Notch Ligands Expressed by Follicular Dendritic Cells Protect Germinal Center B Cells from Apoptosis

The Notch signaling pathway is one of the most conserved mechanisms to regulate cell fate in many tissues during development and postnatal life. In the immune system, Notch signaling regulates T and B cell development and modulates the differentiation of T and B cells. In this study, we investigated the functional roles of Notch signaling in human B cell differentiation within the germinal center (GC). Notch ligands, Delta-like 1 (Dll1) and Jagged 1 (Jg1), are expressed by follicular dendritic cells (FDC) but not by B cells in the GC, while GC-B cells express the Notch receptors, Notch1 and Notch2. The blockade of Notch signaling pathways using a gamma-secretase inhibitor, DAPT (N-[N-(3,5-difluorophenacetyl-l-alanyl)]-S-phenylglycine t-butyl ester), reduces the survival of GC-B cells in the presence of FDC/HK cells. Jg1 has a dominant effect on GC-B cell survival mediated by Notch signaling. Furthermore, Notch cooperates with another anti-apoptotic factor, BAFF/Blys produced by FDC to support GC-B cell growth. Taken together, our data shows the important role of Notch signaling provided by FDC in the survival of GC-B cells in vitro.