LXR nuclear receptors are transcriptional regulators of dendritic cell chemotaxis

The liver X receptors (LXRs) are ligand-activated nuclear receptors with established roles in the maintenance of lipid homeostasis in multiple tissues. LXRs exert additional biological functions as negative regulators of inflammation, particularly in macrophages. However, the transcriptional responses controlled by LXRs in other myeloid cells, such as dendritic cells (DC), are still poorly understood. Here we used gain- and loss-of-function models to characterize the impact of LXR deficiency on DC activation programs. Our results identified an LXR-dependent pathway that is important for DC chemotaxis. LXR-deficient mature DCs are defective in stimulus-induced migration in vitro and in vivo. Mechanistically, we show that LXRs facilitate DC chemotactic signaling by regulating the expression of CD38, an ectoenzyme important for leukocyte trafficking. Pharmacological or genetic inactivation of CD38 activity abolished LXR-dependent induction of DC chemotaxis. Using the LDLR-/- mouse model of atherosclerosis, we also demonstrated that hematopoietic CD38 expression is important for the accumulation of lipid-laden myeloid cells in lesions, suggesting that CD38 is a key factor in leukocyte migration during atherogenesis. Collectively, our results demonstrate that LXRs are required for efficient emigration of DCs in response to chemotactic signals during inflammation. BM-derived DCs (BMDCs) from LXR WT and LXR double knockout (combined deficiency of LXRalpha and LXRbeta) mice were generated in vitro as described by Inaba et al. (J Exp Med. 1992 Dec 1;176(6):1693-702) with modifications. BM cells were cultured for 6-7 days in RPMI 1640 medium containing 10% fetal bovine serum (FBS) supplemented with mouse GM-CSF (20 ng/mL, Peprotech, UK) every two days. Non-adherent cells were collected and further enriched in CD11c positive DCs by positive selection with CD11c microbeads (MiltenyiBiotec) following the manufacturer instructions. Cells were then further cultured with GMCSF (10ng/ml) as control cells or GMCSF + LPS (100ng/ml) for 24 hours as a model for DC maturation. After treatment, RNA was harvest by column purification with RNAeasy mini-kit (Qiagen). RNA was quantified and submitted for microarray analysis to the Unidad de Genomica, Universidad Complutense de Madrid (UCM)