain excessive

inflammatory responses through the secretion of IL-

10. Studies showed that in the inflammatory stage of

RA, intestinal microbes can regulate the

differentiation of Breg cells colonized in immune

tissues and organs, such as spleen and mesenteric

lymph nodes, by releasing IL-1β and IL-6.

Antibiotic-treated mice (ABX mice) and specific

pathogen-free mice (SPF mice) developed a lower

number of IL-1β and IL-6, compared to the

conventionally housed mice (CNV mice), in response

to the induction of arthritis. Besides, compared to the

control, ABX mice showed severer arthritis

symptoms with a lower-functional small number of

Breg cells due to changes in the composition of

intestine-colonized microbiota. It indicated the

participation of microorganisms in regulating

immune responses, inducing Breg cells

differentiation and retraining excessive inflammation

could alleviate RA progression (Rosser, Elizabeth C

et al. 2014). Another study showed that germ-free

mice (GF mice) developed milder arthritis (WuHJ,

IvanovII, DarceJ, etal. 2010). In addition, GF mice

had lower number of splenic autoantibody-secreting

cells, Th17 cells and Treg cells, as well as lower

serum self-reactive antibody titers. After the infection

of intestinal-specifically colonized single commensal

SFB, Th17 cell populations in lamina propria and

serum autoantibody titers have recovered.

Colonization of SFB can induce the proliferation of

CD4+ T helper cells (Th17 cells), which produce IL-

17 and IL-22 in lamina propria. Additionally,

together with the up-regulation of acute subtype

serum amyloid, activation of dendritic cells and

production of other immune mediators lead to mouse

joints destruction (Ivanov II, Atarashi K, Manel N, et

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241

al. 2009). Animals fed with Lactobacillus casei

developed milder level of pro-inflammatory factors

(IL-1β, IL-2, IL-6, IL-12, etc.), TNF-α, and IFN-γ;

together with increased population of IL-10 and TGF-

β ameliorate arthritis (Vaghef-Mehrabany, Elnaz et

al. 2013). It suggested the beneficial effects of

intestinal-specifically colonization of certain

microbes on the amelioration of RA symptoms via

regulating immune functions. In conclusion,

intestine-planted microbes have significant impacts

on the maintenance of normal immune function;

certain or conditioned pathogens can disrupt host

immune structure and affect immune tolerance,

leading to the occurrence and development of

diseases.

Intestinal microorganisms can affect host

immunity via their metabolites. Short-chain fatty

acids (SCFAs), mainly produced by the

decomposition of carbohydrates, are among the most

important microbial metabolites linked with immune

regulation. As a key energic source for intestinal

epithelial cells and intestinal microbes, SCFAs are

also involved in cell differentiation, anti-

inflammatory responses, and many other critical

metabolic processes and are therefore of great

significance in regulating host immunity. Studies

showed that SCFAs recruit granulocytes, thus

aggravate local inflammation. The host can detect

and respond to the appearance of SCFAs using the

surface located G-protein-coupled receptor 41 and

43, which then promotes Treg cells differentiation

and clustering, enhances the IL-10 production, and

thereby inhibits inflammatory responses (Lopez,

Christopher A et al. 2014). SCFAs can also stimulate

mucin 2 expression in intestinal epithelial cells. It

may play an important role in mucoprotective

function (Willemsen, L E M et al. 2003).

Above, microbes may affect the host immune

metabolism process mainly in two ways:

microorganisms and microbial metabolites. Both can

enhance the susceptibility of RA through, for

example, inducing immune cell differentiation, pro-

inflammatory mediator releases. Microbes and their

metabolic products may become new therapeutic

targets in RA clinical treatment.

6 MICROBIOME IN RA

TREATMENT

Probiotics, living microorganisms that are believed to

be beneficial, change the microbial composition of

the human body, influence disease progression.

According to clinical researches, eight weeks of oral

administration of Lactobacillus casei leads to

significantly reduced disease activity index of RA

patients. Also, patients developed lower serum

inflammatory factors (TNF-α, IL-6, IL-12) with

increased serum regulatory cytokine IL-10 (Vaghef-

Mehrabany, Elnaz et al. 2013). Many other studies,

however, demonstrated that probiotics could not

significantly change inflammatory parameters (such

as erythrocyte sedimentation rate, TNF-α, IL-6, IL-

10) and oxidative stress indicators (total antioxidant

capacity and malondialdehyde); while it did improve

the RA disease activity index (Aqaeinezhad Rudbane,

Seyed Mohammad et al. 2018). The current research

evidence is not sufficient to prove the effectiveness of

probiotic interventions in RA treatment, which still

needs extended animal and clinical trials for further

evaluation. Besides, screening for the best bacteria

species and optimizing the dose intake is also

necessary.

Probiotics can change intestinal flora. While, due

to the larger quantities of microorganisms that

colonize gastrointestinal tracts, oral probiotics can

sometimes be difficult to achieve the desired effects.

Therefore, fecal flora transplantation has become an

alternative way to adjust patient’s intestinal microbial

composition. It helps reorganize the micro-

environment of intestinal tract via transplanting

healthy people’s intestinal flora into the patients in

need, thereby improving disease conditions caused by

disorders of intestinal microbes.

In recent years, traditional DMARDs,

methotrexate represented, treatments have played a

certain role in controlling RA development.

However, such therapeutic methods cannot cure the

disease, the long-term use of which will also increase

the risk of infections and cancers. By contrast, oral

probiotics and fecal transplantation are much safer,

with more advantages in inhibiting inflammation and

improving arthritis symptoms.

7 CONCLUSIONS

Human intestinal microbiota is closely related to the

host immune system. It participates in the occurrence

and development of autoimmune diseases such as RA

by interfering with immune cell differentiation,

inflammatory mediator, and self-reactive antibody

production, while the specific mechanism of which

remains to be explored. And due to the relationship

between intestinal flora disorders and incidence of

RA, it’s plausible to predict the disease occurrence

and evaluate disease stages via detecting the human

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intestinal microenvironment, which is helpful to

disease control. In addition, advanced techniques,

such as high-throughput sequencing, make precision

medicine achieved. Patients can obtain targeted

treatments, targeted use of certain probiotics, specific

changes in daily diet, etc. Future studies could

combine multiple analyses (microbiota, genomics,

and proteomics) to explore mechanisms involved

intestinal microorganisms with host immune system.

It would deepen the current understanding of the

intestinal microenvironment, leading to discovering

new biomarkers for RA diagnosis and prognosis,

providing an innovative reference for staged

treatment of RA.

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