U343 cocultures induce Mer expression, Galectin-3 secretion, and survival signaling. (A) MSCs and U343 cells were cultured for the indicated time points in full serum conditions, and Gas6 was measured in culture supernatants by ELISA. (B) Mer ligand mRNA as indicated was measured in U343 as compared with MSC cells by qRT-PCR. (C) Galectin-3 protein levels were measured in cell culture supernatants of the indicated monocultures and cocultures after 72 hours. (D) Leukemic cell lines were cultured alone (−) or with U343 for 48 hours (+), then MACS sorted, and Mer expression was measured by western blotting in TCLs and IPs enriched for Mer. GAPDH was used as a loading control. (E) Leukemic cell lines were cultured alone (−) or with MSCs (+) for 48 hours. Surface staining of Mer was performed and measured via FACS (gated on leukemic cells). (F) Leukemic cell lines were cultured alone (−) or with U343 for 48 hours (+), then MACS sorted by positive selection for CD19 or CD45 and lysed. The indicated (phosphorylated) proteins were measured by western blotting. GAPDH was used as a loading control. *P < .05; **P < .01; ***P < .001. IP, immunoprecipitate; qRT-PCR, quantitative reverse transcription PCR; TCL, total cell lysate.