[CD19 antigen loss after treatment of Bispecific T-cell Engager and effective response to salvage bispecific CAR-T therapy in B cell acute lymphoblastic leukemia: a case report and literature review].

To analyze the influence of CD19 isoforms to the efficacy of CD19/CD3 Bispecific T-cell Engager (BiTE) antibody, and explore the resistance mechanism of BiTE immunotherapy. Semi-quantitative RT-PCR (qRT-PCR) was used to detect the expression of CD19 mRNA isoforms before and after BiTE treatment in a patient with CD19(+) B cell acute lymphoblastic leukemia (ALL) . CD19 isoforms were analyzed by Sanger sequencing. Flow cytometry and transcriptome sequencing were performed to analyze the expression of cell lineage specific molecules before and after BiTE treatment. The expression of CD19 isoform with exon 2 deletion was identified at diagnosis. After relapsed and treatment of BiTE antibody, the patient did not achieve remission and CD19 antigen on leukemic cells turned negative detected by flow cytometry after BiTE treatment. However the expression ratio of CD19 isoform with exon 2 deletion was not increased. Flow cytometry phenotype and transcriptome sequencing confirmed that no linage switching developed, which suggested the expression of CD19 isoform caused by exon alternative splicing and lineage switching was not related to CD19 epitope loss in this patient. This patient achieved complete remission by sequential administration of self-developed CD22 CAR-T and CD19 CAR-T after disease progression. Targeting or combining an alternative antigen specific CAR-T may be a promising treatment option after losing CD19 expression in relapsed ALL.