An important characteristic of immune cells is their ability to circulate through the body scouting for pathogens, transformed cells or other potential insults. However, helper‐like innate lymphoid cells (ILCs) have been identified as primarily tissue resident. Helper‐like ILCs are a recently described set of cell populations composed of three functionally diverse subgroups: ILC1s, ILC2s and ILC3s.¹ ILCs exhibit striking functional similarities to adaptive T cells including expression of subgroup‐specific signature cytokines and transcription factors. However, ILCs do not express rearranged antigen receptors and are activated in an antigen‐independent manner by cytokines, neuropeptides, leukotrienes and other immunomodulators. With their strategic positioning at barrier surfaces and their immediate way of initiating effector mechanisms, ILCs are able to rapidly shape their tissue microenvironment and orchestrate innate as well as adaptive immune responses. Importantly, ILCs themselves are imprinted by local environmental cues and adopt tissue‐specific phenotypes that allow them to tailor their functional capacities to the anatomical niche they reside in. While considered mainly tissue resident, ILC progenitors as well as mature ILCs exhibit limited migratory potential to home to their respective organ during development, to strategically position themselves within an organ or to replenish the exhausted ILC tissue pool. In addition, interorgan trafficking of ILCs has been described.² Moreover, identification of human circulating ILC progenitors has led to further discussion about ILC motility.³ Increasing evidence is also emerging that ILCs are able to directly or indirectly trigger adaptive immune responses, which could be promoted by an ILC migration potential. Whereas T‐cell trafficking is well documented, the understanding of ILC motility remains incompletely understood.

ILCs represent a rare cell population and thus addressing their migration is experimentally extremely challenging. In a recent issue of Mucosal Immunology, Kästele et al.⁴ studied ILC migration by using Kaede photoconvertible mice. Kaede mice are genetically manipulated transgenic mice, which express Kaede protein. In Kaede mice photoconversion takes place upon exposure to low‐intensity violet light and red labelled cells can be identified as resident cells by the Kaede red protein, whereas migrating and thus not photoconverted cells are identified by the Kaede green protein. Kaede mice are therefore important in vivo imaging models to monitor cellular motility within an organ or between different organs. Strikingly, all ILCs within Kaede mice can actively migrate to lymph nodes, a fundamental cellular process previously unknown.

However, the extent of motility by the ILC groups is different depending on the health status. It has been previously shown that ILCs can be found in the lymph. The lymph and the lymphatics build an important network and connect different organs, yet determining the origin of cells in the lymph remains highly elusive. Kästele et al. applied an advanced technique by cannulating the thoracic duct and harvesting migrating cells, enabling lymph to be collected from the efferent lymphatics. Lymph was also collected after removal of the mesenteric lymph node, mimicking pseudo‐afferent lymphatics. This elegant method enabled cells entering the lymphatics from the tissue or the lymph node to be distinguished, which has never been investigated before. Applying these novel models, a significant population of migratory ILCs could be identified in the lymph node, even if at a lower frequency compared with T cells, their adaptive counterpart. With these elegant and novel techniques, the researchers could not only investigate the motility of ILCs but also encourage further research into cell migratory patterns between as well as within organs.

The number of migrating ILCs during infection or conditions of inflammation remains poorly understood, as does a complete understanding of migration potential of all ILC populations. Kästele et al. quantitatively addressed which intestinal ILC populations are migrating at steady state as well as under acute inflammatory conditions, demonstrating for the first time that all ILC subsets could migrate under homeostasis. ILC1s represented the main ILC population trafficking at steady state, confirming previous data, with most migrating from the mesenteric lymph nodes.⁵ Interestingly, ILC2s, ILC3s and T‐bet⁺ ILC3s were also trafficking to a similar extent from the mesenteric lymph nodes. During infection, interferon‐γ (IFN‐γ) responses are crucial in the defense against Salmonella.⁶ Indeed, Kästele et al. identified that migratory ILCs increased expression of several IFN regulatory factors, indicating ILCs may contribute to the resolution of an intestinal bacterial infection through their IFN signature. Whereas the immune activation phenotype of migrating ILCs at steady state and upon infection remained similar, IFN‐γ and granulocyte–macrophage colony‐stimulating factor (GM‐CSF) coexpression were more pronounced upon infection in the draining lymph node, suggesting that ILCs are actively participating in creating a microenvironment in the lymph node to trigger immune responses and resolution of an intestinal bacterial infection. Altogether, the research team has provided a dynamic overview and thus key insights into spatial–temporal patterns of trafficking of ILCs as rare cell populations, which were typically considered tissue resident only (Figure 1).

The work by Kästele et al. revealed for the first time that ILCs, albeit at small numbers, are entering the lymph and can migrate to the draining lymph node of the intestine under homeostatic conditions. Importantly, the ability of ILCs to traffic to the mesenteric lymph node was observed to be independent of the state of health, although upon inflammation the composition of migrating ILC subgroups as well as their activation profile subsequently changed. This indicates that the migration and activation profile is indeed influenced by infection; however, the number may be limited by intercellular dynamics. The ability of migratory ILC3s to express IFN‐γ alone as well as in combination with GM‐CSF suggests that they may directly contribute to the defense against Salmonella Typhimurium. Indeed, IFN‐γ production is key in S. Typhimurium infection.⁶ Here, ILC3‐derived IFN‐γ has been shown to regulate goblet cell formation and inflammatory response upon Salmonella infection.⁷ In addition, ILC3‐derived granulocyte–macrophage colony‐stimulating factor GM‐CSF is important to recruit inflammatory monocytes, trigger dendritic cells and promote acute intestinal inflammation.⁸ Kästele et al. now in detail investigated the activation and cytokine profile of migratory ILC3s, linking their capacity to dislocate to the LN as well as their characteristic cytokine profile to fight off intestinal pathogens. Importantly, previous work^{9, 10} has shown that ILCs, including ILC2s and ILC3s, are located in the interfollicular regions of lymph nodes. Kästele et al. confirmed this observation and additionally demonstrated that ILCs migrate via the lymph to reach this area. CCR7 and CD62L have been previously implicated in the migration of ILC progenitors as well as trafficking of ILC1s to the lymph node.^{5, 11} Importantly, migration of Lymphoid Tissue inducer (LTI)‐like ILC3s, a distinct subpopulation of ILC3s, was shown to be CCR7 dependent upon infection with the parasite Heligmosomoides polygyrus.⁹ Upregulation of CCR7 transcript expression was also observed by unbiased RNA‐Seq analysis comparing tissue‐resident ILCs in the lamina propria with migratory lymph ILCs, further highlighting the potential role of this chemokine receptor in ILC migration via the lymph. It is evident now that ILCs utilize the lymphatic system to traffic to other organs. However, the precise functional role of ILCs in the lymph node remains elusive. During Citrobacter rodentium infection, ILC3s have been shown to play a key role by triggering T‐follicular helper responses and immunoglobulin A production.¹⁰ Based on their specific location, ILCs could influence T‐cell responses as well as the development and recruitment of myeloid cells in Salmonella infection by production of IFN‐γ or GM‐CSF, respectively. Moreover, splenic IFN‐γ has been recently reported to trigger MHC‐II expression by LTi‐like ILC3s and thereby T‐cell activation.¹² T‐bet⁺ ILC3‐released IFN‐γ may thereby impact LTi‐like ILC3 responses in the intestinal lymph node upon Salmonella infection. Interestingly, ILC3s are also present in other lymph nodes including the lung‐draining mediastinal lymph nodes,⁹ although ILC2s represent the dominant helper ILC population at this site, highlighting their potential to direct adaptive immune responses.

In this study, Kästele et al. clearly demonstrate that all investigated ILC populations are able to migrate from the intestinal tissue to the lymph node at steady state as well as under inflammatory conditions. However, not all ILC subgroups traffic to the same extent and the exact underlying mechanisms that trigger migration of ILC3s specifically upon Salmonella infection and retention in, but also egress of ILCs from the lymph node remain unclear. Interestingly, Kästele et al. identified an IFN signature on migrating ILC3s upon infection. IFN may be an important candidate, which could influence restraining of ILC2s and affect the migration of ILC3s in a direct or indirect manner. However, to what extent IFN affects the migration of ILCs still needs to be elucidated. The acquisition of antigen by ILC3s together with additional (maturation) signals may be important in Salmonella infection. Interestingly, ILC migration and positioning in the lymph node has been shown to be regulated by the receptor GPR183 sensing cholesterol metabolites such as oxysterol.¹⁰ Whether this is also the case for ILC3s upon Salmonella infection is not known. Furthermore, whether changes in nutrient or microbiota composition affect ILC3 migration and positioning in the lymph node upon Salmonella infection, and how their retention in the lymph node may regulate adaptive immune responses require further investigation. Overall, the ability of ILCs to migrate to the LN, their expression of cytokines and their positioning are of great interest. Active regulation of these processes may be a target of scientific and clinical interest to counteract adverse immunopathologies of the intestine.

免疫细胞的一个重要特征是它们在体内搜寻病原体，转化细胞或其他潜在侵害的能力。但是，已经鉴定出类辅助性先天淋巴样细胞（ILC）是主要的组织驻留细胞。类似于Helper的ILC是最近描述的一组细胞群，由三个功能多样的亚组组成：ILC1，ILC2和ILC3。^1个ILC与适应性T细胞具有惊人的功能相似性，包括亚组特异性标志性细胞因子和转录因子的表达。但是，ILC不表达重排的抗原受体，并以不依赖抗原的方式被细胞因子，神经肽，白三烯和其他免疫调节剂激活。凭借其在屏障表面的战略定位和启动效应器机制的直接途径，ILC能够快速塑造其组织微环境并编排先天性和适应性免疫反应。重要的是，ILC本身会受到当地环境线索的印记，并采用特定于组织的表型，从而使它们能够根据其所居住的解剖位置调整其功能能力。ILC祖细胞和成熟的ILC在发育过程中迁徙到各自器官的迁移潜力有限，无法战略性地将自己安置在器官内或补充疲惫的ILC组织库。另外，已经描述了ILC的器官间贩运。²此外，人类循环ILC祖细胞的鉴定导致了关于ILC运动性的进一步讨论。³越来越多的证据表明，ILC能够直接或间接触发适应性免疫应答，ILC迁移潜力可能会促进这种免疫应答。尽管对T细胞的贩运已有充分的文献记载，但对ILC运动性的了解仍不完全清楚。

ILC代表稀有细胞群，因此解决其迁移在实验上极具挑战性。在最近的问题粘膜免疫，Kästele等 人。⁴研究了通过使用Kaede光变小鼠的ILC迁移。Kaede小鼠是表达Kaede蛋白的经基因改造的转基因小鼠。在Kaede小鼠中，光转化发生在暴露于低强度紫光下时，Kaede红色蛋白可将红色标记的细胞识别为驻留细胞，而Kaede绿色蛋白可识别正在迁移且因此未进行光转化的细胞。因此，Kaede小鼠在体内很重要成像模型来监视器官内或不同器官之间的细胞运动。令人惊讶的是，Kaede小鼠体内的所有ILC都可以主动迁移至淋巴结，这是以前未知的基本细胞过程。

但是，ILC组的运动程度因健康状况而异。先前已经证明ILCs可以在淋巴中发现。淋巴和淋巴管建立了重要的网络并连接不同的器官，但是确定淋巴中细胞的起源仍然很难捉摸。卡斯特勒等。通过插管胸腔导管和收集迁移细胞应用了一项先进技术，从而能够从传出的淋巴管中收集淋巴液。去除肠系膜淋巴结后，还收集了淋巴，模仿了假冒淋巴结。这种优雅的方法可以区分从组织或淋巴结进入淋巴管的细胞，这是以前从未研究过的。应用这些新颖的模型，即使与T细胞相比，它们的适应性配对物的频率较低，也可以在淋巴结中识别出大量的迁徙性ILC。利用这些优雅而新颖的技术，研究人员不仅可以研究ILC的运动性，而且可以鼓励人们进一步研究器官之间以及器官内部的细胞迁移模式。

人们对感染或炎症状况期间迁移的ILC数量仍然知之甚少，对所有ILC人群迁移潜能的全面了解也鲜为人知。Kästele等。定量分析了哪些肠道ILC群体在稳态以及在急性炎症条件下正在迁移，这首次证明了所有ILC亚群都可以在体内稳态下迁移。ILC1s是稳定状态下主要的ILC人口贩运活动，证实了先前的数据，其中大部分是从肠系膜淋巴结转移的。⁵有趣的是，ILC2，ILC3和T-bet ⁺ILC3s也从肠系膜淋巴结以类似程度贩运。在感染过程中，干扰素-γ（IFN-γ）反应对于沙门氏菌的防御至关重要。⁶确实，Kästele等人。研究表明，迁徙ILC增加了几种IFN调节因子的表达，这表明ILC可能通过其IFN标记而有助于解决肠道细菌感染。迁移的ILC在稳态和感染后的免疫激活表型保持相似，而IFN-γ和粒细胞-巨噬细胞集落刺激因子（GM-CSF）在表达的引流淋巴结中更明显，这表明ILC是活跃的参与在淋巴结中创建微环境以触发免疫反应和解决肠道细菌感染。总之，研究小组提供了动态概述，从而提供了对作为稀有细胞群的ILC贩运的时空格局的关键见解，

工作b ý Kästele等。首次揭示了ILC，尽管数量很少，但已进入体内淋巴结，并且在体内平衡条件下可以迁移至肠的引流淋巴结。重要的是，观察到ILC运输到肠系膜淋巴结的能力与健康状况无关，尽管在炎症时，迁移的ILC亚组的组成及其激活特性随后发生了变化。这表明迁移和激活特性确实受到感染的影响。但是，该数目可能受到细胞间动力学的限制。迁移ILC3s单独表达IFN-γ以及与GM-CSF结合表达的能力表明它们可能直接有助于防御沙门氏菌鼠伤寒。确实，IFN-γ的产生是S中的关键。鼠伤寒感染。⁶在这里，已证明ILC3衍生的IFN-γ可调节沙门氏菌感染后的杯状细胞形成和炎症反应。⁷此外，ILC3衍生的粒细胞-巨噬细胞集落刺激因子GM-CSF对于募集炎性单核细胞，触发树突状细胞并促进急性肠道炎症也很重要。⁸ Kästele等。现在详细研究了迁移性ILC3的活化和细胞因子谱，并将其迁移至LN的能力以及其与肠道病原体对抗的特征性细胞因子谱联系起来。重要的是，以前的作品^9、10已经表明ILC，包括ILC2和ILC3，位于淋巴结的小泡间区域。Kästele等。证实了这一观察结果，并进一步证明了ILC通过淋巴迁移到该区域。CCR7和CD62L以前已牵涉到ILC祖细胞的迁移以及ILC1s向淋巴结的运输。^5，11重要的是，淋巴组织诱导物（LTI）样ILC3s，ILC3s的不同亚群的迁移，被证明是CCR7依赖于感染寄生虫Heligmosomoides polygyrus。⁹通过将固有层中的组织驻留ILC与迁移性淋巴ILC进行比较，通过无偏RNA-Seq分析还观察到了CCR7转录表达的上调，进一步突显了该趋化因子受体在ILC通过淋巴迁移中的潜在作用。现在已经很明显，ILC利用淋巴系统运送到其他器官。但是，ILC在淋巴结中的确切功能作用仍然难以捉摸。在啮齿动物柠檬酸杆菌感染期间，ILC3已显示出通过触发T卵泡辅助反应和免疫球蛋白A的产生起关键作用。¹⁰根据其特定位置，ILC可能会影响沙门氏菌中的T细胞反应以及骨髓细胞的发育和募集分别通过产生IFN-γ或GM-CSF感染。此外，最近有报道称脾脏IFN-γ可通过类LTi ILC3s触发MHC-II表达，从而激活T细胞。沙门氏菌感染后¹² T-bet ⁺ ILC3释放的IFN-γ可能会影响肠淋巴结中LTi样ILC3反应。有趣的是，ILC3也存在于其他淋巴结中，包括引流纵隔的肺部淋巴结[ ^9]，尽管ILC2代表了该部位ILC的主要辅助人群，突显了其指导适应性免疫反应的潜力。

在这项研究中，Kästele等人。清楚地表明，所有研究的ILC种群均能够在稳定状态以及在炎症条件下从肠组织迁移至淋巴结。但是，并非所有ILC亚组的流量都达到相同的程度，确切地说是在沙门氏菌感染和滞留后触发ILC3迁移的确切潜在机制仍然不清楚，而且还不清楚ILC是否从淋巴结流出。有趣的是，Kästele等。在感染后ILC3迁移时鉴定出IFN标记。IFN可能是重要的候选物，其可以影响ILC2的抑制并以直接或间接的方式影响ILC3的迁移。但是，仍然需要阐明IFN在多大程度上影响ILC的迁移。在沙门氏菌感染中，ILC3与其他（成熟）信号一起获得抗原可能很重要。有趣的是，ILC在淋巴结中的迁移和定位已显示出受受体GPR183感应胆固醇代谢产物如氧固醇的调节。¹⁰沙门氏菌感染时是否也存在ILC3感染尚不清楚。此外，沙门氏菌感染后营养素或微生物群组成的变化是否会影响ILC3迁移和在淋巴结中的定位，以及它们在淋巴结中的滞留如何调节适应性免疫反应，还需要进一步研究。总体而言，ILC迁移至LN的能力，其细胞因子的表达及其位置引起了人们的极大兴趣。这些过程的主动调节可能是抵消肠道不利的免疫病理学的科学和临床兴趣的目标。