Extracellular cathepsin S and intracellular caspase 1 activation are surrogate biomarkers of particulate-induced lysosomal disruption in macrophages

BACKGROUND: Particulate matter has been shown to stimulate the innate immune system and induce acute inflammation. Therefore, while nanotechnology has the potential to provide therapeutic formulations with improved efficacy, there are concerns such pharmaceutical preparations could induce unwanted inflammatory side effects. Accordingly, we aim to examine the utility of using the proteolytic activity signatures of cysteine proteases, caspase 1 and cathepsin S (CTSS), as biomarkers to assess particulate-induced inflammation.

METHODS: Primary peritoneal macrophages and bone marrow-derived macrophages from C57BL/6 mice and ctss(-/-) mice were exposed to micro- and nanoparticulates and also the lysosomotropic agent, L-leucyl-L-leucine methyl ester (LLOME). ELISA and immunoblot analyses were used to measure the IL-1β response in cells, generated by lysosomal rupture. Affinity-binding probes (ABPs), which irreversibly bind to the active site thiol of cysteine proteases, were then used to detect active caspase 1 and CTSS following lysosomal rupture. Reporter substrates were also used to quantify the proteolytic activity of these enzymes, as measured by substrate turnover.

RESULTS: We demonstrate that exposure to silica, alum and polystyrene particulates induces IL-1β release from macrophages, through lysosomal destabilization. IL-1β secretion positively correlated with an increase in the proteolytic activity signatures of intracellular caspase 1 and extracellular CTSS, which were detected using ABPs and reporter substrates. Interestingly IL-1β release was significantly reduced in primary macrophages from ctss(-/-) mice.

CONCLUSIONS: This study supports the emerging significance of CTSS as a regulator of the innate immune response, highlighting its role in regulating IL-1β release. Crucially, the results demonstrate the utility of intracellular caspase 1 and extracellular CTSS proteolytic activities as surrogate biomarkers of lysosomal rupture and acute inflammation. In the future, activity-based detection of these enzymes may prove useful for the real-time assessment of particle-induced inflammation and toxicity assessment during the development of nanotherapeutics.

Functional characterization of SjB10, an intracellular serpin from Schistosoma japonicum

Serine protease inhibitors (serpin) play essential roles in many organisms. Mammalian serpins regulate the blood coagulation, fibrinolysis, inflammation and complement activation pathways. In parasitic helminths, serpins are less well characterized, but may also be involved in evasion of the host immune response. In this study, a Schistosoma japonicum serpin (SjB10), containing a 1212 bp open reading frame (ORF), was cloned, expressed and functionally characterized. Sequence analysis, comparative modelling and structural-based alignment revealed that SjB10 contains the essential structural motifs and consensus secondary structures of inhibitory serpins. Transcriptional profiling demonstrated that SjB10 is expressed in adult males, schistosomula and eggs but particularly in the cercariae, suggesting a possible role in cercarial penetration of mammalian host skin. Recombinant SjB10 (rSjB10) inhibited pancreatic elastase (PE) in a dose-dependent manner. rSjB10 was recognized strongly by experimentally infected rat sera indicating that native SjB10 is released into host tissue and induces an immune response. By immunochemistry, SjB10 localized in the S. japonicum adult foregut and extra-embryonic layer of the egg. This study provides a comprehensive demonstration of sequence and structural-based analysis of a functional S. japonicum serpin. Furthermore, our findings suggest that SjB10 may be associated with important functional roles in S. japonicum particularly in host-parasite interactions.