Data are pooled from six (C) and three (D) independent experiments. (E) Titers of LCMV-binding serum IgG in and control variable regions in GC B cells in (B). promoted repeated rounds of divisions of selected GC B cells. B cell-specific deletion of AP4 resulted in reduced GC sizes and reduced somatic hypermutation coupled with a failure to control chronic viral infection. These results indicate that AP4 integrates T cell-mediated selection and sustained expansion of GC B cells for humoral immunity. Graphical Abstract Introduction Upon infection or vaccination, antigen (Ag)-specific lymphocytes that are present at low frequencies under steady-state conditions undergo rapid clonal expansion to increase the magnitude of adaptive immune responses. While expansion (S)-(-)-Bay-K-8644 of Ag-specific B cells ensures that sufficient quantities of antibodies are made, it also serves as a template for somatic hypermutation (SHM), affinity maturation, and the subsequent generation of protective humoral memory for long-term immunity (Basso and Dalla-Favera, 2015; Victora and Nussenzweig, 2012). These proliferating Ag-specific B cells form a specialized compartment in peripheral lymphoid organs, (S)-(-)-Bay-K-8644 the germinal centers (GCs), in which B cells cyclically migrate between the light zone (LZ) and the dark zone (DZ) for selection and subsequent clonal expansion, respectively (Allen et al., 2007; Basso and Dalla-Favera, 2015; Corcoran and Tarlinton, 2016; Victora and Nussenzweig, 2012; Victora et al., 2010). In the LZ, signals from follicular helper T (Tfh) cells facilitate the selection of clones that express B cell receptors (BCRs) with higher affinity for their cognate Ag relative to neighboring clones (Allen et al., 2007; MacLennan, 1994; Rajewsky, 1996; Shih et al., 2002; Victora and Nussenzweig, 2012). The important cues for clonal selection in the LZ include ligation of CD40 on B cells by CD40L on Tfh cells, as well as cytokines, especially interleukin-21 (IL-21) (Basso and Dalla-Favera, 2015; Castigli et al., 1994; Kawabe et al., 1994; Linterman et al., 2010; Renshaw et al., 1994; Victora and Nussenzweig, 2012; Xu et al., 1994; Zotos et al., 2010). GC B cells that receive these signals migrate to the DZ, where they rapidly divide multiple times and accumulate somatic mutations in their immunoglobulin (Ig) genes (Gitlin et al., 2014). GC B cells can then re-enter the LZ for additional rounds of selection followed by clonal expansion for further affinity maturation (Victora et al., 2010). This process allows for the emergence of B cell clones expressing high affinity antibodies that carry multiple Ig mutations (Kocks and Rajewsky, 1988). The magnitude of expansion of selected B cell clones is programmed by T cell help that B cells receive during their transient interaction with Tfh cells in the Rabbit Polyclonal to Cytochrome P450 3A7 LZ. Increased amounts of the cognate peptide Ags presented by B cells to Tfh cells in the context of MHC class II molecules induce elevated production of cytokines, IL-4 and IL-21 by Tfh cells (Shulman et al., 2014), and facilitate rapid expansion of selected B cells in the DZ and affinity maturation (Gitlin et al., 2015; Gitlin et al., 2014). Thus, the transient T-B interaction in the LZ induces gene expression programs that allow selected B cells to sustain their proliferation in the DZ and establish a diverse BCR repertoire. The transcription factor c-MYC regulates proliferation of both pre-GC B cells and GC B cells, while mutations or translocations of the gene are causally linked to GC-derived B cell lymphomas (Basso and Dalla-Favera, 2015; Calado et al., 2012; Dominguez-Sola et al., 2012). Although T cell help controls cell cycle progression of selected B cells by inducing c-MYC, expression of this proto-oncogene is detected only transiently in LZ B cells prior to their proliferation in the DZ (Calado et al., 2012; Dominguez-Sola et al., 2012; Gitlin et al., 2015; Victora et al., 2010). Thus, the identity of nuclear factors that are induced during the T-B interaction in the LZ and continue to be expressed in the DZ to potentiate proliferation (S)-(-)-Bay-K-8644 of selected B cells remains unknown. In this study, we dissected the genetic program that is activated in selected B cells during their transient interaction with Tfh cells in the LZ and supports sustained expansion of B cells in the DZ. Our data demonstrated that the transcription factor AP4 was essential for amplification of GC responses. AP4 was induced by c-MYC in B cells that received T cell help through the CD40-CD40L interaction and maintained by IL-21 as they migrated toward the DZ. Although AP4 was dispensable for selection of B cells and their cell cycle progression in the.