B7/CD28 Family

Background

About B7/CD28 Family

The B7/CD28 family is a group of cell surface molecules involved in regulating immune responses and maintaining immune homeostasis. This family consists of several members, including B7-1 (CD80), B7-2 (CD86), cytotoxic T-lymphocyte antigen 4 (CTLA-4), and programmed death ligand 1 (PD-L1, also known as B7-H1) and PD-L2 (B7-DC, B7-H2). The B7/CD28 family molecules are critical for regulating the balance between immune activation and tolerance. Dysregulation of this pathway can have significant implications for immune-mediated diseases, including autoimmune disorders, chronic infections, and cancer. Therefore, targeting these molecules and their interactions has become an important area of research for developing immunotherapeutic strategies, such as immune checkpoint blockade therapies, which aim to enhance anti-tumor immune responses by blocking inhibitory signals mediated by molecules like CTLA-4 or PD-1.

Co-stimulatory and Co-inhibitory Pathways of the B7/CD28 Family

The B7/CD28 family consists of both co-stimulatory and co-inhibitory pathways that play crucial roles in regulating immune responses. These pathways involve interactions between specific members of the B7/CD28 family and their corresponding receptors. Here's an overview of the co-stimulatory and co-inhibitory pathways within the B7/CD28 family:

Pathways	Functions
Co-stimulatory Pathways	B7-1 (CD80) - CD28	B7-1 is expressed on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. CD28 is present on T cells. Interaction between B7-1 and CD28 provides a co-stimulatory signal that enhances T cell activation, proliferation, cytokine production, and survival. This interaction plays a critical role in promoting effective immune responses.
	B7-2 (CD86) - CD28	B7-2 is also expressed on APCs. CD86 is another receptor expressed on T cells. Binding of B7-2 to CD28 delivers a co-stimulatory signal similar to the B7-1/CD28 interaction. It enhances T cell activation, proliferation, cytokine production, and survival.
Co-inhibitory Pathways	CTLA-4 - B7-1/B7-2	CTLA-4 is an inhibitory receptor expressed on activated T cells. It competes with CD28 for binding to B7-1 and B7-2 with higher affinity. Engagement of CTLA-4 with B7-1 or B7-2 delivers inhibitory signals that dampen T cell activation and suppress immune responses. This pathway helps prevent excessive immune reactions and contributes to immune regulation and tolerance.
	PD-1 - PD-L1/PD-L2	PD-1 is an inhibitory receptor expressed on activated T cells, B cells, and other immune cells. PD-L1 and PD-L2 are ligands expressed on various cell types, including APCs and tumor cells. Binding of PD-1 to PD-L1 or PD-L2 delivers inhibitory signals that attenuate T cell activation and dampen immune responses. This pathway is involved in immune checkpoint regulation, maintaining peripheral tolerance, and can be exploited by tumors to evade immune surveillance.

These co-stimulatory and co-inhibitory pathways of the B7/CD28 family help maintain immune homeostasis and regulate the magnitude and duration of immune responses. The balance between co-stimulatory and co-inhibitory signals is crucial for proper immune function, preventing autoimmunity, and avoiding excessive immune activation or tolerance. The understanding of these pathways has led to the development of immunotherapeutic approaches, such as immune checkpoint blockade, to modulate immune responses and treat various diseases, including cancer and autoimmune disorders.

Fig.1 Coinhibitory pathways in the B7-CD28 family. (Schildberg FA, et al., 2016)

B7/CD28 Family Molecules

The B7/CD28 family consists of several molecules that play critical roles in regulating immune responses. Here are the key members of the B7/CD28 family:

Key molecule types	Functions
B7-1 (CD80) and B7-2 (CD86)	B7-1 (CD80) is a cell surface molecule primarily expressed on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. It interacts with the CD28 receptor on T cells, providing a co-stimulatory signal that enhances T cell activation and effector functions. B7-2 (CD86) is another molecule expressed on APCs, particularly dendritic cells, macrophages, and B cells. It also binds to CD28 on T cells and delivers a co-stimulatory signal, promoting T cell activation, proliferation, and cytokine production.
Cytotoxic T-lymphocyte antigen 4 (CTLA-4)	CTLA-4 is an inhibitory receptor expressed on activated T cells. It competes with CD28 for binding to B7-1 and B7-2 with higher affinity. CTLA-4 engagement delivers inhibitory signals that dampen T cell activation and limit T cell responses. This regulatory mechanism helps prevent excessive immune reactions and maintain immune tolerance.
Programmed death ligand 1 (PD-L1, B7-H1)	PD-L1 is a cell surface protein expressed on various cell types, including tumor cells, APCs, and non-immune cells. It is a ligand for the programmed death 1 (PD-1) receptor on T cells. PD-L1 binding to PD-1 delivers inhibitory signals that suppress T cell activation and effector functions. This pathway is involved in immune checkpoint regulation, maintaining peripheral tolerance, and can be exploited by tumors to evade immune surveillance.
Programmed death ligand 2 (PD-L2, B7-DC, B7-H2)	PD-L2 is another ligand for PD-1, similar to PD-L1. It is expressed on APCs and can interact with PD-1 on T cells, contributing to immune regulation and tolerance.

The interactions between B7 family molecules (B7-1, B7-2, PD-L1, PD-L2) and their receptors (CD28, CTLA-4, PD-1) play a crucial role in modulating T cell activation, co-stimulation, and immune checkpoint pathways. These interactions regulate the balance between immune activation and tolerance, impacting immune responses in various physiological and pathological conditions.

Fig.2 Comparison of intracellular signaling by CTLA-4 and PD-1.(Schildberg FA, et al., 2016)

B7/CD28 Family in Immunomodulation, Tumor Immunity and Autoimmune Diseases

The B7/CD28 family has been extensively studied in the context of immunomodulation, tumor immunity, and autoimmune diseases. Here is an overview of the research progress in these areas:

Immunomodulation

• Co-stimulation: The interaction between B7-1/B7-2 and CD28 provides a co-stimulatory signal necessary for optimal T cell activation. Research has focused on understanding the molecular mechanisms underlying this interaction and its role in regulating immune responses.
• Immune checkpoint blockade: The inhibitory pathways involving CTLA-4, PD-1, and their respective ligands (PD-L1, PD-L2) have been targeted for immune checkpoint blockade therapies. Antibodies blocking CTLA-4 or PD-1/PD-L1 have shown significant clinical success in enhancing anti-tumor immune responses and improving outcomes in various cancers.
• Combination therapies: Researchers are exploring the potential of combining immune checkpoint blockade with other immunomodulatory agents, such as cytokines, immune adjuvants, and targeted therapies, to further enhance anti-tumor immune responses.

Tumor immunity

• Immune evasion mechanisms: Tumor cells often exploit the B7/CD28 family pathways to evade immune surveillance. They can upregulate inhibitory ligands like PD-L1 to suppress T cell responses. Understanding these mechanisms has led to the development of strategies to counteract immune evasion and enhance anti-tumor immunity.
• Biomarkers: Researchers are investigating the expression patterns of B7/CD28 family molecules in tumors as potential biomarkers for predicting response to immune checkpoint blockade therapies. Identifying reliable biomarkers can help personalize treatment decisions and improve patient outcomes.

Autoimmune diseases

• Dysregulation of B7/CD28 family: Alterations in the expression and function of B7/CD28 family members have been implicated in autoimmune diseases. For example, aberrant expression of PD-L1 and PD-L2 has been observed in autoimmune disorders such as rheumatoid arthritis and systemic lupus erythematosus.
• Therapeutic targeting: Modulating the B7/CD28 family pathways has emerged as a potential therapeutic approach for autoimmune diseases. Blocking the inhibitory signals mediated by CTLA-4 or PD-1/PD-L1 has shown promise in preclinical and clinical studies, offering new avenues for treating these conditions.

The progress in this field holds promise for the development of novel immunotherapies and targeted treatments that can modulate immune responses, enhance anti-tumor immunity, and alleviate autoimmune disorders.

Fig.3 Co-stimulatory and co-inhibitory pathways in atopic dermatitis (AD). (Zheng C, et al., 2023)

Case Study

Case 1: Tang ZS, Hao YH, Zhang EJ, et al. CD28 family of receptors on T cells in chronic HBV infection: Expression characteristics, clinical significance and correlations with PD-1 blockade. Mol Med Rep. 2016;14(2):1107-1116.

The authors investigated the effects of blocking PD-1 on the levels of co-stimulatory and co-inhibitory receptors. The expression of the CD28 family receptors and intracellular IFN-γ were measured simultaneously in T cells in HLA-A2+ patients with CHB following exposure to anti-PD-L1 or a control antibody. Increased expression of the receptors with PD-1 blockade or isotype antibody treatment are demonstrated by flow cytometric dot plots of peripheral CD4+ T cells from a patient with CHB (A). Following anti-PD-L1 exposure, the expression levels of CD28, ICOS, PD-1 and CTLA-4 were increased in the CD4+ T cells (B). No significant impact on the expression of the CD28 family receptors was observed on the CD8+ T cells following anti-PD-L1 exposure. In addition, following anti-PD-L1 exposure, a significant positive correlation was detected among the HBV DNA titers and the fold changes of PD-1, with the highest fold-change of PD-1 in CD4+ T cells corresponding to viremia levels >106 IU/ml (C).

Fig.1 Anti-PD-L1 treatment and expression of CD28 family receptors.

Case 2: Deng X, Chen K, Ren J, et al. A B7-CD28 family-based signature demonstrates significantly different prognosis and immunological characteristics in diffuse gliomas. Front Mol Biosci. 2022;9:849723.

The authors performed ESTIMATE and MCP analyses to investigate the relationship between prognostic characteristics and the glioma microenvironment. The authors found that based on the ESTIMATE algorithm, risk scores were positively correlated with immune and stromal scores across all datasets (A-D). Through the MCP approach, the authors further explored the association of genetic features with specific cell populations in the tumor microenvironment. Findings showed that risk scores were significantly correlated with immune cell populations, particularly myeloid dendritic cells, monocyte lineages, and fibroblasts (E, F).

Fig.2 The B7-CD28 family-based signature was tightly associated with immune score (A,B), stromal score (C,D) and infiltrated cells in tumor microenvironment (E,F) in TCGA and mRNAseq_325 cohorts.

References

• Schildberg FA, Klein SR, Freeman GJ, Sharpe AH. Coinhibitory pathways in the B7-CD28 ligand-receptor family. Immunity. 2016;44(5):955-972.
• Zheng C, Shi Y, Zou Y. T cell co-stimulatory and co-inhibitory pathways in atopic dermatitis. Front Immunol. 2023;14:1081999.
• Sharpe AH, Freeman GJ. The B7-CD28 superfamily. Nat Rev Immunol. 2002;2(2):116-126.
• Keir ME, Sharpe AH. The B7/CD28 costimulatory family in autoimmunity. Immunol Rev. 2005;204:128-143.