The reduced transcriptional activity of the haplotype I Fcgr2b promoter is caused by three single nucleotide substitutions leading to defective AP-1 binding. (A) The transcriptional activity of the WT, KI, NZB, and NZW promoter of Fcgr2b was determined in the Bal17 B cell line stimulated with LPS for 48 h. WT versus: KI, P = 0.015; NZW, P = 0.037; and NZB, P = 0.034. (B) The transcriptional activity of the WT promoter mutated at the indicated positions was determined as in A. WT versus: KI/NZB, P = 0.02; WT GG_−1/+2AA, P = 0.1; WT GG_−1/+2AA/T₋₁₆₁C, P = 0.38; WT GG_−1/+2AA/G₋₇₉C, P = 0.0025; WT GG_−1/+2AA/C₋₅₉T, P = 0.88; WT GG_−1/+2AA/A₋₁₉C, P = 0.5; and WT G₋₇₉C, P = 0.3. (C) The transcriptional activity of the KI/NZB promoter mutated at the indicated positions was determined as in A. WT versus: KI/NZB, P = 0.001; KI/NZB AA_−1/+2GG/C₋₇₉G, P = 0.07; KI/NZB C₋₇₉G, P = 0.02; and KI/NZB AA_−1/+2GG, P = 0.94. KI/NZB versus: KI/NZB AA_−1/+2GG/C₋₇₉G, P = 0.03; KI/NZB C₋₇₉G, P = 0.06; and KI/NZB AA_−1/+2GG, P = 0.001. (B and C) The dashed lines represent the luciferase activity of the Fcgr2b KI promoter. (D) Bal17 cells were stimulated for 24 h with anti-Ig or LPS before ChIP with anti–c-Fos or –c-Jun or an isotype control. The region of the Fcgr2b promoter encompassing position −79 was amplified by PCR from input and coimmunoprecipitated DNA. (E) WT and FcγRIIb^wild/H1 KI splenic CD19⁺ B cells were stimulated for 8 h with anti-Ig or LPS and processed as in D. The region of the Fcgr2b promoter encompassing position −79 was amplified by SYBR green quantitative PCR, and the ΔCT of isotype and c-Fos or c-Jun was plotted for each condition. In all panels, error bars represent SEM, and p-values were determined using the Mann–Whitney two-tailed test with a risk of 5%.