The IL-20 receptor axis in immune-mediated inflammatory arthritis: novel links between innate immune recognition and bone homeostasis

Abstract

The treatment of rheumatoid arthritis (RA) and spondyloarthritis (SpA) was transformed a little over a decade ago by the introduction of agents neutralizing the pro-inflammatory cytokine tumour necrosis factor (TNF)-α. Nevertheless, some patients do not achieve remission and the inhibition of the normal immune system with current drugs increases the risk of infection.

The interleukin (IL)-20 receptor (IL-20R) axis is pivotal for tissue homeostasis. By contrast, this axis does not seem to directly activate cells of the immune system. Thus, modulation of the IL-20R axis might not result in increased risk of infection. The IL-20R axis consists of the three cytokines IL-19, IL-20, and IL-24 (termed the IL-20R cytokines) and their shared receptors. All three cytokines bind the receptor complex of IL-20R2/IL-20R1 whereas only IL-20 and IL-24 also bind the receptor complex of IL-20R2/IL-22R1.

This short review describes how the IL-20R axis could be a novel link between innate immune recognition and bone homeostasis. The IL-20R cytokines are produced in response to both danger-associated molecular patterns and immune complexes formed by RA-associated autoantibodies. This could be of importance because these mediators can thus be present even in situations without inflammation.

IL-19 shows anti-inflammatory properties in arthritis through IL-20R1. IL-20 and IL-24 through IL-22R seem to participate in the recruitment of mononuclear cells to the synovial joint and to sites of bone erosion in particular. Our results indicate that dual inhibition of IL-20 and IL-24 or attenuation of the shared IL-22R subunit could have a beneficial effect on radiographic progression, especially in seropositive RA.

Acknowledgements

This work was supported by the Danish Rheumatism Association, the Ministry of Higher Education and Science’s EliteForsk award, and the Aage Bang Foundation.

I wish to thak my graduate supervisor Professor Bent Deleuran. Also, a special thanks to Suzy Chang, Division of Immunology, Stanford University, to change my primitive drawings and scribbles into artful illustrations.

Additional information

Funding

This work was supported by the The Danish Rheumatism Association; Ministry of Higher Education and Science’s EliteForsk award and The Aage Bang Foundation.