Original Aims: The original aims were to determine if there were inflammation in the intestines of rodents resulting from space flight, measure signs of lymphatic insufficiency in the intestine of space flown animals, examine lymphatic vessel/immune cell interaction in tissues of ground based analogs of microgravity, and determine if shifts in the microbiome of ground based microgravity analogs are analogous to those in space flight and check associated nutrition status.

Key Findings: We found evidence of inflammation in the intestines of rats flown for short duration (9 day), low Earth orbit (shuttle PARE 0.3) including edema and immune cell infiltration (via histopathology). This was supported by cytokine analysis that revealed that several key cytokine protein levels (IL-4, IL-5, IP-10, RANTES, etc.) were up-regulated in the intestinal tissues of space flown animals and the profile suggests the initial phases of an inflammatory state. We also observed signs of lymphatic dysfunction with increased lipid droplet staining in the villi of the ileum and collapsed lymphatic vessels in the bowel wall of the colon that when considered together in the context of inflammation suggest a lymphatic functional deficiency. It is also likely that this dysfunction, at least in acute space flight, originates at the capillary lymphatic level given our observations of failed lipid uptake and collapsed pre-collecting lymphatic vessels with no other obvious signs of structural aberrations (no changes in perimeter of breaks in lymphatic endothelial staining). We were able to observe changes in the nature of lymphatic vessel/immune cell interactions in ground based models of microgravity in rats and mice (this is a critical regulator of lymphatic function) but not in space flown rats as the tissue was not available. We found that the number of MHCII positive antigen presenting cells (APCs) were decreased along mesenteric lymphatic collecting vessels in both mice and rats after 4 weeks of suspension. This was accompanied by a concomitant increase in number of potentially fibrotic CD146/CD206(int/low) macrophages along the vessels. We unfortunately were not able to test the antigen uptake and processing efficiency of the remaining APCs. We were able to perform limited micobiomic analysis of rat and mouse feces from simulated microgravity but have not yet received the comparison data for similar space flight animals.

Our data suggests that rats in simulated microgravity have a shift in the microbiome similar to that of an animal fed a high fat diet with some aspect of immune suppression while similar samples from mice appear to resemble that from inflammatory bowel diseases. We were not able to complete nutritional analysis of the stools of space flown animals; however, lipid staining of the colon revealed that there was excessive lipid in the colon of space flown animals suggesting that fats are not being efficiently absorbed.

Impact: Our data suggests that there is the very real possibility that aspects of space flight can interfere with lymphatic function in the visceral organs. While the effects are more pronounced in small rodent models (due to time and sensitivities) this is a factor that we need to be examining in human subjects as lymphatic function is critical to nutrition, wound healing, immune surveillance, and fluid balance.

Proposed Research Plan: As this is a one year fellowship there are no current plans to expand upon this research with the National Space Biomedical Research Institute (NSBRI). However we have secured funding through NASA (myself as a Co-Investigator) to examine these factors (lymphatic dysfunction and inflammation) in animals flown for longer duration on the International Space Station (ISS). This will offer us a mechanism to fulfill several key parts of this project that we were not able to due to lack of appropriate tissue samples as well as extend our observations to longer time points.

The lymphatic system is critical to the uptake of lipids from the diet, returning protein from the intercellular spaces back to the blood, and maintaining proper immune responses. We know that spaceflight affects nutritional uptake, muscle density, and the body’s response to foreign pathogens, both new and reoccurring. These effects might be related to spaceflight environments impacts on the lymphatic system. We know from our previous work on a rat model of fluid shift, which simulates some of the effects of microgravity, that the function of the lymphatic system is depressed. We recently found that this is associated with profound intestinal inflammation and nutritional imbalance, resulting in a general wasting effect despite increased food intake. We hypothesize that lymphatic function is depressed in spaceflight and that this leads to alterations in the inflammatory status of the gut, altered intestinal microbiota, and altered nutritional uptake. We will measure shifts in bacterial populations of the gut microbiota, the host’s response to the microbiota, as well as antigen trafficking and nutritional absorption changes in both space-flown and hind limb suspended animals. Understanding these changes is key to the development of practical countermeasures for GI (gastrointestinal) health.