TNF Receptor Superfamily Member 9
The gene encoding the 4-1BB cell surface glycoprotein (alias CD137) gives rise to a precursor protein of 255 amino acids, with a calculated molecular mass of approximately 28 kDa. Following signal peptide cleavage, the mature protein comprises 232 amino acids; due to multiple N‑glycosylation sites (predominantly Asn138, Asn149, etc.), the observed molecular mass is approximately 30–40 kDa.
4‑1BB is a type I transmembrane protein and a member of the tumour necrosis factor receptor superfamily. Its domain architecture consists of four parts: (i) a signal peptide (1‑23 aa); (ii) an extracellular domain (24‑186 aa), which contains four cysteine‑rich repeats (CRDs) responsible for ligand (4‑1BBL) binding and features internal disulfide bonds; (iii) a transmembrane domain (187‑213 aa, 27 aa); and (iv) an intracellular domain (214‑255 aa, 42 aa), harbouring key functional motifs, including a TRAF‑binding site (the EEEEE motif, approximately at positions 233‑237) that recruits TRAF1/2/3 to mediate NF‑κB and MAPK signalling, as well as potential phosphorylation sites (e.g., on Tyr and Ser residues). Post‑translational modifications of these intracellular motifs are critical for T‑cell co‑stimulation and survival signal transduction.
The overall amino acid sequence homology between human and mouse 4‑1BB proteins is approximately 57%, and this moderate divergence results in a marked absence of cross‑species reactivity. Structural biology studies have revealed that, although both proteins contain four cysteine‑rich domains (CRDs), a key difference lies in the fact that human 4‑1BBL exists as a non‑covalent homotrimer, whereas mouse 4‑1BBL forms a disulfide‑linked dimer via a unique Cys246 residue. This difference fundamentally alters the mechanisms of receptor clustering and downstream TRAF‑mediated signal transduction induced by the respective ligands. Furthermore, mouse 4‑1BB is capable of binding to extracellular matrix components such as laminin, a property not shared by its human counterpart.
4-1BB primarily functions as an inducible co‑stimulatory receptor, with extremely low baseline expression levels under normal physiological conditions. This protein is predominantly distributed on the surface of activated immune cells, including CD4⁺ and CD8⁺ T cells following antigen stimulation, natural killer (NK) cells, regulatory T cells (Tregs), and dendritic cells (DCs).
4‑1BB (CD137/TNFRSF9), a member of the TNF receptor superfamily, has a well‑established biological profile. Its endogenous ligand, 4‑1BBL (TNFSF9/CD137L), is predominantly expressed on activated antigen‑presenting cells (e.g., B cells, macrophages and dendritic cells) and mediates signal transduction through receptor–ligand engagement. The signalling mechanisms are clearly defined, relying primarily on the recruitment of TRAF1/2/3 and cIAP1/2 to activate downstream pathways including NF‑κB, PI3K/AKT, and MAPK (ERK/p38/JNK), which collectively promote T‑cell proliferation, survival, anti‑apoptotic responses, enhanced effector functions, and memory differentiation. In normal tissues, 4‑1BB expression is inducible and largely restricted to activated T cells (CD4⁺ and CD8⁺), NK cells, and a subset of dendritic cells, with little or no constitutive expression on resting immune cells or other tissues, conferring a high degree of tissue selectivity. Its physiological roles are well defined and include enhancing CD8⁺ T‑cell effector functions, promoting anti‑infectious immunity, and modulating inflammatory responses. Under pathological conditions, particularly in the tumour microenvironment, 4‑1BB expression is markedly upregulated on tumour‑infiltrating T cells (including exhausted precursors and regulatory T cells), NK cells, and on some tumour or endothelial cells, exerting a dual role: on one hand, it positively promotes anti‑tumour immunity (e.g., by restoring exhausted T‑cell function, enhancing IFN‑γ secretion and tumour killing); on the other hand, under certain circumstances, it may support regulatory T‑cell survival or promote tumour cell proliferation/metastasis. The functional dependence on this target has moved beyond the exploratory stage, and it has been widely applied in CAR‑T constructs (e.g., the 4‑1BB costimulatory domain in tisagenlecleucel) and in the development of agonistic antibodies, with typical examples including the clinical trials of urelumab and utomilumab, as well as mouse model studies demonstrating that 4‑1BB agonism significantly suppresses tumour growth and synergises with PD‑1 inhibitors.
The core physiological function of 4‑1BB lies in the fine‑tuned regulation of immune responses. Upon binding to its ligand (4‑1BBL), 4‑1BB recruits TNFR‑associated factors (TRAF1/2/3) to activate NF‑κB, PI3K/Akt, and MAPK signalling pathways, which significantly enhance T‑cell clonal expansion and the secretion of pro‑inflammatory cytokines (e.g., IFN‑γ, IL‑2), while effectively inhibiting activation‑induced cell death (AICD), thereby sustaining effective adaptive immune memory. In addition, 4‑1BB is also expressed at low levels in non‑immune cells, such as vascular endothelial cells and haematopoietic progenitor cells, where it participates in regulating cell adhesion and haematopoietic microenvironment homeostasis.
Under pathological (particularly tumour) conditions, the expression profile of 4‑1BB exhibits pronounced spatial heterogeneity and functional complexity. Within the tumour microenvironment (TME), 4‑1BB is significantly upregulated on the surface of tumour‑infiltrating lymphocytes (TILs), especially on exhausted yet still functional CD8⁺ T cells, and has emerged as a key biomarker for identifying tumour‑reactive T cells. However, 4‑1BB can also be ectopically expressed on certain tumour cells, such as those of melanoma, lymphoma, and hepatocellular carcinoma, where it contributes to immune evasion and tumour progression through the inhibition of apoptotic pathways or the induction of epithelial–mesenchymal transition (EMT). In addition, elevated serum levels of soluble 4‑1BB (s4‑1BB) in cancer patients are generally associated with poor prognosis, potentially acting as a 'molecular decoy' that neutralises endogenous 4‑1BBL, thereby impairing anti‑tumour immune surveillance.
LVGN6051 is a humanised IgG1‑subtype agonistic monoclonal antibody targeting 4‑1BB. The core principle of its design is to balance 'intrinsic agonistic potency' with 'FcγR binding affinity' to achieve conditional activation. This antibody possesses weak intrinsic agonistic activity, and its activation is strictly dependent on Fcγ receptor (particularly FcγRIIB)‑mediated crosslinking within the tumour microenvironment (TME). This design is intended to avoid systemic toxicity resulting from overactivation. The antibody exhibits a binding affinity (KD) of approximately 4.56 nM for human 4‑1BB, with high target specificity. Its Fc region retains and optimises binding capacity for FcγRIIB, exploiting cells that abundantly express FcγRIIB in the tumour stroma as scaffolds to induce local clustering and activation of the 4‑1BB receptor at the tumour site.
In vitro experiments were primarily conducted using a HEK293 cell‑based reporter system transfected with human 4‑1BB, as well as human PBMCs isolated from healthy donors. The purpose of the experiments was to verify the necessity of FcγR‑mediated crosslinking for 4‑1BB activation. Key results showed that, in the absence of FcγR crosslinking, LVGN6051 exhibited extremely low agonistic activity; however, upon the addition of cells expressing FcγRIIB (such as CHO‑FcγRIIB cells) or upon anti‑Fc coating, the drug significantly induced NF‑κB signalling and the secretion of IL‑2 and IFN‑γ, with EC₅₀ values showing a clear dose‑dependent manner. Furthermore, in vitro validation confirmed that LVGN6051 does not mediate ADCC effects, thereby protecting 4‑1BB⁺ effector T cells from elimination and ensuring sustained immune activation.
In vivo studies were extensively conducted using h4‑1BB transgenic mice and FcγR‑humanised mouse models. The experimental design encompassed syngeneic transplantation models inoculated with various tumour cell lines, including MC38, CT26, B16‑F10, and Pan02. The dosing regimen typically consisted of 3–10 mg/kg administered twice weekly for 2–3 weeks. Key pharmacodynamic endpoints demonstrated that LVGN6051 as a monotherapy induced significant tumour growth inhibition (TGI), with complete tumour regression observed in some models. Mechanistic analyses confirmed that the drug markedly increased the ratio of CD8⁺ T cells to regulatory T cells (Tregs) within tumour infiltrates and elicited robust tumour‑specific immunological memory. When combined with anti‑PD‑1 antibodies, LVGN6051 exhibited pronounced synergistic efficacy and effectively overcame resistance to anti‑PD‑1 monotherapy.
The 4‑1BB (CD137)‑targeted therapy market is primarily focused on solid tumours and haematological malignancies, including melanoma, non‑small cell lung cancer, and lymphoma. The potential cumulative sales are projected to reach $5 billion by 2034, with a compound annual growth rate (CAGR) of approximately 15% over the period 2025–2034, and major regions (North America and Europe) accounting for over 70% of the market. Driving factors include the unmet need for immune activation, the development of novel mechanisms such as conditional agonists, and the potential for combination with PD‑1/PD‑L1 inhibitors. Representative product peak sales projections, such as those for utomilumab (Pfizer), are estimated at approximately $800 million, while overall market growth is propelled by over 90 clinical‑stage therapies. In the competitive landscape, Pfizer, Bristol Myers Squibb, Agenus, and Adagene are among the dominant players in marketed/clinical‑stage products (none yet approved, with >25 in Phase I/II). Key business development (BD) transactions include the collaboration between Pfizer and BioNTech, as well as AstraZeneca's investment in immune platforms. The competitive landscape encompasses TCEs (e.g., PD‑L1×4‑1BB bispecific antibodies), CAR‑T (optimisation of the 4‑1BB costimulatory domain), and ADC modalities, with technological differentiation emphasising tumour‑targeted bispecifics to reduce hepatotoxicity. Current R&D trends are focused on safety optimisation (e.g., IgG2 isotypes to reduce FcγR crosslinking and conditional activation strategies) and target combinations (e.g., synergy with CTLA‑4). Industrial interest remains high, with investment hotspots shifting towards multimodal immunotherapy and emerging Asia‑Pacific markets.
The clinical positioning of the 4‑1BB target in major indications, including melanoma, non‑small cell lung cancer, and lymphoma, centres on T‑cell co‑stimulatory activation to enhance anti‑tumour immunity. It addresses unmet needs such as immune checkpoint inhibitor (ICI) resistance, insufficient durable responses, and hepatotoxicity risk, offering conditional activation strategies to broaden the therapeutic window. Current dominant therapeutic modalities include T‑cell engagers (TCEs) (e.g., PD‑L1×4‑1BB bispecific antibodies providing tumour‑dependent activation), trispecific antibodies (e.g., CD3×4‑1BB×TAA to enhance T‑cell proliferation), CAR‑T (incorporating the 4‑1BB domain to support memory function), ADCs (conjugating 4‑1BB agonists for targeted delivery), and monoclonal antibodies (mAbs) with IgG4/IgG2 optimisation to reduce FcγR crosslinking. Technological features emphasise Fc engineering and multivalent architectures to balance efficacy and safety. Representative drug pipelines include Pfizer's utomilumab, BMS's urelumab, Agenus's AGEN2373, and Genmab/BioNTech's GEN1046, with no approved products to date; the majority of clinically leading programmes are focused on solid tumours. Major competitors' positioning strategies emphasise combinations with ICIs (e.g., PD‑1 inhibitors), dual/triple‑targeting designs (e.g., CLDN18.2×4‑1BB), and structural innovations (e.g., nanobody fusions). Key business development (BD) events include the Pfizer‑BioNTech collaboration to develop multivalent agonists and Roche's acquisition of a 4‑1BB fusion‑protein platform. Regional competitive dynamics are characterised by US dominance and growing Chinese presence.
To date, among the active 4‑1BB antibody‑based therapeutic pipelines, bispecific antibodies constitute the predominant modality, followed by monoclonal antibodies, trispecific antibodies, trispecific T‑cell engagers (TCEs), and tetraspecific antibodies. At present, no 4‑1BB‑targeting antibody drug has received marketing approval. In terms of target specificity, 4‑1BB‑monospecific agents remain the majority; among dual‑target and multi‑target combinations, the most frequently partnered targets include PD‑L1, CD3, CLDN18.2, as well as other tumour‑associated antigens (TAAs) such as DLL3, EGFR, HER2, FAP, and CD19, in addition to immune checkpoint or co‑stimulatory molecules including PD‑1 and CD40. Regarding indications, the current clinical‑stage 4‑1BB agonist antibody pipelines are heavily concentrated on the treatment of various solid tumours, with a subset of programmes also addressing haematological malignancies.
4‑1BB (TNFRSF9/CD137) is a type I transmembrane member of the TNF receptor superfamily, characterised structurally by an extracellular region containing four cysteine‑rich domains (CRDs). As an inducible T‑cell co‑stimulatory receptor, 4‑1BB recruits TRAF1/2/3 to activate NF‑κB, PI3K/AKT and MAPK signalling cascades, thereby promoting T‑cell proliferation, secretion of IL‑2 and IFN‑γ, anti‑apoptotic effects, and memory T‑cell differentiation. The expression profile of this target is highly inducible, with low baseline levels on activated CD4⁺/CD8⁺ T cells, NK cells, Tregs, and DCs. Within the tumour microenvironment, it is upregulated on exhausted precursor T cells and on certain tumour cells, such as melanoma and hepatocellular carcinoma, whereas its low expression in normal tissues provides a selective therapeutic window.
The core mechanism of 4‑1BB involves enhancing anti‑tumour immunity through 4‑1BBL binding; however, first‑generation agonists such as urelumab triggered severe hepatotoxicity, which prompted the development of second‑generation "conditional activation" strategies. Currently, LVGN6051 and GEN1046 have preliminarily validated both the safety and synergistic efficacy of local activation in clinical settings. 4‑1BB‑targeted therapies have evolved from early agonistic monoclonal antibodies to tumour‑targeted bi‑/trispecific antibodies and ligand‑fusion proteins. Furthermore, its intracellular domain has become a core costimulatory component of second‑generation CAR‑T constructs, establishing it as a standard in cellular therapy. Representative products include Pfizer's utomilumab, Eli Lilly's Exlinkibart/LVGN6051, and Genmab/BioNTech's Acasunlimab, among others.