IL-1 is a potent pro-inflammatory cytokine that was first discovered as a fever- inducing molecule in 1974. It is produced during innate immune responses to infections and injuries. IL-1 drives inflammation by recruiting and activating immune cells, and inducing the production of various inflammatory mediators, including itself in a self-amplifying autoinflammatory loop.1-4
There are two different IL-1 proteins, IL-1α and IL-1β: IL-1α is an alarmin that is found in many different cell types and is released during cell death, ‘alarming’ the immune system to the presence of danger and triggering local (sterile) inflammation; by contrast, IL-1β is mainly produced by activated innate immune cells such as monocytes, macrophages, and neutrophils. IL-1α and IL-1β bind to the same receptor, IL-1R1, and induce the same pro-inflammatory effects.1-4
While IL-1 plays an important role in protective immune responses, abnormal or inadequately regulated IL-1 signaling may lead to excessive inflammation and tissue damage. Given the potent pro-inflammatory effects of IL-1, physiological negative regulators exist, for example the naturally-occurring IL-1 receptor antagonist that blocks IL-1 from binding to IL-1R1. However, if too much IL-1 is produced and/or regulating mechanisms are insufficient, IL-1-driven inflammation may have harmful consequences and contribute to the pathogenesis of inflammatory conditions.1-4
Typical signs and symptoms of IL-1-mediated inflammation include fever, eleva- tion of acute phase reactants (e.g. C-reactive protein, serum amyloid A), neutrop- hilia, and tissue inflammation commonly affecting the skin and musculoskeletal system, but various other organs may also be affected.1
The effects of unopposed, excessive IL-1 signaling are exemplified by the syste- mic, potentially life-threatening, sterile inflammation seen in patients with defi- ciency of IL-1 receptor antagonist (DIRA), who present with neutrophilia, elevated inflammatory markers, and inflammation of the skin, joints, and bones.1-4
In patients with the monogenic periodic fever syndromes cryopyrin-associated periodic syndromes (CAPS) and familial Mediterranean fever (FMF), abnormal, increased production of IL-1 due gain-of-function mutations in the NLRP3 and pyrin inflammasomes lead to recurrent fever episodes, elevation of acute phase reactants, and various inflammatory manifestations involving e.g. the joints (arthritis/arthralgia), skin (rash), eyes (conjunctivitis), serosal membranes (perito- neum, pleura, leading to abdominal and chest pain), and other organs.1-3
IL-1-driven inflammation also plays an important role in Still’s disease, which is characterized by spiking fever, rash, and joint involvement (arthralgia and/or arthritis), as well as neutrophilia and elevated CRP and ferritin.1-3
In patients with rheumatoid arthritis, IL-1 contributes to joint inflammation and tissue damage by stimulating the production of pro-inflammatory mediators and cartilage-degrading enzymes, and activating bone-resorbing osteoclasts.2,3
IL-1 has also been implicated in the hyperinflammatory response observed in se- vere cases of COVID-19. Blocking IL-1 activity has been shown to mitigate the risk of developing severe respiratory failure in COVID-19 patients hospitalised with pneumonia and plasma suPAR ≥6 ng/ml.4-7
Abbreviations
IL-1, interleukin-1; IL-1R1, Interleukin-1 receptor type 1; CRP, C-reactive protein; NLRP3, NOD-like receptor family pyrin domain-containing 3.