The ability of CD40L, but not lipopolysaccharide, to initiate immunoglobulin switching to immunoglobulin G1 is explained by differential induction of NF-kappaB/Rel proteins.

After activation by immunization or infection, naive resting B cells expressing IgM and IgD switch to expression of IgG, IgE, and IgA isotypes. Isotype, or class, switching is mediated by a DNA recombination event called class switch recombination. Recombination occurs between two switch regions, one located 5′ to the Cμ gene and the other located 5′ to one of the downstream heavy chain constant region (CH) genes. Class switching does not alter the antigen specificity of the antibody isotype but does alter its effector function, e.g., the ability to bind to complement, to transcytose across epithelial cells, or to mediate an allergic reaction (41, 44). Cytokines, B-cell mitogens, and the nature of antigen determine the choice of isotype. For example, the IgG1 and IgG3/IgG2b antibody classes are preferentially induced in mice by T-cell-dependent and -independent antigens, e.g., proteins and bacterial LPS, respectively (31, 40). An initiating event in class switching is the induction of transcription of the unrearranged, or GL, CH gene to which the cell will switch. Transcription from the GL CH gene initiates at exon I, upstream of the switch region of each CH gene (44). Subsequent RNA splicing to produce mature GL transcripts, also called switch transcripts, appears to be required for switch recombination (26). The effect of targeted disruption of the promoter and exon I of GL CH gene transcripts provides solid evidence for the requirement for GL transcripts in class switching (4, 20, 54). Since GL transcripts direct switch recombination, an understanding of the mechanisms of regulation of GL transcripts is necessary for understanding the regulation of class switching. Expression of GL transcripts is regulated at the transcriptional level by cytokines, such as IL-4, gamma interferon, and transforming growth factor β, and by B-cell activators, such as LPS, CD40L, and stimuli that induce signaling through surface Ig (19, 27, 37, 44, 45, 48, 53). For example, IL-4 and CD40L induce GL γ1 transcripts, whereas LPS, in the absence of IL-4, induces GL γ2b and γ3 transcripts (27, 37, 48). We previously reported that CD40L, but not LPS, induces the activity of the GL γ1 promoter in a reporter plasmid (25). This induction by CD40L is mediated by activating NF-κB/Rel proteins to bind to three tandem κB sites located in the CD40RR of the GL γ1 promoter, which is located just downstream of an IL-4-responsive Stat6 binding site (3). Overexpression of NF-κB/p50 together with RelA or RelB is sufficient to transactivate the GL γ1 promoter or the CD40RR inserted upstream of a minimal c-fos promoter, whereas overexpression of NF-κB/p50 together with c-Rel fails to induce even though this heterodimer binds to the GL γ1 promoter. These observations suggest that NF-κB/Rel proteins are important for initiating class switching to IgG1 in response to T-dependent antigens. Although LPS is an activator of NF-κB, LPS is a poor inducer of GL γ1 transcripts. In this study, we have addressed the reason for the differential effects of LPS and CD40L on induction of GL γ1 promoter and transcripts. We have tested the hypothesis that GL γ1 transcription depends on the composition of the induced NF-κB/Rel dimers.