Compositions and methods are provided for the cell-mediated targeted killing of diseased cells based on the presence of an intracellular antigen, rather than a surface-bound marker. The targeting cells are modified to express a cytotoxic protein that is delivered into a targeted cell, and after delivery is selectively activated by the presence of a cytoplasmic protein of interest. In one embodiment of the invention, the cytotoxic molecule is a Granzyme B (GrB) polypeptide. In the compositions of the invention, GrB is modified to render its cytotoxic enzymatic functions inactive, until the presence of an intracellular antigen unlocks the GrB molecule to enable enzymatic activities.
