Recombinant Human Tumor necrosis factor ligand superfamily member 9 (TNFSF9)

What is TNFSF9 and how is it structurally characterized?

TNFSF9, also known as 4-1BB ligand or CD137 ligand, is a type II glycoprotein with a molecular mass of approximately 34 kDa. It was identified as the ligand of TNFRSF9 (CD137, 4-1BB) through an expression cloning approach . The protein belongs to the tumor necrosis factor superfamily, characterized by its trimeric structure and type II transmembrane topology (intracellular N-terminus and extracellular C-terminus). The extracellular domain contains the TNF homology domain that mediates receptor binding and biological activity.

The human TNFSF9 gene is located on chromosome 19p13.3 and encodes a 254-amino acid protein. The protein contains a short cytoplasmic domain (N-terminal), a transmembrane region, and a larger extracellular domain (C-terminal). Unlike many proteins that require post-translational modifications for function, TNFSF9 produced in prokaryotic expression systems generally retains its biological activity, suggesting that glycosylation is not critical for its receptor-binding capabilities.

What is the expression pattern of TNFSF9 in normal and pathological conditions?

TNFSF9 displays a distinct expression pattern across various cell types in both normal physiology and pathological states. Under normal conditions, TNFSF9 is expressed on the surface of activated T cells, macrophages, monocytes, dendritic cells, B cells, and B lymphomas . The expression is typically inducible rather than constitutive, responding to cellular activation signals such as antigen recognition or inflammatory stimuli.

In pathological conditions, particularly in cancer microenvironments, TNFSF9 expression patterns may be altered. Research indicates that TNFSF9 expression in renal cell carcinoma (RCC) correlates with increased immune cell infiltration and may predict responsiveness to immune checkpoint inhibitor therapy . Specifically, high TNFSF9 expression is associated with significantly higher immune scores and ESTIMATE scores in kidney renal clear cell carcinoma (KIRC), indicating greater immune cell infiltration in these tumors . Contrastingly, in pancreatic cancer, TNFSF9 expression has been negatively correlated with CD8+ T cell infiltration, highlighting tissue-specific differences in TNFSF9's immunomodulatory effects .

How does TNFSF9 interact with its receptor TNFRSF9, and what are the functional consequences?

The interaction between TNFSF9 and TNFRSF9 represents a complex bidirectional signaling system with diverse immunological effects. When TNFSF9 engages with TNFRSF9 (CD137) expressed on T cells, it provides costimulatory signals that enhance T cell activation, proliferation, survival, and effector functions . This forward signaling occurs through the recruitment of tumor necrosis factor receptor-associated factors (TRAFs), particularly TRAF1 and TRAF2, which activate downstream pathways including NF-κB, PI3K/Akt, and MAP kinases .

Importantly, TNFSF9-TNFRSF9 interaction also initiates reverse signaling, where TNFRSF9 acts as a ligand that triggers signaling cascades through TNFSF9 on antigen-presenting cells (APCs) . This reverse signaling enhances the antigen-presenting capacity of APCs, creating a feedback loop that amplifies immune responses. The reverse signaling pathways in human cells involve multiple kinases including Src tyrosine kinase, PI3K, p38 MAPK, ERK1/2, and can activate NF-κB .

The functional consequences of these interactions are context-dependent and sometimes paradoxical. While TNFRSF9 activation generally provides strong survival signals to T cells, it can also inhibit CD4+ T-cell responses in autoimmune disease models and B-cell responses in vivo . Conversely, TNFSF9 deficiency has been shown to lower CD8+ T-cell responses in virus infection models and reduce cytotoxic T lymphocyte activity against vesicular stomatitis virus .

What are the molecular mechanisms of TNFSF9-mediated reverse signaling, and how do they differ between human and murine models?

TNFSF9-mediated reverse signaling represents one of the more complex and less understood aspects of TNFSF9 biology. In human cells, TNFSF9 reverse signaling involves multiple kinase pathways that collectively modulate immune cell function. Upon engagement with TNFRSF9, TNFSF9 initiates signaling cascades involving Src tyrosine kinase, phosphatidylinositol 3-kinase (PI3K), p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinases 1/2 (ERK1/2), and nuclear factor-κB (NF-κB) . These pathways ultimately regulate gene expression, cellular activation, cytokine production, and APC function.

The experimental approach to study these differences typically involves comparative phosphoproteomic analysis after TNFSF9 crosslinking in human and murine cells, followed by pathway inhibition studies to validate key signaling nodes. Despite these advances, the specific associations between signaling components remain incompletely understood, presenting an active area for future research.

How does TNFSF9-TNFRSF9 bidirectional signaling shape the tumor immune microenvironment?

The bidirectional signaling between TNFSF9 and TNFRSF9 profoundly influences the tumor immune microenvironment (TIME) through multiple mechanisms affecting both innate and adaptive immunity. In tumors with high TNFSF9 expression, transcriptomic analysis reveals upregulation of pathways related to adaptive immune responses, response to bacteria, and extracellular matrix organization . This correlates with increased infiltration of B cells, CD8+ T cells, and myeloid dendritic cells, indicating enhanced antigen presentation and immune recognition .

The analysis of T cell populations in TNFSF9-high tumors reveals a complex landscape. While these tumors show significantly more CD8+ T cells and higher levels of interferon-gamma, indicative of active anti-tumor immunity, they also contain more dysfunctional T cells and regulatory T cells (Tregs) . This paradoxical increase in both effector and regulatory immune components suggests that TNFSF9-high tumors exist in a state of immune recognition counterbalanced by immunosuppression.

Particularly noteworthy is the observed increase in T follicular helper cells and plasma B cells in TNFSF9-high tumors, suggesting enhanced T-B cell interactions leading to humoral immunity . The following table summarizes the key immune cell populations associated with high TNFSF9 expression in renal cell carcinoma:

These findings suggest that tumors with high TNFSF9 expression may respond better to combination immunotherapies that target both the PD-1/PD-L1 and CTLA-4 pathways, as observed in metastatic renal cell carcinoma patients treated with nivolumab and ipilimumab .

What experimental approaches are most effective for studying TNFRSF9-TNFSF9 interactions in different cellular contexts?

Understanding TNFRSF9-TNFSF9 interactions requires a multi-modal experimental approach that captures both structural binding and functional outcomes across different cell types. For protein-protein interaction studies, surface plasmon resonance (SPR) and biolayer interferometry (BLI) provide quantitative binding kinetics between recombinant TNFSF9 and TNFRSF9 proteins. These approaches can be complemented by co-immunoprecipitation assays in cellular systems to verify interactions in a more physiological context.

For functional studies, the use of agonistic anti-TNFRSF9 antibodies or recombinant TNFSF9 in cell culture models allows for the assessment of downstream signaling activation. Phosphoproteomic analysis can then identify the key signaling nodes activated in different cell types. RNA-sequencing following TNFRSF9 stimulation provides insights into transcriptional changes, while CRISPR-Cas9 knockout or knockdown approaches help establish the necessity of specific signaling components.

The context-dependent effects of TNFSF9-TNFRSF9 interactions are best studied in co-culture systems that recapitulate the cellular diversity of the immune microenvironment. For instance, co-cultures of dendritic cells and T cells can reveal how TNFSF9 reverse signaling in APCs influences T cell activation. These in vitro findings should be validated in vivo using conditional knockout mouse models that allow cell type-specific deletion of either TNFSF9 or TNFRSF9.

Given the emerging therapeutic potential of this pathway, the development of bispecific antibodies or engineered TNFSF9 variants with altered receptor-binding properties represents an innovative approach to modulate TNFSF9-TNFRSF9 signaling for therapeutic purposes.

How does TNFSF9 expression correlate with response to immune checkpoint inhibitor therapy?

Recent research has identified TNFSF9 expression as a potential predictive biomarker for response to combination immune checkpoint inhibitor therapy, particularly in metastatic renal cell carcinoma (mRCC). A study examining tumor microenvironment-related gene expression in mRCC patients treated with nivolumab and ipilimumab (NIVO + IPI) found that TNFSF9 expression was significantly upregulated in the responder group compared to non-responders . Remarkably, TNFSF9 expression discriminated between response and non-response groups with 88.89% sensitivity and 87.50% specificity (AUC = 0.9444) , outperforming PD-L1 expression (AUC = 0.75) as a predictive biomarker.

The mechanistic basis for this correlation may lie in the immune landscape of TNFSF9-high tumors. These tumors are characterized by higher immune scores and greater infiltration of immune cells, particularly CD8+ T cells, B cells, and myeloid dendritic cells . Despite these favorable immune features, TNFSF9-high tumors also show increased regulatory T cells and dysfunctional T cells, potentially explaining why they might be especially responsive to combination therapy targeting both PD-1 (nivolumab) and CTLA-4 (ipilimumab) pathways .

Interestingly, the TIDE (Tumor Immune Dysfunction and Exclusion) scores for TNFSF9-high tumors suggest that these patients might be less likely to respond to immune checkpoint inhibitor monotherapy . This indicates that TNFSF9 expression might specifically predict response to combination immunotherapy approaches rather than single-agent treatments, highlighting the importance of targeting multiple immunosuppressive mechanisms in these tumors.

What are the experimental considerations when developing TNFSF9-targeting therapeutics for cancer?

Developing TNFSF9-targeting therapeutics requires careful consideration of its complex biology and potential for both stimulatory and inhibitory effects on immune responses. When designing experiments to evaluate TNFSF9-targeted approaches, researchers should consider several key factors:

First, the bidirectional nature of TNFSF9-TNFRSF9 signaling necessitates careful assessment of effects on both the cells expressing TNFSF9 and those expressing TNFRSF9. Experiments should evaluate how TNFSF9 modulation affects not only T cell activation but also antigen-presenting cell function. Flow cytometric analysis of immune cell activation markers, cytokine production assays, and functional assays like T cell proliferation and cytotoxicity are essential components of a comprehensive assessment.

Second, since TNFSF9 effects are context-dependent, experiments should incorporate relevant tumor microenvironment factors. Co-culture systems that include tumor cells, T cells, and antigen-presenting cells more accurately model the complex interactions in vivo. Three-dimensional organoid cultures or humanized mouse models may provide even more physiologically relevant systems for evaluating TNFSF9-targeted approaches.

Third, combination approaches should be systematically evaluated, as TNFSF9-high tumors may respond better to dual checkpoint blockade. Experimental designs should include comparative arms testing TNFSF9-targeting agents alone and in combination with established immunotherapies like PD-1, PD-L1, or CTLA-4 inhibitors. The table below outlines potential experimental approaches:

Finally, researchers should be aware of potential off-target effects and toxicities. TNFRSF9 agonists have shown hepatotoxicity in clinical trials, likely due to TNFRSF9 expression on liver-resident Kupffer cells. Safety evaluations should include comprehensive analysis of liver function and systemic cytokine levels.

How do TNFSF9 agonists synergize with other immunotherapeutic approaches?

The synergistic potential of TNFSF9 agonists with other immunotherapeutic approaches stems from their complementary mechanisms of action. Recent research indicates that combining TNFRSF9 agonists with PD-L1 inhibitors increases anti-tumor activity , highlighting the value of simultaneous targeting of multiple immune regulatory pathways. This synergy can be explained by several mechanistic principles that should inform experimental design and clinical development.

First, TNFRSF9 agonism primarily provides costimulatory signals to T cells that have recognized tumor antigens, enhancing their survival, proliferation, and effector functions through the upregulation of anti-apoptotic proteins like Bcl-2, Bcl-XL, and Bfl-1 . This mechanism fundamentally differs from checkpoint inhibitors like PD-1/PD-L1 or CTLA-4 blockers, which primarily remove inhibitory signals rather than providing positive stimulation. The combination thus addresses two distinct aspects of T cell regulation: removing the brakes (checkpoint inhibitors) while simultaneously stepping on the accelerator (TNFRSF9 agonists).

Second, the reverse signaling through TNFSF9 on antigen-presenting cells enhances their function, potentially improving tumor antigen presentation and initiating more effective primary immune responses. This effect may be particularly important in immunologically "cold" tumors where antigen presentation is suboptimal. Experimental models should assess dendritic cell maturation and function when evaluating combination approaches.

Third, research in renal cell carcinoma has shown that tumors with high TNFSF9 expression contain more regulatory T cells despite increased CD8+ T cell infiltration . This suggests that TNFSF9-high tumors may exist in a state of immune recognition counterbalanced by immunosuppression. In this context, CTLA-4 inhibitors like ipilimumab, which can deplete regulatory T cells within the tumor microenvironment, may be particularly synergistic with TNFRSF9 agonism.

The optimal sequencing and dosing of combination therapies remain critical research questions. Sequential administration (e.g., checkpoint inhibitor followed by TNFRSF9 agonist) might prove more effective than concurrent administration by first releasing T cells from inhibition before providing costimulatory signals. Rigorous preclinical studies comparing different combination strategies and schedules are essential for maximizing therapeutic efficacy while minimizing toxicity.

What is the paradoxical role of TNFSF9 in different autoimmune diseases?

The role of TNFSF9 in autoimmune diseases exhibits remarkable complexity and disease-specific effects, creating seemingly paradoxical outcomes across different conditions. Research indicates that disruption of TNFRSF9/TNFSF9 signaling ameliorates the severity of certain autoimmune conditions while exacerbating others . This paradoxical behavior presents both challenges and opportunities for therapeutic development.

In herpetic stromal keratitis (HSK) and rheumatoid arthritis (RA), inhibition of TNFRSF9/TNFSF9 signaling reduces disease severity . The mechanism likely involves suppression of pathological T cell responses that drive inflammation in these conditions. TNFRSF9 agonists in these contexts may enhance effector T cell activity and exacerbate disease progression. Experimental models of RA have demonstrated that blocking TNFRSF9-TNFSF9 interactions reduces joint inflammation and bone erosion by limiting the expansion and activation of autoreactive T cells.

Conversely, in lacrimal gland disease and systemic lupus erythematosus (SLE), disruption of TNFRSF9/TNFSF9 signaling increases disease severity . This unexpected effect may relate to the role of TNFRSF9 in supporting regulatory T cell function or in promoting activation-induced cell death of autoreactive lymphocytes in these specific disease contexts. For SLE, experimental evidence suggests that TNFRSF9 signaling may be important for maintaining tolerance to nuclear antigens, with its absence leading to enhanced autoantibody production.

What methodological approaches can effectively distinguish the pro-inflammatory versus regulatory functions of TNFSF9?

Distinguishing between the pro-inflammatory and regulatory functions of TNFSF9 requires sophisticated experimental approaches that account for cellular context, timing, and the complex interplay between various immune cell populations. Several methodological strategies have proven valuable for dissecting these dual roles:

Cell type-specific genetic manipulation represents a powerful approach for understanding TNFSF9's cell-specific functions. Conditional knockout systems using Cre-lox technology allow for the deletion of either TNFSF9 or TNFRSF9 in specific cell populations (e.g., CD4+ T cells, CD8+ T cells, dendritic cells, or regulatory T cells). By comparing the immune responses in these different conditional knockout models, researchers can determine how TNFSF9 signaling in each cell type contributes to either inflammatory or regulatory outcomes.

Temporal manipulation using inducible systems provides insights into the timing-dependent effects of TNFSF9. Since TNFSF9's function may differ during the initiation versus the resolution phase of immune responses, inducible knockout or overexpression systems allow researchers to modulate TNFSF9 activity at different stages of the immune response. This approach has revealed that early TNFRSF9 stimulation may promote effector T cell expansion, while later stimulation can support regulatory T cell function in some contexts.

Single-cell analysis technologies, particularly single-cell RNA sequencing combined with protein expression analysis (CITE-seq), enable comprehensive characterization of how TNFSF9 signaling affects different immune cell subpopulations simultaneously. This approach can identify specific cellular subsets that respond to TNFSF9 with either pro-inflammatory or regulatory transcriptional programs, even within seemingly homogeneous populations like CD4+ T cells.

Functional assays remain essential for establishing the immunological outcomes of TNFSF9 manipulation. These include in vitro suppression assays to assess regulatory T cell function, cytotoxicity assays for CD8+ T cells, and adoptive transfer experiments to evaluate how TNFSF9-modified cells behave in vivo. By comprehensively assessing multiple functional endpoints, researchers can build a more complete picture of TNFSF9's context-dependent effects.

How can researchers model the species-specific differences in TNFSF9 function when translating from animal models to human applications?

The significant differences in TNFSF9 biology between species present a methodological challenge for translational research. Mouse models, while invaluable for in vivo studies, may not accurately reflect human TNFSF9 function, particularly regarding reverse signaling mechanisms . Addressing these species-specific differences requires thoughtful experimental design and complementary approaches.

Humanized mouse models represent one approach to bridge this translational gap. These models, created by engrafting human immune cells or hematopoietic stem cells into immunodeficient mice, allow for the study of human TNFSF9 in an in vivo context. More sophisticated models might involve knockin mice where mouse TNFSF9 is replaced with human TNFSF9, enabling assessment of human-specific signaling in an otherwise intact immune system. These approaches allow researchers to evaluate how human TNFSF9 functions in complex immune responses while maintaining the experimental advantages of mouse models.

Comparative signaling studies are crucial for identifying conserved and divergent aspects of TNFSF9 function across species. Detailed phosphoproteomic analysis after TNFSF9 stimulation in both human and mouse cells can map species-specific signaling pathways. The human TNFSF9 reverse signaling has been reported to involve Src tyrosine kinase, PI3K, p38 MAPK, ERK1/2, and NF-κB pathways, while murine signaling may involve different mediators . Understanding these differences is essential for predicting whether findings in mouse models will translate to human applications.

Ex vivo studies using primary human samples provide directly relevant information about TNFSF9 function in humans. Peripheral blood mononuclear cells or tissue samples from patients with various conditions can be treated with TNFSF9 modulators ex vivo to assess functional outcomes. These studies can be particularly informative when samples are obtained from patients with the specific disease being targeted, as they capture the relevant disease context.

The table below summarizes the strengths and limitations of different approaches for addressing species-specific differences in TNFSF9 biology:

By employing multiple complementary approaches, researchers can develop a more comprehensive understanding of how TNFSF9 functions in humans and make more accurate predictions about the translational potential of TNFSF9-targeted therapies.

How might single-cell technologies advance our understanding of cell-specific TNFSF9-TNFRSF9 signaling dynamics?

Single-cell technologies offer unprecedented opportunities to unravel the complexity of TNFSF9-TNFRSF9 signaling across diverse cell populations within heterogeneous tissues. These approaches can transform our understanding of this pathway by revealing cell-specific responses that are masked in bulk analysis.

Single-cell RNA sequencing (scRNA-seq) combined with protein expression analysis (CITE-seq) can simultaneously profile transcriptional responses and surface protein levels, including TNFSF9 and TNFRSF9, across thousands of individual cells. This approach can identify previously unrecognized cell subpopulations that differentially express or respond to TNFSF9, potentially explaining some of the context-dependent effects observed in different disease models. By applying trajectory analysis to scRNA-seq data, researchers can also track how TNFSF9-TNFRSF9 signaling influences cell fate decisions and differentiation pathways in dynamic immune responses.

Spatial transcriptomics and imaging mass cytometry add critical spatial context to understanding TNFSF9 function. These techniques can map the expression of TNFSF9 and TNFRSF9 within tissue microenvironments, revealing cellular neighborhoods and interaction patterns that may determine functional outcomes. For instance, in tumor samples, spatial analysis might reveal whether TNFSF9-expressing cells preferentially localize with TNFRSF9-expressing effector T cells or regulatory T cells, providing insights into the predominant signaling direction in different regions of the tumor.

Single-cell phosphoproteomics, though technically challenging, offers the potential to directly measure signaling events downstream of TNFSF9-TNFRSF9 engagement at the individual cell level. This approach could resolve conflicting reports about signaling outcomes by revealing how the same receptor-ligand interaction triggers different signaling cascades in distinct cell types or activation states.

Cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) with oligo-conjugated TNFSF9 or anti-TNFRSF9 antibodies could enable direct detection of TNFSF9-TNFRSF9 interactions at the single-cell level while simultaneously capturing transcriptional responses. This innovative approach would provide direct evidence linking receptor-ligand engagement to functional outcomes in heterogeneous cell populations.

What are the research gaps in understanding TNFSF9 as a predictive biomarker for immunotherapy response?

Despite promising results showing TNFSF9 as a potential predictive biomarker for response to combination immunotherapy in renal cell carcinoma , several critical research gaps must be addressed before clinical implementation. These gaps present opportunities for researchers to develop more robust biomarker approaches.

First, validation in larger, prospective cohorts is essential. The initial observation of TNFSF9 as a predictive biomarker was made in a relatively small cohort of mRCC patients . Validation studies should include diverse patient populations across multiple institutions to establish the generalizability of TNFSF9's predictive value. These studies should also assess whether TNFSF9 expression predicts response to other immunotherapy combinations beyond nivolumab plus ipilimumab.

Second, standardization of TNFSF9 measurement methodologies requires attention. For clinical application, researchers must establish standardized protocols for TNFSF9 detection, including tissue processing, staining procedures, and quantification methods. Comparison of different detection platforms—including immunohistochemistry, quantitative PCR, NanoString technology, and RNA sequencing—would help identify the most robust and clinically feasible approach for TNFSF9 assessment.

Third, the relationship between TNFSF9 expression and specific tumor immune microenvironment features warrants deeper investigation. While TNFSF9-high tumors show greater immune cell infiltration , the causal relationship remains unclear. Does TNFSF9 expression drive immune infiltration, or is it merely a consequence of an inflamed tumor microenvironment? Mechanistic studies using in vitro co-culture systems and in vivo models with conditional TNFSF9 expression could address this fundamental question.

Fourth, integration with other biomarkers may enhance predictive accuracy. TNFSF9 expression should be evaluated alongside established biomarkers like PD-L1 expression, tumor mutational burden, and immune cell infiltration patterns to develop composite signatures with improved predictive power. Machine learning approaches applied to multiparametric data could identify optimal biomarker combinations for patient stratification.

Finally, exploring the potential of soluble TNFSF9 as a less invasive biomarker represents an untapped opportunity. TNFSF9 can be shed from the plasma membrane by metalloproteases and function as a soluble ligand . Investigating whether soluble TNFSF9 levels in peripheral blood correlate with tissue expression and treatment response could lead to the development of liquid biopsy approaches that allow for more frequent monitoring during treatment.

What novel therapeutic approaches could target the TNFSF9-TNFRSF9 axis more precisely to reduce off-target effects?

The development of next-generation therapeutics targeting the TNFSF9-TNFRSF9 axis focuses on enhancing precision to maximize efficacy while minimizing off-target effects. Several innovative approaches hold promise for advancing this field.

Bispecific and trispecific antibodies represent a sophisticated approach to targeting TNFRSF9 in specific cellular contexts. By combining TNFRSF9 targeting with recognition of a second antigen present on specific T cell subsets or tumor-infiltrating lymphocytes, these antibodies could deliver TNFRSF9 agonism selectively to cells engaged in anti-tumor responses. For example, a bispecific antibody targeting both TNFRSF9 and a tumor-associated antigen could localize TNFRSF9 agonism to the tumor microenvironment, reducing systemic activation and associated toxicities.

Conditionally active biologics that become activated only within the tumor microenvironment offer another precision approach. These molecules can be designed to remain inactive in circulation but undergo conformational changes that enable TNFRSF9 agonism when exposed to tumor-specific conditions, such as the acidic pH or specific proteases found in the tumor microenvironment. This approach could substantially improve the therapeutic window by concentrating activity where it's most needed.

Cell type-selective TNFRSF9 agonists exploit subtle differences in TNFRSF9 expression or signaling between different immune cell populations. Through structure-guided engineering and directed evolution approaches, researchers can develop variants of TNFSF9 or antibodies that preferentially activate TNFRSF9 on effector T cells rather than regulatory T cells or other immune populations. This selectivity could enhance anti-tumor effects while limiting undesirable immunosuppressive activation.

Temporal control strategies recognize that the optimal timing of TNFRSF9 agonism may differ depending on the stage of the immune response. Programmable drug delivery systems, such as injectable hydrogels that release TNFRSF9 agonists according to predetermined kinetics or in response to specific immune signals, could provide temporal precision that matches therapeutic delivery to the evolving immune response against tumors.

The table below summarizes these novel therapeutic approaches:

Additionally, developing approaches that selectively modulate TNFSF9 reverse signaling represents an underexplored therapeutic strategy. Since reverse signaling enhances antigen presentation and APC function , agents that specifically enhance this direction of signaling without triggering TNFRSF9 forward signaling could potentially boost anti-tumor immunity through improved T cell priming rather than direct effector enhancement.