CTLA4 - cytotoxic T-lymphocyte associated protein 4 |Elisa - Clia - Antibody - Protein

Background

Cytotoxic T-lymphocyte associated protein 4 (CTLA-4), encoded by the CTLA4 gene on chromosome 2, is an essential immune checkpoint receptor primarily expressed on activated T cells and regulatory T cells (Tregs). CTLA-4 plays a crucial role in maintaining immune homeostasis by delivering inhibitory signals to downregulate T cell activation and proliferation. This receptor functions by binding to the ligands CD80 (B7-1) and CD86 (B7-2), which are expressed on antigen-presenting cells (APCs). By inhibiting excessive immune responses, CTLA-4 prevents autoimmunity and maintains immune tolerance to self-antigens. CTLA-4 is the first immune checkpoint to have been targeted for immunotherapy, exemplified by the development of the monoclonal antibody ipilimumab, which blocks CTLA-4 to boost anti-tumor immune responses. Given its significance in regulating immune responses, CTLA-4 is a focus of research in fields ranging from oncology to autoimmune disorders.

Protein Structure

CTLA-4 is a transmembrane glycoprotein of approximately 223 amino acids, with a structure comprising the following key domains:

Extracellular Domain (ECD):

• The extracellular region is essential for ligand binding and includes an immunoglobulin variable (IgV)-like domain. This IgV-like structure allows CTLA-4 to bind CD80 and CD86 on APCs with high affinity, outcompeting the stimulatory receptor CD28.
• The ECD contains a unique MYPPPY motif, a short sequence of amino acids critical for high-affinity binding to its ligands.

Transmembrane Domain (TMD):

• The transmembrane domain anchors CTLA-4 within the cell membrane and facilitates the receptor’s localization on the T cell surface.
• The TMD also contributes to the formation of CTLA-4 homodimers, which are believed to have increased binding affinity for CD80/CD86, thereby enhancing inhibitory signaling.

Cytoplasmic Domain (CD):

• The cytoplasmic tail is rich in tyrosine residues, which contain signaling motifs crucial for CTLA-4’s inhibitory activity. This region lacks any known enzymatic activity but contains a YVKM motif, which is essential for interacting with intracellular proteins like protein phosphatase 2A (PP2A) and clathrin-associated adapter protein complexes.
• The cytoplasmic domain is highly regulated through phosphorylation, determining whether CTLA-4 is retained in intracellular compartments or translocated to the cell surface, modulating its availability and inhibitory functions.

Classification and Subtypes

CTLA-4 is classified as a member of the immunoglobulin superfamily, specifically as an immune checkpoint receptor. Structurally and functionally, it is closely related to CD28, a costimulatory receptor with which CTLA-4 shares ligand binding specificity (CD80/CD86). However, unlike CD28, which provides stimulatory signals to T cells, CTLA-4 transmits inhibitory signals to limit T cell responses. CTLA-4 exists primarily in two isoforms due to alternative splicing:

Full-Length CTLA-4:

• This membrane-bound isoform contains the full extracellular, transmembrane, and cytoplasmic domains.
• Full-length CTLA-4 is responsible for most of the inhibitory effects observed on activated T cells and Tregs.

Soluble CTLA-4 (sCTLA-4):

• The soluble isoform lacks the transmembrane domain and circulates freely in the plasma, where it can bind to CD80 and CD86, competitively inhibiting their interaction with both CD28 and membrane-bound CTLA-4.
• sCTLA-4 is thought to modulate immune responses in a different manner from membrane-bound CTLA-4 and has been implicated in autoimmune conditions.

Function and Biological Significance

Inhibition of T Cell Activation:

• CTLA-4’s primary function is to inhibit T cell activation. When bound to CD80/CD86, CTLA-4 transmits inhibitory signals that reduce T cell proliferation, cytokine production, and cell cycle progression.
• This mechanism is essential for avoiding excessive immune activation and protecting against autoimmunity. CTLA-4 expression on Tregs also enhances their suppressive capabilities, reinforcing immune tolerance.

Competition with CD28:

• CTLA-4 has a significantly higher binding affinity for CD80 and CD86 compared to CD28, enabling it to outcompete CD28 for ligand binding.
• By limiting CD28-mediated costimulatory signaling, CTLA-4 dampens immune responses, providing a checkpoint that helps balance immune activation and inhibition.

Regulation of Immune Homeostasis:

• CTLA-4 is a critical player in immune homeostasis. Constitutive expression of CTLA-4 on Tregs is essential for their suppressive function, ensuring immune tolerance and preventing autoimmunity.
• Beyond direct inhibition, CTLA-4 expression on Tregs can facilitate the removal of CD80/CD86 from the surface of APCs, reducing available ligands for CD28 and enhancing immune regulation.

Modulation in Disease Contexts:

• CTLA-4 expression and function are dynamically regulated in response to immune challenges, such as infections and malignancies. CTLA-4 upregulation on activated T cells provides a natural feedback mechanism to control immune responses, while in tumors, CTLA-4 expression can limit anti-tumor immunity, promoting immune evasion.

Clinical Issues

Cancer and Immunotherapy:

• The inhibitory nature of CTLA-4 has made it a target for cancer immunotherapy, aiming to boost T cell activity against tumors. CTLA-4 blockade, notably with ipilimumab, has shown effectiveness in treating cancers like metastatic melanoma by enhancing T cell activation and promoting anti-tumor immunity.
• CTLA-4 inhibition, however, can lead to immune-related adverse events (irAEs), reflecting the receptor's role in immune homeostasis. These effects include dermatitis, colitis, hepatitis, and endocrinopathies, necessitating careful monitoring and management in clinical settings.

Autoimmune Diseases:

• Genetic polymorphisms in the CTLA4 gene have been associated with several autoimmune diseases, including type 1 diabetes, rheumatoid arthritis, systemic lupus erythematosus, and multiple sclerosis. These polymorphisms may lead to reduced CTLA-4 expression or function, resulting in insufficient immune regulation and an increased risk of autoimmunity.
• Therapeutic strategies aimed at enhancing CTLA-4 signaling or function may offer potential benefits for managing autoimmune conditions, given CTLA-4’s role in maintaining tolerance.

Infectious Diseases:

• CTLA-4’s inhibitory effects can also influence the immune response during infections. Reduced CTLA-4 function may lead to enhanced pathogen clearance, while increased CTLA-4 expression could limit effective immune responses against chronic infections, potentially promoting pathogen persistence.

Transplantation:

• CTLA-4 modulation has implications for organ transplantation. Strategies to upregulate CTLA-4 expression or function are being explored to prevent graft rejection by inducing immune tolerance, while CTLA-4 blockade might be avoided due to the risk of enhancing alloimmune responses.

Summary

CTLA-4 is a critical immune checkpoint receptor that controls T cell activation and immune homeostasis by binding to CD80 and CD86 on APCs. Structurally, CTLA-4 comprises an IgV-like extracellular domain, a transmembrane domain, and a cytoplasmic domain with motifs essential for ligand binding, signal transduction, and intracellular trafficking. This protein competes with CD28 for ligand binding, providing inhibitory signals that dampen T cell responses, thereby preventing autoimmune activation and promoting immune tolerance. Its role is particularly pronounced in regulatory T cells, where it enhances immune suppression. CTLA-4’s involvement in immune checkpoint pathways makes it a key therapeutic target in cancer immunotherapy, though blockade can lead to immune-related adverse effects. Beyond oncology, CTLA-4 is implicated in autoimmune diseases, infectious disease immunity, and organ transplantation tolerance. Its modulation remains an active area of research for therapeutic interventions aimed at balancing immune activation and inhibition.