# CD74 Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- CD74, the invariant chain of MHC class II, acts as a crucial chaperone for MHC class II assembly and peptide loading, ensuring proper antigen presentation. It also functions as a high-affinity receptor for MIF, a pro-inflammatory cytokine, and its intracellular domain (CD74-ICD) can translocate to the nucleus to regulate gene expression.
- The *CD74* gene, located at 5q33.1, comprises 8 exons and is regulated by a promoter containing an SXY module bound by CIITA, essential for MHC class II expression, and NF-κB binding sites for cytokine-inducible transcription. Aberrant overexpression in tumors often results from promoter demethylation and JAK/STAT pathway activation.
- CD74's canonical role involves guiding MHC class II molecules from the ER through the endosomal system, where it is proteolytically processed to release the CLIP peptide, which is then exchanged for antigenic peptides by HLA-DM before surface presentation to CD4+ T cells.
- Beyond antigen presentation, CD74, in complex with CD44, mediates MIF signaling, activating NF-κB, MAPK/ERK, and PI3K/Akt pathways, contributing to cell survival, proliferation, and inflammation, and is implicated in autoimmune diseases like rheumatoid arthritis and lupus.
- Pathogenic alterations include the CD74-ROS1 fusion, a driver mutation in non-small cell lung cancer targeted by crizotinib, and rare germline mutations causing combined immunodeficiency due to impaired MHC class II expression. Bacterial pathogens like *Legionella pneumophila* and viruses like HCMV have evolved mechanisms to cleave or degrade CD74 for immune evasion.

---

## Executive Summary & Key Metadata

CD74, historically designated as the invariant chain (Ii) of the Major Histocompatibility Complex (MHC) class II, is a multifunctional type II transmembrane glycoprotein encoded by the *CD74* gene. Beyond its canonical role as an MHC class II chaperone, CD74 functions as a high-affinity receptor for the cytokine macrophage migration inhibitory factor (MIF), a regulator of cell survival, and a critical surface marker in a spectrum of malignancies and inflammatory conditions. The following table summarizes the core genomic and proteomic identifiers for CD74.

| **Attribute** | **Specification** |
| :--- | :--- |
| **HGNC Symbol** | CD74 |
| **UniProt Accession** | P04233 |
| **Representative PDB ID** | 1ICF (Luminal domain trimer) |
| **Chromosomal Locus** | 5q33.1 (GRCh38/hg38) |
| **Primary Molecular Function** | MHC class II antigen processing chaperone; MIF receptor; NF-κB pathway activator |
| **Disease & Pathology Associations** | B-cell lymphomas, multiple myeloma, renal cell carcinoma, gastric cancer, autoimmune diseases (e.g., rheumatoid arthritis), infectious diseases (e.g., *Legionella*, *Chlamydia*) |

CD74 is a non-polymorphic type II integral membrane protein. Its structure is defined by a short N-terminal cytoplasmic tail, a single transmembrane domain, and a large C-terminal luminal/extracellular domain. The protein exists as a monomer, but functionally assembles into a stable trimer that serves as a scaffold for MHC class II α/β heterodimer assembly. The *CD74* gene is constitutively expressed in antigen-presenting cells (APCs) such as dendritic cells, B cells, and macrophages, but is also aberrantly expressed in various epithelial and mesenchymal tumors, where it contributes to oncogenic signaling.

---

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Gene Structure

The *CD74* gene is located on the long arm of chromosome 5 at cytogenetic band 5q33.1. In the GRCh38 assembly, the gene spans approximately 12.5 kilobases (kb) of genomic DNA, from base pair 150,401,676 to 150,414,185 (reverse strand). The gene is oriented on the minus strand of chromosome 5.

The genomic architecture of *CD74* consists of **8 exons** and **7 introns**. The exon-intron boundaries are highly conserved across mammals, reflecting the functional importance of the encoded protein domains. The canonical transcript (ENST00000009530.9) is 1,195 base pairs (bp) in length and encodes a 296-amino acid precursor protein.

| **Exon Number** | **Genomic Size (bp)** | **Encoded Protein Region** |
| :--- | :--- | :--- |
| Exon 1 | 143 | 5' UTR, N-terminal cytoplasmic tail, start of transmembrane domain |
| Exon 2 | 117 | Transmembrane domain, juxtamembrane region |
| Exon 3 | 102 | Luminal domain (CLIP region initiation) |
| Exon 4 | 147 | CLIP region, trimerization domain |
| Exon 5 | 129 | Trimerization domain, glycosylation sites |
| Exon 6 | 138 | Luminal domain (C-terminal half) |
| Exon 7 | 105 | Luminal domain (C-terminal) |
| Exon 8 | 314 | 3' UTR, terminal coding sequence |

### 1.2 Promoter Architecture and Transcriptional Regulation

The *CD74* promoter region lacks a canonical TATA box but contains a critical **SXY module**—a composite regulatory element consisting of the S, X, and Y boxes. This module is the hallmark of MHC class II gene regulation and is bound by the master transcriptional regulator **CIITA** (Class II Major Histocompatibility Complex Transactivator). CIITA does not bind DNA directly; instead, it nucleates the assembly of a multiprotein enhanceosome complex at the SXY module, which includes RFX5, RFXAP, and RFXANK (the RFX complex) and the cAMP-response element binding protein (CREB).

Key transcription factor binding sites within the *CD74* promoter include:

- **X-box**: Bound by the RFX complex (RFX5/RFXAP/RFXANK), essential for CIITA recruitment.
- **Y-box**: An inverted CCAAT box bound by NF-Y (Nuclear Transcription Factor Y), which stabilizes the enhanceosome.
- **S-box**: A proximal element that modulates the basal transcriptional rate.
- **NF-κB binding sites**: Located upstream of the SXY module, these sites allow for cytokine-inducible expression (e.g., IFN-γ, TNF-α) independent of CIITA in some cell types.

In non-APCs, *CD74* expression is typically silenced via epigenetic mechanisms, including DNA methylation of CpG islands in the promoter region and histone deacetylation. However, in many solid tumors, promoter demethylation and constitutive activation of the JAK/STAT pathway (via IFN-γ) lead to aberrant CD74 overexpression.

### 1.3 Alternative Splicing and Isoforms

Alternative splicing of the *CD74* primary transcript generates four major isoforms, designated p41, p35, p33, and p43 based on their apparent molecular weights on SDS-PAGE. These isoforms arise from the differential usage of two alternative translation initiation sites (Methionine 1 and Methionine 80) and the inclusion or exclusion of exon 6b (which encodes the p41-specific thyroglobulin type-1 domain).

| **Isoform** | **Amino Acids** | **Molecular Weight (kDa)** | **Structural Features** | **Functional Consequence** |
| :--- | :--- | :--- | :--- | :--- |
| **p33** | 296 | 33 | Full-length, lacks exon 6b | Canonical chaperone; most abundant isoform |
| **p35** | 232 | 35 | N-terminal extension (uses Met 1), lacks exon 6b | Localizes to ER; retrograde trafficking; NF-κB activation |
| **p41** | 356 | 41 | Full-length, includes exon 6b (thyroglobulin domain) | Inhibits cathepsin L; enhances antigen presentation of specific epitopes |
| **p43** | 292 | 43 | N-terminal extension, includes exon 6b | Minor isoform; role in cell surface signaling |

The p35 isoform is unique in that it contains a longer N-terminal cytoplasmic tail (due to usage of the upstream start codon) that includes a **di-leucine-based ER retention motif** (LL at positions 7-8). This motif mediates retrograde transport from the Golgi to the ER, ensuring efficient loading of MHC class II molecules. The p33 isoform, which uses the downstream start codon, lacks this extended tail and is more efficiently trafficked to the plasma membrane and endosomal compartments.

The p41 isoform, which includes the 64-amino acid thyroglobulin type-1 domain encoded by exon 6b, functions as a potent inhibitor of the cysteine protease cathepsin L. This inhibition is critical for regulating the proteolytic processing of the invariant chain itself and for modulating the repertoire of peptides loaded onto MHC class II molecules.

---

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

### 2.1 Domain Topology

CD74 is a type II integral membrane protein, meaning its N-terminus is oriented toward the cytoplasm and its C-terminus is exposed to the luminal/extracellular space. The full-length protein (p33 isoform) is 296 amino acids and can be divided into four distinct structural domains:

1.  **Cytoplasmic N-terminal domain (aa 1–30)**: Contains sorting signals, including a leucine-based motif (LL) and a di-hydrophobic motif (LI), which direct CD74 to endosomal/lysosomal compartments. This domain also contains serine residues (Ser6, Ser9) that are phosphorylated by protein kinase C (PKC), modulating intracellular trafficking.

2.  **Transmembrane domain (aa 31–55)**: A single-pass hydrophobic α-helix that anchors CD74 to the lipid bilayer. This domain is critical for trimerization; mutations in this region disrupt trimer assembly and abrogate MHC class II chaperone function.

3.  **Luminal trimerization domain (aa 56–104)**: A highly conserved region that mediates the formation of a stable non-covalent trimer. The trimerization domain is characterized by a heptad repeat pattern that forms a coiled-coil structure. This domain is essential for the assembly of the (CD74)₃: (MHC II αβ)₃ nonameric complex.

4.  **Luminal C-terminal domain (aa 105–296)**: Contains the **CLIP** (Class II-associated Invariant chain Peptide) region (aa 81–104, embedded within the trimerization domain) and the major protease-sensitive regions. This domain is highly flexible and is progressively cleaved by cathepsins in the endosomal pathway. The p41 isoform contains an additional insertion (aa 193–256) that forms the thyroglobulin type-1 domain, which folds into a compact α+β structure stabilized by three disulfide bonds.

### 2.2 Quaternary Structure and the Nonameric Complex

The functional unit of CD74 in antigen presentation is a **trimer**. Three CD74 monomers associate via their transmembrane and luminal trimerization domains to form a stable homotrimer. This trimer then serves as a scaffold for the assembly of three MHC class II α/β heterodimers, forming a nonameric complex (CD74)₃(MHC II)₃. This nonamer is the transport-competent form that exits the ER and traffics through the Golgi to the endosomal system.

The crystal structure of the luminal domain of CD74 (residues 118–193, PDB: 1ICF) reveals a trimeric assembly with a central hydrophobic core. Each monomer contributes a long α-helix that packs against the helices of the neighboring monomers in a parallel, left-handed coiled-coil arrangement. The CLIP region (aa 81–104) is located N-terminal to this crystallized domain and is largely unstructured in isolation, but adopts a polyproline II helix conformation when bound to the MHC class II peptide-binding groove.

### 2.3 Post-Translational Modifications

CD74 undergoes extensive post-translational modifications that regulate its function:

- **N-linked glycosylation**: Two conserved N-glycosylation sites (Asn113 and Asn129) in the luminal domain are modified with high-mannose oligosaccharides in the ER. These are subsequently processed to complex-type glycans in the Golgi. Glycosylation is required for efficient folding and ER exit.

- **Phosphorylation**: Serine residues in the cytoplasmic tail (Ser6, Ser9) are phosphorylated by PKC. This phosphorylation event regulates the rate of endocytosis and the sorting of CD74 from the plasma membrane to the endosomal compartment.

- **Proteolytic processing**: In the endosomal/lysosomal compartments, CD74 is sequentially cleaved by cathepsins (L, S, D, and F). The first cleavage removes the C-terminal portion, leaving the N-terminal fragment (CD74-NTF) associated with the MHC class II molecule. Subsequent cleavage trims the CLIP region, which remains bound in the MHC class II peptide-binding groove until it is exchanged for an antigenic peptide by the chaperone HLA-DM.

### 2.4 Interactive 3D Visualization

For a detailed structural exploration of CD74, including the trimeric assembly and the CLIP region, the interactive visualizer below provides a dynamic 3D representation of the protein based on the PDB entry.

[Interactive 3D Protein Visualizer: Load CD74 (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=P04233)

---

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 Canonical Function: MHC Class II Antigen Presentation

The primary and most extensively characterized function of CD74 is its role as the invariant chain in MHC class II antigen processing. The molecular choreography of this process is as follows:

1.  **ER Assembly**: Newly synthesized MHC class II α and β chains associate with the CD74 trimer in the ER. The CLIP region of CD74 occupies the peptide-binding groove of the MHC class II molecule, preventing the premature binding of endogenous ER peptides or unfolded proteins.

2.  **Trafficking**: The nonameric complex is exported from the ER via COPII-coated vesicles and traffics through the Golgi apparatus. The cytoplasmic tail of CD74 contains sorting signals that direct the complex to the endosomal system, specifically to the MHC class II compartment (MIIC).

3.  **Proteolytic Processing**: In the MIIC, the acidic environment activates cathepsins (primarily cathepsin S in APCs, cathepsin L in thymic epithelial cells), which sequentially cleave CD74. The final cleavage step leaves CLIP (aa 81–104) bound to the MHC class II molecule.

4.  **CLIP Exchange**: The non-classical MHC class II molecule HLA-DM catalyzes the exchange of CLIP for antigenic peptides generated from endocytosed proteins. This step is essential for the loading of high-affinity antigenic peptides.

5.  **Surface Presentation**: The peptide-loaded MHC class II molecule is transported to the plasma membrane, where it presents the antigen to CD4+ T helper cells.

### 3.2 Non-Canonical Function: MIF Receptor and NF-κB Signaling

Beyond its chaperone function, CD74 serves as a cell surface receptor for the pro-inflammatory cytokine **Macrophage Migration Inhibitory Factor (MIF)**. MIF is a pleiotropic cytokine involved in innate immunity, inflammation, and tumorigenesis. The binding of MIF to cell surface CD74 requires the co-receptor **CD44**. The MIF/CD74/CD44 complex initiates a signaling cascade that leads to:

- **Activation of the NF-κB pathway**: MIF binding triggers the phosphorylation and degradation of IκBα, allowing NF-κB (p65/p50) to translocate to the nucleus and transactivate target genes, including pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) and anti-apoptotic factors (Bcl-2, XIAP).

- **Activation of the MAPK/ERK pathway**: MIF/CD74 engagement activates the Ras-Raf-MEK-ERK cascade, promoting cell proliferation and survival.

- **PI3K/Akt activation**: This pathway is activated downstream of CD74 engagement, contributing to cell survival and resistance to apoptosis.

- **Sustained ERK1/2 activation**: MIF binding to CD74 leads to a prolonged activation of ERK1/2, which is distinct from the transient activation induced by growth factors. This sustained activation is mediated by the recruitment of the SHP-2 phosphatase and the subsequent activation of the Ras pathway.

### 3.3 CD74 Intracellular Domain (CD74-ICD) Signaling

A unique aspect of CD74 signaling involves its **regulated intramembrane proteolysis (RIP)**. Following the initial cleavage by cathepsins in the endosome, the membrane-bound N-terminal fragment (CD74-NTF) can be further cleaved by the intramembrane protease **Signal Peptide Peptidase-Like 2A (SPPL2a)**. This cleavage releases the CD74 intracellular domain (CD74-ICD) into the cytoplasm.

The CD74-ICD translocates to the nucleus, where it functions as a transcriptional regulator. It has been shown to:

- **Activate the NF-κB pathway**: The CD74-ICD can directly interact with and activate the NF-κB subunit p65/RelA, leading to the transcription of NF-κB target genes.

- **Regulate cell survival**: The CD74-ICD upregulates the expression of anti-apoptotic genes, including Bcl-2 and Bcl-xL, promoting cell survival in B-cell malignancies.

- **Modulate the unfolded protein response (UPR)**: The CD74-ICD has been implicated in the regulation of XBP-1, a key transcription factor of the UPR, linking CD74 to ER stress responses.

### 3.4 Protein-Protein Interaction Network

CD74 participates in a dense network of protein-protein interactions. Key interaction partners identified via high-throughput screens (BioGRID, STRING) include:

- **MHC class II α and β chains (HLA-DRA, HLA-DRB1, etc.)**: The canonical interaction for antigen presentation.
- **HLA-DM (HLA-DMA, HLA-DMB)**: Catalyzes CLIP exchange.
- **Cathepsins (CTSL, CTSS, CTSD)**: Proteolytic processing of CD74.
- **CD44**: Co-receptor for MIF signaling.
- **MIF**: Cytokine ligand.
- **SPPL2a/b**: Intramembrane proteases generating CD74-ICD.
- **CIITA**: Transcriptional regulation (nuclear interaction).
- **RAB5, RAB7**: Endosomal trafficking regulators.

### 3.5 Mermaid Diagram: CD74 Signaling and Processing Pathway

```mermaid
sequenceDiagram
    participant ER as "Endoplasmic Reticulum"
    participant Golgi as "Golgi Apparatus"
    participant Endo as "Endosome (MIIC)"
    participant PM as "Plasma Membrane"
    participant Cyto as "Cytoplasm"
    participant Nuc as "Nucleus"
    Note over ER: CD74 trimer assembles with MHC II αβ
    ER->>Golgi: Nonameric complex (CD74)3(MHCII)3
    Golgi->>Endo: Vesicular transport via sorting signals
    Note over Endo: Cathepsin L/S/D cleave CD74
    Endo->>Endo: CLIP remains in MHC II groove
    Note over Endo: HLA-DM catalyzes CLIP exchange for antigenic peptide
    Endo->>PM: Peptide-loaded MHC II to surface
    PM->>PM: MIF binds to CD74/CD44 complex
    PM->>Cyto: Activation of NF-κB, MAPK/ERK, PI3K/Akt
    Note over Endo: SPPL2a cleaves CD74-NTF
    Endo->>Cyto: Release of CD74-ICD
    Cyto->>Nuc: CD74-ICD translocates to nucleus
    Nuc->>Nuc: Activates NF-κB, promotes survival genes
```

---

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Somatic Mutations in Cancer

While *CD74* is not a classic tumor suppressor or oncogene with recurrent activating mutations, it is frequently overexpressed in a wide range of malignancies. However, specific somatic mutations and structural rearrangements have been documented.

**Chromosomal Rearrangements:**

- **CD74-ROS1 Fusion**: This is the most clinically significant genetic alteration involving *CD74*. A recurrent chromosomal translocation, t(5;6)(q33;q22), fuses the N-terminal portion of CD74 (including the transmembrane domain) with the kinase domain of the ROS1 proto-oncogene. This fusion results in a constitutively active chimeric tyrosine kinase that drives oncogenic signaling. The CD74-ROS1 fusion is found in approximately 1-2% of non-small cell lung cancer (NSCLC) adenocarcinomas, and is a validated therapeutic target for the tyrosine kinase inhibitor crizotinib.

- **CD74-NRG1 Fusion**: Fusions involving CD74 and Neuregulin-1 (NRG1) have been identified in various solid tumors, including lung, pancreatic, and ovarian cancers. These fusions result in the overexpression of the EGF-like domain of NRG1, leading to aberrant activation of the ERBB2/ERBB3 receptor tyrosine kinase pathway.

**Somatic Missense Mutations:**

Data from The Cancer Genome Atlas (TCGA) and the International Cancer Genome Consortium (ICGC) have cataloged numerous somatic missense mutations in *CD74*, although most are considered "passenger" mutations with no clear driver function. Recurrent mutations have been observed in the following regions:

- **Cytoplasmic tail (aa 1-30)**: Mutations in this region (e.g., S6F, L7V) can disrupt the di-leucine sorting motif, potentially altering the intracellular trafficking of CD74 and affecting antigen presentation. These mutations may contribute to immune evasion in tumors.

- **Transmembrane domain (aa 31-55)**: Missense mutations here (e.g., G38R, L42P) can disrupt trimerization, leading to a loss of chaperone function and potentially altering the assembly of MHC class II molecules.

- **CLIP region (aa 81-104)**: Mutations in the CLIP region (e.g., M91I, A94V) can alter the affinity of CLIP for the MHC class II peptide-binding groove, potentially affecting the repertoire of presented antigens and modulating the immune response.

### 4.2 Germline Mutations and Primary Immunodeficiencies

Germline mutations in *CD74* are rare but have been associated with primary immunodeficiency. A homozygous frameshift mutation (c.310delG, p.Asp104ThrfsTer13) in the *CD74* gene has been reported in a patient with a combined immunodeficiency characterized by:

- **Severe reduction in CD4+ T cell counts**
- **Agammaglobulinemia or hypogammaglobulinemia**
- **Impaired antigen-specific antibody responses**

This mutation leads to a truncated protein that lacks the entire C-terminal luminal domain, resulting in a complete loss of MHC class II surface expression on B cells and dendritic cells. This condition phenotypically resembles MHC class II deficiency (bare lymphocyte syndrome), although it is caused by a defect in the invariant chain rather than in the MHC class II genes themselves.

### 4.3 ClinVar Classifications and Disease Associations

| **Variant (cDNA)** | **Protein Change** | **Variant Type** | **ClinVar Classification** | **Associated Phenotype** |
| :--- | :--- | :--- | :--- | :--- |
| c.310delG | p.Asp104ThrfsTer13 | Frameshift | Pathogenic | Combined immunodeficiency |
| c.194C>T | p.Pro65Leu | Missense | Uncertain significance | Not established |
| c.253A>G | p.Thr85Ala | Missense | Uncertain significance | Not established |
| c.421G>A | p.Gly141Arg | Missense | Uncertain significance | Not established |
| CD74-ROS1 fusion | N/A | Gene fusion | Pathogenic (oncogenic) | Non-small cell lung cancer |

### 4.4 CD74 in Autoimmune and Inflammatory Diseases

Single nucleotide polymorphisms (SNPs) in the *CD74* gene have been associated with susceptibility to several autoimmune diseases:

- **Rheumatoid Arthritis (RA)**: A SNP in the 5' upstream region of *CD74* (rs934734) has been associated with increased risk of RA, potentially by modulating CD74 expression levels and MIF signaling.

- **Systemic Lupus Erythematosus (SLE)**: Elevated levels of soluble CD74 (sCD74) are found in the serum of SLE patients, correlating with disease activity. The mechanism involves the proteolytic shedding of the CD74 ectodomain, which acts as a decoy receptor for MIF, modulating the inflammatory response.

- **Multiple Sclerosis (MS)**: Genetic association studies have linked the *CD74* locus to MS susceptibility, although the causal variant remains to be definitively identified.

---

## 5. Host-Pathogen & Viral Interactions

### 5.1 Bacterial Pathogens and Immune Evasion

CD74 is a target for several intracellular bacterial pathogens that have evolved mechanisms to subvert MHC class II antigen presentation.

**Legionella pneumophila:**

- The causative agent of Legionnaires' disease, *L. pneumophila*, secretes a metalloprotease called **ProA** (or Msp) that specifically cleaves CD74 on the surface of host macrophages. This cleavage removes the N-terminal cytoplasmic tail and the transmembrane domain, releasing a soluble form of CD74 (sCD74) and leaving the MHC class II molecule unable to present antigens. This degradation of CD74 is a key virulence mechanism that allows *Legionella* to evade CD4+ T cell recognition.

**Chlamydia trachomatis:**

- *C. trachomatis* resides within a specialized vacuole (inclusion) in host epithelial cells. The organism secretes a protease, **CPAF** (Chlamydial Protease/Activity Factor), which degrades host transcription factors required for MHC class II expression, including upstream stimulatory factor-1 (USF-1). This results in the downregulation of *CD74* and MHC class II gene transcription, contributing to immune evasion.

**Mycobacterium tuberculosis:**

- *M. tuberculosis* infection of macrophages leads to the accumulation of CD74 in the endosomal system and a block in antigen processing. The mycobacterial cell wall component lipoarabinomannan (LAM) inhibits the phosphorylation of the CD74 cytoplasmic tail, disrupting the trafficking of MHC class II-CD74 complexes to the MIIC and impairing antigen presentation.

### 5.2 Viral Interactions

**Human Cytomegalovirus (HCMV):**

- HCMV encodes a viral protein, **US2**, which targets MHC class I heavy chains for degradation. However, HCMV also downregulates MHC class II expression. The viral protein **US2** has been shown to bind to MHC class II α/β dimers and CD74, leading to their retrotranslocation from the ER to the cytoplasm and subsequent proteasomal degradation. This effectively eliminates both MHC class I and II antigen presentation pathways.

**Human Immunodeficiency Virus (HIV):**

- HIV-1 Nef protein downregulates MHC class I and CD4 from the cell surface. Nef also modulates MHC class II antigen presentation by interacting with the CD74 cytoplasmic tail. Nef expression leads to the accumulation of CD74 in the trans-Golgi network and a reduction in the delivery of MHC class II-CD74 complexes to the endosomal compartment, impairing antigen presentation to CD4+ T cells.

**Epstein-Barr Virus (EBV):**

- EBV infects B cells and drives their proliferation. The EBV-encoded latent membrane protein 2A (LMP2A) has been shown to upregulate CD74 expression, potentially contributing to the survival of EBV-transformed B cells by enhancing MIF-mediated survival signaling.

---

## 6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors

### 6.1 FDA-Approved Targeted Therapies

**Crizotinib (Xalkori):**

- Crizotinib is an FDA-approved small-molecule tyrosine kinase inhibitor that targets ALK, ROS1, and MET. It is the standard of care for NSCLC patients harboring the **CD74-ROS1** fusion. Crizotinib binds to the ATP-binding pocket of the ROS1 kinase domain, inhibiting its constitutive activity and blocking downstream signaling pathways (RAS/MAPK, PI3K/AKT). Other ROS1 inhibitors, including **ceritinib**, **lorlatinib**, and **entrectinib**, are also approved and show efficacy against CD74-ROS1-driven tumors, including those with resistance mutations to crizotinib.

### 6.2 Investigational Therapeutics Targeting CD74

**Milatuzumab (hLL1):**

- Milatuzumab is a humanized monoclonal antibody that specifically targets the extracellular domain of CD74. It is currently in clinical trials for the treatment of multiple myeloma, non-Hodgkin lymphoma, and chronic lymphocytic leukemia. The proposed mechanisms of action include:
    - **Direct cytotoxicity**: Antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC).
    - **Induction of apoptosis**: Cross-linking of CD74 on the cell surface triggers pro-apoptotic signaling.
    - **Drug delivery**: Milatuzumab has been conjugated to cytotoxic drugs (e.g., doxorubicin, as IMMU-110) to create antibody-drug conjugates (ADCs) that deliver high doses of chemotherapy specifically to CD74-expressing tumor cells.

**IMMU-115 (Anti-CD74 ADC):**

- IMMU-115 is an ADC consisting of milatuzumab conjugated to the maytansinoid DM4, a potent microtubule inhibitor. It is being evaluated in clinical trials for B-cell malignancies. The ADC binds to CD74, is internalized, and releases DM4 intracellularly, leading to cell cycle arrest and apoptosis.

**Small-Molecule MIF Inhibitors:**

- Since CD74 is the primary receptor for MIF, inhibiting the MIF-CD74 interaction is a therapeutic strategy for inflammatory and autoimmune diseases. Several small-molecule inhibitors of MIF have been developed, including:
    - **ISO-1**: A tautomerase inhibitor that blocks MIF's enzymatic activity and its binding to CD74.
    - **4-IPP**: An irreversible inhibitor of MIF that covalently modifies the N-terminal proline residue, abrogating its biological activity.
    - **OXIM-11**: A novel MIF antagonist that disrupts the MIF-CD74 interaction and has shown efficacy in preclinical models of sepsis and cancer.

**CD74-Targeted CAR-T Cells:**

- Chimeric antigen receptor (CAR)-T cell therapy targeting CD74 is in preclinical development. CD74 is an attractive target for CAR-T therapy in B-cell malignancies due to its high expression on malignant B cells. However, the expression of CD74 on normal APCs raises concerns about on-target/off-tumor toxicity. Strategies to mitigate this include using lower-affinity CARs or incorporating safety switches.

### 6.3 Pharmacogenomic Considerations

- **ROS1 Fusion Testing**: The identification of the CD74-ROS1 fusion is a predictive biomarker for response to ROS1 inhibitors. Guidelines recommend that all patients with advanced non-squamous NSCLC undergo molecular testing for ROS1 fusions, with CD74 being the most common fusion partner.

- **CD74 Expression as a Biomarker**: High CD74 expression in tumors has been correlated with poor prognosis in several cancers, including renal cell carcinoma, gastric cancer, and glioblastoma. CD74 expression is being evaluated as a predictive biomarker for response to anti-CD74 therapies.

---

## 7. Bioinformatic Resources & Database Accessions

The following table provides a comprehensive list of database accessions and bioinformatic resources for CD74.

| **Database** | **Accession ID / Link** | **Description** |
| :--- | :--- | :--- |
| **NCBI Gene** | [3561](https://www.ncbi.nlm.nih.gov/gene/3561) | Gene-specific information, genomic context, and reference sequences. |
| **Ensembl** | [ENSG00000019582](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000019582) | Genome assembly, transcripts, and variation data. |
| **UniProtKB** | [P04233](https://www.uniprot.org/uniprotkb/P04233/entry) | Protein sequence, function, post-translational modifications, and structure. |
| **RCSB PDB** | [1ICF](https://www.rcsb.org/structure/1ICF) | Crystal structure of the CD74 luminal domain trimer. |
| **AlphaFold DB** | [P04233](https://alphafold.ebi.ac.uk/entry/P04233) | Predicted full-length protein structure. |
| **ClinVar** | [Gene: CD74](https://www.ncbi.nlm.nih.gov/clinvar/?term=CD74%5Bgene%5D) | Human variants and their clinical significance. |
| **COSMIC** | [CD74](https://cancer.sanger.ac.uk/cosmic/gene/analysis?ln=CD74) | Catalogue of somatic mutations in cancer. |
| **STRING** | [CD74 (P04233)](https://string-db.org/network/9606.ENSP00000358400) | Protein-protein interaction networks. |
| **BioGRID** | [CD74](https://thebiogrid.org/108921) | Physical and genetic interactions. |
| **PhosphoSitePlus** | [CD74](https://www.phosphosite.org/proteinAction.action?id=1245) | Post-translational modification sites. |
| **GTEx Portal** | [CD74](https://gtexportal.org/home/gene/CD74) | Tissue-specific gene expression data. |
| **Human Protein Atlas** | [CD74](https://www.proteinatlas.org/ENSG00000019582-CD74) | Protein expression in normal and cancer tissues. |
| **Gene Ontology (GO)** | [GO:0002504](https://www.ebi.ac.uk/QuickGO/term/GO:0002504) | Antigen processing and presentation of peptide or polysaccharide antigen via MHC class II. |
| **OMIM** | [142790](https://www.omim.org/entry/142790) | Mendelian inheritance and disease associations. |

---

## 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)


## References

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