Evidence of CD1d pathway of lipid antigen presentation in mouse primary lung epithelial cells and its up-regulation upon <i>Mycobacterium bovis</i> BCG infection

Presentation of a prototype lipid antigen α-Galactosylceramide (αGC) was examined on primary epithelial cells derived from mouse lungs and on bronchoalveolar lavage (BAL) cells that essentially comprise alveolar macrophages. Presence of CD1d molecules coupled to αGC was demonstrated on both types of cells pre-treated with αGC, suggesting that both cell types are equipped to present lipid antigens. Internalization of Mycobacterium bovis Bacillus Calmette–Guérin (BCG: a prototype pathogen), a pre-requisite to the processing and presentation of protein as well as lipid antigens, was clearly demonstrated in primary lung epithelial (PLE) cells as well as BAL cells. Both PLE and BAL cells expressed CD1d molecule and a significant up-regulation of its expression occurred upon infection of these cells with BCG. Besides CD1d, the expression of other important molecules that participate in lipid antigen presentation pathway (i.e. microsomal triglyceride transfer protein (MTTP), scavenger receptor B1 (SR-B1) and Saposin) was also significantly upregulated in PLE and BAL cells upon BCG infection. In situ up-regulation of CD1d expression on lung epithelial cells was also demonstrated in the lungs of mice exposed intra-tracheally to BCG. Taken together these results suggest that lung epithelial cells may have the ability to present lipid antigens and this pathway seems to get significantly upregulated in response to BCG infection.本研究针对脂质抗原α-半乳糖基神经酰胺（α-Galactosylceramide, αGC）的呈递情况展开检测，检测对象包括小鼠肺部原代上皮细胞，以及以肺泡巨噬细胞为主要组分的支气管肺泡灌洗（bronchoalveolar lavage, BAL）细胞。经αGC预处理的两类细胞上均检出结合了αGC的CD1d分子，证实这两类细胞均具备脂质抗原呈递能力。作为蛋白质与脂质抗原加工及呈递的先决条件，牛分枝杆菌卡介苗（Mycobacterium bovis Bacillus Calmette–Guérin, BCG，简称BCG，一种模式病原体）的内化过程，在原代肺上皮（primary lung epithelial, PLE）细胞与BAL细胞中均得到明确验证。PLE细胞与BAL细胞均表达CD1d分子，且经BCG感染后，该分子的表达水平出现显著上调。除CD1d外，参与脂质抗原呈递通路的其他关键分子——微粒体甘油三酯转运蛋白（microsomal triglyceride transfer protein, MTTP）、清道夫受体B1（scavenger receptor B1, SR-B1）及鞘脂激活蛋白（Saposin）——在BCG感染后的PLE细胞与BAL细胞中同样出现显著上调表达。经气管内接种BCG的小鼠肺部组织中，也检测到肺上皮细胞上CD1d表达的原位上调现象。综合上述结果，本研究提示肺上皮细胞或具备脂质抗原呈递能力，且该呈递通路在BCG感染后可被显著激活上调。