BCP-ALL blasts are not dependent on CD19 expression for leukaemic maintenance

Introduction: The B-lymphocyte surface marker CD19 is the focus of attention for the development of immunotherapies due to its high expression on leukemic blasts and B-lineage specificity. The monoclonal antibody blinatumomab and chimeric antigen receptor modified T-cells targeting CD19 have shown early promise in clinical trials in the treatment of B cell precursor acute lymphoblastic leukaemia (BCP-ALL). However, the precise role of CD19 in leukaemic maintenance is unclear. Results: To investigate this we silenced CD19 expression using RNA interference in BCP-ALL cell lines and BCP-ALL primograft samples. Assays in suspension culture and on feeder cells did not demonstrate any proliferative disadvantage of CD19 - leukemic cells, compared to CD19 + blasts. In support of this, we obtained a patient sample which relapsed with CD19 - BCP-ALL following Blinatumomab treatment and was able to engraft NOD/LtSz-scid IL-2Rγ null (NSG) with CD19 negative disease. Notably, we present data showing that the loss of CD19 in this patient does not involve exon skipping, as reported in a recent study, demonstrating the existence of a novel mechanism of CD19 depletion. Conclusion: Our results suggest that ALL blasts do not require CD19 for survival, which increase the likelihood of CD19 negative populations emerging in response to anti-CD19 therapies.