A novel recombinant anti-CD22 immunokinase delivers proapoptotic activity of death-associated protein kinase (DAPK) and mediates cytotoxicity in neoplastic B cells

The serine/threonine death-associated protein kinases (DAPK) provide pro-death signals in response to (oncogenic) cellular stresses. Lost DAPK expression due to (epi)genetic silencing is found in a broad spectrum of cancers.Within B-cell lymphomas, deficiency of the prototypic family member DAPK1 represents a predisposing or early tumorigenic lesion and high-frequency promotermethylation marksmore aggressive diseases. On the basis of protein studies and meta-analyzed gene expression profiling data, we show here that within the low-level context of B-lymphocytic DAPK, particularly CLL cells have lost DAPK1 expression. To target this potential vulnerability, we conceptualized B-cell-specific cytotoxic reconstitution of the DAPK1 tumor suppressor in the format of an immunokinase. After rounds of selections for its most potent cytolytic moiety and optimal ligand part, a DK1KD-SGIII fusion protein containing a constitutiveDAPK1mutant,DK1KD, linked to the scFv SGIII against the B-cell-exclusive endocytic glyco-receptor CD22 was created. Its high purity and large-scale recombinant production provided a stable, selectively binding, and efficiently internalizing construct with preserved robust catalytic activity. DK1KD-SGIII specifically and efficiently killed CD22-positive cells of lymphoma lines and primary CLL samples, sparing healthy donor- or CLL patient-derived non-B cells. The mode of cell death was predominantly PARP-mediated and caspase-dependent conventional apoptosis as well as triggering of an autophagic program. The notoriously high apoptotic threshold of CLL could be overcome by DK1KD-SGIII in vitro also in cases with poor prognostic features, such as therapy resistance. Themanufacturing feasibility of the novel CD22-targetingDAPK immunokinase and its selective antileukemic efficiency encourage intensified studies towards specific clinical application.