TNF-α (Tumor Necrosis Factor-alpha) is a pro-inflammatory primarily produced by macrophages and other immune cells such as T lymphocytes, natural killer (NK) cells, and neutrophils in response to stimuli like bacterial endotoxins, tissue injury, or infection [1]. It exists as a trimeric transmembrane protein (tmTNF-α) that can be cleaved by the enzyme TACE to release a soluble form (sTNF-α), both of which bind to two distinct receptors: and [2]. TNF-α plays a central role in regulating the , promoting inflammation by stimulating the expression of adhesion molecules, recruiting leukocytes, and inducing the production of other cytokines such as and . It also triggers in tumor and infected cells, contributing to its name. However, dysregulated or excessive TNF-α production is implicated in chronic inflammatory diseases including , , , and neurodegenerative disorders such as and . The critical balance between its protective and pathological roles has made TNF-α a major therapeutic target, leading to the development of biologic drugs such as , , and , which block TNF-α activity and are used to treat autoimmune conditions. Despite their efficacy, these therapies carry risks such as increased susceptibility to infections, including reactivation of latent , highlighting the complex dual nature of TNF-α in both host defense and disease pathogenesis.
Structure and Isoforms of TNF-α
Tumor Necrosis Factor-alpha (TNF-α) exists in two primary molecular forms: a transmembrane precursor (tmTNF-α) and a soluble form (sTNF-α), both of which are biologically active and play distinct roles in immune signaling. The protein is initially synthesized as a 26 kDa type II transmembrane protein anchored to the cell surface, constituting the tmTNF-α isoform [3]. This membrane-bound form consists of a cytoplasmic domain, a transmembrane region, and an extracellular domain that self-assembles into a functional trimeric structure, which is the active conformation required for receptor binding and signal transduction [3].
Soluble TNF-α and the Role of TACE
The soluble form of TNF-α (sTNF-α), with a molecular weight of 17 kDa, is generated through proteolytic cleavage of the extracellular domain of tmTNF-α. This process is mediated by the metalloprotease enzyme known as TNF-α converting enzyme (TACE), also referred to as ADAM17 [2]. TACE cleaves tmTNF-α near the cell membrane, releasing the trimeric sTNF-α into the extracellular space. Once liberated, sTNF-α can diffuse through tissues and the bloodstream, enabling it to exert systemic effects and act at distant sites from its cellular source [6]. This capacity for long-range signaling makes sTNF-α a key mediator of systemic inflammatory responses, including the induction of fever, cachexia, and acute-phase protein production.
Functional Differences Between tmTNF-α and sTNF-α
The two isoforms of TNF-α exhibit significant functional differences due to their distinct modes of action. tmTNF-α primarily mediates juxtacrine signaling, which requires direct cell-to-cell contact. This localized signaling is crucial for organizing immune responses, regulating cell survival, and facilitating bidirectional communication between immune cells such as macrophages, T lymphocytes, and dendritic cells [7]. In contrast, sTNF-α functions as a systemic hormone-like molecule, capable of amplifying inflammation throughout the body by binding to its receptors on a wide variety of cell types [6].
Selective Receptor Activation by TNF-α Isoforms
The functional divergence of the two isoforms is further defined by their preferential activation of two distinct receptors: TNFR1 (p55) and TNFR2 (p75). sTNF-α has a higher affinity for TNFR1, which is ubiquitously expressed on most cell types and contains a death domain in its cytoplasmic tail. Activation of TNFR1 primarily drives pro-inflammatory signaling, including the activation of the pathway and the induction of apoptosis through caspase activation [9]. This makes sTNF-α a major contributor to the pathological inflammation observed in chronic diseases such as , , and [3].
Conversely, tmTNF-α is the preferred ligand for TNFR2, which is predominantly expressed on immune cells, including Treg cells and myeloid cells, as well as on endothelial cells [9]. TNFR2 lacks a death domain and instead mediates signals associated with cell survival, proliferation, and immunoregulation. Activation of the tmTNF-α/TNFR2 axis promotes the expansion and function of Treg cells, supports tissue regeneration, and provides neuroprotective effects [12]. This pathway represents an immunoregulatory and protective arm of the TNF-α system, which can counterbalance the pro-inflammatory actions of the sTNF-α/TNFR1 axis.
The structural and functional duality of TNF-α isoforms allows for a finely tuned regulation of the immune response, balancing potent inflammatory activation with mechanisms for control and tissue repair. This understanding has profound implications for the development of targeted therapies, as drugs that indiscriminately block both forms of TNF-α may inadvertently disrupt beneficial TNFR2-mediated signaling while aiming to suppress harmful TNFR1-driven inflammation [13].
Cellular Sources and Regulation of Expression
The cellular sources of TNF-α (Tumor Necrosis Factor-alpha) are diverse, reflecting its central role in coordinating the immune response. The primary source of TNF-α is the , which releases the cytokine in response to stimuli such as tissue injury, infection, or bacterial endotoxins like lipopolysaccharide [1]. In addition to macrophages, multiple other immune cells contribute to TNF-α production. T lymphocytes, particularly when activated by antigenic stimulation, are significant producers [6]. Natural killer (NK) cells also secrete TNF-α as part of their defense against infected or tumor cells [16]. Other contributing cells include neutrophils, which are active in the early phases of inflammation [6], monocytes, which are precursors to macrophages and are especially active in chronic inflammatory conditions like , mast cells, eosinophils, and dendritic cells, which are crucial for antigen presentation and immune coordination [2]. Notably, even non-immune cells such as neurons in the central nervous system can produce TNF-α during neuroinflammatory processes [2].
Regulation of TNF-α Gene Expression
The expression of the TNF-α gene is tightly regulated at both the transcriptional and post-transcriptional levels to ensure a rapid yet controlled immune response. Transcriptional regulation is primarily driven by inflammatory stimuli, most notably LPS, which activates Toll-like receptors (TLRs) on immune cells [20]. This activation triggers a signaling cascade involving adaptor proteins like and kinases such as IKK, leading to the phosphorylation and degradation of IκB, the inhibitor of the transcription factor NF-κB [21]. The liberated NF-κB translocates to the nucleus and binds to the promoter region of the TNF gene, initiating its transcription. Other transcription factors, including AP-1 (activated by MAP kinase pathways like JNK and p38) and C/EBP, also contribute to gene induction [22]. The process is further modulated by other cytokines; for instance, IL-1β and IFN-γ can synergize with LPS to amplify TNF-α expression, while anti-inflammatory cytokines like IL-10 suppress it [23].
Post-transcriptional regulation is equally critical for controlling the duration and intensity of the TNF-α response. The mRNA of TNF-α contains AU-rich elements (AREs) in its 3'-untranslated region (3'-UTR), which serve as binding sites for regulatory proteins that influence mRNA stability and translation. Two key proteins are tristetraprolin (TTP) and HuR. TTP promotes the degradation of TNF-α mRNA, acting as a natural brake on its production, and its expression can be induced by anti-inflammatory signals [24]. Conversely, HuR stabilizes the mRNA, prolonging its half-life and increasing translation. The balance between TTP and HuR determines the net level of TNF-α produced [25]. The pathway regulates TTP activity through phosphorylation, linking signal transduction directly to mRNA stability [26]. Furthermore, microRNAs (miRNAs), such as miR-125b, miR-146a, and miR-155, provide an additional layer of control by directly binding to TNF-α mRNA to inhibit its translation or by targeting components of the TLR-NF-κB pathway to create negative feedback loops [25]. Recent research has also highlighted the role of epitranscriptional modifications, particularly N6-adenosine (m6A) methylation, in regulating TNF-α mRNA degradation, adding another sophisticated level of control to this critical inflammatory mediator [28].
Signaling Pathways and Biological Effects
The biological effects of TNF-α are mediated through its interaction with two distinct cell surface receptors, and , which activate complex intracellular signaling pathways. These pathways regulate a wide array of cellular responses, including inflammation, cell survival, apoptosis, and immune activation, and their precise outcome depends on the receptor engaged, the cellular context, and the duration and intensity of the signal. The dual nature of TNF-α signaling—capable of both promoting tissue homeostasis and driving pathological damage—stems from the distinct functions of these two receptors and the two molecular forms of the cytokine: the soluble (sTNF-α) and transmembrane (tmTNF-α) isoforms.
Receptor-Specific Signaling: TNFR1 and TNFR2
The primary receptors for TNF-α, TNFR1 (p55) and TNFR2 (p75), mediate divergent biological effects due to differences in their expression patterns and intracellular signaling domains. TNFR1 is ubiquitously expressed on nearly all cell types and contains a "death domain" in its cytoplasmic tail, which allows it to initiate both pro-inflammatory and pro-apoptotic signaling cascades [29]. In contrast, TNFR2 is expressed primarily on immune cells such as T lymphocytes, Treg cells, and myeloid cells, as well as on endothelial cells. It lacks a death domain and primarily mediates signals related to cell survival, proliferation, and immune regulation [9].
The form of TNF-α also dictates receptor preference. Soluble TNF-α (sTNF-α), generated by the proteolytic cleavage of tmTNF-α by the enzyme TACE, has a higher affinity for TNFR1 and is the main activator of its downstream pathways [9]. This form is responsible for systemic effects such as fever, cachexia, and widespread endothelial activation. Conversely, transmembrane TNF-α (tmTNF-α) acts through juxtacrine signaling, requiring direct cell-to-cell contact, and preferentially activates TNFR2. This interaction promotes immunoregulatory functions, including the expansion and function of Treg cells, tissue regeneration, and neuroprotection [12]. This functional dichotomy allows TNF-α to balance potent immune activation with mechanisms to control and resolve inflammation.
Key Intracellular Signaling Pathways: NF-κB and MAPK
The binding of TNF-α to its receptors, particularly TNFR1, triggers two major signaling cascades: the (nuclear factor kappa B) pathway and the (mitogen-activated protein kinase) pathway. These pathways are central to the cytokine's pro-inflammatory actions. Upon TNF-α binding, TNFR1 recruits adaptor proteins such as (TNF receptor-associated death domain) and (TNF receptor-associated factor 2), leading to the activation of the (IκB kinase). IKK phosphorylates the inhibitor protein IκB, targeting it for degradation by the . This releases NF-κB, allowing it to translocate to the nucleus and activate the transcription of numerous genes involved in inflammation, including other cytokines like and , chemokines, adhesion molecules such as and , and enzymes like (cyclooxygenase-2) and (inducible nitric oxide synthase) [33]. This transcriptional program is essential for initiating and amplifying the immune response.
Simultaneously, TNF-α activates the MAPK cascade, which includes three main sub-pathways: (Extracellular signal-Regulated Kinase), (c-Jun N-terminal Kinase), and . These kinases are also activated through the TRADD-TRAF2 complex and regulate a variety of cellular processes [34]. The p38 and JNK pathways are particularly important in the inflammatory response, as they regulate the production of inflammatory mediators and can induce apoptosis under conditions of cellular stress. The ERK pathway is more commonly associated with cell proliferation and survival. The coordinated activation of NF-κB and MAPK pathways by TNF-α ensures a robust and multifaceted inflammatory response.
Biological Effects on Cells and Tissues
The activation of these signaling pathways translates into a range of biological effects that orchestrate the inflammatory response. One of the most immediate effects is on the vascular endothelium. TNF-α induces the expression of adhesion molecules like E-selectin, ICAM-1, and VCAM-1 on endothelial cells, which facilitates the adhesion and transmigration of leukocytes from the bloodstream into the site of inflammation [35]. It also increases vascular permeability, allowing plasma proteins and immune cells to enter the affected tissue, contributing to edema and swelling [36]. Furthermore, TNF-α acts as a potent pyrogen, contributing to the development of fever through its action on the .
TNF-α is also a key orchestrator of the immune response, bridging innate and adaptive immunity. It activates macrophages and dendritic cells, enhancing their phagocytic activity and their ability to present antigens to T cells [37]. It promotes the differentiation of T helper cells into pro-inflammatory subsets such as and cells, which produce and , respectively [38]. Additionally, TNF-α can induce apoptosis in certain target cells, particularly through TNFR1, which is a key mechanism for eliminating infected or tumor cells [39]. However, chronic or excessive TNF-α signaling can lead to detrimental effects, including the destruction of cartilage and bone in joints, as seen in , and systemic symptoms like anorexia and anemia.
Dual Roles in Health and Disease
The biological effects of TNF-α are a double-edged sword. In the context of acute inflammation, its actions are protective, essential for containing and eliminating pathogens and initiating tissue repair. The rapid activation of NF-κB and MAPK pathways, the recruitment of immune cells, and the induction of fever are all part of a coordinated defense mechanism. However, when TNF-α production becomes dysregulated and chronic, these same pathways drive tissue destruction and contribute to the pathogenesis of numerous diseases. The persistent activation of NF-κB leads to a continuous production of inflammatory cytokines and tissue-degrading enzymes, which underlies the joint destruction in and the transmural inflammation in . The dual roles of TNF-α in promoting both cell death and survival, and in activating both destructive and protective pathways, highlight its central and complex role in maintaining immune balance.
Role in Inflammatory and Autoimmune Diseases
Tumor Necrosis Factor-alpha (TNF-α) plays a central and complex role in the pathogenesis of numerous inflammatory and autoimmune diseases. As a key pro-inflammatory , its overproduction or dysregulated signaling contributes to chronic inflammation, tissue damage, and disease progression across a spectrum of conditions. The therapeutic success of anti-TNF agents has unequivocally established TNF-α as a critical mediator in these disorders, highlighting its dual nature as both a vital component of host defense and a potent driver of pathological inflammation [1].
Key Inflammatory and Autoimmune Conditions
The pathogenic role of TNF-α is most prominent in several well-characterized chronic inflammatory and autoimmune diseases.
Rheumatoid Arthritis (RA)
In , TNF-α is a pivotal driver of synovial inflammation and joint destruction. It is produced primarily by infiltrating macrophages and T lymphocytes within the synovial membrane. Its overexpression initiates a vicious cycle of inflammation by stimulating the production of other pro-inflammatory cytokines like and , and by promoting the expression of adhesion molecules (e.g., ICAM-1, VCAM-1) that facilitate the recruitment of additional leukocytes into the joint. This leads to synovial hyperplasia, formation of an invasive pannus, and the activation of osteoclasts, which are responsible for bone erosion and cartilage degradation. The profound impact of TNF-α on joint pathology is evidenced by the dramatic clinical improvement seen with anti-TNF therapies, which reduce inflammation, alleviate pain, and slow or halt structural damage [41].
Psoriasis and Psoriatic Arthritis
TNF-α is critically involved in the pathogenesis of both psoriasis and . In psoriasis, it contributes to the hyperproliferation of keratinocytes and the inflammatory infiltration of the skin. It activates dermal cells and promotes the release of chemokines that recruit immune cells, sustaining the inflammatory cascade. In psoriatic arthritis, TNF-α drives inflammation in the joints and entheses (sites of tendon and ligament insertion into bone), leading to pain, swelling, and structural damage. Anti-TNF agents are highly effective in reducing skin lesions and improving joint symptoms, confirming the central role of this cytokine in the disease process [1].
Crohn's Disease
, a form of inflammatory bowel disease, is characterized by transmural inflammation of the gastrointestinal tract. TNF-α is a major contributor to the chronic inflammation and tissue damage seen in this condition. It is produced by macrophages and other immune cells in the intestinal mucosa and perpetuates the inflammatory response by stimulating the production of other cytokines and enzymes that degrade the extracellular matrix. It is also involved in the formation of granulomas, a hallmark of Crohn's disease. The efficacy of anti-TNF agents like infliximab and adalimumab in inducing and maintaining remission in Crohn's disease underscores the critical pathogenic role of TNF-α in this disorder [43].
Ankylosing Spondylitis
In , a type of spondyloarthritis, TNF-α is highly expressed at the entheses, particularly in the sacroiliac joints and spine. It drives local inflammation, causing pain and stiffness, and stimulates osteoclast activity, leading to bone resorption. Paradoxically, it is also implicated in the subsequent pathological bone formation (syndesmophytes) that results in spinal fusion and rigidity. Anti-TNF therapy is highly effective in controlling disease activity, reducing pain, and improving function, demonstrating the central role of TNF-α in the inflammatory phase of this disease [44].
Mechanisms of Tissue Damage
The tissue damage observed in these diseases is a direct consequence of the biological effects of TNF-α. Its binding to its receptors, and , activates multiple intracellular signaling pathways. The activation of the pathway is particularly significant, as it regulates the expression of genes involved in inflammation, cell proliferation, and survival. This leads to a sustained production of inflammatory mediators. TNF-α also induces the production of matrix metalloproteinases (MMPs) by synoviocytes and chondrocytes, which degrade cartilage and other connective tissues. Furthermore, it disrupts the balance of bone remodeling by promoting osteoclastogenesis (bone resorption) while inhibiting osteoblast function (bone formation), resulting in progressive bone loss and erosions [45].
Genetic and Syndromic Associations
Beyond common autoimmune conditions, TNF-α is implicated in rare genetic syndromes. A prime example is the Tumor Necrosis Factor Receptor-Associated Periodic Syndrome (TRAPS), an autosomal dominant autoinflammatory disorder caused by mutations in the TNFRSF1A gene, which encodes the TNFR1 receptor. These mutations lead to abnormal receptor folding and impaired shedding, resulting in prolonged and spontaneous activation of the inflammatory pathway. This causes recurrent episodes of fever, myalgia, and skin rashes, highlighting how a single defect in the TNF-α signaling system can lead to severe inflammatory disease [46].
Broader Pathological Implications
Elevated levels of TNF-α are also associated with a range of other conditions, reflecting its broad influence on physiological processes. It is linked to metabolic alterations such as obesity and insulin resistance, contributing to the inflammatory component of metabolic syndrome. It plays a role in cardiovascular risk by promoting atherosclerosis through endothelial activation and inflammation. Furthermore, TNF-α is a key mediator of neuroinflammation, with implications for neurodegenerative disorders such as and [47][48].
In summary, TNF-α is a master regulator of inflammation whose dysregulation is a common thread in a wide array of chronic inflammatory and autoimmune diseases. Its actions, from coordinating the recruitment of immune cells to directly causing tissue destruction, make it a fundamental pathogenic factor. The development of effective anti-TNF therapies has not only revolutionized the treatment of these conditions but has also provided profound insights into the molecular mechanisms of chronic inflammation.
TNF-α in Cancer: Dual Roles in Tumor Suppression and Progression
The cytokine TNF-α (Tumor Necrosis Factor-alpha) exhibits a profound dualism in oncogenesis, functioning as both a tumor suppressor and a promoter of cancer progression. This paradoxical role is determined by a complex interplay of molecular, cellular, and microenvironmental factors, making TNF-α a pivotal yet challenging target in cancer biology and therapy. Its actions are mediated through two distinct receptors, and , which trigger opposing signaling pathways that can either induce cancer cell death or foster tumor growth, invasion, and immune evasion.
Tumor-Suppressive Effects of TNF-α
TNF-α was originally named for its ability to cause hemorrhagic necrosis in tumors, highlighting its potent anti-tumor activity. This tumor-suppressive function is primarily mediated through the activation of , which contains a death domain in its cytoplasmic tail. Upon binding of TNF-α, TNFR1 recruits adaptor proteins such as (TNF receptor-associated death domain) and (Fas-associated death domain), leading to the activation of cascades, particularly caspase-8 and caspase-3, which initiate the process of in susceptible tumor cells [29]. This mechanism is particularly effective in experimental settings and forms the rationale for some localized cancer therapies.
A notable clinical application of this principle is isolated limb perfusion (ILP), a technique used to treat advanced melanoma or sarcoma in a limb. In this procedure, high concentrations of TNF-α are delivered directly to the affected area, often in combination with chemotherapy. The localized high-dose TNF-α exerts its cytotoxic effects by directly killing tumor cells and by disrupting the tumor vasculature, leading to massive hemorrhagic necrosis of the tumor mass [50]. This approach maximizes the anti-tumor effect while minimizing the severe systemic toxicity that would occur with intravenous administration.
Furthermore, TNF-α contributes to tumor suppression by modulating the . It can enhance the anti-tumor immune response by stimulating the activation and cytotoxic activity of T lymphocytes and macrophages, which then target and destroy cancer cells. This immune-mediated clearance is a crucial component of the body's natural defense against cancer [51].
Pro-Tumor Effects of TNF-α
Despite its potential as a direct cytotoxin, in the context of chronic inflammation, TNF-α predominantly acts as a powerful promoter of cancer progression. Persistent, low-level expression of TNF-α, often produced by tumor-infiltrating immune cells such as tumor-associated macrophages (TAMs) and lymphocytes, creates a microenvironment that is highly conducive to tumor growth, survival, and spread [6].
One of the key pro-tumor mechanisms is the activation of the (nuclear factor kappa B) signaling pathway. When TNF-α binds to TNFR1 in a chronic setting, it can trigger a signaling cascade that leads to the nuclear translocation of NF-κB. NF-κB is a master transcription factor that upregulates the expression of a wide array of genes involved in cell survival, proliferation, and resistance to apoptosis [53]. This effectively shields tumor cells from various forms of cell death, including the very apoptotic signals that TNF-α can induce in other contexts. The balance between apoptosis and survival is often tipped towards survival by the expression of anti-apoptotic proteins like c-FLIP, which are also regulated by NF-κB [29].
TNF-α also plays a central role in promoting , the formation of new blood vessels that are essential for tumor growth beyond a minimal size. It stimulates the production of pro-angiogenic factors such as (Vascular Endothelial Growth Factor) and induces a pro-angiogenic phenotype in endothelial cells. Specifically, TNF-α primes endothelial cells for sprouting by inducing the expression of Jagged-1, a ligand in the Notch signaling pathway, which is critical for the formation of new vascular networks within the tumor [55].
Moreover, TNF-α is a key driver of . It promotes the in cancer cells, a process where epithelial cells lose their adhesion and gain migratory and invasive properties. This transition is largely mediated by the TNF-α/NF-κB/Snail signaling axis, which downregulates the cell adhesion molecule E-cadherin and upregulates mesenchymal markers like vimentin [53]. TNF-α also enhances the expression of , enzymes that degrade the extracellular matrix, allowing cancer cells to invade surrounding tissues and enter the bloodstream [57].
TNF-α and the Immunosuppressive Tumor Microenvironment
Perhaps one of the most significant pro-tumor roles of TNF-α is its contribution to the creation of an immunosuppressive tumor microenvironment (TME). Chronic TNF-α signaling recruits and activates myeloid-derived suppressor cells (MDSCs), which inhibit the proliferation and function of cytotoxic T cells by depleting essential amino acids like arginine and producing reactive oxygen species [58]. TNF-α also polarizes macrophages towards an M2 phenotype (TAMs), which secrete anti-inflammatory cytokines like and , further dampening the anti-tumor immune response and promoting tissue repair that inadvertently supports tumor stroma formation [59].
TNF-α can also modulate immune checkpoint molecules. It has been shown to stabilize the expression of (programmed death-ligand 1) on tumor cells and macrophages, thereby enhancing the PD-1/PD-L1 checkpoint pathway that inhibits T cell activity, allowing the tumor to evade immune surveillance [60]. This complex manipulation of the immune system transforms TNF-α from a potential anti-tumor agent into a key enabler of immune escape.
Factors Determining the Dual Role of TNF-α
The ultimate effect of TNF-α in cancer—whether it suppresses or promotes the tumor—is determined by several critical factors. The concentration and duration of TNF-α exposure are paramount. High, acute levels, as achieved in localized therapies, favor apoptosis, while low, chronic levels, typical of an inflamed tumor, promote survival and proliferation via NF-κB. The balance between TNFR1 and TNFR2 signaling is another crucial determinant. TNFR1 is primarily responsible for the pro-apoptotic signal, whereas TNFR2, which lacks a death domain, predominantly activates pro-survival and proliferative pathways [61].
The cellular context and the composition of the tumor microenvironment are equally important. The presence of other inflammatory mediators, such as or , can amplify the pro-tumor effects of TNF-α [62]. Furthermore, cancer cells themselves can develop resistance to TNF-α-induced apoptosis by downregulating TNFR1 expression or upregulating anti-apoptotic proteins, allowing them to benefit from the growth-promoting signals of the cytokine without succumbing to its cytotoxic effects [63].
Therapeutic Implications and Challenges
The dual nature of TNF-α presents a significant challenge for cancer therapy. While preclinical studies support the potential of TNF-α inhibitors to block angiogenesis, metastasis, and immunosuppression in cancers driven by chronic inflammation (e.g., colitis-associated colorectal cancer), their use in oncology is limited and controversial [64]. A major concern is that systemic inhibition of TNF-α might remove a natural anti-tumor surveillance mechanism, potentially promoting cancer progression.
Indeed, long-term use of anti-TNF-α drugs for autoimmune diseases has been associated with an increased risk of certain malignancies, particularly non-melanoma skin cancers (NMSC) and lymphomas, although the direct causal link remains complex and may be confounded by the underlying inflammatory disease and concomitant immunosuppressive therapies [65]. This risk underscores the delicate balance between controlling harmful inflammation and preserving beneficial immune functions.
Conversely, the use of TNF-α as a direct anti-cancer agent is restricted by its severe systemic toxicity, including hypotension, organ failure, and septic shock. Therefore, the future of TNF-α-targeted cancer therapy likely lies in highly selective strategies. These include the development of drugs that inhibit the pro-tumor TNFR1/NF-κB pathway while sparing or even enhancing the potentially protective TNFR2 pathway, or the use of targeted delivery systems to achieve high local concentrations of TNF-α within the tumor, as in ILP, to maximize efficacy and minimize systemic side effects [50].
Involvement in Neurodegenerative Diseases
The dualistic role of TNF-α in the central nervous system (CNS) is a pivotal factor in the pathogenesis of several neurodegenerative diseases, including , , and . TNF-α, a key pro-inflammatory , exerts both neuroprotective and neurotoxic effects depending on the context, receptor activation, duration of exposure, and concentration. This complex duality presents significant challenges for therapeutic targeting, as systemic inhibition of TNF-α may disrupt essential physiological functions while attempting to mitigate pathological inflammation [67].
Dualistic Role of TNF-α in the Central Nervous System
TNF-α’s actions in the CNS are primarily mediated through two distinct receptors: and . The balance between signaling through these receptors determines whether the outcome is neuroprotection or neurodegeneration. TNFR1, which contains a death domain in its cytoplasmic region, is ubiquitously expressed and predominantly mediates pro-inflammatory and pro-apoptotic signals. Activation of TNFR1 recruits adaptor proteins such as TRADD, TRAF, and RIP, leading to the activation of caspases and subsequent neuronal apoptosis [29]. This pathway is particularly implicated in chronic neuroinflammatory conditions where sustained TNF-α expression contributes to neuronal loss.
Conversely, TNFR2 is mainly expressed on immune cells, endothelial cells, and neurons, and is associated with neuroprotective, regenerative, and anti-inflammatory signaling. Activation of TNFR2 promotes cell survival through the NF-κB pathway, supports remyelination, and enhances the function of regulatory T cells (Tregs), which help maintain immune homeostasis in the CNS [29]. For example, in acute injury models such as traumatic brain injury or ischemia, TNF-α acting via TNFR2 has been shown to protect neurons and support tissue repair [67].
The shift from protective to toxic effects often depends on the duration and level of TNF-α expression. Transient, low-level TNF-α signaling can support synaptic plasticity and microglial clearance of cellular debris, contributing to CNS homeostasis. However, chronic overexpression, commonly observed in neurodegenerative disorders, leads to persistent activation of TNFR1, resulting in excitotoxicity, oxidative stress, and disruption of neuronal networks [71].
TNF-α in Multiple Sclerosis
In (MS), an autoimmune disorder characterized by demyelination and neuroinflammation, TNF-α plays a central role in promoting disease progression. TNF-α is produced by infiltrating T cells, activated microglia, and astrocytes within CNS lesions. It contributes to the breakdown of the (BBB) by downregulating tight junction proteins such as occludin and claudin-5, thereby facilitating the entry of autoreactive immune cells into the CNS [72]. Elevated levels of TNF-α and adhesion molecules like ICAM-1 in the serum and cerebrospinal fluid correlate with disease activity and BBB permeability in MS patients [73].
Despite its pro-inflammatory role, TNF-α also exhibits reparative functions in MS. Microglia activated by TNF-α can prepare to repair myelin damage, highlighting the cytokine’s dual nature [74]. However, clinical trials using systemic anti-TNF-α therapies have shown mixed or detrimental outcomes, with some reports indicating exacerbation of MS symptoms, likely due to interference with TNFR2-mediated protective mechanisms [75].
TNF-α in Alzheimer's Disease
In (AD), TNF-α is a key mediator of chronic neuroinflammation driven by amyloid-beta (Aβ) plaques and neurofibrillary tangles of hyperphosphorylated tau protein. Activated microglia surrounding Aβ deposits release TNF-α, which amplifies the inflammatory cascade and contributes to synaptic dysfunction and neuronal death [76]. TNF-α can increase Aβ aggregation and modulate tau phosphorylation, accelerating disease pathology [77].
Moreover, Aβ itself can alter neuronal responses to TNF-α by reducing the expression of FAIM-L, a protein that mediates neuroprotective signaling, thereby shifting the balance toward cell death [78]. Preclinical studies suggest that selective inhibition of soluble TNF-α (sTNF-α), the primary activator of TNFR1, may prevent Aβ-associated neuropathology without compromising the beneficial effects of transmembrane TNF-α (tmTNF-α) signaling through TNFR2 [79].
TNF-α in Parkinson's Disease
In (PD), TNF-α contributes to the progressive degeneration of dopaminergic neurons in the substantia nigra. Chronic microglial activation leads to sustained TNF-α release, which induces oxidative stress, mitochondrial dysfunction, and neurotoxicity [80]. Experimental models show that persistent low-level expression of TNF-α in the substantia nigra results in progressive neuronal loss and glial activation [81].
A recently identified mechanism involves TNF-α in the propagation of pathological α-synuclein. TNF-α promotes lysosomal exocytosis associated with cellular senescence, facilitating the release of misfolded α-synuclein and its spread to healthy neurons, thus driving disease progression [82]. This highlights TNF-α not only as a mediator of inflammation but also as a facilitator of prion-like protein transmission in PD.
Therapeutic Challenges and Emerging Strategies
Targeting TNF-α in neurological disorders faces major hurdles, primarily due to the limited ability of current anti-TNF agents—such as , , and —to cross the intact blood-brain barrier (BBB) [83]. These large protein-based drugs remain largely confined to the periphery, limiting their efficacy in modulating CNS inflammation. Furthermore, systemic TNF-α inhibition carries risks of serious adverse effects, including increased susceptibility to infections such as reactivation of latent , due to the cytokine’s essential role in granuloma formation [84].
Paradoxically, anti-TNF therapies have been linked to the onset or worsening of demyelinating diseases like MS, underscoring the potential neuroprotective roles of TNF-α that are disrupted by global inhibition [85]. This has led to a shift in therapeutic focus toward selective modulation of TNF-α signaling pathways.
Emerging strategies aim to selectively inhibit TNFR1 while sparing or even enhancing TNFR2 activity. For instance, XPro1595, a dominant-negative inhibitor that neutralizes soluble TNF-α, has shown neuroprotective effects in animal models of PD and AD by reducing neuroinflammation without impairing host defense mechanisms [86]. Similarly, nanobodies and small molecules designed to specifically block TNFR1 have demonstrated efficacy in reducing disease severity in experimental autoimmune encephalomyelitis (EAE), a model of MS [87].
Another promising avenue involves the use of targeted delivery systems, such as -based carriers or viral vectors, to transport anti-TNF agents across the BBB and deliver them directly to affected brain regions [88]. These approaches could enable precise modulation of TNF-α signaling within the CNS while minimizing systemic side effects.
In conclusion, while TNF-α is undeniably involved in the pathogenesis of major neurodegenerative diseases, its dual role necessitates a nuanced therapeutic approach. Future treatments are likely to move beyond broad TNF-α suppression toward receptor-specific modulation and targeted delivery, aiming to suppress harmful inflammation while preserving essential neuroprotective and regenerative functions.
Therapeutic Targeting with Anti-TNF Agents
The development of anti-TNF agents represents a landmark achievement in the treatment of chronic inflammatory and autoimmune diseases. These biologic drugs function by neutralizing the activity of tumor necrosis factor-alpha (TNF-α), a central pro-inflammatory implicated in the pathogenesis of conditions such as , , and . By blocking TNF-α, these therapies disrupt the inflammatory cascade, reducing symptoms, slowing structural damage, and improving quality of life for millions of patients. The success of anti-TNF therapy has validated the concept of targeted immunomodulation and has revolutionized the management of autoimmune disorders [84].
Mechanisms of Action and Classes of Anti-TNF Agents
Anti-TNF agents exert their therapeutic effects primarily by preventing TNF-α from binding to its cellular receptors, and , thereby inhibiting downstream inflammatory signaling pathways such as and . This blockade leads to a reduction in the expression of adhesion molecules, decreased recruitment of leukocytes, and diminished production of other pro-inflammatory cytokines like and . However, different classes of anti-TNF drugs achieve this through distinct mechanisms, largely determined by their molecular structure [90].
The two main classes are monoclonal antibodies and soluble receptor fusion proteins. Monoclonal antibodies, such as infliximab, adalimumab, golimumab, and certolizumab pegol, are engineered immunoglobulins that bind with high affinity to both the soluble (sTNF-α) and transmembrane (tmTNF-α) forms of the cytokine. Their IgG1 structure allows them to induce additional effector functions, including antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC), which can lead to the apoptosis of cells expressing tmTNF-α, such as activated T cells and macrophages [91].
In contrast, etanercept is a soluble receptor fusion protein composed of the extracellular ligand-binding portion of the human TNFR2 (p75) linked to the Fc region of human IgG1. It functions primarily as a "molecular sponge," trapping soluble TNF-α and lymphotoxin-alpha (LT-α) to prevent receptor engagement. Unlike the monoclonal antibodies, etanercept has a lower affinity for tmTNF-α and does not effectively induce ADCC or CDC, resulting in a more limited mechanism of action focused on neutralizing the soluble cytokine [92].
Structural and Functional Differences Among Agents
The structural differences between anti-TNF agents translate into significant functional and pharmacokinetic variations that influence clinical use. Infliximab is a chimeric (partially murine) monoclonal antibody, which makes it more immunogenic and prone to inducing anti-drug antibodies (ADAb) compared to fully human agents like adalimumab and golimumab [83]. Certolizumab pegol is unique as a PEGylated Fab' fragment, lacking the Fc portion entirely, which eliminates the risk of Fc-mediated effects like CDC and may contribute to a different safety profile [83].
The presence or absence of the Fc region also affects the drugs' ability to cross the placenta, which is a critical consideration for pregnant patients. Furthermore, the pharmacokinetics vary: etanercept has a shorter half-life (approximately 4-5 days) and requires more frequent dosing (weekly or bi-weekly), while agents like adalimumab, golimumab, and certolizumab pegol have longer half-lives (10-20 days), allowing for dosing every two to four weeks [95].
Clinical Efficacy Across Inflammatory Diseases
Anti-TNF agents are highly effective across a spectrum of inflammatory conditions. In , they significantly reduce disease activity, inhibit joint erosion, and improve physical function, especially when used in combination with [96]. For and other spondyloarthropathies, they are particularly effective at controlling spinal inflammation, pain, and stiffness, as measured by indices like BASDAI [97]. In , agents like adalimumab and certolizumab pegol have shown strong efficacy for both joint and skin manifestations [98].
Their role is especially transformative in . Infliximab and adalimumab are approved for both Crohn's disease and ulcerative colitis, where they are used to induce and maintain remission, reduce the need for surgery, and promote mucosal healing [99]. The ability of monoclonal antibodies to induce apoptosis in lamina propria T cells may explain their superior efficacy in these conditions compared to etanercept, which has not shown significant benefit in Crohn's disease [100].
Pharmacokinetics, Immunogenicity, and Therapeutic Drug Monitoring
The pharmacokinetics of anti-TNF agents are critical determinants of their efficacy and dosing frequency. Factors such as systemic inflammation, body weight, and the presence of anti-drug antibodies (ADAb) can increase drug clearance, leading to subtherapeutic serum concentrations and loss of response [101]. This phenomenon is particularly relevant for monoclonal antibodies, where immunogenicity is a major cause of secondary treatment failure.
To address this, therapeutic drug monitoring (TDM) has become an essential tool in clinical practice. By measuring serum drug levels and the presence of ADAb, clinicians can distinguish between pharmacokinetic failure (low drug levels due to rapid clearance or ADAb) and pharmacodynamic failure (adequate drug levels but no clinical response). This allows for personalized interventions, such as dose escalation, shortening the dosing interval, or switching to a different agent or drug class (e.g., a or an anti-IL-12/23 agent) [102].
Patient Selection and Treatment Optimization
The selection of patients for anti-TNF therapy is guided by strict criteria to ensure appropriate use and maximize benefit. According to international guidelines from organizations like and , these drugs are typically indicated for patients with moderate-to-severe disease activity who have had an inadequate response to conventional synthetic disease-modifying antirheumatic drugs (csDMARDs), such as methotrexate [103]. Disease activity is assessed using validated tools like the DAS28 for rheumatoid arthritis or the ASDAS for ankylosing spondylitis.
Before initiating therapy, a comprehensive evaluation is mandatory to assess safety. This includes screening for latent with a tuberculin skin test (TST) or interferon-gamma release assay (IGRA), as well as a chest X-ray. Screening for hepatitis B and C, HIV, and a thorough neurological and oncological history is also required to mitigate risks [104]. Vaccination status should be reviewed and updated, with particular attention to ensuring protection against pneumococcal and influenza infections before starting treatment [105].
Strategies for Overcoming Treatment Resistance
Resistance to anti-TNF therapy, either primary (no initial response) or secondary (loss of response over time), is a significant clinical challenge. The primary mechanisms include immunogenicity (formation of ADAb), altered pharmacokinetics due to inflammation, and the activation of alternative inflammatory pathways (e.g., IL-6, IL-17/IL-23) that bypass TNF-α blockade [106].
Strategies to overcome resistance are guided by TDM. For pharmacokinetic failure, options include optimizing the dose, shortening the dosing interval, or combining the anti-TNF agent with an immunomodulator like methotrexate to reduce immunogenicity. For pharmacodynamic failure, a switch to a different class of biologic or targeted synthetic DMARD is recommended. This can be an "in-class" switch (e.g., from infliximab to adalimumab) or an "out-of-class" switch to agents like (an IL-6 inhibitor), (a T-cell co-stimulation modulator), or a [107]. Emerging strategies involve using biomarkers and molecular profiling from tissue biopsies to predict the best therapeutic choice from the outset, moving towards true precision medicine [108].
Adverse Effects and Clinical Monitoring of Anti-TNF Therapy
The use of anti-TNF-α agents, while highly effective in managing chronic inflammatory diseases such as , , and , is associated with a range of adverse effects that necessitate careful clinical monitoring. These biological therapies, including , , and , modulate the immune system by neutralizing the pro-inflammatory cytokine TNF-α, which plays a critical role in host defense, particularly against infections. Consequently, their suppression of TNF-α activity can compromise immune surveillance, leading to increased susceptibility to infections, autoimmune phenomena, and malignancies [84]. A comprehensive understanding of these risks and the implementation of structured monitoring protocols are essential for ensuring patient safety and optimizing long-term outcomes.
Infections and Opportunistic Pathogens
One of the most significant and well-documented risks associated with anti-TNF therapy is the increased incidence of infections, particularly opportunistic infections. The most prominent of these is the reactivation of latent (TB), a consequence of the essential role TNF-α plays in the formation and maintenance of granulomas, the immune structures that contain Mycobacterium tuberculosis [84]. The risk of TB reactivation varies among different anti-TNF agents, with monoclonal antibodies like and generally posing a higher risk compared to the soluble receptor , likely due to their more potent inhibition of transmembrane TNF-α [104]. A systematic review has reported an odds ratio of approximately 1.92 for developing active TB in patients receiving anti-TNF therapy, underscoring the critical importance of pre-treatment screening [112].
Beyond TB, patients are also at an increased risk for other serious infections, including invasive fungal infections (e.g., histoplasmosis), bacterial sepsis, and reactivation of (HBV). Common infections such as upper respiratory tract infections, sinusitis, and urinary tract infections are also more frequent [113]. The risk of infection is influenced by factors such as the patient's underlying disease activity, concomitant use of immunosuppressants like , and the specific pharmacokinetic profile of the anti-TNF agent used [114].
Autoimmune and Paradoxical Reactions
Paradoxically, while anti-TNF agents are used to treat autoimmune diseases, they can themselves induce new autoimmune or autoinflammatory conditions, a phenomenon known as "paradoxical reactions." The most frequently observed is the development of a drug-induced -like syndrome, characterized by the presence of anti-nuclear antibodies (ANA) and anti-double-stranded DNA (anti-dsDNA) antibodies, sometimes accompanied by clinical symptoms such as rash, serositis, or arthritis [115]. Another notable paradoxical reaction is the induction of , which has been described in patients treated with etanercept or adalimumab, even in the absence of significant muscle involvement, and often resolves upon discontinuation of the drug [116].
Other cutaneous reactions include the development or exacerbation of psoriasis, particularly in patients with inflammatory bowel disease. The pathogenesis of these paradoxical reactions is not fully understood but may involve an imbalance in cytokine networks, such as a shift towards interferon-alpha production, or the induction of apoptosis leading to the release of autoantigens [117].
Neoplastic Risk
The potential association between anti-TNF therapy and an increased risk of malignancy has been a subject of extensive study. The primary concern has been an elevated risk of , particularly non-Hodgkin lymphoma. However, the interpretation of this risk is complicated by the fact that patients with chronic inflammatory diseases like rheumatoid arthritis already have a higher baseline risk of lymphoma compared to the general population [118]. While some studies suggest a modest increase in risk with anti-TNF use, the overall consensus is that the absolute risk remains low, and the benefits of controlling severe inflammation often outweigh this potential risk [119].
A more consistently observed risk is the increased incidence of non-melanoma skin cancers (NMSC), such as basal cell and squamous cell carcinomas. This has led to recommendations for regular dermatological screening and sun protection for patients on long-term anti-TNF therapy [119]. The risk of other solid tumors does not appear to be significantly increased, but ongoing pharmacovigilance is crucial.
Other Adverse Effects and Tolerability
Other common adverse effects are related to the route of administration. Subcutaneous agents like adalimumab and etanercept frequently cause injection site reactions, including pain, erythema, and pruritus [113]. Intravenous agents like infliximab carry a risk of infusion reactions, which can range from mild (fever, chills, headache) to severe anaphylactoid reactions [122]. The development of anti-drug antibodies (ADAbs) is a key factor contributing to both infusion reactions and loss of response to therapy [123].
Rare but serious adverse events include the development of demyelinating disorders of the central nervous system, such as multiple sclerosis or optic neuritis, which are more commonly associated with monoclonal antibodies than with etanercept [117]. There is also a documented risk of exacerbating or unmasking pre-existing heart failure, particularly with infliximab [125].
Clinical Monitoring and Management Strategies
Given these risks, a rigorous monitoring protocol is essential before, during, and after anti-TNF therapy. Pre-treatment screening is mandatory and includes testing for latent tuberculosis using either a tuberculin skin test (TST) or interferon-gamma release assay (IGRA), along with a chest X-ray [84]. Patients with positive tests for latent TB must undergo prophylactic treatment, typically with isoniazid, for at least nine months before starting anti-TNF therapy. Screening for hepatitis B and C, as well as HIV, is also recommended [127].
Vaccination status should be reviewed and updated before initiating therapy, with an emphasis on completing all necessary vaccines, particularly against pneumococcus, influenza, and SARS-CoV-2, while avoiding live vaccines during treatment [105]. Baseline laboratory tests, including a complete blood count, liver function tests, and markers of inflammation (e.g., C-reactive protein, erythrocyte sedimentation rate), are also required.
During treatment, regular clinical assessments every 3–6 months are standard to evaluate disease activity using validated indices such as DAS28 for rheumatoid arthritis or BASDAI for ankylosing spondylitis [129]. Laboratory monitoring for hematological and hepatic toxicity should be performed at regular intervals [130]. Therapeutic Drug Monitoring (TDM), which involves measuring serum drug levels and anti-drug antibodies, is increasingly used to manage patients who lose response to therapy. This allows clinicians to distinguish between pharmacokinetic failure (low drug levels due to clearance or immunogenicity) and pharmacodynamic failure (adequate drug levels but no response), guiding decisions on dose escalation or switching to a different agent [131].
In conclusion, while anti-TNF agents have revolutionized the treatment of chronic inflammatory diseases, their use requires a personalized and vigilant approach to patient management. A thorough pre-treatment evaluation, ongoing clinical and laboratory monitoring, and a proactive strategy for managing adverse events are critical components of safe and effective therapy. The balance between achieving optimal disease control and minimizing the risk of serious complications remains the cornerstone of clinical practice in this field [132].