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

Abstract

Ab-secreting cells survive in niche microenvironments, but cellular responses driven by particular niche signals are incompletely defined. The TNF superfamily member a proliferation-inducing ligand (APRIL) can support the maturation of transitory plasmablasts into long-lived plasma cells. In this study, we explore the biological programs established by APRIL in human plasmablasts. Under conditions allowing the maturation of ex vivo– or in vitro–generated plasmablasts, we find that APRIL drives activation of ERK, p38, and JNK, accompanied by a classical NF-κB response and activation of the AKT/FOXO1 pathway. Time-course gene expression data resolve coordinated transcriptional responses propagated via immediate early genes and NF-κB targets and converging onto modules of genes enriched for MYC targets and metabolism/cell growth–related pathways. This response is shared between APRIL and an alternate TNF superfamily member CD40L but is not a feature of alternative niche signals delivered by IFN-α or SDF1. However, APRIL and CD40L responses also diverge. CD40L drives expression of genes related to the activated B cell state whereas APRIL does not. Thus, APRIL establishes a broad foundation for plasma cell longevity with features of cellular refueling while being uncoupled from support of the B cell state.

Enforced MYC expression selectively redirects transcriptional programs during human plasma cell differentiation

“…[59] A distinct pattern of NFκB activation is subsequently reintroduced upon addition of APRIL at day 6 which supports differentiation to the PC stage. [57] In the context of this model peripheral blood memory B cells were transduced on day 2 of activation with MYC T58I-t2A-BCL2 retroviral vector (henceforth T58I-t2A-BCL2) and were then returned to CD40 stimulating conditions for 24h before progressing into the differentiation protocol with removal of CD40 stimulation at day 3 (Figure 1a, Sup. Figure 1a).…”

“…[15,53,54] We have developed models of human B-cell activation that are permissive for differentiation to a long-lived PC state. [55][56][57] recapitulating physiological PC differentiation. [55] At the heart of the process of PC differentiation is a coordinated reorganization of transcription factors.…”

Integration of chromatin accessibility and gene expression data with cisREAD reveals a switch from PU.1/SPIB-driven to AP-1-driven gene regulation during B cell activation

“…Cells were sampled by careful removal from the stromal cell layer at indicated time points. Cells were transferred at day 3 to conditions with cytokines IL-2 and IL-21 alone, and plasmablasts at day 6 were driven towards a long-lived plasma cell phenotype through further cytokine signalling (IL-21, IL-6 and APRIL) and the addition of γ-Secretase Inhibitor (GSI) (19). ATAC-seq and RNA-seq experiments, measuring chromatin accessibility and gene expression, were performed at 9 time-points across the in vitro differentiation process, yielding a dataset of 19 samples (Figure 1B).…”

“…ATAC-seq and RNA-seq datasets have been deposited in GEO under the accession GSE219012 (Password/token for reviewers: ehghwggelhgxzmf) ATAC-seq signal tracks for the 19 factor at CREs, in each cluster, with PU.1/SPIB footprints only (yellow), PU.1/SPIB and AP-1 footprints (blue) and AP-1 footprints (pink) to all DARs without the given footprint. D) Gene sets enriched (one-sided fisher test, FDR < 0.1) in genes in each cluster, linked to regulatory elements with PU.1/SPIB (orange), PU.1/SPIB + AP-1 (blue) and AP-1 (red) footprints, compared to genes in other clusters.…”

“…Here we introduce a new method, based on correlating differential cis-regulatory element (CRE) accessibility with differential gene expression, designed to identify and prioritize transcription-factor bound cis-regulatory elements across differentiation (cisREAD). We apply this method to 19 sample-matched ATAC-seq and RNA-seq samples taken from up to three donors at fine-grained intervals across human in vitro B cell activation stimulated by CD40L and plasma cell differentiation induced by APRIL (18, 19).…”