Engraftment of cells from patients with myelofibrosis, but not healthy donors, results in organoid “niche remodeling” and fibrosis. A, Cryopreserved peripheral blood or bone marrow cells from healthy donors and patients with blood cancers were fluorescently labeled, and 5,000 donor cells were seeded into each well of a 96-well plate containing individual organoids. Schematic created using BioRender.com. B, Maximum-intensity projection of confocal Z-stack of a whole engrafted organoid 72 hours after seeding of the wells with donor cells, indicating donor cells engrafted throughout the volume of the organoids. C, Soluble TGFβ in organoids engrafted by cells from patients with myelofibrosis and controls. D–G, Comparison of organoids engrafted with healthy donor and myelofibrosis cells for collagen 1 (COL1A1) and αSMA immunofluorescence (D) and representative images (E), Col1A1 and ACTA2 gene expression (F), and CDH5 and TIE2 expression (G).n = 4 healthy donors; n = 7 myelofibrosis samples for qRT-PCR; n = 4 healthy donors; n = 6 myelofibrosis samples for imaging and quantification of cryosections. MFI, mean fluorescence intensity. H and I, Increased reticulin deposition with a concomitant reduction in vascular area in organoids engrafted with myelofibrosis (MF) cells versus nonengrafted control organoids with paired t tests; each datapoint corresponds to a single organoid engrafted with cells from 3 donors. J, Variant allele frequencies (VAF) of mutations detected by next-generation sequencing of cells from patients with myelofibrosisbefore seeding in organoids (day 0) compared with cells isolated by flow cytometry 12 days after culture in organoids, indicating maintenance of clonal architecture. K, Workflow for organoid generation, engraftment with cells from patients with myelofibrosis, and treatment with inhibitors. L, αSMA and collagen 1 expression in nonengrafted organoids, and organoids engrafted with myelofibrosis cells treated with DMSO (control), SB431542, JQ1, and ruxolitinib. Each data point corresponds to total measurements per organoid within a block (n = 3 donors). One-way ANOVA with multiple comparisons (Fisher least significant difference). M, Representative images from L. Rux, ruxolitinib. *, P < 0.01; **, P < 0.05; ***, P < 0.001 for Mann–Whitney test. See also Supplementary Fig. S8.