T follicular helper cell differentiation, function, and roles in disease

Although T cell help for B cells was described several decades ago, it was the identification of CXCR5 expression by B follicular helper T (Tfh) cells and the subsequent discovery of their dependence on BCL6 that led to the recognition of Tfh cells as an independent helper subset and accelerated the pace of discovery. More than 20 transcription factors, together with RNA-binding proteins and microRNAs, control the expression of chemotactic receptors and molecules important for the function and homeostasis of Tfh cells. Tfh cells prime B cells to initiate extrafollicular and germinal center antibody responses and are crucial for affinity maturation and maintenance of humoral memory. In addition to the roles that Tfh cells have in antimicrobial defense, in cancer, and as HIV reservoirs, regulation of these cells is critical to prevent autoimmunity. The realization that follicular T cells are heterogeneous, comprising helper and regulatory subsets, has raised questions regarding a possible division of labor in germinal center B cell selection and elimination.

Follicular Helper T Cells

T Follicular Helper Cell Biology: A Decade of Discovery and Diseases

Humoral immunity depends on the germinal centre (GC) reaction during which somatically mutated high-affinity memory B cells and plasma cells are generated. Recent studies have uncovered crucial cues that are required for the formation and the maintenance of GCs and for the selection of high-affinity antibody mutants. In addition, it is now clear that these events are promoted by the dynamic movements of cells within and between GCs. These findings have resolved the complexities of the GC reaction in greater detail than ever before. This Review focuses on these recent advances and discusses their implications for the establishment of humoral immunity.

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Dynamics of B cells in germinal centres

Germinal Center B Cell Dynamics

Germinal centres support antibody affinity maturation and memory formation. Follicular T-helper cells promote proliferation and differentiation of antigen-specific B cells inside the follicle. A genetic deficiency in the inducible co-stimulator (ICOS), a classic CD28 family co-stimulatory molecule highly expressed by follicular T-helper cells, causes profound germinal centre defects, leading to the view that ICOS specifically co-stimulates the follicular T-helper cell differentiation program. Here we show that ICOS directly controls follicular recruitment of activated T-helper cells in mice. This effect is independent from ICOS ligand (ICOSL)-mediated co-stimulation provided by antigen-presenting dendritic cells or cognate B cells, and does not rely on Bcl6-mediated programming as an intermediate step. Instead, it requires ICOSL expression by follicular bystander B cells, which do not present cognate antigen to T-helper cells but collectively form an ICOS-engaging field. Dynamic imaging reveals ICOS engagement drives coordinated pseudopod formation and promotes persistent T-cell migration at the border between the T-cell zone and the B-cell follicle in vivo. When follicular bystander B cells cannot express ICOSL, otherwise competent T-helper cells fail to develop into follicular T-helper cells normally, and fail to promote optimal germinal centre responses. These results demonstrate a co-stimulation-independent function of ICOS, uncover a key role for bystander B cells in promoting the development of follicular T-helper cells, and reveal unsuspected sophistication in dynamic T-cell positioning in vivo.

Follicular T-helper cell recruitment governed by bystander B cells and ICOS-driven motility

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.

T–B-cell entanglement and ICOSL-driven feed-forward regulation of germinal centre reaction

Antibody affinity maturation depends on positive selection in germinal centres (GCs) of rare B cell clones that acquire higher-affinity B cell receptors via somatic hypermutation, present more antigen to follicular helper T (TFH) cells and, consequently, receive more contact-dependent T cell help1. As these GC B cells and TFH cells do not maintain long-lasting contacts in the chaotic GC environment2–4, it is unclear how sufficient T cell help is cumulatively focused onto those rare clones. Here we show that, upon stimulation of CD40, GC B cells upregulate the chemokine CCL22 and to a lesser extent CCL17. By engaging the chemokine receptor CCR4 on TFH cells, CCL22 and CCL17 can attract multiple helper cells from a distance, thus increasing the chance of productive help. During a GC response, B cells that acquire higher antigen-binding affinities express higher levels of CCL22, which in turn ‘highlight’ these high-affinity GC B cells. Acute increase or blockade of TFH cells helps to rapidly increase or decrease CCL22 expression by GC B cells, respectively. Therefore, a chemokine-based intercellular reaction circuit links the amount of T cell help that individual B cells have received recently to their subsequent ability to attract more help. When CCL22 and CCL17 are ablated in B cells, GCs form but B cells are not affinity-matured efficiently. When competing with wild-type B cells in the same reaction, B cells lacking CCL22 and CCL17 receive less T cell help to maintain GC participation or develop into bone-marrow plasma cells. By uncovering a chemokine-mediated mechanism that highlights affinity-improved B cells for preferential help from TFH cells, our study reveals a principle of spatiotemporal orchestration of GC positive selection. CCL22 promotes positive selection in germinal centres by highlighting affinity-enhanced B cells for helper T cells to sense and seek remotely.

Dan Liu

Papers

Follicular T-helper cell recruitment governed by bystander B cells and ICOS-driven motility

T–B-cell entanglement and ICOSL-driven feed-forward regulation of germinal centre reaction

Affinity-coupled CCL22 promotes positive selection in germinal centres

Bcl-6 controlled TFH polarization and memory: the known unknowns

Follicular dendritic cells restrict interleukin-4 availability in germinal centers and foster memory B cell generation