In-depth Analysis of CD3E and CD3D Targets: Molecular Mechanisms, Immunodeficiency Diseases, and Latest Advances in TCE Bispecific Antibodies

Keywords: CD3E, CD3D, T-cell engager, TCE bispecific antibodies, TCR-CD3 complex, tumor immunotherapy, autoimmune diseases, severe combined immunodeficiency

CD3E and CD3D are core subunits of the T-cell receptor-associated CD3 complex, playing indispensable roles in T-cell development, antigen recognition, and immune activation. With the rapid breakthroughs in T-cell engager (TCE) bispecific antibodies for tumors and autoimmune diseases, CD3 has become one of the most transformative therapeutic targets in immunotherapy. In April 2026, the world's first DLL3×CD3 TCE for solid tumors—tarlatamab—was approved in China, marking the entry of CD3-targeted therapies into a new era of solid tumor treatment. This article systematically explores the biological functions and clinical translation progress of CD3E and CD3D from four dimensions: molecular structure, signaling mechanisms, association with immunodeficiency diseases, and the current state of TCE drug development, providing a comprehensive reference for professionals in the biopharmaceutical industry.

The TCR-CD3 complex consists of TCRαβ (or TCRγδ) paired with CD3γε, CD3δε, and CD3ζζ dimers, forming the core molecular machinery for T-cell antigen recognition and immune response initiation. Within this complex, CD3E (CD3ε) serves as a structural hub: it pairs with CD3G to form the CD3γε dimer and with CD3D to form the CD3δε dimer, thereby bridging CD3γ and CD3δ and facilitating transmembrane assembly of the entire complex.

All CD3 subunits contain immunoreceptor tyrosine-based activation motifs (ITAMs) in their cytoplasmic tails. CD3E, CD3D, and CD3γ each have one ITAM, while CD3ζ has three ITAMs, totaling 10 ITAMs (20 phosphorylatable tyrosine residues) in the entire complex. Upon TCR recognition of MHC-peptide complexes on antigen-presenting cells, Src-family kinases Lck and Fyn rapidly phosphorylate the tyrosine residues in ITAMs. Phosphorylated ITAMs recruit and activate the downstream kinase ZAP-70, which then initiates multiple signaling pathways—including PLCγ, Ras/MAPK, Ca²⁺/NFAT, and PKCθ/NF-κB—ultimately driving T-cell activation, proliferation, differentiation, and effector functions.

Additionally, the cytoplasmic region of CD3E contains endocytic sequences that mediate internalization and surface downregulation of the TCR-CD3 complex, providing a critical regulatory node for signal termination and receptor recycling.

Recent breakthroughs have elucidated the fine-tuned regulation of CD3 complex signaling. In 2025, a landmark study by Xu Chenqi, Shi Xiaoshan, and Wang Zhaopeng's team at the Chinese Academy of Sciences, published as a cover story in Molecular Cell, used NMR and quantitative mass spectrometry to reveal the "N-to-C-terminal gradient membrane insertion" heterogeneity of CD3ζ ITAMs—ITAM1 inserts most shallowly, ITAM2 moderately, and ITAM3 most deeply—leading to sequential phosphorylation from N- to C-terminus in acidic lipid environments. The study further identified insufficient CD3ζ phosphorylation as a key contributor to T-cell exhaustion, providing a theoretical foundation for rational CAR-T cell engineering.

Loss-of-function mutations in CD3E or CD3D genes severely impair T-cell development, clinically manifesting as severe combined immunodeficiency (SCID) with a T-B+NK+ phenotype. These patients exhibit near-complete absence of thymic T-cell production and face life-threatening infections in early infancy.

A systematic review of 49 patients with CD3 subunit deficiencies revealed that CD3δ, CD3ε, and CD3ζ defects typically present as classic SCID, while CD3γ defects may manifest as either SCID or milder combined immunodeficiency (CID). Approximately 45% of CID patients experience recurrent sino-pulmonary infections, and an equal proportion develop thyroiditis. Notably, autoimmune abnormalities may be the sole clinical manifestation of CD3γ deficiency, underscoring the need for combined genetic testing and immunophenotyping in diagnosis.

Allogeneic hematopoietic stem cell transplantation remains the primary curative treatment for SCID patients. Meanwhile, gene therapy trials for CD3D-deficient SCID are advancing steadily. A study led by UCLA uses lentiviral vectors to deliver functional CD3D genes into patients' autologous hematopoietic stem cells, demonstrating promising safety and efficacy in preclinical models. The associated clinical trial has secured funding and entered patient recruitment.

TCE bispecific antibodies are designed to simultaneously bind tumor-associated antigens and CD3 on T cells, directly bridging tumor cells with T cells and bypassing MHC-I restriction to activate endogenous T-cell cytotoxicity. Since the approval of blinatumomab, the first BiTE molecule, for B-cell acute lymphoblastic leukemia in 2014, over 10 CD3-targeted bispecific antibodies have been approved for hematologic malignancies, including diffuse large B-cell lymphoma and multiple myeloma.

A landmark breakthrough in solid tumors occurred in April 2026 when BeiGene and Amgen's tarlatamab (Antais®) received conditional approval from China's NMPA for extensive-stage small-cell lung cancer (ES-SCLC) in adults who failed at least two prior systemic therapies. Tarlatamab is the first and only DLL3×CD3 TCE bispecific antibody approved globally. The pivotal DeLLphi-301 trial demonstrated a 40% objective response rate, median duration of response of 9.7 months, and median overall survival of 13.6 months in heavily pretreated ES-SCLC patients. This approval signifies the successful expansion of CD3-targeted TCE therapies beyond hematologic malignancies into solid tumors.

Innovative molecular designs are enhancing safety. Mabwell's 6MW5311, the first LILRB4×CD3 TCE bispecific antibody, targets acute myeloid leukemia, chronic myelomonocytic leukemia, and multiple myeloma. Its "2+1" asymmetric structure incorporates steric hindrance to minimize off-tumor CD3 binding, activating T-cell cytotoxicity only in the presence of LILRB4-high tumor cells. In April 2026, 6MW5311's clinical trial application was accepted by China's NMPA, marking the entry of next-generation conditionally activated TCE molecules into clinical translation.

CD3-targeted TCEs are expanding from oncology into autoimmune diseases, leveraging their ability to selectively deplete pathogenic B cells or plasma cells and restore immune homeostasis.

In March 2026, Antengene and UCB signed a global licensing agreement for ATG-201, a CD19×CD3 TCE, with Antengene receiving a $60 million upfront payment and up to $1.1 billion in milestones. ATG-201 employs AnTenGager™ technology with steric hindrance to mask the CD3-binding arm in the absence of antigen, preventing systemic T-cell overactivation. It targets B-cell-driven autoimmune diseases.

Also in March 2026, InnoCare's partner Prolium secured $50 million in Series A funding to advance ICP-B02/PRO-203, a CD20×CD3 TCE, for systemic sclerosis and lupus erythematosus. The molecule achieved an 82% overall response rate in non-Hodgkin lymphoma trials, providing a strong foundation for autoimmune applications.

In February 2026, Nature Medicine published studies on CD19×CD3 (blinatumomab) and BCMA×CD3 (teclistamab) bispecific antibodies for refractory antisynthetase syndrome and systemic sclerosis, offering proof-of-concept for CD3-targeted TCEs in autoimmunity. These advances are shifting autoimmune treatment from broad immunosuppression to selective depletion of pathogenic immune cells.

The global CD3 antibody market is entering a phase of rapid growth. Market research estimates its value at $3.1 billion in 2024, projected to reach $10 billion by 2035 with an 11.3% CAGR. The CD3-targeted bispecific antibody segment is expected to grow at a 14.5% CAGR from 2026 to 2035, increasing its share of the bispecific antibody market.

Future innovations will focus on tumor microenvironment-conditioned activation to reduce toxicity, multi-targeting strategies to address antigen heterogeneity, incorporation of cytokine or checkpoint modulators to enhance T-cell function, and process optimization to improve affordability. As core molecular switches in T-cell signaling, CD3E and CD3D will continue to drive immunotherapy's evolution from "one-way targeting" to "dynamic bridging" paradigms.