Plasminogen activation by normal B lymphocytes, a function associated with the cell membrane

When purified murine plasminogen was added to cultures of mouse spleen B cells, active plasmin progressively appeared in the supernatants. This reaction, resulting from the specific cleavage of the plasminogen by lymphocyte plasminogen activator (LPA), was measured in a fibrinolysis assay using 125I-fibrinogen. T cells were totally ineffective; under certain conditions, they could even antagonize the B cell action. Of the various B populations studied, i.e., obtained from spleen, lymph nodes, or blood of various mouse strains, all expressed the same property of plasminogen activation, which concerned mainly medium-sized B cells. Since only slight activities were detected in extracellular or intracellular compartments, a membrane-associated proteolytic enzyme may be responsible for plasminogen activation. Submitted to a series of group-specific antiproteases, the lymphocyte plasminogen activator essentially behaved as a serine-protease, with sensitivity to diisopropyl fluorophosphate, phenyl methyl sulfonyl fluoride, and nitro phenyl guanidino benzoate. The fast renewal of the enzyme in the membrane was also demonstrated by different techniques, using modifiers of cell physiology, like cycloheximide and dexamethasone, or following the reexpression of the enzyme by the cell kinetically.