ABSTRACT: Long-term humoral immunity is partly maintained by memory B cells (MBCs). MBCs can be recalled to produce antibody-producing plasma cells (PCs) or form secondary germinal centers (GCs). The former process underlies markedly increased antigen-specific antibodies seen in the secondary response, while the latter process allows continuous somatic hypermutation and affinity maturation. MBC re-participation in the GC reaction is thought to be important for generating broadly neutralizing antibodies against highly mutating viruses such as influenza and HIV. However, how MBC recall is transcriptionally or epigenetically programmed to produce secondary PCs or GCs is not yet understood. Here we show that BACH2, a transcription factor required for GC formation and maintenance1,2, decreases across post-activation stages of B cells, from the naïve state to GC, memory, and the terminal PC state. In contrast, BLIMP-1, a transcription factor that promotes PC formation3,4 and is antagonistic to BACH2 (ref. 5), increases as B cells traverse the same course. The relative strength between BLIMP-1 and BACH2 progressively increases in favor of BLIMP-1 in GCs and MBCs through the primary response. MBCs of the isotype or subset identity that preferentially give rise to secondary GCs exhibit comparatively higher BACH2 but lower BLIMP-1 expression than those predisposed for PC formation. Using a tetracycline-controlled circuit implanted in MBCs, we show that skewing the BLIMP-1-BACH2 balance in favor of BACH2 markedly increases GC formation by otherwise PC-predisposed MBCs, whereas skewing the balance in favor of BLIMP-1 drives PC formation by GC-prone MBCs, indicating that the potential for GC re-participation or PC development rests in the relative strength of BLIMP-1 over BACH2 within individual MBCs. Underneath this relative BLIMP-1-over-BACH2 strength in naïve B cells, GCs, MBCs and PCs, we observe progressively increased accessibilities to chromatin loci that are specifically opened in PCs, particularly those that contain ISRE elements and are controlled by IRF-4. IRF-4 is universally upregulated in B cells when stimulated through the BCR, the CD40 receptor, or by innate stimuli. By analyzing time-stamped GC B cells, we demonstrate a progressive increase in chromatin accessibilities at PC-specific, IRF-4-controlled gene loci over time. Our results support a model of progressive IRF-4 imprinting in which the cumulative stimulation history of B cells is epigenetically recorded in an IRF-4-dependent manner, determines the relative strength between BLIMP-1 and BACH2 in individual MBCs, and thereby dictates their individual probabilities to develop into GCs or PCs upon re-stimulation.
