Alterations of BM differentiation after 5-HT2BR inhibition. (A) In mouse BM cells, methyl cellulose cultures, RS-127445 reduced CFU-GEMM/CFU-GEM/BFU-E/CFU-E (n = 3 independent cultures). Statistical difference by unpaired t test versus control is indicated by ***P < .001, *P < .05 versus Vehicle. (B) Similarly, in c-kit⁺ BM cells, methyl cellulose cultures, RS-127445 reduced CFU-GEMM/CFU-GEM/BFU-E/CFU-E (n = 3 independent cultures). Statistical difference by unpaired t test versus control is indicated by ***P < .001, *P < .05 versus Vehicle. (C) On methyl cellulose cultures of human blood cord CD34⁺ cells, the 5-HT_2BR antagonist RS-127445 significantly reduced expansion of BFU-E/CFU-E, burst-forming unit/colony-forming unit-erythroid, but not the 5-HT_1BR antagonist SB-269970 (n = 4 independent cultures). Any statistical difference by unpaired t test versus control is indicated by ***P < .001 versus Vehicle, #P < .05 versus SB-269970. (D) After flow cytometric purification of BM cells, expression of 5-HT_2BRs was detected in c-kit⁺ (CD117 positive) cells (n = 8), but remained undetectable in c-kit⁻ cells (n = 5), c-kit⁺ cells after STI-571 (STI) treatment (n = 6) or c-kit⁺ cells from 5-HT_2B^−/− mice (n = 5). (E) On ex vivo expansion of total BM cells in the presence of GM-CSF driving the monocyte/macrophages and all granulocytes differentiation, the lack of 5-HT_2BRs significantly reduced the number of CD11b⁺/Gr⁺ cells (n = 3-5 mice of each genotype). Any statistical difference by unpaired t test versus control is indicated by ***P < .001 versus 5-HT_2B^+/+ culture.