Public exposure to radiation due to large-scale radiation incidents is a rising global concern. Acute
radiation syndrome (ARS) is associated with high morbidity and mortality, but no FDA-approved
therapeutics for gastrointestinal (GI) ARS. Therefore, delineating the mechanisms underlying radiation
injury to develop targeted medical countermeasures (MCM) is a high priority. The GI mucosal immune
system is susceptible to ionizing radiation, and dysfunctional mucosal immunity is a major contributing
factor in the pathogenesis of ARS. The gap in this field is that the precise mechanisms by which radiation
impairs the mucosal immune system and immune dysfunction-mediated dysbiosis of gut microbiota and
multi-organ injury (MOI) are poorly defined. The long-term goal of our research is to identify the radiation-
sensitive immune-specific pathways and test and develop novel immune dysfunction-targeted MCM for
radiation exposure. Endotoxemia and systemic inflammation are common conditions associated with
morbidity and mortality in ARS. Clinical and experimental evidence indicates that intestinal dysbiosis
(depleted beneficial species, increased pathobionts, and decreased diversity) is a prerequisite for
developing endotoxemia, systemic inflammation, and MOI. a-Defensins are antibacterial peptides secreted
from Paneth cells, the highly specialized intestinal epithelial cells, to maintain microbiota homeostasis.
Human Paneth cells produce two a-defensins - defensin 5 (HD5) and 6 (HD6). Our preliminary data show
that ionizing radiation in mouse intestine 1) depletes Paneth cell a-defensins, 2) reduces mucosal Tcf4
mRNA, 3) alters microbiota composition, 4) disrupts epithelial barrier, and 5) consequent mucosal
inflammatory response, endotoxemia, and systemic inflammation. Importantly, HD5 administered in the
diet at 24 h post-irradiation mitigates altered gut microbiota, gut barrier dysfunction, and endotoxemia.
These findings form the scientific premise (FIG 1) and support the central hypothesis that “HD5 mitigates
GI-ARS by reversing dysbiosis of gut microbiota and epithelial barrier dysfunction, leading to mitigation of
endotoxemia and systemic inflammation.” We will test this hypothesis by determining that 1) Ionizing
radiation downregulates Wnt signaling in intestinal Paneth cells, 2) TCF4 down-regulation mediates
radiation-induced a-defensin depletion and consequent dysbiosis, 3) a-Defensin supplementation reverses
radiation-induced dysbiosis of gut microbiota, 4) Radiation-induced dysbiosis drives gut barrier dysfunction,
endotoxemia, and systemic inflammation, 5) the lowest and most effective dose of HD5 in mitigating GI-
ARS, 6) the ideal time window for post-exposure (+24-96 h) effectiveness of HD5 to reverse GI-ARS, and
7) the HD5 treatment paradigm to increase the survival rates from lethal dose radiation.
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