A functional role for the Y box in regulating an MHC class II B gene promoter in chicken lymphocytes

The conserved S, X, X2, and Y boxes located in the proximal regions of MHC class II gene promoters are necessary and sufficient to control both constitutive and induced MHC class II expression in mammals (Benoist and Mathis 1990; Glimcher and Kara 1992; Mach et al. 1996). The chicken immune system shares many of the features possessed by its mammalian counterparts, but has also evolved major points of divergence, such as the mechanism of antibody diversification (Reynaud et al. 1985, 1987; Thompson and Neiman 1987), and a different MHC genomic structure with a separate Rfp-Y region (Briles et al. 1993; Miller et al. 1994) and interspersed class I and class II genes (Guillemot et al. 1989; Kaufman and Lamont 1996). Sequence analysis of the promoters from chicken class II B genes (Xu et al. 1989; Zoorob et al. 1990) detected S, X, X2, and Y boxes in the proximal promoter region similar to those in mammals. Surprisingly, deletion of these conserved sequences did not significantly affect promoter activity in a chicken macrophage cell line (Chen et al. 1997). By characterizing the role of these DNA elements in class II gene regulation in chicken lymphocyte cell lines, the current study investigated whether the lack of function of these conserved boxes represents another major evolutionary divergence of the chicken immune system or is a macrophage-specific phenomenon.