THE SERINE-PROTEASE GRANZYME-A DOES NOT INDUCE PLATELET-AGGREGATION BUT INHIBITS RESPONSES TRIGGERED BY THROMBIN

Granzyme A is a serine protease stored in cytoplasmic granules of cyto toxic and helper T lymphocytes. This protease seems to elicit thrombin receptor-mediated responses in neural cells, thereby triggering neuri te retraction and reversal of astrocyte stellation. Here we report tha t granzyme A does not cause platelet aggregation even at concentration s that are more than two orders of magnitude higher than the EC(50) fo r granzyme A in causing morphological changes in neural cells, However , granzyme A blocks thrombin-induced platelet aggregation in a dose-de pendent manner without affecting the response to either ADP or to the peptide agonist of the thrombin receptor SFLLRN that corresponds in se quence to the tethered ligand domain. The inability of granzyme A to c ause aggregation and its inhibition of thrombin-induced aggregation we re seen in platelets from man, rat and mouse. Granzyme A does not affe ct the catalytic activity of thrombin in cleaving a chromogenic substr ate or the macromolecular substrate fibrinogen. However, granzyme A do es seem to cleave the thrombin receptor on platelets to produce a weak Ca2+ signal and reduce the response to subsequent challenge with thro mbin, but does not induce a signal in thrombin-stimulated platelets. I t is proposed that granzyme A interacts with the thrombin receptor fou nd on platelets in a manner that is insufficient to cause aggregation, but sufficient to compete with thrombin for the receptor. These resul ts suggest that granzyme A cleaves the thrombin receptor at a rate tha t is insufficient to cause platelet aggregation but is sufficient to c ause morphological changes in neural cells. Furthermore, these observa tions demonstrate that granzyme A release occurring during immune resp onses within blood vessels would not directly cause platelet aggregati on.