GZMB and mastocytoma: To determine whether these changes in substrate preference translate into a variable capacity to kill target cells, we exposed P815 (mouse mastocytoma), EL-4 (mouse thymoma), HeLa (human cervical cancer) and Jurkat (human T lymphoma) cells to graded doses of each GzmB form and very low (‘sublytic’) quantities of purified recombinant Pfp (Fig. 3).