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Tatyasaheb Kore College of Pharmacy, Warananager, Kolhapur-416113, Maharashtra, India
A chronic autoimmune disease that can affect numerous organs, rheumatoid arthritis (RA) most commonly affects the synovial joints. The cartilage and bone in these joints are gradually eroded by persistent inflammation, which finally results in obvious abnormalities and loss of joint function. Genetic vulnerability, environmental exposures, and anomalies in immune regulation interact in a complex way to cause the condition. These elements sustain a milieu of chronic inflammation and interfere with proper cytokine signalling. The Janus kinase/signal transducer and activator of transcription (JAK/STAT) system, which regulates a variety of cytokine-driven immune responses, is one of the key pathways involved. Excessive activation of this route promotes the development of autoantibodies, upsets the balance of immune cells, and quickens the deterioration of joints. Targeted synthetic disease-modifying antirheumatic medications, including upadacitinib, baricitinib, and tofacitinib, have become viable substitutes for biologic DMARDs in recent years due to their potent therapeutic efficacy. Notwithstanding their advantages, ongoing studies are assessing their long-term safety profiles, paying special emphasis to hazards like infections, cancers, blood-related problems, and thromboembolic events. Organisations like the European Medicines Agency (EMA), the European League Against Rheumatism (EULAR), and the U.S. Food and Drug Administration (FDA) frequently update treatment recommendations and clinical guidelines to encourage the safe and effective use of these treatments. In this review, the molecular framework of rheumatoid arthritis is analyzed in detail, focusing on the JAK/STAT signaling mechanism, cytokine receptor communication and the evolving role of JAK inhibitors in precision-targeted therapy.
Rheumatoid arthritis is a chronic autoimmune disorder that affects millions globally, leading to significant joint dysfunction and a substantial decline in overall quality of life [1]. The incidence of rheumatoid arthritis (RA) rose by 13.2% to 11.8 per 100,000 individuals worldwide between 1990 and 2021. Ireland and West Berkshire, UK, reported the highest rates, while Oman experienced the fastest growth and Africa the lowest. In 2021, RA affected over 17.9 million individuals, a 14.4% rise since 1990. Although RA was most common in those over 55, cases among younger adults (20–54) have grown since 2015, particularly in high-SDI regions, with some areas showing bimodal age patterns [2]. Rheumatoid arthritis has an uncertain natural history, with a variable clinical course and unpredictable prognosis. About 1-3% of people have it, and although women are almost three times more likely than males to have it, this female predominance declines with age. Moreover, evidence suggests that the disease has a genetic component [3]. It involves gradual and irreversible damage to joints lined with synovium, leading to loss of joint space, bone deterioration, impaired function and deformities. Breakdown of the extracellular matrix is a key process in rheumatoid arthritis, contributing to the gradual damage of cartilage, ligaments, tendons, and bone. RA typically presents as a symmetrical arthritis. Affected joints and surrounding tissues often exhibit swelling and tenderness, accompanied by morning stiffness and significant limitation in movement [4].
Figure 1: Key Joints Prone to Pain and Swelling in Arthritis
Environmental causes and a genetic predisposition combine to cause rheumatoid arthritis (RA). When CD4+ T cells identify antigens in the synovial tissue, macrophages, monocytes, and fibroblasts are activated, which initiates the disease. These immune cells release extracellular matrix-degrading enzymes and pro-inflammatory cytokines, such as TNF-α, IL-1, and IL-6, which contribute to the deterioration of bone and cartilage. Other T cell-produced cytokines, including interferon-γ, IL-2, IL-10, and IL-12, show an immunological response that is mostly Th1-driven. By activating resident joint cells, TNF-α and IL-1 intensify the inflammatory process and maintain an ongoing cycle of inflammation. Understanding these processes has influenced the creation of treatments that target immune cells and cytokines to prevent joint deterioration [5]. The primary aims of rheumatoid arthritis (RA) treatment are to reduce pain, prevent lasting joint damage, and maintain patients’ daily functioning and overall quality of life. Disease-modifying antirheumatic medications (DMARDs) must be administered as soon as possible; methotrexate is frequently advised as the initial treatment. Depending on the severity and response of the illness, other traditional DMARDs such hydroxychloroquine, leflunomide, sulfasalazine, azathioprine, and cyclosporine may be given singly or in combination. Biologic agents including TNF blockers and therapies that inhibit T-cell co-stimulation provide valuable options for individuals who do not achieve adequate improvement with standard treatments. However, their use is associated with an increased susceptibility to infections, particularly the reactivation of tuberculosis. For patients whose rheumatoid arthritis remains difficult to control, newer targeted synthetic DMARDs have been introduced, such as the Janus kinase (JAK) inhibitors tofacitinib, baricitinib, upadacitinib, filgotinib and ruxolitinib [6]. Protein phosphorylation, mediated by signaling kinases, is a key mechanism in intracellular communication [7]. The human genome encodes 518 kinases grouped into eight families, with 90 classified as tyrosine kinases, including Janus kinases (JAKs). Janus kinases (JAKs) play a major role in the pathophysiology of rheumatoid arthritis (RA) and are crucial parts of the signalling pathways utilised by growth factors, hormones, and cytokines. In the treatment of RA, tofacitinib, which mainly targets JAK1 and JAK3 with some activity on JAK2 and Tyk2, has shown remarkable efficacy and good tolerability. Other JAK inhibitors, such as baricitinib, filgotinib, and peficitinib, have been developed as a result of its clinical success [8] [9]. Focusing on rheumatoid arthritis, this review examines how JAK inhibitors modulate disease pathways, their clinical effectiveness, safety considerations and their emerging role in tailoring individualized therapies.
Pathophysiology
Genetics of Rheumatoid Arthritis
Recent genome-wide association studies (GWAS) have greatly expanded our knowledge of rheumatoid arthritis (RA) by uncovering numerous single nucleotide polymorphisms (SNPs) that contribute to an individual’s genetic risk for developing the condition. Most of these variations influence immune system regulation and are often shared with other inflammatory conditions. The HLA gene complex shows the strongest connection to RA. Specific HLA alleles, especially within the HLA-DR region, are thought to affect how antigens are presented to immune cells. This interaction not only influences the likelihood of developing RA but also helps predict disease progression, complications, and outcomes. Several non-HLA genes also contribute to disease development. Variants in genes such as CD28, CD40, and PTPN22 are involved in immune signaling, cell activation, and inflammatory pathways, which can disrupt immune balance and promote chronic inflammation. Beyond genetic factors, autoantibodies significantly influence rheumatoid arthritis (RA) development and diagnosis. Rheumatoid factor targets IgG and is common in RA, whereas highly specific ACPAs often appear before symptoms and play an important role in disease progression [10]. Epigenetic mechanisms, which reflect the interplay of genetic and environmental factors, significantly influence the onset of this disease. Recent large-scale research has identified ten genetic locations associated with heightened disease risk. Normally, DNA methylation and histone acetylation regulate leukocyte and fibroblast functions, but disturbances in these processes are linked to rheumatoid arthritis [11].
Figure 2: Integrated molecular framework of rheumatoid arthritis pathogenesis.
Immune System Dysregulation in Rheumatoid Arthritis
A severe inflammatory condition associated with substantial joint destruction and increased mortality is seropositive rheumatoid arthritis (RA). Complement activation and inflammation are brought on by immunological complex formation, which is linked to its severity. A major diagnostic breakthrough was the discovery of anti-citrullinated protein antibodies (ACPAs), which recognize citrullinated self-antigens and may appear nearly a decade before clinical symptoms, indicating an early preclinical phase of rheumatoid arthritis. Increasing ACPA levels and pro-inflammatory cytokines drive disease progression. ACPAs also activate osteoclasts and macrophages, contributing to bone erosion and inflammation. Effective treatment reduces rheumatoid factor (RF) and ACPAs. Additional autoantibodies, including anti-acetylated peptide and anti-carbamylated antibodies, further reflect RA’s complex autoimmune nature [12] [13]. Joint inflammation arises from immune system activation, leading to synovial membrane inflammation and infiltration of leukocytes into the synovial cavity. Joint damage in rheumatoid arthritis develops through the combined actions of innate and adaptive immune mechanisms. Analysis of synovial tissue at the molecular and histological levels has identified lymphoid, fibroid, and myeloid-predominant inflammatory patterns. Recognizing these subtypes is crucial for guiding targeted treatments and achieving better clinical outcomes [14]. Cytokines and chemokines such as GM-CSF, TNF, and IL-6 are major regulators of inflammatory activity, while IL-1 has a comparatively limited influence within this intricate immune signaling system. These mediators trigger endothelial activation, cellular infiltration and fibroblast proliferation. Activated fibroblasts promote osteoclastogenesis through RANKL signaling, leading to bone resorption. Simultaneously, cytokine induced activation of chondrocytes enhances matrix degradation via metalloproteinases, ultimately resulting in cartilage and joint tissue destruction [15].
Figure 3: Multisystem Impact of Rheumatoid Arthritis: An Illustrated Overview
JAK/STAT Pathway
Essential signaling cascade regulating immune responses, cellular development, and tissue homeostasis. Consists of the following four Janus kinases (JAKs): TYK2, JAK1, JAK2 and JAK3.
Contains seven STAT proteins: STAT1–STAT6 (including STAT5a and STAT5b).
Cytokine binding activates JAKs.
Receptors are phosphorylated by active JAKs.
STAT proteins undergo dimerisation, phosphorylation and recruitment. To control gene expression, STAT dimers go to the nucleus.
JAK1: Promotes immune cell activation and hematopoiesis.
JAK2: Responds to growth factors.
JAK3: Involved in endothelial signaling.
TYK2: Mediates interferon and interleukin pathways.
Control Th1, Th2, Th17, and regulatory T cell development. Regulate the activity of natural killer and dendritic cells.
Abnormal activation drives inflammation, joint damage, and tissue fibrosis. Dysregulated STAT activity and reduced negative regulators highlight therapeutic potential [16].
Dysregulated JAK/STAT signaling can result from JAK mutations or continuous TYK2 activity.
Linked to
JAK inhibitors (Jakinibs) modulate
These processes are important in autoimmune disease onset and progression [18]. Type I and II cytokine transmission are disrupted by pharmacological JAK inhibition [19].
Effective in treating autoimmune conditions
Contain JAK homology (JH) regions: JH1 to JH7 (C-terminal to N-terminal) [21] [22].
JH2 domain: previously considered catalytic, now a pseudo-kinase (limited catalytic function) [23].
JH4 to JH7 domains
Normally inactive in cytoplasm.
These proteins connect to cytokine–receptor complexes via SH2 domains (for example, IL-6 interacting with IL-6Rα/gp130). After being phosphorylated, STAT proteins pair up and migrate into the nucleus, where they attach to defined DNA sequences and direct the activation or suppression of target genes [25] [26].
Table 1: Cytokine Recognition by Receptors and Their JAK/STAT Signaling Profiles [27] [28]
|
Cytokine(s) |
Receptor Family |
JAKs |
STATs |
|
IL-4 |
γc family |
Jak3, Jak1 |
STAT6 |
|
EGF, PDGF |
Receptor tyrosine kinases |
Jak2 |
STAT5, STAT1, STAT3 |
|
GM-CSF |
ßc family |
Jak2 |
STAT5 |
|
IL-12 |
gp130 |
Jak2, Tyk2 |
STAT4 |
|
IFN-α/β |
Interferon family |
Jak1, Tyk2 |
STAT1, STAT3, STAT4, STAT5, STAT2 |
|
IFN-γ |
Interferon family |
Jak1, Jak2 |
STAT1 |
|
IL-10 |
Interferon family |
Jak1, Tyk2 |
STAT3 |
|
IL-11, IL-6, OSM, LIF |
gp130 |
Jak1, Tyk2, Jak2 |
STAT3, STAT1 |
|
IL-7, IL-2, IL-15, IL-9, |
γc family |
Jak1, Jak3 |
STAT3, STAT5 |
Figure 4: JAK–STAT Dynamics in RA
Ongoing inflammation within the synovial tissue is a defining feature of rheumatoid arthritis (RA), a long-lasting autoimmune disorder in which continuous immune-driven damage steadily wears down cartilage and bone, ultimately leading to joint deformities and impaired mobility [29-31]. Excessive release of pro-inflammatory cytokines from activated synovial cells such as tumour necrosis factor (TNF), interleukin-1 (IL-1), and IL-6 serves as a central driver of rheumatoid arthritis, fueling its underlying disease processes [32-34]. These cytokines exert much of their inflammatory effect by stimulating the Janus kinase signal transducer and activator of transcription (JAK–STAT) pathway, a crucial intracellular signaling route that orchestrates immune regulation [35-38] . Within the arthritic joint, synovial fibroblasts play a central role in driving inflammation and structural damage. Activation of STAT4 prompts these cells to secrete large amounts of IL-6, intensifying persistent inflammation and speeding up joint degradation [39]. Mori and colleagues reported that elevated cytokine activity in RA maintains a self-reinforcing IL-6–STAT3 signaling cycle, which perpetuates tissue injury and chronic inflammatory responses [40]. In addition, fibroblast-like synoviocytes (FLSs) from patients with active RA show increased STAT1 expression, reinforcing ongoing immune activation within the joint [41] [42]. Enhanced STAT3 activity in synovial CD4? T cells disrupts the Th17/Treg equilibrium and is linked to more aggressive synovial inflammation. Additionally, increased expression of JAK3, STAT4, and STAT6 has been observed in CD1a? dendritic cells, indicating potential molecular signatures of RA within the synovial environment [43-47]. Studies in proteoglycan-induced arthritis and other experimental models demonstrate that IL-4 inhibits IL-12–STAT4 signaling, underscoring its anti-inflammatory properties. In rheumatoid arthritis, transcriptomic and pathway analyses show marked elevation of type I interferon–responsive genes in peripheral blood, connecting heightened interferon activity to ongoing inflammation and worsening disease [48] [49]. Moreover, this regulatory network is strongly influenced by noncoding RNAs. For instance, miR-17 suppresses STAT3 and JAK1, thereby lowering the release of IL-6 and IL-1β, while LINC-PINT enhances SOCS1 expression, diminishing TNF-α–driven fibroblast activation [50]. The introduction of JAK inhibitors as targeted disease-modifying antirheumatic drugs (DMARDs) has significantly reshaped the management of RA. These medications effectively diminish synovial inflammation, overall inflammatory burden, and functional decline. The 2019 European League Against Rheumatism (EULAR) recommendations regard JAK inhibitors as comparable to biologic DMARDs (bDMARDs) in patients who do not achieve adequate control with conventional synthetic DMARDs (csDMARDs) [51-53]. Real-world studies indicate that JAK inhibitors deliver superior therapeutic benefits compared to methotrexate when used as second-line treatment options [54-57]. Chen et al. discovered the JAK3-selective inhibitor Z583, which covalently attaches to the cysteine 909 (Cys909) residue on JAK3, blocking its activation and markedly slowing the advancement of rheumatoid arthritis [58].
Table 2: Intersections of Cytokine–Jak/STAT Signaling with Neural Pain and Cognitive Circuits [59]
|
Cytokine (Ligand/Receptor) |
Jak/STAT Components Affected |
Observed or Proposed Effects on Pain and Cognition |
|
GM-CSF / GM-CSFR |
Jak2, STAT3, STAT5 |
Enhances pain sensitivity (hyperalgesia), Encourages sodium channel expression to rise. Nav1.7–Nav1.9. |
|
IL-4R / IL-4 |
Jak3, Jak1, STAT6, STAT1, STAT3 |
Pain hypersensitivity is lessened by overexpression, while IL-4 shortage is associated with social improvement but may also cause mechanical allodynia and cognitive deterioration. |
|
IFN-γR / IFN-γ |
Jak2, Jak1, STAT3, STAT1, |
Implicated in onset and persistence of pain, Deficiency in IFN-γ or its receptor alters social behavior, May have an impact on inhibitory GABAergic neurones. |
|
IL-1R / IL-1β |
IL-6–STAT3 axis-based indirect activation |
Contributes to pain initiation, May impair cognitive processes. |
|
gp130 / IL-27 |
STAT5a/b, STAT3 TYK2, STAT1 |
employs gp130 for signalling; pain perception is reduced when gp130 expression is lower. |
|
sIL-6R, IL-6 / IL-6R |
Jak1, Jak2, TYK2, STAT3 |
Acts as a pro-nociceptive mediator, Drives hyperalgesia/allodynia in animal models, May also cause cognitive impairment. |
|
IL-18R / IL-18 |
STAT4, TYK2 |
Determined to be a pro-nociceptive cytokine. |
|
IL-17RA / IL-17 |
STAT3, Jak2, STAT1 |
Encourages tactile allodynia. |
|
IL-10R / IL-10 |
STAT3, Jak1 |
It has pain-relieving (anti-nociceptive) properties. |
|
IL-12R / IL-12 |
STAT4, TYK2 |
Triggers the release of pro-nociceptive cytokines such IFN-γ and TNF. |
|
TNF-α / TNFR1, TNFR2 |
Indirect via IFN-β–Jak/STAT and STAT1 signaling |
Initiates and sensitizes pain pathways (mechanical and thermal), Contributes to neuropathic pain, Negatively impacts cognition. |
|
IL-15Rα / IL-15, γC, IL-2Rβ |
Jak1, STAT5, Jak3 |
Interacts with osteoarthritis pain intensity, |
|
IL-22R / IL-22 |
STAT3, Jak1 |
Inhibiting IL-22, which is upregulated in arthritic models, lessens pain reactions. |
Shital Devkar*, Shruti Sahekar, Amol Sherikar, Ajit Patil, Harnessing JAK-STAT Pathway Insights in Rheumatoid Arthritis: Merging Molecular Mechanisms with Cutting-Edge Therapeutic Approaches, Int. J. Med. Pharm. Sci., 2026, 2 (2), 85-104. https://doi.org/10.5281/zenodo.18514439
10.5281/zenodo.18514439