CCL1: The Overlooked Immune Signal "Messenger"

In the intricate immune communication network of the human body, chemokines function like precise GPS navigation signals, directing the migration and deployment of immune cells. CCL1 (also known as I-309) is a unique and critical member of this vast signaling family. Although it may not be as renowned as "star" inflammatory cytokines like IL-6 or TNF-α, as the primary ligand for the CCR8 receptor, CCL1 plays an irreplaceable role as a "coordinator" in tumor immune evasion, chronic inflammation regulation, and defense against specific pathogens. In-depth research on CCL1 is opening a new window for understanding cancer treatment resistance, autoimmune balance, and the pathology of chronic infections.

What is the CCL1/I-309 Protein?

Core Definition and Historical Naming
CCL1, systematically named C-C motif chemokine ligand 1, is a small secreted protein produced by immune cells such as activated T cells (primarily CD4⁺ T cells), monocytes/macrophages, and mast cells. Its alternative name, I-309, originates from its initial discovery—it was identified as the 309th clone gene product with chemotactic activity in the supernatant of activated T cell cultures. It belongs to the CC chemokine subfamily, characterized by two adjacent conserved cysteine residues.

Molecular Structure and Mechanism of Action
CCL1 functions as a monomer, with its core activity mediated by binding to the specific receptor CCR8 on cell membranes.

The core signaling pathway is as follows:
Signal Emission: Under specific inflammatory or tumor microenvironment stimuli, immune cells synthesize and secrete CCL1.
Receptor Recognition: CCL1, as a ligand, binds with high affinity to the seven-transmembrane G protein-coupled receptor CCR8 on target cell surfaces.
Signal Transduction: The CCL1-CCR8 interaction triggers intracellular G protein dissociation, activating downstream pathways such as PI3K/Akt and MAPK/ERK.
Cellular Response: Ultimately, this induces a series of biological effects in CCR8-expressing cells, including chemotactic migration (directional movement), enhanced survival, and functional modulation.

Core Biological Functions: The "Recruiter" and "Modulator" of Immune Cells

CCL1's primary function revolves around the fine-tuned regulation of specific immune cell subsets:

Core Recruitment Function: It serves as an effective chemoattractant for CCR8-expressing regulatory T cells (Tregs), recruiting them to inflammatory or tumor sites where they exert potent immunosuppressive effects.
Th2-Type Immune Response: It participates in recruiting Th2 cells and eosinophils, playing a role in allergic reactions and anti-parasitic immunity.
Monocyte/Macrophage Regulation: It influences monocyte migration and macrophage polarization states.
Neuro-Immune Communication: Evidence suggests that the CCL1-CCR8 axis may be involved in regulating neuroinflammation and pain perception.

Deep Associations Between CCL1/I-309 and Diseases

Dysregulated CCL1 expression is closely linked to the pathological processes of various diseases, particularly in shaping immunosuppressive microenvironments.

1. Cancer: Building a Protective "Firewall" for Tumors
The CCL1-CCR8 axis is a hot topic in tumor immunology research, recognized as a key mechanism for tumor immune evasion.

Role in the Tumor Immune Microenvironment:
Recruitment of Immunosuppressive Cells: Cancer cells or tumor-associated macrophages in various solid tumors (e.g., breast cancer, lung cancer, colorectal cancer, melanoma) and lymphomas can secrete CCL1.
Creation of an Immunosuppressive Environment: CCL1 specifically recruits CCR8-high tumor-infiltrating Tregs from peripheral circulation into the tumor core. Once inside, these Tregs strongly suppress the anti-tumor activity of cytotoxic T cells (CD8⁺ T cells), acting like a "firewall" that shields tumor cells from immune attack.
Prognostic Implications: High levels of CCL1 or CCR8⁺ Treg infiltration in tumor tissues are often associated with poorer clinical outcomes, faster disease progression, and primary or secondary resistance to immune checkpoint inhibitors (e.g., PD-1 antibodies).

Therapeutic Targeting Potential:
CCR8-Targeting Monoclonal Antibodies: These drugs aim to selectively deplete tumor-infiltrating Tregs while preserving peripheral and normal tissue Tregs to maintain immune tolerance. This is considered a safer and more promising strategy than non-selective Treg inhibitors. Currently, several anti-CCR8 monoclonal antibodies (e.g., BMS-986340) are in clinical trials, designed to be combined with PD-1 inhibitors to overcome immunotherapy resistance.

2. Chronic Inflammatory and Autoimmune Diseases
CCL1 plays a dual role in maintaining immune balance, and its dysregulation can lead to uncontrolled inflammation or excessive suppression.

Psoriasis and Atopic Dermatitis: In skin inflammation, CCL1 may recruit Th2 cells and Tregs, influencing the balance between inflammation progression and resolution. Its exact role—pro-inflammatory or anti-inflammatory—may depend on the disease stage and microenvironment context.
Rheumatoid Arthritis: CCL1 is detectable in the synovial tissue and fluid of RA patients, where it may modulate T cell and monocyte infiltration, contributing to chronic joint inflammation.
Multiple Sclerosis: In experimental autoimmune encephalomyelitis models, the CCL1-CCR8 axis exhibits complex regulatory effects, potentially influencing pathogenic T cell entry into the central nervous system.

3. Infectious Diseases
Viral Infections: In HIV infection, CCL1 can inhibit infection of macrophages by certain HIV strains. In herpesvirus infection models, CCL1 exhibits protective effects, possibly by modulating antiviral immune responses.
Bacterial Infections and Sepsis: In severe systemic infections like sepsis, CCL1 expression changes, potentially regulating excessive inflammatory responses or immunosuppressive states.

4. Fibrotic Diseases
Pulmonary and Hepatic Fibrosis: Studies suggest that CCL1 may participate in tissue repair and fibrosis regulation by recruiting regulatory cells or influencing macrophage function.

Clinical Significance and Research Frontiers

Potential as a Biomarker
Predicting Tumor Immunotherapy Response: Detecting CCL1/CCR8-related signals in tumor tissue or peripheral blood may help predict patient responses to PD-1/PD-L1 inhibitors and identify those most likely to benefit from combined anti-CCR8 therapy.
Monitoring Disease Activity: In some autoimmune diseases, CCL1 levels could serve as a potential indicator of immune status or treatment response.

Drug Development as a Therapeutic Target
Monoclonal Antibodies:
Anti-CCL1 Antibodies: Block ligand function.
Anti-CCR8 Antibodies: The current mainstream approach. Among these, anti-CCR8 monoclonal antibodies with antibody-dependent cell-mediated cytotoxicity (ADCC) functionality, which selectively deplete tumor microenvironment Tregs, are a key focus.
Small-Molecule Inhibitors: Developing CCR8-targeting small-molecule antagonists for oral administration could broaden therapeutic applications.
Combination Strategies: Pairing CCR8 inhibitors with existing immune checkpoint inhibitors, chemotherapy, or radiotherapy aims to synergistically activate anti-tumor immunity and overcome resistance.

Future Prospects and Challenges

Context-Dependent Functionality: CCL1 may exert opposing roles (pro-inflammatory vs. anti-inflammatory) in different diseases or stages. Future research must dissect its specific functional networks in distinct microenvironments.
Tissue Specificity and Safety: A core challenge in drug development is selectively targeting CCL1/CCR8 signaling in tumors or lesions without disrupting systemic immune homeostasis (e.g., immune tolerance in the gut or skin).
Biomarker System Establishment: Large-scale clinical studies are needed to validate the reliability of CCL1/CCR8 as predictive and prognostic markers and to standardize detection methods.
Exploring New Disease Areas: Beyond cancer, CCL1's roles in chronic inflammation, fibrosis, and metabolic diseases (e.g., obesity-related chronic inflammation) warrant further investigation.

Conclusion

CCL1/I-309, once a relatively understated chemokine, has gained prominence as the primary ligand for CCR8 and a central player in tumor immunosuppressive microenvironments. Like a shrewd "dispatcher," it precisely recruits immunosuppressive regulatory T cells, profoundly influencing the progression of cancer, chronic inflammation, and other diseases. Targeting the CCL1-CCR8 axis—particularly through drugs that selectively deplete tumor-infiltrating Tregs—has emerged as a highly promising frontier in next-generation cancer immunotherapy. From basic research to clinical translation, the in-depth exploration of CCL1 not only unveils new layers of immune regulation but also heralds the potential to "fine-tune" immune microenvironments, offering new therapeutic hope for patients with refractory diseases.