Autoimmune inner ear disease patient–associated 28-kDa proinflammatory IL-1β fragment results from caspase-7–mediated cleavage in vitro

Interleukin-1beta (IL-1beta) is a key proinflammatory cytokine involved in the progression of many autoinflammatory and autoimmune diseases, including autoimmune inner ear disease (AIED). IL-1beta inhibition has been shown to result in clinical hearing improvement in a small cohort of corticosteroid-resistant patients with AIED. Canonical processing of pro-IL-1beta by caspase-1 generates an active 17-kDa fragment, capable of instigating a proinflammatory microenvironment. However, in response to LPS, PBMCs from patients with AIED uniquely express a 28-kDa IL-1beta fragment, as compared with PBMCs from control subjects. We synthesized and compared the biologic activity of the 28-kDa fragment to the 17-kDa IL-1beta product and the pro-IL-1 31-kDa protein. The 28-kDa IL-1beta fragment induces IL-6, TNF-alpha, and CCL3 in PBMCs. Uniquely, only caspase-7 treatment showed a dose- and time-dependent increase in 28-kDa band generation. Mass spectrometry confirmed the putative caspase-7 cleavage site of pro-IL-1beta, which was used to generate the 28-kDa fragment used for PBMC stimulation studies. Collectively, these results provide insight into the function of a poorly understood, processed 28-kDa form of IL-1beta in patients with AIED that is uniquely generated by caspase-7 and is capable of activating further downstream proinflammatory cytokines. Further investigation may provide novel pharmacologic targets for the treatment of this rare disease.