Caspase-3 is a central execution caspase contributing to and finalizing the process of induced cell death, also know as apoptosis. Caspase-3 is localized in the cytoplasm and proteolytically activated by the apoptotic initiator caspases, caspase-8 and caspase-9. Moreover, the protease granzyme B, which is secreted by cytotoxic CD8+ T-lymphocytes, can enter target cells in a perforin-dependent manner and activate caspase-3.
We could show previously that mast cells express granzyme B and store it in their secretory lysosomes. In our recent publication by Carolin Zorn et al. "Secretory lysosomes of mouse mast cells store and exocytose active caspase-3 in a strictly granzyme B dependent manner" (Eur. J. Immunol. 2013; 43: 3209-3218) Link: http://www.ncbi.nlm.nih.gov/pubmed/24414824 we demonstrate that viable mast cells, in a granzyme B dependent manner, express active caspase-3 in their secretory lysosomes. Thus, mast cells can secrete active caspase-3 in response to antigen/allergen contact. How extracellular active caspase-3 contributes to patho-physiological situations that involve mast cells and which extracellular proteins might serve a caspase-3 substrates will be exciting future questions.