Abnormal development of intestinal intraepithelial lymphocytes and peripheral natural killer cells in mice lacking the IL-2 receptor beta chain

Bridging therapy-induced phenotypes and genetic immune dysregulation to study interleukin-2-induced immunotoxicology

Interleukin-2 (IL-2) holds promise for the treatment of cancer and autoimmune diseases, but its high-dose usage is associated with systemic immunotoxicity. Differential IL-2 receptor (IL-2R) regulation might impact function of cells upon IL-2 stimulation, possibly inducing cellular changes similar to patients with hypomorphic IL2RB mutations, presenting with multiorgan autoimmunity. Here, we show that sustained high-dose IL-2 stimulation of human lymphocytes drastically reduces IL-2Rβ surface expression especially on T cells, resulting in impaired IL-2R signaling which correlates with high IL-2Rα baseline expression. IL-2R signaling in NK cells is maintained. CD4+ T cells, especially regulatory T cells are more broadly affected than CD8+ T cells, consistent with lineage-specific differences in IL-2 responsiveness. Given the resemblance of cellular characteristics of high-dose IL-2-stimulated cells and cells from patients with IL-2Rβ defects, impact of continuous IL-2 stimulation on IL-2R signaling should be considered in the onset of clinical adverse events during IL-2 therapy.

A single-agent fusion of human IL-2 and anti-IL-2 antibody that selectively expands regulatory T cells

The occurrence of many autoimmune diseases takes root on the disrupted balance among Treg cells, Teff cells, etc. Low-dose interleukin-2 (IL-2) cytokine demonstrates promising clinical efficacy in the expansion of Treg cells and the treatment of autoimmune diseases. However, its clinical application is hindered by the small therapeutic index and short half-life. Previous studies have shown that non-covalent complex of human IL-2 and anti-IL-2 antibody biases cytokine activity towards Treg cells and extends IL-2's half-life. The clinical translation of such complex is non-trivial. In this study, we discover an anti-human IL-2 antibody and engineer a covalently-linked single-agent fusion of human IL-2 and its antibody that selectively expands Treg cells and exhibits superior disease control activity in animal models of ulcerative colitis and systemic lupus erythematosus, with proper safety profile and good developability. These studies pave the road for its clinical development in diverse autoimmune diseases.

Immunological Phenotyping of Mice with a Point Mutation in Cdk4

Cyclin-dependent kinases (CDKs) play a crucial role in regulation of the mammalian cell cycle. CDK4 and CDK6 control the G1/S restriction checkpoint through their ability to associate with cyclin D proteins in response to growth factor signals. CDK4 deficiency in mice gives rise to a range of endocrine-specific phenotypes including diabetes, infertility, dwarfism, and atrophy of the anterior pituitary. Although CDK6 deficiency can cause thymic atrophy due to a block in the double-negative (DN) to double-positive (DP) stage of T cell development, there are no overt defects in immune cell development reported for CDK4-deficient mice. Here, we examined the impact of a novel N-ethyl-N-nitrosourea-induced point mutation in the gene encoding CDK4 on immune cell development. Mutant mice (Cdk4wnch/wnch) showed normal development and differentiation of major immune cell subsets in the thymus and spleen. Moreover, T cells from Cdk4wnch/wnch mice exhibited normal cytokine production in response to in vitro stimulation. However, analysis of the mixed bone marrow chimeras revealed that Cdk4wnch/wnch-derived T cell subsets and NK cells are at a competitive disadvantage compared to Cdk4+/+-derived cells in the thymus and periphery of recipients. These results suggest a possible role for the CDK4wnch mutation in the development of some immune cells, which only becomes apparent when the Cdk4wnch/wnch mutant cells are in direct competition with wild-type immune cells in the mixed bone marrow chimera.

FOXO1 and FOXO3 Cooperatively Regulate Innate Lymphoid Cell Development

Natural killer (NK) cells play roles in viral clearance and early surveillance against malignant transformation, yet our knowledge of the underlying mechanisms controlling their development and functions remain incomplete. To reveal cell fate-determining pathways in NK cell progenitors (NKP), we utilized an unbiased approach and generated comprehensive gene expression profiles of NK cell progenitors. We found that the NK cell program was gradually established in the CLP to preNKP and preNKP to rNKP transitions. In line with FOXO1 and FOXO3 being co-expressed through the NK developmental trajectory, the loss of both perturbed the establishment of the NK cell program and caused stalling in both NK cell development and maturation. In addition, we found that the combined loss of FOXO1 and FOXO3 caused specific changes to the composition of the non-cytotoxic innate lymphoid cell (ILC) subsets in bone marrow, spleen, and thymus. By combining transcriptome and chromatin profiling, we revealed that FOXO TFs ensure proper NK cell development at various lineage-commitment stages through orchestrating distinct molecular mechanisms. Combined FOXO1 and FOXO3 deficiency in common and innate lymphoid cell progenitors resulted in reduced expression of genes associated with NK cell development including ETS-1 and their downstream target genes. Lastly, we found that FOXO1 and FOXO3 controlled the survival of committed NK cells via gene regulation of IL-15Rβ (CD122) on rNKPs and bone marrow NK cells. Overall, we revealed that FOXO1 and FOXO3 function in a coordinated manner to regulate essential developmental genes at multiple stages during murine NK cell and ILC lineage commitment.

The RNA helicase DHX15 is a critical regulator of natural killer-cell homeostasis and functions

The RNA helicase DHX15 is widely expressed in immune cells and traditionally thought to be an RNA splicing factor or a viral RNA sensor. However, the role of DHX15 in NK-cell activities has not been studied thus far. Here, we generated Dhx15-floxed mice and found that conditional deletion of Dhx15 in NK cells (Ncr1^CreDhx15^fl/fl mice) resulted in a marked reduction in NK cells in the periphery and that the remaining Dhx15-deleted NK cells failed to acquire a mature phenotype. As a result, Dhx15-deleted NK cells exhibited profound defects in their cytolytic functions. We also found that deletion of Dhx15 in NK cells abrogated their responsiveness to IL-15, which was associated with inhibition of IL-2/IL-15Rβ (CD122) expression and IL-15R signaling. The defects in Dhx15-deleted NK cells were rescued by ectopic expression of a constitutively active form of STAT5. Mechanistically, DHX15 did not affect CD122 mRNA splicing and stability in NK cells but instead facilitated the surface expression of CD122, likely through interaction with its 3'UTR, which was dependent on the ATPase domain of DHX15 rather than its splicing domain. Collectively, our data identify a key role for DHX15 in regulating NK-cell activities and provide novel mechanistic insights into how DHX15 regulates the IL-15 signaling pathway in NK cells.

Immune Evasion Mechanism and AXL

Extensive interest in cancer immunotherapy is reported according to the clinical importance of CTLA-4 and (PD-1/PD-L1) [programmed death (PD) and programmed death-ligand (PD-L1)] in immune checkpoint therapies. AXL is a receptor tyrosine kinase expressed in different types of cancer and in relation to resistance against various anticancer therapeutics due to poor clinical prognosis. AXL and its ligand, i.e., growth arrest-specific 6 (GAS6) proteins, are expressed on many cancer cells, and the GAS6/AXL pathway is reported to promote cancer cell proliferation, survival, migration, invasion, angiogenesis, and immune evasion. AXL is an attractive and novel therapeutic target for impairing tumor progression from immune cell contracts in the tumor microenvironment. The GAS6/AXL pathway is also of interest immunologically because it targets fewer antitumor immune responses. In effect, several targeted therapies are selective and nonselective for AXL, which are in preclinical and clinical development in multiple cancer types. Therefore, this review focuses on the role of the GAS6/AXL signaling pathway in triggering the immunosuppressive tumor microenvironment as immune evasion. This includes regulating its composition and activating T-cell exclusion with the immune-suppressive activity of regulatory T cells, which is related to one of the hallmarks of cancer survival. Finally, this article discusses the GAS6/AXL signaling pathway in the context of several immune responses such as NK cell activation, apoptosis, and tumor-specific immunity, especially PD-1/PDL-1 signaling.

Clean up by aisle 2: roles for IL-2 receptors in host defense and tolerance

Although IL-2 was first recognized as growth factor for T cells, it is now also appreciated to be a key regulator of T cells through its effects on regulatory T cells (Treg). The IL-2 receptor (IL-2R) subunits' different (i) ligand affinities, (ii) dimerization or trimerization relationships with other cytokine subunits, (iii) expression across multiple cell types, and (iv) downstream signaling effects, largely dictate cellular tolerance and antimicrobial processes. Defects in IL-2Rγ result in profound and almost universally fatal immune deficiency, unless treated with hematopoietic stem cell transplantation (HSCT). Defects in IL-2Rα and IL-2Rβ result in more limited infection susceptibility, particularly to herpesviruses. However, the most prominent clinical symptomatology for IL-2Rα and IL-2Rβ defects include multi-organ autoimmunity and inflammation, consistent with the critical role of IL-2 in establishing and maintaining immune tolerance. Here, we review how we have arrived at our current understanding of the complex roles of IL-2/2R in host defense and tolerance focusing on the insights gained from human clinical immunology.

The Potential of Harnessing IL-2-Mediated Immunosuppression to Prevent Pathogenic B Cell Responses

Immunosuppressive drugs can partially control Antibody (Ab)-dependent pathology. However, these therapeutic regimens must be maintained for the patient's lifetime, which is often associated with severe side effects. As research advances, our understanding of the cellular and molecular mechanisms underlying the development and maintenance of auto-reactive B cell responses has significantly advanced. As a result, novel immunotherapies aimed to restore immune tolerance and prevent disease progression in autoimmune patients are underway. In this regard, encouraging results from clinical and preclinical studies demonstrate that subcutaneous administration of low-doses of recombinant Interleukin-2 (r-IL2) has potent immunosuppressive effects in patients with autoimmune pathologies. Although the exact mechanism by which IL-2 induces immunosuppression remains unclear, the clinical benefits of the current IL-2-based immunotherapies are attributed to its effect on bolstering T regulatory (Treg) cells, which are known to suppress overactive immune responses. In addition to Tregs, however, rIL-2 also directly prevent the T follicular helper cells (Tfh), T helper 17 cells (Th17), and Double Negative (DN) T cell responses, which play critical roles in the development of autoimmune disorders and have the ability to help pathogenic B cells. Here we discuss the broader effects of rIL-2 immunotherapy and the potential of combining rIL-2 with other cytokine-based therapies to more efficiently target Tfh cells, Th17, and DN T cells and subsequently inhibit auto-antibody (ab) production in autoimmune patients.

IL-2Rβγ signalling in lymphocytes promotes systemic inflammation and reduces plasma cholesterol in atherosclerotic mice

BACKGROUND AND AIMS

The relationship between inflammation and lipid metabolism is complex and bidirectional. Lymphocyte-driven inflammation has been shown to modulate both atherosclerotic plaque development and cholesterol levels, but the mechanisms are incompletely understood.

METHODS

The cardiometabolic effects of IL-2Rβγ signalling in atherosclerotic Apoe^-/- mice were investigated by treatment with an agonistic IL-2Rβγ-targeting IL-2/anti-IL-2 complex or a monoclonal anti-CD122 (IL-2Rβ) blocking antibody.

RESULTS

Administration of IL-2Rβγ agonistic IL-2/anti-IL-2 complexes to Apoe^-/- mice augmented opposing arms of the adaptive immune system. Expansion of effector/memory T cells and increased levels of circulating pro-inflammatory cytokines were observed along with elevated levels of regulatory T cells and IL-10. Notably, IL-2/anti-IL-2 treatment did not affect plaque size but decreased levels of plasma cholesterol. The cholesterol lowering effect of IL-2Rβγ agonism was not affected by anti-CD8 or anti-NK1.1 depleting antibody treatment but was contingent on the presence of adaptive immunity. Expression of multiple liver X receptor (LXR)-related genes, including Pltp and Srebp1c in the liver, was decreased by IL-2/anti-IL-2 treatment. Although IL-2Rβγ agonism lowered cholesterol levels, blocking IL-2Rβγ signalling using an anti-CD122 monoclonal antibody did not impact cholesterol levels or plaque burden in Apoe^-/- mice.

CONCLUSIONS

Elevated IL-2Rβγ signalling results in activation of both inflammatory and regulatory lymphocytes with a net zero effect on atherosclerosis and decreased plasma cholesterol levels. Changes in cholesterol levels were associated with reductions in hepatic LXR-related gene expression. Further studies are needed to investigate the clinical significance of IL-2 mediated modulation of hepatic LXR signalling in inflammatory disorders.

Transcription Factors Associated With IL-15 Cytokine Signaling During NK Cell Development

Natural killer (NK) cells are lymphocytes primarily involved in innate immunity and possess important functional properties in anti-viral and anti-tumor responses; thus, these cells have broad potential for clinical utilization. NK cells originate from hematopoietic stem cells (HSCs) through the following two independent and continuous processes: early commitment from HSCs to IL-15-responsive NK cell progenitors (NKPs) and subsequent differentiation into mature NK cells in response to IL-15. IL-15 is the most important cytokine for NK cell development, is produced by both hematopoietic and nonhematopoietic cells, and functions through a distinct delivery process termed transpresentation. Upon being transpresented to NK cells, IL-15 contributes to NK cell development via the activation of several downstream signaling pathways, including the Ras-MEK-MAPK, JAK-STAT5, and PI3K-ATK-mTOR pathways. Nonetheless, the exact role of IL-15 in NK cell development has not been discussed in a consecutive and comprehensive manner. Here, we review current knowledge about the indispensable role of IL-15 in NK cell development and address which cells produce IL-15 to support NK cell development and when IL-15 exerts its function during multiple developmental stages. Specifically, we highlight how IL-15 supports NK cell development by elucidating the distinct transpresentation of IL-15 to NK cells and revealing the downstream target of IL-15 signaling during NK cell development.

A High-Salt Diet Disturbs the Development and Function of Natural Killer Cells in Mice

A high-salt diet (HSD) is common worldwide and can lead to cardiovascular disease, chronic inflammation, and autoimmune diseases. Moreover, increasing evidence shows that HSD is closely related to a variety of immune diseases. Natural killer (NK) cells are important innate immune cells that directly kill their targets via degranulation and secretion of interferon gamma (IFN-γ). NK cells play a vital role in resisting viruses and preventing the malignant transformation of cells; however, whether HSD affects the development and function of NK cells has not yet been elucidated. Therefore, the purpose of the present study was to understand the effects of HSD on the development and function of NK cells, in addition to investigating the underlying molecular mechanism. Our results show that the number of NK cells in the spleen and lungs of HSD-fed mice was significantly reduced, which may be due to the inhibition of NK cell proliferation. Further, the development of NK cells in mice was evaluated, and it was found that HSD reduced the effective NK cell subset (CD27⁺CD11b⁻). Moreover, it was also found that the ability of NK cells to secrete CD107a and IFN-γ in HSD-fed mice was decreased following stimulation with RMA-S and YAC-1 tumor cells. Finally, the underlying molecular mechanism was evaluated, and it was found that HSD increased the production of reactive oxygen species (ROS) by NK cells, while the expression of CD122 was decreased, suggesting that HSD downregulates CD122 expression in NK cells via ROS signaling, thereby reducing the responsiveness to IL-15 and ultimately inhibiting NK cell function. The present research discovered a novel mechanism by which HSD inhibits the function of NK cells, providing an alternative avenue for the treatment of immune diseases caused by HSD.

Intestinal epithelial cell-derived IL-15 determines local maintenance and maturation of intra-epithelial lymphocytes in the intestine

Interleukin-15 (IL-15) is a cytokine critical for maintenance of intestinal intra-epithelial lymphocytes (IELs), especially CD8αα + IELs (CD8αα IELs). In the intestine, IL-15 is produced by intestinal epithelial cells (IECs), blood vascular endothelial cells (BECs) and hematopoietic cells. However, the precise role of intestinal IL-15 on IELs is still unknown. To address the question, we generated two kinds of IL-15 conditional knockout (IL-15cKO) mice: villin-Cre (Vil-Cre) and Tie2-Cre IL-15cKO mice. IEC-derived IL-15 was specifically deleted in Vil-Cre IL-15cKO mice, whereas IL-15 produced by BECs and hematopoietic cells was deleted in Tie2-Cre IL-15cKO mice. The cell number and frequency of CD8αα IELs and NK IELs were significantly reduced in Vil-Cre IL-15cKO mice. By contrast, CD8αα IELs were unchanged in Tie2-Cre IL-15cKO mice, indicating that IL-15 produced by BECs and hematopoietic cells is dispensable for CD8αα IELs. Expression of an anti-apoptotic factor, Bcl-2, was decreased, whereas Fas expression was increased in CD8αα IELs of Vil-Cre IL-15cKO mice. Forced expression of Bcl-2 by a Bcl-2 transgene partially restored CD8αα IELs in Vil-Cre IL-15cKO mice, suggesting that some IL-15 signal other than Bcl-2 is required for maintenance of CD8αα IELs. Furthermore, granzyme B production was reduced, whereas PD-1 expression was increased in CD8αα IELs of Vil-Cre IL-15cKO mice. These results collectively suggested that IEC-derived IL-15 is essential for homeostasis of IELs by promoting their survival and functional maturation.

Hematopoietic Stem Cell Niches and Signals Controlling Immune Cell Development and Maintenance of Immunological Memory

Studies over the last couple of decades have shown that hematopoietic stem cells (HSCs) are critically dependent on cytokines such as Stem Cell Factor and other signals provided by bone marrow niches comprising of mesenchymal stem and progenitor cells (MSPCs) and endothelial cells (ECs). Because of their critical roles in HSC maintenance the niches formed by MSPCs and ECs are commonly referred to as HSC niches. For the most part, the signals required for HSC maintenance act in a short-range manner, which imposes the necessity for directional and positional cues in order for HSCs to localize and be retained properly in stem cell niches. The chemokine CXCL12 and its Gαi protein coupled receptor CXCR4, besides promoting HSC quiescence directly, also play instrumental roles in enabling HSCs to access bone marrow stem cell niches. Recent studies have revealed, however, that HSC niches also provide a constellation of hematopoietic cytokines that are critical for the production of most, if not all, blood cell types. Some hematopoietic cytokines, namely IL-7 and IL-15 produced by HSC niches, are not only required for lymphopoiesis but are also essential for memory T cell maintenance. Consequently, hematopoietic progenitors and differentiated immune cells, such as memory T cell subsets, also depend on the CXCL12/CXCR4 axis for migration into bone marrow and interactions with MSPCs and ECs. Similarly, subsets of antibody-secreting plasma cells also reside in close association with CXCL12-producing MSPCs in the bone marrow and require the CXCR4/CXCL12 axis for survival and long-term maintenance. Collectively, these studies demonstrate a broad range of key physiological roles, spanning blood cell production and maintenance of immunological memory, that are orchestrated by stem cell niches through a common and simple mechanism: CXCL12/CXCR4-mediated cell recruitment followed by receipt of a maintenance and/or instructive signal. A fundamental flaw of this type of cellular organization is revealed by myeloid and lymphoid leukemias, which target stem cell niches and induce profound transcriptomic changes that result in reduced hematopoietic activity and altered mesenchymal cell differentiation.

Gut γδ T cells as guardians, disruptors, and instigators of cancer

Colorectal cancer is the third most common cancer worldwide with nearly 2 million cases per year. Immune cells and inflammation are a critical component of colorectal cancer progression, and they are used as reliable prognostic indicators of patient outcome. With the growing appreciation for immunology in colorectal cancer, interest is growing on the role γδ T cells have to play, as they represent one of the most prominent immune cell populations in gut tissue. This group of cells consists of both resident populations-γδ intraepithelial lymphocytes (γδ IELs)-and transient populations that each has unique functions. The homeostatic role of these γδ T cell subsets is to maintain barrier integrity and prevent microorganisms from breaching the mucosal layer, which is accomplished through crosstalk with enterocytes and other immune cells. Recent years have seen a surge in discoveries regarding the regulation of γδ IELs in the intestine and the colon with particular new insights into the butyrophilin family. In this review, we discuss the development, specialities, and functions of γδ T cell subsets during cancer progression. We discuss how these cells may be used to predict patient outcome, as well as how to exploit their behavior for cancer immunotherapy.

γδ T cell migration: Separating trafficking from surveillance behaviors at barrier surfaces

γδ T cells are found in highest numbers at barrier surfaces throughout the body, including the skin, intestine, lung, gingiva, and uterus. Under homeostatic conditions, γδ T cells provide immune surveillance of the epidermis, intestinal, and oral mucosa, whereas the presence of pathogenic microorganisms in the dermis or lungs elicits a robust γδ17 response to clear the infection. Although T cell migration is most frequently defined in the context of trafficking, analysis of specific migratory behaviors of lymphocytes within the tissue microenvironment can provide valuable insight into their function. Intravital imaging and computational analyses have been used to define "search" behavior associated with conventional αβ T cells; however, based on the known role of γδ T cells as immune sentinels at barrier surfaces and their TCR-independent functions, we put forth the need to classify distinct migratory patterns that reflect the surveillance capacity of these unconventional lymphocytes. This review will focus on how γδ T cells traffic to various barrier surfaces and how recent investigation into their migratory behavior has provided unique insight into the contribution of γδ T cells to barrier immunity.

The Role of Tissue-resident T Cells in Stress Surveillance and Tissue Maintenance

While forming a minor population in the blood and lymphoid compartments, T cells are significantly enriched within barrier tissues. In addition to providing protection against infection, these tissue-resident T cells play critical roles in tissue homeostasis and repair. T cells in the epidermis and intestinal epithelium produce growth factors and cytokines that are important for the normal turnover and maintenance of surrounding epithelial cells and are additionally required for the efficient recognition of, and response to, tissue damage. A role for tissue-resident T cells is emerging outside of the traditional barrier tissues as well, with recent research indicating that adipose tissue-resident T cells are required for the normal maintenance and function of the adipose tissue compartment. Here we review the functions of tissue-resident T cells in the epidermis, intestinal epithelium, and adipose tissue, and compare the mechanisms of their activation between these sites.

Current progress in NK cell biology and NK cell-based cancer immunotherapy

A better understanding of the complex interactions between the immune system and tumour cells from different origins has opened the possibility to design novel procedures of antitumoral immunotherapy. One of these novel approaches is based on the use of autologous or allogeneic natural killer (NK) cells to treat cancer. In the last decade, different strategies to activate NK cells and their use in adoptive NK cell-based therapy have been established. Although NK cells are often considered as a uniform cell population, several phenotypic and functionally distinct NK cells subsets exist in healthy individuals, that are differentially affected by ageing or by apparently innocuous viruses such as cytomegalovirus (CMV). In addition, further alterations in the expression of activating and inhibitory receptors are found in NK cells from cancer patients, likely because of their interaction with tumour cells. Thus, NK cells represent a promising strategy for adoptive immunotherapy of cancer already tested in phase 1/2 clinical trials. However, the existence of NK cell subpopulations expressing different patterns of activating and inhibitory receptors and different functional capacities, that can be found to be altered not only in cancer patients but also in healthy individuals stratified by age or CMV infection, makes necessary a personalized definition of the procedures used in the selection, expansion, and activation of the relevant NK cell subsets to be successfully used in NK cell-based immunotherapy.

Dysregulation of T Follicular Helper Cells in Lupus

Although multiple and overlapping mechanisms are ultimately responsible for the immunopathology observed in patients with systemic lupus erythematosus, autoreactive Abs secreted by autoreactive plasma cells (PCs) are considered to play a critical role in disease progression and immunopathology. Given that PCs derive from the germinal centers (GC), long-term dysregulated GC reactions are often associated with the development of spontaneous autoantibody responses and immunopathology in systemic lupus erythematosus patients. In this review, we summarize the emerging evidence concerning the roles of T follicular helper cells in regulating pathogenic GC and autoreactive PC responses in lupus.

Trans-endocytosis of intact IL-15Rα-IL-15 complex from presenting cells into NK cells favors signaling for proliferation

Interleukin 15 (IL-15) is an essential cytokine for the survival and proliferation of natural killer (NK) cells. IL-15 activates signaling by the β and common γ (γ_c) chain heterodimer of the IL-2 receptor through trans-presentation by cells expressing IL-15 bound to the α chain of the IL-15 receptor (IL-15Rα). We show here that membrane-associated IL-15Rα-IL-15 complexes are transferred from presenting cells to NK cells through trans-endocytosis and contribute to the phosphorylation of ribosomal protein S6 and NK cell proliferation. NK cell interaction with soluble or surface-bound IL-15Rα-IL-15 complex resulted in Stat5 phosphorylation and NK cell survival at a concentration or density of the complex much lower than required to stimulate S6 phosphorylation. Despite this efficient response, Stat5 phosphorylation was reduced after inhibition of metalloprotease-induced IL-15Rα-IL-15 shedding from trans-presenting cells, whereas S6 phosphorylation was unaffected. Conversely, inhibition of trans-endocytosis by silencing of the small GTPase TC21 or expression of a dominant-negative TC21 reduced S6 phosphorylation but not Stat5 phosphorylation. Thus, trans-endocytosis of membrane-associated IL-15Rα-IL-15 provides a mode of regulating NK cells that is not afforded to IL-2 and is distinct from activation by soluble IL-15. These results may explain the strict IL-15 dependence of NK cells and illustrate how the cellular compartment in which receptor-ligand interaction occurs can influence functional outcome.

Progress in interleukin-2 therapy for rheumatic immune diseases by regulating the immune balance of T cells

Breaking the balance between effector T cells, including Th17 (T helper cell 17) cells, and regulatory T cells (Tregs) is a key link in the pathogenesis of rheumatic immune diseases, which lead to a new concept of regulating immune balance in the treatment of rheumatic immune diseases. Interleukin (IL)-2 can effectively regulate the differentiation, development and functional activity of regulatory T cells, thus restoring the immune balance between regulatory T cells and effector T cells. Therefore, low-dose IL-2 has been used in the treatment of rheumatic immune diseases, and it has become a promising new choice to achieve therapeutic purpose by regulating the immune balance of T cell. Here, we discuss the role of T cells immune imbalance in the pathogenesis of rheumatic immune diseases and the mechanism of IL-2 in the treatment of rheumatic immune diseases by regulating T cells immune balance and summarize the relevant clinical trials.

Natural killer cells protect white matter integrity in bipolar disorder

BACKGROUND

Bipolar Disorder (BD) associates with disrupted white matter (WM) microstructure and functional connectivity, and with a perturbation of the immune system. Higher cytokines, and reduced T cells, correlated with WM disruption and fMRI responses. A core component of the innate immune system, natural killer (NK) cells were detected in brain parenchyma, but never studied in BD.

METHODS

We studied Diffusion Tensor Imaging (DTI) measures of water diffusion, fMRI corticolimbic functional response and connectivity, and multi-parameter cytofluorometry analysis of NK (CD56⁺) subpopulations, in 30 inpatients with active Bipolar Disorder type I. NK cells were also obtained in 36 healthy controls.

RESULTS

Patients had significantly higher circulating counts of CD56⁺GMCSF⁺, CD56⁺INFγ⁺, and CD56⁺IL17⁺. NK cell levels positively associated to fractional anisotropy (FA) measures. CD56⁺TNFα⁺, CD56⁺INFγ⁺, and CD56⁺GMCSF⁺ directly correlated with FA, and inversely with radial (RD) and mean (MD) diffusivity. Duration of lithium treatment associated with higher CD56⁺TNFα⁺, CD56⁺IL2⁺, and CD56⁺IL4⁺, and positively associated with FA in tracts were NKs had significant effects. A mediation model suggested a partial mediation of CD56⁺TNFα⁺ cells, higher in patients on lithium, on the effects of lithium on FA. Frequencies of the same cytokine-producing NK cells also influenced fMRI cortico-limbic functional connectivity during processing of both, emotional and non-emotional stimuli.

DISCUSSION

Higher circulating cytokine-producing NK cells associated with lithium treatment, and with DTI measures of WM integrity, partially mediating the effect of lithium on WM. The same cells associated with fMRI responses and connectivity, thus suggesting an effect on structural and functional connectomics in BD.

NK Cell Hyporesponsiveness: More Is Not Always Better

Natural Killer (NK) cells are a type of cytotoxic lymphocytes that play an important role in the innate immune system. They are of particular interest for their role in elimination of intracellular pathogens, viral infection and tumor cells. As such, numerous strategies are being investigated in order to potentiate their functions. One of these techniques aims at promoting the function of their activating receptors. However, different observations have revealed that providing activation signals could actually be counterproductive and lead to NK cells’ hyporesponsiveness. This phenomenon can occur during the NK cell education process, under pathological conditions, but also after treatment with different agents, including cytokines, that are promising tools to boost NK cell function. In this review, we aim to highlight the different circumstances where NK cells become hyporesponsive and the methods that could be used to restore their functionality.

Dynamics and clinical significance of intestinal intraepithelial lymphocytes

Intestinal intraepithelial lymphocytes (IELs) are one of the largest populations of lymphocytes and comprised of heterogeneous populations with varying phenotypes and physiological/pathological functions. IELs located between the basolateral surfaces of the epithelial cells and then potentially provide a first line of immune defense against enteric pathogens, although, the precise roles of each IEL populations are not well defined. A variety of molecules are involved in the IEL-homing to the intestinal epithelium. Conventional IELs originate from circulating T cells activated in lymphoid organs and imprinted for gut homing. On the other hand, unconventional IELs derive from thymocytes and migrate to the intestinal epithelium, although, some of them may arise extrathymically. Regarding the interaction between IELs and epithelial cells, IELs are known to be highly motile and actively migrate along the basement membrane, suggesting their roles in immune surveillance. In addition, there has been growing evidence to support that IELs are involved in the pathogenesis of gut disorders such as celiac disease and inflammatory bowel diseases. In this review, we provide a comprehensive overview of IEL dynamics and their clinical significance.

Strategies to enhance NK cell function for the treatment of tumors and infections

Natural killer (NK) cells are innate immune cells equipped with the ability to rapidly kill stressed cells that are neoplastic or virally infected. These cells are especially important in settings where these stressed cells downregulate MHC class I molecules and evade recognition by cytotoxic T cells. However, the activity of NK cells alone is often suboptimal to fully control tumor growth or to clear viral infections. Thus, the enhancement of NK cell function is necessary to fully harness their antitumor or antiviral potential. In this review, we discuss how NK cell function can be augmented by the modulation of signal transduction pathways, by the manipulation of inhibitory/activating receptors on NK cells, and by cytokine-induced activation. We also discuss how some of these strategies are currently impacting NK cells in the treatment of cancer and infections.

CD137 (4-1BB) Engagement Fine-Tunes Synergistic IL-15- and IL-21-Driven NK Cell Proliferation

To understand and dissect the mechanisms driving human NK cell proliferation, we exploited the methodology used in cell therapy to numerically expand NK cells in the presence of K562-derived artificial APC (aAPCs) and cytokines. For four consecutive weeks, high expression of CD137L by a K562-derived aAPC cell line could sustain NK cell expansion by 3 × 10⁵-fold, whereas low expression of CD137L by the parental K562 cell line only supported the expansion by 2 × 10³-fold. The level of expression of CD137L, however, did not modulate the sensitivity of K562 cells to the intrinsic cytotoxicity of NK cells. Similarly, the low NK cell proliferation in the presence of the parental K562 cell line and cytokines was increased by adding agonistic anti-CD137 Abs to levels similar to CD137L-expressing K562-derived aAPCs. Finally, synergy between IL-15 and IL-21 was observed only upon CD137 engagement and the presence of aAPCs. Therefore, we conclude that NK cell proliferation requires cell-to-cell contact, activation of the CD137 axis, and presence of IL-15 (or its membranous form) and IL-21. By analogy with the three-signal model required to activate T cells, we speculate that the cell-to-cell contact represents "signal 1," CD137 represents "signal 2," and cytokines represent "signal 3." The precise nature of signal 1 remains to be defined.

IL2RB maintains immune harmony

How the IL-2 receptor β-chain specifically shapes immunity has remained enigmatic. In this issue of JEM, Zhang et al. (https://doi.org/10.1084/jem.20182304) and Fernandez et al. (https://doi.org/10.1084/jem.20182015) independently report the first observations of autosomal recessive mutations in IL2RB, revealing a requirement for IL2RB in immunity and peripheral immune tolerance.

Human interleukin-2 receptor β mutations associated with defects in immunity and peripheral tolerance

Zhang et al. identify human IL-2Rβ deficiency as a cause of severe immune dysregulation. The hypomorphic gene mutations reveal variable IL-2Rβ expression and function between different lymphocyte subsets as a means of selectively modulating immune responses.

Interleukin-2, which conveys essential signals for immunity, operates through a heterotrimeric receptor. Here we identify human interleukin-2 receptor (IL-2R) β chain (IL2RB) gene defects as a cause of life-threatening immune dysregulation. We report three homozygous mutations in the IL2RB gene of eight individuals from four consanguineous families that cause disease by distinct mechanisms. Nearly all patients presented with autoantibodies, hypergammaglobulinemia, bowel inflammation, dermatological abnormalities, lymphadenopathy, and cytomegalovirus disease. Patient T lymphocytes lacked surface expression of IL-2Rβ and were unable to respond to IL-2 stimulation. By contrast, natural killer cells retained partial IL-2Rβ expression and function. IL-2Rβ loss of function was recapitulated in a recombinant system in which IL2RB mutations caused reduced surface expression and IL-2 binding. Stem cell transplant ameliorated clinical symptoms in one patient; forced expression of wild-type IL-2Rβ also increased the IL-2 responsiveness of patient T lymphocytes in vitro. Insights from these patients can inform the development of IL-2–based therapeutics for immunological diseases and cancer.

An Id2RFP-Reporter Mouse Redefines Innate Lymphoid Cell Precursor Potentials

Innate lymphoid cell (ILC) development proposes that ILC precursors (ILCPs) segregate along natural killer (NK) cell versus helper cell (ILC1, ILC2, ILC3) pathways, the latter depending on expression of Id2, Zbtb16, and Gata3. We have developed an Id2-reporter strain expressing red fluorescent protein (RFP) in the context of normal Id2 expression to re-examine ILCP phenotype and function. We show that bone-marrow ILCPs were heterogeneous and harbored extensive NK-cell potential in vivo and in vitro. By multiplexing Id2RFP with Zbtb16CreGFP and Bcl11btdTomato strains, we made a single-cell dissection of the ILCP compartment. In contrast with the current model, we have demonstrated that Id2+Zbtb16+ ILCPs included multi-potent ILCPs that retained NK-cell potential. Late-stage ILC2P and ILC3P compartments could be defined by differential Zbtb16 and Bcl11b expression. We suggest a revised model for ILC differentiation that redefines the cell-fate potential of helper-ILC-restricted Zbtb16+ ILCPs.

Human intraepithelial lymphocytes

The location of intraepithelial lymphocytes (IEL) between epithelial cells, their effector memory, cytolytic and inflammatory phenotype positions them to kill infected epithelial cells and protect the intestine against pathogens. Human TCRαβ+CD8αβ+ IEL have the dual capacity to recognize modified self via natural killer (NK) receptors (autoreactivity) as well as foreign antigen via the T cell receptor (TCR), which is accomplished in mouse by two cell subsets, the naturally occurring TCRαβ+CD8αα+ and adaptively induced TCRαβ+CD8αβ+ IEL subsets, respectively. The private/oligoclonal nature of the TCR repertoire of both human and mouse IEL suggests local environmental factors dictate the specificity of IEL responses. The line between sensing of foreign antigens and autoreactivity is blurred for IEL in celiac disease, where recognition of stress ligands by induced activating NK receptors in conjunction with inflammatory signals such as IL-15 can result in low-affinity TCR/non-cognate antigen and NK receptor/stress ligand interactions triggering destruction of intestinal epithelial cells.

Epithelial IL-15 Is a Critical Regulator of γδ Intraepithelial Lymphocyte Motility within the Intestinal Mucosa

Intraepithelial lymphocytes (IELs) expressing the γδ TCR (γδ IELs) provide continuous surveillance of the intestinal epithelium. However, the mechanisms regulating the basal motility of these cells within the epithelial compartment have not been well defined. We investigated whether IL-15 contributes to γδ IEL localization and migratory behavior in addition to its role in IEL differentiation and survival. Using advanced live cell imaging techniques in mice, we find that compartmentalized overexpression of IL-15 in the lamina propria shifts the distribution of γδ T cells from the epithelial compartment to the lamina propria. This mislocalization could be rescued by epithelial IL-15 overexpression, indicating that epithelial IL-15 is essential for γδ IEL migration into the epithelium. Furthermore, in vitro analyses demonstrated that exogenous IL-15 stimulates γδ IEL migration into cultured epithelial monolayers, and inhibition of IL-2Rβ significantly attenuates the basal motility of these cells. Intravital microscopy showed that impaired IL-2Rβ signaling induced γδ IEL idling within the lateral intercellular space, which resulted in increased early pathogen invasion. Similarly, the redistribution of γδ T cells to the lamina propria due to local IL-15 overproduction also enhanced bacterial translocation. These findings thus reveal a novel role for IL-15 in mediating γδ T cell localization within the intestinal mucosa and regulating γδ IEL motility and patrolling behavior as a critical component of host defense.

Emergence of NK Cell Hyporesponsiveness after Two IL-15 Stimulation Cycles

IL-15 is a cytokine playing a crucial role in the function of immune cells, including NK and CD8 T cells. In this study, we demonstrated that in vivo, in mice, IL-15-prestimulated NK cells were no longer able to respond to a second cycle of IL-15 stimulation. This was illustrated by defects in cell maturation, proliferation, and activation, seemingly linked to the environment surrounding NK cells but not related to the presence of CD4 regulatory T cells, TGF-β, or IL-10. Moreover, NK cells from immunodeficient mice could respond to two cycles of IL-15 stimulation, whereas an adoptive transfer of CD44⁺CD8⁺ cells impaired their responsiveness to the second cycle. Conversely, in immunocompetent mice, NK cell responsiveness to a second IL-15 stimulation was restored by the depletion of CD8⁺ cells. These biological findings refine our understanding of the complex mode of action of NK cells in vivo, and they should be taken into consideration for IL-15-based therapy.

mTORC1 and mTORC2 differentially promote natural killer cell development

Natural killer (NK) cells are innate lymphoid cells that are essential for innate and adaptive immunity. Mechanistic target of rapamycin (mTOR) is critical for NK cell development; however, the independent roles of mTORC1 or mTORC2 in regulating this process remain unknown. Ncr1^iCre-mediated deletion of Rptor or Rictor in mice results in altered homeostatic NK cellularity and impaired development at distinct stages. The transition from the CD27⁺CD11b⁻ to the CD27⁺CD11b⁺ stage is impaired in Rptor cKO mice, while, the terminal maturation from the CD27⁺CD11b⁺ to the CD27⁻CD11b⁺ stage is compromised in Rictor cKO mice. Mechanistically, Raptor-deficiency renders substantial alteration of the gene expression profile including transcription factors governing early NK cell development. Comparatively, loss of Rictor causes more restricted transcriptome changes. The reduced expression of T-bet correlates with the terminal maturation defects and results from impaired mTORC2-Akt^S473-FoxO1 signaling. Collectively, our results reveal the divergent roles of mTORC1 and mTORC2 in NK cell development.

SOCS-mediated immunomodulation of natural killer cells

Natural killer (NK) cells are innate immune cells with an intrinsic ability to detect and kill infected and cancerous cells. The success of therapies targeting immune checkpoints on CD8 cells has intensified interest in harnessing the cytolytic effector functions of NK cells for new cancer treatments. NK cell development, survival and effector activity is dependent on exposure to the cytokine interleukin (IL)-15. The suppressor of cytokine (SOCS) proteins (CIS; SOCS1-7) are important negative regulators of cytokine signaling, and both CIS and SOCS2 are reported to have roles in regulating NK cell responses. Their immunomodulatory effects on NK cells suggest that these SOCS proteins are promising targets that can potentially form the basis of novel cancer therapies. Here we discuss the role of NK cells in tumor immunity as well as review the role of the SOCS proteins in regulating IL-15 signaling and NK cell function.

Out-sourcing for Trans-presentation: Assessing T Cell Intrinsic and Extrinsic IL-15 Expression with Il15 Gene Reporter Mice

IL-15 is a cytokine of the common γ-chain family that is critical for natural killer (NK), invariant natural killer T (iNKT), and CD8 memory T cell development and homeostasis. The role of IL-15 in regulating effector T cell subsets, however, remains incompletely understood. IL-15 is mostly expressed by stromal cells, myeloid cells, and dendritic cells (DCs). Whether T cells themselves can express IL-15, and if so, whether such T cell-derived IL-15 could play an autocrine role in T cells are interesting questions that were previously addressed but answered with mixed results. Recently, three independent studies described the generation of IL-15 reporter mice which facilitated the identification of IL-15-producing cells and helped to clarify the role of IL-15 both in vitro and in vivo. Here, we review the findings of these studies and place them in context of recent reports that examined T cell-intrinsic IL-15 expression during CD4 effector T cell differentiation.

Natural killer cells and anti-tumor immunity

Immune checkpoint inhibitors harness the power of the immune system to fight cancer. The clinical success achieved with antibodies against the inhibitory T cell receptors PD-1 and CTLA4 has focused attention on the possibility of manipulating other immune cells, in particular those involved in innate immunity. Here we review the role of innate lymphoid cells (ILCs) and their contribution to tumor immunity. As the prototypical ILC, the natural killer (NK) cell has an intrinsic ability to detect and kill cancer cells. NK cells are dependent on the cytokine interleukin (IL)-15 for their development and effector activity. We discuss the role of the Suppressor of cytokine (SOCS) proteins in negatively regulating IL-15 and NK cell responses and the potential for targeting these small intracellular regulators as new immune checkpoints.

Long-term maintenance of peripheral blood derived human NK cells in a novel human IL-15- transgenic NOG mouse

We generated a novel mouse strain expressing transgenic human interleukin-15 (IL-15) using the severe immunodeficient NOD/Shi-scid-IL-2Rγ^null (NOG) mouse genetic background (NOG-IL-15 Tg). Human natural killer (NK) cells, purified from the peripheral blood (hu-PB-NK) of normal healthy donors, proliferated when transferred into NOG-IL-15 Tg mice. In addition, the cell number increased, and the hu-PB-NK cells persisted for 3 months without signs of xenogeneic graft versus host diseases (xGVHD). These in vivo-expanded hu-PB-NK cells maintained the original expression patterns of various surface antigens, including NK receptors and killer cell immunoglobulin-like receptor (KIR) molecules. They also contained significant amounts of granzyme A and perforin. Inoculation of K562 leukemia cells into hu-PB-NK-transplanted NOG-IL-15 Tg mice resulted in significant suppression of tumor growth compared with non-transplanted mice. Furthermore, NOG-IL-15 Tg mice allowed for engraftment of in vitro-expanded NK cells prepared for clinical cell therapy. These cells exerted antibody-dependent cell-mediated cytotoxicity (ADCC) on Her2-positive gastric cancer cells in the presence of therapeutic anti-Her2 antibody, and subsequently suppressed tumor growth. Our results collectively suggest that the NOG-IL-15 Tg mice are a useful model for studying human NK biology and evaluating human NK cell-mediated in vivo cytotoxicity.

Allogeneic bone marrow transplant in the absence of cytoreductive conditioning rescues mice with β-thalassemia major

β-thalassemia is a group of inherited blood disorders that result in defects in β-globin chain production. Cooley anemia (CA), or β-thalassemia major, is the most severe form of the disease and occurs when an individual has mutations in both copies of the adult β-globin gene. Patients with CA fail to make adult hemoglobin, exhibit ineffective erythropoiesis, experience severe anemia, and are transfusion dependent for life. Currently, allogeneic bone marrow transplantation (BMT) is the only cure; however, few patients have suitable donors for this procedure, which has significant morbidity and mortality. In this study, a novel humanized murine model of CA is rescued from lethal anemia by allogeneic BMT in the absence of cytoreductive conditioning. A single intravenous postnatal injection of allogeneic bone marrow results in stable, mixed hematopoietic chimerism. Five months after transplantation, donor cells accounted for approximately 90% of circulating erythrocytes and up to 15% of hematopoietic stem and progenitor cells. Transplanted mice are transfusion independent, have marked improvement of hematological indices, exhibit no growth retardation or signs of graft-versus-host disease, and are fertile. This study describes a method for the consistent engraftment of allogeneic donor hematopoietic cells that rescues a humanized mouse model of CA from lethal anemia, all in the absence of toxic cytoreductive conditioning.

Natural killer-cell deficiency alters placental development in rats

Natural killer (NK) cells are the most prevalent leukocyte population in the uterus during early pregnancy. Natural killer cells contribute to uterine vascular (spiral artery) remodeling in preparation for the increased demand on these vessels later in pregnancy. A second wave of spiral artery modification is directed by invasive trophoblast cells. The significance of the initial wave of NK-cell-mediated vascular remodeling in species exhibiting deep trophoblast invasion such as humans and rats is not known. The purpose of this study was to generate a genetic model of NK-cell deficiency in rats, and determine the consequences of NK-cell deficiency on spiral artery remodeling and reproductive outcomes. To accomplish this task, we utilized zinc finger nuclease-mediated genome editing of the rat interleukin-15 (Il15) gene. Il15 encodes a cytokine required for NK-cell lineage development. Using this strategy, a founder rat was generated containing a frameshift deletion in Il15. Uteri of females harboring a homozygous mutation at the Il15 locus contained no detectable NK cells. NK-cell deficiency did not impact fetal growth or viability. However, NK-cell deficiency caused major structural changes to the placenta, including expansion of the junctional zone and robust, early-onset activation of invasive trophoblast-guided spiral artery remodeling. In summary, we successfully generated an NK-cell-deficient rat and showed, using this model, that NK cells dampen the extent of trophoblast invasion and delay trophoblast-directed spiral artery remodeling. This study furthers our understanding of the role of NK cells on uterine vascular remodeling, trophoblast invasion, and placental development.

Critical functions for STAT5 tetramers in the maturation and survival of natural killer cells

Interleukin-15 (IL-15) is essential for the development and maintenance of natural killer (NK) cells. IL-15 activates STAT5 proteins, which can form dimers or tetramers. We previously found that NK cell numbers are decreased in Stat5a-Stat5b tetramer-deficient double knockin (DKI) mice, but the mechanism was not investigated. Here we show that STAT5 dimers are sufficient for NK cell development, whereas STAT5 tetramers mediate NK cell maturation and the expression of maturation-associated genes. Unlike the defective proliferation of Stat5 DKI CD8⁺ T cells, Stat5 DKI NK cells have normal proliferation to IL-15 but are susceptible to death upon cytokine withdrawal, with lower Bcl2 and increased active caspases. These findings underscore the importance of STAT5 tetramers in maintaining NK cell homoeostasis. Moreover, defective STAT5 tetramer formation could represent a cause of NK cell immunodeficiency, and interrupting STAT5 tetramer formation might serve to control NK leukaemia.

Subset- and tissue-defined STAT5 thresholds control homeostasis and function of innate lymphoid cells

Innate lymphoid cells (ILCs) patrol environmental interfaces to defend against infection and protect barrier integrity. Using a genetic tuning model, we demonstrate that the signal-dependent transcription factor (TF) STAT5 is critical for accumulation of all known ILC subsets in mice and reveal a hierarchy of STAT5 dependency for populating lymphoid and nonlymphoid tissues. We apply transcriptome and genomic distribution analyses to define a STAT5 gene signature in natural killer (NK) cells, the prototypical ILC subset, and provide a systems-based molecular rationale for its key functions downstream of IL-15. We also uncover surprising features of STAT5 behavior, most notably the wholesale redistribution that occurs when NK cells shift from tonic signaling to acute cytokine-driven signaling, and genome-wide coordination with T-bet, another key TF in ILC biology. Collectively, our data position STAT5 as a central node in the TF network that instructs ILC development, homeostasis, and function and provide mechanistic insights on how it works at cellular and molecular levels.

Tissue adaptation: Implications for gut immunity and tolerance

Faria et al. discuss the concept that immune cells undergo specialized adaptation to tissue-specific conditions and its potential implications for tolerance and immunity.

Tissue adaptation is an intrinsic component of immune cell development, influencing both resistance to pathogens and tolerance. Chronically stimulated surfaces of the body, in particular the gut mucosa, are the major sites where immune cells traffic and reside. Their adaptation to these environments requires constant discrimination between natural stimulation coming from harmless microbiota and food, and pathogens that need to be cleared. This review will focus on the adaptation of lymphocytes to the gut mucosa, a highly specialized environment that can help us understand the plasticity of leukocytes arriving at various tissue sites and how tissue-related factors operate to shape immune cell fate and function.

Suppressor of Cytokine Signaling 2 Negatively Regulates NK Cell Differentiation by Inhibiting JAK2 Activity

Suppressor of cytokine signaling (SOCS) proteins are negative regulators of cytokine responses. Although recent reports have shown regulatory roles for SOCS proteins in innate and adaptive immunity, their roles in natural killer (NK) cell development are largely unknown. Here, we show that SOCS2 is involved in NK cell development. SOCS2^−/− mice showed a high frequency of NK cells in the bone marrow and spleen. Knockdown of SOCS2 was associated with enhanced differentiation of NK cells in vitro, and the transplantation of hematopoietic stem cells (HSCs) into congenic mice resulted in enhanced differentiation in SOCS2^−/− HSCs. We found that SOCS2 could inhibit Janus kinase 2 (JAK2) activity and JAK2-STAT5 signaling pathways via direct interaction with JAK2. Furthermore, SOCS2^−/− mice showed a reduction in lung metastases and an increase in survival following melanoma challenge. Overall, our findings suggest that SOCS2 negatively regulates the development of NK cells by inhibiting JAK2 activity via direct interaction.

The checkpoint for agonist selection precedes conventional selection in human thymus

The thymus plays a central role in self-tolerance, partly by eliminating precursors with a T cell receptor (TCR) that binds strongly to self-antigens. However, the generation of self-agonist-selected lineages also relies on strong TCR signaling. How thymocytes discriminate between these opposite outcomes remains elusive. Here, we identified a human agonist-selected PD-1⁺ CD8αα⁺ subset of mature CD8αβ⁺ T cells that displays an effector phenotype associated with agonist selection. TCR stimulation of immature post-β-selection thymocyte blasts specifically gives rise to this innate subset and fixes early T cell receptor alpha variable (TRAV) and T cell receptor alpha joining (TRAJ) rearrangements in the TCR repertoire. These findings suggest that the checkpoint for agonist selection precedes conventional selection in the human thymus.

A 33D1+ Dendritic Cell/Autoreactive CD4+ T Cell Circuit Maintains IL-2-Dependent Regulatory T Cells in the Spleen

Phenotypically and functionally diverse regulatory T (Tr) cell subsets populate lymphoid and nonlymphoid tissues, where their maintenance and function are governed by unique homeostatic signals. Whereas Tr cells resident in nonlymphoid tissues depend on continual TCR signaling for their survival and function, phenotypically naive Tr cells occupying secondary lymphoid organs are largely supported by paracrine IL-2 signaling. Crucially, the absence of either of these distinct Tr cell subsets results in pathogenic autoimmunity, underscoring their nonredundant roles in the preservation of self-tolerance. However, the cellular and molecular factors precipitating IL-2 release and subsequent maintenance of secondary lymphoid organ-resident Tr cells are still poorly understood. In this study, we report that IL-2-dependent Tr cells in the spleen compete for a limiting supply of paracrine IL-2 generated by autoreactive CD4(+) T cells in response to MHC class II-restricted autoantigen activation by 33D1(+)CD11b(int) dendritic cells. Manipulating this cellular circuit culminating in IL-2 production could have clinical benefits in settings in which diminished Tr cell abundance is desired.

IL-15 activates mTOR and primes stress-activated gene expression leading to prolonged antitumor capacity of NK cells

Treatment of hematological malignancies by adoptive transfer of activated natural killer (NK) cells is limited by poor postinfusion persistence. We compared the ability of interleukin-2 (IL-2) and IL-15 to sustain human NK-cell functions following cytokine withdrawal to model postinfusion performance. In contrast to IL-2, IL-15 mediated stronger signaling through the IL-2/15 receptor complex and provided cell function advantages. Genome-wide analysis of cytosolic and polysome-associated messenger RNA (mRNA) revealed not only cytokine-dependent differential mRNA levels and translation during cytokine activation but also that most gene expression differences were primed by IL-15 and only manifested after cytokine withdrawal. IL-15 augmented mammalian target of rapamycin (mTOR) signaling, which correlated with increased expression of genes related to cell metabolism and respiration. Consistently, mTOR inhibition abrogated IL-15-induced cell function advantages. Moreover, mTOR-independent STAT-5 signaling contributed to improved NK-cell function during cytokine activation but not following cytokine withdrawal. The superior performance of IL-15-stimulated NK cells was also observed using a clinically applicable protocol for NK-cell expansion in vitro and in vivo. Finally, expression of IL-15 correlated with cytolytic immune functions in patients with B-cell lymphoma and favorable clinical outcome. These findings highlight the importance of mTOR-regulated metabolic processes for immune cell functions and argue for implementation of IL-15 in adoptive NK-cell cancer therapy.

Natural Killer Cell-Based Therapies Targeting Cancer: Possible Strategies to Gain and Sustain Anti-Tumor Activity

Natural killer (NK) cells were discovered 40 years ago, by their ability to recognize and kill tumor cells without the requirement of prior antigen exposure. Since then, NK cells have been seen as promising agents for cell-based cancer therapies. However, NK cells represent only a minor fraction of the human lymphocyte population. Their skewed phenotype and impaired functionality during cancer progression necessitates the development of clinical protocols to activate and expand to high numbers ex vivo to be able to infuse sufficient numbers of functional NK cells to the cancer patients. Initial NK cell-based clinical trials suggested that NK cell-infusion is safe and feasible with almost no NK cell-related toxicity, including graft-versus-host disease. Complete remission and increased disease-free survival is shown in a small number of patients with hematological malignances. Furthermore, successful adoptive NK cell-based therapies from haploidentical donors have been demonstrated. Disappointingly, only limited anti-tumor effects have been demonstrated following NK cell infusion in patients with solid tumors. While NK cells have great potential in targeting tumor cells, the efficiency of NK cell functions in the tumor microenvironment is yet unclear. The failure of immune surveillance may in part be due to sustained immunological pressure on tumor cells resulting in the development of tumor escape variants that are invisible to the immune system. Alternatively, this could be due to the complex network of immune-suppressive compartments in the tumor microenvironment, including myeloid-derived suppressor cells, tumor-associated macrophages, and regulatory T cells. Although the negative effect of the tumor microenvironment on NK cells can be transiently reverted by ex vivo expansion and long-term activation, the aforementioned NK cell/tumor microenvironment interactions upon reinfusion are not fully elucidated. Within this context, genetic modification of NK cells may provide new possibilities for developing effective cancer immunotherapies by improving NK cell responses and making them less susceptible to the tumor microenvironment. Within this review, we will discuss clinical trials using NK cells with a specific reflection on novel potential strategies, such as genetic modification of NK cells and complementary therapies aimed at improving the clinical outcome of NK cell-based immune therapies.

NK Cell Proliferation Induced by IL-15 Transpresentation Is Negatively Regulated by Inhibitory Receptors

IL-15 bound to the IL-15Rα-chain (IL-15Rα) is presented in trans to cells bearing the IL-2Rβ-chain and common γ-chain. As IL-15 transpresentation occurs in the context of cell-to-cell contacts, it has the potential for regulation by and of other receptor-ligand interactions. In this study, human NK cells were tested for the sensitivity of IL-15 transpresentation to inhibitory receptors. Human cells expressing HLA class I ligands for inhibitory receptors KIR2DL1, KIR2DL2/3, or CD94-NKG2A were transfected with IL-15Rα. Proliferation of primary NK cells in response to transpresented IL-15 was reduced by engagement of either KIR2DL1 or KIR2DL2/3 by cognate HLA-C ligands. Inhibitory KIR-HLA-C interactions did not reduce the proliferation induced by soluble IL-15. Therefore, transpresentation of IL-15 is subject to downregulation by MHC class I-specific inhibitory receptors. Similarly, proliferation of the NKG2A(+) cell line NKL induced by IL-15 transpresentation was inhibited by HLA-E. Coengagement of inhibitory receptors, either KIR2DL1 or CD94-NKG2A, did not inhibit phosphorylation of Stat5 but inhibited selectively phosphorylation of Akt and S6 ribosomal protein. IL-15Rα was not excluded from, but was evenly distributed across, inhibitory synapses. These findings demonstrate a novel mechanism to attenuate IL-15-dependent NK cell proliferation and suggest that inhibitory NK cell receptors contribute to NK cell homeostasis.

Models to Study NK Cell Biology and Possible Clinical Application

Natural killer (NK) cells are large granular lymphocytes of the innate immune system, responsible for direct targeting and killing of both virally infected and transformed cells. NK cells rapidly recognize and respond to abnormal cells in the absence of prior sensitization due to their wide array of germline-encoded inhibitory and activating receptors, which differs from the receptor diversity found in B and T lymphocytes that is due to the use of recombination-activation gene (RAG) enzymes. Although NK cells have traditionally been described as natural killers that provide a first line of defense prior to the induction of adaptive immunity, a more complex view of NK cells is beginning to emerge, indicating they may also function in various immunoregulatory roles and have the capacity to shape adaptive immune responses. With the growing appreciation for the diverse functions of NK cells, and recent technological advancements that allow for a more in-depth understanding of NK cell biology, we can now begin to explore new ways to manipulate NK cells to increase their clinical utility. In this overview unit, we introduce the reader to various aspects of NK cell biology by reviewing topics ranging from NK cell diversity and function, mouse models, and the roles of NK cells in health and disease, to potential clinical applications. © 2015 by John Wiley & Sons, Inc.