Radiation-induced gastrointestinal syndrome (RIGS) remains one of the major limitations for delivering tumoricidal doses of abdominal radiation therapy (RT). It could also limit survival of victims in a mass casualty setting from nuclear accidents or terrorism. While supportive care with antibiotics, hydration and bone marrow transplantation can rescue radiation-induced bone marrow syndrome, currently there are no approved therapy for protecting or mitigating RIGS. Manifestations of RIGS are influenced by radiation dose with a short prodromal syndrome of nausea and vomiting upon exposure to whole body irradiation of 1.5 Gy. With higher doses (≥6 Gy), the prodromal syndrome is more marked, followed by a subacute syndrome of diarrhea and gastrointestinal bleeding in 2–5 days post-exposure. As doses reach over 8–10 Gy, the full-blown syndrome of RIGS with diarrhea, dehydration, sepsis and intestinal bleeding ensues with eventual mortality. In the clinic, higher doses from fractionated RT or large single fractions during sterotactic body radiation therapy (SBRT) can cause diarrhea and gastrointestinal bleeding, ulcer or fistula from breakdown of irradiated intestinal mucosa. The efficacy of cancer treatment by radiation and chemotherapeutic drugs is often limited by severe side effects that primarily affect the hematopoietic system and the epithelium of the gastrointestinal tract. Progress in understanding differences in the mechanisms involved in the responses of normal and tumor cells to genotoxic stress has led to the development of new rational approaches to selective protection of normal cells, such as suppression of apoptosis by pharmacological inhibition of p53 or activation of NF-k B. Another promising approach presented in this issue by Johnson et al. is based on the idea of using pharmacological inhibitors of cyclin-dependent kinases (CDKs) to convert normal cells into a radioresistant state by inducing reversible cell cycle arrest at the G1/S transition.
The pathophysiological mechanisms of RIGS is complex and involves loss of clonogenic crypt cells with eventual depopulation of the intestinal villi, defective regeneration of the irradiated intestinal stem cells and a systemic inflammatory response syndrome (SIRS) from a host of cytokines and growth factors released in the serum, following exposure to radiation and gut microbes 
. Survival from RIGS depends on the rate of the crypt depopulation and the efficiency and number of the residual clonogens, capable of regenerating crypt-villus units. Therefore, growth factors that can promote proliferation of intestinal crypt cells, such as, keratinocyte growth factor and interleukin-11 have been shown to be radioprotective for RIGS 
. Similarly, pre-treatment with growth factors that inhibit cell cycle in regenerating crypts, such as, transforming growth factor-β1 (TGFβ1) and transforming growth factor-β3 (TGFβ3), promoted radioresistance in these cells. Besides the intestinal crypts where the putative intestinal stem cell (ISC) resides, the stroma or the ISC niche has also been postulated to be a target in RIGS. Thus, endothelial cells residing in the ISC niche, is particularly vulnerable to radiation. Growth factors, such as, basic fibroblast growth factor (bFGF) that prevented radiation-induced endothelial 
cell death, also conferred radioprotection from RIGS.
Besides the endothelial cell the intestinal subepithelial immune effector cells, such as, gd-T cells and macrophages interact with the host enteric microbiota and modulate the intestinal regenerative response following radiation exposure 
. Cellular signaling via the Toll-like receptor (TLR) and other pathogen-associated molecular pattern (PAMP) recognition molecules plays a critical role in the dynamic interactions between the host's enteric microbiota and innate immune system 
. Intestinal macrophages function as mobile “cellular transceivers” that “recognize” invading bacteria entering through the disrupted intestinal mucosa via the TLR receptors and transmit regenerative and repair signals to the neighboring intestinal epithelial progenitors in the ISC niche by forming immune synapses with the ISC 
. Recently, a peptide derived from Salmonella Flagellin, which is a ligand for TLR5, was found to be radioprotective against RIGS in murine and primate models, possibly via NFκB activation 
. TLR9 is present in the basolateral surface of crypt epithelial cells and could potentially sense bacterial invasion via TLR ligand binding. Earlier reports demonstrated that activation of TLR9 pathway by bacterial CpG-rich oligonucleotides could be beneficial in protecting against chemical models of intestinal injury 
. TLR stimulation could also contribute to generation of anti-inflammatory signal and dampen intestinal sepsis 
. We, therefore, hypothesized that systemic administration of TLR9 ligands could activate TLR signaling in the intestine and confer radioprotection and/or mitigation from RIGS. In this study we have used a novel TLR9 agonist containing synthetic immunomodulatory CpR (R
2′-deoxy-7-dezaguanosine) dinucleotide and 3′-3′-attached novel structures that has been shown to induce potent TLR9-mediated immune responses 
. The presence of 3′-3′-attached structure provides higher metabolic stability and also optimal 5′-end presentation required for TLR9 recognition 
. These novel TLR9 agonists have been shown to induce potent Th1-type immune responses and a broad spectrum of antitumor activity in a number of tumor models 
. A human selective TLR9 agonist, referred to as IMO-2055, is in phase II clinical trials for cancer 
. Here we demonstrate that activation of the TLR9 pathway by a novel TLR9 agonist promoted intestinal crypt cell survival by inhibiting radiation-induced apoptosis and stimulated regeneration of the irradiated intestine, thereby, improving survival in a murine model of RIGS.