A trypsin-like serine protease activity on activated human B cells and various B cell lines

We have studied the trypsin-like serine protease activity of human tonsillar B lymphocytes. The lysate of the low-density, in vivo activated B cells as well as the lysate of cells stimulated with anti-human IgM F(ab')2 show elevated trypsin-like serine protease activity compared to the resting subset as monitored by the cleavage of Tos-Gly-Pro-Arg-pNA. The cleavage is sensitive to N-tosyl-L-lysyl-chloromethyl ketone and benzamidine but not to iodoacetamide. Experiments with intact cells give similar results. The finding that the intact cells hydrolyze the substrate, while their supernatant does not, suggests that the protease activity is cell membrane associated. It is possible that C3 is a substrate of the enzyme since the activated B cells cleave C3, whereas the resting B cells do not, and also C3 inhibits the enzyme-substrate reaction. In addition to the ex vivo B cells, we studied the serine protease activity of certain well-characterized B cell lines. The results show a correlation between the phenotype and the enzyme expression of the cell lines. BL41, an Epstein-Barr virus (EBV)-negative Burkitt lymphoma line, with a resting phenotype, has low activity, while its EBV genome-carrying convertants E95-A-BL41, E95-C-BL41, EHR-A-BL41 and BL41/95 that have the phenotype of activated B cells, have high proteolytic activity. The lymphoblastoid cell line WW-1-LCL which has the phenotype of an immunoblast, has the highest serine protease activity. On the basis of the above data, we suggest that a rather tight correlation exists between the degree of activation and the appearance of serine protease(s) on the surface of human B cells.