Isolation Of Gastrointestinal Interstitial Fluid As A Novel Method To Capture Host-microbiome Crosstalk

Metabolites produced by the microbiome such as short chain fatty acids (SCFAs) and tryptophan metabolites have been shown to impact the pathogenesis of hypertension. The microenvironment of the host gastrointestinal (GI) tract acts as the site-of-action for many host-microbiome interactions, although this space is not readily accessible. Fecal or serum samples are commonly used as a proxy, relying on the assumption that the obtained information should in some respect represent what is seen at the host epithelial interface. We surmised that it would be feasible to overcome the limitations of such indirect analysis by isolating interstitial fluid (IF) from the gut mucosa. We have established two methodologies to isolate IF from small segments of along GI tract, a centrifugation-based and elution-based approach in rats and mice. For rats and mice, 51 Cr-EDTA tracer experiments were used to demonstrate that fluid obtained was from an extracellular origin and can be reliably considered IF. Using GC-MS, we could identify several SCFAs (acetate, butyrate, propionate, valerate) within eluted IF samples. We were able to empirically measure the enrichment of these metabolites in eluted IF from the colon of rats compared to the duodenum (for propionate; mean difference=59.57 μM, p-value < 0.0001) or the serum (for propionate; mean difference= 60.25 μM, p-value < 0.0001). LC-MS based shotgun proteomics revealed that proteins annotated to the extracellular phase were site-specifically identifiable in IF and were differentially expressed when compared to matched serum samples. Furthermore, we could demonstrate that the use of tissue for IF isolation does not impede the use of other methods such as immunophenotyping or histology. Absolute CD45+ cells from the lamina propria measured using flow cytometry were not influenced by IF methods (p-value = 0.8344) but were unsurprisingly influenced by the gut segment which was analyzed (p-value = 0.0010). The ability to collect IF and directly measure metabolites at the site-of-action overcomes the limitations of indirect analysis of fecal samples or serum from the systemic circulation, and thus offers direct insight into this hitherto underexplored compartment.