Stephen C. Jameson

Checkpoint-responsive exhausted CD8 ⁺ T cells sustain immunity during chronic infection and cancer, but the tissue-derived signals that preserve these cells remain poorly defined. Here, we identify extracellular ATP sensing through P2RX7 as a microenvironmental circuit that maintains progenitor exhausted (T _PEX ) and tissue-resident exhausted (TR _TEX ) CD8 ⁺ T cells. P2RX7 was enriched in T _PEX and TR _TEX cells and was required cell-intrinsically to preserve their abundance, diversity, and developmental competence during chronic viral infection. Loss of P2RX7 restricted early progenitor trajectories and induced mitochondrial dysfunction, lipid peroxidation, and terminal exhaustion-associated phenotypes. Mechanistically, P2RX7-dependent Ca ²⁺ influx activated NFAT/AP-1 signaling, which induced KLF3 to promote mitochondrial fitness, subset development, and persistence of T _PEX and TR _TEX cells. P2RX7 also supported CD8 ⁺ T cell expansion after PD-1 blockade, whereas pharmacologic P2RX7 activation enhanced antitumor immunity in preclinical models. These findings define an extracellular ATP-P2RX7-KLF3 program that couples tissue damage sensing to metabolic adaptation and persistence of checkpoint-responsive exhausted CD8 ⁺ T cells.

Cell migration and strategic positioning within tissues is critical for the rapid mobilization of a T cell response. T cells must remain motile in both lymphoid and nonlymphoid tissues, which vary widely in mechanical properties such as stiffness. Here we showed that activated T cells sensed mechanical cues and responded with changes in cell morphology, nuclear envelope composition and initiation of DNA repair to protect their genomic material from force-mediated damage. Increased mechanical input also drove the transcriptional reprogramming of activated T cells, including changes in many of the core genes shared by tissue-resident memory T cells across diverse tissues, by modulating the expression of the tissue-resident memory T cell-associated transcription factors Klf2, Runx3 and Hic1. Thus, mechanosensing by activated T cells impacted T cell fate, promoting a transcriptional program associated with tissue residency.

Numerous transcriptional regulators have been associated with the differentiation pathways that lead to recirculating vs. tissue-resident memory T cells. However, it is unclear whether independent, coordinated expression of these regulators is required to determine residency vs. recirculation or whether there is a hierarchy, with some factors playing a dominant role in controlling T cell trafficking. We report that ablation of the gene encoding Kruppel-like factor 2 (KLF2) during CD8 ⁺ T cell activation leads to rapid transcriptional reprogramming, such that effector T cells fail to recirculate and prematurely acquire canonical phenotypic and transcriptional characteristics of resident memory cells (T _RM ). Klf2 -deficient memory CD8 ⁺ T cells retained the capacity to undergo recall responses, including in vivo pathogen control. These data suggest that KLF2 diverts CD8 ⁺ T cells from the T _RM differentiation program. In contrast, ablation of another member of the KLF family, KLF3, enhanced differentiation of some recirculating T cell subsets and limited production of T _RM in lymphoid tissues. However, both KLF2 and KLF3 were required for differentiation of long-lived effector cells, suggesting cooperation between these factors in some situations. These findings indicate that KLFs occupy a central nexus in coordinating activated CD8 ⁺ T cell differentiation and trafficking.

Memory CD8⁺ T (T_mem) cells are activated into innate-like killers by cytokines, including interleukin-12 (IL-12), IL-15 and IL-18; but mechanisms regulating this phenomenon (termed bystander activation) are unclear. Here we show that basal IL-4 signals antagonize IL-18 sensing and subsequent interferon-γ production during T_mem cell bystander activation. IL-4 treatment can act directly on T_mem cells in a STAT6-dependent manner to limit interferon-γ-mediated control of a bystander bacterial infection. IL-4 does not simply block bystander activation but tunes effector molecule expression. Strain-specific defects in bystander activation of homeostatic T_mem cells partially relates to IL-4 exposure, but these differences are erased in T_mem cells produced by T cell antigen receptor activation, leading to uniform IL-18 receptor expression and capacity for bystander activation/cytotoxicity. Our data demonstrate that bystander activation by inflammatory cytokines is subject to regulation by both IL-4 and prior antigen experience. These findings underscore the importance of the cytokine milieu in dictating bystander-mediated pathogen control.

Adoptive transfer of natural killer (NK) cells can induce complete remissions in 30% to 50% of patients with refractory acute myeloid leukemia and lymphoma. Although blood chimerism occurs, attaining functional homing to the site of tumor without exhaustion has been elusive. During chronic infections and tumorigenesis, exposure to activating stimuli weakens the effector activity of NK cells. Despite this knowledge, there is little known about the mechanisms that govern this dysregulation and whether these disparate activating stimuli use distinct pathways to downregulate effector immunity. In this study, we reveal that chronic NK cell activation receptor (NKAR) stimulation and chronic interleukin-15 exposure impart distinct modes of dysregulation, with NKAR stimulation inducing a tissue resident–like state that resembles that of tumor-infiltrating NK cells in patients with cancer. Using loss- and gain-of-function studies, we identify the transcription factor KLF2 as a master regulator of the NK cell response to chronic activation and provide evidence that KLF2 overexpression promotes NK cell cytotoxicity, cytokine production, and chemotaxis and inhibits the development of dysfunctional, tissue resident–like features. Using KLF2 reporter mice, we show that in certain tissues, tissue resident NK cells are predominantly KLF2–, whereas circulating NK cells in these tissues are overwhelmingly KLF2+. Lastly, using mixed bone marrow chimeras, we demonstrate that conditional KLF2 deficiency in NK cells leads to altered homing and the acquisition of tissue resident–like features in vivo. Together, these findings highlight the profound changes NK cells undergo during prolonged activation and advance our understanding of how some NK cell therapies fail during malignant relapse.

Advances in T cell biology have revealed heterogeneity among T cell populations that is not captured by existing general nomenclature. This issue has caused an ad hoc broadening of core T cell subset definitions and the invention of new subset designations that have not been uniformly delineated. To address this issue, in this Consensus Statement, we propose guidelines that serve three goals. First, they advocate that primary research reports define the experimental basis by which relevant subsets are designated in the methods section of each study. Second, they provide standardized definitions for existing subset designations in popular use, and common experimental criteria for defining each subset are noted. Last, they present an alternative ‘modular nomenclature’ paradigm. The newly proposed modular nomenclature eschews conceptualization of antigen-experienced T cells as belonging to a few idealized subsets, and the nomenclature instead simply indicates individual biological properties present in a T cell population with brief descriptors. Collectively, these guidelines intend to enhance transparency in the literature while facilitating clearer communication of findings and concepts to researchers, students and clinicians.

Many mouse models of SARS-CoV-2 infection involve expression of the human ACE2 protein, the entry receptor for SARS-CoV-2 Spike protein, in mouse tissues. However, most of these models suffer from nonphysiological regulation of ACE2 expression, which can lead to atypically severe infections and aberrant sites of viral replication. In this report, we developed and characterized an ACE2 gene replacement (ACE2-GR) mouse strain in which the mouse Ace2 genomic locus was replaced by the entire human ACE2 gene locus, and we investigated the ability of these animals to respond to SARS-CoV-2 infection. We show that ACE2-GR mice support SARS-CoV-2 viral replication, but, in stark contrast to the widely used K18-hACE2 transgenic model, this infection leads to a mild disease with no detectable involvement of the CNS. Thus, ACE2-GR mice provide a novel, to our knowledge, model to explore immune responses and long-term consequences of SARS-CoV-2 infection.

KLRG1 ⁺ CD8 T cells persist for months after clearance of acute infections and maintain high levels of effector molecules, contributing protective immunity against systemic pathogens. Upon secondary infection, these long-lived effector cells (LLECs) are incapable of forming other circulating KLRG1 ⁻ memory subsets such as central and effector memory T cells. Thus, KLRG1 ⁺ memory T cells are frequently referred to as a terminally differentiated population that is relatively short lived. Here, we show that after viral infection of mice, effector cells derived from LLECs rapidly enter nonlymphoid tissues and reduce pathogen burden but are largely dependent on receiving antigen cues from vascular endothelial cells. Single-cell RNA sequencing reveals that secondary memory cells in nonlymphoid tissues arising from either KLRG1 ⁺ or KLRG1 ⁻ memory precursors develop a similar resident memory transcriptional signature. Thus, although LLECs cannot differentiate into other circulating memory populations, they still retain the flexibility to enter tissues and establish residency.