Defining AP4- and MYC-regulated gene expression programs in germinal center B cells during cyclic re-entry

B cells diversify and affinity-mature their antigen receptor repertoire in germinal centers (GC). GC B cells receive help signals during transient interaction with T cells, yet it remains unknown how these transient T-B interactions sustain the subsequent proliferative program of selected B cells. Here we show that the transcription factor AP4 is required for sustained GC B cell proliferation and subsequent establishment of a diverse and protective antibody repertoire. AP4 is induced by c-MYC during the T-B interactions and maintained by T cell-derived IL-21. B cell-specific deletion of AP4 resulted in reduced GC sizes and somatic hypermutation and 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. Splenic B cells activated with anti-IgM and CD40L in vitro and germinal center B cells sorted from C57BL/6 mice eight days after immunization with sheep red blood cells (SRBCs) were obtained and genome-wide occupancy of c-MYC, AP4 and active histone marks was profiled by chromatin immunoprecipitation and high-throughput sequencing. Gene expression in MYC–AP4– LZ, MYC+AP4+ LZ, AP4+ DZ, and AP– DZ GC B cells from SRBC-immunized AP4-mCherry / c-MYC-GFP dual reporter mice was profiled by RNA-seq.