GSE195698
Post-mitotic G1 phase survivin drives mitogen-independent cell divisions of B lymphocytes
The prevailing dogma is that renewed mitogenic signaling is essential to traverse G1 phase of the cell cycle after each division. B lymphocytes undergo multiple mitotic divisions, termed clonal expansion, to expand antigen-specific cells that mediate effective immunity. We explored mechanisms by which primary murine B lymphocytes make cell division decisions. We demonstrate that commitment to DNA replication _S phase_ programs B cells to divide multiple times in the absence of overt mitogenic signaling. The extent of division is limited by cell death rather than by return to quiescence, and circumventing cell death permits up to 4 rounds of division in 72h. Mitogen-independent cell cycle progression is driven by unique S- and G2/M- like characteristics of the G1 phase of cells that have divided once, such as, large cell size, low levels of p27, phosphorylated Rb and expression of G2/M markers survivin, Cenp-A and Aim1/Aurora B. Pharmacologic inhibition of survivin blocked G1 progression in a B cell line and in primary B cells undergoing mitogen-independent division. These observations indicate that, in contrast to textbook models of the cell cycle, B cells inherit a partially active G1 phase after cell division that permits them to move quickly to the next S phase in the absence of exogenous G1 progression signals. Our studies provide direct evidence for Pardee's hypothesis that retention of features of G2/M in post-mitotic cells could trigger a second round of cell cycle progression. We propose that these mechanisms may assist in rapid cell division without differentiation that is required for clonal expansion in response to antigens. Overall design: RNA-seq from mouse splenic B cells. Naïve, activated and not-divided vs activated and divided