CD34⁺ cells secreting only CXCL8 are enriched in a subset of patients with MF, and this correlates with clinical features, including grade reticulin fibrosis. (A) Uniform manifold approximation and projection visualization of individual HSCs colored based on the patient (see supplemental Table 1). (B, top) GSEA of differentially expressed genes (DEGs) based on clustering of patient HSCs; (B, bottom) most DEGs and their pathway associations (percent expressed: percentage of cells expressing listed gene; average expression scale: Z score of normalized read counts, with blue, positive values and gray, negative values). UPR, unfolded protein response. (C) Heatmap demonstrating frequency of individual cytokine-secreting CD34⁺ cells detected across MPN subtypes MF, PV, and ET among individual patients as a percentage of total cytokine-secreting cells (from 0% in gray to 100% in dark blue). Four cytokines presented: IL-6, MIP-1β, TNFα, and CXCL8. (D) Violin plot depicting the correlation between MPN subtype and percent fraction of cells secreting only CXCL8, as detected via single-cell cytokine analysis. (E) Ratio of total cell output relative to untreated cultured healthy donor (HD) (light blue) vs MF (dark blue) CD34⁺ cells in response to exogenous CXCL8 (50 or 100 ng). Representative of triplicate experiments from N = 3 HD and N = 6 MF samples. Data shown represent mean ± standard deviation (SD. (F) Percent of total CD34⁺ cells expressing CXCR1 (left) or CXCR2 (right) via flow cytometry of HD (control [CTRL]; N = 13) vs patients with MF (N = 15). Data shown represent mean ± SD. ∗P < .05; ∗∗∗P < .001. NES, normalized enrichment score.