LAUSR

Significance High-affinity antibody responses involve selection of B cells in the germinal center (GC) by cognate interactions with T follicular helper (TFH) cells, which in turn must first be activated by classical dendritic cells (cDCs). We observe that Notch2-dependent cDC2s are required in vivo for induction of TFH cells, GC B cells, and specific antibody production in response to sheep red blood cell (SRBC) immunization. Notch2 signaling impacted a broad transcriptional program in cDC2s both at homeostasis and after SRBC immunization, although we have not identified a target gene that mediates TFH differentiation. Thus, Notch2 is a transcription factor that acts in cDCs and is selectively required for support of the GC reaction. CD4+ T follicular helper (TFH) cells support germinal center (GC) reactions promoting humoral immunity. Dendritic cell (DC) diversification into genetically distinct subsets allows for specialization in promoting responses against several types of pathogens. Whether any classical DC (cDC) subset is required for humoral immunity is unknown, however. We tested several genetic models that selectively ablate distinct DC subsets in mice for their impact on splenic GC reactions. We identified a requirement for Notch2-dependent cDC2s, but not Batf3-dependent cDC1s or Klf4-dependent cDC2s, in promoting TFH and GC B cell formation in response to sheep red blood cells and inactivated Listeria monocytogenes. This effect was mediated independent of Il2ra and several Notch2-dependent genes expressed in cDC2s, including Stat4 and Havcr2. Notch2 signaling during cDC2 development also substantially reduced the efficiency of cDC2s for presentation of MHC class II-restricted antigens, limiting the strength of CD4 T cell activation. Together, these results demonstrate a nonredundant role for the Notch2-dependent cDC2 subset in supporting humoral immune responses.