of triplicate samples and are representative of two independent experiments. their B cells, have impaired TI-1 antibody responses but normal T cell-dependent and TI-2 antibody responses. The absence of IB- in B cells also impaired proliferation triggered by Toll-like receptor (TLR) activation, plasma cell differentiation, and class switch recombination (CSR). Mechanistically, IB–deficient B cells could not induce TLR-mediated induction of activation-induced cytidine deaminase (AID), a class-switch DNA recombinase. Retroviral transduction of AID in IB–deficient B cells restored CSR activity. Furthermore, acetylation OSMI-4 of histone H3 in the vicinity of the transcription start site of the gene that encodes AID was reduced in IB–deficient B cells relative to IB–expressing B cells. These results indicate that IB- regulates TLR-mediated CSR by inducing AID. Moreover, IB- defines differences in the transcriptional regulation of different antibody responses. == Introduction == Antibody responses are central to protecting hosts from pathogen infection. After B cells recognize antigens, they initiate three steps, proliferation, class switch recombination (CSR),2and plasma cell differentiation, that are required for antibody production. In terms of antibody responses, antigens are typically classified as being either T cell-dependent (TD) or T cell-independent OSMI-4 (TI) antigens (1). TD antigens are soluble proteins or peptides that are recognized by specific B cell receptors and induce clonal activation of B cells; TD antibody responses require the interaction of the CD40 ligand on a T cell with a CD40 receptor on a B cell (2,3). In contrast, TI antigens can initiate antibody responses independently of T cells. TI antibody responses are classically defined as TI type 1 (TI-1) antigens and TI type 2 (TI-2) antigens, depending on their requirement for Btk, which is a key kinase needed for B cell antigen receptor (BCR) signaling (4,5). The TI-1 antigen TNP-LPS, but not the TI-2 antigen TNP-Ficoll, can elicit anti-TNP OSMI-4 plaque-forming cell responses in Btk-deficient mice (4). Thus, BCR signaling is necessary for responses triggered by TI-2 antigen but dispensable for responses triggered by the TI-1 antigen. TI-2 antigens, which contain a repetitive epitope such as capsular polysaccharide, induce strong BCR signaling by engaging multiple BCRs, which induces clonal B cell activation and antigen-specific immunoglobulin (Ig) production (6). TI-1 antigens, such as LPS, are considered to act as mitogens that stimulate B cells to produce polyclonal antibodies following Toll-like receptor (TLR) stimulation (7,8). However, the polysaccharide moiety of the LPS binds to the BCRs of multiple B cells (9). As a consequence, LPS can induce the production MKK6 of not only polyclonal Igs but also antigen-specific Igs by co-engaging TLR4 and BCR. In addition, co-stimulation of other TLR ligands and BCR induces strong activation-induced cytidine deaminase (AID) expression and a high rate of CSR. Thus, TLR-mediated antibody responses are divided into BCR-independent polyclonal responses and BCR-dependent clonal responses. Although the mechanism of antibody responses varies widely between the types of antigens described above, it remains unclear whether common transcriptional factors regulate both TD and TI antibody responses. CSR in B cells switches one isotype of an antibody to another. AID is thought to be a master regulator of CSR, which is regulated by transcriptional factors that include Bach2, IRF4, and Hoxc4 (1016). The observation that deficiencies in any of these four transcriptional factors impair both TD- and OSMI-4 TI-induced AID and CSR suggests that the same mechanisms of transcriptional regulation operate both in TD and TI antibody responses. However, the signaling pathway activated by CD40, which is a key receptor for the TD antibody response, clearly differs from that triggered by TLR activation. Thus, it is possible that transcriptional regulation of AID is regulated by.
