4-1BB Ligand in Immune Response and Cancer Research

The immune system relies on a complex network of signals to regulate responses against infections and abnormal cell growth. Among these, 4-1BB ligand (4-1BBL) plays a key role in immune activation, particularly through interactions with T cells. Its function has gained attention in cancer immunotherapy, where modulating immune pathways can influence tumor progression.

Research into 4-1BBL provides insights into fine-tuning immune responses for therapeutic purposes. Understanding its role in immune signaling is crucial for developing targeted treatments in oncology and other diseases.

Expression And Structural Features

4-1BBL is a type II transmembrane glycoprotein in the tumor necrosis factor (TNF) superfamily. It is primarily expressed on antigen-presenting cells like dendritic cells, macrophages, and B cells, though it can also appear on activated T cells and certain non-hematopoietic cells under specific conditions. Its expression is tightly regulated and typically induced following immune activation, preventing excessive immune responses. Unlike constitutively expressed TNF superfamily members, 4-1BBL is transiently upregulated in response to inflammatory stimuli.

Structurally, 4-1BBL consists of an extracellular TNF homology domain for receptor binding, a transmembrane region anchoring it to the cell surface, and a short cytoplasmic tail with minimal signaling capacity. The extracellular domain forms a homotrimeric structure, a characteristic of TNF family ligands, essential for high-affinity binding to its receptor, 4-1BB (CD137). This trimeric arrangement facilitates receptor clustering, a prerequisite for downstream signaling. X-ray crystallography studies have shown that 4-1BBL adopts a jelly-roll β-sandwich fold, similar to other TNF ligands, providing a stable framework for receptor engagement.

Post-translational modifications influence 4-1BBL’s function. Glycosylation stabilizes its structure and modulates receptor interactions, while proteolytic cleavage generates a soluble form of the ligand. Though less potent than the membrane-bound version, the soluble variant can still engage 4-1BB and affect signaling dynamics. The balance between these forms is an active area of research with potential therapeutic implications.

Signaling Mechanisms

Upon binding to 4-1BB (CD137), 4-1BBL triggers intracellular signaling cascades that regulate cellular function. Unlike receptor tyrosine kinases, which have intrinsic enzymatic activity, 4-1BB relies on adaptor proteins to propagate signals. The recruitment of TNF receptor-associated factors (TRAFs), particularly TRAF1 and TRAF2, is central to 4-1BB signaling. These adaptor proteins link the receptor to downstream kinases, enabling the activation of multiple pathways. TRAF2, in particular, facilitates interactions with ubiquitin ligases that regulate signal activation and duration.

A primary pathway activated by 4-1BB signaling is the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway. TRAF2 recruits and activates the inhibitor of kappa B kinase (IKK) complex, leading to the phosphorylation and degradation of inhibitor of kappa B (IκB). This releases NF-κB dimers, allowing them to enter the nucleus and promote transcription of genes involved in survival, proliferation, and metabolism. Negative feedback mechanisms, including deubiquitinating enzymes, regulate NF-κB activation to prevent excessive signaling. Dysregulation of this pathway has been linked to inflammatory disorders and malignancies.

4-1BB signaling also engages the phosphoinositide 3-kinase (PI3K)-Akt pathway, promoting cell survival and metabolic adaptation. TRAF2 recruitment activates PI3K, leading to Akt phosphorylation, which supports glycolytic metabolism and mitochondrial function. The mammalian target of rapamycin (mTOR) complex is also influenced by this pathway, coordinating protein synthesis and autophagy. Given its broad impact, targeting PI3K-Akt signaling is being explored for therapeutic modulation of 4-1BB-driven responses.

Additionally, 4-1BB influences mitogen-activated protein kinase (MAPK) signaling. TRAF-mediated activation of transforming growth factor-beta-activated kinase 1 (TAK1) leads to phosphorylation of MAPK family members, including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. These kinases regulate gene expression linked to differentiation, stress responses, and apoptosis. The balance between survival and apoptotic signals within this pathway depends on cellular context, with ERK favoring proliferation and JNK/p38 contributing to stress adaptation. The integration of MAPK with NF-κB and PI3K-Akt pathways highlights the complexity of 4-1BB-mediated signaling.

Interactions With T Cells And Other Leukocytes

4-1BBL engagement with 4-1BB (CD137) significantly impacts T cell activity, particularly in CD8⁺ T cells. When antigen-presenting cells expressing 4-1BBL interact with CD8⁺ T cells, they enhance proliferation, survival, and cytokine production. This interaction expands effector T cells and supports long-lived memory populations. 4-1BB signaling also upregulates anti-apoptotic proteins like Bcl-xL, reinforcing T cell persistence even in low-antigen environments. Metabolic shifts toward oxidative phosphorylation and glycolysis further sustain T cell function under changing energy demands.

CD4⁺ T cells also respond to 4-1BBL, though effects vary by subset. T helper 1 (Th1) and T follicular helper (Tfh) cells exhibit increased cytokine secretion and enhanced differentiation upon 4-1BB engagement. Regulatory T cells (Tregs) show mixed responses, with some evidence suggesting 4-1BB signaling suppresses their immunosuppressive activity, while other studies indicate it stabilizes their function in specific conditions.

Beyond T cells, 4-1BBL interactions extend to other leukocyte populations. Natural killer (NK) cells, which target infected or transformed cells, show enhanced cytotoxicity and interferon-gamma (IFN-γ) production with 4-1BB stimulation, relevant for tumor clearance. Dendritic cells and macrophages expressing 4-1BBL provide co-stimulatory signals and receive reciprocal activation through reverse signaling, influencing cytokine secretion and antigen presentation.

Tumor Microenvironment

The tumor microenvironment (TME) is a dynamic landscape affecting cancer progression, therapeutic resistance, and metastasis. Within this environment, 4-1BBL modulates cellular interactions and signaling that shape tumor behavior. Tumor cells, stromal fibroblasts, and infiltrating immune populations contribute to 4-1BBL expression, creating a complex signaling network that can either suppress or promote tumor growth. The spatial distribution of 4-1BBL within the TME determines its impact, as localized expression on stromal or immune cells may lead to distinct signaling outcomes compared to soluble forms circulating in the tumor milieu.

Hypoxic conditions and metabolic constraints in the TME influence 4-1BBL function, altering its signaling dynamics. Low oxygen levels, a hallmark of aggressive tumors, have been linked to changes in 4-1BBL interactions, particularly through stabilization of hypoxia-inducible factors (HIFs) that regulate gene transcription. These adaptations affect tumor metabolism, angiogenesis, and extracellular matrix remodeling, contributing to immune evasion. Targeting these pathways may offer therapeutic opportunities in tumors where 4-1BBL signaling is dysregulated.

Experimental Observations In Oncology

Preclinical and clinical studies highlight 4-1BBL’s potential in cancer treatment. Mouse models of melanoma, lymphoma, and colorectal cancer show that 4-1BB agonism enhances tumor-specific CD8⁺ T cell expansion and persistence, improving tumor clearance. Tumors engineered to express 4-1BBL exhibit increased cytotoxic lymphocyte infiltration, correlating with delayed progression and extended survival. These findings suggest that targeting 4-1BB signaling could strengthen antitumor immunity, particularly in immunosuppressive cancers.

Clinical trials have explored 4-1BB-targeting therapies, including agonistic antibodies and engineered cell therapies. Urelumab, a monoclonal antibody targeting 4-1BB, showed strong T cell activation in early trials but was associated with dose-dependent hepatotoxicity, highlighting challenges in balancing immune stimulation with safety. Another antibody, Utomilumab, had a better safety profile but limited efficacy as a monotherapy, prompting combination strategies with checkpoint inhibitors like anti-PD-1 or anti-CTLA-4 agents. These studies underscore the need for further optimization to maximize therapeutic benefits while minimizing toxicity.