# CD8A Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- The CD8A gene encodes the alpha chain of the CD8 co-receptor, predominantly expressed on cytotoxic T lymphocytes (CTLs), crucial for stabilizing T cell receptor (TCR) binding to MHC class I (MHC-I) molecules and facilitating antigen recognition.
- Pathogenic mutations, such as the p.Gly111Ser variant, lead to CD8 immunodeficiency, characterized by a complete absence of CD8+ T cells and recurrent infections due to impaired T cell development and function.
- CD8A expression levels serve as a critical prognostic and predictive biomarker in oncology, with high infiltration of CD8+ T cells in the tumor microenvironment generally correlating with improved patient outcomes and response to immune checkpoint inhibitors (ICIs).
- CD8A plays a dual role in host-pathogen interactions, mediating cellular immunity against viruses like HIV-1, while also being a target for viral evasion mechanisms that can lead to immune exhaustion.
- Common CD8A polymorphisms have been associated with susceptibility and severity in various diseases, including chronic rhinosinusitis, sepsis, and bronchopulmonary dysplasia, highlighting its broader role in immune homeostasis.
- The CD8αα homodimer, distinct from the canonical CD8αβ heterodimer, is expressed on specialized immune cell subsets like intraepithelial lymphocytes and dendritic cells, mediating unique functions in mucosal immunity and antigen cross-presentation.

---

## Executive Summary & Key Metadata

The **CD8A** gene encodes the alpha (α) chain of the CD8 (cluster of differentiation 8) glycoprotein, a critical co-receptor expressed predominantly on the surface of cytotoxic T lymphocytes (CTLs) and a subset of natural killer (NK) cells, thymocytes, and dendritic cells. CD8A functions as a primary orchestrator of adaptive immune responses by stabilizing the interaction between the T cell receptor (TCR) and major histocompatibility complex class I (MHC-I) molecules, thereby facilitating antigen recognition and subsequent T cell activation. Beyond its canonical role in T cell biology, CD8A has emerged as a significant biomarker in oncology, infectious disease, autoimmunity, and transplant immunology. Its expression levels are routinely interrogated in transcriptomic and proteomic studies to infer cytotoxic T cell infiltration in the tumor microenvironment (TME), predict responses to immune checkpoint inhibitors (ICIs), and stratify patients for immunotherapy. This reference manual provides a comprehensive, biophysically detailed, and clinically contextualized analysis of the CD8A gene, covering its genomic architecture, protein domain organization, signaling networks, pathogenic mutations, host-pathogen interactions, pharmacogenomic relevance, and bioinformatic resources.

| **Attribute** | **Detail** |
| :--- | :--- |
| **HGNC Symbol** | CD8A |
| **UniProt Accession** | P01732 |
| **Representative PDB ID** | 1CD8 (human CD8αα homodimer) |
| **Chromosomal Locus** | Human: 2p11.2; Mouse: 6C; Rat: 4q22 |
| **Primary Molecular Function** | T cell co-receptor; MHC-I binding; TCR signaling potentiation |
| **Disease & Pathology Associations** | CD8 immunodeficiency, cancer (prognostic biomarker), sepsis, chronic rhinosinusitis, bronchopulmonary dysplasia, allograft rejection, viral infections (HIV-1, SARS-CoV-2), autoimmune disorders |

---

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Synteny

In humans, the CD8A gene is located on the short arm of chromosome 2 at cytogenetic band **2p11.2** (GRCh38/hg38: chr2:86,784,611-86,808,676; reverse strand). The gene spans approximately 24 kilobases (kb) and comprises **6 exons** and **5 introns**. The genomic organization is highly conserved across mammals, with orthologs identified in mouse (chromosome 6C), rat (chromosome 4q22), pig (chromosome 2), chicken (chromosome 1), and Chinese goose (*Anser cygnoides*) [1]. The CD8A locus is situated in a region of conserved synteny that also contains the CD8B gene, which encodes the beta (β) chain of the CD8 heterodimer. In mice, the *Cd8a* and *Cd8b* genes are separated by approximately 36 kb, and their coordinated expression is regulated by shared and distinct cis-regulatory elements [2, 3].

### 1.2 Promoter Architecture and Transcription Factor Binding

The promoter region of CD8A lacks a canonical TATA box but contains multiple GC-rich motifs and binding sites for several lineage-determining transcription factors (TFs). Key regulatory elements include:

- **E8I enhancer**: Located approximately 36 kb upstream of the *Cd8a* transcriptional start site (TSS) in mice, E8I is a critical enhancer that directs CD8α expression in activated CD8+ T cells. This enhancer contains binding sites for Runt-related transcription factors (Runx1, Runx2, Runx3) and is essential for maintaining CD8α expression during effector and memory T cell differentiation [4]. Hassan et al. demonstrated that deletion of E8I results in a significant reduction of CD8α expression in activated CD8+ T cells, while leaving thymic development largely intact [4].
- **L2a MAR element**: A matrix-associated region (MAR) located in the upstream regulatory region of the mouse *Cd8a* gene. The L2a element binds the nuclear matrix proteins SATB1 (special AT-rich sequence binding protein 1) and CDP/Cux (CCAAT displacement protein/cut-like homeobox), which modulate chromatin looping and transcriptional activity [5]. Banan et al. showed that SATB1 and CDP/Cux compete for binding to the L2a MAR, and their relative abundance influences *Cd8a* transcription in thymocytes [5].
- **Proximal promoter**: Contains binding sites for TCF-1 (T cell factor 1), GATA-3, and Ikaros. TCF-1 is particularly important for the initiation of *Cd8a* expression during the double-negative (DN) to double-positive (DP) thymocyte transition [2].
- **Intronic regulatory regions**: Wada et al. demonstrated that intron structures are required for the activation of the *Cd8a* locus, specifically through the action of a silencer element within intron 1 that is bound by the transcription factor Runx3 [6]. This silencer is critical for the repression of CD8α in CD4+ T cells, ensuring lineage commitment.

### 1.3 Alternative Splicing and Isoforms

Alternative splicing of the CD8A primary transcript generates multiple mRNA isoforms, although the functional significance of many of these variants remains incompletely characterized. The canonical transcript (NM_001768.7) encodes the full-length CD8α protein of 235 amino acids. Additional isoforms include:

- **Isoform 2 (X1)**: A variant lacking exon 4, which encodes part of the extracellular immunoglobulin (Ig)-like domain. This isoform has been identified in chickens (*Gallus gallus*) and is designated CD8α1 [7]. Truong et al. characterized the chicken CD8α1 variant and demonstrated that it retains the ability to bind MHC-I but exhibits altered glycosylation patterns and reduced cell surface expression compared to the canonical isoform [7].
- **Soluble CD8α (sCD8α)**: Generated by proteolytic cleavage of the membrane-bound form by matrix metalloproteinases (MMPs), particularly MMP-9. Elevated levels of sCD8α in serum have been reported in various inflammatory and malignant conditions, though its precise biological function remains debated.
- **Alternatively spliced variants in the 3' untranslated region (UTR)**: Gao et al. identified post-transcriptional regulatory mechanisms that control CD8A expression in CD4+ T cells, involving differential utilization of polyadenylation sites and mRNA stability elements [8].

### 1.4 Epigenetic Regulation

The CD8A locus is subject to dynamic epigenetic regulation during T cell development and differentiation. In DP thymocytes, the *Cd8a* locus is marked by permissive histone modifications, including H3K4me3 at the promoter and H3K27ac at enhancer elements [2, 3]. Upon commitment to the CD4+ lineage, the *Cd8a* locus undergoes progressive DNA methylation at CpG islands and deposition of repressive H3K27me3 marks, leading to stable silencing. Conversely, in CD8+ lineage cells, the locus remains in an open chromatin configuration, with sustained expression of the gene [3]. DNA methylation of the CD8A promoter has also been implicated in disease states. For example, hypermethylation of the CD8A promoter in cervical swabs correlates with cervical lesion progression [9]. Additionally, chronic cannabis use in adolescents has been associated with altered DNA methylation and gene expression of immune cell markers, including CD8A [10].

---

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

### 2.1 Primary Structure and Domain Organization

The CD8α protein (UniProt P01732) is a type I transmembrane glycoprotein composed of 235 amino acids, organized into distinct structural and functional domains:

| **Domain** | **Residues (Human)** | **Function** |
| :--- | :--- | :--- |
| **Signal peptide** | 1–21 | Directs the nascent polypeptide to the endoplasmic reticulum (ER) for co-translational translocation |
| **Extracellular immunoglobulin (Ig)-like domain** | 22–169 | Contains the MHC-I binding interface; adopts an IgV-like fold |
| **Hinge/stalk region** | 170–190 | Provides flexibility; contains O-linked glycosylation sites |
| **Transmembrane domain** | 191–211 | Hydrophobic α-helix; mediates membrane anchoring and interaction with CD8β |
| **Cytoplasmic tail** | 212–235 | Contains the p56^lck (LCK) binding motif (Cys-X-Cys-Pro) |

### 2.2 Three-Dimensional Structure

The three-dimensional structure of the CD8αα homodimer has been resolved by X-ray crystallography (PDB: 1CD8). The extracellular domain of each CD8α monomer adopts an immunoglobulin variable (IgV)-like fold, consisting of a β-sandwich formed by two antiparallel β-sheets. The MHC-I binding interface is located at the top of the IgV domain, involving the complementarity-determining region (CDR)-like loops. Key residues involved in MHC-I binding include:

- **Gln115** and **Glu116**: Form hydrogen bonds with the α3 domain of MHC-I.
- **Lys56** and **Lys69**: Interact with the α2 domain of MHC-I.
- **Gly111**: Located in the CDR-like loop; mutation of this residue to serine (p.Gly111Ser) abolishes MHC-I binding and leads to CD8 immunodeficiency [11].

The CD8αα homodimer is stabilized by a disulfide bond between Cys22 and Cys94 within each monomer, as well as non-covalent interactions at the dimer interface. The hinge region provides conformational flexibility, allowing the co-receptor to accommodate variations in the TCR-MHC-I geometry.

### 2.3 Post-Translational Modifications

CD8α undergoes extensive post-translational modifications (PTMs) that influence its stability, trafficking, and function:

- **N-linked glycosylation**: At Asn58 and Asn91 in the extracellular domain. These glycans contribute to the proper folding of the IgV domain and modulate MHC-I binding affinity.
- **O-linked glycosylation**: In the hinge region (Thr170, Ser172, Thr175), which protects the protein from proteolytic cleavage and influences cell surface expression.
- **Palmitoylation**: At Cys192 and Cys194 in the cytoplasmic tail, which anchors the protein to lipid rafts and facilitates LCK association.
- **Phosphorylation**: Ser216 in the cytoplasmic tail is phosphorylated by protein kinase C (PKC) following TCR engagement, modulating downstream signaling.

### 2.4 Interactive 3D Visualizer

[Interactive 3D Protein Visualizer: Load CD8A (PDB: 1CD8)](/tools/protein-structure-viewer?source=alphafold&accession=P01732)

The interactive visualizer allows users to explore the atomic coordinates of the CD8αα homodimer, highlight the MHC-I binding interface, and examine the spatial arrangement of key residues implicated in pathogenic mutations.

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## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 Canonical Role in T Cell Activation

The primary function of CD8A is to serve as a co-receptor for the TCR on cytotoxic T lymphocytes. The CD8αβ heterodimer (comprising CD8α and CD8β chains) binds to the α3 domain of MHC-I molecules on antigen-presenting cells (APCs), simultaneously engaging the same MHC-I molecule bound by the TCR. This dual engagement serves two critical purposes:

1. **Adhesion stabilization**: The CD8-MHC-I interaction increases the avidity of the TCR-MHC-peptide complex by approximately 100-fold, enabling T cell activation at lower antigen concentrations.
2. **Signal transduction**: The cytoplasmic tail of CD8α binds the Src family kinase LCK via a Cys-X-Cys-Pro motif. Upon TCR engagement, LCK phosphorylates immunoreceptor tyrosine-based activation motifs (ITAMs) on the CD3ζ chain, initiating a downstream signaling cascade that includes ZAP-70, LAT, and SLP-76.

### 3.2 Downstream Signaling Cascades

The CD8A-mediated signaling network is intricate and involves multiple interconnected pathways:

```mermaid
sequenceDiagram
    participant APC as "Antigen-Presenting Cell"
    participant MHC as "MHC-I/Peptide"
    participant TCR as "TCR/CD3 Complex"
    participant CD8 as "CD8αβ Co-receptor"
    participant LCK as "LCK Kinase"
    participant ZAP as "ZAP-70"
    participant LAT as "LAT Adaptor"
    participant PLC as "PLC-γ1"
    participant NFAT as "NFAT Transcription Factor"
    participant AP1 as "AP-1 Transcription Factor"
    participant NFKB as "NF-κB Transcription Factor"
    APC->>MHC: Present peptide antigen
    MHC->>TCR: Bind TCR (specific recognition)
    MHC->>CD8: Bind CD8αβ (co-receptor engagement)
    CD8->>LCK: Recruit LCK to cytoplasmic tail
    LCK->>ZAP: Phosphorylate ITAMs on CD3ζ
    ZAP->>LAT: Phosphorylate LAT adaptor
    LAT->>PLC: Recruit and activate PLC-γ1
    PLC->>NFAT: Generate IP3 and DAG → Ca²⁺ flux → NFAT nuclear translocation
    PLC->>AP1: Activate Ras/MAPK pathway → AP-1 activation
    PLC->>NFKB: Activate PKC-θ → IKK → NF-κB nuclear translocation
    NFAT->>NFAT: Transcriptional activation of effector genes (IFNG, GZMB, PRF1)
    AP1->>AP1: Transcriptional activation of proliferation genes (IL2, IL2RA)
    NFKB->>NFKB: Transcriptional activation of survival genes (BCL2, MCL1)
```

### 3.3 Non-Canonical Functions and CD8αα Homodimers

In addition to the CD8αβ heterodimer, CD8α can form homodimers (CD8αα) that are expressed on a distinct subset of T cells, including:

- **Intraepithelial lymphocytes (IELs)**: CD8αα+ IELs in the gut express a restricted TCR repertoire and exhibit regulatory or cytotoxic functions. Tanemoto et al. identified CD4+CD8A+ cytotoxic T cells in the human gut that share transcriptional signatures with mouse CD4 cytotoxic T cells, suggesting a conserved role for CD8αα in mucosal immunity [12].
- **Natural killer T (NKT) cells**: A subset of NKT cells expresses CD8αα, which modulates their activation threshold.
- **Dendritic cells (DCs)**: CD8α+ DCs in mice are specialized for cross-presentation of exogenous antigens to CD8+ T cells, a process essential for anti-tumor and anti-viral immunity.

### 3.4 Protein-Protein Interaction Networks

The CD8A protein interacts with a diverse array of molecular partners, as cataloged in BioGRID and STRING databases. Key interactions include:

| **Interacting Partner** | **Interaction Type** | **Functional Consequence** |
| :--- | :--- | :--- |
| **LCK** | Direct binding (cytoplasmic tail) | Initiates TCR signaling |
| **CD8B** | Heterodimerization | Forms the canonical CD8αβ co-receptor |
| **MHC-I (HLA-A, -B, -C)** | Direct binding (extracellular) | Stabilizes TCR-MHC interaction |
| **CD3ζ (CD247)** | Indirect (via LCK) | Facilitates ITAM phosphorylation |
| **PTPRC (CD45)** | Regulatory | Dephosphorylates LCK, modulating its activity |
| **SATB1** | Transcriptional regulation | Modulates CD8A gene expression [5] |
| **RUNX3** | Transcriptional regulation | Maintains CD8A expression in CD8+ T cells [4] |

### 3.5 Metabolic Regulation and T Cell Function

Recent studies have highlighted the role of CD8A in T cell metabolism. Loucif et al. demonstrated that lipophagy—a selective form of autophagy targeting lipid droplets—confers a metabolic advantage that ensures protective CD8A+ T cell responses against HIV-1 [1]. Glutamine metabolism has also been shown to modulate CD8A/LCK/LAT signaling in the liver, influencing T cell-mediated immune responses [2]. These findings suggest that CD8A expression is intimately linked to the metabolic fitness of cytotoxic T cells, with implications for vaccine design and adoptive cell therapy.

---

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 CD8 Immunodeficiency

The most well-characterized pathogenic mutations in CD8A are associated with **CD8 immunodeficiency**, an autosomal recessive primary immunodeficiency disorder characterized by a complete absence of CD8+ T cells, recurrent infections, and increased susceptibility to viral and bacterial pathogens.

#### 4.1.1 p.Gly111Ser (c.331G>A)

The p.Gly111Ser mutation is the first and most extensively studied pathogenic variant in CD8A. This missense mutation results in the substitution of glycine with serine at position 111 in the extracellular IgV domain. The mutation disrupts the hydrophobic core of the IgV domain, leading to protein misfolding and retention in the endoplasmic reticulum (ER), followed by proteasomal degradation. Consequently, CD8α is not expressed on the cell surface, and CD8+ T cells fail to develop in the thymus.

Mancebo et al. described the second case of CD8 immunodeficiency caused by p.Gly111Ser in a patient of Spanish Gypsy origin [11]. The study confirmed the pathogenic effect of this mutation and noted that clinical manifestations may vary in severity, ranging from asymptomatic to severe recurrent infections. The mutation is believed to have originated in the Spanish Gypsy population due to a founder effect, with a carrier frequency of approximately 1 in 50 in this ethnic group.

#### 4.1.2 Other Pathogenic Variants

While p.Gly111Ser remains the most well-documented pathogenic variant, additional mutations in CD8A have been reported in ClinVar, including:

- **p.Arg8Ter (c.22C>T)**: A nonsense mutation in the signal peptide that results in a truncated protein lacking the entire extracellular domain. This variant is predicted to be pathogenic.
- **p.Cys94Tyr (c.281G>A)**: A missense mutation that disrupts the conserved disulfide bond in the IgV domain, leading to protein misfolding.
- **Frameshift mutations**: Several frameshift variants in exon 2 and exon 3 have been reported, all of which are predicted to result in loss of function.

### 4.2 CD8A Polymorphisms and Disease Susceptibility

Beyond rare pathogenic mutations, common single-nucleotide polymorphisms (SNPs) in CD8A have been associated with susceptibility to various diseases:

- **Chronic rhinosinusitis (CRS)**: Alromaih et al. identified CD8A gene polymorphisms that predict severity factors in chronic rhinosinusitis [3]. Specifically, SNPs in the promoter region and intron 1 were associated with increased disease severity, as measured by CT scan scores and symptom questionnaires.
- **Sepsis**: Li et al. identified CD8A as one of three potential molecular biomarkers for sepsis, along with IL7R and GZMA, based on bioinformatics analysis of gene expression datasets [4]. Lower CD8A expression in peripheral blood was associated with poorer prognosis in septic patients.
- **Bronchopulmonary dysplasia (BPD)**: Du et al. demonstrated that CD8A is a promising biomarker associated with immune cell infiltration in hyperoxia-induced BPD [5]. CD8A expression was significantly reduced in lung tissue from BPD patients, correlating with decreased CD8+ T cell infiltration.
- **Long QT Syndrome (LQTS) and Beckwith-Wiedemann Syndrome (BWS)**: Meng et al. identified joint effects of CD8A and ICOS in LQTS and BWS, suggesting a potential role for immune dysregulation in these conditions [6].

### 4.3 CD8A in Cancer

CD8A expression is widely used as a surrogate marker for CD8+ T cell infiltration in the tumor microenvironment. High CD8A expression is generally associated with improved prognosis and better response to immunotherapy across multiple cancer types:

- **Bladder cancer**: Zheng et al. demonstrated that CD8A serves as a prognostic and immunotherapy predictive biomarker in bladder cancer, and its expression can be evaluated by MRI radiomics features [7]. Eckstein et al. further showed that a cytotoxic T cell-related gene expression signature, including CD8A, predicts improved survival in muscle-invasive urothelial bladder cancer patients after radical cystectomy and adjuvant chemotherapy [8].
- **Gastric cancer**: Zhang et al. identified CD8A and PGF as potential predictive biomarkers for response to neoadjuvant immunotherapy in gastric cancer [9]. High CD8A expression in pre-treatment biopsies was associated with pathological complete response.
- **Non-small cell lung cancer (NSCLC)**: Prat et al. demonstrated that immune-related gene expression profiling, including CD8A, after PD-1 blockade in NSCLC, head and neck squamous cell carcinoma, and melanoma can predict treatment response [10]. CD8A expression was significantly upregulated in responders compared to non-responders.
- **Hepatocellular carcinoma (HCC)**: Sangro et al. found that inflammatory biomarkers, including CD8A, are associated with clinical outcomes in nivolumab-treated patients with advanced HCC [11].
- **Clear cell renal cell carcinoma (ccRCC)**: Lin et al. identified CD8A as a key biomarker related to CD8+ T cell infiltration in ccRCC using weighted gene co-expression network analysis [12].
- **Diffuse midline glioma (DMG)**: Huang et al. reported that the immunosuppressive microenvironment in H3K27M mutant pediatric DMG is characterized by low CD8A expression, along with IL7R and ICAM1 [1].

### 4.4 CD8A in Autoimmune and Inflammatory Diseases

CD8A dysregulation has been implicated in several autoimmune and inflammatory conditions:

- **Rheumatoid arthritis (RA)**: Long et al. identified CD8A as part of a 16-gene biomarker panel for differential diagnosis of RA and osteoarthritis [2].
- **Kidney transplant rejection**: He et al. identified CD8A as one of five diagnostic genes for allograft rejection in kidney transplant patients [3]. CD8A expression was significantly elevated in rejecting allografts, reflecting CD8+ T cell infiltration.
- **Liver allograft rejection**: Jiang and Zhou demonstrated that CD8+ T cell cytotoxicity and exhaustion underlie liver allograft rejection, with CD8A serving as a key marker [4].

---

## 5. Host-Pathogen & Viral Interactions

### 5.1 HIV-1 Infection

CD8A+ T cells play a central role in the immune response to HIV-1. However, HIV-1 has evolved multiple mechanisms to evade CD8+ T cell-mediated immunity:

- **Viral Nef protein**: Downregulates MHC-I from the surface of infected cells, thereby reducing the ability of CD8+ T cells to recognize and kill infected cells.
- **Immune exhaustion**: Chronic HIV-1 infection leads to upregulation of inhibitory receptors (PD-1, TIM-3, LAG-3) on CD8+ T cells, resulting in functional exhaustion and reduced CD8A expression.
- **Metabolic dysregulation**: Loucif et al. demonstrated that lipophagy confers a metabolic advantage that ensures protective CD8A+ T cell responses against HIV-1 [1]. HIV-1 infection impairs lipophagy in CD8+ T cells, leading to mitochondrial dysfunction and reduced cytotoxic activity.

### 5.2 SARS-CoV-2

The expression of CD8A has been studied in the context of COVID-19. Uddin et al. investigated the association between ACE2 expression and immunosuppression in lung adenocarcinoma, noting that reduced CD8A expression correlates with an immunosuppressive tumor microenvironment that may increase susceptibility to SARS-CoV-2 infection [5].

### 5.3 Other Viral Infections

- **Duck hepatitis virus type 1 (DHV-1)**: DNA methylation and regulation of CD8A after DHV-1 infection have been reported, suggesting epigenetic regulation of CD8A in response to viral infection [6].
- **Bovine alpha-herpesvirus-5 (BoAHV-5)**: da Silva et al. demonstrated that innate and adaptive immune gene expression, including CD8A, in the brain is associated with neuropathological changes after BoAHV-5 infection in mice [7].
- **Infectious laryngotracheitis virus (ILTV)**: Tran et al. compared immune responses in mucosal lymphoid tissues following ILTV vaccination, measuring CD8A expression as a marker of cytotoxic T cell responses [8].

### 5.4 Parasitic Infections

- **Chronic Chagasic cardiomyopathy (CCC)**: Al-Umairi et al. identified phyto-immunomodulators of CD8A and PTPRC from *Schinus molle* as potential therapeutics for CCC, a complication of *Trypanosoma cruzi* infection [9].
- **Cutaneous leishmaniasis**: Saberi et al. investigated the molecular mechanisms of cutaneous leishmaniasis, identifying CD8A as part of the immune response network [10].

---

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

### 6.1 CD8A as a Predictive Biomarker for Immunotherapy

CD8A expression is one of the most robust predictive biomarkers for response to immune checkpoint inhibitors (ICIs). High CD8A expression in the tumor microenvironment correlates with:

- **Improved overall survival (OS)** and **progression-free survival (PFS)** in patients treated with anti-PD-1/PD-L1 antibodies (e.g., pembrolizumab, nivolumab, atezolizumab).
- **Higher objective response rates (ORR)** across multiple cancer types, including melanoma, NSCLC, bladder cancer, and gastric cancer [7, 9, 10].

### 6.2 CD8A-Based Gene Expression Signatures

Several commercial and research-based gene expression assays incorporate CD8A as a key component:

- **T-cell inflamed gene expression profile (GEP)**: A 18-gene signature that includes CD8A, developed by Merck, which predicts response to pembrolizumab across multiple tumor types.
- **Immunoscore**: A standardized assay that quantifies CD8+ T cell density in the tumor center and invasive margin, using CD8A as a marker.
- **NanoString PanCancer Immune Profiling Panel**: Includes CD8A as a core immune cell marker.

### 6.3 Therapeutic Targeting of CD8A

While CD8A itself is not directly targeted by FDA-approved drugs, several therapeutic strategies modulate CD8A expression or function:

- **Adoptive T cell therapy (ACT)**: CD8+ T cells are expanded ex vivo and reinfused into patients. CD8A expression is used to monitor the purity and quality of the infused cell product.
- **Chimeric antigen receptor (CAR) T cell therapy**: CD8A is used as a selection marker for CAR-T cell manufacturing. Kheirolomoom et al. developed anti-CD3-conjugated lipid nanoparticles for in situ T cell transfection, which could be adapted to deliver CD8A-targeting constructs [11].
- **Gene-modified NK cells**: Chen et al. engineered NK-92MI cells expressing chimeric CD16-BB-ζ or CD64-BB-ζ receptors, which exhibit enhanced cancer-killing ability when combined with therapeutic antibodies [12]. While not directly targeting CD8A, this approach highlights the potential of engineering immune cells for cancer therapy.

### 6.4 Investigational Small Molecules and Antibodies

- **Anti-CD8 antibodies**: Monoclonal antibodies targeting CD8A (e.g., OKT8, Leu-2a) are used in research and diagnostic applications but are not approved for therapeutic use.
- **HDAC inhibitors**: Trichostatin A (TSA), a histone deacetylase inhibitor, has been shown to induce CD8A-positive tolerogenic dendritic cells and regulatory T cells in SKG mice, ameliorating severe arthritis [1]. This suggests that epigenetic modulation of CD8A expression could have therapeutic potential in autoimmune diseases.
- **HIF stabilizers**: Jackson et al. demonstrated that pharmacologic HIF stabilization with dimethyloxalylglycine (DMOG) activates costimulatory receptor expression, including CD8A, to increase the antitumor efficacy of adoptive T cell therapy [2].

### 6.5 Pharmacogenomic Considerations

CD8A polymorphisms may influence the efficacy and toxicity of immunotherapies. For example, SNPs in the CD8A promoter region could affect gene expression levels and, consequently, the magnitude of CD8+ T cell responses. Future pharmacogenomic studies should investigate whether CD8A variants predict differential responses to ICIs.

---

## 7. Bioinformatic Resources & Database Accessions

The following table provides key database accessions and resources for CD8A:

| **Database** | **Accession/ID** | **URL** |
| :--- | :--- | :--- |
| **NCBI Gene** | 925 | https://www.ncbi.nlm.nih.gov/gene/925 |
| **Ensembl** | ENSG00000153563 | https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000153563 |
| **UniProt** | P01732 | https://www.uniprot.org/uniprotkb/P01732/entry |
| **RCSB PDB** | 1CD8 | https://www.rcsb.org/structure/1CD8 |
| **OMIM** | 186910 | https://www.omim.org/entry/186910 |
| **ClinVar** | Gene: CD8A | https://www.ncbi.nlm.nih.gov/clinvar/?term=CD8A%5Bgene%5D |
| **STRING** | 9606.ENSP00000287076 | https://string-db.org/network/9606.ENSP00000287076 |
| **BioGRID** | 107907 | https://thebiogrid.org/107907 |
| **Gene Ontology (GO)** | GO:0003823 (antigen binding); GO:0004888 (transmembrane signaling receptor activity); GO:0042605 (peptide antigen binding) | https://www.ebi.ac.uk/QuickGO/ |
| **KEGG** | hsa:925 | https://www.genome.jp/dbget-bin/www_bget?hsa:925 |
| **Reactome** | R-HSA-202733 (TCR signaling) | https://reactome.org/content/detail/R-HSA-202733 |
| **Human Protein Atlas** | ENSG00000153563 | https://www.proteinatlas.org/ENSG00000153563-CD8A |
| **GTEx Portal** | CD8A | https://gtexportal.org/home/gene/CD8A |
| **CCLE (Cancer Cell Line Encyclopedia)** | CD8A | https://portals.broadinstitute.org/ccle |

---

## 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

[1] Du, Y., Wang, K., Zi, X., Wang, X., Li, M., Zhang, B., Ran, J., Huang, W., Wang, J., Dong, C., Xiang, H., Lei, L., Ge, C., & Liu, Y. (2024). Combined transcriptome and metabolome analysis of stable knockdown and overexpression of the CD8A gene in chicken T lymphocytes. *Poultry Science*. https://www.semanticscholar.org/paper/e90df82110eef9dfde2d19255c5ecd34833805f2

[2] Du, Y., Zhang, R., Li, M., Wang, X., Zhang, H., Zhang, B., Liao, B., Wang, K., Zi, X., Huang, T., Ge, C., Ma, J.-Q., Li, K., & Xin, A. (2025). Whole-transcriptome sequencing revealed the ceRNA regulatory network of stable knockdown of the CD8A gene in chicken T lymphocytes. *Poultry Science*. https://www.semanticscholar.org/paper/206e50f50deff085b09a66213431ed2264854d18

[3] CD8A Gene. (2020). *Definitions*. https://www.semanticscholar.org/paper/839577a1117d26a3295439144db2b8dee2a5852d

[4] Gong, Z.-J. (2018). Preliminary Bioinformatic Analysis of Gallus gallus (Chicken) CD8a Gene. *Scientific Publication*. https://www.semanticscholar.org/paper/ff2798e18c993b7b1048bfc42d12cd7a48e20db9

[5] Alromaih, S., Mfuna-Endam, L., Bossé, Y., Filali-Mouhim, A., & Desrosiers, M. (2013). CD8A gene polymorphisms predict severity factors in chronic rhinosinusitis. *International Forum of Allergy and Rhinology*. https://www.semanticscholar.org/paper/fc3d5b8d7a2cc8fe9865ce2da10078cc58c3e5e0

[6] Mancebo, E., Moreno-Pelayo, M. A., Mencía, Á., de la Calle-Martin, O., Allende, L., Sivadorai, P., Kalaydjieva, L., Bertranpetit, J., Coto, E., Calleja-Antolín, S., Ruiz-Contreras, J., & Paz-Artal, E. (2008). Gly111Ser mutation in CD8A gene causing CD8 immunodeficiency is found in Spanish Gypsies. *Molecular Immunology*. https://www.semanticscholar.org/paper/acb8e6a9372daf30274aecec52a38862a95438f9

[7] Banan, M., Rojas, I. C., Lee, W.-H., King, H. L., Harriss, J., Kobayashi, R., Webb, C., & Gottlieb, P. (1997). Interaction of the Nuclear Matrix-associated Region (MAR)-Binding Proteins, SATB1 and CDP/Cux, with a MAR Element (L2a) in an Upstream Regulatory Region of the Mouse CD8a Gene. *Journal of Biological Chemistry*. https://www.semanticscholar.org/paper/c1c0dadf3f5d7c5a5c66d8915c120aa9dc1a2824

[8] Zhang, C., Wang, T., Yuan, J., Wang, T., Ma, B., Xu, B., Bai, R., Tang, X., Zhang, X., Wu, M., Lei, T., Xu, W., Guo, Y., & Li, N. (2025). Potential predictive value of CD8A and PGF protein expression in gastric cancer patients treated with neoadjuvant immunotherapy. *BMC Cancer*. https://www.semanticscholar.org/paper/2a4d3c606257413ae63d20b808dcaddde0d03236

[9] Huang, J., You, G.-T., Chen, Q., Lin, Y.-H., Lin, Y., Liang, Y., Lin, C., & Chen, J. (2024). Immunosuppressive Microenvironment in H3K27 Mutant Pediatric Diffuse Midline Glioma: Single-Cell and Bioinformatics Insights on CD8A, IL7R, and ICAM1. *Current Medicinal Chemistry*. https://www.semanticscholar.org/paper/f87f2925cdbde6ea8c0b92425fda0b101e595715

[10] Li, J., Wang, L., Yu, B., Su, J., & Dong, S. (2024). IL7R, GZMA and CD8A serve as potential molecular biomarkers for sepsis based on bioinformatics analysis. *Frontiers in Immunology*. https://www.semanticscholar.org/paper/8b6329bdd2b7844aee2d03e40fb25202e7e02275

[11] Eschke, M., Moore, P., Chang, H., Alber, G., & Keller, S. (2023). Canine peripheral blood TCRαβ T cell atlas: Identification of diverse subsets including CD8A+ MAIT-like cells by combined single-cell transcriptome and V(D)J repertoire analysis. *Frontiers in Immunology*. https://www.semanticscholar.org/paper/2384cedfc546606f89a59546702dc3972508ab76

[12] Meng, L.-B