An ephrin B1-based guidance system

Precise control over the migration of T cells and B cells to and from germinal centres (GCs) is crucial for effective humoral immunity. In particular, appropriate GC recruitment, retention and provision of help by T follicular helper (T_FH) cells are required to support GC B cell proliferation and differentiation. A recent study in Science shows that ephrin B1 (EFNB1) limits the retention of T_FH cells in GCs and yet promotes the production of IL-21 to support GC maintenance and plasma cell formation.

Credit: David R. Frazier Photolibrary, Inc./Alamy

First, the authors noted that EFNB1 — which is best known for its role in guiding cell migration during development — is expressed at much higher levels on GC B cells than on naive B cells. However, wild-type mice reconstituted with B cells lacking EFNB1 did not show any impairment in GC formation or T_FH cell development in the draining lymph nodes 5 days after antigenic stimulation. However, at 7 days after stimulation, there were marked differences in the follicular and GC distribution of T_FH cells in the absence of B cell-expressed EFNB1 — in particular, more T_FH cells were found in GCs containing EFNB1-deficient B cells. Close tracking of T_FH cells in the two groups of mice using intravital microscopy revealed that T_FH cells entered the EFNB1-deficient GCs with a slightly increased frequency and left with a markedly reduced frequency, leading to an accumulation of T_FH cells in the absence of EFNB1.

Next, the authors tested whether EFNB1 might limit the retention of T_FH cells in GCs by disrupting antigen-specific T cell–B cell adhesion. Indeed, pre-activated T cells were less efficient in adhering to antigen-presenting B cells that expressed a high level of EFNB1 than to control B cells. Conversely, in a competitive GC reaction, EFNB1-deficient B cells more efficiently interacted with T_FH cells. To better understand the mechanism of EFNB1-mediated repulsion of GC T_FH cells, the authors used short hairpin RNA (shRNA) to knock down the expression of the T cell-expressed EFNB1 receptors EPHB4 and EPHB6. EPHB6 knockdown, but not EPHB4 knockdown, led to an accumulation of T_FH cells in the GCs, which suggests that EPHB6 mediates EFNB1-dependent repulsion of T_FH cells.

more T_FH cells were found in GCs containing EFNB1-deficient B cells

Contrary to expectation, the prolonged GC residence of T_FH cells occurring in the absence of the EFNB1–EPHB6 axis did not result in improved GC responses. Instead, the abundance of plasma cells in the spleen and bone marrow was reduced, which was accompanied by a decrease in serum antibody affinity for antigen. These defects were shown to be due to reduced levels of the T_FH cell-derived cytokine IL-21, which drives plasma cell development; T_FH cells from EFNB1-deficient GCs produced less IL-21 than those from control GCs. Knockdown of EFNB1 receptor expression indicated that EPHB4 is the receptor that transmits EFNB1 signals to stimulate IL-21 production by T_FH cells in the GC.

The combination of negative regulation of GC residence and positive regulation of effector functions of T_FH cells by EFNB1 seems counterintuitive, but the authors suggest that it may be a mechanism to ensure that the potentially dangerous GC reaction is productive but self-limiting.