Reduced tumor burden and fibrosis in PMF do not require effector functions of anti-GARP:TGF-β1 mAbs. (A) MPL^W508A mice were treated with wild-type or Fc-dead mAbs as indicated in Figure 2 (n = 8-10 mice per group). Blood was taken on day 21 and mice were euthanized on day 32 to collect femurs and spleens. Blood cell counts, GFP expression, and spleen fibrosis were measured as indicated in Figure 2. Data points: values in individual mice; horizontal lines: mean ± SEM per group; stacked bars: mean + SEM. P values were calculated using a two-tailed Student t test. (B) 32D cells were transduced with Mpl^WT-IRES-GFP or Mpl^W508A-IRES-GFP RV and GFP⁺ cells were sorted by flow cytometry. Parental 32D cells, sorted MPL^WT/GFP⁺, or MPL^W508A/GFP⁺ bulk cell populations, or clones (supplemental Figure 6) were cultured in complete medium, in the absence or presence of recombinant IL-3, TPO, or TGF-β1, as indicated in the graphical legend. Proliferation was measured in ³H-thymidine incorporation assays. Maximum growth inhibition (%) in the presence of rTGF-β1 is indicated in colored text on the right and was calculated as follows: (cpm w/o rTGF-β1 – cpm at the highest concentration of rTGF-β1) ÷ (cpm w/o rTGF-β1). Data points: mean cpm ± SD (triplicates). Results are representative of 2 independent experiments. cpm w/o rTGF-β1, counts per minute in the absence of rTGF-β1; SD, standard deviation.