The CD19 antigen is a promising target for immunotherapy of acute lymphoblastic leukemia (ALL), but CD19-negative relapses remain a major challenge in about 10-20% of patients. Here, we analyzed four CD19-negative ALL relapses after treatment with the CD19/CD3 bispecific T-cell engager blinatumomab. Three were on-drug relapses, with the CD19-negative escape variant first detected after only two treatment courses. In one patient, the CD19-negative clone appeared as a late relapse 19 months after completion of blinatumomab treatment. All four cases showed a cellular phenotype identical to the primary diagnosis except for CD19-negativity. This argued strongly in favor of an isolated molecular event and against a common lymphoid CD19-negative progenitor cell or myeloid lineage shift driving resistance. A thorough molecular workup of one of the cases with early relapse confirmed this hypothesis by revealing a disrupted CD19 membrane export in the post endoplasmic reticulum compartment as molecular basis for blinatumomab resistance.
