B Lymphocytes Exit Lymph Nodes through Cortical Lymphatic Sinusoids by a Mechanism Independent of Sphingosine-1-Phosphate-Mediated Chemotaxis

Abstract

Sphingosine-1 Phosphate (S1P) helps mediate lymphocyte egress from lymph nodes, yet significant mechanistic questions remain. Here we show that B lymphocyte egress sites exist close to lymph node follicles. Recent B cell emigrants localize towards follicle centers, while longer-term residents tend towards cortical sinusoids. Exiting B lymphocytes squeeze through apparent portals in the lymphatic endothelium. Treatment with the S1P receptor agonist FTY720 empties the cortical sinusoids of lymphocytes, blocks lymphatic endothelial penetration, and displaces B lymphocytes into the T cell zone. S1P3−/− B cells, which lack chemoattractant responses to S1P, transit lymph nodes normally, while Gnai2−/− B cells, which have impaired responses to chemokines and S1P, transit more rapidly than do wild type cells. This study identifies a major site of B lymphocyte lymph node egress, shows that FTY720 treatment blocks passage through the cortical lymphatic endothelium, and argues against a functional role for S1P chemotaxis in B lymphocyte egress.

Down-regulation of S1P1 Receptor Surface Expression by Protein Kinase C Inhibition

“…7F), indicating that S1P 1 receptor stimulation mostly accounts for FTY720-induced egress inhibition, and S1P 1 receptor downregulation is a secondary, less important event, at least in case of FTY720 treatment. This is in line with recent reports showing the requirement of FTY720 phosphorylation by SK2 for efficient lymphopenia at low FTY720 concentrations (30,31) and the independence of B cell exit from S1P-mediated chemotaxis (40). It also explains the observation that S1P 1 receptor agonists and not antagonists were able to induce lymphopenia in vivo, although antagonists could reverse lymphopenia induced by agonists (39).…”

“…On the other hand S1P 1 receptor agonists prevent lymphocytes from crossing the lymphatic endothelium, which was blocked by otherwise inactive S1P 1 receptor antagonists (14,39). Furthermore B cells seem to exit lymph nodes independently from S1P-mediated chemotaxis (40). The contribu- 6 HTC 4 cells were incubated for 15 h with indicated concentrations of AAL(R), AAL(S), FTY720, staurosporine, calphostin C, and GF109203X.…”

Expansion of Cortical and Medullary Sinuses Restrains Lymph Node Hypertrophy during Prolonged Inflammation

“…Most research on lymphocyte egress from DLNs has been carried out in homeostatic conditions and focused either on mechanisms that lymphocytes use to reach efferent lymphatic vessels, including regulation of CCR7 (7) and sphingosine 1-phosphate receptor-1 (S1P1) expression (18,29,30,53,54), or on mechanisms operating at the level of lymphatic endothelial barriers (31,55). We showed that the expansion of cortical and medullary sinuses in the late phase of inflammation functionally supports the egress of naive lymphocytes from LNs.…”

“…We hypothesized that the return of lymphocyte egress to baseline levels at day 14 postimmunization compared with day 4 may occur through an expansion of cortical and medullary sinuses, because they are the major exit paths for lymphocytes (29)(30)(31). To investigate this possible scenario, we developed a strategy to distinguish cortical and medullary sinuses from subcapsular sinuses given that specific markers to differentiate them are currently lacking.…”

Local Inactivation of Sphingosine 1-Phosphate in Lymph Nodes Induces Lymphopenia

“…Based on these observations, a multistep model was proposed delineating cortical sinus probing, S1P 1 -dependent entry, and flow-based capture of T cells as critical steps for their lymph node exit. B cell exit was also shown to occur through cortical lymphatic sinusoids, although by a mechanism that is likely independent of sphingosine 1-phosphate-mediated chemotaxis (9).…”