To the lymph node and beyond: migratory ILC3s regulate innate and adaptive immune responses

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. 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 tissuespecific 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. Moreover, identification of human circulating ILC progenitors has led to further discussion about ILC motility. 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, K€astele et al. 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. K€astele 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 pseudoafferent 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 *Correspondence Claudia U Duerr, Department of Microbiology, Infectious Diseases and Immunology, Freie Universit€at Berlin and Humboldt-Universit€at zu Berlin, Charit e – Universit€atsmedizin Berlin, Hindenburgdamm 30, 12003 Berlin, Germany. E-mail: claudia.duerr@charite.de

Immunology & Cell Biology 2021; 99: 442-445; doi: 10.1111/imcb.12457 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, K€ astele 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. K€ astele 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 pseudoafferent 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 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 K€ astele 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-c alone as well as in combination with GM-CSF suggests that they may directly contribute to the defense against Salmonella Typhimurium. Indeed, IFN-c production is key in S. Typhimurium infection. 6 Here, ILC3-derived IFN-c has been shown to regulate goblet cell formation and inflammatory response upon Salmonella infection. 7 In addition, ILC3-derived granulocytemacrophage colony-stimulating factor GM-CSF is important to recruit inflammatory monocytes, trigger dendritic cells and promote acute intestinal inflammation. 8  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-c or GM-CSF, respectively. Moreover, splenic IFN-c has been recently reported to trigger MHC-II expression by LTi-like ILC3s and thereby T-cell activation. 12  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 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.