Little is known about how cells regulate and integrate distinct biosynthetic pathways governing differentiation and cell division. For B-lineage cells it is widely accepted that activated cells must complete several rounds of mitosis before yielding antibody-secreting plasma cells. However, we report that marginal zone (MZ) B cells, innate-like naïve B cells known to generate plasma cells rapidly in response to blood-borne bacteria, generate functional plasma cells despite cell cycle arrest. Further, short-term Notch2 blockade in vivo reversed division-independent differentiation potential and decreased transcript abundance for numerous mTORC1-and Myc-regulated genes. Myc loss compromised plasma cell differentiation for MZ B cells, and reciprocally induced ectopic mTORC1 signaling in follicular B cells enabled division-independent differentiation and plasma cell-affiliated gene expression. We conclude that ongoing in situ Notch2/mTORC1 signaling in MZ B cells establishes a unique cellular state that enables rapid division-independent plasma cell differentiation.

Lymphocyte differentiation is often tightly linked to mitosis. Clonal bursts due to antigen- or TLR-driven responses increase numbers of responding cells and may also facilitate changes in gene expression and chromatin needed for effector cell differentiation. Alternatively, to guard against rapid infection, lymphocyte pools may also contain cells poised for effector differentiation with minimal proliferation. Using cell cycle inhibitors or induced mutation of CDK1, we directly compared the impact of arresting mitosis on early plasma cell (PC) differentiation for naive follicular and marginal zone (MZ) B cells. MZ B cells reside in the marginal sinus of the spleen where they are positioned to respond rapidly to blood borne microbes. Whereas PC differentiation from follicular B cells occurred after only 4 or more divisions and was highly dependent on mitosis, MZ B cells yielded PCs much faster and despite full cell cycle blockade. Furthermore, short-term withdrawal of Notch signaling in MZ B cells in vivo caused rapid and robust down-regulation of large numbers of established Myc-regulated genes, and resulted in division/differentiation kinetics that mirrored follicular B cells. Altogether these results suggest that ongoing Notch signaling establishes a differentiation-poised state in MZ B cells needed for rapid division-independent antibody responses.

No abstract

Lymphocyte differentiation is often tightly linked to mitosis. Clonal bursts due to antigen- or TLR-driven responses increase numbers of responding cells and may also facilitate changes in gene expression and chromatin needed for effector cell differentiation. Alternatively, to guard against rapid infection, lymphocyte pools may also contain cells poised for effector differentiation with minimal proliferation. Since marginal zone (MZ) B cells, which have increased basal mTORC1 activity and c-Myc expression, reside in the marginal sinus of the spleen where they are positioned to respond rapidly to blood borne microbes, we compared their ability to produce plasma cells (PC) with that of follicular B cells in an ex-vivo stimulation assay. We found that MZ B cells are able to produce PCs rapidly and independently of cell division whether exposed to cell cycle inhibitors or with genetic ablation of Cdk1. Furthermore, we found that blockade of Notch signaling rapidly down-regulated c-Myc-driven gene expression and mTORC1 activity in MZ B cells and caused them to adopt differentiation kinetics similar to follicular B cells. Genetic ablation of c-Myc further emphasized the uncoupling of cell division and differentiation in MZ B cells. Whereas acute loss of c-Myc blocked all cell division in ex-vivo stimulation assays, PC differentiation was only blocked with prolonged loss of c-Myc. Finally, using genetic ablation of Tsc1 to disinhibit mTORC1 signaling, we conferred both rapid and division-independent plasma cell differentiation to follicular B cells. These data illustrate that ongoing Notch signaling through c-Myc and mTORC1 allow for rapid division-independent antibody responses in MZ B cells.