Competition for refueling rather than cyclic reentry initiation evident in germinal centers

Abstract

Antibody affinity maturation occurs in germinal centers (GCs) through iterative rounds of somatic hypermutation and proliferation in dark zones (DZs) and selection in light zones (LZs). GC B cells exit cell cycle a number of hours before entering LZs; therefore, continued participation in responses requires that they subsequently reenter cell cycle and move back to DZs, a process known as cyclic reentry. Affinity enhancements are thought to arise by B cells having to compete to initiate cyclic reentry each time they enter LZs, with T cell help being a major determinant; however, direct proof is lacking. Using Fucci2 mice, we confirmed an association between B cell receptor affinity and the first step of cyclic reentry, S phase initiation from a resting LZ state. However, neither T cell ablation nor MHCII deletion prevented resting LZ cells from reentering cell cycle, and this late G 1 -S transition was also not detectably restricted by competition. In contrast, using BATF induction as exemplar, we found that T cells “refueled” LZ cells in an affinity-dependent manner that was limited by both competition and cells’ intrinsic antigen-acquiring abilities. Therefore, cyclic reentry initiation and B cell refueling are independently regulated in GCs, which may contribute to permitting cells of different competencies to be sustained alongside each other and allow T cell support to be provided across a dynamic range commensurate with affinity. We speculate that this less binary selection mechanism could help GCs nurture complex antibody maturation pathways and support the clonal diversity required for countering fast-evolving pathogens.

APRIL Drives a Coordinated but Diverse Response as a Foundation for Plasma Cell Longevity

“…Single cellbased approaches may provide a means to resolve such interesting questions of heterogeneity in response to niche signals in the future. Considering the focus in the present study on the shared response induced by both TNFSF members we note that at a functional level this includes expression features that would be consistent with the recently proposed concept of cellular refueling observed in light zone germinal center B cells at the point of follicular helper T cell encounter (31). We conclude that support delivered by APRIL for PC survival is delivered by a broad range of signaling pathways and converges on growth-related gene expression while being uncoupled from support for the B cell state.…”

IL-21 has a critical role in establishing germinal centers by amplifying early B cell proliferation

“…Our findings that IL-21 increased the speed of passage through and frequency of entry into the cell cycle and promoted GC B cell accumulation over a large range of BCR affinities together with its previously reported roles in maintaining GC [27,28] and the LZ/DZ ratio [29,46] are all indicative of the key role of IL-21 in the initiation of a TD immune response being to promote the proliferation of pre-GC and GC B cells. We previously described the role of IL-21 in GC LZ B cell proliferation [29] while another recent study showed that the cyclic re-entry of LZ GC B cells could still occur when MHC-II or T cells had recently been deleted [72]. These results, with those presented here, suggest to us that the lingering presence of IL-21 may sustain GC B-cell proliferation following its initiation by cell contact mediated mitogenic signals.…”

The metabolic plasticity of B cells

“…Importantly, mTORC1 activity is strictly required to trigger cell growth but is largely dispensable after activated B cells increase in size and start to execute rapid rounds of cell division (Ersching et al, 2017). The inability to shut off mTORC1 activity by means of constitutively-active nutrient or growth factor signaling to mTORC1 is detrimental and decreases fitness of GC B cells (Goldfinger et al, 2011;Ersching et al, 2017;Long et al, 2022). Interestingly, a moderate, partial increase in nutrient-Rag GTPase signaling results in enhanced B-cell activation, enlarged GC and increased high-affinity antibody production (Ortega-Molina et al, 2019).…”