Inhibition of intestinal epithelial apoptosis improves survival in a murine model of radiation combined injury

World conditions place large populations at risk from ionizing radiation (IR) from detonation of dirty bombs or nuclear devices. In a subgroup of patients, ionizing radiation exposure would be followed by a secondary infection. The effects of radiation combined injury are potentially more lethal than either insult in isolation. The purpose of this study was to determine mechanisms of mortality and possible therapeutic targets in radiation combined injury. Mice were exposed to IR with 2.5 Gray (Gy) followed four days later by intratracheal methicillin-resistant Staphylococcus aureus (MRSA). While either IR or MRSA alone yielded 100% survival, animals with radiation combined injury had 53% survival (p = 0.01). Compared to IR or MRSA alone, mice with radiation combined injury had increased gut apoptosis, local and systemic bacterial burden, decreased splenic CD4 T cells, CD8 T cells, B cells, NK cells, and dendritic cells, and increased BAL and systemic IL-6 and G-CSF. In contrast, radiation combined injury did not alter lymphocyte apoptosis, pulmonary injury, or intestinal proliferation compared to IR or MRSA alone. In light of the synergistic increase in gut apoptosis following radiation combined injury, transgenic mice that overexpress Bcl-2 in their intestine and wild type mice were subjected to IR followed by MRSA. Bcl-2 mice had decreased gut apoptosis and improved survival compared to WT mice (92% vs. 42%; p<0.01). These data demonstrate that radiation combined injury results in significantly higher mortality than could be predicted based upon either IR or MRSA infection alone, and that preventing gut apoptosis may be a potential therapeutic target

Radiation combined injury causes increased systemic levels of IL-6, IL-10 and G-CSF.

<p>While neither IR nor MRSA caused alterations in systemic cytokines (except for an increase in G-CSF following MRSA), IL-6 (p<0.005), IL-10 (p<0.01) and G-CSF (p<0.05) were significantly increased in IR/MRSA mice compared to NR/sham, IR/sham, NR/MRSA mice (n = 10−14/group).</p

Radiation combined injury increases mortality compared to IR or MRSA alone.

<p>Mice that received either 2.5% survival. In contrast, when IR was followed four days later by MRSA, survival was only 53% (n = 8−15/group; p = 0.01). Note that day 0 in this survival curve represents when MRSA or sham pneumonia was induced, and IR was given four days earlier.</p

Increased gut apoptosis with radiation combined injury is associated with changes in the mitochondrial pathway.

<p>Real time PCR was performed on mediators in both the mitochondrial pathway and receptor-mediated pathway of apoptosis. Radiation combined injury caused an increase in mRNA levels of Bcl-2 (A), Bcl-x_L (B), and Bax (C, p<0.05 IR/MRSA compared to all other groups). In contrast, no alterations were noted in Bid (D), Fas (E), FADD (F) or TRADD levels (G, n = 7–8/group for all mediators).</p

Gut Bcl-2 overexpression prevents intestinal epithelial apoptosis and improves survival following radiation combined injury.

<p>Gut apoptosis was lower in <i>Fabpl</i>-Bcl-2 mice than WT mice following IR/MRSA by both H&E (A) and active caspase-3 (B) staining (p<0.05 for H&E, p<0.005 for active caspase-3, n = 12–15). Preventing gut apoptosis led to improved outcome, as survival was 92% in <i>Fabpl</i>-Bcl-2 mice subjected to radiation combined injury compared to 42% in WT mice given the same insult (p<0.01, n = 12–13/group). Note that day 0 in this survival curve represents when MRSA pneumonia was induced, and IR was given four days earlier.</p