The cytidine deaminase AID is required for antibody gene diversification. Casali and colleagues show that the transcription factor HoxC4 is expressed in germinal centers and is required for AID expression. The cytidine deaminase AID (encoded by Aicda in mice and AICDA in humans) is critical for immunoglobulin class-switch recombination (CSR) and somatic hypermutation (SHM). Here we show that AID expression was induced by the HoxC4 homeodomain transcription factor, which bound to a highly conserved HoxC4-Oct site in the Aicda or AICDA promoter. This site functioned in synergy with a conserved binding site for the transcription factors Sp1, Sp3 and NF-κB. HoxC4 was 'preferentially' expressed in germinal center B cells and was upregulated by engagement of CD40 by CD154, as well as by lipopolysaccharide and interleukin 4. HoxC4 deficiency resulted in impaired CSR and SHM because of lower AID expression and not some other putative HoxC4-dependent activity. Enforced expression of AID in Hoxc4−/− B cells fully restored CSR. Thus, HoxC4 directly activates the Aicda promoter, thereby inducing AID expression, CSR and SHM.

AID is a DNA cytidine deaminase with a central role in the generation of antibody diversity. AID initiates class switch recombination by modifying DNA within the switch (S) regions in the IgH locus. Now AID is shown to be targeted to the S regions by interaction with adaptor proteins 14-3-3, which in turn bind directly to the DNA motif 5'AGCT. Class switch DNA recombination (CSR) is the mechanism that diversifies the biological effector functions of antibodies. Activation-induced cytidine deaminase (AID), a key protein in CSR, targets immunoglobulin H (IgH) switch regions, which contain 5′-AGCT-3′ repeats in their core. How AID is recruited to switch regions remains unclear. Here we show that 14-3-3 adaptor proteins have an important role in CSR. 14-3-3 proteins specifically bound 5′-AGCT-3′ repeats, were upregulated in B cells undergoing CSR and were recruited with AID to the switch regions that are involved in CSR events (Sμ→Sγ1, Sμ→Sγ3 or Sμ→Sα). Moreover, blocking 14-3-3 by difopein, 14-3-3γ deficiency or expression of a dominant-negative 14-3-3σ mutant impaired recruitment of AID to switch regions and decreased CSR. Finally, 14-3-3 proteins interacted directly with AID and enhanced AID-mediated in vitro DNA deamination, further emphasizing the important role of these adaptors in CSR.

Expression and activity of activation-induced cytidine deaminase (AID) encoded by the aicda gene are essential for immunoglobulin (Ig) gene somatic hypermutation (SHM) and class switch DNA recombination (CSR). SHM and CSR unfold, in general, in germinal centers and/are central to the maturation of effective antibody responses. AID expression is induced by activated B-cell CD40 signaling, which is critical for the germinal center reaction, and is further enhanced by other stimuli, including interleukin-4 (IL-4) secreted from CD4⁺ T cells or Toll-like receptor (TLR)-activating bacterial and/or viral molecules. Integration of different intracellular signal transduction pathways, as activated by these stimuli, leads to a dynamic aicda-regulating program, which involves both positively acting trans-factors, such as Pax5, HoxC4, E47, and Irf8, and negative modulators, such as Blimp1 and Id2, to restrict aicda expression primarily to germinal center B cells. The phosphatidylinositol 3-kinase (PI 3-K), which functions downstream of activated B-cell receptor (BCR) signaling, likely plays an important role in triggering the downregulation of aicda expression in postgerminal center B cells and throughout plasmacytoid differentiation. In B cells undergoing SHM and CSR, AID activity, and, possibly, AID targeting to the Ig locus are regulated at a posttranslational level, including AID dimerization/oligomerization, nuclear/cytoplasmic AID translocation, and phosphorylation of the AID Ser38 residue by protein kinase A (PKA). Here, we discuss the role of B-cell activation signals, transcription regulation programs, and posttranslational modifications in controlling aicda expression and AID activity, thereby delineating an integrated model of modulation of SHM and CSR in the germinal center reaction.

Emerging evidence indicates that aberrant maternal inflammation is associated with several pregnancy-related disorders such as preeclampsia, preterm birth, and intrauterine growth restriction. Sirtuin1 (SIRT1), a class III histone deacetylase, is involved in the regulation of various physiopathological processes including cellular inflammation and metabolism. However, the effect of SIRT1 on the placental proinflammatory environment remains to be elucidated. In this study, we investigated the effect of SIRT1 on lipopolysaccharide (LPS)-induced NLRP3 inflammasome activation and its underlying mechanisms in human first-trimester trophoblasts (Sw.71 and HTR-8/SVneo cells). Treatment with LPS elevated SIRT1 expression and induced NLRP3 inflammasome activation in mouse placental tissues and human trophoblasts. Knockdown of SIRT1 enhanced LPS-induced NLRP3 inflammasome activation, inflammatory signaling, and subsequent interleukin (IL)-1β secretion. Furthermore, knockdown of NLRP3 considerably attenuated the increase of IL-1β secretion in SIRT1-knockdown cells treated with LPS. Moreover, SIRT1 inhibited LPS-induced NLRP3 inflammasome activation by reducing oxidative stress. This study revealed a novel mechanism via which SIRT1 exerts anti-inflammatory effects, suggesting that SIRT1 is a potential therapeutic target for the prevention of inflammation-associated pregnancy-related complications.

Transforming growth factor (TGF)‐β1 is well established as a critical IgA isotype switching factor and Smad molecules have been reported to act as transducers and transcriptional factors in the expression of TGF‐β1‐targeted genes. We examined the involvement of Smad proteins in TGF‐β1‐induced IgA expression. First, we found that TGF‐β1 significantly increases endogenous germ‐line (GL) α transcripts by LPS‐stimulated CH12.LX.4933 (μ⁺) B lymphoma cells. To investigate its signaling mechanisms, the lymphoma cell line was transfected with pFL3 that contains the TGF‐β‐responsive element of the GLα promoter, and stimulated with TGF‐β1. Similar to endogenous GLα transcripts, TGF‐β1 induces GLα promoter activity and overexpression of Smad3 markedly enhances the promoter activity. This activity is further augmented by cotransfected Smad4. On the other hand, Smad7 substantially abrogates the synergistic effect of Smad3/4 onGLα promoter activity. In addition, overexpression of Smad3/4 enhances TGF‐β1‐induced endogenous GLα transcripts in normal spleen B cells. Finally, in the presence of TGF‐β1, overexpression of Smad3/4 selectively increases both surface IgA expression and IgA production. The results from the present study indicate that Smad3, Smad4, and Smad7, at least in part, serve as mediators linking TGF‐β1 to transcriptional regulation of IgA switching related gene and regulation of IgA class switching.

The present study demonstrates that RA has activity of an IgA switch factor and is more specific than TGF-β1. RA independently caused only IgA switching, whereas TGF-β1 caused IgA and IgG2b switching. We found that RA increased IgA production and that this was a result of its ability to increase the frequency of IgA-secreting B cell clones. Increased IgA production was accompanied by an increase of GLTα. RA activity was abrogated by an antagonist of the RAR. Additionally, RA affected intestinal IgA production in mice. Surprisingly, RA, in combination with TGF-β1, notably enhanced not only IgA production and GLTα expression but also CCR9 and α4β7 expression on B cells. These results suggest that RA selectively induces IgA isotype switching through RAR and that RA and TGF-β have important effects on the overall gut IgA antibody response.