Visualization of splenic marginal zone B-cell shuttling and follicular B-cell egress.

The splenic marginal zone is a unique microenvironment where resident immune cells are exposed to the open blood circulation. Even though it has an important role in responses against blood-borne antigens, lymphocyte migration in the marginal zone has not been intravitally visualized due to challenges associated with achieving adequate imaging depth in this abdominal organ. Here we develop a two-photon microscopy procedure to study marginal zone and follicular B-cell movement in the live mouse spleen. We show that marginal zone B cells are highly motile and exhibit long membrane extensions. Marginal zone B cells shuttle between the marginal zone and follicles with at least one-fifth of the cells exchanging between compartments per hour, a behaviour that explains their ability to deliver antigens rapidly from the open blood circulation to the secluded follicles. Follicular B cells also transit from follicles to the marginal zone, but unlike marginal zone B cells, they fail to undergo integrin-mediated adhesion, become caught in fluid flow and are carried into the red pulp. Follicular B-cell egress via the marginal zone is sphingosine-1-phosphate receptor-1 (S1PR1)-dependent. This study shows that marginal zone B cells migrate continually between marginal zone and follicles and establishes the marginal zone as a site of S1PR1-dependent B-cell exit from follicles. The results also show how adhesive differences of similar cells critically influence their behaviour in the same microenvironment.