IL12A - interleukin 12A |Elisa - Clia - Antibody - Protein

Background

Interleukin-12 (IL-12) is a pivotal cytokine in immune responses, primarily produced by antigen-presenting cells such as dendritic cells, macrophages, and B cells. It is a heterodimeric cytokine, meaning it is composed of two distinct subunits: IL-12A (also known as p35) and IL-12B (p40). The combination of these two subunits forms the bioactive IL-12 heterodimer, referred to as IL-12p70. IL-12 plays a critical role in linking innate and adaptive immunity by promoting the differentiation of naïve T cells into T-helper 1 (Th1) cells, which are essential for cellular immune responses.

The primary biological role of IL-12 is to stimulate the production of interferon-gamma (IFN-γ) by T cells and natural killer (NK) cells, which enhances cytotoxic activity and supports pathogen clearance, especially intracellular pathogens like viruses and certain bacteria. Beyond its immune-regulatory functions, IL-12 has significant anti-tumor properties, making it a potential candidate for cancer immunotherapy.

IL-12A, the alpha subunit of IL-12, is crucial for the bioactivity of the IL-12 cytokine. However, IL-12A by itself is not biologically active; its function is dependent on pairing with the IL-12B subunit.

Protein Structure

IL-12A (p35) is a glycosylated protein of 35 kDa, composed of 219 amino acids. It belongs to the type I cytokine superfamily and has structural characteristics typical of cytokines involved in immune signaling.

Key structural features include:

• Four-helix bundle: Like many cytokines, IL-12A adopts a four-helix bundle structure that is stabilized by internal hydrogen bonds and disulfide bridges. This arrangement is characteristic of cytokines that signal via the Jak/STAT pathway.
• Glycosylation: IL-12A is glycosylated at several asparagine residues. Glycosylation is critical for protein stability, proper folding, and secretion. The glycosylated regions also influence how the protein interacts with its receptors.
• Heterodimer formation: IL-12A alone is not biologically active; it forms a heterodimer with the IL-12B (p40) subunit. This dimerization occurs via non-covalent interactions, allowing the two subunits to form the fully functional IL-12p70 cytokine. The IL-12A (p35) subunit contributes the necessary structural elements for receptor binding, while the IL-12B (p40) subunit is primarily involved in interacting with the IL-12 receptor.
• Receptor Binding: IL-12A primarily engages the IL-12 receptor subunit IL-12Rβ1, part of the heterodimeric IL-12 receptor complex. This interaction is essential for downstream signaling via the Jak2 and STAT4 pathways, leading to the production of IFN-γ and the activation of Th1 cells.

The complex quaternary structure of IL-12, with both the IL-12A and IL-12B subunits, is necessary for its full range of immune-modulating effects. Disruption in the formation or function of either subunit leads to impaired immune responses, particularly in the context of Th1-mediated immunity.

Classification and Subtypes

IL-12 belongs to the IL-12 cytokine family, which also includes IL-23, IL-27, and IL-35. These cytokines share structural similarities and some overlap in biological function, particularly in immune regulation and inflammation. They are often involved in controlling T cell differentiation and cytokine production.

• IL-12A (p35): The IL-12A subunit is shared between IL-12 and IL-35. When paired with IL-12B (p40), it forms IL-12, while when paired with EBI3 (Epstein-Barr virus-induced gene 3), it forms IL-35.
• IL-12B (p40): The IL-12B subunit is shared between IL-12 and IL-23. When combined with p19, IL-12B forms IL-23, which has roles in promoting Th17 cell differentiation and inflammatory responses.

There are no true subtypes of IL-12A, but its combination with different subunits (IL-12B for IL-12 and EBI3 for IL-35) allows it to participate in multiple cytokine complexes with distinct biological activities.

Function and Biological Significance

IL-12, as a heterodimer of IL-12A and IL-12B, plays a central role in promoting Th1 responses and cell-mediated immunity. Its key functions and biological significance are outlined below.

Primary Functions of IL-12:

1. Th1 Differentiation: IL-12 is crucial for the differentiation of naïve CD4+ T cells into Th1 cells. Th1 cells are responsible for the production of IFN-γ, a cytokine that enhances macrophage activation and promotes the killing of intracellular pathogens such as Mycobacterium tuberculosis and certain viruses.
2. Stimulation of IFN-γ Production: IL-12 induces the production of IFN-γ by both T cells and NK cells. IFN-γ plays a critical role in activating macrophages, increasing their ability to engulf and destroy pathogens, and enhancing antigen presentation.
3. Enhancement of Cytotoxic Activity: IL-12 enhances the cytotoxic activity of NK cells and cytotoxic T lymphocytes (CTLs), promoting the lysis of infected or malignant cells. This cytotoxic function is vital in the control of viral infections and tumor surveillance.
4. Promotion of Immune Memory: By driving Th1 differentiation and promoting cytotoxic responses, IL-12 also supports the development of long-term immune memory, which is essential for protecting against recurring infections.
5. Anti-Tumor Activity: IL-12 has potent anti-tumor effects due to its ability to stimulate Th1 responses and IFN-γ production, both of which enhance immune-mediated tumor cell killing. IL-12 is a promising candidate for cancer immunotherapy due to its ability to activate the immune system against tumors.

Biological Significance in Disease Contexts:

• Pathogen Defense: IL-12 is indispensable for defense against intracellular pathogens, including viruses, bacteria, and parasites. It orchestrates the immune response required to eliminate pathogens that survive inside cells by enhancing macrophage activation and promoting IFN-γ production.
• Autoimmunity: While IL-12 is essential for defending against pathogens, its overproduction can contribute to the development of autoimmune diseases such as multiple sclerosis, type 1 diabetes, and Crohn's disease. In these conditions, excessive Th1 activation leads to the destruction of host tissues.

Clinical Issues

Dysregulation of IL-12A, either due to overproduction or deficiency, is associated with various clinical conditions. Understanding IL-12’s role in these diseases has opened avenues for therapeutic interventions.

Overexpression of IL-12 in Disease:

1. Autoimmune Diseases: Elevated levels of IL-12 are linked to autoimmune diseases, particularly those characterized by Th1 dominance. In multiple sclerosis (MS), overactive Th1 responses promote inflammation and destruction of the myelin sheath surrounding nerve fibers. Similarly, in type 1 diabetes, excessive IL-12-driven immune responses contribute to the destruction of pancreatic beta cells.
2. Inflammatory Bowel Disease (IBD): Crohn's disease, an IBD subtype, is strongly associated with elevated IL-12 levels, which promote the differentiation of pathogenic Th1 cells in the intestinal mucosa, leading to chronic inflammation.
3. Chronic Infections: In some chronic infections, such as tuberculosis, persistent IL-12 production helps sustain the immune response but can also lead to excessive inflammation and tissue damage.

IL-12 Deficiency:

1. Susceptibility to Infections: Individuals with genetic deficiencies in IL-12 production or signaling are highly susceptible to infections by intracellular pathogens such as Mycobacterium and Listeria monocytogenes. These patients often suffer from recurrent or severe infections due to impaired Th1 responses and inadequate IFN-γ production.
2. Immunodeficiency: Mutations in the IL-12A gene or its receptor (IL-12R) can lead to primary immunodeficiency syndromes, where the immune system's ability to combat intracellular pathogens is compromised.

Therapeutic Applications:

• Cancer Immunotherapy: IL-12 has shown promise as a therapeutic agent in cancer immunotherapy due to its ability to enhance anti-tumor immunity. Clinical trials have investigated IL-12 in various cancers, including melanoma, renal cell carcinoma, and colorectal cancer. However, systemic administration of IL-12 has been associated with significant toxicity, including cytokine release syndrome. Therefore, strategies that target IL-12 directly to the tumor microenvironment or modify its delivery are being explored.
• Autoimmune Disease Treatments: Blocking IL-12 signaling has been proposed as a treatment for autoimmune diseases like psoriasis, Crohn's disease, and rheumatoid arthritis. Inhibitors that target IL-12 or its receptor are already in clinical use (e.g., ustekinumab, which blocks IL-12/IL-23 signaling).

Summary

IL-12A (p35) is an essential component of the IL-12 cytokine, which plays a pivotal role in immune responses by promoting Th1 differentiation, enhancing IFN-γ production, and supporting cytotoxic immune responses. Its structural combination with IL-12B forms the active IL-12p70 cytokine, which is crucial for controlling intracellular infections and regulating immune-mediated tumor surveillance. However, dysregulation of IL-12 can contribute to autoimmune diseases and chronic inflammatory conditions, making it both a potential therapeutic target and a therapeutic agent in certain clinical settings. Advances in IL-12-targeted therapies hold promise for treating a variety of diseases, including infections, autoimmune conditions, and cancer.