MCL cells overexpress a new AXL isoform. (A) Schematic representation of AXL mRNA structure. The different sets of primers used for AXL mRNA amplification (reverse transcription polymerase chain reaction [RT-PCR]) are indicated in purple (specific amplification of isoform 1), green (amplification of isoform 1 and 2), and blue (specific amplification of isoform 3). The new AXL 3 isoform (bottom) is missing the first 4 coding exons of AXL isoforms 1 and 2. (B) Relative mRNA expression of the different AXL transcripts investigated by RT-PCR in MCL cell lines. All the different AXL mRNA isoforms can be detected in MCL cells. MIA-PaCa-2 and K562 were used as positive controls for AXL amplification. (C) Detection of AXL protein expression in MCL using an antibody targeting C-terminal domain of AXL. The C-terminal antibody is able to recognize all AXL isoforms. The AXL N-terminal antibody recognizes the isoform 1 and 2 but not the AXL3 isoform because the antibody epitope is not present in AXL3. K562 and MIA-PaCa-2 served as positive controls. (D) Detection of AXL protein expression in MCL using an antibody targeting the N-terminal. (E) AXL can be detected at the cell surface of MCL cells by flow cytometry staining using the AXL N-terminal antibody. K562 served as a positive control. Experiments were performed in triplicate. (F) AXL mRNA expression was investigated by RT-PCR in primary MCL cells. All the AXL isoforms were expressed in MCL primary cells. K562 and MIA-PaCa-2 served as positive controls for the AXL1 and AXL2. (G-H) AXL is expressed at the protein level in MCL cells. AXL is overexpressed in MCL cells in comparison to peripheral blood mononuclear cells (PBMCs) or CD19⁺ B cells. (I) AXL3 full transcript was cloned and sequenced (J-K) and corresponds to the predicted AXL3 isoform.