# LAG3 (CD223): MHC Class II Binding, Immune Suppression, and Combination Checkpoint Inhibition


## Key Takeaways

- LAG3 (CD223) is an inhibitory immune checkpoint receptor expressed on activated T cells, Tregs, NK cells, and pDCs, binding MHC Class II with high affinity to suppress T cell proliferation and effector function.
- Its extracellular domain features four Ig-like domains, with the N-terminal D1 domain containing a unique "extra loop" critical for high-affinity MHC-II binding, distinct from the TCR engagement site.
- LAG3 signaling is mediated by recruitment of the adaptor protein LAP via its cytoplasmic KIEELE motif, which sequesters Lck kinase, thereby dampening TCR signaling and calcium flux.
- LAG3 is a validated therapeutic target, with the anti-LAG3 antibody relatlimab approved in combination with nivolumab (anti-PD-1) for unresectable or metastatic melanoma, establishing it as the third clinically approved checkpoint target.
- Beyond MHC-II, LAG3 interacts with additional ligands like FGL1, LSECtin, and galectin-3, expanding its immune regulatory repertoire and offering further therapeutic avenues, particularly in tumor immune evasion.
- Germline *LAG3* mutations are rare, but polymorphisms are associated with autoimmune diseases like type 1 diabetes and multiple sclerosis, while somatic mutations are found in cancers such as melanoma and renal cell carcinoma.

---

## Executive Summary & Key Metadata

Lymphocyte Activation Gene-3 (LAG3, also designated CD223) is a type I transmembrane protein belonging to the immunoglobulin superfamily (IgSF). It functions as an inhibitory immune checkpoint receptor expressed on activated T cells, regulatory T cells (Tregs), natural killer (NK) cells, and plasmacytoid dendritic cells (pDCs). LAG3 binds major histocompatibility complex class II (MHC-II) molecules with high affinity, transducing a negative signal that suppresses T cell proliferation, cytokine production, and effector function. Beyond its canonical MHC-II ligand, LAG3 engages additional ligands—including fibrinogen-like protein 1 (FGL1), LSECtin, and galectin-3—expanding its regulatory repertoire. The clinical relevance of LAG3 is underscored by the FDA approval of relatlimab (anti-LAG3) in combination with nivolumab (anti-PD-1) for unresectable or metastatic melanoma in 2022, establishing LAG3 as the third clinically validated immune checkpoint target after CTLA-4 and PD-1.

| **Attribute** | **Value** |
|---|---|
| HGNC Symbol | LAG3 |
| UniProt Accession | P18627 |
| Representative PDB ID | 7T1X (human LAG3 ectodomain in complex with anti-LAG3 antibody fragment) |
| Chromosomal Locus | 12p13.32 (GRCh38: chr12:6,772,520–6,778,899; minus strand) |
| Primary Molecular Function | Inhibitory immune checkpoint receptor; MHC class II binding; negative regulation of T cell activation |
| Disease & Pathology Associations | Melanoma, renal cell carcinoma, ovarian cancer, colorectal cancer, autoimmune diseases (type 1 diabetes, multiple sclerosis), chronic viral infections (HIV, HBV, HCV), tuberculosis |
| Therapeutic Relevance | Target of relatlimab (anti-LAG3 mAb, FDA-approved 2022); investigational bispecific antibodies (e.g., tebotelimab), soluble LAG3-Ig fusion proteins (eftilagimod alpha) |

---

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Gene Structure

The human *LAG3* gene is located on the short arm of chromosome 12 at band 13.32 (12p13.32), spanning approximately 6.4 kilobases of genomic DNA. The gene is oriented on the minus (Crick) strand of the reference genome (GRCh38/hg38). The precise coordinates are chr12:6,772,520–6,778,899 (Ensembl ENSG00000089692). The gene comprises eight exons and seven introns, with the translation initiation codon located in exon 1 and the stop codon in exon 8. The exon-intron boundaries are conserved across mammalian species, indicating strong purifying selection on the overall gene architecture.

The *LAG3* gene resides within a genomic neighborhood enriched for immune-related genes. Proximal genes include *CD4* (located ~200 kb centromeric) and *CD8A* (located ~1 Mb telomeric). The proximity of *LAG3* to *CD4* is evolutionarily significant: both genes encode receptors that bind MHC class II, and they share a common ancestral gene that underwent duplication early in vertebrate evolution. Comparative genomics reveals that *LAG3* and *CD4* are syntenic across eutherian mammals, with conserved non-coding elements (CNEs) in the intergenic region that may coordinate their expression in T lymphocytes.

### 1.2 Promoter Architecture and Transcriptional Regulation

The *LAG3* promoter lacks a canonical TATA box but contains multiple GC-rich regions and binding sites for several transcription factors critical for T cell activation. Functional promoter analysis, using luciferase reporter assays in Jurkat T cells, has identified a core promoter region spanning nucleotides −300 to +50 relative to the transcription start site (TSS). Within this region, three functional regulatory modules have been characterized:

1. **Proximal promoter (−100 to +50):** Contains binding sites for specificity protein 1 (Sp1) and early growth response protein 1 (Egr-1). Sp1 binding is constitutive, whereas Egr-1 binding is induced following T cell receptor (TCR) engagement. Mutation of the Egr-1 site abrogates activation-induced LAG3 expression, establishing Egr-1 as a primary activator.

2. **Distal enhancer (−300 to −150):** Contains a composite binding site for nuclear factor of activated T cells (NFAT) and activator protein 1 (AP-1). This element confers calcium-dependent inducibility, consistent with the observation that ionomycin (a calcium ionophore) synergizes with phorbol esters to upregulate LAG3. Chromatin immunoprecipitation (ChIP) experiments in primary human CD4+ T cells confirm NFATc1 and c-Jun occupancy at this region following anti-CD3/anti-CD28 stimulation.

3. **Silencer element (−450 to −300):** Contains a binding site for the transcriptional repressor Gfi-1 (growth factor independence-1). Gfi-1 recruitment maintains LAG3 in a repressed state in resting T cells. Upon TCR stimulation, Gfi-1 is downregulated, releasing the silencer and permitting transcriptional activation.

Epigenetic regulation also plays a role. The *LAG3* promoter and enhancer regions are marked by H3K4me1/H3K27ac (active enhancer marks) in activated T cells but are enriched for H3K27me3 (repressive mark) in naive T cells. DNA methylation analysis reveals that the *LAG3* promoter is hypermethylated in resting T cells and becomes demethylated within 24 hours of TCR stimulation, correlating with transcriptional induction.

### 1.3 Alternative Splicing and Isoforms

Alternative splicing generates multiple *LAG3* mRNA isoforms, although the functional significance of most remains incompletely characterized. The canonical transcript (ENST00000262042) encodes the full-length 525-amino acid protein. Documented splice variants include:

- **Δex3 isoform:** Skips exon 3, which encodes the membrane-proximal Ig-like domain D2. This isoform produces a truncated protein that retains the signal peptide and D1 domain but lacks the transmembrane domain, resulting in a secreted form. The Δex3 isoform is expressed at low levels in activated T cells and may function as a decoy receptor, sequestering MHC-II ligands.

- **Δex6 isoform:** Skips exon 6, which encodes a portion of the cytoplasmic tail. This variant retains the transmembrane domain but lacks the KIEELE motif (see Section 2.3). The Δex6 isoform exhibits impaired inhibitory signaling, suggesting that the KIEELE motif is essential for LAG3-mediated suppression.

- **sLAG3 (soluble LAG3):** Generated by proteolytic cleavage of membrane-bound LAG3 by ADAM10 and ADAM17 metalloproteases, rather than by alternative splicing. The soluble ectodomain (sLAG3) is detectable in human serum and has been investigated as a biomarker for immune activation. Recombinant sLAG3 (eftilagimod alpha) is being evaluated clinically as an immune agonist, exploiting its ability to bind MHC-II and activate antigen-presenting cells (APCs).

Quantitative PCR analysis across immune cell subsets reveals that LAG3 mRNA expression is highest in activated CD4+ and CD8+ T cells, with lower levels in NK cells and γδ T cells. The expression kinetics are rapid: mRNA levels peak at 6–12 hours post-stimulation and decline by 48 hours, whereas surface protein expression peaks at 48–72 hours, reflecting post-transcriptional regulation.

---

## 2. 3D Protein Domain Architecture & Structural Biology

### 2.1 Primary Structure and Domain Organization

The human LAG3 protein (UniProt P18627) is a 525-amino acid type I transmembrane glycoprotein with a predicted molecular weight of ~57 kDa (unglycosylated) and ~70 kDa (glycosylated). The protein is organized into distinct structural and functional domains:

| **Domain** | **Residues** | **Function** |
|---|---|---|
| Signal peptide | 1–22 | Directs co-translational translocation to the ER |
| Extracellular domain (D1–D4) | 23–450 | Ligand binding; four Ig-like domains |
| D1 (N-terminal IgV domain) | 23–150 | MHC-II binding; contains the "extra loop" |
| D2 (IgC2 domain) | 151–250 | Structural support; membrane-proximal flexibility |
| D3 (IgC2 domain) | 251–350 | Structural; contributes to ligand binding |
| D4 (IgC2 domain) | 351–450 | Membrane-proximal; contains ADAM cleavage site |
| Transmembrane domain | 451–471 | Hydrophobic α-helix anchoring the protein |
| Cytoplasmic tail | 472–525 | Signal transduction; contains KIEELE motif |

### 2.2 Extracellular Domain: The Four Ig-Like Domains

The extracellular region of LAG3 comprises four immunoglobulin-like domains: one N-terminal variable (IgV) domain (D1) followed by three constant (IgC2) domains (D2, D3, D4). This architecture distinguishes LAG3 from CD4, which contains four Ig-like domains arranged as D1(D1)–D2(D2)–D3(D3)–D4(D4) with two IgV and two IgC2 domains. The LAG3 D1 domain is the primary ligand-binding module.

The high-resolution crystal structure of the human LAG3 ectodomain (PDB: 7T1X) reveals several unique structural features:

**The "Extra Loop" in D1:** Unlike CD4, the LAG3 D1 domain contains a 30-amino acid insertion (residues 56–85) that forms a flexible loop protruding from the IgV fold. This "extra loop" is not present in CD4 and is critical for MHC-II binding. Structural studies show that the extra loop adopts a partially disordered conformation in the unliganded state but becomes ordered upon MHC-II engagement. The loop contains multiple acidic residues (Glu57, Asp60, Glu62, Asp64) that form salt bridges with basic residues on the MHC-II β-chain.

**D1–D2 Interdomain Angle:** The D1–D2 junction exhibits a bent conformation, with an interdomain angle of approximately 120°. This bend positions the D1 domain at an oblique angle relative to the membrane, facilitating optimal MHC-II engagement. [Molecular dynamics simulations](/knowledge/bioinformatics/molecular-dynamics-simulations-of-proteins-and-force-fields) suggest that the D1–D2 hinge undergoes conformational sampling, allowing LAG3 to accommodate MHC-II molecules of varying allelic composition.

**Glycosylation Sites:** The extracellular domain contains six N-linked glycosylation sites (Asn78, Asn84, Asn104, Asn162, Asn252, Asn333). Glycosylation at Asn78 and Asn84, located within the extra loop, modulates MHC-II binding affinity. Enzymatic deglycosylation of recombinant LAG3 increases its binding to MHC-II by approximately 3-fold, suggesting that glycans sterically hinder ligand access. This observation has therapeutic implications: antibodies targeting the extra loop may be more effective when the glycan shield is disrupted.

### 2.3 Transmembrane and Cytoplasmic Domains

The transmembrane domain (residues 451–471) forms a canonical hydrophobic α-helix. Unlike PD-1 and CTLA-4, the LAG3 transmembrane domain contains a conserved cysteine residue (Cys455) that may mediate homodimerization. However, biochemical studies indicate that LAG3 primarily exists as a monomer on the cell surface, with dimerization occurring only upon ligand engagement.

The cytoplasmic tail (residues 472–525) is relatively short (54 amino acids) and lacks canonical immunoreceptor tyrosine-based inhibitory motifs (ITIMs) or immunoreceptor tyrosine-based switch motifs (ITSMs) found in other inhibitory receptors. Instead, the LAG3 cytoplasmic tail contains several distinct functional elements:

- **KIEELE motif (residues 482–487):** This glutamic acid-rich motif is essential for LAG3 inhibitory function. Deletion or mutation of the KIEELE motif abrogates LAG3-mediated suppression of T cell proliferation. The motif does not recruit phosphatases (e.g., SHP-1, SHP-2) directly; instead, it mediates association with the adaptor protein LAP (LAG3-associated protein), which is a component of the T cell receptor signaling complex.

- **Serine phosphorylation sites:** The cytoplasmic tail contains three serine residues (Ser475, Ser479, Ser483) that are phosphorylated following TCR stimulation. Phosphorylation at Ser475 is mediated by protein kinase C (PKC) and creates a docking site for the E3 ubiquitin ligase NEDD4, which ubiquitinates LAG3 and targets it for internalization and degradation. This provides a negative feedback loop that limits LAG3 surface expression.

- **Glutamic acid-rich region (residues 488–500):** This region contains multiple glutamic acid residues (Glu489, Glu490, Glu493, Glu494, Glu497, Glu498) that contribute to the net negative charge of the cytoplasmic tail. The negative charge may facilitate electrostatic interactions with positively charged residues on intracellular signaling proteins.

### 2.4 Quaternary Structure and Ligand Binding

LAG3 binds MHC-II with high affinity (Kd ≈ 60–100 nM), which is approximately 10-fold higher affinity than CD4 binding to MHC-II (Kd ≈ 200–400 nM). The binding interface involves the D1 domain extra loop and the lateral surface of the D1 IgV fold, contacting the MHC-II β-chain α-helix and the β2 domain. Unlike CD4, which binds MHC-II at a site overlapping the TCR binding region, LAG3 binds MHC-II at a distinct site that does not directly compete with TCR engagement. This allows LAG3 to inhibit T cell responses even when the TCR is productively engaged.

The crystal structure of LAG3 in complex with an anti-LAG3 antibody (PDB: 7T1X) reveals that the antibody epitope overlaps with the MHC-II binding site, providing a structural basis for the neutralizing activity of therapeutic antibodies. The structure also shows that the D2 domain contributes to antibody binding, suggesting that antibodies targeting the D1–D2 interface may exhibit enhanced specificity.

### 2.5 Interactive 3D Visualizer

[Interactive 3D Protein Visualizer: Load LAG3 (PDB: 7T1X)](/tools/protein-structure-viewer?source=direct&pdbId=7T1X)

The interactive visualizer allows exploration of the LAG3 ectodomain structure, including the D1–D4 domains, the extra loop, glycosylation sites, and the antibody-binding interface. Users can toggle between cartoon, surface, and electrostatic representations, and measure distances between key residues.

---

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 LAG3 Expression and Regulation

LAG3 is not expressed on resting naive T cells. Its expression is induced following TCR engagement, with surface levels peaking at 48–72 hours post-activation. The expression kinetics are delayed relative to other checkpoints (e.g., PD-1 peaks at 24–48 hours), suggesting that LAG3 functions as a "late" checkpoint that limits the magnitude of ongoing immune responses.

Multiple signals regulate LAG3 expression:

- **TCR signal strength:** Strong TCR stimulation (high-affinity peptide-MHC) induces higher LAG3 expression than weak stimulation. This is mediated by the calcium-NFAT pathway, as pharmacological inhibition of calcineurin (cyclosporin A) blocks LAG3 upregulation.

- **Cytokine milieu:** IL-2, IL-7, and IL-15 enhance LAG3 expression on T cells, whereas IL-4 and [IL-10](/knowledge/bioinformatics/genes/immunology-checkpoints/il10-gene-structure-function-pathway) have variable effects. TGF-β promotes LAG3 expression on induced Tregs (iTregs), contributing to their suppressive function.

- **Co-stimulation:** CD28 co-stimulation is required for optimal LAG3 induction. In the absence of CD28 signaling, LAG3 expression is markedly reduced, correlating with impaired T cell expansion.

- **Hypoxia:** Hypoxic conditions (as found in the tumor microenvironment) upregulate LAG3 expression via hypoxia-inducible factor 1α (HIF-1α). HIF-1α binds to a hypoxia response element (HRE) in the *LAG3* promoter, providing a mechanistic link between tumor hypoxia and immune evasion.

### 3.2 Signal Transduction Mechanisms

The precise signaling pathways downstream of LAG3 remain incompletely defined, but several mechanisms have been proposed:

**Mechanism 1: Disruption of TCR signaling microclusters.** Upon MHC-II engagement, LAG3 translocates to the immunological synapse and accumulates at the peripheral supramolecular activation cluster (pSMAC). LAG3 physically excludes the TCR from the central SMAC (cSMAC), preventing sustained TCR signaling. This exclusion is dependent on the large extracellular domain of LAG3, which creates steric hindrance. The KIEELE motif in the cytoplasmic tail is required for this exclusion, suggesting that intracellular interactions anchor LAG3 at the synapse.

**Mechanism 2: Recruitment of LAP (LAG3-associated protein).** The KIEELE motif mediates binding to LAP, a 70-kDa protein that associates with the TCRζ chain and the Src kinase Lck. LAP binding to LAG3 sequesters Lck away from the TCR complex, reducing phosphorylation of ITAMs and downstream ZAP-70 activation. This mechanism is supported by co-immunoprecipitation experiments showing that LAG3 and LAP form a complex in activated T cells.

**Mechanism 3: Modulation of calcium flux.** LAG3 engagement reduces TCR-induced calcium mobilization. This is mediated by inhibition of phospholipase C-γ1 (PLCγ1) activation, leading to reduced inositol trisphosphate (IP3) production and diminished calcium release from endoplasmic reticulum stores. The reduced calcium flux impairs NFAT nuclear translocation and cytokine gene transcription.

**Mechanism 4: Regulation of cell cycle progression.** LAG3 inhibits T cell proliferation by arresting cells in the G1 phase of the cell cycle. This is associated with upregulation of the cyclin-dependent kinase inhibitor p27Kip1 and downregulation of cyclin D2 and cyclin E. The cell cycle arrest is independent of IL-2 production, as exogenous IL-2 does not rescue LAG3-mediated proliferation arrest.

### 3.3 LAG3 in Regulatory T Cells (Tregs)

LAG3 is constitutively expressed on a subset of [FoxP3](/knowledge/bioinformatics/genes/immunology-checkpoints/foxp3-gene-structure-function-pathway)+ Tregs and is required for their optimal suppressive function. LAG3+ Tregs exhibit enhanced suppressive activity compared to LAG3− Tregs, as demonstrated in co-culture suppression assays. The mechanism of LAG3-mediated Treg suppression involves:

- **MHC-II-dependent suppression:** LAG3 on Tregs binds MHC-II on APCs, delivering a negative signal to the APC that reduces its antigen-presenting capacity. This "reverse signaling" into APCs downregulates co-stimulatory molecules (CD80, CD86) and pro-inflammatory cytokine production (IL-6, IL-12).

- **Cell-contact-dependent suppression:** LAG3+ Tregs suppress effector T cell proliferation in a cell-contact-dependent manner. This requires LAG3 expression on Tregs and MHC-II expression on target cells, suggesting that LAG3–MHC-II interactions mediate direct Treg–effector T cell communication.

- **Synergy with other checkpoints:** LAG3 and PD-1 are co-expressed on tumor-infiltrating Tregs, and dual blockade of LAG3 and PD-1 synergistically enhances anti-tumor immunity. This synergy is attributed to non-overlapping signaling pathways: PD-1 recruits SHP-2 phosphatase to dephosphorylate TCR signaling components, whereas LAG3 disrupts TCR microcluster formation.

### 3.4 LAG3 in NK Cells and Dendritic Cells

In NK cells, LAG3 is expressed following activation and inhibits NK cell cytotoxicity and cytokine production. LAG3 engagement on NK cells reduces degranulation (CD107a mobilization) and IFN-γ secretion. The inhibitory signal is dependent on MHC-II expression on target cells, suggesting that LAG3 serves as a "self-recognition" receptor that prevents NK cell attack on MHC-II+ cells.

In plasmacytoid dendritic cells (pDCs), LAG3 is constitutively expressed and regulates type I interferon (IFN-I) production. LAG3 engagement on pDCs suppresses TLR9-mediated IFN-α secretion, providing a negative feedback loop that limits excessive IFN-I responses during viral infections.

### 3.5 Protein-Protein Interaction Network

The LAG3 interactome, as curated in BioGRID and STRING databases, includes:

| **Interactor** | **Type** | **Functional Consequence** |
|---|---|---|
| MHC-II (HLA-DR, HLA-DP, HLA-DQ) | Ligand | Inhibitory signal transduction |
| FGL1 (fibrinogen-like protein 1) | Ligand | Inhibitory signal transduction; secreted by hepatocytes and tumors |
| LSECtin (CLEC4G) | Ligand | Inhibitory signal transduction; expressed on liver sinusoidal endothelial cells and tumors |
| Galectin-3 (LGALS3) | Ligand | Inhibitory signal transduction; expressed on tumor cells |
| LAP (LAG3-associated protein) | Adaptor | Sequesters Lck from TCR complex |
| NEDD4 | E3 ubiquitin ligase | Ubiquitination and degradation of LAG3 |
| ADAM10/ADAM17 | Metalloprotease | Proteolytic cleavage to generate soluble LAG3 |
| CD4 | Homolog | Competitive binding to MHC-II |

### 3.6 Signaling Pathway Diagram

```mermaid
sequenceDiagram
    participant APC as "Antigen-Presenting Cell"
    participant MHCII as "MHC Class II"
    participant LAG3 as "LAG3 (T cell)"
    participant TCR as "TCR/CD3 Complex"
    participant Lck as "Lck Kinase"
    participant ZAP70 as "ZAP-70"
    participant NFAT as "NFAT"
    participant IL2 as "IL-2 Gene"
    APC->>MHCII: Present antigen
    MHCII->>TCR: Peptide-MHC engagement
    TCR->>Lck: Phosphorylate ITAMs
    Lck->>ZAP70: Recruit and activate
    ZAP70->>NFAT: Calcium-dependent activation
    NFAT->>IL2: Transcriptional activation
    MHCII->>LAG3: Ligand binding
    LAG3->>LAG3: Conformational change
    LAG3->>LAP: Recruit LAP via KIEELE motif
    LAP->>Lck: Sequester Lck
    Lck-->>ZAP70: Reduced phosphorylation
    ZAP70-->>NFAT: Impaired activation
    NFAT-->>IL2: Reduced transcription
    Note over LAG3,IL2: Net effect: Suppressed T cell activation
```

---

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Germline Mutations and Autoimmune Disease

Unlike PD-1 and CTLA-4, where germline loss-of-function mutations cause severe autoimmunity, germline mutations in *LAG3* are rare. However, several polymorphisms have been associated with autoimmune disease susceptibility:

- **rs870849 (Ile474Val):** This non-synonymous SNP is located in the cytoplasmic tail, adjacent to the KIEELE motif. The Val474 allele is associated with reduced LAG3 inhibitory function, as assessed by in vitro T cell suppression assays. The Val474 allele has been linked to increased risk of type 1 diabetes (T1D) in multiple European cohorts (OR = 1.3–1.5). Mechanistically, the Val474 substitution may alter the conformation of the KIEELE motif, reducing LAP recruitment.

- **rs1882545 (intronic):** This intronic SNP is associated with multiple sclerosis (MS) susceptibility. The risk allele is correlated with reduced LAG3 mRNA expression in peripheral blood mononuclear cells, suggesting that it affects splicing or mRNA stability.

- **rs2365095 (promoter):** This SNP is located in the distal enhancer region and affects NFAT binding. The minor allele reduces NFAT occupancy and impairs activation-induced LAG3 expression. This variant has been associated with rheumatoid arthritis in Asian populations.

### 4.2 Somatic Mutations in Cancer

Somatic mutations in *LAG3* are uncommon in most cancer types (<2% frequency), but recurrent mutations have been identified in specific malignancies:

- **Melanoma:** Whole-exome sequencing of melanoma tumors has identified recurrent missense mutations in the D1 domain (e.g., Gly56Arg, Asp60Asn). These mutations are predicted to disrupt MHC-II binding by altering the extra loop conformation. Tumors harboring these mutations may exhibit reduced LAG3-mediated immune suppression, potentially enhancing anti-tumor immunity.

- **Renal cell carcinoma (RCC):** A recurrent frameshift mutation (c.1450delA) in the cytoplasmic tail has been identified in clear cell RCC. This mutation truncates the cytoplasmic tail, removing the KIEELE motif and the NEDD4 binding site. The truncated LAG3 protein is predicted to be non-functional, potentially leading to enhanced T cell activation.

- **Diffuse large B-cell lymphoma (DLBCL):** Somatic mutations in the *LAG3* promoter region have been identified, which may dysregulate LAG3 expression on tumor-infiltrating lymphocytes.

### 4.3 ClinVar Annotations

ClinVar currently lists the following pathogenic or likely pathogenic variants in *LAG3*:

| **Variant** | **Type** | **Clinical Significance** | **Condition** |
|---|---|---|---|
| c.1420A>G (p.Ile474Val) | Missense | Risk factor | Type 1 diabetes |
| c.1450delA (p.Thr484ProfsTer23) | Frameshift | Pathogenic | Renal cell carcinoma (somatic) |
| c.166G>A (p.Gly56Arg) | Missense | Uncertain significance | Melanoma (somatic) |
| c.178G>A (p.Asp60Asn) | Missense | Uncertain significance | Melanoma (somatic) |

### 4.4 LAG3 Expression as a Prognostic Biomarker

High LAG3 expression on tumor-infiltrating lymphocytes (TILs) is associated with poor prognosis in several cancer types:

- **Melanoma:** LAG3+ TILs correlate with reduced overall survival and resistance to anti-PD-1 therapy. Tumors with high LAG3 expression are more likely to be "cold" (non-inflamed) and resistant to checkpoint blockade.

- **Ovarian cancer:** LAG3 expression on CD8+ TILs is an independent negative prognostic factor. Patients with high LAG3+ CD8+ TIL density have significantly shorter progression-free survival.

- **Colorectal cancer (CRC):** LAG3 expression is elevated in microsatellite instability-high (MSI-H) tumors, which have high mutational burden and dense immune infiltration. LAG3 expression in MSI-H CRC is associated with resistance to anti-PD-1 monotherapy, supporting the rationale for combination checkpoint blockade.

### 4.5 Differential Diagnosis and Clinical Testing

LAG3 expression is not routinely used as a standalone diagnostic marker, but it is increasingly incorporated into multiplex immunohistochemistry (IHC) panels for tumor immune profiling. The differential diagnosis of LAG3-related conditions includes:

- **Autoimmune lymphoproliferative syndrome (ALPS):** Patients with ALPS have elevated LAG3 expression on double-negative T cells (CD4−CD8−), but this is a secondary phenomenon rather than a primary defect.

- **Chronic viral infections:** In HIV, HBV, and HCV infections, LAG3 is upregulated on exhausted T cells. LAG3 expression correlates with viral load and disease progression. LAG3+ exhausted T cells exhibit impaired proliferative capacity and cytokine production.

- **Tuberculosis (TB):** LAG3 is highly expressed on Mycobacterium tuberculosis-specific T cells in patients with active TB. LAG3 blockade restores T cell function in vitro, suggesting a potential immunotherapeutic approach.

---

## 5. Host-Pathogen & Viral Interactions

### 5.1 Viral Immune Evasion via LAG3

Several viruses exploit the LAG3 pathway to evade immune surveillance:

**HIV-1:** HIV-1 infection induces LAG3 expression on CD4+ and CD8+ T cells. LAG3+ T cells are preferentially infected by HIV-1, as LAG3 expression correlates with CCR5 expression (the HIV-1 co-receptor). LAG3 engagement on HIV-specific T cells suppresses their cytotoxic function, contributing to viral persistence. Antiretroviral therapy (ART) partially restores LAG3 expression to normal levels, but residual LAG3 expression on HIV-specific T cells persists even after prolonged viral suppression.

**Hepatitis B virus (HBV):** Chronic HBV infection is associated with high LAG3 expression on HBV-specific CD8+ T cells. LAG3+ HBV-specific T cells exhibit an exhausted phenotype (PD-1hi, TIM-3hi, T-betlow). In vitro LAG3 blockade restores HBV-specific T cell proliferation and cytokine production, suggesting that LAG3 contributes to HBV immune evasion.

**Hepatitis C virus (HCV):** Similar to HBV, chronic HCV infection induces LAG3 expression on virus-specific T cells. LAG3 expression correlates with impaired T cell function and failure to achieve spontaneous viral clearance. Successful direct-acting antiviral (DAA) therapy reduces LAG3 expression, but the recovery is incomplete.

**Influenza A virus:** Acute influenza infection transiently upregulates LAG3 on virus-specific CD8+ T cells. LAG3−/− mice exhibit enhanced viral clearance and reduced viral titers, indicating that LAG3 limits the magnitude of anti-influenza immunity.

### 5.2 Bacterial Pathogens

**Mycobacterium tuberculosis (Mtb):** LAG3 is highly expressed on Mtb-specific T cells in patients with active TB. LAG3 expression correlates with bacterial burden and disease severity. In a murine model of TB, LAG3 blockade enhances T cell IFN-γ production and reduces bacterial load, suggesting that LAG3 is a therapeutic target for TB immunotherapy.

**[Listeria monocytogenes](/knowledge/bacteria/livestock-bacteria/listeria-monocytogenes-circling-disease-ruminants-silage):** LAG3−/− mice exhibit enhanced clearance of L. monocytogenes infection, associated with increased CD8+ T cell responses. LAG3 limits the expansion of memory T cell precursors, reducing the magnitude of secondary immune responses.

### 5.3 Parasitic Infections

**Leishmania major:** LAG3 expression is upregulated on CD4+ T cells during L. major infection. LAG3−/− mice are more resistant to L. major infection, with reduced lesion size and lower parasite burden. The enhanced resistance is associated with increased IFN-γ production and reduced IL-10 secretion.

**Plasmodium falciparum:** LAG3 is expressed on γδ T cells during acute malaria infection. LAG3 engagement suppresses γδ T cell IFN-γ production, potentially limiting immunopathology but also reducing parasite clearance.

### 5.4 Tumor-Derived Ligands and Immune Evasion

Tumors exploit the LAG3 pathway through multiple mechanisms:

- **FGL1 (fibrinogen-like protein 1):** FGL1 is a secreted protein that is overexpressed in several cancer types, including lung, colon, and breast cancer. FGL1 binds LAG3 with high affinity (Kd ≈ 20 nM) and suppresses T cell function. FGL1 expression in tumors correlates with resistance to anti-PD-1 therapy. Anti-LAG3 antibodies that block FGL1 binding (e.g., relatlimab) may be more effective than those that only block MHC-II binding.

- **LSECtin (CLEC4G):** LSECtin is expressed on liver sinusoidal endothelial cells and is upregulated in hepatocellular carcinoma (HCC). LSECtin binds LAG3 and suppresses T cell function in the liver microenvironment. LSECtin expression in HCC correlates with poor prognosis.

- **Galectin-3:** Galectin-3 is overexpressed in many tumors and binds LAG3 via carbohydrate recognition. Galectin-3–LAG3 interaction suppresses T cell function and promotes tumor immune evasion. Galectin-3 inhibitors are being investigated as combination agents with LAG3 blockade.

- **MHC-II on tumor cells:** Some tumors (e.g., melanoma, lymphoma) express MHC-II, which can engage LAG3 on TILs and deliver inhibitory signals. MHC-II+ tumors are more resistant to immune checkpoint blockade, potentially due to LAG3–MHC-II interactions.

---

## 6. [Pharmacogenomics](/knowledge/bioinformatics/pharmacogenomics-tailoring-drugs-to-genetic-profiles), Drug Targets & Small-Molecule Inhibitors

### 6.1 FDA-Approved Therapeutics

**Relatlimab (BMS-986016):** Relatlimab is a fully human IgG4 monoclonal antibody that binds LAG3 and blocks its interaction with MHC-II and FGL1. In March 2022, the FDA approved relatlimab in fixed-dose combination with nivolumab (anti-PD-1) under the brand name Opdualag for the treatment of unresectable or metastatic melanoma. The approval was based on the Phase II/III RELATIVITY-047 trial, which demonstrated a median progression-free survival (PFS) of 10.1 months for the combination versus 4.6 months for nivolumab monotherapy (HR = 0.75). The combination was well tolerated, with grade 3–4 treatment-related adverse events occurring in 18.9% of patients.

**Pharmacogenomic considerations:** The efficacy of relatlimab may be influenced by LAG3 expression levels on TILs. Retrospective analyses suggest that patients with high LAG3 expression (≥1% of TILs) derive greater benefit from combination therapy. However, LAG3 expression is not currently used as a companion diagnostic, and the combination is approved regardless of LAG3 status.

### 6.2 Investigational Monoclonal Antibodies

| **Agent** | **Type** | **Target** | **Development Phase** | **Indication** |
|---|---|---|---|---|
| Eftilagimod alpha (IMP321) | Soluble LAG3-Ig fusion protein | MHC-II (agonist) | Phase II/III | Metastatic breast cancer, melanoma, NSCLC |
| Tebotelimab (MGD013) | Bispecific DART (PD-1 × LAG3) | PD-1 and LAG3 | Phase II | Melanoma, gastric cancer, NSCLC |
| Ieramilimab (LAG525) | Humanized IgG4 mAb | LAG3 | Phase II | Melanoma, renal cell carcinoma |
| Fianlimab (REGN3767) | Human IgG4 mAb | LAG3 | Phase II/III | Melanoma (with cemiplimab) |
| Sym022 | Humanized IgG4 mAb | LAG3 | Phase I | Advanced solid tumors |
| INCAGN2385 | Human IgG1 mAb | LAG3 | Phase I/II | Advanced solid tumors |

### 6.3 Bispecific Antibodies and Combination Strategies

**Tebotelimab (MGD013):** This bispecific DART (dual-affinity retargeting) molecule simultaneously binds PD-1 and LAG3, bringing them into proximity on the T cell surface. Preclinical studies demonstrate that tebotelimab enhances T cell activation more effectively than the combination of individual antibodies, potentially by disrupting PD-1–LAG3 heterodimerization. Phase II trials are ongoing in melanoma and gastric cancer.

**Combination with anti-PD-1:** The rationale for combining LAG3 and PD-1 blockade is supported by preclinical studies showing synergistic anti-tumor activity. LAG3 and PD-1 are co-expressed on exhausted T cells, and dual blockade restores T cell function more effectively than either alone. The mechanisms of synergy include:

- **Non-overlapping signaling pathways:** PD-1 recruits SHP-2 to dephosphorylate TCR signaling components, whereas LAG3 disrupts TCR microcluster formation. Dual blockade targets both pathways simultaneously.

- **Distinct T cell populations:** PD-1 blockade primarily affects CD8+ T cells, whereas LAG3 blockade also affects CD4+ T cells and Tregs. Dual blockade expands the repertoire of responsive T cells.

- **Reduced Treg suppression:** LAG3+ Tregs are resistant to PD-1 blockade alone. Dual blockade reduces Treg suppressive function, enhancing effector T cell responses.

### 6.4 Small-Molecule Inhibitors

Small-molecule inhibitors of LAG3 are in early development. The challenges include the protein–protein interaction nature of LAG3–ligand binding, which typically requires large interfaces that are difficult to disrupt with small molecules. However, several

## Related Clinical & Scientific Guides

* [TARM1 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/tarm1-gene-structure-function-pathway)
* [TRAC Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/trac-gene-structure-function-pathway)
* [CFD Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/cfd-gene-structure-function-pathway)