Introduction
The tumor necrosis factor superfamily (TNFSF) and their corresponding receptors (TNFRSF) play pivotal roles in regulating the immune system. These ligands and receptors facilitate crucial processes such as cell proliferation, survival, differentiation, and immune cell function through their interactions. The TNFSF comprises 19 ligands, while the TNFRSF includes 29 receptors, forming a diverse and extensive family.
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TNFSF ligands are characterized by type II proteins with a C-terminal TNF homology domain (THD) responsible for ligand trimerization and receptor binding. Conversely, TNFRSF receptors exhibit between one to six cysteine-rich domains (CRD) in their extracellular region, facilitating ligand binding and receptor auto-association. TNFRs can be classified into three groups based on functional and structural disparities: death domain (DD) containing receptors, decoy receptors, and TNF receptor-associated factor (TRAF) binding receptors. DD-containing receptors, housing an 80 amino acid cytoplasmic tail DD, primarily initiate cell death signaling but also participate in other signaling pathways like NF-κB. Decoy receptors lack signal initiation ability and include glycosylphosphatidylinositol (GPI) tethered receptors, soluble receptors, and receptors with non-functional DDs. TRAF binding receptors possess TRAF-interacting motifs (TIF) in their cytoplasmic tail, recruiting TRAFs for downstream signaling upon receptor activation.
Fig. 1 Classification of TNFRSF (Dadas O., et al. 2023).
Upon cell surface expression, certain TNFRSF members self-associate into dimers or multimers prior to ligand binding. While some form covalently linked dimers (e.g., CD27), others rely on the pre-ligand assembly domain (PLAD) covering the N-terminal CRD1, with exceptions like GITR, which dimerizes through interactions within CRD3.
TNFSF ligands exist in soluble or membrane-bound forms. While some TNFRSF members can be activated by soluble ligand trimers (category I), others necessitate interaction with membrane-bound ligands for full activation (category II). For instance, soluble TNFα binds TNFR1 with higher affinity, predominantly activating TNFR1 signaling, while TNFR2 is mainly activated by membrane-bound ligands. CD40 and GITR activation is augmented with higher valency ligands or cross-linking of trimeric molecules, possibly inducing higher-order clustering. Conversely, CD27 and 4-1BB show minimal activation and require higher-order clustering. Activation of TNFRSF members elicits diverse cellular responses including proliferation, survival, and differentiation, with potential therapeutic implications.
Targeting TNFRSF for Enhanced T-Cell Responses
T-cell activation hinges not only on the interaction between the T-cell receptor (TCR) and peptide-MHC (major histocompatibility complex) but also on co-stimulatory signaling, ensuring a robust immune response. TNFRSF (Tumor Necrosis Factor Receptor Superfamily) members like CD27, OX40, 4-1BB, TNFR2, and GITR are vital in this process, expressed on T cells. Equally crucial are co-stimulatory receptors on antigen-presenting cells (APCs) such as CD40, pivotal in activating dendritic cells (DCs) and B lymphocytes for effective adaptive immunity. However, within the tumor microenvironment, DCs might be excluded, hampering T-cell responses due to immunosuppression. Nevertheless, DCs, upon maturation, upregulate TNFSF ligands necessary for optimal T-cell co-stimulation. Hence, targeting TNFRSF for co-stimulation emerges as an attractive strategy to enhance T-cell responses.
After TCR activation, co-stimulatory receptors are upregulated, facilitating engagement with their ligands. Despite diverse downstream signaling pathways, activation mainly involves TRAF recruitment to their cytoplasmic tails, leading to NF-κB and JNK pathway activation. Stimulation of these receptors augments T-cell effector function and survival. For instance, CD27, OX40, and GITR stimulation upregulates cytokines crucial for T-cell activation and survival. Additionally, these signals influence T-cell differentiation into cytotoxic T lymphocytes (CTLs) and memory T cells, crucial for sustained immune surveillance.
Furthermore, deficiencies in TNFRSF members correlate with immunodeficiency and various pathologies, emphasizing their role in immune regulation. In cancer immunotherapy, targeting these receptors has gained traction, aiming to modulate immune responses effectively. Notably, agonistic approaches activating these receptors have shown promise in cancer immunotherapy. Thus, understanding the rationale behind targeting TNFRSF members offers insights into enhancing immunotherapeutic strategies, particularly in cancer treatment.
Targeting the TNFRSF for Therapeutic Purposes
In pre-clinical tumor models, stimulating certain members of the TNFRSF has proven effective. For instance, targeting 4-1BB in liver cancer, squamous cell cancer, colorectal cancer, and lymphoma using monoclonal antibodies (mAb) or recombinant 4-1BBL has triggered robust anti-tumor responses. Studies have shown that anti-4-1BB antibodies induce such responses by stimulating effector T cells and depleting T regulatory (Treg) cells. Interestingly, depleting Tregs before stimulating effector T cells has shown superior results compared to either approach alone. Similarly, targeting OX40 or GITR has led to robust anti-tumor responses through similar mechanisms, either by stimulating effector T cells or depleting Tregs. Notably, the mechanism of action may vary depending on the tumor model. Additionally, targeting CD27 has shown significant anti-tumor responses in pre-clinical models, potentially by either agonizing effector cells or depleting Tregs, depending on the expression level on different cell populations.
Moreover, targeting TNFR2 has emerged as a promising therapeutic strategy. TNFR2, expressed at high levels on immune cells including Tregs, is crucial for their survival. While initially considered for Treg depletion to boost effector T-cell responses, recent studies have shown that agonistic targeting of TNFR2 can stimulate the expansion of tumor-specific CD8+ T cells with improved effector function.
Antibody targeting has been the primary method for TNFRSF targeting, with monoclonal antibodies (mAb) used extensively. These antibodies achieve optimal activation by engaging FcγR, inducing receptor clustering. However, the extent of agonism can depend on various factors, including antibody isotype, subclass, and domain of receptor binding. For instance, antibodies binding to specific domains of the receptor may induce higher agonistic activity.
Clinical trials targeting co-stimulatory members of the TNFRSF have yielded mixed results in terms of tolerability and efficacy. For instance, targeting CD27 with Varlilumab has shown promising clinical efficacy against various tumors, while targeting 4-1BB with Utomilumab or Urelumab has demonstrated modest clinical effects. Similarly, targeting CD40 with CP870,893 or CDX1140 has shown modest clinical effects, especially in combination with other agents. Targeting OX40 and GITR has shown potential in pre-clinical studies but has had limited clinical success so far.
In summary, while targeting TNFRSF members shows promise for cancer immunotherapy, optimizing the approach for maximum efficacy and minimal toxicity remains a challenge. Further research and clinical trials are needed to fully realize the therapeutic potential of TNFRSF targeting in cancer treatment.
Reference
- Dadas O., et al. Delivering co-stimulatory tumor necrosis factor receptor agonism for cancer immunotherapy: past, current and future perspectives. Frontiers in Immunology. 2023, 14: 1147467.
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