MSC^E116K promotes growth of normal and neoplastic T cells. (A) MSC^wt inhibited cell growth in normal CD4⁺ T cells stimulated with anti-CD3/CD28. Growth inhibition was completely reversed by MSC^E116K with an additional trend toward increased growth over baseline control. (B) Western blot of musculin expression in cells from the experiment shown in (A). (C) MSC^wt inhibited cell cycle progression via G1 arrest; G1 arrest was reversed by MSC^E116K. (D) Coexpression of normal CD4⁺ T cells with MSC^wt and MSC^E116K rescued cells from the growth inhibition induced by MSC^wt alone. Cells were flow sorted for ZsGreen and mCherry double positivity prior to evaluation. Representative results from 3 independent experiments. The Student t test was used to compare results. (E) GSEA of genes differentially expressed in CD4⁺ T cells overexpressing MSC^wt vs control-transduced cells showed depletion of genes encoding MYC and E2F targets. (F) Overexpression of MSC^wt inhibited growth in MSC^low Karpas 299 cells, whereas overexpression of MSC^E116K promoted growth. Data show growth relative to vector control in a competitive growth assay; error bars reflect 3 independent experiments. (G) Small interfering RNA (siRNA) knockdown of MSC increased cell growth in MSC^high Mac-1 cells relative to cells treated with a control siRNA. Data reflect 3 independent experiments. The Student t test was used to compare results. (H) Western blot of musculin expression in cells from the experiment shown in (G). *P < .05, MSC^wt vs MSC^E116K.