Cluster of Differentiation 8 (CD8)
CD8 is a typical type I transmembrane glycoprotein, predominantly highly expressed on the surface of cytotoxic T lymphocytes (CTLs), and serves as a critical co-receptor for the T cell receptor (TCR). CD8 protein has two main subtypes: CD8α and CD8β. These two subtypes can form homodimers (CD8αα) or heterodimers (CD8αβ), with the CD8αβ heterodimer being the most common form under physiological conditions. CD8α and CD8β are encoded by the CD8A and CD8B genes, respectively, which are expressed in a tandem manner.
Structurally, both CD8α and CD8β chains contain three key regions: ① N-terminal extracellular domain: contains an immunoglobulin variable (IgV)-like domain responsible for binding to MHC-I molecules. ② Transmembrane domain: anchors the protein to the cell membrane. ③ Intracellular domain: participates in intracellular signal transduction. CD8α has a relatively broader expression range; in addition to CTLs, it is also present on the surface of some natural killer (NK) cells and other non-lymphoid cells. In contrast, CD8β is primarily and specifically expressed on CTLs and mainly forms heterodimers with CD8α.
CD8A and CD8B exhibit low to moderate protein homology across species, with significant sequence divergence between human and mouse. According to the UniProt database (Human CD8A: P01732; Mouse CD8A: P01731; Human CD8B: P10966; Mouse CD8B: P10300) and Clustal Omega sequence alignment, the amino acid sequence identity of the extracellular domain (ECD) between human and mouse CD8A is approximately 49%, with overall cross-species homology ranging from 20% to 60%; CD8B displays a similar pattern. Both chains contain highly conserved residues (CD8A: 24; CD8B: 21), primarily distributed in the Ig-like fold domains (stabilizing structure and dimerization), hinge/transmembrane regions (disulfide bond formation), and cytoplasmic tails (Lck recruitment).
These differences have significant implications for epitope recognition, signal transduction, and antibody development. Species-specific epitopes lead to variations in human-mouse CD8-MHC-I binding modes and affinities, potentially altering T cell co-receptor function and signal transduction efficiency (while the conserved CxC motif maintains Lck association, non-conserved regions affect activation thresholds); most anti-CD8 monoclonal antibodies are species-specific, limiting the translation from mouse models to humans, whereas conserved epitopes support the development of cross-reactive antibodies for immune modulation or therapy.
CD8, as a key co-receptor of the T cell receptor (TCR), plays a central role in T cell-mediated cellular immunity. Its primary function is to bind to MHC class I molecules presented on the surface of antigen-presenting cells (APCs), thereby enhancing the sensitivity of the TCR to antigens and being crucial for the activation of cytotoxic T lymphocytes (CTLs). Specifically:
① CD8α subunit: Primarily responsible for binding to MHC class I molecules on APCs, promoting antigen recognition by stabilizing the interaction between T cells and APCs.
② CD8β subunit: Mainly stabilizes the structure of the CD8α-CD8β heterodimer and assists in intracellular signal transduction within T cells after antigen recognition, thereby enhancing the function of CD8α.
Under normal conditions, CD8 is primarily highly expressed on the surface of cytotoxic T lymphocytes (CTLs), serving as a key co-receptor of the T cell receptor (TCR), and is also present in thymocytes, certain natural killer (NK) cells, and some dendritic cell subsets.
When APCs present antigens via MHC class I molecules, the TCR on the surface of CTLs recognizes the specific antigen, while the CD8 molecule binds to the constant region of the MHC class I molecule. This co-binding stabilizes the TCR-peptide-MHC complex, promotes the recruitment of signaling molecules, and thereby leads to T cell activation. The cytoplasmic domain of CD8 also interacts with the lymphocyte-specific protein tyrosine kinase p56Lck (Lck), recruiting it to the vicinity of the TCR/CD3 complex. Lck phosphorylates the tyrosine-based immunoreceptor activation motifs (ITAMs) in the cytoplasmic tails of the CD3γ, CD3δ, CD3ϵ, and CD3ζ subunits associated with the TCR, helping to initiate TCR signaling, significantly enhancing TCR signal strength, and ultimately activating T cells. Activated CTLs subsequently release cytotoxic granules such as perforin and granzymes, inducing apoptosis in target cells, thereby eliminating diseased cells.
CD8 exhibits a dual mechanism in TCR signaling: first, it stabilizes the TCR-pMHC complex through physical binding, enhancing T cell recognition efficiency of antigens; second, it serves as a recruitment platform for Lck kinase, actively initiating and amplifying the intracellular signaling cascade in T cells. This synergistic effect is crucial for effective T cell activation. Therefore, designing CD8-targeted therapies that simultaneously leverage or enhance both aspects could achieve more potent and specific T cell activation, thereby reducing off-target effects potentially associated with conventional pan-CD3 activation. This is one of the reasons why CD8-biased TCEs offer superior safety compared to CD3-TCEs.
In disease states, the expression levels and functional status of CD8+ T cells undergo changes. For instance, in the tumor microenvironment (TME), nutrient deprivation may lead to reduced CD8 expression, forming a functionally impaired CD8Low T cell subset, whereas CD8High T cells are closer to tumor cells and exhibit stronger effector functions. Additionally, CD8+ tissue-resident memory T cells (TRM) play a critical role in infection and cancer; their expression and functional heterogeneity across different tissues offer possibilities for fine-tuned targeted therapies. CD8+ T cells play a pivotal role in the onset and progression of various diseases, and precise regulation of their functional status is crucial for maintaining organismal health.
① Infectious Diseases: CD8+ T cells are the primary effectors for clearing virus-infected cells. By recognizing viral antigens presented on the surface of infected cells, they directly kill these cells, thereby effectively controlling viral replication and disease progression.
② Cancer: CD8+ T cells play a central role in tumor immune surveillance. They can recognize and eliminate cancer cells, preventing tumor initiation and progression. However, within the tumor microenvironment (TME), CD8+ T cells often become functionally exhausted due to inhibitory signals, metabolic constraints, and physical barriers, leading to immune escape and tumor progression. Therefore, restoring or enhancing the anti-tumor functions of CD8+ T cells is a key strategy in cancer immunotherapy.
③ Autoimmune Diseases: Overactivation or dysfunction of CD8+ T cells can lead to the onset of autoimmune diseases, such as type 1 diabetes and multiple sclerosis. In these cases, CD8+ T cells mistakenly recognize and attack self-tissues, triggering inflammation and tissue damage. Consequently, therapeutic strategies for autoimmune diseases may involve suppressing or modulating CD8+ T cell activity, or restoring their regulatory functions (e.g., regulatory CD8 T cells, CD8 Treg).
The double-edged sword effect of CD8+ T cell dysfunction endows them with vastly different therapeutic potentials across various diseases. In cancer and chronic infections, the goal is to enhance their cytotoxic functions; whereas in autoimmune diseases, it may be necessary to suppress their activity or restore immune regulatory balance. This multifaceted nature implies that CD8 targets hold broad therapeutic application prospects, spanning oncology, infectious diseases, and even autoimmune disorders. Precise activation or inhibition of CD8+ T cells, rather than broad-spectrum immune modulation, represents a crucial direction for the future development of immunotherapy and aligns with the concept of precision medicine.
AZD9793 is an asymmetric tri-specific IgG1 monoclonal antibody designed to enhance anti-tumor effects and reduce the risk of cytokine release syndrome (CRS) through CD8-biased binding. Its structure comprises two Fab domains that bind to GPC3 on the tumor cell surface, one VHH domain that binds to the TCR, and another VHH domain that binds to the CD8 co-receptor. This design enables bivalent GPC3 binding to improve tumor targeting, while low-affinity TCR binding minimizes non-specific T cell activation. Unlike conventional GPC3×CD3 T cell engagers (TCEs), AZD9793 reduces the risk of CRS by limiting CD4+ T cell activation, thereby optimizing the therapeutic index.
In vitro experiments demonstrated that AZD9793 exhibited significant T cell-dependent cytotoxicity across multiple HCC cell lines with varying GPC3 expression levels (including HepG2, Hep3B, Huh7, and PLC/PRF/5). Unlike conventional TCEs that broadly activate CD3+ T cells, AZD9793 achieved preferential activation of CD8+ T cells through CD8/TCR engagement, resulting in potent killing accompanied by reduced CD4+ T cell activation and cytokine secretion. Further experiments utilizing purified T cell subsets and control molecules validated the mechanism: CD4+ T cells were the primary drivers of cytokine release, while CD8-biased binding enhanced cytotoxic effects. Additionally, AZD9793 demonstrated a bystander killing effect in mixed GPC3-positive/negative cell lines, suppressing heterogeneous tumor growth via an IFN-γ-dependent mechanism.
In a humanized mouse HCC xenograft model, AZD9793 demonstrated dose-dependent antitumor activity. At efficacy levels comparable to conventional TCEs, it induced significantly lower systemic cytokine levels, approaching those of the control group (TNF-α, IL-6, IFN-γ, IL-10, IL-2, IL-17A). In both HCC and non-small cell lung cancer models, AZD9793 enhanced intratumoral T-cell infiltration and increased the CD8/CD4 ratio. In patient-derived xenograft (PDX) models, AZD9793 effectively controlled GPC3-heterogeneously expressing tumors, further validating the bystander killing effect. Overall, the results demonstrate robust antitumor activity across multiple disease-relevant models.
CD8-targeted novel therapies are primarily applied in tumor immunology and autoimmune diseases, with potential in the field of infectious diseases. In cancer treatment, their applications include developing CD8-biased T cell engagers (TCEs) to specifically activate CD8+ T cells while reducing the risk of cytokine release syndrome (CRS); utilizing CD8-cis-targeted cytokines (such as IL-2, IL-15, IL-21) to selectively expand and enhance CD8+ T cell function; and employing in vivo CAR-T technology to deliver gene therapy vectors specifically to CD8+ T cells, achieving in vivo T cell engineering. In autoimmune diseases, CD8-targeted therapies aim to restore the function of regulatory CD8 T cells (CD8 Tregs) or eliminate pathogenic B cells through in vivo CAR-T.
The cancer immunotherapy market is large and continues to grow, with a global market size of approximately $101.5 billion to $226.38 billion in 2024, expected to reach $111.1 billion to $226.38 billion in 2025, and grow at a compound annual growth rate (CAGR) of 9.46% to 11.90% through 2030/2033. The T cell therapy market (including CAR-T) is growing even more rapidly, with a market size of approximately $5.1 billion to $7.59 billion in 2024, expected to reach $6.4 billion to $10.3 billion in 2025, and grow at a CAGR of 23.8% to 35.74% through 2034/2037. The in vivo CAR-T market is projected to grow at a CAGR of 32.9% between 2025 and 2034. The autoimmune disease treatment market was valued at $79.76 billion to $79.82 billion in 2025 and is expected to reach $103.01 billion to $124.28 billion by 2030/2033, with a CAGR of 5.25% to 5.69%. These data indicate that CD8-targeted therapies hold significant market potential.
The competitive landscape for CD8-targeted therapies is becoming increasingly intense, with major global pharmaceutical companies and innovative biotechnology firms actively expanding their portfolios. AstraZeneca leads in the field of CD8-biased T-cell engagers (TCEs), with its AZD9793 and AZD5492 pipelines advancing into preclinical and early clinical stages. Capstan Therapeutics (acquired by AbbVie) and Sanofi are competing in the realm of in vivo CAR-T therapies targeting CD8, utilizing LNP-mediated mRNA delivery to engineer T cells within the body. Additionally, Mozart Therapeutics focuses on KIR and CD8 bispecific antibodies aimed at restoring regulatory CD8 T cell function; Asher Biotherapeutics is developing CD8 cis-targeted IL-2/IL-21 cytokine therapies; and MacroGenics is exploring anti-ROR1/CD3/CD8 trispecific antibodies. Although most pipelines remain in preclinical or early clinical stages, the entry of major pharmaceutical companies and the diversity of technological approaches signal fierce market competition in the future.
Currently, the total number of CD8-targeted therapy pipelines in the region exceeds 50, with no approved drugs on the market. These pipelines are primarily concentrated in autoimmune diseases and oncology. Notably, most early high-profile CD8&CD4 dual-targeted drugs have failed. Pipelines currently in clinical and IND (Investigational New Drug application) stages are predominantly composed of small-molecule drugs. Mainstream novel CD8-targeted therapies, such as TCEs and in vivo CAR-Ts, are mostly in preclinical proof-of-concept or early-stage safety clinical trials. These innovative therapies hold promise for overcoming the druggability limitations of CD8 targets and expanding their therapeutic applications.
CD8, as a key co-receptor of the T cell receptor (TCR), is predominantly highly expressed on the surface of cytotoxic T lymphocytes (CTLs) and plays a central role in T cell activation and the mediation of cytotoxic effects via antigen presentation by MHC class I molecules. Its well-defined biological functions reduce development risks and offer broad application prospects across oncology, infectious diseases, and autoimmune disorders.
Currently, CD8-targeted therapies hold significant potential, primarily encompassing novel T cell engagers (TCEs) and in vivo CAR-T therapies. AstraZeneca's CD8-biased TCEs (such as AZD9793 for solid tumors and AZD5492 for hematological malignancies and autoimmune diseases), compared to traditional CD3-TCEs, preferentially activate CD8+ T cells to effectively eliminate tumors or treat autoimmune conditions while significantly reducing the risk of cytokine release syndrome (CRS), demonstrating superior safety. CD8-targeted in vivo CAR-T therapies led by Capstan Therapeutics (acquired by AbbVie) and Sanofi utilize CD8-targeted LNPs to deliver CAR mRNA, simplifying treatment workflows, enhancing accessibility, and substantially lowering costs. Mozart Therapeutics' MTX-101 (a KIR/CD8 bispecific antibody) focuses on restoring the function of regulatory CD8 T cells to treat autoimmune diseases. The cancer immunotherapy and T cell therapy markets are vast and continuously growing, providing immense potential for CD8-targeted therapies.
Although no CD8-targeted therapies have yet received approval, the active strategic positioning by major pharmaceutical companies and diverse technological approaches signal breakthrough progress. Overall, the CD8 target boasts well-defined biological functions with multifaceted roles in oncology, infectious diseases, and autoimmune disorders, while novel therapies have demonstrated superior safety and efficacy advantages.