Decoding IL-2: From Structure to Clinical Significance in Immunotherapy and Beyond

Understanding Interleukin-2 (IL-2)

Interleukin 2 (IL-2) is a protein with a secreted structure weighing 15 kDa, characterized by a 4-α-helical bundle configuration. Operating as a growth factor linked to T cells, IL-2 plays a crucial role in enhancing the cytotoxicity of NK cells and facilitating the synthesis of immunoglobulins by B cells. Furthermore, it contributes to the formation of regulatory T cells (Treg), a process pivotal in establishing immune tolerance among peripheral T cells. IL-2 also takes charge of controlling the proliferation and differentiation of activated T cells.

The Complex Structure of IL-2

The molecular architecture of IL-2 is remarkably intricate, comprising a protein molecule consisting of around 133 amino acids. This molecular arrangement can be primarily dissected into three distinct segments:

1. Signal Peptide (N-Terminal): Positioned at the molecule's N-terminal, the signal peptide guides IL-2 into cells. During protein synthesis, this signal peptide is cleaved, giving rise to the mature IL-2 protein.
2. Core Peptide (Main Segment): The core peptide constitutes the primary portion of the IL-2 molecule and encompasses the majority of the amino acid chain. Through multiple non-covalent bonds and hydrogen interactions, the core peptide assumes a distinct three-dimensional structure. This structure critically determines the biological function and activity of IL-2.
3. C-Terminal (Terminal Segment): Occupying the terminal end of the molecule, the C-terminal potentially engages in vital interactions with other proteins. Moreover, the C-terminus contributes to the stability and functionality of IL-2.

The intricate structural arrangement of IL-2 significantly influences its interactions with receptors and other cytokines. IL-2 is capable of binding with its receptor components: IL-2 receptor α chain (IL-2Rα), IL-2 receptor β chain (IL-2Rβ), and γ chain (IL-2Rγ). This binding triggers intricate signal transduction pathways, fostering the proliferation and activation of immune cells.

The Vital Roles of IL-2 in Immunity and Tolerance

IL-2 plays a pivotal role in fundamental immune system functions, contributing significantly to both tolerance and immunity, primarily through its direct impact on T cells. In the thymus, the site of T cell maturation, IL-2 takes on a critical role by promoting the differentiation of specific immature T cells into regulatory T cells. These regulatory T cells play a suppressive role, preventing other T cells from targeting healthy cells and thus mitigating the risk of autoimmune diseases. Additionally, IL-2 enhances a process known as activation-induced cell death (AICD)[2], aiding in maintaining immune balance.

IL-2 also has a profound influence on the differentiation of T cells. When initial T cells encounter antigens, IL-2 promotes their differentiation into effector T cells and memory T cells, thereby bolstering the body's ability to combat infections[3]. In concert with other key cytokines, IL-2 stimulates the differentiation of naive CD4+ T cells into distinct Th1 and Th2 lymphocyte subsets, while concurrently suppressing differentiation into Th17 and follicular Th lymphocytes[4]. Moreover, IL-2 heightens the cell-killing capabilities of natural killer cells and cytotoxic T cells, contributing to the immune response against threats[5].

The expression and secretion of IL-2 are under stringent regulation, serving as integral components of transient positive and negative feedback loops. These mechanisms play a crucial role in orchestrating the initiation and attenuation of immune responses. Beyond these immediate effects, IL-2 is instrumental in cultivating T cell immunologic memory. This process hinges on the expansion in both number and function of antigen-selected T cell clones, pivotal for the establishment of enduring cell-mediated immunity[6].

IL-2 Receptor Family: Orchestrating Immune Responses

IL-2, a pivotal immunoregulatory cytokine, finds its primary source in activated CD4+ T cells, although it can also be induced by innate lymphoid cells, activated CD8+ T cells, dendritic cells, B cells, and mast cells. The orchestration of IL-2 secretion is governed by key transcription factors such as AP-1, NFκB, and NFAT. This cytokine communicates its effects via dimeric or trimeric IL-2 receptors (IL-2R)[7].

The IL-2 receptor, IL-2R, is a complex heteromer consisting of α, β, and γ chains. Signaling cascades emanate from all IL-2 receptors, utilizing JAK1 and JAK3 to activate downstream STATs, including STAT5. Within this receptor system, three subunit defects have been identified, all associated with immunodeficiency and immune dysregulation/autoimmune diseases. The first instances of IL-2R-related disorders gained public attention with the emergence of patients exhibiting IL-2Rγ (CD132) deficiency, marked by severe T-B+NK-combined immunodeficiency (X-SCID). Subsequently, cases of human IL-2Rα (CD25) deficiency and IL-2Rβ (CD122) deficiency were discovered[8].

Remarkably, IL-2Rγ serves not only as the receptor for IL-2 but also for IL-4, IL-7, IL-15, and IL-21. IL-4 assumes the role of activating B cell proliferation and driving the differentiation of CD4+T cells into Th2 cells. Meanwhile, IL-7 exerts varying effects on T cell survival and proliferation. IL-15, as an early stimulant, fuels the proliferation and activation of T cells while counteracting T cell apoptosis induced by IL-2. This function is pivotal for maintaining sustained immune levels[9].

Clinical Applications of IL-2: From Autoimmunity to Tumor Immunotherapy

IL-2 is Used in the Treatment of Autoimmune Diseases by Enhancing Treg

The significance of IL-2 comes to the forefront in the treatment of autoimmune disorders, primarily through its enhancement of regulatory T cells (Treg). While IL-2 relies on the growth and development of Treg cells, these specialized cells cannot independently synthesize IL-2 and must obtain it through paracrine signaling. Notably, Treg cells prominently display the IL-2Rα-IL-2Rβ-IL-2Rγ trimer on their surface. In the quiescent state of T cells, factors such as PTEN, PD-1, and CTLA-4 restrain proliferation and differentiation by influencing glucose metabolism and blocking the PI3K pathway. The sustained high expression of high-affinity receptors on Treg cell surfaces enables IL-2-activated Tregs to disrupt intracellular signal equilibrium when IL-2 signal levels fall below the CD8+ cell excitation threshold. This bias towards Treg cell proliferation and differentiation ensues.

Confirmed by research, the application of low-dose IL-2 has yielded positive outcomes in conditions like systemic lupus erythematosus (SLE), primary Sjögren's syndrome, and dermatomyositis/polymyositis. This approach effectively augments the Treg cell count, achieving the desired therapeutic goals.

IL-2 for the Treatment of Tumor Immunity

The role of IL-2 in the immune response of T cells and natural killer (NK) cells underscores its significance in tumor immunotherapy. However, the challenge lies in optimizing the activation of IL-2 in cytotoxic T lymphocytes (CTLs) and NK cells, while concurrently mitigating the activation of Treg cells. In this context, strategies focus on bolstering the affinity of IL-2 for the IL-2R βγ dimer, while weakening its binding to the IL-2R αβγ trimer or IL-2Rα.

Efforts in utilizing IL-2 for tumor immunity have revolved around cytokine and receptor engineering to fine-tune therapeutic efficacy. For instance, fusion of the extracellular domain of the IL2Rα subunit to the C-terminus of IL-2 or the introduction of PEG modifications in IL-2 aims to impede its interaction with the high-affinity IL-2Rαβγ subunit. These modifications, however, do not interfere with IL-2Rαβγ subunit binding, thus enabling the selective activation of non-Treg lymphocytes.

Unveiling the IL-2 Signaling Pathways

IL-2's influence on T cell proliferation and function is orchestrated through three distinct pathways:

1. Jak-STAT Pathway:

Upon binding to medium/high-affinity IL-2R, IL-2 initiates a complex cascade. Jak1, linked to the IL-2Rβ chain, becomes proximal to Jak3 on the γ chain, triggering their mutual phosphorylation and activation. The activated Jak1 and Jak3 then proceed to phosphorylate tyrosine residues on the IL-2Rβ chain, creating binding sites for the STAT5 protein. This STAT5 protein, once bound, becomes activated and undergoes dissociation through Jak kinase phosphorylation. This leads to the generation of dimers which subsequently migrate to the nucleus. There, they orchestrate the transcription of target genes, impacting T cell proliferation and function. Remarkably, the Jak-STAT pathway predominates, accounting for over 90% of IL-2's effects across the three pathways.

1. Raf-Ras-MAPK Pathway:

Activated Jak1's influence extends further as it triggers the Raf-Ras-MAPK signaling pathway. By binding to the adapter protein (Grb2) and the guanine nucleotide exchange factor (SOS), Jak1 activation leads to the regulation of cell proliferation and differentiation. This intricate mechanism underscores IL-2's diverse impact on cellular processes.

1. Jak-PI3K Pathway:

The third signaling pathway involves Jak1 activation propelling the PI3K pathway into action. This activation triggers downstream kinase activities, actively participating in T cell proliferation and anti-apoptosis functions. This intricate network highlights the multifaceted ways in which IL-2 orchestrates the immune response, underscoring its central role in immune regulation.

Clinical Significance of IL-2

The clinical significance of Interleukin-2 (IL-2) stems from its pivotal role as an immunomodulatory cytokine, yielding profound implications across various medical domains, particularly within immunotherapy and cancer treatment. Below, we delve into key facets of IL-2's clinical importance:

1. Revolutionizing Cancer Immunotherapy: The potential of IL-2 in cancer treatment has garnered extensive scrutiny. Its therapeutic promise shines particularly bright in immunotherapy scenarios, notably metastatic melanoma and renal cell carcinoma. IL-2 serves as a catalyst, fostering the activation and proliferation of cytotoxic T cells and natural killer (NK) cells. This augmentation of the immune response accentuates the body's fight against cancer cells. High-dose IL-2 therapy has demonstrated remarkable outcomes, propelling immune cells to prompt sustained regression of tumors in select instances.
2. Elevating Immune Response: IL-2's indispensable role in the growth and function of various immune cells, including T cells and NK cells, cannot be overstated. The administration of exogenous IL-2 offers a compelling avenue for fortifying the immune response. This versatility positions it as a potential ally in amplifying the efficacy of vaccines and other immunotherapies.
3. Harboring Promise for Autoimmune Diseases: Beyond its immunostimulatory prowess, IL-2 exhibits the capability to rein in hyperactive immune responses. Investigative strides have been taken to explore low-dose IL-2's potential in tackling autoimmune diseases like type 1 diabetes and systemic lupus erythematosus. This endeavor centers on nurturing regulatory T cells that orchestrate immune reactions.
4. Empowering Stem Cell Transplantation: In the realm of stem cell transplantation, IL-2 assumes a crucial role. By propelling the growth and functionality of immune cells post-transplantation, IL-2 contributes to averting complications such as graft-versus-host disease. This, in turn, supports the recovery of the immune system.
5. Synergistic Combo Therapies: IL-2 finds its place in synergy with other therapies, notably immune checkpoint inhibitors. The fusion of IL-2 with these agents generates a harmonious collaboration that bolsters immune responses, thereby heightening the onslaught against cancer cells.

In summation, IL-2's clinical significance lies in its capacity to navigate and modulate immune responses. Its multi-pronged applications span cancer treatment, immunotherapy, autoimmune disorders, stem cell transplantation, and synergistic combination therapies. With ongoing strides in immunology and personalized medicine, IL-2 persists as a beacon of promise, poised to enhance immune-driven treatments and usher in improved patient outcomes.

IL-2 Protein

Recombinant Human IL-2 Protein

Click here for more IL-2

Synonym:Interleukin-2; IL-2; T-Cell Growth Factor; TCGF; Aldesleukin; IL2

References:

[1] Wilson CG, Arkin MR. Small-molecule inhibitors of IL-2/IL-2R: lessons learned and applied. Curr Top Microbiol Immunol. 2011;348:25-59. doi:10.1007/82_2010_93
[2] Arenas-Ramirez N, Woytschak J, Boyman O. Interleukin-2: Biology, Design and Application. Trends Immunol. 2015;36(12):763-777. doi:10.1016/j.it.2015.10.003
[3] Liao W, Lin JX, Leonard WJ. IL-2 family cytokines: new insights into the complex roles of IL-2 as a broad regulator of T helper cell differentiation. Curr Opin Immunol. 2011;23(5):598-604. doi:10.1016/j.coi.2011.08.003
[4] Liao W, Lin JX, Leonard WJ. Interleukin-2 at the crossroads of effector responses, tolerance, and immunotherapy. Immunity. 2013;38(1):13-25. doi:10.1016/j.immuni.2013.01.004
[5] Spolski R, Li P, Leonard WJ. Biology and regulation of IL-2: from molecular mechanisms to human therapy. Nat Rev Immunol. 2018;18(10):648-659. doi:10.1038/s41577-018-0046-y
[6] Malek TR, Castro I. Interleukin-2 receptor signaling: at the interface between tolerance and immunity. Immunity. 2010;33(2):153-165. doi:10.1016/j.immuni.2010.08.004
[7] Zhou P. Emerging mechanisms and applications of low-dose IL-2 therapy in autoimmunity. Cytokine Growth Factor Rev. 2022;67:80-88. doi:10.1016/j.cytogfr.2022.06.003
[8] Hsieh EW, Hernandez JD. Clean up by aisle 2: roles for IL-2 receptors in host defense and tolerance. Curr Opin Immunol. 2021;72:298-308. doi:10.1016/j.coi.2021.07.010
[9] Toumi R, Yuzefpolskiy Y, Vegaraju A, et al. Autocrine and paracrine IL-2 signals collaborate to regulate distinct phases of CD8 T cell memory. Cell Rep. 2022;39(2):110632. doi:10.1016/j.celrep.2022.110632
[10] Liao W, Lin JX, Leonard WJ. Interleukin-2 at the crossroads of effector responses, tolerance, and immunotherapy. Immunity. 2013;38(1):13-25. doi:10.1016/j.immuni.2013.01.004