The constitutive proteasome and its inflammation-driven derivative, the immunoproteasome (IP), perform important intracellular proteolytic functions. However, the IP is increasingly recognised as being present in the extracellular space during pathology, though its mechanisms of release and functions are unknown. We hypothesised that extracellular IP is a feature of, and plays a role in, the acute respiratory distress syndrome (ARDS).

We show that the levels and activity of IP are elevated in bronchoalveolar lavage fluid from patients with ARDS, the human healthy volunteer LPS model and the murine inhaled LPS model. In a series of in vitro experiments, we demonstrate that IP is released constitutively from macrophages, though this release can be exacerbated by activation of the NLRP3 and AIM2 inflammasomes, and that IP release can be abrogated by pharmacological or genetic targeting of the inflammasome pathway. We have found that direct instillation of IP alone into the lungs of mice does not induce inflammation. However, when administered in combination with bacterial endotoxin, IP appears to alter the inflammatory cell population within the lung. We plan to further characterise these changes in order to understand the role of extracellular IP in ARDS. Closely linked to this, since the extracellular substrates of IP are not known, identifying such targets represents a key aspect of our future work.

In conclusion, extracellular IP is a feature in human ARDS and murine ARDS-like disease. We have identified a potential mechanism of release of IP, which is closely linked to inflammasome activation, and are exploring the relevance of extracellular IP in the acutely inflamed lung.