The Biology of CCR8: Signal Transduction and Immune Surveillance

The chemokine receptor family consists of key transmembrane G-protein coupled receptors (GPCRs) that primarily regulate the chemotactic properties of cells. These receptors, by recognizing small molecule chemokines, control the migration of various immune cells, playing a crucial role in maintaining immune surveillance, inflammatory responses, and cellular localization during tissue repair.

The chemokine receptor family is broadly classified into four types: CC, CXC, CX3C, and XC, based on the number of amino acids between the first and second cysteine residues in their ligands—the chemokines. For instance, chemokines in the CC family have no other amino acids between the cysteines, whereas those in the CXC family have one amino acid separating the two cysteines.

Each type of chemokine receptor specifically binds to one or more chemokines, thereby activating specific signaling pathways. These pathways often involve changes in intracellular calcium levels and the activation of various protein kinases. These signals ultimately lead to chemotactic responses, where cells move towards higher concentrations of chemokines.

In subsequent sections, we will primarily explore the biological functions of CCR8 and its applications in therapy.

Fig.1 The chemokine family of chemokine ligands and chemokine receptors. [10]

What is CCR8?

CCR8 in humans was first identified in 1997 using either low-stringency PCR screening methods [1] or through functional screening of orphan GPCRs [2]. This receptor interacts selectively with two ligands: CCL1, first identified in 1989 as a protein secreted by T cells, and CCL18, which was characterized 25 years later. Both human and mouse CCR8 share structural similarities with their primary ligand, CCL1 [3]. The mouse variant of CCR8 additionally binds to CCL8, a chemokine with higher affinity and functional potency compared to human CCL18, despite structural differences [4].

Research into CCR8 has faced challenges due to its limited expression on immune cells in human peripheral blood and the absence of notable phenotypic changes in CCR8-deficient mouse models. However, recent in vivo studies have demonstrated a definitive role for CCR8 in mediating type-2 inflammatory conditions such as atopic dermatitis and allergic enteritis [8]. Whether these findings are applicable to human inflammatory conditions remains uncertain.

Additionally, the CCR8/CCL1 pathway is essential for the movement of cutaneous dendritic cells (DCs) in mice, although CCR8 is not found on human skin DCs. Current knowledge is more comprehensive regarding the cellular sources of CCL1 compared to the more recently identified ligands, mouse CCL8 and human CCL18. Consistent with earlier studies [5-7], CCL1 is predominantly produced by T cells and is also secreted by various other cells including tissue phagocytes (macrophages, DCs), mast cells, and tumor cells [9].

Structure and Function of CCR8

CCR8, is also known as CC chemokine receptor 8, is a Class A G-protein coupled receptor that plays a crucial role in the chemokine-mediated regulatory pathways of the immune system. This receptor is predominantly expressed on T regulatory (Treg) cells and type 2 helper T (Th2) cells, orchestrating a myriad of immune responses, especially those related to inflammation and tissue homeostasis [11].

Molecular Structure

CCR8 is characterized by its structure which consists of seven transmembrane alpha-helices, typical of G-protein-coupled receptors, that span the cellular membrane. Its external loops enable the attachment to its ligand, the chemokine CCL1, while its internal loops engage with G-proteins to relay signals within the cell. The receptor’s N-terminal domain undergoes glycosylation, which is affect ligand recognition and the dynamics of the response [12]. Advanced techniques such as X-ray crystallography and cryo-electron microscopy have recently provided insights into the arrangement of these domains, elucidating how ligands trigger receptor activation.

Ligand Binding and Signaling

CCR8 is specifically activated by the chemokine CCL1. This interaction is highly precise, facilitating exact regulation of immune cell movement and activation. When CCL1 binds to CCR8, the receptor alters its shape, enhancing its interaction with G-proteins. It predominantly interacts with Gα_i proteins, leading to the suppression of adenylate cyclase and reducing levels of cAMP within the cell, which are essential for pathways that control immune cell movement and activation [13].

The activation of CCR8 triggers pathways such as phosphoinositide 3-kinases (PI3K) and protein kinase B (PKB/Akt), which support cell survival and growth. Additionally, the β-arrestin pathway is activated, contributing to the receptor’s internalization and the desensitization of signaling, which helps regulate the immune response strictly.

Immune Function and Regulation

The presence of CCR8 on T regulatory (Treg) and type 2 helper T (Th2) cells underlines its crucial role in adjusting immune responses. In Th2 cells, CCR8 affects the secretion of cytokines like IL-4, IL-5, and IL-13, essential for fighting parasitic infections and managing allergic reactions. In Treg cells, it bolsters their inhibitory function [14], aiding in sustaining immune tolerance and preventing autoimmune conditions.

Furthermore, CCR8 is involved in directing immune cells to sites of inflammation or injury. It is upregulated in response to inflammatory signals, steering cells to critical areas to amplify or mitigate the inflammatory response.

Therapeutic Applications of CCR8

CCR8, the CC chemokine receptor 8, plays a pivotal role in the recruitment and regulation of immune cells, presenting significant therapeutic possibilities for a range of diseases, such as autoimmune conditions, inflammatory disorders, and various cancers. Adjusting the activity or expression of CCR8 could modulate immune reactions in ways that may alleviate disease symptoms and optimize therapeutic effects.

Autoimmune Conditions

In autoimmune disorders where immune tolerance is compromised, CCR8 is instrumental through its regulation of Treg cells. These cells, expressing CCR8, are crucial for sustaining immune equilibrium by mitigating autoimmune activity. In conditions such as multiple sclerosis, rheumatoid arthritis, and inflammatory bowel disease, uncontrolled Treg responses play a role in exacerbating disease [15]. Altering CCR8 function could rebalance Treg activity, enabling them to more effectively suppress inflammatory immune cells, potentially stopping or even reversing the progression of autoimmune disorders.

Studies have indicated that altering the activity of CCR8 impacts the movement of Treg cells to inflammation sites, potentially lessening the intensity of autoimmune reactions. For instance, in experimental models like experimental autoimmune encephalomyelitis, a form of multiple sclerosis, using antagonists to target CCR8 has been shown to curtail the influx of harmful T cells into the central nervous system, thus easing symptoms.

Cancer Immunotherapy

Within the tumor microenvironment, CCR8 expression on Treg cells helps maintain an immunosuppressive environment that enables tumor evasion from the immune system. Targeting CCR8, through blocking or depleting cells that express the receptor, may boost the effectiveness of immunotherapies. For example, monoclonal antibodies against CCR8 can specifically eliminate Treg cells within tumors, potentially enhancing the immune system's capacity to combat cancer cells [16].

Recent studies have illuminated the potential of therapies targeting CCR8 to enhance outcomes in cancers like melanoma, ovarian, and breast cancer, where Treg cells suppress anti-tumor immune responses. Early clinical trials are exploring the safety and efficacy of these methods, with early results suggesting that inhibiting CCR8 could significantly benefit cancer immunotherapy protocols.

Fig.2 The significance of the CCL1→CCR8 axis in cancer processes. [17]

Allergic Diseases

CCR8 also plays a role in allergic disease mechanisms, where Th2 cells dominate. By affecting the migration and cytokine production of Th2 cells, targeting CCR8 could help manage allergic symptoms. This is especially pertinent in conditions such as asthma and atopic dermatitis, where excessive Th2 activity results in ongoing inflammation and tissue damage.

Strategies that block CCR8 or its ligand, CCL1, could inhibit Th2 cell migration to inflamed areas, potentially diminishing symptoms and enhancing the life quality of individuals with allergic diseases. Support for this approach comes from early-stage research and animal studies, which show decreased inflammatory markers and reduced severity of symptoms following CCR8 inhibition [18].

Detailed Analysis of CCR8 Signaling Pathway

The main ligand of CCR8 is CCL1, also known as I-309, which is a small molecule chemokine that activates downstream signaling pathways by specifically binding to CCR8. After CCL1 binds to CCR8, it causes conformational changes within the receptor, which is the first step in signal transduction.

Conformational changes trigger activation of G proteins on the inner side of the cell membrane. G protein is a heterotrimer composed of three subunits: α, β and γ. Upon activation of the CCR8 receptor, the α subunit of the G protein dissociates from the βγ complex. This separation further amplifies the signal, affecting multiple signaling pathways within the cell [19].

branches of signaling pathways

1. Protein kinase A (PKA) pathway: The α subunit of G protein activates intracellular cyclic AMP (cAMP) production, thereby activating PKA. PKA is a serine/threonine kinase that can phosphorylate a variety of target proteins and regulate cellular metabolism and survival signals.
2. Phosphatidylinositol 3-kinase (PI3K)-Akt pathway: The βγ complex of G protein can activate PI3K. Activated PI3K catalyzes the production of phosphatidylinositol-3,4,5-trisphosphate (PIP3), which then activates Akt. Akt kinase is a key regulator of cell survival and proliferation and is essential for T cell function and survival.
3. Activation of small GTPases and Rho family: The βγ complex can also activate small GTPases of the Rho family, such as Rac and Rho. These small molecule GTPases play a key role in cytoskeletal reorganization and cell migration.

Physiological functions of signaling pathways

Through these signaling pathways, the activation of CCR8 regulates the migration, differentiation and function of T cells. Especially in regulatory T cells, CCR8 signaling can enhance their immunosuppressive function and help maintain immune homeostasis and self-tolerance. In addition, CCR8 signaling can also regulate the activity of effector T cells in inflammatory responses and affect the production and release of cytokines.

The role of the CCR8 signaling pathway in immune regulation is complex and far-reaching. By further studying the specific mechanisms of these signaling pathways, we can not only better understand how the immune system works, but also provide new strategies for treating related immune and inflammatory diseases. As more and more knowledge about the function and regulation of CCR8 is revealed, its clinical application prospects will become clearer.

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