LAG3: The Next Frontier in Tumor Immunotherapy

2. Mechanisms of LAG3

2.1 Interaction with the TCR-CD3 Complex

LAG3 can continuously associate with the T cell receptor (TCR)-CD3 complex and migrate to the immunological synapse (IS) after T cell activation. The EP repeat sequence in its cytoplasmic tail lowers the local pH at the immunological synapse, disrupting the zinc ion (Zn²⁺) binding between the CD4/CD8 co-receptors and the tyrosine kinase Lck. This leads to the dissociation of the co-receptor-Lck complex, ultimately inhibiting downstream TCR signaling (such as phosphorylation of ZAP70, ERK, and AKT), thereby restricting T cell activation. This process does not depend on its classical ligand, MHC class II molecules, and is the core mechanism by which LAG3 independently inhibits T cell function ^[3].

2.2 Ligand Binding and Dimerization Mechanism

The main ligands of LAG3 include MHC class II molecules, fibrinogen-like protein 1 (FGL1), and liver sinusoidal endothelial cell lectin (LSECtin) ^[2]. The Loop 2 region (Gly¹⁰⁷-Pro¹¹⁵) in the D1 domain is the key interface for ligand binding, and binding to both MHC class II molecules and FGL1 depends on this region ^[4]. In addition, LAG3 forms homodimers through its D2 domain. The murine LAG3 dimer is mediated by interactions between Trp¹⁸⁰ and a hydrophobic pocket in the adjacent protomer, while the human LAG3 dimer relies on residues such as Phe²²⁷ and Trp¹⁸⁴. Dimerization enhances its binding capacity with ligands.

2.3 Synergistic Effects with Other Immune Checkpoints

LAG3 and PD1 exhibit synergistic effects in T cell exhaustion. In chronic infection and tumor models, blocking either LAG3 or PD1 alone has limited effects on restoring T cell function, while combined blockade significantly reverses T cell exhaustion, enhancing cytokine secretion and proliferation. This synergy may be due to their distinct mechanisms of inhibiting TCR signaling: LAG3 primarily interferes with the co-receptor-Lck interaction, while PD1 inhibits downstream signaling through SHP-1/2 ^[1,3].

3. LAG3-Related Signaling Pathways

LAG3 mainly inhibits immune responses through the following pathways:

• TCR Signaling Inhibition: By lowering local pH via the EP repeat sequence, it disrupts the binding between CD4/CD8 and Lck, reducing the phosphorylation of molecules such as ZAP70 and PLCγ, inhibiting calcium influx and the activation of transcription factors like NFAT ^[3].
• Enhancement of Treg Cell Function: High expression of LAG3 on Treg cells enhances their immunosuppressive functions by binding to MHC class II molecules, thereby inhibiting the proliferation of effector T cells ^[2].
• Regulation of Antigen-Presenting Cells (APCs): Binding of LAG3 to MHC class II molecules on DCs inhibits DC maturation, reduces the secretion of pro-inflammatory cytokines such as IL-12, and promotes the production of anti-inflammatory factors like IL-10 ^[1,2].

4. LAG3-Related Diseases

Abnormal regulation of IL2 can lead to immune imbalance and disordered signaling pathways, which are closely related to the occurrence and development of various diseases:

4.1 Tumors

4.1.1 Uveal Melanoma (UM)

In 43 UM tissue samples, LAG3 expression was detected in 25 cases (58.1%), present in both tumor cells and tumor-infiltrating lymphocytes (TILs). In UM cases with high-risk histopathological factors (such as large cell type and necrosis), LAG3 gene expression was significantly upregulated. Patients with LAG3-positive TILs and high mRNA expression had significantly lower disease-free survival (DFS) (p<0.0001), suggesting that LAG3 can serve as a prognostic biomarker for high-risk UM patients. The mechanism may involve LAG3 inhibiting the antitumor activity of TILs, allowing tumor cells to evade immune surveillance ^[5].

4.1.2 Gastroenteropancreatic Neuroendocrine Tumors (GEP-NETs)

Analysis of 61 GEP-NETs patients showed no LAG3-positive expression (0/61). However, other immune markers such as PD-L1 and MSI-H exhibited expression differences: PD-L1 had the highest positivity rate in G3 tumors (50%), and MSI-H was present in 80% of G1 tumors, with both more common in localized lesions. Although LAG3 was not positive in this tumor type, its co-absence with other immune checkpoints may contribute to immune evasion in GEP-NETs, with specific mechanisms requiring further research ^[6].

4.1.3 Breast Cancer

In breast cancer tissues, the presence of LAG3⁺ intraepithelial tumor-infiltrating lymphocytes (iTILs) is associated with improved breast cancer-specific survival (BCSS) (HR 0.71; 95% CI 0.56–0.90). Approximately 53% of PD-L1⁺ tumors and 61% of PD-1⁺ tumors had LAG3⁺ iTILs, and BCSS was better when LAG3⁺CD8⁺ iTILs co-infiltrated. This suggests that LAG3⁺ iTILs may reflect the body's antitumor immune activity, and their presence indicates a more effective immune response ^[7].

4.1.4 Non-Small Cell Lung Cancer (NSCLC)

LAG3 is highly expressed on TILs in the NSCLC tumor microenvironment and is associated with tumor stage: patients with stage III had a higher proportion of LAG3⁺ cells in the tumor microcirculation than those with stage IV. LAG3 expression also correlated positively with PD-L1 expression. Patients with high LAG3 expression had lower response rates to PD-1 inhibitors and shorter progression-free survival (PFS), suggesting that LAG3 may be a marker of immune therapy resistance in NSCLC. The mechanism may involve LAG3 and PD1 synergistically inhibiting T cell function, weakening the effectiveness of immune therapy ^[8].

4.1.5 Gastric Cancer

In patients with advanced gastric cancer treated with nivolumab monotherapy, the presence of LAG3⁺ TILs was associated with lower treatment response rates, and patients with high LAG3 mRNA levels had significantly shorter overall survival (OS). Additionally, LAG3 was negatively correlated with tumor mutation burden (TMB), suggesting that high LAG3 expression may inhibit T cell infiltration, reducing immune therapy sensitivity ^[9,10].

4.1.6 Colorectal Cancer

In locally advanced colorectal cancer with mismatch repair deficiency (MMR-deficient), neoadjuvant nivolumab combined with relatlimab (anti-LAG3) treatment induced significant pathological complete response (pCR), with increased CD8⁺ T cell infiltration and decreased LAG3⁺ cell proportion in the tumor microenvironment. This indicates that LAG3 in colorectal cancer promotes tumor progression by inhibiting cytotoxic T cell function, and blocking LAG3 can restore immune response ^[11].

4.2 Autoimmune Diseases

Deficiency or dysfunction of LAG3 is associated with increased susceptibility to autoimmune diseases. In a type 1 diabetes model, Lag3⁻/⁻ mice had significantly increased numbers of CD4⁺ and CD8⁺ T cells infiltrating the islets, with accelerated disease progression. In patients with rheumatoid arthritis (RA), the proportion of LAG3⁺ Tregs was lower and negatively correlated with disease activity, suggesting that impaired immunosuppressive function of LAG3⁺ Tregs may exacerbate joint inflammation. The mechanisms mainly involve weakened inhibition of effector T cells by LAG3 and decreased Treg function, leading to excessive activation of autoreactive T cells ^[2].

4.3 Chronic Infections

In chronic hepatitis B virus (HBV) infection, LAG3⁺ erythroid progenitor cells inhibit HBsAg seroclearance by secreting TGF-β, and LAG3 expression levels correlate positively with HBsAg titers. The mechanism involves LAG3⁺ cells interacting with T cells through LAG3, while simultaneously releasing TGF-β to inhibit T cell clearance of HBV, delaying viral clearance ^[12].

5. Research Progress on LAG3-Targeted Drugs

The research progress on LAG3 (lymphocyte activation gene 3)-targeted drugs is rapid, with several drugs now in clinical trial stages. Among them, BMS-986016 (relatlimab), the first LAG3 blocker, has conducted multiple phase I to III clinical trials, showing good tolerability and antitumor activity. Other anti-LAG3 monoclonal antibodies such as LAG525, TSR-033, and INCAGN02385 are also in clinical trials, with preliminary data indicating that these drugs have good safety and potential antitumor effects. In addition, bispecific antibodies such as MGD013 (targeting both LAG3 and PD-1) have also shown good tolerability and antitumor activity. LAG3-targeted drugs have shown potential in both monotherapy and combination therapy, with some of the ongoing drug pipelines listed below:

(Data sourced from patsnap)

6. CUSABIO Products Related to LAG3

As a key immune checkpoint, LAG3 plays an important role in tumor immune evasion, autoimmune diseases, and chronic infections by interacting with the TCR-CD3 complex, interfering with co-receptor signaling, and binding to ligands. HUAMBI offers LAG3 recombinant proteins, antibodies, and ELISA kits to support your mechanism research or drug development.