Peterlin B M, Gonwa T A, Stobo J D
Howard Hughes Medical Institute, San Francisco, California.
J Mol Cell Immunol. 1984;1(3):191-200.
Several studies have demonstrated that the expression of Ia molecules by macrophages is not constitutive but can be enhanced by soluble factors from activated T cells. This induced expression of Ia appears to be causally important in certain accessory functions such as antigen presentation. While the phenomenon of Ia induction is clear, the mechanism by which this occurs has not been determined. Therefore, experiments were designed to investigate the molecular events leading to expression of the human Ia molecule, HLA-DR. To accomplish this, the human monocytoid cell line U 937, which does not express any detectable HLA-DR molecules were used. Utilizing a cDNA probe for the alpha chain of HLA-DR and total cellular RNA, it could be demonstrated that resting U 937 lacked detectable HLA-DR transcripts. Digestion of genomic DNA from U 937 with the isoschizomers Msp I and Hpa II followed by analysis of the restriction fragments on Southern blots demonstrated the HLA-DR alpha chain genes to be methylated. Addition of 5-azacytidine, an analogue of cytidine which causes hypomethylation of DNA to U 937 caused hypomethylation of HLA-DR alpha chain genes but did not, by itself, lend to the appearance of HLA-DR molecules or transcripts. However, treatment of U 937 with 5-azacytidine followed by addition of either culture fluids from activated T cells or human recombinant gamma interferon did lead to the rapid appearance of abundant, mature HLA-DR transcripts and surface HLA-DR molecules. The results of these studies provide the first demonstration that methylation plays a role in the expression of human Ir genes and that induced expression of Ia molecules by soluble factors from T cells, including gamma interferon, is accompanied by the rapid appearance of Ir gene transcripts.
多项研究表明，巨噬细胞Ia分子的表达并非组成性的，而是可被活化T细胞产生的可溶性因子增强。Ia分子的这种诱导表达在某些辅助功能如抗原呈递中似乎具有因果重要性。虽然Ia诱导现象很清楚，但发生这种现象的机制尚未确定。因此，设计了实验来研究导致人类Ia分子HLA - DR表达的分子事件。为实现这一目标，使用了不表达任何可检测到的HLA - DR分子的人类单核细胞系U 937。利用针对HLA - DRα链的cDNA探针和总细胞RNA，可证明静止的U 937缺乏可检测到的HLA - DR转录本。用同裂酶Msp I和Hpa II消化U 937的基因组DNA，然后在Southern印迹上分析限制性片段，结果表明HLA - DRα链基因被甲基化。向U 937中添加5 - 氮杂胞苷（一种导致DNA低甲基化的胞苷类似物），导致HLA - DRα链基因低甲基化，但它本身并未导致HLA - DR分子或转录本的出现。然而，先用5 - 氮杂胞苷处理U 937，然后添加活化T细胞的培养液或人重组γ干扰素，确实导致了大量成熟的HLA - DR转录本和表面HLA - DR分子迅速出现。这些研究结果首次证明甲基化在人类Ir基因的表达中起作用，并且T细胞产生的可溶性因子（包括γ干扰素）诱导Ia分子表达时，Ir基因转录本会迅速出现。
