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Lung-tissue-resident memory (T RM ) CD8 + T cells are critical for heterosubtypic immunity against influenza virus (IAV) reinfection. How T RM cells surveil the lung, respond to infection, and interact with other cells remains unresolved. Here, we used IAV infection of mice in combination with intravital and static imaging to define the spatiotemporal dynamics of lung T RM cells before and after recall infection. CD69 + CD103 + T RM cells preferentially localized to lung sites of prior IAV infection, where they exhibited patrolling behavior. After rechallenge, lung T RM cells formed tight clusters in an antigen-dependent manner. Transcriptomic analysis of IAV-specific T RM cells revealed the expression of several factors that regulate myeloid cell biology. In vivo rechallenge experiments demonstrated that protection elicited by T RM cells is orchestrated in part by interferon (IFN)-γ-mediated recruitment of inflammatory monocytes into the lungs. Overall, these data illustrate the dynamic landscapes of CD103 + lung T RM cells that mediate early protective immunity against IAV infection., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Exhausted T cells are largely hampered by epigenetically enforced mechanisms that limit their effector potential. In this issue of Immunity, Beltra et al. found that Stat5 can alter these epigenetic profiles when T cells transition from the Tpex precursor stage into differentiated cells. At this stage, enforced Stat5 expression increases the number of intermediate exhausted T cells and induces durable effector cells with superior anti-tumor activity., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 Elsevier Inc. All rights reserved.)

T cell factor 1 (Tcf-1) expressing CD8 + T cells exhibit stem-like self-renewing capacity, rendering them key for immune defense against chronic viral infection and cancer. Yet, the signals that promote the formation and maintenance of these stem-like CD8 + T cells (CD8 + SL) remain poorly defined. Studying CD8 + T cell differentiation in mice with chronic viral infection, we identified the alarmin interleukin-33 (IL-33) as pivotal for the expansion and stem-like functioning of CD8 + SL as well as for virus control. IL-33 receptor (ST2)-deficient CD8 + T cells exhibited biased end differentiation and premature loss of Tcf-1. ST2-deficient CD8 + SL responses were restored by blockade of type I interferon signaling, suggesting that IL-33 balances IFN-I effects to control CD8 + SL formation in chronic infection. IL-33 signals broadly augmented chromatin accessibility in CD8 + SL and determined these cells' re-expansion potential. Our study identifies the IL-33-ST2 axis as an important CD8 + SL-promoting pathway in the context of chronic viral infection., Competing Interests: Declaration of interests D.D.P. is a founder, consultant, and shareholder of Hookipa Pharma Inc. commercializing arenavirus-based vector technology, and he as well as W.V.B., S.M.K., and D.M. are listed as inventor on corresponding patents., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Reinvigoration of exhausted CD8 + T (Tex) cells by checkpoint immunotherapy depends on the activation of precursors of exhausted T (Tpex) cells, but the local anatomical context of their maintenance, differentiation, and interplay with other cells is not well understood. Here, we identified transcriptionally distinct Tpex subpopulations, mapped their differentiation trajectories via transitory cellular states toward Tex cells, and localized these cell states to specific splenic niches. Conventional dendritic cells (cDCs) were critical for successful αPD-L1 therapy and were required to mediate viral control. cDC1s were dispensable for Tpex cell expansion but provided an essential niche to promote Tpex cell maintenance, preventing their overactivation and T-cell-mediated immunopathology. Mechanistically, cDC1s insulated Tpex cells via MHC-I-dependent interactions to prevent their activation within other inflammatory environments that further aggravated their exhaustion. Our findings reveal that cDC1s maintain and safeguard Tpex cells within distinct anatomical niches to balance viral control, exhaustion, and immunopathology., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Exposure to chronic infection or malignant tumors triggers the differentiation of functionally impaired, so-called exhausted T cells. In this issue of Immunity, Beltra et al. reveal a multi-step developmental process that defines how T cells transition from proliferating progenitors into terminally differentiated exhausted T cells., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Infections are thought to trigger CD8 + cytotoxic T lymphocyte (CTL) responses during autoimmunity. However, the transcriptional programs governing the tissue-destructive potential of CTLs remain poorly defined. In a model of central nervous system (CNS) inflammation, we found that infection with lymphocytic choriomeningitis virus (LCMV), but not Listeria monocytogenes (Lm), drove autoimmunity. The DNA-binding factor TOX was induced in CTLs during LCMV infection and was essential for their encephalitogenic properties, and its expression was inhibited by interleukin-12 during Lm infection. TOX repressed the activity of several transcription factors (including Id2, TCF-1, and Notch) that are known to drive CTL differentiation. TOX also reduced immune checkpoint sensitivity by restraining the expression of the inhibitory checkpoint receptor CD244 on the surface of CTLs, leading to increased CTL-mediated damage in the CNS. Our results identify TOX as a transcriptional regulator of tissue-destructive CTLs in autoimmunity, offering a potential mechanistic link to microbial triggers., (Copyright © 2018 Elsevier Inc. All rights reserved.)

During chronic stimulation, CD8 + T cells acquire an exhausted phenotype characterized by expression of inhibitory receptors, down-modulation of effector function, and metabolic impairments. T cell exhaustion protects from excessive immunopathology but limits clearance of virus-infected or tumor cells. We transcriptionally profiled antigen-specific T cells from mice infected with lymphocytic choriomeningitis virus strains that cause acute or chronic disease. T cell exhaustion during chronic infection was driven by high amounts of T cell receptor (TCR)-induced transcription factors IRF4, BATF, and NFATc1. These regulators promoted expression of inhibitory receptors, including PD-1, and mediated impaired cellular metabolism. Furthermore, they repressed the expression of TCF1, a transcription factor required for memory T cell differentiation. Reducing IRF4 expression restored the functional and metabolic properties of antigen-specific T cells and promoted memory-like T cell development. These findings indicate that IRF4 functions as a central node in a TCR-responsive transcriptional circuit that establishes and sustains T cell exhaustion during chronic infection., (Crown Copyright © 2017. Published by Elsevier Inc. All rights reserved.)

Chronic infections promote the terminal differentiation (or "exhaustion") of T cells and are thought to preclude the formation of memory T cells. In contrast, we discovered a small subpopulation of virus-specific CD8(+) T cells that sustained the T cell response during chronic infections. These cells were defined by, and depended on, the expression of the transcription factor Tcf1. Transcriptome analysis revealed that this population shared key characteristics of central memory cells but lacked an effector signature. Unlike conventional memory cells, Tcf1-expressing T cells displayed hallmarks of an "exhausted" phenotype, including the expression of inhibitory receptors such as PD-1 and Lag-3. This population was crucial for the T cell expansion that occurred in response to inhibitory receptor blockade during chronic infection. These findings identify a memory-like T cell population that sustains T cell responses and is a prime target for therapeutic interventions to improve the immune response in chronic infections., (Copyright © 2016 Elsevier Inc. All rights reserved.)

MicroRNAs (miRNAs) regulate the function of several immune cells, but their role in promoting CD8(+) T cell immunity remains unknown. Here we report that miRNA-155 is required for CD8(+) T cell responses to both virus and cancer. In the absence of miRNA-155, accumulation of effector CD8(+) T cells was severely reduced during acute and chronic viral infections and control of virus replication was impaired. Similarly, Mir155(-/-) CD8(+) T cells were ineffective at controlling tumor growth, whereas miRNA-155 overexpression enhanced the antitumor response. miRNA-155 deficiency resulted in accumulation of suppressor of cytokine signaling-1 (SOCS-1) causing defective cytokine signaling through STAT5. Consistently, enforced expression of SOCS-1 in CD8(+) T cells phenocopied the miRNA-155 deficiency, whereas SOCS-1 silencing augmented tumor destruction. These findings identify miRNA-155 and its target SOCS-1 as key regulators of effector CD8(+) T cells that can be modulated to potentiate immunotherapies for infectious diseases and cancer., (Copyright © 2013 Elsevier Inc. All rights reserved.)

The strength of interactions between T cell receptors and the peptide-major histocompatibility complex (pMHC) directly modulates T cell fitness, clonal expansion, and acquisition of effector properties. Here we show that asymmetric T cell division is an important mechanistic link between increased signal strength, effector differentiation, and the ability to induce tissue pathology. Recognition of pMHC above a threshold affinity drove responding T cells into asymmetric cell division. The ensuing proximal daughters underwent extensive division and differentiated into short-lived effector cells expressing the integrin VLA-4, allowing the activated T cell to infiltrate and mediate destruction of peripheral target tissues. In contrast, T cells activated by below-threshold antigens underwent symmetric division, leading to abortive clonal expansion and failure to fully differentiate into tissue-infiltrating effector cells. Antigen affinity and asymmetric division are important factors that regulate fate specification in CD8(+) T cells and predict the potential of a self-reactive T cell to mediate tissue pathology., (Copyright © 2012 Elsevier Inc. All rights reserved.)

T cells causing autoimmunity must escape tolerance. We observed that CD8(+) T cells with high avidity for an antigen expressed in the pancreas, kidney, and thymic medulla were efficiently removed from a polyclonal repertoire by central and peripheral tolerance mechanisms. However, both mechanisms spared low-avidity T cells from elimination. Neither the introduction of activated, self-antigen-specific CD4(+) helper T cells nor a global inflammatory stimulus were sufficient to activate the low-avidity CD8(+) T cells and did not break tolerance. In contrast, challenge with a recombinant bacterium expressing the self antigen primed the low-avidity T cells, and the animals rapidly developed autoimmune diabetes. We suggest that whereas thymic and peripheral tolerance mechanisms remove cells that can be primed by endogenous amounts of self antigen, they do not guard against tissue destruction by low-avidity effector T cells, which have been primed by higher amounts of self antigen or by crossreactive antigens.