The germinal centre (GC) reaction supports affinity-based B-cell competition and generates high-affinity bone-marrow plasma cells (BMPCs). How follicular T-helper (TFH) cells regulate GC selection is not clear. Using competitive mixed chimaera, we show here that, beyond the role in promoting TFH development, ICOSL (inducible T-cell co-stimulator ligand, also known as ICOSLG) is important for individual B cells to competitively participate in the GC reaction and to develop into BMPCs. Using intravital imaging aided by a calcium reporter, we further show that ICOSL promotes an 'entangled' mode of TFH-B-cell interactions, characterized by brief but extensive surface engagement, productive T-cell calcium spikes, and B-cell acquisition of CD40 signals. Reiterated entanglement promotes outer-zone co-localization of outcompeting GC B cells together with TFH cells, affording the former increased access to T-cell help. ICOSL on GC B cells is upregulated by CD40 signals. Such an intercellular positive feedback between contact-dependent help and ICOSL-controlled entanglement promotes positive selection and BMPC development, as evidenced by observations that higher-affinity B-cell receptor variants are enriched in the ICOSL(high) fraction, that numerically disadvantaged ICOSL-deficient GC B cells or BMPCs exhibit strong affinity compensation in competitive chimaera, and that when GC competition proceeds without ICOSL, selection of high-affinity variants in otherwise normal GC reactions is impaired. By demonstrating entanglement as the basic form of GC TFH-B-cell interactions, identifying ICOSL as a molecular linkage between T-B interactional dynamics and positive selection for high-affinity BMPC formation, our study reveals a pathway by which TFH cells control the quality of long-lived humoral immunity.

Follicular T helper (T) cells orchestrate the germinal center (GC) reaction locally. Local mechanisms regulating their dynamics and helper functions are not well defined. Here we found that GC-expressed ephrin B1 (EFNB1) repulsively inhibited T cell to B cell adhesion and GC T retention by signaling through T-expressed EPHB6 receptor. At the same time, EFNB1 promoted interleukin-21 production from GC T cells by signaling predominantly through EPHB4. Consequently, EFNB1-null GCs were associated with defective production of plasma cells despite harboring excessive T cells. In a competitive GC reaction, EFNB1-deficient B cells more efficiently interacted with T cells and produced more bone-marrow plasma cells, likely as a result of gaining more contact-dependent help. Our results reveal a contact-dependent repulsive guidance system that controls GC T dynamics and effector functions locally.

Follicular T helper (T) cells orchestrate the germinal center (GC) response locally. T localization in GCs is controlled by chemo-guidance cues and antigen-specific adhesion. Here. we define an antigen-independent, contact-dependent, adhesive guidance system for T cells. Unusual for amoeboid cell migration, the system is composed of transmembrane plexin B2 (PlxnB2) molecule, which is highly expressed by GC B cells, and its transmembrane binding partner semaphorin 4C (Sema4C), which is upregulated on T cells. Sema4C on T cells serves as a receptor to sense the GC-presented PlxnB2 cue and biases T migration inwards at the GC edge to promote GC access. The absence of PlxnB2 from the GC or Sema4C from T cells causes T accumulation along the GC border, impairs T-B cell interactions in the GC, and is associated with defective plasma cell production and affinity maturation. Therefore, Sema4C and PlxnB2 regulate GC T recruitment and function and optimize antibody responses.