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

Abstract

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.

Circulating CXCR5+CXCR3+PD-1lo Tfh-like cells in HIV-1 controllers with neutralizing antibody breadth

“…HIV-1 controllers who had maintained <1,500 copies/ml HIV-1 viral loads (VL) for a median of 5 years (range [2][3][4][5][6][7][8][9][10][11][12][13][14] in the absence of antiretroviral therapy, with (neutralizers [NT], n = 25, median VL 123 copies/ml, range 20-1,400 copies/ml; median CD4 counts 769.5 cells/ml, range 418-1545 cells/ml) or without (non-neutralizers [NN], n = 20; median VL 75 copies/ml, range 48-1,470 copies/ml; median CD4 counts 844.5 cells/ml, range 407-2,117 cells/ml) neutralizing antibodies against Tier-2/3 HIV-1 viruses; untreated chronic progressors (CP; n = 14, median VL 28,352.5 copies/ml, range 2,368-90,500 copies/ml; median CD4 + T cell counts 313 cells/ml, range 88-955 cells/ml); ART-treated chronically HIV-1-infected patients with suppressed HIV-1 viremia (H; n = 14, median VL <49 copies/ml; median CD4 + T cell counts 490 cells/ml, range 183-964 cells/ml); and HIV-1-seronegative healthy persons (NG; n = 14) were recruited for this study. Samples of mononuclear cells extracted from inguinal lymph nodes were obtained by surgical excision from HIV-1-negative study persons.…”

“…Tfh cells are phenotypically characterized by the expression of the surface receptor CXCR5, and their developmental program is regulated by the master transcription factor Bcl-6 (5, 6). Functionally, Tfh cells enhance maturation, Ig class switching, and affinity maturation in B cells by secreting cytokines such as IL-21 and IL-4 (7,8), and through contact-dependent mechanisms (9,10). The molecular and cellular signals necessary for Tfh development represent an area of active investigation, but current data from experimental animal models suggest that antigen presentation by DCs is necessary and sufficient to initiate a Tfh development program (11,12), while cognate interactions with activated B cells seem required to sustain DC-primed Tfh cells (13).…”

The transcriptional coactivator Bob1 promotes the development of follicular T helper cells via Bcl6

“…Activated B cells and Tfh cells that eventually move from the T‐B border into the follicle seed GCs in which GC‐Tfh cells promote the growth and survival of high‐affinity antibody producing GC B cells. At the same time, GC‐Tfh cells are thought to be a major driver of antibody affinity maturation by governing the positive selection of GC B cells with highest affinity for a given antigen (Allen et al , 2007; Victora et al , 2010; Liu et al , 2014a). …”

A Stochastic Model of the Germinal Center Integrating Local Antigen Competition, Individualistic T–B Interactions, and B Cell Receptor Signaling

“…In contrast, it has recently been shown that ICOS ligand (ICOSL), an important costimulatory ligand expressed by all GC B cells, promotes T-B entanglement, a form of contact that involves more extensive surface engagement and thus more efficient delivery of CD40L from the Tfh cells to the B cells (21). Furthermore, together with Tfh-derived CD40L, a B cell-expressed ICOSL constitutes an intercellular positive-feedback loop that accelerates affinity maturation (21). These results suggest that accessory molecules regulating individual cross-talks between Tfh and GC B cells may significantly impinge on qualitative and quantitative nature of affinity maturation.…”

“…In particular, Meyer-Hermann models apparently do not allow individual T-B contacts be characterized or modulated (15), leaving mechanistically how T-B interactions in GCs promote affinity maturation largely unexplored in sillico. In contrast, it has recently been shown that ICOS ligand (ICOSL), an important costimulatory ligand expressed by all GC B cells, promotes T-B entanglement, a form of contact that involves more extensive surface engagement and thus more efficient delivery of CD40L from the Tfh cells to the B cells (21). Furthermore, together with Tfh-derived CD40L, a B cell-expressed ICOSL constitutes an intercellular positive-feedback loop that accelerates affinity maturation (21).…”