Hindlimb Unloading in Rodents Induces Gastrointestinal Inflammation. Running head: Simulated Microgravity in Rodents Induces GI Inflammation

The hindlimb unloading (HU) rodent model is used to simulate changes incurred in spaceflight due to microgravity. We have previously shown 2 weeks of rat HU impairs lymphatic function systemically (42). We also previously demonstrated diminished vasoconstrictor responses of both mesenteric arteries and veins (8). However, gut biological adaptations in response to spaceflight have been minimally investigated, a concern given gut biology’s involvement in maintaining overall biological health as well as dietary absorption; moreover, intestinal inflammation and gut immunological alterations have not been systemically measured in microgravity models before. Recently we characterized and observed rat gastrointestinal (GI) inflammation and mesenteric lymphatic vessel associated immune cells after 4 weeks of HU. Body weight and food intake differences were notable, with HU animals weighing less as well as having increased daily food intake ( p < 0.05 ). Colon histopathology indicated elevated damage in HU compared to controls ( p < 0.05 ). Fecal calprotectin (a clinical IBD marker of GI inflammation) was significantly increased at 2-weeks of HU and trended towards elevation at 4-weeks. Furthermore, we noted shifts in innate immune cell populations (CD163 ⁺ and MHCII ⁺ ) localized with mesenteric lymphatics. CD163 ⁺ cells increased in both numbers and localization with lymphatics after 4 weeks of HU ( p < 0.05 ). Conversely MHCII ⁺ immune cells were reduced in both total number and their association with lymphatics in HU, suggesting altered antigen presentation capacity ( p < 0.05) . These patterns are similar to our observations in models of gut inflammation. These findings may provide insight with adaptations astronauts may experience related with immune dysregulation, nutrient malabsorption, and GI adaptations. These observations also present an unrecognized inflammatory stress response that may explain physiological adaptations occurring between HU and space-flown rodent studies.