LAG-3 signaling pathway: How does the discovery of its ligand FGL1 guide the next generation of immunotherapy?

I. Why is LAG-3 as an immune checkpoint receiving significant attention?

Lymphocyte-activation gene 3 (LAG-3, CD223) is an important immune checkpoint molecule expressed on the surface of activated T cells, natural killer cells, and other immune cells. Similar to the Nobel Prize-recognized PD-1 and CTLA-4, LAG-3 is also a critical immunosuppressive receptor, or "immune brake." When LAG-3 is activated by its ligands, it transmits inhibitory signals that weaken T cell proliferation, cytokine production, and killing function, thereby helping tumor cells evade immune surveillance and clearance. Given the revolutionary success of immune checkpoint inhibitors targeting PD-1/PD-L1 and CTLA-4 in cancer treatment, targeting LAG-3 to release T cell activity and overcome existing treatment resistance has naturally become a highly promising new direction in tumor immunotherapy.

II. Why did early clinical trials targeting LAG-3 fail to meet expectations?

Over the past few years, several institutions have developed LAG-3 blocking antibodies and advanced them to clinical trials. However, preliminary clinical data did not replicate the "breakthrough" efficacy seen in early PD-1/PD-L1 inhibitors, with overall response rates being limited. This phenomenon has prompted deep reflection on the underlying biological mechanisms. A core question is: What is the primary functional ligand of LAG-3? For a long time, the scientific community widely believed that major histocompatibility complex class II molecules (MHC-II) were the primary ligands of LAG-3. However, MHC-II is mainly expressed on antigen-presenting cells. In certain tumors lacking MHC-II expression, how is LAG-3 continuously activated to suppress T cell function? This contradiction suggests the possible existence of unknown critical ligands.

III. How has the discovery of FGL1 redefined the LAG-3 signaling pathway?

Recent breakthrough research has revealed a new and primary functional ligand of LAG-3—fibrinogen-like protein 1 (FGL1). FGL1 is primarily secreted by the liver and is a plasma protein. However, studies have found that tumor cells and tumor-associated fibroblasts in various human cancers can abnormally overexpress FGL1. Rigorous biochemical and cell biology experiments have confirmed that FGL1 can directly bind LAG-3 with high affinity and trigger downstream inhibitory signals, leading to T cell dysfunction.

This discovery has multiple important implications:

1. Explains a new mechanism of tumor immune evasion: It reveals that tumor cells actively activate LAG-3 on infiltrating T cells by secreting FGL1, thereby constructing a local immunosuppressive microenvironment. This is a new evasion mechanism independent of the PD-1/PD-L1 and MHC-II pathways.

2. Provides a potential explanation for limited clinical efficacy: Many LAG-3 antibodies entering clinical trials were designed before the discovery of FGL1. Their epitopes may primarily block the binding of LAG-3 to MHC-II but fail to effectively interfere with the interaction between LAG-3 and FGL1. Therefore, these antibodies may not fully解除 the immune suppression mediated by the FGL1-LAG-3 axis, which could be a key reason for their limited clinical activity.

3. Points to new therapeutic targets: FGL1 itself becomes a全新的 and actionable target for immunotherapy. Developing FGL1-neutralizing antibodies or small molecules that disrupt FGL1-LAG-3 binding could more彻底 block this pathway.

IV. What is the core value of human LAG-3 protein in related research and development?

Whether for深入 understanding LAG-3 biology or developing new-generation targeted drugs, high-purity, high-activity human LAG-3 protein is an indispensable key tool. Its applications span the entire链条 from basic research to drug development:

1. Ligand identification and mechanism research: It was precisely by using recombinant human LAG-3 protein as a "molecular probe" that the new ligand FGL1 was ultimately identified through proteomics methods from complex samples. This protein is also used to validate the direct binding of LAG-3 to FGL1 or other potential ligands, measure binding affinity (KD values), and map binding domains.

2. Antibody characterization and optimization: In developing new LAG-3 antibodies, human LAG-3 protein is used to evaluate the binding activity, affinity, and epitope mapping of candidate antibodies. More importantly, competitive experiments can be designed to test whether antibodies can effectively block the binding of LAG-3 to FGL1 (and MHC-II), thereby筛选出 antibodies that can simultaneously block multiple inhibitory pathways—"fully blocking" antibodies.

3. Biomarker development and detection: Based on human LAG-3 protein, detection reagents can be developed to quantify soluble LAG-3 levels in patient tumor tissues or peripheral blood, exploring its potential as a predictive biomarker.

4. Safety evaluation: In preclinical studies, human LAG-3 protein can be used to assess the immunogenicity risks that candidate drugs may引发.

V. What do current clinical data on LAG-3-targeted therapies reveal?

Despite challenges, existing clinical trials of LAG-3 inhibitors have provided valuable insights:

- Synergistic potential with PD-1: In patients who failed PD-1/PD-L1 inhibitor therapy (especially melanoma), the combination of LAG-3 antibodies (e.g., Relatlimab) with PD-1 antibodies仍能 observed objective responses in some patients (particularly the LAG-3 high-expression subgroup), suggesting this combination has the potential to overcome部分 PD-1 resistance.

- Preliminary efficacy signals: Different companies' LAG-3 inhibitors (monotherapy or combined with PD-1) have shown some activity in early trials for advanced solid tumors. Although response rates are generally low (lower for monotherapy, slightly improved for combination therapy), they confirm that targeting this pathway can indeed yield clinical benefits.

- Generally manageable safety: Current data show that the safety profiles of LAG-3 inhibitors alone or in combination with PD-1 are, with no unexpected severe unique toxicities.

VI. Summary and Outlook

As an important immune checkpoint, the therapeutic potential of LAG-3 has been and illuminated by the discovery of its new ligand FGL1. The limitations of early clinical data likely stem from incomplete understanding of the pathway's mechanisms. Moving forward, based on insights into the FGL1-LAG-3 axis, drug development strategies need adjustment: developing new-generation antibodies or bispecific molecules that can effectively block the interaction between LAG-3 and FGL1;, targeting FGL1 itself has become an attractive new direction. Human LAG-3 protein, as a core research tool, will continue to play a foundational role in mechanism, drug screening, and biomarker development. With the emergence of more precise targeting strategies, the LAG-3 pathway is poised to become the next important immune after PD-1/PD-L1, bringing benefits to more cancer patients.

VII. Which manufacturers provide human LAG-3 protein?

Hangzhou Starter Biotech Co., Ltd. has independently developed "Human LAG-3 Protein (His tag)" (product name: Human LAG-3 Protein, His tag, catalog number: S0A1131), a highly bioactive, high-purity, and stable immune checkpoint receptor protein. This product is recombinantly expressed in mammalian systems, fusing the extracellular domain of human LAG-3 protein with a His tag. It holds significant application value in LAG-3-targeted drug screening, immune checkpoint mechanism research, and in vitro T cell function regulation.

Professional technical support: We provide detailed product technical, including comprehensive purity analysis reports, binding activity validation data, recommended application protocols, and specialized technical consultation, assisting clients in achieving progress in drug development and mechanism research.

Hangzhou Starter Biotech Co., Ltd. is committed to providing high-quality, high-value biological reagents and solutions to global innovative pharmaceutical companies and research institutions. For more details about "Human LAG-3 Protein (His tag)" (catalog number S0A1131) or to request sample testing, please feel free to contact us.

Product Information