CD96 - CD96 molecule |Elisa - Clia - Antibody - Protein

Background

The CD96 molecule, also known as TACTILE (T-cell-activated increased late expression), is a transmembrane glycoprotein predominantly expressed on the surface of NK cells and certain subsets of T cells. This protein plays a crucial role in the regulation of immune responses, specifically within the innate immune system, by modulating the activity of NK cells. CD96 is part of the immunoglobulin superfamily and is closely related to other checkpoint receptors such as TIGIT (T cell immunoreceptor with Ig and ITIM domains) and CD226. It has structural and functional similarities with these receptors, which are involved in immune cell interactions and responses to cellular stress signals, infection, and malignant transformations.

CD96 binds to the ligand CD155, also known as poliovirus receptor (PVR), which is broadly expressed on various cell types, including endothelial cells and tumor cells. Through its interaction with CD155, CD96 modulates NK cell cytotoxicity and plays a part in controlling the inflammatory response and immunological synapse formation between NK cells and target cells. By balancing activating and inhibitory signals, CD96 ensures immune homeostasis, minimizing the risk of autoimmune responses while enabling effective pathogen and tumor surveillance.

The CD96 gene is located on chromosome 3 in humans, and its expression is upregulated in response to immune activation signals, especially in the tumor microenvironment and during viral infections. Given its regulatory function, CD96 has become a focal point in cancer immunotherapy, particularly as a potential target for enhancing NK cell activity against tumors.

Protein Structure

The CD96 molecule is a type I transmembrane glycoprotein approximately 585 amino acids in length. The structure of CD96 comprises multiple domains that facilitate its role in ligand binding, cellular signaling, and immune regulation:

Extracellular Domain:

• CD96 features an extracellular domain with three Ig-like domains:
• The first domain (IgV-like) at the N-terminal end is primarily responsible for ligand binding, particularly with CD155.
• The second and third domains are IgC2-like domains that contribute to the stability and flexibility of the receptor, allowing effective engagement with ligands in the immune synapse.
• The extracellular region also undergoes glycosylation, enhancing ligand affinity and stability, which are critical for efficient receptor-ligand interaction.

Transmembrane Domain:

• CD96 possesses a single hydrophobic transmembrane domain, which anchors the protein within the cell membrane of NK and T cells. This domain stabilizes CD96 and facilitates its positioning within the immunological synapse for optimal ligand binding and signal transduction.

Cytoplasmic Tail:

• The cytoplasmic tail of CD96 contains a conserved immunoreceptor tyrosine-based inhibitory motif (ITIM), an essential component for transmitting inhibitory signals within NK cells.
• The cytoplasmic region also contains proline-rich motifs and other signaling motifs that can recruit adaptor proteins, amplifying or modulating intracellular signaling cascades. These motifs allow CD96 to act as a co-stimulatory receptor in specific contexts, balancing NK cell activation and inhibition depending on the surrounding environment and interaction with other receptors.

Classification and Subtypes

CD96 belongs to the immunoglobulin superfamily (IgSF) and is classified as a checkpoint receptor involved in regulating immune cell interactions, particularly in NK and T cells. There are no known subtypes of CD96, but it shares functional and structural similarities with receptors like TIGIT and CD226, which also interact with the same ligands (such as CD155) but yield different immune outcomes. The competitive and cooperative interactions among CD96, TIGIT, and CD226 determine the final immune response, with TIGIT and CD96 generally transmitting inhibitory signals, while CD226 primarily provides activating signals.

Function and Biological Significance

The primary function of CD96 is to modulate NK cell activity through its interaction with ligands like CD155. CD96’s role encompasses multiple immune-related activities:

Immune Regulation:

• CD96 modulates NK cell activation by binding to CD155, creating a regulatory checkpoint that balances NK cell cytotoxicity. This balance is critical to prevent unwarranted NK cell activation, which can lead to tissue damage or autoimmune responses.

Tumor Surveillance:

• In the tumor microenvironment, CD96 plays a significant role by modulating NK cell responses against tumor cells expressing CD155. Tumors often upregulate CD155 as an immune evasion mechanism. CD96 binding to CD155 can inhibit NK cell-mediated cytotoxicity, allowing tumors to escape immune detection.
• CD96 can also contribute to an anti-tumor response indirectly by regulating the expression and activity of other immune receptors, making it an attractive target for immunotherapy, where blocking CD96 might enhance NK cell response against cancer cells.

Inflammation and Infection Control:

• During infections, especially viral infections, CD96 helps regulate NK cell responses, minimizing tissue damage by preventing excessive cytotoxicity. Inflammatory cytokines in infection sites can modulate CD96 expression, indirectly influencing the balance between immune activation and tolerance.

Regulation of the Immunological Synapse:

• CD96 is involved in forming and maintaining the immunological synapse between NK cells and their targets. Through its interactions with CD155, CD96 stabilizes the synapse, facilitating controlled immune cell engagement. This regulation is essential for efficient antigen presentation and the activation of other immune cells, contributing to a coordinated immune response.

Clinical Issues

Cancer:

• CD96 has emerged as a target of interest in cancer immunotherapy due to its role in NK cell inhibition. Tumor cells often express high levels of CD155 to engage inhibitory receptors like CD96 on NK cells, creating an immune-suppressive environment.
• Blockade of CD96 in preclinical cancer models has shown potential in enhancing NK cell-mediated killing of tumor cells, leading to a reduction in tumor growth. As a result, CD96 inhibitors or antibodies are being explored as adjunctive therapies to standard immune checkpoint inhibitors.

Infectious Diseases:

• CD96’s role in regulating NK cell responses to infections has implications for infectious disease control, particularly for viruses that cause chronic infection. Viral infections, such as HIV or hepatitis, can alter the expression of CD155 on infected cells, allowing CD96 to modulate NK cell responses. Strategies to modulate CD96 function in infections may support better immune control of viral replication.

Autoimmune Diseases:

• Aberrant CD96 signaling is implicated in autoimmune conditions, where dysregulated NK cell activity contributes to excessive tissue inflammation. In autoimmune diseases such as rheumatoid arthritis, altered CD96 expression may exacerbate inflammation, and therapeutic modulation of CD96 signaling could potentially help mitigate inflammatory damage.

Transplantation:

• Given its role in immune modulation, CD96 could influence outcomes in organ and tissue transplantation. Since NK cells are involved in transplant rejection, modulating CD96 interactions with CD155 may improve graft survival by reducing NK cell-mediated cytotoxicity against the transplanted tissue.

Summary

CD96 is a critical immunoregulatory receptor expressed on NK and T cells that modulates immune responses through interactions with its primary ligand, CD155. Structurally, CD96 contains three Ig-like domains in its extracellular region, a single-pass transmembrane domain, and a cytoplasmic tail with an ITIM motif, which enables it to transmit inhibitory signals. Functionally, CD96 contributes to immune homeostasis by balancing NK cell activation and inhibition, especially in tumor surveillance and response to infections. CD96's interactions with CD155 on target cells play a significant role in tumor immunity, immune tolerance, and infection control, making it a promising target for immunotherapeutic approaches in cancer, autoimmune conditions, and transplantation. Blocking CD96 to enhance NK cell responses holds potential for boosting anti-tumor immunity, while its modulatory role on T cells and NK cells presents therapeutic options across a broad range of immune-related diseases.