The NFκB signaling system in the generation of B-cell subsets: from germinal center B cells to memory B cells and plasma cells

Memory B cells and antibody-secreting cells are the two prime effector B cell populations that drive infection- and vaccine-induced long-term antibody-mediated immunity. The antibody-mediated immunity mostly relies on the formation of specialized structures within secondary lymphoid organs, called germinal centers (GCs), that facilitate the interactions between B cells, T cells, and antigen-presenting cells. Antigen-activated B cells may proliferate and differentiate into GC-independent plasmablasts and memory B cells or differentiate into GC B cells. The GC B cells undergo proliferation coupled to somatic hypermutation of their immunoglobulin genes for antibody affinity maturation. Subsequently, affinity mature GC B cells differentiate into GC-dependent plasma cells and memory B cells. Here, we review how the NFκB signaling system controls B cell proliferation and the generation of GC B cells, plasmablasts/plasma cells, and memory B cells. We also identify and discuss some important unanswered questions in this connection.

AID to overcome the limitations of genomic information by introducing somatic DNA alterations

The immune system has adopted somatic DNA alterations to overcome the limitations of the genomic information. Activation induced cytidine deaminase (AID) is an essential enzyme to regulate class switch recombination (CSR), somatic hypermutation (SHM) and gene conversion (GC) of the immunoglobulin gene. AID is known to be required for DNA cleavage of S regions in CSR and V regions in SHM. However, its molecular mechanism is a focus of extensive debate. RNA editing hypothesis postulates that AID edits yet unknown mRNA, to generate specific endonucleases for CSR and SHM. By contrast, DNA deamination hypothesis assumes that AID deaminates cytosine in DNA, followed by DNA cleavage by base excision repair enzymes. We summarize the basic knowledge for molecular mechanisms for CSR and SHM and then discuss the importance of AID not only in the immune regulation but also in the genome instability.

The Evolutionarily Conserved Sequence Upstream of the Human Ig Heavy Chain Sγ3 Region Is an Inducible Promoter: Synergistic Activation by CD40 Ligand and IL-4 Via Cooperative NF-κB and STAT-6 Binding Sites

Germline Cγ gene transcription is a crucial event in the process that leads to switch DNA recombination to IgG, but its regulation in the human is poorly understood. We took advantage of our monoclonal model of germinal center B cell differentiation, IgM+ IgD+ CL-01 cells, to define the role of the Iγ3 evolutionarily conserved sequence (ECS) in the germline transcriptional activation of the human Cγ3 gene. The Iγ3 ECS lies upstream of the major Iγ3 transcription initiation site and displays more than 90% identity with the corresponding human Iγ1, Iγ2, and Iγ4 regions. Reporter luciferase gene vectors containing the human γ3 ECS were used to transfect CL-01 cells, which have been shown to undergo Sμ→Sγ3 DNA recombination, upon engagement of CD40 by CD40 ligand (CD40L) and exposure to IL-4. In these transfected CL-01 cells, CD40:CD40L engagement and exposure to IL-4 synergistically induced γ3 ECS-dependent luciferase reporter gene activation. Targeted mutational analysis demonstrated that a tandem NF-κB/Rel binding motif is critical for the γ3 ECS responsiveness to both CD40L and IL-4, while a STAT-6-binding site is additionally required for IL-4 inducibility. Electrophoretic mobility shift assays showed that p50/p65/c-Rel and STAT-6 are effectively induced by CD40L and IL-4, respectively, and bind to specific DNA motifs within the ECS. These partially overlapping CD40L and IL-4 responsive elements are functionally cooperative as the disruption of one of them prevents synergistic promoter activation. Thus, the γ3 ECS is an inducible promoter containing cis elements that critically mediate CD40L and IL-4-triggered transcriptional activation of the human Cγ3 gene.

Requirement for Nuclear Factor-κB Activation by a Distinct Subset of CD40-Mediated Effector Functions in B Lymphocytes

CD40 stimulation, which is crucial for generating an effective T-dependent humoral response, leads to the activation of transcription factors NF-AT (nuclear factor of activated T cells), AP-1 (activator protein-1), and NF-κB (nuclear factor-κB). However, which CD40-mediated B cell functions actually require activation of specific transcription factors is unknown. We examined the causal relationship between NF-κB activation and CD40 effector functions by evaluating CD40 functions in the presence of an inducible mutant inhibitory κBα (IκBα) superrepressor. IκBαAA inhibited nuclear translocation of multiple NF-κB dimers without the complicating effect of depriving cells of NF-κB during development. This approach complements studies that use mice genetically deficient in single or multiple NF-κB subunits. Interestingly, only a subset of CD40 effector functions was found to require NF-κB activation. Both CD40-induced Ab secretion and B7-1 up-regulation were completely abrogated by expression of IκBαAA. Surprisingly, up-regulation of Fas, CD23, and ICAM-1 was partially independent, and up-regulation of LFA-1 was completely independent, of CD40-induced NF-κB activation. For the first time, it is clear that distinct transcription factors are required for the dynamic regulation of CD40 functions.

The p65 Subunit of NF-κB Is Redundant with p50 During B Cell Proliferative Responses, and Is Required for Germline CH Transcription and Class Switching to IgG3

B cells lacking individual NF-κB/Rel family members exhibit defects in activation programs. We generated small resting B cells lacking p65 or p50 alone, or lacking both p50 and p65, then evaluated the ability of these cells to proliferate, secrete Ig, and undergo Ig class switching. B cells lacking p65 proliferated well in response to all stimuli tested. However, these cells demonstrated an isolated defect in switching to IgG3, which was associated with a decrease in γ3 germline CH gene expression. Whereas, previously reported, B cells lacking p50 alone had a severe proliferative defect in response to LPS, a moderate defect in response to CD40 ligand (CD40L), and normal proliferation to Ag receptor cross-linking using dextran-conjugated anti-IgD Abs (αδ-dex), B cells lacking both p50 and p65 exhibited severely impaired proliferation in response to LPS, αδ-dex, and CD40L. This defect could be overcome by simultaneous administration of αδ-dex and CD40L. In response to this latter combination of stimuli, B cells lacking both p50 and p65 secreted Ig and underwent isotype switching to IgG1 as efficiently as B cells lacking p50 alone. These data demonstrate a role for the p65 subunit of NF-κB in germline CH gene expression as well as functional redundancy between p50 and p65 during proliferative responses.