Proteolytic activity in the culture supernatants of mouse hybridoma cells

When serum-reduced or serum-free culture supernatants were incubated for 16 hours at 37 degrees C, more than 70-80% of original proteins were digested as measured by gel electrophoretic analysis. The proteolytic activity, which was only observed at pH values lower than 4.5, was reduced in conditioned medium containing higher concentrations of fetal calf serum. During incubation large amounts of the monoclonal antibody (IgG1) were cleaved giving F(ab')2 fragments. The results reported here strongly suggest that the conditioned medium of mouse hybridoma cultures contain probably two cellular (acid) proteases with apparent MW of 45-50 K and 90-95 K, respectively. The similarities with the lysosomal cathepsin D are discussed.