Jennifer Londregan (Mentor: David Allman, PhD)
“SNotch2 signals amplify BCR-driven responses”
Jennifer Londregan, Brian Gaudette and David Allman
The timing and efficiency with which B cells respond to activation signals vary depending on the subset. Whereas marginal zone (MZ) B cells exhibit rapid activation and kinetics following stimulation, follicular (FO) B cells adopt delayed effector programs. The development and maintenance of MZ B cells uniquely requires signaling through transmembrane receptor family member Notch2, but whether this signal underlies the functional distinction among B cell subsets is not known. To examine the functional consequences of Notch2 signals on B cells with no distinct Notch requirement, I cocultured FO B cells on OP9 stromal cells expressing Notch2 ligand delta-like 1 (DLL1) and stimulated them via B cell receptor (BCR) crosslinking to track the division kinetics over time. Notch2-experienced FO B cells displayed a marked enhancement of several aspects of proliferation. For one, DLL1 was found to drive two-fold more rounds of division than non-Notch2 experienced FO B cells. Further, sensitivity to BCR crosslinking was enhanced by DLL1, enabling FO B cells to enter cell cycle 24 hours earlier and in response to lower concentrations of anti-BCR. Finally, Notch2 was found to augment a higher precursor frequency and a higher fold-expansion of those responders compared to Notch2-inexperienced B cells. To get at a mechanism by which Notch2 modifies the proliferation capacity of B cells, I examined the timing and magnitude of Myc induction, a mitotic regulator that is a known Notch target, in conjunction with DLL1 and BCR ligation. Whereas the magnitude of Myc appears identical regardless of Notch2 experience, Notch2 signals appear to increase the percentage cells that induce Myc and sustains their Myc signal for longer. Together these observations highlight a novel role for Notch2 signals in modifying the proliferative response to antigen receptor crosslinking in B cells, potentially by enhancing the frequency of cells that turn on Myc.