Microbiome-derived indoles mitigate morbidity and mortality in a mouse model of GVHD. (A) Left panel: Changes in urinary 3-IS before (day −1) and after (day 1, 2, 7) lethal TBI at 11 Gy (2 × 5.5 Gy) without subsequent BMT (combined data from 2 experiments, n = 10, sampled at multiple time points). Right panel: 3-IS before (day −6) and 1 day after (day 1) completion of 6-day chemo regimen with busulfan (80 mg/kg total) and cyclophosphamide (200 mg/kg total), without subsequent BMT. Combined data from 2 experiments, n = 10, sampled at multiple time points. (B-F) B10.BR recipient mice were treated with streptomycin and then colonized with streptomycin and nalidixic acid resistant K12 or K12ΔTnaA E coli 1 week prior to lethal irradiation and allo-BMT with TCD-BM + T cells from C57Bl/6 donor mice. Various parameters were tracked before irradiation/allo-BMT (day −1) and then weekly for 5 weeks following transplant. Single experiment with n = 15 per condition and 5 mice censored per condition on day 21 for various assays. (B) Colonization as measured by colony-forming unit (CFU)/g of bacteria in feces assessed on E coli selective plates containing streptomycin and nalidixic acid (n = 15 per condition). (C) 3-IS levels in urine (n = 15 per condition). (D) Weight loss. (E) Bacterial translocation to MLN (day 21, n = 5 per condition). (F) Kaplan-Meier survival curve with ticks indicating mice censored on day 21. Statistics: Mantel Cox Log-rank (survival curve), Mann-Whitney or Kruskal-Wallis ANOVA. ****P < .0001; ***P = .0001 to .001; **P = .001 to .01; *P = .01 to .05.