Interleukin-2 (IL-2) is a common γ chain (CD132)-dependent cytokine that acts during immune responses, and in particular by the cells of adaptive immunity. It is mainly produced from activated CD4+ T lymphocytes and exerts multiple effects on several types of immune system cells, including T cells, B cells, natural killer (NK) cells, and regulatory T cells.
IL-2 was the first interleukin to be discovered in 1976 during studies on T-cell expansion in vitro. This discovery marked a significant advancement in immunology, revealing a molecular mechanism behind clonal T-cell expansion. It has since become a vital cytokine for clinical immunotherapy and lab research.
This article explores the role of IL-2 in immune system regulation, its cellular targets, and how it informs both immunological research and medical treatment.
Drives T Lymphocyte Expansion and Activation
T lymphocytes are important for the cellular immune response. Naive T cells need additional stimuli to become activated when they encounter antigens presented by antigen-presenting cells.
Interleukin 2, secreted by activated T helper cells, binds to the high-affinity IL-2 receptor (composed of α [CD25], β [CD122], and γ [CD132] chains) expressed on T cells following antigen recognition. IL-2 initiates intracellular signaling primarily through the JAK1/3–STAT5 pathway, promoting cell cycle progression, survival (via upregulation of anti-apoptotic proteins such as Bcl-2), and differentiation into effector-T cells.
Without IL-2, there is not an adequate clonal expansion of T cells, and therefore, there are not enough immune responses. This is especially important in the early phases of infection when massive T-cell proliferation is needed to rein in pathogens. Furthermore, IL-2 modulates T cell fate decisions by influencing the balance between effector and memory T cell differentiation, aiding immediate response and long-term immunity.
Enhances Natural Killer (NK) Cell Cytotoxicity
NK cells are innate immune cells that can recognize and destroy virally infected or malignant cells without prior sensitization. IL-2 significantly enhances NK cell activity by increasing their cytolytic function, promoting cytokine production, and prolonging cellular survival.
When activated by IL-2, NK cells increase expression of the activation receptors and cytotoxic granule proteins like perforin and granzymes, which help in killing the abnormal cells.
IL-2 has been used worldwide and also in the laboratory for ex vivo activation and expansion of NK cells for adoptive cell transfer therapies. Such innate immune-enhancing properties of IL-2 have made it a valuable adjunct to immunotherapy, particularly in cancers with hematologic malignancies and solid tumors.
Maintains Regulatory T Cell (Treg) Function and Immune Tolerance
Although IL-2 is viewed as an immune activator, it is essential for the maintenance of immune tolerance. Tregs that suppress over-reactivity of the immune system and autoimmune diseases are highly dependent on IL-2 for their survival and function.
These cells express high levels of the IL-2 receptor alpha chain (CD25) and rely entirely on exogenous acquisition of IL-2 since they do not produce this cytokine. IL-2 is also required for Treg lineage commitment and maintenance via promoting FOXP3 expression, a transcription factor critical for their suppressive phenotype.
In murine models, loss of either IL-2 or IL-2 receptor leads to lethal autoimmune diseases, highlighting the delicate balance maintained in immune homeostasis. Currently, low-dose IL-2 therapy is being explored to expand Tregs in malignancies and autoimmune diseases such as systemic lupus erythematosus, type 1 diabetes, and graft-versus-host disease (GVHD).
Supports B Cell Responses and Antibody-Mediated Immunity
B cells are not primary targets of IL-2, but this cytokine is indirectly critical for humoral immunity. IL-2 regulates T follicular helper (Tfh) cell activity required for B cell activation, class switching, and affinity maturation in germinal centers.
This process generates high-affinity antibodies, memory B cells, and long-lived plasma cells, which provide long-term immunity after infection or immunization. IL-2 helps to establish immune memory by influencing the interactions of helper T cells with B cells to provide enhanced and faster protection against re-infection with a pathogen.
Facilitates Immunotherapeutic Development and Laboratory Research
In immunologic research, IL-2 has been used for years to grow lymphocyte cultures and study cell-mediated immunity. Its clinical use expanded in the 1990s when recombinant IL-2 (aldesleukin) was approved for treating metastatic renal cell carcinoma and malignant melanoma.
Despite recent clinical success with immune checkpoint inhibitors and CAR-T cell therapy, IL-2 remains a critical component of the immunotherapeutic regimen. This is especially true for treatments for certain cancers, including melanoma and renal cell carcinoma.
However, high doses of IL-2 can cause side effects such as systemic toxicities (vascular leak syndrome/cytokine release) and a lack of specific intratumoral T-cell activation. This has driven interest in developing both IL-2 variants with improved pharmacokinetic profiles or receptor selectivity and combination strategies.
Conclusion
Interleukin-2 (IL-2) is a multi-function cytokine that regulates the immune system. It is central to T cell proliferation, natural killer (NK) cell cytotoxicity, maintenance of regulatory T cells (Treg), and generation of effective B cell responses.
Despite progress in understanding these physiological functions, IL-2 has remained an essential agent in immunotherapy research, focusing on its role in cancer and autoimmunity.
The dichotomy of immune activation versus regulation by this cytokine underscores the importance and potential for its use as a therapeutic agent. Ongoing research continues to refine applications of IL-based therapy while uncovering additional mechanisms on the horizon.
