Further studies on the mitogenic and immune-modulating effects of plasminogen activator.

Plasminogen activator (PA), an enzyme which is secreted in large quantities by certain types of tumour cells, is an apparent lymphocyte mitogen. Treatment of mouse spleen cells with anti-Thy-1.2 serum plus complement prevents responsiveness, indicating that T cells are being stimulated to divide. In addition to the foregoing, we have shown that PA has the capacity to interfere with certain types of cell-mediated cytotoxicity and natural killer cell reactions in which tumour cells are employed as targets. These results suggest that PA may play an immune-modulating role which may be of importance in tumour self-defense.