Interleukin-2 (IL-2) is a cytokine, a secreted signaling protein that coordinates the activity of the immune system. Produced principally by activated T cells, it promotes lymphocyte proliferation and supports both immune responses and mechanisms that restrain them. Its functions include maintaining regulatory T cells, which help prevent inappropriate reactions against the body’s own tissues. Recombinant IL-2 also has applications in laboratory research and cancer immunotherapy. (ncbi.nlm.nih.gov)
Discovery and molecular identity
An important step toward identifying IL-2 was a 1976 study showing that medium conditioned by stimulated human lymphocytes could sustain the selective growth of T cells from human bone marrow for months. The growth-promoting activity became known as T-cell growth factor. In 1983, researchers cloned a complementary DNA encoding human IL-2, establishing its sequence and enabling production through recombinant DNA technology. (pubmed.ncbi.nlm.nih.gov)
The human IL2 gene lies at position 4q27 on chromosome 4. It encodes a precursor containing 153 amino acids, including a signal sequence that directs secretion. IL-2 interacts with its receptor through distinct molecular surfaces, allowing several receptor components to assemble around a single cytokine molecule. Structural studies have resolved this complex and explained how ligand binding brings the signaling components together. (ncbi.nlm.nih.gov)
Production and regulation
IL-2 production is closely linked to T-cell activation. Recognition of an antigen through the T-cell receptor, together with costimulatory signals such as those delivered through CD28, induces the molecular machinery responsible for IL-2 expression. This couples growth-factor production to the recognition and activation events that initiate an immune response. (pubmed.ncbi.nlm.nih.gov)
The IL2 promoter integrates signals from several transcription factors, including NFAT, AP-1, and members of the NF-κB family. Experiments with promoter mutations and DNA-binding assays have demonstrated cooperation between these factors rather than control by a single molecular switch. Consequently, IL-2 production reflects the combination of signals received by a T cell, not simply the presence of one activating stimulus. (pubmed.ncbi.nlm.nih.gov)
Receptor and intracellular signaling
The IL-2 receptor comprises three possible subunits: IL-2Rα, or CD25; IL-2Rβ, or CD122; and the common γ chain, or CD132. Together, these form the high-affinity receptor. CD25 facilitates cytokine capture, whereas the β and γ components provide the intracellular signaling machinery. Structural analysis shows that IL-2 binding creates a composite interface that recruits the common γ chain into the complex. (pubmed.ncbi.nlm.nih.gov)
Receptor engagement activates the associated Janus kinases JAK1 and JAK3 and downstream STAT proteins, particularly STAT5. This forms part of the JAK–STAT signaling pathway, which connects extracellular cytokine recognition to changes in gene expression. IL-2 signaling therefore does more than trigger cell multiplication: it changes transcriptional programs governing lymphocyte function. (pubmed.ncbi.nlm.nih.gov)
The importance of STAT5 is especially apparent in regulatory T cells. Mouse experiments have shown that STAT5 binds regulatory regions of the Foxp3 gene and is required for normal regulatory T-cell development. These findings link IL-2 receptor signaling to FOXP3, a transcription factor central to regulatory T-cell identity and function. (pubmed.ncbi.nlm.nih.gov)
Immune activation and tolerance
IL-2 was initially characterized through its ability to sustain T-cell growth. Laboratory studies also demonstrate enhanced lymphocyte cytotoxicity, activation of natural killer cells, and induction of interferon-γ production. These activities underpin its use as an immune-stimulating agent in cancer treatment. Its antitumor action, however, cannot be reduced to a single fully characterized mechanism. (pubmed.ncbi.nlm.nih.gov)
Equally important is its role in immune tolerance. Regulatory T cells commonly express high levels of CD25 and depend on IL-2 for normal maintenance and function. In mouse studies, neutralizing IL-2 reduced regulatory T-cell populations and induced autoimmune disease; IL-2-deficient mice also developed uncontrolled T-cell activation and autoimmunity. Thus, loss of a lymphocyte growth factor can produce excessive, rather than merely weakened, immune activity. (pubmed.ncbi.nlm.nih.gov)
IL-2 also influences differentiation among other T-cell populations. Experimental disruption of IL-2–STAT5 signaling increases the development of IL-17-producing helper T cells, illustrating how this cytokine regulates the composition as well as the magnitude of an immune response. (pubmed.ncbi.nlm.nih.gov)
Therapeutic forms and clinical research
Aldesleukin, marketed as Proleukin, is a recombinant form of human IL-2. It differs from native IL-2 by lacking an N-terminal alanine, substituting serine for cysteine at position 125, and lacking glycosylation. Its United States prescribing information lists metastatic renal cell carcinoma and metastatic melanoma as indications. The label carries boxed warnings for potentially fatal capillary leak syndrome, neurological toxicity, and serious infections. (dailymed.nlm.nih.gov)
Low-dose IL-2 has also been investigated as a means of expanding regulatory T cells in autoimmune diseases, including systemic lupus erythematosus. Clinical results must be interpreted by trial and endpoint: a 60-patient randomized controlled trial published in 2020 did not meet its primary response endpoint at week 12, although a difference was observed at week 24. Engineered, longer-acting IL-2 preparations have additionally been studied for selective regulatory T-cell expansion. Early trials demonstrate biological effects, but such effects alone do not establish clinical benefit across diseases. (pubmed.ncbi.nlm.nih.gov)