In-depth Analysis of the IL-2 Target: From T-cell Growth Factor to Engineered Immunotherapy Breakthroughs

Keywords: IL-2, Interleukin-2, biased mutants, regulatory T cells, Treg, tumor immunotherapy, autoimmune diseases, engineered cytokines, mRNA therapeutics

In October 2025, the Nobel Prize in Physiology or Medicine was awarded to the discoverers of regulatory T cells (Treg), once again bringing the underlying mechanisms of immune regulation into the spotlight. Around the same time, IL-2—a cytokine deeply intertwined with the fate of Treg—was undergoing a transformation from "crude" to "precise" in both understanding and technological innovation. From its early days as the first approved cancer immunotherapy to its long-standing limitations due to severe toxicity and paradoxical Treg expansion, and now to its resurgence in clinical value through engineered modifications, the evolution of IL-2 reflects the typical challenges and breakthroughs in cytokine drug development. This article systematically reviews the biological foundations and translational frontiers of the IL-2 target from four dimensions: molecular mechanisms, clinical dilemmas, engineering strategies, and the latest R&D progress, providing a comprehensive framework for professionals in the biopharmaceutical industry.

IL-2 is a 15.5-kDa cytokine primarily secreted by activated CD4+ T cells, and its biological functions are mediated by a heterotrimeric receptor complex on the cell surface. The IL-2 receptor consists of three subunits: the α chain (CD25), the β chain (CD122), and the common γ chain (γc, CD132). These subunits combine in different configurations to form three receptor complexes with varying affinities: the low-affinity receptor (CD25 monomer alone, Kd ~ 10 nM, no signaling function), the intermediate-affinity receptor (CD122-CD132 dimer, Kd ~ 1 nM, with basic signaling capability), and the high-affinity receptor (CD25-CD122-CD132 heterotrimer, Kd ~ 10 pM). This hierarchical structure forms the molecular basis for its precise signal regulation.

The differential expression profiles of IL-2R subunits on various immune cells determine the "dual identity" of IL-2 signaling. Treg constitutively express high levels of CD25, enabling them to preferentially capture IL-2 at extremely low concentrations, thereby maintaining their survival and function. In contrast, effector T cells and NK cells primarily rely on the intermediate-affinity receptor and require higher IL-2 concentrations for activation. This property allows IL-2 to serve as an "immune tolerance" tool for Treg expansion in autoimmune diseases and transplant rejection (low-dose strategy), while high-dose IL-2 activates effector T cells for "immune activation" in tumor therapy. However, this duality also presents a therapeutic dilemma: exogenous IL-2 activates antitumor effects while simultaneously expanding immunosuppressive Treg in the tumor microenvironment, creating a self-counteracting "double-edged sword" effect.

Although high-dose IL-2 has demonstrated durable and even curative efficacy in melanoma and renal cell carcinoma, its clinical application has long been hampered by severe dose-limiting toxicities. Vascular leak syndrome is one of the most life-threatening adverse reactions to IL-2 therapy, manifesting as edema, hypotension, and organ toxicity, which can be fatal in severe cases. Additionally, IL-2 can induce cytokine release syndrome, further exacerbating systemic inflammatory responses. These issues have confined traditional IL-2 therapy to a limited number of medical centers equipped with intensive care capabilities, significantly restricting its accessibility and applicability.

The root cause of these toxicities lies in the inability of exogenous IL-2 to distinguish between high- and intermediate-affinity receptor expression. Treg exploit their high-affinity receptor advantage in the tumor microenvironment to capture IL-2, not only undermining antitumor efficacy but also expanding the immunosuppressive cell population. Over the past three decades, extensive frontline clinical trials have largely ended in failure, indicating that traditional protein engineering alone has not resolved this dilemma.

To overcome the toxicity limitations and enhance the therapeutic window of IL-2, academia and industry have explored multiple engineering pathways.

1. Biased IL-2 Mutants
The most mainstream strategy involves reducing IL-2's binding affinity for CD25, enabling preferential activation of effector T cells and NK cells expressing intermediate-affinity receptors while avoiding Treg expansion. Representative molecules include NKTR-214 (bempegaldesleukin). However, this molecule failed to meet the primary endpoint in a Phase III melanoma trial, suggesting that while "non-α" biased designs reduce vascular leak syndrome risk, they introduce new dose-limiting toxicities and insufficient efficacy. Subsequent studies further revealed that CD25 engagement is indispensable for optimal activation of tumor-specific CD8+ T cells. Wild-type IL-2 is nearly 100 times more potent than non-α mutants in activating primary human tumor-infiltrating lymphocytes. Meanwhile, another class of biased designs is exploring the opposite approach—preserving or even enhancing CD25 binding to preferentially expand Treg for autoimmune diseases. Nektar's rezpegaldesleukin is currently under evaluation in Phase III trials for atopic dermatitis and alopecia areata.

2. Conditionally Activated IL-2
As biased strategies face bottlenecks, conditionally activated IL-2 has emerged as a more promising direction. WTX-124 is a prodrug form of wild-type IL-2 designed to "mask" the active site of IL-2 in peripheral circulation, releasing it only when the molecule reaches the tumor microenvironment via tumor-specific protease cleavage. This strategy maintains the full activity of wild-type IL-2 while achieving pharmacokinetic separation between low peripheral exposure and high intratumoral exposure, resulting in low receptor occupancy on peripheral lymphocytes and high receptor occupancy on tumor-infiltrating lymphocytes. Preclinical studies show that WTX-124 retains CD25-mediated optimal effector cell activation while significantly reducing systemic toxicity, offering a new paradigm for balancing efficacy and safety.

3. IL-2/IL-10 Synergistic Design
Another breakthrough approach involves conjugating IL-2 with IL-10. A 2025 study published in Cell Reports Medicine demonstrated that IL-2 toxicity is primarily driven by cytokine release syndrome, while IL-10 naturally possesses anti-inflammatory and Treg-suppressing functions. By coupling IL-2 with an IL-10 mutant to form a tumor-targeting fusion protein (e.g., EGFR-targeting DK210), the design preserves IL-2's cytotoxic T cell activation function while suppressing cytokine release syndrome and Treg expansion. In syngeneic mouse tumor models, this approach achieved potent antitumor activity without peripheral inflammatory toxicity.

From 2025 to 2026, the IL-2 target landscape is bustling with R&D activity, reflecting diverse technological approaches and clinical positioning.

Innovent Biologics: PD-1/IL-2 bispecific fusion protein. IBI363 is a PD-1-targeted IL-2 bispecific fusion protein with a precisely engineered IL-2 arm: it retains high affinity for IL-2Rα but reduces binding to IL-2Rβ and IL-2Rγ, significantly lowering IL-2-related systemic toxicity. The drug has received three breakthrough therapy designations in China for indications such as non-small cell lung cancer and melanoma, as well as two FDA Fast Track designations in the U.S., demonstrating breakthrough potential in addressing immune resistance and "cold tumor" treatment challenges.

Nektar Therapeutics: α-biased IL-2. Nektar raised $325 million to advance rezpegaldesleukin in Phase III trials for atopic dermatitis and alopecia areata. Its latest maintenance-phase data show promising efficacy, but the market remains cautious about the prospects of α-biased strategies in oncology indications.

Merck's acquisition of Pandion. In 2026, Merck acquired Pandion for $1.85 billion, with PT101—a βγ-biased IL-2R agonist—as the core asset. PT101 has completed Phase Ia trials and is poised to initiate follow-up studies in ulcerative colitis and systemic lupus erythematosus.

Genetic Leap: Oral small-molecule IL-2 mRNA modulator. GL-IL2-138 is an orally available small molecule designed via an AI platform. It targets and binds endogenous IL-2 mRNA, regulating immune cells to produce natural IL-2 protein physiologically rather than delivering exogenous recombinant IL-2. The molecule has received FDA IND approval and is entering Phase I trials, potentially circumventing the systemic toxicity and immunogenicity issues associated with traditional IL-2 therapies.

mRNA technology offers a novel pathway for the precise delivery of IL-2. From 2025 to 2026, researchers developed apolipoprotein nanoparticles (aNPs) to deliver IL-2-encoding mRNA to monocytes in secondary lymphoid organs. In mouse B16F10 melanoma and MC38 colon cancer models, intravenous aNP-mRNA-IL-2 achieved sustained IL-2 production in the spleen and lymph nodes, significantly inhibiting tumor growth. This approach has also demonstrated clinical translation potential in non-human primates. Additionally, the mRNA-LNP platform can be used to transiently reprogram cytotoxic effector T cells in vivo to express IL-2 and other proteins, offering a new paradigm for in vivo T cell engineering.

IL-2 is evolving from a single cytokine into a platform target for diverse engineering strategies. Future innovations will focus on: more refined artificial designs to optimize receptor selectivity and signal bias, leveraging the tumor microenvironment for conditional activation to reduce systemic toxicity, adopting combination therapies to counteract Treg-mediated immunosuppression, employing mRNA delivery for in situ expression, and enhancing therapeutic indices through biomarker-guided personalized dosing. Currently, the global IL-2 market is growing steadily, projected to maintain a high single-digit compound annual growth rate (CAGR) through 2035, while the IL-2 ELISA kit market is expected to expand from $300 million in 2025 to $500 million by 2035. Just as engineering is reshaping the clinical value of this classic target, IL-2 is redefining the boundaries of cytokine drug development.