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Recent evidence indicates that adaptive T cell-mediated immune responses can regulate innate lymphocytes (natural killer cells and innate lymphoid cells) in an interleukin-2-dependent manner. The authors propose a model in which adaptive T cells function as peripheral antigen-specific sensors that recruit and activate innate lymphocytes to amplify and coordinate local immune responses. Innate lymphocytes — including natural killer cells and the recently discovered innate lymphoid cells — have crucial roles during infection, tissue injury and inflammation. Innate signals regulate the activation and homeostasis of innate lymphocytes. The contribution of the adaptive immune system to the coordination of innate lymphocyte responses is less well understood. In this Opinion article, we review our current understanding of the interactions between adaptive and innate lymphocytes, and propose a model in which T cells of the adaptive immune system function as antigen-specific sensors for the activation of innate lymphocytes to amplify and instruct local immune responses. We highlight the potential roles of regulatory and helper T cells in these processes, and discuss major questions in the emerging area of crosstalk between adaptive and innate lymphocytes.

Innate lymphoid cells (ILCs) contribute to barrier immunity, tissue homeostasis, and immune regulation at various anatomical sites throughout the body. How ILCs maintain their presence in lymphoid and peripheral tissues thus far has been unclear. We found that in the lymphoid and nonlymphoid organs of adult mice, ILCs are tissue-resident cells that were maintained and expanded locally under physiologic conditions, upon systemic perturbation of immune homeostasis and during acute helminth infection. However, at later time points after infection, cells from hematogenous sources helped to partially replenish the pool of resident ILCs. Thus, ILCs are maintained by self-renewal in broadly different microenvironments and physiological settings. Such an extreme \"sedentary\" lifestyle is consistent with the proposed roles of ILCs as sentinels and local keepers of tissue function.

The cytokine receptor IL-2R is essential for the development of T reg cells; therefore, it has been difficult to separate this from its role in the suppressive function of T reg cells. Rudensky and colleagues use various genetic systems to show that capture of IL-2 by IL-2R is important for suppression of CD8 + T cells but not that of CD4 + T cells. Regulatory T cells (T reg cells), which have abundant expression of the interleukin 2 receptor (IL-2R), are reliant on IL-2 produced by activated T cells. This feature indicates a key role for a simple network based on the consumption of IL-2 by T reg cells in their suppressor function. However, congenital deficiency in IL-2R results in reduced expression of the T reg cell lineage–specification factor Foxp3, which has confounded experimental efforts to understand the role of IL-2R expression and signaling in the suppressor function of T reg cells. Using genetic gain- and loss-of-function approaches, we found that capture of IL-2 was dispensable for the control of CD4 + T cells but was important for limiting the activation of CD8 + T cells, and that IL-2R-dependent activation of the transcription factor STAT5 had an essential role in the suppressor function of T reg cells separable from signaling via the T cell antigen receptor.

Innate lymphoid cells (ILCs) participate in tissue homeostasis, inflammation, and early immunity against infection. It is unclear how ILCs acquire effector function and whether these mechanisms differ between organs. Through multiplexed single-cell mRNA sequencing, we identified cKit + CD127 hi TCF-1 hi early differentiation stages of T-bet + ILC1s. These cells were present across different organs and had the potential to mature toward CD127 int TCF-1 int and CD127 − TCF-1 − ILC1s. Paralleling a gradual loss of TCF-1, differentiating ILC1s forfeited their expansion potential while increasing expression of effector molecules, reminiscent of T cell differentiation in secondary lymphoid organs. The transcription factor Hobit was induced in TCF-1 hi ILC1s and was required for their effector differentiation. These findings reveal sequential mechanisms of ILC1 lineage commitment and effector differentiation that are conserved across tissues. Our analyses suggest that ILC1s emerge as TCF-1 hi cells in the periphery and acquire a spectrum of organ-specific effector phenotypes through a uniform Hobit-dependent differentiation pathway driven by local cues. ILCs are considered preformed effector cells. Gasteiger and colleagues report that ILC1s undergo effector differentiation after lineage commitment. Hobit + ILC1s emerge as cKit + TCF-1 hi cells that generate tissue-specific helper- and cytotoxic-like cells along a Hobit-dependent trajectory.

T cell exhaustion is a hallmark of cancer and persistent infections, marked by inhibitory receptor upregulation, diminished cytokine secretion, and impaired cytolytic activity. Terminally exhausted T cells are steadily replenished by a precursor population (Tpex), but the metabolic principles governing Tpex maintenance and the regulatory circuits that control their exhaustion remain incompletely understood. Using a combination of gene-deficient mice, single-cell transcriptomics, and metabolomic analyses, we show that mitochondrial insufficiency is a cell-intrinsic trigger that initiates the functional exhaustion of T cells. At the molecular level, we find that mitochondrial dysfunction causes redox stress, which inhibits the proteasomal degradation of hypoxia-inducible factor 1α (HIF-1α) and promotes the transcriptional and metabolic reprogramming of Tpex cells into terminally exhausted T cells. Our findings also bear clinical significance, as metabolic engineering of chimeric antigen receptor (CAR) T cells is a promising strategy to enhance the stemness and functionality of Tpex cells for cancer immunotherapy. Exhaustion is the functional deterioration of T cells following chronic stimulation. Here, Wu et al. show that mitochondrial dysfunction drives T cell exhaustion by inhibiting HIF-1α degradation and transcriptional metabolic reprogramming.

Establishing and maintaining tolerance to self-antigens or innocuous foreign antigens is vital for the preservation of organismal health. Within the thymus, medullary thymic epithelial cells (mTECs) expressing autoimmune regulator (AIRE) have a critical role in self-tolerance through deletion of autoreactive T cells and promotion of thymic regulatory T (T reg ) cell development 1 – 4 . Within weeks of birth, a separate wave of T reg cell differentiation occurs in the periphery upon exposure to antigens derived from the diet and commensal microbiota 5 – 8 , yet the cell types responsible for the generation of peripheral T reg (pT reg ) cells have not been identified. Here we describe the identification of a class of RORγt + antigen-presenting cells called Thetis cells, with transcriptional features of both mTECs and dendritic cells, comprising four major sub-groups (TC I–TC IV). We uncover a developmental wave of Thetis cells within intestinal lymph nodes during a critical window in early life, coinciding with the wave of pT reg cell differentiation. Whereas TC I and TC III expressed the signature mTEC nuclear factor AIRE, TC IV lacked AIRE expression and was enriched for molecules required for pT reg generation, including the TGF-β-activating integrin αvβ8. Loss of either major histocompatibility complex class II (MHCII) or ITGB8 by Thetis cells led to a profound impairment in intestinal pT reg differentiation, with ensuing colitis. By contrast, MHCII expression by RORγt + group 3 innate lymphoid cells (ILC3) and classical dendritic cells was neither sufficient nor required for pT reg generation, further implicating TC IV as the tolerogenic RORγt + antigen-presenting cell with an essential function in early life. Our studies reveal parallel pathways for the establishment of tolerance to self and foreign antigens in the thymus and periphery, respectively, marked by the involvement of shared cellular and transcriptional programmes. Single-cell transcriptomic and epigenetic analysis has enabled the identification of Thetis cells, a class of RORγt + antigen-presenting cells with a key role in the differentiation of commensal microbiota-induced peripheral regulatory T cells.

Interferon-γ (IFN-γ) is an immunoregulatory cytokine essential for cellular immunity against intracellular pathogens, including Chlamydia . Interferon-stimulated gene (ISG) 15, a member of the ubiquitin family, contributes to host resistance to viral and bacterial infections. ISG15 can exist either in an unconjugated form or covalently attached to host proteins through a process known as ISGylation. Here, we show that infection with Chlamydia trachomatis (Ct) induces the expression and secretion of ISG15 in human primary cells and mouse female genital tract (FGT) organoids. ISG15 secretion by genital tract epithelial cells resulted in increased IFN-γ release from natural killer (NK) cells. The production of IFN-γ by NK cells in response to ISG15 was completely abolished in NK cells lacking the interleukin-18 receptor alpha (IL-18Ra), demonstrating a co-stimulatory effect of ISG15 with IL-18 in enhancing IFN-γ release. ISG15 was secreted into the FGT and was involved in controlling bacterial load in a murine infection model. Furthermore, ISG15 reduced macrophage responsiveness to IFN-γ as an M1-polarizing signal for pro-inflammatory responses, potentially “shielding” macrophages from excessive IFN-γ. Evidence of uterine horn pathology and reduced IL-10 levels in the FGT of infected ISG15 −/− mice further supports a critical dual function of ISG15 in controlling chlamydial infection and modulating the resulting inflammatory responses.

In pancreatic ductal adenocarcinoma (PDAC), endogenous MYC is required for S-phase progression and escape from immune surveillance. Here we show that MYC in PDAC cells is needed for the recruitment of the PAF1c transcription elongation complex to RNA polymerase and that depletion of CTR9, a PAF1c subunit, enables long-term survival of PDAC-bearing mice. PAF1c is largely dispensable for normal proliferation and regulation of MYC target genes. Instead, PAF1c limits DNA damage associated with S-phase progression by being essential for the expression of long genes involved in replication and DNA repair. Surprisingly, the survival benefit conferred by CTR9 depletion is not due to DNA damage, but to T-cell activation and restoration of immune surveillance. This is because CTR9 depletion releases RNA polymerase and elongation factors from the body of long genes and promotes the transcription of short genes, including MHC class I genes. The data argue that functionally distinct gene sets compete for elongation factors and directly link MYC-driven S-phase progression to tumor immune evasion. MYC drives S-phase progression and immune invasion in pancreatic ductal adenocarcinoma (PDAC), but the underlying mechanisms are not fully understood. Here, the authors show that the transcription elongation complex PAF1c controls the competition of different gene sets for RNA polymerase and elongation factors to regulate these MYC-associated mechanisms in PDAC.

Alterations of the airway microbiome are often associated with pulmonary diseases. For example, detection of the bacterial pathogen in the upper airways is linked with an increased risk to develop or exacerbate asthma. However, the mechanisms by which augments allergic airway inflammation (AAI) remain unclear. We here characterized the cellular and soluble mediators of triggered excacerbation of AAI in wt and IL-17 deficient as well as in animals treated with TNF-α and IL-6 neutralizing antibodies. We compared the type of inflammatory response in infected, house dust mite (HDM)-allergic and animals infected with at different time points of HDM sensitization. We found that airway infection of mice with triggers a strong inflammatory response with massive neutrophilic infiltrates, high amounts of IL-6 and TNF-α and moderate levels of CD4 T-cell-derived IFN-γ and IL-17. If bacterial infection occurred during HDM allergen sensitization, the allergic airway response was exacerbated, particularly by the expansion of Th17 cells and increased TNF-α levels. Neutralization of IL-17 or TNF-α but not IL-6 resulted in accelerated clearance of and effectively prevented infection-induced exacerbation of AAI. Taken together, our data demonstrate an essential role for TNF-α and IL-17 in infection-triggered exacerbation of AAI.

Infection of specific pathogen-free mice with lymphocytic choriomeningitis virus (LCMV) is a widely used model to study antiviral T-cell immunity. Infections in the real world, however, are often accompanied by coinfections with unrelated pathogens. Here we show that in mice, systemic coinfection with E. coli suppresses the LCMV-specific cytotoxic T-lymphocyte (CTL) response and virus elimination in a NK cell- and TLR2/4-dependent manner. Soluble TLR4 ligand LPS also induces NK cell-mediated negative CTL regulation during LCMV infection. NK cells in LPS-treated mice suppress clonal expansion of LCMV-specific CTLs by a NKG2D- or NCR1-independent but perforin-dependent mechanism. These results suggest a TLR4-mediated immunoregulatory role of NK cells during viral-bacterial coinfections. Exposure to multiple pathogens is common in nature, yet interactions between the immune components targeting bacterial and viral pathogens during co-infection are poorly understood. Here the authors show that bacteria-derived LPS induces cytotoxic NK cells that suppress antiviral CD8 T cell response.