P aeruginosa's LPS is sufficient and necessary to induce the BM alterations observed during sepsis. Cohorts of CD1 and C57BL/6J mice were injected intraperitoneally with P aeruginosa's LPS (0.8 mg/kg) or phosphate-buffered saline (PBS) and compared with mice challenged with PA14. Mice were killed at the indicated time points, and their BM cells were analyzed. (A) BM cells harvested at 24 hours from LPS or PBS injection, or PA14 challenge, were labeled with antibodies anti-Gr1 and anti-Mac1. Bar graph shows percentage of Gr1⁺/Mac1⁺ cells in total BM. Values are average of 6 mice in 3 independent experiments. Error bars indicate SD. *P < .001. (B) BM smears from PBS- or LPS-challenged mice. Samples were stained by hematoxylin/eosin, and morphology was evaluated by light microscopy (×100 magnification). Circled area indicates the presence of neutrophils in PBS condition and of immature cells in LPS (arrows). (C) Bar graph indicates percentages of LSK cells in the S+G₂M phase of the cell cycle, as determined by Hoechst staining. Values indicate average of 2 mice at 24 hours from challenge in a representative experiment. Error bars indicate SD. (D) Each sample of sorted LSK was derived from a pool of BM of 4 to 6 mice. RNA was extracted from LSK (IL-7R⁻) cells from PBS- and LPS-challenged mice at 24 hours. Samples were analyzed for expression of SKP2 by qRT-PCR. Bar graphs represent averages of fold changes in expression in 3 independent samples from 2 independent experiments. Error bars indicate SD: LPS versus PBS, P = .04. (E) BM cells were analyzed for colony-forming ability by methylcellulose colony assay. Bar graph shows average number of myeloid colonies (CFU-GM + CFU-G + CFU-M) per 10 000 cells in 3 independent samples, each of them in quadruplicate. *P < .001.