Lymph node cortical sinus organization and relationship to lymphocyte egress dynamics and antigen exposure

Recent studies have identified cortical sinuses as sites of sphingosine-1-phosphate receptor-1 (S1P1)-dependent T- and B-cell egress from the lymph node (LN) parenchyma. However, the distribution of cortical sinuses in the entire LN and the extent of lymph flow within them has been unclear. Using 3D reconstruction and intravital two-photon microscopy we describe the branched organization of the cortical sinus network within the inguinal LN and show that lymphocyte flow begins within blunt-ended sinuses. Many cortical sinuses are situated adjacent to high endothelial venules, and some lymphocytes access these sinuses within minutes of entering a LN. However, upon entry to inflamed LNs, lymphocytes rapidly up-regulate CD69 and are prevented from accessing cortical sinuses. Using the LN reconstruction data and knowledge of lymphocyte migration and cortical sinus entry dynamics, we developed a mathematical model of T-cell egress from LNs. The model suggests that random walk encounters with lymphatic sinuses are the major factor contributing to LN transit times. A slight discrepancy between predictions of the model and the measured transit times may be explained by lymphocytes undergoing a few rounds of migration between the parenchyma and sinuses before departing from the LN. Because large soluble antigens gain rapid access to cortical sinuses, such parenchyma–sinus shuttling may facilitate antibody responses.