The Ability of CD40L, but Not Lipopolysaccharide, To Initiate Immunoglobulin Switching to Immunoglobulin G1 Is Explained by Differential Induction of NF-κB/Rel Proteins

Antibodies of the immunoglobulin G1 class are induced in mice by T-cell-dependent antigens but not by lipopolysaccharide (LPS). CD40 engagement contributes to this preferential isotype production by activating NF-κB/Rel to induce germ line γ1 transcripts, which are essential for class switch recombination. Although LPS also activates NF-κB, it poorly induces germ line γ1 transcripts. Western blot analyses show that CD40 ligand (CD40L) induces all NF-κB/Rel proteins, whereas LPS activates predominantly p50 and c-Rel. Electrophoretic mobility shift assays show that in CD40L-treated cells, p50-RelA and p50-RelB dimers are the major NF-κB complexes binding to the germ line γ1 promoter, whereas in LPS-treated cells, p50–c-Rel and p50-p50 dimers are the major binding complexes. Transfection of expression plasmids for NF-κB/Rel fusion proteins (forced dimers) indicates that p50-RelA and p50-RelB dimers activate the germ line γ1 promoter and that p50–c-Rel and p50-p50 dimers inhibit this activation by competitively binding to the promoter without activating the promoter. Therefore, germ line γ1 transcription depends on the composition of NF-κB/Rel proteins.