Intestinal epithelial cells (IECs) constitute the border between the vast antigen load present in the intestinal lumen and the mucosal immune compartment. Their ability to express antigen processing and presentation machinery evokes the question whether IECs function as non‐conventional antigen‐presenting cells. Major histocompatibility complex (MHC) class II expression by non‐haematopoietic cells, such as IECs, is tightly regulated by the class II transactivator (CIITA) and is classically induced by IFN‐γ. As MHC class II expression by IECs is upregulated under inflammatory conditions, it has been proposed to activate effector CD4+ T (Teff) cells. However, other studies have reported contradictory results and instead suggested a suppressive role of antigen presentation by IECs, through regulatory T (Treg)‐cell activation. Recent studies investigating the role of MHC class II + exosomes released by IECs also reported conflicting findings of either immune enhancing or immunosuppressive activities. Moreover, in addition to modulating inflammatory responses, recent findings suggest that MHC class II expression by intestinal stem cells may elicit crosstalk that promotes epithelial renewal. A more complete understanding of the different consequences of IEC MHC class II antigen presentation will guide future efforts to modulate this pathway to selectively invoke protective immunity while maintaining tolerance to beneficial antigens.

中文翻译：

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为什么肠上皮细胞表达 MHC II 类？

肠上皮细胞 (IEC) 构成了肠腔和粘膜免疫区室中存在的大量抗原负载之间的边界。它们表达抗原加工和呈递机制的能力引发了 IEC 是否作为非常规抗原呈递细胞发挥作用的问题。非造血细胞（如 IEC）表达的主要组织相容性复合体 (MHC) II 类受到 II 类反式激活因子 (CIITA) 的严格调控，并且通常由 IFN-γ 诱导。由于 IEC 的 MHC II 类表达在炎症条件下上调，因此有人提议激活效应 CD4+ T (Teff) 细胞。然而，其他研究报告了相互矛盾的结果，而是表明 IEC 通过调节性 T (Treg) 细胞激活抑制抗原呈递的作用。最近对 IEC 释放的 MHC II 类 + 外泌体作用的研究也报告了免疫增强或免疫抑制活性的相互矛盾的发现。此外，除了调节炎症反应外，最近的研究结果表明，肠干细胞表达的 MHC II 类可能引发促进上皮更新的串扰。更全面地了解 IEC MHC II 类抗原呈递的不同后果将指导未来努力调节该途径以选择性地调用保护性免疫，同时保持对有益抗原的耐受性。最近的研究结果表明，肠干细胞表达的 MHC II 类可能引发促进上皮更新的串扰。更全面地了解 IEC MHC II 类抗原呈递的不同后果将指导未来努力调节该途径以选择性地调用保护性免疫，同时保持对有益抗原的耐受性。最近的研究结果表明，肠干细胞表达的 MHC II 类可能引发促进上皮更新的串扰。更全面地了解 IEC MHC II 类抗原呈递的不同后果将指导未来努力调节该途径以选择性地调用保护性免疫，同时保持对有益抗原的耐受性。