Project Summary The intestinal epithelium regulates the development of adaptive immunity to gut microorganisms, yet little is known about the underlying mechanisms. Filling this knowledge gap is crucial, as many human intestinal diseases arise from dysregulated intestinal immunity. Dietary vitamin A absorbed by the intestinal epithelium is essential for key intestinal adaptive immune responses, including the homing of T and B cells to the intestine and immunoglobulin A production. These vitamin A-dependent immune responses depend on specialized intestinal myeloid cells that enzymatically convert the vitamin A derivative retinol to retinoic acid (RA). In the previous project period we unraveled a molecular mechanism by which intestinal myeloid cells acquire retinol. We discovered that serum amyloid A (SAA) proteins – microbiota-inducible epithelial cell proteins – are retinol chaperones that deliver retinol to myeloid cells via the endocytic receptor LRP1 (low density lipoprotein-related receptor 1). We found that this mechanism is essential for the development of intestinal adaptive immunity and for immune protection against pathogenic bacterial infection. This R01 renewal application will build on these findings to further define the biochemical, cell biological, and microbiological mechanisms that regulate retinol delivery to intestinal myeloid cells and thus promote vitamin A-dependent immunity. In Aim 1, we will determine how SAA-retinol complexes are assembled and secreted from intestinal epithelial cells. In Aim 2, we will identify the SAA-LRP1 interface and determine its importance for myeloid cell retinol uptake and vitamin A-dependent immunity. In Aim 3, we will define the role of the intestinal microbiota in regulating myeloid cell retinol acquisition and vitamin A-dependent immunity. These studies will provide mechanistic insight into how vitamin A is mobilized to intestinal immune cells and advance our understanding of how the gut microbiota regulates the intestinal adaptive immune system. Our findings will help identify new strategies for inhibiting or enhancing vitamin A mobilization to the intestinal immune system in order to treat infection and inflammation.