For Caspase-3 activity assay, 50μl of host cell containing supernatant was transferred to a new tube and lysed with Cell Lysis Buffer according to the instructions in EnzChekTM Caspase-3 Assay Kit #1, Z-DEVD-AMC substrate (ThermoFisher Scientific). Protein concentration was measured with Qubit Protein Assay Kit (Life Technologies). 50μg of protein in volume of 50μl or lower was used to measure caspase 3 activity following the kit instructions. Fluorescence was measured with SpectraMax2 (Molecular devices) using SoftMax Pro v4.8 software.