Although predominantly expressed on lymphocytic and hematopoietic cells, the role of sphin‐gosine‐1‐phospate receptor 4 (S1P ₄ ) in immune homeostasis is still poorly understood. In this report, we used a S1P ₄ ‐deficient murine model to characterize the biological role of S1P ₄ ‐mediated S1P signaling in the immune system. S1p ₄ ^‐/‐ animals showed normal peripheral lymphocyte numbers and a regular architecture of secondary lymphoid organs. Interestingly, S1P ₄ only marginally affects T‐cell function in vivo. In contrast, dendritic cell (DC) migration and cytokine secretion are profoundly affected by S1P ₄ deficiency. Lack of S1P ₄ expression on DCs significantly reduces T _H 17 differentiation of T _H cells. Furthermore, in various in vivo models of T _H 1‐ or T _H 2‐dominated immune reactions, S1P ₄ deficiency consistently increased the amplitude of T _H 2‐dominated immune responses, while those depending on T _H 1‐dominated mechanisms were diminished. Finally, S1p ₄ ^‐/‐ mice showed decreased pathology in a model of dextran sulfate sodium‐induced colitis. In summary, for the first time, we show that S1P ₄ signaling is involved in the regulation of DC function and T _H 17 T‐cell differentiation. S1P ₄ ‐mediated S1P signaling also modifies the course of various immune diseases in a murine model. We propose that S1P ₄ may constitute an interesting target to influence the course of various autoimmune pathologies.—Schulze, T., Golfier, S., Tabeling, C., Räbel, K., Gräler, M. H., Witzenrath, M., Lipp, M. Sphingosine‐1‐phospate receptor 4 (S1P ₄ ) deficiency profoundly affects dendritic cell function and T _H 17‐cell differentiation in a murine model. FASEB J. 25, 4024–4036 (2011). www.fasebj.org

Introduction. Macrophages are key players in complex biological processes. In response to environmental signals, macrophages undergo polarization towards a proinflammatory (M1) or anti-inflammatory (M2) phenotype. Sphingosine 1-phosphate (S1P) is a bioactive lysophospholipid that acts via 5 G-protein coupled receptors (S1P1–5) in order to influence a broad spectrum of biological processes. This study assesses S1P receptor expression on macrophages before and after M1 and M2 polarization and performs a comparative analysis of S1P signalling in the two activational states of macrophages. Methods. Bone marrow derived macrophages (BMDM) from C57 BL/6 mice were cultured under either M1- or M2-polarizing conditions. S1P-receptor expression was determined by quantitative RT-PCR. Influence of S1P on macrophage activation, migration, phagocytosis, and cytokine secretion was assessed in vitro. Results. All 5 S1P receptor subclasses were expressed in macrophages. Culture under both M1- and M2-polarizing conditions led to significant downregulation of S1P₁. In contrast, M1-polarized macrophages significantly downregulated S1P₄. The expression of the remaining three S1P receptors did not change. S1P increased expression of iNOS under M2-polarizing conditions. Furthermore, S1P induced chemotaxis in M1 macrophages and changed cytokine production in M2 macrophages. Phagocytosis was not affected by S1P-signalling. Discussion. The expression of different specific S1P receptor profiles may provide a possibility to selectively influence M1- or M2-polarized macrophages.