Physiological microbial exposure transiently inhibits mouse lung ILC2 responses to allergens.

Lung group 2 innate lymphoid cells (ILC2s) control the nature of immune responses to airway allergens. Some microbial products, including those that stimulate interferons, block ILC2 activation, but whether this occurs after natural infections or causes durable ILC2 inhibition is unclear. In the present study, we cohoused laboratory and pet store mice as a model of physiological microbial exposure. Laboratory mice cohoused for 2 weeks had impaired ILC2 responses and reduced lung eosinophilia to intranasal allergens, whereas these responses were restored in mice cohoused for ≥2 months. ILC2 inhibition at 2 weeks correlated with increased interferon receptor signaling, which waned by 2 months of cohousing. Reinduction of interferons in 2-month cohoused mice blocked ILC2 activation. These findings suggest that ILC2s respond dynamically to environmental cues and that microbial exposures do not control long-term desensitization of innate type 2 responses to allergens.

Selection of self-reactive T cells in the thymus.

On the whole, the healthy adaptive immune system is responsive to foreign antigens and tolerant to self. However, many individual lymphocytes have, and even require, substantial self-reactivity for their particular functions in immunity. In this review, we discuss several populations of lymphocytes that are thought to experience agonist stimulation through the T cell receptor during selection: nTreg cells, iNKT cells, nIELs, and nTh17s. We discuss the nature of this self-reactivity, how it compares with conventional T cells, and why it is important for overall immune health. We also outline molecular pathways unique to each lineage and consider possible commonalities to their development and survival.

CD8+ T cell tolerance: It doesn't translate.

The mechanisms that make and break CD8+ T cell tolerance to self-antigens remain unclear. In this issue of Immunity, Van Der Byl et al. show that tolerant CD8+ T cells rapidly adopt an epigenetically and transcriptionally distinct cell state and exhibit impaired protein translation. Breaking tolerance requires both inflammation and increased antigen exposure to augment MYC expression and restore translation.

Engagement of the costimulatory molecule ICOS in tissues promotes establishment of CD8+ tissue-resident memory T cells.

Elevated gene expression of the costimulatory receptor Icos is a hallmark of CD8+ tissue-resident memory (Trm) T cells. Here, we examined the contribution of ICOS in Trm cell differentiation. Upon transfer into WT mice, Icos-/- CD8+ T cells exhibited defective Trm generation but produced recirculating memory populations normally. ICOS deficiency or ICOS-L blockade compromised establishment of CD8+ Trm cells but not their maintenance. ICOS ligation during CD8+ T cell priming did not determine Trm induction; rather, effector CD8+ T cells showed reduced Trm differentiation after seeding into Icosl-/- mice. IcosYF/YF CD8+ T cells were compromised in Trm generation, indicating a critical role for PI3K signaling. Modest transcriptional changes in the few Icos-/- Trm cells suggest that ICOS-PI3K signaling primarily enhances the efficiency of CD8+ T cell tissue residency. Thus, local ICOS signaling promotes production of Trm cells, providing insight into the contribution of costimulatory signals in the generation of tissue-resident populations.

T Cell Memory: Understanding COVID-19.

As the SARS-CoV-2 pandemic has progressed, increasing attention has focused on establishing natural and vaccine-induced immunity against this coronavirus and the disease, COVID-19, that it causes. In this Primer, we explain the fundamental features of T cell memory and their potential relevance for effective immunity to SARS-CoV-2.

CD8αα intraepithelial lymphocytes arise from two main thymic precursors.

TCRαß+CD4-CD8α+CD8ß- intestinal intraepithelial lymphocytes (CD8αα IELs) are an abundant population of thymus-derived T cells that protect the gut barrier surface. We sought to better define the thymic IEL precursor (IELp) through analysis of its maturation, localization and emigration. We defined two precursor populations among TCRß+CD4-CD8- thymocytes by dependence on the kinase TAK1 and rigorous lineage-exclusion criteria. Those IELp populations included a nascent PD-1+ population and a T-bet+ population that accumulated with age. Both gave rise to intestinal CD8αα IELs after adoptive transfer. The PD-1+ IELp population included more strongly self-reactive clones and was largely restricted by classical major histocompatibility complex (MHC) molecules. Those cells localized to the cortex and efficiently emigrated in a manner dependent on the receptor S1PR1. The T-bet+ IELp population localized to the medulla, included cells restricted by non-classical MHC molecules and expressed the receptor NK1.1, the integrin CD103 and the chemokine receptor CXCR3. The two IELp populations further differed in their use of the T cell antigen receptor (TCR) α-chain variable region (Vα) and ß-chain variable region (Vß). These data provide a foundation for understanding the biology of CD8αα IELs.

Sensing of ATP via the Purinergic Receptor P2RX7 Promotes CD8+ Trm Cell Generation by Enhancing Their Sensitivity to the Cytokine TGF-ß.

Tissue-resident memory (Trm) CD8+ T cells mediate protective immunity in barrier tissues, but the cues promoting Trm cell generation are poorly understood. Sensing of extracellular adenosine triphosphate (eATP) by the purinergic receptor P2RX7 is needed for recirculating CD8+ T cell memory, but its role for Trm cells is unclear. Here we showed that P2RX7 supported Trm cell generation by enhancing CD8+ T cell sensing of TGF-ß, which was necessary for tissue residency. P2RX7-deficient Trm cells progressively decayed in non-lymphoid tissues and expressed dysregulated Trm-specific markers. P2RX7 was required for efficient re-expression of the receptor TGF-ßRII through calcineurin signaling. Forced Tgfbr2 expression rescued P2RX7-deficient Trm cell generation, and TGF-ß sensitivity was dictated by P2RX7 agonists and antagonists. Forced Tgfbr2 also rescued P2RX7-deficient Trm cell mitochondrial function. Sustained P2RX7 signaling was required for long-term Trm cell maintenance, indicating that P2RX7 signaling drives induction and CD8+ T cell durability in barrier sites.

Late stages of T cell maturation in the thymus involve NF-κB and tonic type I interferon signaling.

Positive selection occurs in the thymic cortex, but critical maturation events occur later in the medulla. Here we defined the precise stage at which T cells acquired competence to proliferate and emigrate. Transcriptome analysis of late gene changes suggested roles for the transcription factor NF-κB and interferon signaling. Mice lacking the inhibitor of NF-κB (IκB) kinase (IKK) kinase TAK1 underwent normal positive selection but exhibited a specific block in functional maturation. NF-κB signaling provided protection from death mediated by the cytokine TNF and was required for proliferation and emigration. The interferon signature was independent of NF-κB; however, thymocytes deficient in the interferon-α (IFN-α) receptor IFN-αR showed reduced expression of the transcription factor STAT1 and phenotypic abnormality but were able to proliferate. Thus, both NF-κB and tonic interferon signals are involved in the final maturation of thymocytes into naive T cells.

The TCR's sensitivity to self peptide-MHC dictates the ability of naive CD8(+) T cells to respond to foreign antigens.

The strength with which complexes of self peptide and major histocompatibility complex (MHC) proteins are recognized by the T cell antigen receptor (TCR) dictates the homeostasis of naive CD8(+) T cells, but its effect on reactivity to foreign antigens is controversial. As expression of the negative regulator CD5 correlates with self-recognition, we studied CD5(lo) and CD5(hi) naive CD8(+) T cells. Gene-expression characteristics suggested CD5(hi) cells were better poised for reactivity and differentiation than were CD5(lo) cells, and we found that the CD5(hi) pool also exhibited more efficient clonal recruitment and expansion, as well as enhanced reactivity to inflammatory cues, during the recognition of foreign antigen. However, the recognition of complexes of foreign peptide and MHC was similar for both subsets. Thus, CD8(+) T cells with higher self-reactivity dominate the immune response to foreign antigens, with implications for T cell repertoire diversity and autoimmunity.

TCR affinity for thymoproteasome-dependent positively selecting peptides conditions antigen responsiveness in CD8(+) T cells.

In the thymus, low-affinity T cell antigen receptor (TCR) engagement facilitates positive selection of a useful T cell repertoire. Here we report that TCR responsiveness of mature CD8(+) T cells is fine tuned by their affinity for positively selecting peptides in the thymus and that optimal TCR responsiveness requires positive selection on major histocompatibility complex class I-associated peptides produced by the thymoproteasome, which is specifically expressed in the thymic cortical epithelium. Thymoproteasome-independent positive selection of monoclonal CD8(+) T cells results in aberrant TCR responsiveness, homeostatic maintenance and immune responses to infection. These results demonstrate a novel aspect of positive selection, in which TCR affinity for positively selecting peptides produced by thymic epithelium determines the subsequent antigen responsiveness of mature CD8(+) T cells in the periphery.

Understanding Subset Diversity in T Cell Memory.

Considerable advances have been made in recent years in understanding the generation and function of memory T cells. Memory T cells are typically parsed into discreet subsets based on phenotypic definitions that connote distinct roles in immunity. Here we consider new developments in the field and focus on how emerging differences between memory cells with respect to their trafficking, metabolism, epigenetic regulation, and longevity may fail to fit into small groups of "memory subsets." Rather, the properties of individual memory T cells fall on a continuum within each of these and other parameters. We discuss how this continuum influences the way that the efficacy of vaccination is assessed, as well as the suitability of a memory population for protective immunity.

Interleukin-15 Complex Treatment Protects Mice from Cerebral Malaria by Inducing Interleukin-10-Producing Natural Killer Cells.

Cerebral malaria is a deadly complication of Plasmodium infection and involves blood brain barrier (BBB) disruption following infiltration of white blood cells. During experimental cerebral malaria (ECM), mice inoculated with Plasmodium berghei ANKA-infected red blood cells develop a fatal CM-like disease caused by CD8+ T cell-mediated pathology. We found that treatment with interleukin-15 complex (IL-15C) prevented ECM, whereas IL-2C treatment had no effect. IL-15C-expanded natural killer (NK) cells were necessary and sufficient for protection against ECM. IL-15C treatment also decreased CD8+ T cell activation in the brain and prevented BBB breakdown without influencing parasite load. IL-15C induced NK cells to express IL-10, which was required for IL-15C-mediated protection against ECM. Finally, we show that ALT-803, a modified human IL-15C, mediates similar induction of IL-10 in NK cells and protection against ECM. These data identify a regulatory role for cytokine-stimulated NK cells in the prevention of a pathogenic immune response.

The self-obsession of T cells: how TCR signaling thresholds affect fate 'decisions' and effector function.

Self-reactivity was once seen as a potential characteristic of T cells that was eliminated by clonal selection to protect the host from autoimmune pathology. It is now understood that the T cell repertoire is in fact broadly self-reactive, even self-centered. The strength with which a T cell reacts to self ligands and the environmental context in which this reaction occurs influence almost every aspect of T cell biology, from development to differentiation to effector function. Here we highlight recent advances and discoveries that relate to T cell self-reactivity, with a particular emphasis on T cell antigen receptor (TCR) signaling thresholds.

Steady-state production of IL-4 modulates immunity in mouse strains and is determined by lineage diversity of iNKT cells.

Invariant natural killer T cells (iNKT cells) can produce copious amounts of interleukin 4 (IL-4) early during infection. However, indirect evidence suggests they may produce this immunomodulatory cytokine in the steady state. Through intracellular staining for transcription factors, we have defined three subsets of iNKT cells (NKT1, NKT2 and NKT17) that produced distinct cytokines; these represented diverse lineages and not developmental stages, as previously thought. These subsets exhibited substantial interstrain variation in numbers. In several mouse strains, including BALB/c, NKT2 cells were abundant and were stimulated by self ligands to produce IL-4. In those strains, steady-state IL-4 conditioned CD8(+) T cells to become 'memory-like', increased serum concentrations of immunoglobulin E (IgE) and caused dendritic cells to produce chemokines. Thus, iNKT cell-derived IL-4 altered immunological properties under normal steady-state conditions.

Transcriptional downregulation of S1pr1 is required for the establishment of resident memory CD8+ T cells.

Cell-mediated immunity critically depends on the localization of lymphocytes at sites of infection. While some memory T cells recirculate, a distinct lineage (resident memory T cells (T(RM) cells)) are embedded in nonlymphoid tissues (NLTs) and mediate potent protective immunity. However, the defining transcriptional basis for the establishment of T(RM) cells is unknown. We found that CD8(+) T(RM) cells lacked expression of the transcription factor KLF2 and its target gene S1pr1 (which encodes S1P1, a receptor for sphingosine 1-phosphate). Forced expression of S1P1 prevented the establishment of T(RM) cells. Cytokines that induced a T(RM) cell phenotype (including transforming growth factor-ß (TGF-ß), interleukin 33 (IL-33) and tumor-necrosis factor) elicited downregulation of KLF2 expression in a pathway dependent on phosphatidylinositol-3-OH kinase (PI(3)K) and the kinase Akt, which suggested environmental regulation. Hence, regulation of KLF2 and S1P1 provides a switch that dictates whether CD8(+) T cells commit to recirculating or tissue-resident memory populations.

Responsiveness to interleukin-15 therapy is shared between tissue-resident and circulating memory CD8+ T cell subsets.

Interleukin-15 (IL-15) is often considered a central regulator of memory CD8+ T cells, based primarily on studies of recirculating subsets. However, recent work identified IL-15-independent CD8+ T cell memory populations, including tissue-resident memory CD8+ T cells (TRM) in some nonlymphoid tissues (NLTs). Whether this reflects the existence of IL-15-insensitive memory CD8+ T cells is unclear. We report that IL-15 complexes (IL-15c) stimulate rapid proliferation and expansion of both tissue-resident and circulating memory CD8+ T cell subsets across lymphoid and nonlymphoid tissues with varying magnitude by tissue and memory subset, in some sites correlating with differing levels of the IL-2Rß. This was conserved for memory CD8+ T cells recognizing distinct antigens and elicited by different pathogens. Following IL-15c-induced expansion, divided cells contracted to baseline numbers and only slowly returned to basal proliferation, suggesting a mechanism to transiently amplify memory populations. Through parabiosis, we showed that IL-15c drive local proliferation of TRM, with a degree of recruitment of circulating cells to some NLTs. Hence, irrespective of homeostatic IL-15 dependence, IL-15 sensitivity is a defining feature of memory CD8+ T cell populations, with therapeutic potential for expansion of TRM and other memory subsets in an antigen-agnostic and temporally controlled fashion.