Global collaboration is the cornerstone of scientific advancement.? Frontiers in Immunology has organized a series of special edition Research Topics, with the goal of highlighting the latest advancements in Cellular Immunology across the globe, showcasing the academic excellence and high-quality work of internationally recognized researchers.? These collections aim to shed light on the recent progress made across the entire breadth of the Cellular Immunology field, and reflect on the future challenges faced by researchers across borders.? Please note, contributions to the collection are by invitation only. Please inform the Editorial Office at immunology.submissions@frontiersin.org once you are prepared to submit.

Heinrich Heine University of Düsseldorf

German Rheumatism Research Center (DRFZ)

Frontiers in Immunology

B Cell Biology

T Cell Biology

Immunological Memory

NK and Innate Lymphoid Cell Biology

Antigen Presenting Cell Biology

Global Excellence in Cellular Immunology: Europe 2021

This Research Topic was set up to highlight recent progress in cellular immunology in Europe. Rather than focus on a particular smaller topic, it aims to shed light on recent progress made across the entire breadth of the cellular immunology field in Europe. Considering this, the present Research Topic hosts a small but nice collection of four articles highlighting the present key aspects of T cell and Natural Killer (NK) cell immunology: memory compartments, tissue-residency, checkpoint regulation, and mechanism of cytotoxicity. Of note, all articles focus on the human immune system, which we in general do understand less well than the murine immune system. Given the genetic and functional differences between human and mouse immune cells already known (<a href="#B1">1</a>, <a href="#B2">2</a>), the need for a better understanding of the human immune system is obvious, in order to solve the many burning questions involved in avoiding and controlling infectious diseases, autoimmunity, and cancer. The field is hampered by the logistic and ethical constraints that are attached to working with tissue biopsies other than blood from patients or even harder, from healthy individuals. The research work presented here reflects the ongoing efforts to better understand how the human immune system works on the systemic level.In the study by <a href="https://doi.org/10.3389/fimmu.2023.1130370">Haugstøyl et&nbsp;al.</a>, the surface proteome of NK cells residing in adipose tissue was characterized. Adipose tissue-resident NK cells have recently gained much interest from studies suggesting them to be mediators of inflammation and insulin resistance in obesity (<a href="#B3">3</a>). So far, NK cells residing in human adipose tissue are poorly defined. The present study performed high-dimensional flow cytometric analyses on NK cells from subcutaneous (SAT) as well as visceral adipose tissue (VAT). The authors nicely illustrate how tissue-resident NK cells adapt to their specific microenvironment by defining protein signatures specific for VAT vs. SAT. Notably, VAT contained a higher frequency of immature tissue-resident NK cells compared to SAT, which is interesting given that the VAT/SAT ratio correlates with risk of cardiovascular disease and insulin resistance (<a href="#B4">4</a>). Although a link between the presence of NK cell subsets and insulin resistance has not yet been established in the present study, the newly defined surface markers will be a valuable asset to explore if and how NK cells contribute to obesity-induced inflammatory stress and diabetes.Another type of lymphocyte, absent from peripheral blood and a challenge to analyze, are human tissue-resident memory T cells (Trm) (<a href="#B5">5</a>). When T cells enter a nonlymphoid organ in the effector phase of an immune response, a subset of them can develop in situ into Trm cells that remain in the tissue and do not recirculate anymore. Trm cells ensure their tissue-residency by acquisition of a specific set of receptors that varies between organs but typically involves upregulation of CD69 and downregulation of S1PR1 as well as other receptors enabling recirculation such as CCR7. In the tissue, upon reinfection, Trm can mount rapid antigen-specific recall responses, i.e. immunosurveillance at an early stage of infection. The focus of the present review by <a href="https://doi.org/10.3389/fimmu.2023.1111521">Ginsberg et&nbsp;al.</a>, are Trm cells residing in the kidney. The work illustrates the Janus-faced nature of this highly-sensitive memory T cell compartment, which provides antigen-specific immune protection against pathogens, but if autoreactive, can also promote autoimmune diseases. The review summarizes our knowledge on the role of pathogenic renal Trm in glomerulonephritis, emphasizing that most T cells in the kidney have a Trm phenotype and that their abundance correlates with disease severity. Finally, the authors discuss possible strategies to target pathogenic Trm in renal autoimmune disease based on metabolic inhibition, cytokine deprivation, or induction of antigen-specific tolerance with autoantigen-coated reagents.Besides tissue resident Trm, effector memory (Tem) and central memory (Tcm) T cells circulating through blood and lymph provide systemic protection, by patrolling secondary lymphoid organs and migrating to inflamed nonlymphoid tissues (<a href="#B6">6</a>). It is believed that Tem are more cytotoxic than Tcm cells, but a quantitative comparison including dissection of the cytotoxic mechanisms on the single cell level has been lacking. In their present work, <a href="https://doi.org/10.3389/fimmu.2022.838484">Knörck et&nbsp;al.</a> describe a FRET (Förster Resonance Energy Transfer)-based assay to distinguish between target cell apoptosis and necrosis on the single cell level. The study exemplifies the importance of cooperation across system boundaries, in this case a team of biophysicists employing a FRET reporter system developed by chemists (<a href="#B7">7</a>) and applying it to basic immunology questions. The authors used a Casper3 FRET reporter with a caspase 3 cleavage site to monitor apoptosis and necrosis event, simultaneously tracking effector/target cell contact times by real-time microscopy. Tem were more efficient killers than Tcm and this correlated to higher perforin content, faster target cell contact and more efficient immune synapse formation. The new technique will aid to evaluate how different T cell subsets, for example in CAR-T cell therapy, contribute to tumor control.Umbilical cord blood (CB) is another home of immune and hematopoietic cells of basic and applied interest. It not only is a source for allogeneic stem cells used for transplantation, but also holds great promise as a source for cell-based tumor therapy and the in vitro generation of immune cells with therapeutic potential that are rare in peripheral blood such as innate lymphoid cells (ILCs) and plasmacytoid dendritic cells (<a href="#B8">8</a>–<a href="#B11">11</a>). Here, <a href="https://doi.org/10.3389/fimmu.2023.1183215">Greppi et&nbsp;al.</a> have identified a novel subpopulation of NK cells in CB that express the checkpoint inhibitor programmed death receptor 1 (PD1). The same group had previously shown that so-called adaptive NK cells, a population of memory NK cells specifically arising upon infection with human Cytomegalovirus (HCMV) could express PD-1 (<a href="#B12">12</a>). It thus came as a surprise that PD1+ NK cells are also present in CB, which except for rare cases of mother-to-child transmission, had never been exposed to HCMV. Importantly, the authors were able to obtain CB from the child together with peripheral blood from the mother and could show that PD1+ NK cells arise independent of HCMV seropositivity of the mother. Notably, PD1+ NK cells were generally more responsive to target cell and cytokine stimulation whereas direct triggering of PD1 led to inhibition of PD1+ NK cells as expected. Given that PD1 checkpoint inhibition is widely used in treatment of cancer, it will be important to find out in how far this affects tumor-specific responses of CB-derived NK cells.Altogether, the different contributions in this Research Topic provide an excellent display of recent advances in cellular immunology in Europe.MU: Conceptualization, Writing &#x2013; original draft, Writing &#x2013; review &amp; editing. AR: Conceptualization, Writing &#x2013; original draft, Writing &#x2013; review &amp; editing.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Editorial: Global excellence in cellular immunology: Europe 2021

BackgroundNatural Killer cells (NKs) represent the innate counterpart of TCRαβ lymphocytes and are characterized by a high anti-tumor and an anti-viral cytotoxic activity. Recently, it has been demonstrated that NKs can express PD-1 as an additional inhibitory receptor. Specifically, PD-1 was identified on a subpopulation of terminally differentiated NKs from healthy adults with previous HCMV infection. So far it is unknown whether PD-1 appears during NK-cell development and whether this process is directly or indirectly related to HCMV infection.</sec>MethodsIn this study, we analyzed the expression and function of PD-1 on Cord Blood derived NKs (CB-NKs) on a large cohort of newborns through multiparametric cytofluorimetric analysis.</sec>ResultsWe identified PD-1 on CB-NKs in more than of half the newborns analyzed. PD-1 was present on CD56dim NKs, and particularly abundant on CD56neg NKs, but only rarely present on CD56bright NKs. Importantly, unlike in adult healthy donors, in CB-NKs PD-1 is co-expressed not only with KIR, but also with NKG2A. PD-1 expression was independent of HCMV mother seropositivity and occurs in the absence of HCMV infection/reactivation during pregnancy. Notably, PD-1 expressed on CB-NKs was functional and mediated negative signals when triggered.</sec>ConclusionTo our understanding, this study is the first to report PD-1 expression on CB derived NKs and its features in perinatal conditions. These data may prove important in selecting the most suitable CB derived NK cell population for the development of different immunotherapeutic treatments.</sec>

Background: Natural Killer cells (NKs) represent the innate counterpart of TCRαβ lymphocytes and are characterized by a high anti-tumor and an anti-viral cytotoxic activity. Recently, it has been demonstrated that NKs can express PD-1 as an additional inhibitory receptor. Specifically, PD-1 was identified on a subpopulation of terminally differentiated NKs from healthy adults with previous HCMV infection. So far it is unknown whether PD-1 appears during NK-cell development and whether this process is directly or indirectly related to HCMV infection.Methods: In this study, we analyzed the expression and function of PD-1 on Cord Blood derived NKs (CB-NKs) on a large cohort of newborns through multiparametric cytofluorimetric analysis.Results: We identified PD-1 on CB-NKs in more than of half the newborns analyzed. PD-1 was present on CD56dim NKs, and particularly abundant on CD56neg NKs, but only rarely present on CD56bright NKs. Importantly, unlike in adult healthy donors, in CB-NKs PD-1 is co-expressed not only with KIR, but also with NKG2A. PD-1 expression was independent of HCMV mother seropositivity and occurs in the absence of HCMV infection/reactivation during pregnancy. Notably, PD-1 expressed on CB-NKs was functional and mediated negative signals when triggered.Conclusion: To our understanding, this study is the first to report PD-1 expression on CB derived NKs and its features in perinatal conditions. These data may prove important in selecting the most suitable CB derived NK cell population for the development of different immunotherapeutic treatments.

Identification of a novel cord blood NK cell subpopulation expressing functional programmed death receptor-1

Natural killer (NK) cells have emerged as key mediators of obesity-related adipose tissue inflammation. However, the phenotype of NK cell subsets residing in human adipose tissue are poorly defined, preventing a detailed understanding of their role in metabolic disorders. In this study, we applied multicolor flow cytometry to characterize CD56bright and CD56dim NK cells in blood and adipose tissue depots in individuals with obesity and identified surface proteins enriched on adipose tissue-resident CD56bright NK cells. Particularly, we found that adipose tissue harbored clusters of tissue-resident CD56bright NK cells signatured by the expression of CD26, CCR5 and CD63, possibly reflecting an adaptation to the microenvironment. Together, our findings provide broad insights into the identity of NK cells in blood and adipose tissue in relation to obesity.

Natural killer (NK) cells have emerged as key mediators of obesity-related adipose tissue inflammation. However, the phenotype of NK cell subsets residing in human adipose tissue are poorly defined, preventing a detailed understanding of their role in metabolic disorders. In this study, we applied multicolor flow cytometry to characterize CD56bright and CD56dim NK cells in blood and adipose tissue depots in individuals with obesity and identified surface proteins enriched on adipose tissue-resident CD56bright NK cells. Particularly, we found that adipose tissue harbored clusters of tissue-resident CD56bright NK cells signatured by the expression of CD26, CCR5 and CD63, possibly reflecting an adaptation to the microenvironment. Together, our findings provide broad insights into the identity of NK cells in blood and adipose tissue in relation to obesity.

Phenotypic diversity of human adipose tissue-resident NK cells in obesity

The discovery of tissue-resident memory T cells (TRM cells) reinterpreted the potential of human tissue-specific immunity. Following T cell receptor (TCR) activation and clonal expansion, effector T cells migrate to peripheral tissues where they remain long-term and differentiate to TRM cells after antigen clearance. This allows for prompt immunological responses upon antigen re-encounter. In addition to their protective properties in acute infections, recent studies have revealed that TRM cells might lead to aggravation of autoimmune diseases, such as lupus nephritis (LN) and anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis (GN). These diseases present as proliferative and crescentic glomerulonephritis (cGN), which is a life-threatening condition leading to end-stage renal disease (ESRD) if left untreated. A better understanding of renal TRM cells might lead to identifying new therapeutic targets for relapsing autoimmune diseases of the kidney. In this review, we summarize the current knowledge of renal TRM cells and discuss their potential pathophysiological roles in renal autoimmune diseases.

The discovery of tissue-resident memory T cells (TRM cells) reinterpreted the potential of human tissue-specific immunity. Following T cell receptor (TCR) activation and clonal expansion, effector T cells migrate to peripheral tissues where they remain long-term and differentiate to TRM cells after antigen clearance. This allows for prompt immunological responses upon antigen re-encounter. In addition to their protective properties in acute infections, recent studies have revealed that TRM cells might lead to aggravation of autoimmune diseases, such as lupus nephritis (LN) and anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis (GN). These diseases present as proliferative and crescentic glomerulonephritis (cGN), which is a life-threatening condition leading to end-stage renal disease (ESRD) if left untreated. A better understanding of renal TRM cells might lead to identifying new therapeutic targets for relapsing autoimmune diseases of the kidney. In this review, we summarize the current knowledge of renal TRM cells and discuss their potential pathophysiological roles in renal autoimmune diseases.

Tissue-resident memory T cells in renal autoimmune diseases

Immunological memory is important to protect humans against recurring diseases. Memory CD8+ T cells are required for quick expansion into effector cells but also provide immediate cytotoxicity against their targets. Whereas many functions of the two main cytotoxic subtypes, effector memory CD8+ T cells (TEM) and central memory CD8+ T cells (TCM), are well defined, single TEM and TCM cell cytotoxicity has not been quantified. To quantify cytotoxic efficiency of TEM and TCM, we developed a FRET-based single cell fluorescent assay with NALM6 target cells which allows analysis of target cell apoptosis, secondary necrosis following apoptosis, and primary necrosis after TEM- or TCM-target cell contact. Both, single cell and population cytotoxicity assays reveal a higher cytotoxic efficiency of TEM compared to TCM, as quantified by target cell apoptosis and secondary necrosis. Perforin, granzyme B, FasL, but not TRAIL expression are higher in TEM compared to TCM. Higher perforin levels (likely in combination with higher granzyme levels) mediate higher cytotoxic efficiency of TEM compared to TCM. Both, TEM and TCM need the same time to find their targets, however contact time between CTL and target, time to induce apoptosis, and time to induce secondary necrosis are all shorter for TEM. In addition, immune synapse formation in TEM appears to be slightly more efficient than in TCM. Defining and quantifying single TEM and TCM cytotoxicity and the respective mechanisms is important to optimize future subset-based immune therapies.

Immunological memory is important to protect humans against recurring diseases. Memory CD8+ T cells are required for quick expansion into effector cells but also provide immediate cytotoxicity against their targets. Whereas many functions of the two main cytotoxic subtypes, effector memory CD8+ T cells (TEM) and central memory CD8+ T cells (TCM), are well defined, single TEM and TCM cell cytotoxicity has not been quantified. To quantify cytotoxic efficiency of TEM and TCM, we developed a FRET-based single cell fluorescent assay with NALM6 target cells which allows analysis of target cell apoptosis, secondary necrosis following apoptosis, and primary necrosis after TEM- or TCM-target cell contact. Both, single cell and population cytotoxicity assays reveal a higher cytotoxic efficiency of TEM compared to TCM, as quantified by target cell apoptosis and secondary necrosis. Perforin, granzyme B, FasL, but not TRAIL expression are higher in TEM compared to TCM. Higher perforin levels (likely in combination with higher granzyme levels) mediate higher cytotoxic efficiency of TEM compared to TCM. Both, TEM and TCM need the same time to find their targets, however contact time between CTL and target, time to induce apoptosis, and time to induce secondary necrosis are all shorter for TEM. In addition, immune synapse formation in TEM appears to be slightly more efficient than in TCM. Defining and quantifying single TEM and TCM cytotoxicity and the respective mechanisms is important to optimize future subset-based immune therapies.