Class-switch DNA recombination (CSR) and somatic hypermutation (SHM), which require AID, and plasma cell differentiation, which requires Blimp-1, are critical for the generation of class-switched and hypermutated (mature) antibody and autoantibody responses. We showed here that the histone deacetylase (HDAC) inhibitors (HDI) valproic acid (VPA) and butyrate upregulated miR-155, miR-181b and miR-361, which silenced AICDA/Aicda (AID) mRNA, and miR-23b, miR-30a and miR-125b, which silenced PRDM1/Prdm1 (Blimp-1) mRNA, in human and mouse B cells. This led to downregulation of AID, Blimp-1 and Xbp-1 expression, thereby dampening CSR, SHM and plasma cell differentiation without altering B cell viability or proliferation. The selectivity of HDI-mediated silencing of AICDA/Aicda and PRDM1/Prdm1 was emphasized by unchanged expression of HoxC4 and Irf4 (important inducers/modulators of AICDA/Aicda), Rev1 and Ung (central elements for CSR/SHM), and Bcl6, Bach2 or Pax5 (repressors of PRDM1/Prdm1 expression), as well as unchanged expression of miR-19a/b, miR-20a and miR-25, which are not known to regulate AICDA/Aicda or PRDM1/Prdm1. Through these B cell intrinsic epigenetic mechanisms, VPA blunted class-switched and hypermutated T-dependent and T-independent antibody responses in C57BL/6 mice. In addition, it decreased class-switched and hypermutated autoantibodies, ameliorated disease and extended survival in lupus MRL/Faslpr/lpr mice. Our findings outline epigenetic mechanisms that modulate expression of an enzyme (AID) and transcription factors (Blimp-1 and Xbp-1) that critical to the B cell differentiation processes that underpin antibody and autoantibody responses. They also provide therapeutics proof-of-principle in autoantibody-mediated autoimmunity.

SUMMARY Class switch DNA recombination (CSR) is central to the antibody response, as it changes the immunoglobulin heavy chain (IgH) constant region, thereby diversifying biological effector functions of antibodies. The AID-centered CSR machinery excises and rejoins DNA between an upstream (donor) and a downstream (acceptor) S region, which precede the respective constant region DNA. AID is stabilized on S regions by 14-3-3 adaptors, which display a high affinity for 5’-AGCT-3’ repeats, as recurring in all S regions. However, how 14-3-3, AID and the CSR machinery target exclusively the donor and acceptor S regions is poorly understood. Here we showed that histone methyltransferases and acetyltransferases were induced by CD40- or TLR-signaling and catalyzed H3K4me3 and H3K9ac/K14ac histone modifications, which were enriched in S regions but did not specify the S region target of CSR. By contrast, the combinatorial H3K9acS10ph modification specifically marked the S regions set to recombine and directly recruited 14-3-3 adaptors for AID stabilization there. Inhibition of the enzymatic activity of GCN5 and PCAF histone acetyltransferases reduced H3K9acS10ph in S regions, 14-3-3 and AID stabilization, and CSR. Thus, H3K9acS10ph is a histone code that is specifically “written” in S regions and “read” by 14-3-3 adaptors to target AID for CSR as an important biological outcome.

Ig class switch DNA recombination (CSR) in B cells is crucial to the maturation of antibody responses. It requires IgH germline IH-CH transcription and expression of AID, both of which are induced by engagement of CD40 or dual engagement of a Toll-like receptor (TLR) and B cell receptor (BCR). Here, we have addressed cross-regulation between two different TLRs or between a TLR and CD40 in CSR induction by using a B cell stimulation system involving lipopolysaccharides (LPS). LPS mediated long-term primary class-switched antibody responses and memory-like antibody responses in vivo and induced generation of class-switched B cells and plasma cells in vitro. Consistent with the requirement for dual TLR and BCR engagement in CSR induction, LPS, which engages TLR4 through its lipid A moiety, triggered cytosolic Ca2+ flux in B cells through its BCR-engaging polysaccharidic moiety. In the presence of BCR crosslinking, LPS synergized with a TLR1/2 ligand (Pam3CSK4) in CSR induction, but much less efficiently with a TLR7 (R-848) or TLR9 (CpG) ligand. In the absence of BCR crosslinking, R-848 and CpG, which per se induced marginal CSR, virtually abrogated CSR to IgG1, IgG2a, IgG2b, IgG3 and/or IgA, as induced by LPS or CD154 (CD40 ligand) plus IL-4, IFN-γ or TGF-β, and reduced secretion of class-switched Igs, without affecting B cell proliferation or IgM expression. The CSR inhibition by TLR9 was associated with the reduction in AID expression and/or IgH germline IH-S-CH transcription, and required co-stimulation of B cells by CpG with LPS or CD154. Unexpectedly, B cells also failed to undergo CSR or plasma cell differentiation when co-stimulated by LPS and CD154. Overall, by addressing the interaction of TLR1/2, TLR4, TLR7 and TLR9 in the induction of CSR and modulation of TLR-dependent CSR by BCR and CD40, our study suggests the complexity of how different stimuli cross-regulate an important B cell differentiation process and an important role of TLRs in inducing effective T-independent antibody responses to microbial pathogens, allergens and vaccines.