The Impact of 4-1BB in Cancer Immunotherapy
What is 4-1BB？
4-1BB, a vital glycoprotein receptor belonging to the tumor necrosis factor superfamily (TNFRSF), was first identified by Kwon et al. in 1989. This receptor protein was found on activated cytolytic and helper T lymphocytes[1]. Subsequent research by Pollok et al. shed light on 4-1BB's nature as an inducible cell surface receptor. It becomes expressed in the presence of activating stimuli, playing a crucial role in cell signaling during T cell activation and proliferation[2].
The natural counterpart to 4-1BB, 4-1BBL (CD137L/TNFSF9), was later discovered by Goodwin et al. Their work highlighted that the interaction between 4-1BB and its ligand results in thymocyte and splenic T cell proliferation, underlining its significance in the body's immune response[3]. Subsequent research by Hurtado et al. demonstrated that blocking the 4-1BB/4-1BBL interaction with antibodies inhibits T cell proliferation and activation, suggesting a promising avenue for immunotherapy development[4].
Further studies have shown that antibodies targeting 4-1BB can provide essential costimulatory signals to T cells, mimicking the natural agonistic effect of endogenous 4-1BBL[5].
The Structure and Function of 4-1BB
4-1BB, classified as a type I transmembrane glycoprotein, exhibits a distinctive structure comprising four cysteine-rich domains (CRDs) in its extracellular domain, a helical transmembrane domain, and a cytoplasmic signaling domain[6]. In contrast, 4-1BBL, a type II transmembrane protein, features an extracellular domain forming a homotrimer resembling an extended, three-bladed propeller structure. When they interact, the resulting 4-1BB/4-1BBL complex adopts a structure akin to established TNF receptor-ligand complexes. In this arrangement, the ligand trimer takes on a bell shape, while the three monomeric receptors encircle the ligand trimer, forming a hetero-hexamer signaling axis[7].
Upon binding with 4-1BBL and subsequent activation, 4-1BB signaling primarily involves the recruitment of TNFR-associated factors 1 and 2 (TRAF1 and TRAF2) to the TRAF-binding motif in the 4-1BB cytoplasmic tail[8-10]. These TRAF proteins act as scaffold proteins, activating downstream effectors in diverse signaling cascades, leading to various cellular responses. While the involvement of TRAF1 and TRAF2 in the 4-1BB signalosome is well-established, TRAF3's role is suggestive and requires further investigation, particularly in cell types beyond 4-1BB-based chimeric antigen receptor (CAR) T cells[11].
4-1BB expression is mostly transient and activation-induced, notably in activated T and NK cells. Resting T cells typically lack 4-1BB expression; however, it is transiently induced upon T cell receptor (TCR) stimulation and CD3 signaling[12]. Upon ligand binding, the 4-1BB/4-1BBL interaction delivers a costimulatory signal via TRAF1 and TRAF2 trimers, which recruit ubiquitin ligases cIAP1/2. These ligases activate downstream signaling pathways, including nuclear factor kappa B (NF-κB), extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAPK), and c-Jun N-terminal kinase (JNK). Activation of these pathways augments CD4+ and CD8+ T cell proliferation, differentiation, and effector functions. Moreover, 4-1BB signaling induces the expression of anti-apoptotic proteins Bcl-2-related protein A1 (Bfl-1) and B cell lymphoma-extra-large (Bcl-xL) in CD8+ T cells, enhancing cell survival through the transcription factor NF-κB. Another survival-promoting mechanism of 4-1BB signaling involves the downregulation of pro-apoptotic Bcl-2-like protein 11 (Bim), downstream of the ERK pathway. It is noteworthy that this costimulatory function is predominantly observed in CD8+ T cells compared to CD4+ T cells[13-14].
Fig.1 Overall Structure of the 4-1BB/4-1BBL Complex and the CRDs of 4-1BB.[15]
Role of 4-1BB in the Tumor Microenvironment
Both IL-15 and IL-2 play pivotal roles in promoting 4-1BB expression on NK cells, triggering NK cell proliferation and IFN-γ production. This activation subsequently leads to T cell activation[16]. Furthermore, 4-1BB supports the proliferation of CD8+ T cells, generating memory T (Tm) cells[17]. Stimulation by 4-1BB increases IL-2 and IFN-γ expression in both CD4+ and CD8+ T cells. However, the effect of 4-1BB on T regulatory cells (Treg) is controversial, as it promotes Treg proliferation while altering their functionality toward cytotoxic or helper effects. Notably, 4-1BBL inhibits the conversion of CD4+FOXP3- cells to CD4+FOXP3+[18].
In monocytes, 4-1BB expression leads to the upregulation of IL-8 and TNF-α while downregulating IL-10. Additionally, 4-1BB promotes monocyte differentiation into dendritic cells, which subsequently secrete IL-6 and IL-12. However, 4-1BB stimulation differentiates monocytes into M2 macrophages and accelerates B-cell apoptosis, consequently enhancing the expression of TNF-α/β in B cells[19].
4-1BB in Cancer Progression
In Hodgkin and Reed–Sternberg cells, the EBV protein LMP1 induces 4-1BB expression via the PI3K/AKT/mTOR pathway, facilitating immune evasion[20]. In patients with AML, low levels of soluble 4-1BBL correlate with a better prognosis, notably longer disease-free survival. Abnormal expression of both 4-1BB L and 4-1BB in tumor cells in hematopoietic malignancies promotes tumor growth in cutaneous T-cell lymphoma[21]. Leukemic cells overexpressing 4-1BB are significantly associated with a poor prognosis. Studies in 4-1BB-knockout mice have shown enhanced anti-tumor activity, and in mice injected with CT26 cells, tumor growth was significantly inhibited in 4-1BB knockout mice, highlighting the crucial role of 4-1BB-4-1BBL interaction in tumor development[22].
Signaling Pathway of 4-1BB
Upon binding with its ligand, 4-1BB (CD137L), the 4-1BB signaling pathway is initiated, playing a pivotal role in the immune system. This pathway serves as a crucial regulator, enhancing the survival, proliferation, and effector functions of both T cells and NK cells. Furthermore, it boosts the activity of antigen-specific T cells, prompting increased cytokine secretion, heightened cytotoxicity, and improved tumor cell-killing capabilities.
In the process of signal transduction, 4-1BB activates multiple signaling pathways, including the NF-κB (nuclear factor-kB) pathway, the MAPK (mitogen-activated protein kinase) pathway, and the PI3K (phosphoinositide 3-kinase) pathway. Activation of these pathways leads to the phosphorylation and activation of intracellular signaling molecules. Ultimately, this cascade profoundly influences cellular survival, proliferation, differentiation, and cytokine secretion, orchestrating a sophisticated response within the immune system.
Fig.2 4-1BB Expression and Signaling.[23]
4-1BB Protein
Recombinant Human 4-1BB Protein (C-6His)
Synonym : CD137; ILA; TNFRSF9; 4-1BB ligand receptor; CDw137; T-cell antigen 4-1BB homolog; T-cell antigen ILA
Recombinant Human 4-1BBL Protein
Synonym : 4 1BB L 4 1BB ligand 4 1BBL 4-1BB ligand 4-1BBL Cd137l Cd157l Homolog of mouse 4 1BB L Homolog of mouse 4 1BBL ILA ligand (TNF related) Ly63l Receptor 4 1BB ligand TNF superfamily member 9 TNFL9_HUMAN Tnfsf9 TNLG5A Tumor necrosis factor (ligand) superfamily member 9 Tumor necrosis factor ligand 5A Tumor necrosis factor ligand superfamily member 9 Tumor necrosis factor superfamily member 9
References:
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