B7-H6 in Immune Checkpoints: Role and Clinical Insights

Immune checkpoints help maintain immune balance by regulating immune cell activity. Among them, B7-H6 has gained attention for its role in tumor immune evasion and surveillance, making it a key focus in cancer immunotherapy research.

Understanding its molecular identity, expression patterns, and regulatory mechanisms sheds light on how B7-H6 influences immune interactions and contributes to disease progression.

Molecular Identity

B7-H6 is a glycosylated transmembrane protein in the B7 family, a group of immunoregulatory ligands that modulate immune responses. Structurally, it has two extracellular immunoglobulin-like domains, a single-pass transmembrane region, and a short cytoplasmic tail. Unlike other B7 family members, typically found on antigen-presenting cells, B7-H6 is absent in healthy tissues and is instead induced in malignant cells, making it a tumor-associated ligand with implications for immune recognition.

The extracellular domain of B7-H6 binds to NKp30, an activating receptor on natural killer (NK) cells, triggering immune responses. Structural studies have shown that B7-H6 has a V-type and C1-type immunoglobulin fold, with unique surface charge distributions that enhance its specificity for NKp30. These characteristics set it apart from other immune checkpoint ligands.

Post-translational modifications, particularly glycosylation, affect B7-H6’s stability and function. Glycosylation influences its binding affinity to NKp30 and immune recognition. Abnormal glycosylation in tumor cells can alter its immunogenicity, potentially aiding immune evasion. Additionally, soluble forms of B7-H6, generated by proteolytic cleavage, have been detected in cancer patients’ serum. These soluble variants act as decoys, interfering with NK cell cytotoxicity by binding to NKp30 and dampening immune responses.

Expression Patterns

B7-H6 expression is highly restricted under normal conditions, unlike other B7 family members found on antigen-presenting cells. Healthy tissues, including peripheral blood cells and major organs, do not exhibit detectable B7-H6 mRNA or protein, suggesting its expression is tightly controlled and linked to pathological conditions. Its absence in normal tissues also makes it a potential biomarker for abnormal cellular activity, particularly in cancer.

In contrast, tumors frequently upregulate B7-H6, particularly in hematologic cancers like acute myeloid leukemia (AML) and lymphomas, as well as solid tumors such as glioblastomas and neuroblastomas. The mechanisms behind this selective expression are still being studied, but factors like cellular stress, genomic instability, and inflammatory signals contribute to its induction. Hypoxia, a common tumor microenvironment feature, promotes B7-H6 expression through hypoxia-inducible factor (HIF) signaling. Epigenetic modifications, including DNA methylation and histone acetylation, also influence its transcriptional activation in malignant cells.

Beyond cancer, B7-H6 has been detected in some inflammatory conditions, though at lower levels. Certain infections, particularly those caused by intracellular pathogens, are associated with transient B7-H6 upregulation. For example, Mycobacterium tuberculosis and Staphylococcus aureus infections have been linked to increased B7-H6 presence on infected cells, likely due to pathogen-induced stress. While B7-H6 is mainly tumor-associated, it may also be triggered by non-malignant inflammatory stimuli, though its role in these contexts remains unclear.

Regulation Mechanisms

B7-H6 expression is controlled by genetic, epigenetic, and environmental factors. Unlike other B7 family members with active promoter elements in immune cells, B7-H6 transcription is largely absent under normal conditions, indicating tight regulation. Genomic analyses have identified enhancer regions and transcription factor binding sites that activate in response to cellular stress, particularly in oncogenic and inflammatory environments. Transcription factors like NF-κB and STAT3 drive B7-H6 expression in response to cytokine signaling.

Epigenetic modifications further regulate B7-H6 at the chromatin level. DNA methylation in its promoter suppresses transcription in normal tissues, while hypomethylation in cancer cells correlates with upregulation. Histone modifications, such as H3K27 acetylation, are associated with active transcription. Pharmacological inhibitors targeting DNA methyltransferases (DNMTs) and histone deacetylases (HDACs) have been shown to modulate B7-H6 levels, suggesting potential therapeutic strategies.

Extracellular signals in the tumor microenvironment also influence B7-H6 expression. Hypoxia enhances its expression through HIF-1α, which binds to hypoxia response elements in the B7-H6 promoter. Inflammatory cytokines like IL-6 and TNF-α also upregulate B7-H6, linking its expression to chronic inflammation and tumor progression. Cellular stress responses, including oxidative stress and DNA damage, further modulate its levels, showing that its expression is shaped by both genetic alterations and environmental pressures.

Role in Immune Response

B7-H6 affects immune interactions, particularly in tumor surveillance and immune evasion. Its engagement with immune cells influences their activation, cytotoxicity, and signaling pathways, shaping immune responses in disease settings.

NK Cell Interactions

Natural killer (NK) cells are the primary immune effectors influenced by B7-H6. As a ligand for NKp30, B7-H6 on tumor cells binds to NKp30, activating NK cells and triggering the release of cytotoxic molecules like perforin and granzyme B, leading to tumor cell lysis. However, tumors evade this response by shedding soluble B7-H6, which competes for NKp30 binding and weakens NK cell-mediated cytotoxicity. High levels of soluble B7-H6 in patient sera correlate with poor prognosis in cancers such as neuroblastoma and gastrointestinal stromal tumors. Additionally, some tumors suppress NKp30 expression, further impairing NK cell function.

T Lymphocyte Dynamics

While B7-H6 primarily interacts with NK cells, it also indirectly affects T lymphocytes. NKp30 is expressed on subsets of activated T cells, including γδ T cells and some CD8+ T cells, which may respond to B7-H6 engagement. In the tumor microenvironment, NK cell activation via B7-H6 leads to cytokine secretion, including IFN-γ and TNF-α, which promote T cell recruitment and activation. However, tumors that shed soluble B7-H6 or modulate NKp30 signaling may suppress T cell responses by reducing NK cell-derived cytokines. B7-H6 expression has also been linked to immune evasion strategies involving regulatory T cell (Treg) expansion, further dampening anti-tumor T cell activity. While B7-H6 does not directly engage classical T cell co-stimulatory or inhibitory pathways, its impact on NK cells influences T cell-mediated immunity in cancer.

Crosstalk With Dendritic Cells

Dendritic cells (DCs) play a key role in antigen presentation and immune activation, and their interactions with B7-H6-expressing cells can affect immune responses. While DCs do not express NKp30 at the same levels as NK cells, certain subsets, particularly plasmacytoid dendritic cells (pDCs), respond to NKp30 engagement. NK cells interacting with B7-H6-expressing tumor cells influence DC maturation and cytokine production, affecting antigen presentation and adaptive immune responses. NK-DC interactions via NKp30 signaling enhance IL-12 and IFN-α production, crucial for T cell priming. However, tumors that shed soluble B7-H6 or suppress NKp30 signaling disrupt this interaction, impairing DC function and reducing antigen presentation. This contributes to immune evasion by limiting tumor-specific T cell activation.

Co Expression With PD L1

The relationship between B7-H6 and programmed death-ligand 1 (PD-L1) is a complex mechanism that enhances tumor immune evasion. Though these ligands belong to distinct immune checkpoint pathways, their co-expression in aggressive cancers, including glioblastomas and triple-negative breast cancer, is increasingly documented. The simultaneous presence of B7-H6 and PD-L1 suggests tumors employ multiple immune suppression strategies, targeting both innate and adaptive responses to enhance survival. This dual expression is often linked to highly immunosuppressive tumor microenvironments.

Environmental factors in tumors drive the concurrent upregulation of B7-H6 and PD-L1. Hypoxia, a hallmark of advanced tumors, promotes both ligands through HIF signaling. Inflammatory cytokines such as IL-6 and IFN-γ, commonly elevated in chronic inflammation and cancer, also increase their expression. Tumors under sustained immune pressure may activate multiple evasion pathways to counteract host defenses. Clinically, patients with tumors co-expressing these ligands often respond poorly to immunotherapy, particularly PD-1/PD-L1 blockade, as B7-H6 may sustain immune evasion even when PD-L1 signaling is inhibited.