Imipenem-cilastatin treatment, compared to aztreonam treatment, suppresses anaerobic commensals and elevates GVHD severity in mice. (A) Intestinal microbiota composition analysis using 16S rRNA sequencing prior to and after treatment with the indicated antibiotics in healthy C57BL/6 mice. Mice were treated with subcutaneous (SC) injections of each antibiotic twice a day for two days (100 mg/kg) and stool specimens were collected the following day. Values represent mean ± SEM (n = 6–7). *, P < 0.05; **, P < 0.01 by multiple comparisons corrected by Holm-Sidak test. Data are combined from two independent experiments. (B to D) Lethally irradiated 129S1 recipients were transplanted with MHC-matched C57BL/6 T-cell depleted bone marrow (“BM” in the figure) cells and 1 × 106 C57BL/6 T cells (“T” in the figure). Recipients were treated with aztreonam, cefepime, imipenem-cilastatin or piperacillin-tazobactam (pip-tazo) (100 mg/kg, SC, 3 times a week from day 10 to day 24 following allo-HSCT). (B) Comparison of overall survival, with combined data from two independent experiments (n = 17–18). *, P < 0.05; **, P < 0.01 by Mantel-Cox logrank test. (C) Comparison with control mice with T-cell depleted BM only (no GVHD). Overall survival, with combined data from three independent experiments (n = 15–43). ****, P < 0.0001 by Mantel-Cox logrank test. Tx indicates the period of antibiotics treatment in B and C. (D) Mice with GVHD treated with antibiotics were sacrificed on day 21 and GVHD histology scores in target organs were quantified by a blinded pathologist. Data are combined from three independent experiments (n = 5–20). *, P < 0.05 by Mann-Whitney U test.