It is well established that TGF-beta1 induces IgA and IgG2b class-switching recombination in murine B cells. In the present study, we assessed the activity of IL-21 along with TGF-beta1 in Ig synthesis by murine spleen B cells. IL-21 showed antiproliferative activity on LPS-activated splenic B cells, comparable with that of TGF-beta1. IL-21 alone had little effect on IgA secretion and decreased other isotypes. Likewise, IL-21 also did not alter the TGF-beta1-induced IgA synthesis and concurrently diminished the syntheses of IgM and IgG2a, which were repressed by TGF-beta1. Unexpectedly, IL-21 inhibited the TGF-beta1-induced IgG2b production. This IL-21 effect was examined using B cells from IL-21R knockout mice, where the IgA production profile was paralleled by that seen in wild-type B cells. However, the inhibitory effect of IL-21 on TGF-beta1-induced IgG2b synthesis was not seen in the IL-21R(-/-) mouse, suggesting that IL-21 causes TGF-beta1-stimulated B cells to decrease IgG2b synthesis. Expression patterns of Ig germ-line alpha(GL alpha)/GL gamma 2b transcripts under the influence of TGF-beta1 and IL-21 were paralleled by IgA/IgG2b secretion. This was also observed in the activities of GL(alpha) and GL(gamma 2b) promoters. These results indicate that IL-21 decreases IgG2b secretion mainly through inhibition of GL(gamma 2b) transcription and is ultimately associated with selective IgA secretion induced by TGF-beta1. Our results showed that IL-21 was expressed in greater magnitude in Peyer's patches (PP) than in spleen. These results suggest that IL-21 has an important effect on selective IgA(+) B cell commitment in PP.

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Activation-induced cytidine deaminase (AID) plays a key role in B cell immunoglobulin (Ig) class switch recombination (CSR) and somatic hypermutation (SHM). We have previously reported that the highly conserved homeodomain HoxC4 transcription factor binds to the Aicda (AID gene) promoter to induce AID expression. Here, we investigated the regulation of HoxC4 transcription by a proliferation-inducing ligand (APRIL) and B cell-activating factor belonging to the TNF family (BAFF) in mouse B cells. APRIL substantially increased both HoxC4 and AID expression, whereas BAFF induced the expression of AID but not HoxC4. To elucidate the underlying mechanisms, we constructed a HoxC4 gene promoter reporter vector and analyzed the promoter induction after APRIL stimulation. APRIL enhanced the HoxC4 promoter activity by 2.3-fold, and this increase disappeared when the second putative NF-κB-binding promoter element (NBE2) was mutated. Based on ChIP assays, we found that NF-κB bound to the HoxC4 promoter NBE2 region. Furthermore, the overexpression of NF-κB augmented the APRIL-induced HoxC4 promoter activity, while the expression of dominant negative-IkBa suppressed it. Taken together, our findings suggest that NF-κB mediates APRIL-induced HoxC4 transcription.