A Spontaneous Mutation of the Rat Themis Gene Leads to Impaired Function of Regulatory T Cells Linked to Inflammatory Bowel Disease

Spontaneous or chemically induced germline mutations, which lead to Mendelian phenotypes, are powerful tools to discover new genes and their functions. Here, we report an autosomal recessive mutation that occurred spontaneously in a Brown-Norway (BN) rat colony and was identified as causing marked T cell lymphopenia. This mutation was stabilized in a new rat strain, named BNm for “BN mutated.” In BNm rats, we found that the T cell lymphopenia originated in the thymus, was intrinsic to CD4 T lymphocytes, and was associated with the development of an inflammatory bowel disease. Furthermore, we demonstrate that the suppressive activity of both peripheral and thymic CD4+ CD25bright regulatory T cells (Treg) is defective in BNm rats. Complementation of mutant animals with BN Treg decreases disease incidence and severity, thus suggesting that the impaired Treg function is involved in the development of inflammatory bowel disease in BNm rats. Moreover, the cytokine profile of effector CD4 T cells is skewed toward Th2 and Th17 phenotypes in BNm rats. Linkage analysis and genetic dissection of the CD4 T cell lymphopenia in rats issued from BNm×DA crosses allowed the localization of the mutation on chromosome 1, within a 1.5 megabase interval. Gene expression and sequencing studies identified a frameshift mutation caused by a four-nucleotide insertion in the Themis gene, leading to its disruption. This result is the first to link Themis to the suppressive function of Treg and to suggest that, in Themis-deficient animals, defect of this function is involved in intestinal inflammation. Thus, this study highlights the importance of Themis as a new target gene that could participate in the pathogenesis of immune diseases characterized by chronic inflammation resulting from a defect in the Treg compartment.可引发孟德尔表型的自发或化学诱导种系突变，是发掘新基因及其功能的强效研究工具。本研究报道了一种在布朗-挪威（Brown-Norway, BN）大鼠饲养种群中自发产生的常染色体隐性突变，该突变可引发显著的T细胞淋巴细胞减少症。该突变被稳定传代培育为全新大鼠品系，命名为BNm（取"BN mutated"之意）。研究发现，BNm大鼠的T细胞淋巴细胞减少症起源于胸腺，且为CD4阳性T淋巴细胞所固有，同时与炎症性肠病的发生发展相关。此外，本研究证实，BNm大鼠外周及胸腺来源的CD4+CD25bright调节性T细胞（regulatory T cells, Treg）的抑制功能存在缺陷。向突变大鼠体内回补BN来源的Treg可降低疾病的发生率与严重程度，这表明Treg功能受损参与了BNm大鼠炎症性肠病的发生发展。此外，BNm大鼠效应性CD4 T细胞的细胞因子谱向Th2及Th17表型偏移。通过对BNm×DA（Dark Agouti）杂交子代大鼠的CD4 T细胞淋巴细胞减少症进行连锁分析与遗传解析，研究人员将该突变定位至1号染色体上1.5兆碱基的区间内。基因表达与测序研究发现，Themis基因中存在由4个核苷酸插入引发的移码突变，导致该基因功能失活。本研究首次将Themis基因与Treg的抑制功能联系起来，并表明在Themis基因缺陷的动物体内，Treg功能缺陷参与了肠道炎症的发生。因此，本研究凸显了Themis作为全新靶基因的重要性，该基因可能参与由Treg细胞池缺陷引发的慢性炎症性免疫疾病的发病机制。