Cytokine Response in Mice After Turpentine Injection

Overview of the IL-1 Family

• The interleukin-1 (IL-1) family of cytokines and receptors plays a unique role in immunology, sharing similar functions with the Toll-like receptor (TLR) families.
• The family is primarily associated with innate immunity, which is used by over 95% of living organisms for survival.
• Innate immunity relies on inflammation as a host defense mechanism, but unchecked inflammation is detrimental.
• Each member of the IL-1 receptor and TLR families contains a cytoplasmic Toll/IL-1-Receptor (TIR) domain, highly homologous to the Toll protein in Drosophila.
• IL-1 family cytokines trigger innate inflammation via IL-1 receptors, while TLRs respond to bacteria, microbial products, viruses, nucleic acids, and damage-associated molecular patterns (DAMPs). Note IL-1a and IL-33 also function as DAMPs.
• While primarily associated with innate immunity, IL-1 family members can also play a role in acquired immunity.

The IL-1 Family of Cytokines and the Innate Immune System

• There are 11 members of the IL-1 family of cytokines and 10 members of the IL-1 family of receptors.
• These members are closely linked to inflammation, but also increase nonspecific resistance to infection and immune development.

Organization of the IL-1 Family of Cytokines and Receptors

• The eleven members of the IL-1 family are divided into three subfamilies based on their consensus sequence and primary ligand binding receptor.
• Except for IL-1Ra, members lack a signal peptide and are not readily secreted.
• They exist as precursors in the cytoplasm, containing a conserved consensus sequence (A-X-D), where A is an aliphatic amino acid, X is any amino acid, and D is aspartic acid.

IL-1a

• IL-1α is a "dual-function" cytokine.
• It has a nuclear localization sequence and functions as a transcription factor in the nucleus.
• In the nucleus, it triggers gene expression, for example, that of the chemokine IL-8.
• It has a pro-inflammatory role when released from necrotic cells. This is when it acts as a DAMP.

Constitutive IL-1a

• IL-1α is constitutively present in epithelial and mesenchymal cells in healthy subjects, unlike IL-1β, which is primarily induced under disease states.
• IL-1α is present in vesicles (apoptotic bodies) contained within endothelial cells; it acts as an alarmin.
• In cell death via necrosis, IL-1α triggers chemokine production which results in neutrophil infiltration followed by monocytes.

Processing of IL-1a

• IL-1α processing from cultured cells reveals an N-terminus at serine 113, initially considered unusual for the consensus sequence.
• The processing of human IL-1α from LPS-stimulated macrophages is calcium-dependent and neutral protease-mediated, blocked by calpain inhibitors.
• Copper is also involved in the processing via S100A13.

Membrane IL-1a

• IL-1a precursor is found on the surface of several cells, particularly monocytes and B lymphocytes.
• Membrane IL-1a is biologically active and crucial in inflammation.

IL-1β

• IL-1β has a long history of investigation; first purified in 1977, it was subsequently cloned in 1984.
• Clinical trials using antibodies to neutralize IL-1β have underscored its role in atherosclerosis and cancer progression.

Transcription, Translation, and Synthesis of IL-1β

• Initially, IL-1β is synthesized as an inactive precursor, processed later by NLRP3 inflammasome and caspase-1.
• TLR ligands induce IL-1β production.
• Following LPS stimulation in human monocytes, IL-1β mRNA rises rapidly and peaks around 4 hours, declining thereafter due to mRNA half-life.

NLRP3 Inflammasome-Dependent Processing and Secretion of IL-1β

• NLRP3 inflammasome, a complex of intracellular proteins, is crucial for the processing and secretion of mature IL-1β.
• Caspase-1 activation is the rate-limiting step within the inflammasome.

IL-1 Receptor Antagonist (IL-1Ra)

• IL-1Ra acts as an inhibitor, specifically blocking the IL-1R1, and thus the effects of IL-1α and IL-1β.
• It is used therapeutically for various inflammatory diseases (arthritis, gout, pericarditis to rare diseases).

Anakinra and Autoinflammation

• Anakinra (recombinant IL-1Ra) has been proven effective for treatment of diverse autoinflammatory conditions, notably familial Mediterranean fever (FMF) and cryopyrin-associated periodic syndromes (CAPS).
• Anakinra is effective in treating acute myocardial infarctions; early clinical trials demonstrated a role in reducing inflammatory markers and thus improving cardiovascular outcomes.

IL-33

• IL-33 is a cytokine that belongs to the IL-1 subfamily, but it can also act as an alarmin and exerts dual function as a transcription factor.
• It has been identified as a ligand for IL-1 receptor-related protein (ST2), playing a role in inflammation.
• Studies in mice with deleted nuclear localization sequences reveal the importance of this location for reduced inflammation.

IL-18

• IL-18 was initially termed 'IFN-γ-inducing factor' because of its role in initiating IFN-γ production.
• Similar to IL-1β, IL-18 begins as a pre-cursor protein that gets processed into an active form through caspase-1 activation.
• There are differences in how IL-18 and IL-1β activate signaling; IL-18 requires a higher concentration for cell activation.

IL-18 Binding Protein

• IL-18BP is a soluble decoy receptor that binds IL-18, thereby mitigating its effects.
• Higher levels of IL-18BP relative to IL-18 have been observed in several inflammatory conditions.

IL-36 Subfamily

• The IL-36 subfamily encompasses five ligands (IL-36α, IL-36β, IL-36γ, and IL-36Ra) with a common receptor (IL-1R6), which is expressed mainly in the skin.
• All family members exhibit varied involvement in inflammatory and autoimmune conditions, with IL-36a being implicated in psoriasis and hidradenitis suppurativa (HS).

IL-38

• IL-38 is a member of the IL-1 family that acts as an anti-inflammatory cytokine.
• It has been involved in mediating the inflammatory pathology in several conditions that are also influenced by the other IL-1 family members. For example, it has been studied in SLE, Crohn's disease, and other conditions.

This quiz explores the immune response in wildtype and genetically modified mice following subcutaneous turpentine injection. It focuses on the roles of IL-1β and IL-1α in inflammation and their mechanisms of action, as well as the effects of cytokine deficiencies. Test your knowledge on cytokine biology and sterile inflammation mechanisms.