A Promoter Haplotype of the Immunoreceptor Tyrosine-Based Inhibitory Motif-Bearing FcγRIIb Alters Receptor Expression and Associates with Autoimmunity. II. Differential Binding of GATA4 and Yin-Yang1 Transcription Factors and Correlated Receptor Expression and Function

Kaihong Su; Xiaoli Li; Jeffrey C Edberg; Jianming Wu; Polly Ferguson; Robert P Kimberly

doi:10.4049/jimmunol.172.11.7192

The immunoreceptor tyrosine-based inhibitory motif-containing FcγRIIb modulates immune function on multiple cell types including B cells, monocytes/macrophages, and dendritic cells. The promoter for the human FCGR2B is polymorphic, and the less frequent 2B.4 promoter haplotype is associated with the autoimmune phenotype of systemic lupus erythematosus. In the present study, we demonstrate that the 2B.4 promoter haplotype of FCGR2B has increased binding capacity for GATA4 and Yin-Yang1 (YY1) transcription factors in both B lymphocytes and monocytes, and that overexpression of GATA4 or YY1 enhances the FCGR2B promoter activity. The 2B.4 haplotype leads to elevated expression of the endogenous receptor in heterozygous donors by ≈1.5-fold as assessed on EBV-transformed cells, primary B lymphocytes, and CD14+ monocytes. This increased expression accentuates the inhibitory effect of FcγRIIb on B cell Ag receptor signaling, measured by Ca2+ influx and cell viability in B cells. Our results indicate that transcription factors GATA4 and YY1 are involved in the regulation of FcγRIIb expression, and that the expression variants of FcγRIIb lead to altered cell signaling, which may contribute to autoimmune pathogenesis in humans. The IgG FcRs play an important role in regulating immune system by bridging the humoral and cellular immune responses (1, 2, 3, 4). On mouse follicular dendritic cells (FDCs),5 FcγRIIb is the highly expressed IgG FcR and can mediate the retention and conversion of immune complexes on FDCs to a highly immunogenic form (5, 6), which may play a role in affinity maturation and memory B cell development (7, 8). Similarly, on Langerhans cells, FcγRIIb mediates Ag internalization and presentation (9, 10, 11). On B cells, at least in part by recruitment of phosphatases to its immunoreceptor tyrosine-based inhibitory motif, FcγRIIb engagement can shape the Ab repertoire through modulation of B cell Ag receptor (BCR)-mediated cell activation and proliferation (12, 13), through signals for apoptosis independent of BCR (14), and through down-regulation of pre-BCR-mediated apoptosis (15). On myeloid lineage cells, FcγRIIb down-regulates Ab-mediated phagocytosis and inflammatory responses when clustered with the activating FcγRs, such as FcγRIa, FcγRIIa, and FcγRIIIa (16, 17). Thus, through its roles in facilitating Ag presentation and in regulating B cell survival and proliferation, FcγRIIb has a significant role in maintaining immune homeostasis, which makes FcγRIIb an attractive functional candidate for autoimmune diseases. We have demonstrated that a functional promoter haplotype in the human FCGR2B gene is associated with systemic lupus erythematosus (SLE) (18), suggesting that FcγRIIb contributes to susceptibility for autoimmune disease. To address the underlining molecular mechanism in relation to the in vivo function of these FCGR2B haplotypes, we have explored the transcription factor-binding capability of the polymorphic sites within the FCGR2B promoter haplotypes. Computer-based searches suggested that the single-nucleotide polymorphisms (SNPs) were located in putative GATA family and Yin-Yang1 (YY1) transcription factor-binding elements. Direct assessment of binding indicated that the allelic variants from the less frequent 2B.4 haplotype have increased binding capacity for both GATA4 and YY1 transcription factors in B lymphocytes and monocytes. Overexpression of either GATA4 or YY1 up-regulates FcγRIIb promoter activity, suggesting that GATA4 and YY1 are involved in the regulation of FcγRIIb expression. Among genotyped donors, the 2B.4 haplotype leads to higher expression of endogenous FcγRIIb on both primary B lymphocytes and monocytes. This increased receptor expression accentuates the FcγRIIb function as measured by BCR-induced Ca2+ influx and cell viability in B cells. Thus, our data indicate that the FCGR2B promoter SNPs occur in transcription factor-binding elements and alter transcription factor binding, that GATA4 and YY1 transcription factors regulate FcγRIIb expression, and that the resultant change in expression can alter cell function. Given the several roles that FcγRIIb may play in the pathogenesis of autoimmunity, the specific function for FcγRIIb may vary according to the nature and stage of the disease.

A Promoter Haplotype of the Immunoreceptor Tyrosine-Based Inhibitory Motif-Bearing FcγRIIb Alters Receptor Expression and Associates with Autoimmunity. II. Differential Binding of GATA4 and Yin-Yang1 Transcription Factors and Correlated Receptor Expression and Function

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