interleukin-1

interleukin-1 is a pivotal cytokine primarily involved in the regulation of immune and inflammatory responses. It is produced predominantly by activated macrophages, as well as other cell types including epithelial cells and dendritic cells. The interleukin-1 family encompasses several key members, with interleukin-1α and interleukin-1β being the most studied agonists, and the interleukin-1 receptor antagonist acting as a natural inhibitor. Its dysregulation is implicated in a wide spectrum of autoimmune diseases and chronic inflammatory conditions.

Structure and isoforms

The interleukin-1 family consists of multiple structurally related proteins encoded by distinct genes. The two primary agonist isoforms, interleukin-1α and interleukin-1β, are initially synthesized as inactive precursor molecules, or proproteins. Mature interleukin-1β is generated through proteolytic cleavage by the caspase-1 enzyme within a multi-protein complex known as the inflammasome. In contrast, interleukin-1α often remains membrane-associated and biologically active in its precursor form. Other important family members include interleukin-18 and interleukin-33, which also signal through related receptor systems. The natural antagonist, interleukin-1 receptor antagonist, competitively binds to the signaling receptor without activating it, providing a crucial regulatory mechanism.

Function and signaling pathway

Interleukin-1 functions as a master regulator of innate immunity and inflammation. Upon binding to its cognate type I interleukin-1 receptor, it recruits the interleukin-1 receptor accessory protein to form a high-affinity signaling complex at the cell membrane. This recruitment triggers the association of intracellular adaptor proteins such as MyD88, initiating a signaling cascade that ultimately activates key transcription factors, including nuclear factor-κB and activator protein 1. These factors translocate to the nucleus and drive the expression of numerous pro-inflammatory genes, leading to the production of other cytokines like interleukin-6 and tumor necrosis factor, cyclooxygenase-2, and adhesion molecules. This amplifies the immune response, induces fever through actions on the hypothalamus, and stimulates the production of acute-phase proteins by the liver.

Role in disease and inflammation

Dysregulated interleukin-1 activity is a central driver in many pathological states. It plays a critical role in the pathogenesis of classic autoinflammatory diseases such as familial Mediterranean fever and cryopyrin-associated periodic syndrome. Its involvement extends to common rheumatic diseases, including rheumatoid arthritis and gout, where it promotes synovitis and cartilage destruction. In metabolic disorders, interleukin-1β contributes to insulin resistance and the progression of type 2 diabetes mellitus. Furthermore, its actions are implicated in the inflammatory components of atherosclerosis, Alzheimer's disease, and certain cancers. Excessive production, often due to inflammasome hyperactivation, can lead to life-threatening conditions like macrophage activation syndrome.

Discovery and history

The discovery of interleukin-1 originated from research into endogenous pyrogens, substances that cause fever. In the 1940s and 1950s, work by Paul Beeson and others identified that leukocytes released a fever-inducing factor. The modern era of interleukin-1 research began in the 1970s, with pivotal contributions from Charles Dinarello, who purified and characterized the protein. Dinarello's group at the University of Colorado successfully cloned the cDNA for interleukin-1β in 1984, a milestone that opened the door to molecular studies. The identification of the interleukin-1 receptor antagonist in the late 1980s by William Arend and colleagues revealed a natural regulatory mechanism, further highlighting the cytokine's biological significance.

Therapeutic targeting

Given its central role in inflammation, interleukin-1 has become a major therapeutic target. The development of the recombinant interleukin-1 receptor antagonist, anakinra, provided the first specific blockade and is approved for conditions like rheumatoid arthritis and cryopyrin-associated periodic syndrome. Longer-acting biologic agents have since been developed, including the monoclonal antibody canakinumab, which neutralizes interleukin-1β, and rilonacept, a soluble decoy receptor fusion protein. Notably, the CANTOS trial, a large-scale clinical study, demonstrated that canakinumab significantly reduced cardiovascular events in patients with a history of myocardial infarction, directly linking interleukin-1β inhibition with improved atherosclerotic outcomes. Research continues into small molecule inhibitors targeting components of the interleukin-1 signaling pathway or the inflammasome.