P1.069: The Absence of Farnesoid-X Receptor Impacts the Myeloid Population in Colitis

Background: Farnesoid-X receptor (FXR) is a nuclear bile acid receptor known to regulate intestinal inflammation. We have previously shown that the small bowel barrier of FXR-KO (knockout) animals is preserved in acute injury. Macrophages express FXR and are essential in intestinal injury response. We have also shown that macrophage FXR-KO (mpFXR-KO) in culture have increased low level pro-inflammatory cytokine expression in M0 and M2 phenotypes, however reduced pro-inflammatory cytokine expression in the M1 phenotypes. We hypothesized that the absence of FXR within macrophages would alter the immune population in acute colitis by reducing pro-inflammatory macrophages.

Methods: C57BL/6 and mpFXR-KO mice were given 3% dextran sulfate sodium (DSS) or water for four days and harvested on day seven. Whole colon samples were processed for histology and F4/80 staining to label myeloid cells. Samples also underwent lamina propria isolation, which were then stained for FACS and flow cytometry performed on the Cytek Aurora. Macrophage gating and analysis was performed with FlowJo and Prism software.

Results: Both WT and mpFXR-KO controls did not show a difference in F4/80 staining. WT mice given DSS reflected a significant proportion of F4/80 staining in the descending colon, however this response was greatly reduced in DSS-treated mpFXR-KO mice. Flow cytometry showed no significant difference in %CD45 positive (+) cells detected between WT and mpFXR-KO mice at baseline or after DSS. CD11b+Ly6C- cells were significantly increased after DSS in both WT and mpFXR-KO mice compared to H2O; however, no significant increase was seen in CD11b+Ly6C+ cells. No significant difference was noted in MHCII+CXRCR1+ cell positivity. A significant increase in CD11b+CD206+ cells was seen in WT mice after DSS, but this was not seen in mpFXR-KO mice after DSS.

Conclusion: The relative absence of F4/80 staining in DSS mpFXR-KO animals as compared to WT suggests FXR may be influencing the overall migration of myeloid cells after injury. CD11b+Ly6C- cells were elevated after DSS in both WT and mpFXR-KO animals, however, no difference was seen in CD11b+Ly6C+, indicating a more differentiated and mature population of monocytes after DSS. While it does not appear tissue resident macrophages are affected given no difference was noted in %MHCII+CXRCR1+ cells, there was a decrease in CD11b+CD206+ cells in mpFXR-KO mice after DSS as compared to WT mice, indicating less M2 phenotype or wound healing macrophages in the absence of FXR. This may explain the lack of histologically F4/80 positive cells in DSS mpFXR-KO mice. These data suggest that FXR in macrophages plays a critical role in attenuating the inflammatory response in colitis.