Multifaceted role of TNF-α during the pathogenesis of rheumatoid arthritis

doi:10.4236/abb.2013.410123

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Advances in Bioscience and Biotechnology, 2013, 4, 937-940 ABB

http://dx.doi.org/10.4236/abb.2013.410123 Published Online October 2013 (http://www.scirp.org/journal/abb/)

Multifaceted role of TNF-α during the pathogenesis of

rheumatoid arthritis

Ramanjaneya V. R. Mula, Rangaiah Shashidharamurthy*

Department of Pharmaceutical Sciences, Philadelphia College of Osteopathic Medicine, School of Pharmacy, Suwanee, USA

Email: *rangaiahsh@pcom.edu

Received 9 July 2013; revised 9 August 2013; accepted 10 September 2013

tive Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the

original work is properly cited.

ABSTRACT

Tumor necrosis factor alpha (TNF-α) a cytokine has

been shown to be the key player during the patho-

genesis of several autoimmune inflammatory disor-

ders (presumably sterile inflammation) including rheu-

matoid arthritis (RA). Several studies have shown

that TNF-α is mainly involved in the proinflamma-

tory responses. However recent studies have reported

multifunctional role of TNF-α during the develop-

ment of RA. Therefore, in this article we have high-

lighted the distinct functions of TNF-α during patho-

genesis of RA.

Keywords: Cytokines; Tumor Necrosis Factor-α;

Rheumatoid Arthritis; Inflammation

1. TNF-α AS CHEMO-ATTRACTANT AT

SYNOVIAL JOINTS FOR CD4+ T

CELLS

The organization of infiltrated cells in the arthritic mi-

croenvironment resembles the secondary lymphoid organ

including the presence of CD4+ T cells [1]. Typically, in

secondary lymphoid organs the presence of chemokine

gradient is necessary for the localization of T cells, espe-

cially to interact maximally with antigen presenting cells

[2]. The cytokine TNF-α plays a crucial role during the

pathogenesis of RA [3]. The focal immunological func-

tion of TNF-α is thought to be the induction of several

proinflammatory cytokines and stimulate the effector im-

mune cells through interaction with TNF-α receptors. In

addition TNF-α is also known to act as chemoattractant

for various cells including T cells [4,5]. However, the

additional role of TNF-α in early phase of T cell migra-

tion to arthritic microenvironment, especially adhesion

and transmigration through endothelium is not clear.

Therefore, Rossol et al. [6] observed that the interactions

of TNF-α and TNFRI are necessary for T-cell migration

into synovium using in vitro and ex vivo experiments.

“The results of Rossol et al. [6] study show that the in-

teraction of TNF-α with receptor type1 (TNFR1) is in-

deed a migratory stimulus for CD4+ T cells in RA pa-

tients which is a TNF-α gradient dependent”.

In this article, the authors used horizontally oscillating

microtome for synovial sections and confocal as well as

fluorescent microscopic methods to follow the T lym-

phocyte migration. They also collected the migrated T

lymphocytes for phenotyping. The in vitro transwell ex-

periment was performed to ascertain that TNF-α is in-

deed required for T cell migration. In support of this,

they also carried out the ex vivo T cell migration assay

using synovial tissue section from RA patients by co-

incubating with autologous peripheral CD4+ T cells. The

migrated CD4+ T cells in the tissue sections were visu-

alized by Fluorescent microscopy. The results showed

that only the CD4+ T cells form RA patients migrated

into synovial tissue but not from the healthy controls. In

continuation with this observation, they found that the

expression of TNFR1 and ICAM-1 is necessary for acti-

vated CD4+ T cells to migrate into synovium. Subse-

quent flow cytometry results of cells isolated from syno-

vial tissue show that expression of TNFR1 is specific in

transmigrated activated CD4+ T cells but not in control

healthy volunteer CD4+ T cells. Taken together, these

results suggest that the TNF-α is necessary for CD4+ T

cell migration in arthritic joints. At the conclusive point

the goal of the investigation on the role of TNF-α in

synovium which was proven concretely as a potential

chemoattractnat may possibly pave way to develop ther-

apy of TNF-α inhibitors as well as new approaches that

can be more focused towards targeting T-cells that ex-

press TNFR1 in excess.

*Corresponding author.

OPEN ACCESS

R. V. R. Mula, R. Shashidharamurthy / Advances in Bioscience and Biotechnology 4 (2013) 937-940

938

2. DISTINCT EFFECT OF TNF-α

ON MACROPHAGES AND

FIBROBLAST-LIKE

SYNOVIOCYTES DURING RA

A step-by-step manifestation of synovial inflammation in

RA quietly transforms into chronic inflammation through

elevated levels of many inflammatory mediators [7] of

which TNF-α plays a key role during synovial inflamma-

tion. Both macrophages and fibroblast-like synovial cells

(FLS) are most prominent among multiple cell types for

prolonged synovial inflammation [8,9] in RA. In addition,

TNF-α secreted at the arthritic microenvironment is

known to influence these two cells differently. Activa-

tion of macrophages through TNF-α is transient, while

the FLS are responsible for prolonged synovial inflam-

mation [10]. The distinct role of TNF-α in activating the

macrophages and FLS is not clearly understood. How-

ever, it can be postulated that the main function of ma-

crophages is to promote the inflammation and later

dampen the inflammation by triggering the tissue repair

mechanism as protective role to avoid the local and sys-

temic toxicity caused by high levels of inflammatory

mediators [11]. Therefore, they may transiently respond

to the TNF-α to initiate the inflammation in the early

phase. Whereas, since FLS normal function is to create a

mechanical platform of synovium [12] and usually never

exposes to foreign antigens, therefore they may lack the

haemostatic mechanism to control their activation during

RA. However, the molecular mechanism of FLS activa-

tion leading to prolonged synovial inflammation is not

clearly understood. An attempt to investigate the possible

mechanism was carried out by Lee et al. [13] have

shown that TNF-α induced FLS activation leads to un-

controlled NF-κB signaling in FLS resulting in secretion

of cytokines, chemokines and metalloproteinases for

longer period of time leading to persistent synovial in-

flammation.

“TNF-α induces prolonged canonical NF-κB signaling

resulting in uncontrolled production of inflammatory

mediators contributing to the persistent synovial inflam-

mation during RA through FLS but not macrophages. In

other words FLS may be the contributing cell type for

chronic inflammation during RA”.

The authors studied the distinct functional role of

TNF-α using macrophages and FLS. The signaling me-

chanisms of NF-κB were elucidated by western blot

analyses and the protein involvement in transcriptional

regulation was analyzed by histone acetylation, chroma-

tin accessibility, and NF-κB, p65 and RNA polymerase II

(Pol II) occupancy at the interleukin-6 (IL-6) promoter

using chromatin immunoprecipitation and restriction

enzyme accessibility assays. Lee et al. have shown ma-

crophage and FLS stimulation by TNF-α is transient and

sustained respectively during the inflammatory RA con-

dition. The feedback inhibition of inflammatory re-

sponses mediated by ABIN-3, IRAK-M, SOCS-3, ATF-3

and STAT-3 are expressed at high level in macrophages

while at a low level in FLS. This may be a contributing

factor for persistence of synovial inflammation in RA

through FLS. Further, TNF-α facilitated histone modifi-

cation and increased accessibility for RNA polymerase II

at IL-6 promoter site resulting in secretion of proin-

flammatory cytokine IL-6 in FLS. Taken together this

investigation has shown that sustained inflammatory

response by FLS through TNF-α mediated inflammatory

gene transcriptions and protein expressions prolong the

synovial inflammation. This may pave a way to make

FLS as an additional target in countering the unresolved

synovitis.

3. DIFFERENTIAL FUNCTION OF TNF-α

DURING INFLAMMATION AND

OSTEOCLASTOGENESIS IN RA

TNF-α, a proinflammatory cytokine initiates the inflam-

matory response leading to edematic joint and subse-

quent bone destruction during the development of rheu-

matoid arthritis [3]. The bone destruction is mainly me-

diated by osteoclast, a cell of myeloid origin (OC). OC

differentiation, maturation and activation are primarily

mediated by RANKL and its receptor RANK. Recent

studies have shown that TNF-α can magnify the process

of osteoclastogenesis in the presence of RANKL [14-16]

as an additional role apart from its proinflammatory re-

sponse. It has been shown that monoclonal antibody

therapy (infliximab, etanercept and adalimumab) against

TNF-α, though inhibited/reduced the bone erosion after

the treatment, the clinical symptoms of inflammation

were not affected [17]. Nonetheless, the mechanism of

the biphasic effect of biological TNF-α inhibitors was

not yet fully understood. Therefore, Binder et al. [18]

investigated the fundamental mechanism of TNF-α dur-

ing RA by in vitro studies and human TNF-α (hTNF)

transgenic destructive arthritis mouse model. Binder et al.

have shown the differential effects of TNF-α on osteo-

clastogenesis and bone erosion activity using above men-

tioned mouse model of experimental arthritis.

“High level of TNF-α is required for osteoclastogene-

sis than to initiate the inflammation at the synovial joints.

Hence, low doses of TNF-α inhibitor used in present in-

vestigation lead to expression of low levels of TNF-α,

which impede osteoclastogenesis but do not inhibit syno-

vial inflammation. Therefore, high levels of TNF-α in-

hibitors are required to inhibit both inflammation and

bone resorption”.

The authors have chosen monocytes derived from

spleen cells and tested the osteoclastogenesis in vitro us-

R. V. R. Mula, R. Shashidharamurthy / Advances in Bioscience and Biotechnology 4 (2013) 937-940 939

ing different concentrations of TNF-α inhibitor adalimu-

mab and activator RANKL. An hTNF-transgenic de-

structive arthritis mouse model was employed for in vivo

studies of inhibition of osteoclast precursors by the ther-

apeutic. The dose dependent differential effect of TNF-α

on osteoclastogenesis though countered with low doses

of immunotherapeutic inhibitor adalimumab even in the

absence of RANKL, the inflammation of synovium was

not affected. These data suggested that TNF-α alone can

activate the osteoclasts. The mRNA levels of proinflam-

matory mediators such as IL-1, matrix metalloproteinase

(MMP) 3, and MMP13 are significantly upregulated in

untreated hTNF-transgenic mice compared to hTNF-

transgenic mice treated with high concentration (10

mg/kg body weight) of adalimumab. Whereas low dose

(0.1 mg/kg) did not have any affect. Osteoclast associ-

ated genes such as NF-ATc1, cathepsin K, c-Fms, and

M-CSF were also upregulated at higher dose but, inter-

estingly not at lower dose. These data suggest that anti-

resorptive activity of TNF-α was not affected at lower

concentration but not synovial inflammation. Taken to-

gether Binder et al. [18] exemplified the need of control-

ling both cartilage destruction as well as inflammation

mediated by TNF-α during the treatment of rheumatoid

arthritis.

4. ACKNOWLEDGEMENTS

This work was supported by American Heart Association award

(11SDG5710004) to R.S. The authors declare no competing financial

interests.

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