Immunological memory, defined as the ability to respond in an enhanced manner upon secondary encounter with the same pathogen, can provide substantial protection against infectious disease. The improved protection is mediated in part by different populations of memory CD8 T cells that are retained after primary infection. Memory cells persist in the absence of pathogen-derived antigens and enable secondary CD8 T-cell responses with accelerated kinetics and of larger magnitude after reencounter with the same pathogen. At least three subsets of memory T cells have been defined that are referred to as central memory CD8 T cells (Tcm), effector memory CD8 T cells (Tem), and tissue-resident memory CD8 T cells (Trm). Tcm and Tem are circulating memory T cells that mediate bodywide immune surveillance in search of invading pathogens. In contrast, Trm permanently reside in peripheral barrier tissues, where they form a stationary defensive line of sentinels that alert the immune system upon pathogen reencounter. The characterization of these different subsets has been instrumental in our understanding of the strategies that memory T cells employ to counter invading pathogens. It is clear that memory T cells not only have a numerical advantage over naive T cells resulting in improved protection in secondary responses, but also acquire distinct sets of competencies that assist in pathogen clearance. Nevertheless, inherent challenges are associated with the allocation of memory T cells to a limited number of subsets. The classification of memory T cells into Tcm, Tem, and Trm may not take into account the full extent of the heterogeneity that is observed in the memory population. Therefore, in this review, we will revisit the current classification of memory subsets, elaborate on functional and migratory properties attributed to Tcm, Tem, and Trm, and discuss how potential heterogeneity within these populations arises and persists.

中文翻译：

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免疫监视领域：记忆 CD8 T 细胞亚群的专门作用。

免疫记忆，定义为在第二次遇到相同病原体时以增强的方式做出反应的能力，可以提供针对传染病的实质性保护。增强的保护作用部分是由原发感染后保留的不同记忆 CD8 T 细胞群介导的。记忆细胞在没有病原体衍生抗原的情况下持续存在，并且在再次遇到相同病原体后能够以加速的动力学和更大幅度的二次 CD8 T 细胞反应。至少三个记忆 T 细胞亚群已被定义，称为中央记忆 CD8 T 细胞 (Tcm)、效应记忆 CD8 T 细胞 (Tem) 和组织驻留记忆 CD8 T 细胞 (Trm)。Tcm 和 Tem 是循环记忆 T 细胞，它们介导全身免疫监视以寻找入侵的病原体。相比之下，Trm 永久存在于外周屏障组织中，在那里它们形成固定的哨兵防御线，在病原体再次遇到时提醒免疫系统。这些不同亚群的特征有助于我们理解记忆 T 细胞用来对抗入侵病原体的策略。很明显，记忆 T 细胞不仅比幼稚 T 细胞具有数量优势，从而提高了对二级反应的保护，而且还获得了有助于清除病原体的独特能力。然而，固有的挑战与将记忆 T 细胞分配给有限数量的子集有关。将记忆 T 细胞分类为 Tcm、Tem 和 Trm 可能没有考虑到在记忆群体中观察到的异质性的全部程度。所以，