CXCR4 and a cell-extrinsic mechanism control immature B lymphocyte egress from bone marrow

HDL-bound sphingosine-1-phosphate restrains lymphopoiesis and neuroinflammation

Lipid mediators influence immunity in myriad ways. For example, circulating sphingosine 1-phosphate (S1P) is a key regulator of lymphocyte egress^1,2. Although the majority of plasma S1P is bound to apolipoprotein M (ApoM) in the high-density lipoprotein (HDL) particle³, immunological functions of the ApoM-S1P complex are unknown. Here, we show that ApoM-S1P is dispensable for lymphocyte trafficking yet restrains lymphopoiesis by activating the S1P₁ receptor on bone marrow (BM) lymphocyte progenitors. Mice that lacked ApoM (Apom^−/−) had increased proliferation of Lin⁻Sca1⁺cKit⁺ hematopoietic progenitor cells (LSK) and common lymphoid progenitors (CLP) in BM. Pharmacologic activation or genetic overexpression of S1P₁ suppressed LSK and CLP proliferation in vivo. ApoM was stably associated with BM CLPs, which showed active S1P₁ signaling in vivo⁴. Moreover, ApoM⁺HDL, but not albumin-bound S1P, inhibited lymphopoiesis in vitro. Upon immune stimulation, Apom^−/− mice developed more severe experimental autoimmune encephalomyelitis⁵, characterized by increased lymphocytes in the central nervous system (CNS) and breakdown of the blood-brain barrier. Thus, the ApoM-S1P-S1P₁ signaling axis restrains the lymphocyte compartment and subsequently, adaptive immune responses. Unique biological functions imparted by specific S1P chaperones could be exploited for novel therapeutic opportunities.