[Show abstract][Hide abstract] ABSTRACT: Inhibition of Cdk4/Cdk6 by p18(INK4c) (p18) is pivotal for generation of noncycling immunoglobulin (Ig)-secreting plasma cells (PCs). In the absence of p18, CD138(+) plasmacytoid cells continue to cycle and turnover rapidly, suggesting that p18 controls PC homeostasis. We now show that p18 selectively acts in a rare population of rapidly cycling CD138(hi)/B220(hi) intermediate PCs (iPCs). While retaining certain B-cell signatures, iPCs are poised to differentiate to end-stage PCs although the majority undergo apoptosis. p18 is dispensable for the development of the PC transcriptional circuitry, and Blimp-1 and Bcl-6 are expressed fully and mutually exclusively in individual iPCs. However, a minor proportion of iPCs express both, and they are preferentially protected by p18 or Bcl-xL overexpression, consistent with expansion of the iPC pool by Bcl-xL overexpression, or loss of proapoptotic Bim or Noxa. Expression of Noxa is induced during B-cell activation, peaks in iPCs, and selectively repressed by p18. It is required to promote apoptosis of cycling B cells, especially in the absence of p18. These findings define the first physiologic function for Noxa and suggest that by repressing Noxa, induction of G₁ arrest by p18 bypasses a homeostatic cell-cycle checkpoint in iPCs for PC differentiation.
[Show abstract][Hide abstract] ABSTRACT: Cell-cycle entry is critical for homeostatic control in physiologic response of higher organisms but is not well understood. The antibody response begins with induction of naive mature B cells, which are naturally arrested in G(0)/G(1) phase of the cell cycle, to enter the cell cycle in response to antigen and cytokine. BLyS (BAFF), a cytokine essential for mature B cell development and survival, is thought to act mainly by attenuation of apoptosis. Here, we show that BLyS alone induces cell-cycle entry and early G(1) cell-cycle progression, but not S-phase entry, in opposition to the cyclin-dependent kinase inhibitors p18(INK4c). Independent of its survival function, BLyS enhances the synthesis of cyclin D2, in part through activation of NF-kappaB, as well as CDK4 and retinoblastoma protein phosphorylation. By convergent activation of the same cell-cycle regulators in opposition to p18(INK4c), B cell receptor signaling induces cell-cycle entry and G(1) progression in synergy with BLyS, but also DNA replication. The failure of BLyS to induce S-phase cell-cycle entry lies in its inability to increase cyclin E and reduce p27(Kip1) expression. Antagonistic cell-cycle regulation by BLyS and p18(INK4c) is functionally linked to apoptotic control and conserved from B cell activation in vitro to antibody response in vivo, further indicating a physiologic role in homeostasis.
Full-text · Article · Jan 2005 · Proceedings of the National Academy of Sciences