GDF15 expands MM TICs. (A) Relative cell numbers after treatment of unfractionated, CD138^neg, and CD138⁺ NCI-H929 cells with GDF15 for 7 days. (B) Absolute number of CD138^neg NCI-H929 cells quantified by flow cytometry after treatment with GDF15. (C) Relative expansion of prospectively isolated CD138^neg NCI-H929 cells by GDF15. (D) Relative clonogenic recovery of CD138^neg NCI-H929 cells after treatment with GDF15 for 7 days. (E) Relative clonogenic recovery of CD138⁺ NCI-H929 cells after treatment with GDF15 for 7 days. (F) Absolute number of ALDH⁺ NCI-H929 cells quantified by flow cytometry after treatment with GDF15. *P < .05 compared with 0 ng/mL GDF15.