ObjectivesTo evaluate whether the up-regulation in sepsis-induced gut epithelial apoptosis is balanced by an increase in intestinal proliferation and to assess mechanisms affecting the gut’s regenerative response to overwhelming infection.DesignProspective, randomized, controlled study.SettingAnimal laboratory in a university medical center.InterventionsMice were subjected to intratracheal injection ofPseudomonas aeruginosaand killed between 1.5 and 24 hrs after induction of pneumonia-induced sepsis to assess for gut epithelial proliferation and cell division and for apoptosis. Animals were compared with sham-operation controls, septic transgenic mice that overexpress Bcl-2 throughout their small intestinal epithelium, and septic p53−/-mice.Measurements and Main ResultsProliferation and cell division were assessed by measuring S-phase and M-phase cells in intestinal crypts. The number of S-phase cells showed a progressive decline at all time points measured, with a 5-fold decrease in proliferation between control animals and septic mice 24 hrs after intratracheal injection of pathogenic bacteria (p< .0001). In contrast, cells in M-phase remained constant for the first 12 hrs after the onset of sepsis, but increased nearly 50% at 24 hrs after instillation ofP. aeruginosa(p< .005). Both the decrease in S-phase cells and the increase in M-phase cells were partially suppressible in Bcl-2 overexpressors, but cellular proliferation and division were similar between septic p53−/-and p53+/+mice. Crypt apoptosis was increased at all time points, with maximal death occurring between 12 and 24 hrs.ConclusionsSepsis fromP. aeruginosapneumonia induces a p53-independent decrease in gut epithelial proliferation. Despite an increase in sepsis-induced intestinal apoptosis, there is no compensatory increase in intestinal epithelial proliferation, and there is evidence of a cell cycle block with an accumulation of cells in M-phase. Decreasing gut apoptosis by overexpression of Bcl-2 is associated with a partial reversal of the effect of sepsis on the cell cycle.