TRM Generation Research Articles

The diverse landscape of memory T-cell heterogeneity.

The diverse landscape of memory T-cell heterogeneity.

Axes of heterogeneity in human tissue-resident memory T cells.

Tissue-resident memory T cells (TRM) represent a dedicated layer of localized immune memory in virtually every organ throughout the human body. By virtue of their long-term residence in disparate tissues, TRM are shaped by a myriad of site-specific influences and exhibit remarkable heterogeneity in form and function. Here, we review the major axes by which TRM vary, including their surface phenotypes, transcriptional programming, and the tissue-specific adaptations that accrue over their tenancy. We discuss how localization within distinct anatomic niches both within and across major organ systems shapes TRM identity and examine mechanisms and prevailing models for TRM generation. Understanding the drivers of differentiation, function and maintenance of the various subpopulations that together define the TRM lineage may hold the key to unlocking the full potential of TRM to promote localized and protective tissue immunity throughout the body.

A173 ELUCIDATING THE EFFECTS OF NOD2-MEDIATED SIGNALLING ON INTESTINAL RESIDENT-MEMORY T-CELL FORMATION AND FUNCTION

Abstract Background Aberrant resident memory T-cell (TRM) responses have been associated with increased intestinal inflammation and Crohn’s disease (CD) pathology in humans. Intestinal TRM cells are not only important for maintaining the integrity of the intestinal epithelial barrier, but also for the rapid clearance of pathogens in the intestine during infection. Understanding the signals received by the intestinal immune system to generate TRM responses is paramount to elucidating treatments for CD. Genetic mutations in NOD2 are associated with the highest risk of CD development. As a host intracellular sensor of bacterial peptidoglycan, NOD2 is critical for initiating both innate and adaptive immune responses. Furthermore, work from our lab as well as those of our collaborators suggest that NOD2 deficiency reduces systemic memory B and T-cell responses. However, the role of NOD2 in establishing memory T-cell responses in the intestine remains unclear. This work will therefore establish the role of NOD2 signaling in initiating and maintaining optimal TRM responses to achieve intestinal homeostasis and resilience to intestinal inflammation. Purpose It is the main objective of this project to determine whether NOD2-mediated signalling affects: 1. Antigen-specific T-cell priming in vivo 2. Bona fide intestinal TRM generation 3. Bona fide intestinal TRM function Method To address the effects of NOD2-signalling on intestinal T-cell priming in vivo, wildtype (WT) mice were adoptively transferred 50,000 naïve LCMV-specific (SMARTA) CD4+ T-cells. Mice were subsequently infected with LCMV-Armstrong in the presence or absence of the NOD2 agonist; MDP. 5-days following infection, the numbers and percentage of LCMV-specific T-cells in the mesenteric lymph nodes and spleen were examined. To examine the effects of NOD2 on intestinal TRM generation, littermate WT and NOD2 KO mice were infected with LCMV-Armstrong. Thirty-six-days following infection, the percentage and number of LCMV-specific CD4+ T-cells were profiled in the small and large intestinal lamina-propria by means of gp66-77 class-II MHC-tetramer staining. In another set of experiments, littermate WT and NOD2 KO mice were re-infected with LCMV-C13 30-days following LCMV-Armstrong immunization, and the interferon-response in the small intestine was profiled by quantitative PCR to assess the effect of NOD2-deficiency on antigen recall responses. Result(s) NOD2-stimulation by means of MDP injection increased the percentage and number of adoptively transferred SMARTA CD4+ T-cells in the mesenteric lymph nodes upon LCMV infection. Furthermore, NOD2-deficiency did not alter intestinal LCMV-specific CD4+ TRM seeding in the small and large intestinal lamina propria 36 days after infection. However, in vivo antigen recall experiments showed a decreased intestinal IFN response in NOD2 KO mice. Conclusion(s) Our findings reveal a potential role of NOD2 in the intestinal CD4+ T-cell priming and subsequent Ag-specific memory response. Disclosure of Interest None Declared

The link between tissue-resident memory T cells and graft-versus-host disease

In the context of allogeneic hematopoietic stem cell transplantation (allo-HSCT), this review outlines and analyzes recent developments in the understanding of the properties of tissue-resident memory T cells (TRMs). The paper first discusses commonly recognized TRM characteristics and key TRM phenotypic markers and then covers the master transcription factors controlling TRM generation and maintenance. The TRM distribution in graft-versus-host disease (GVHD)-targeted organs and correlations between allo-HSCT outcomes and various GVHD subtypes were also reviewed, mainly focusing on skin and gut GVHD. This review discusses the organ- and tissue-specific characteristics of donor- and recipient-derived TRMs after allo-HSCT. It also highlights investigations using murine GVHD models, nonhuman primates, and cutting-edge technologies to track clonotypes, establish transcriptome profiles, and identify donor- and recipient-derived TRMs. Furthermore, this review discusses significant results for TRM functions in GVHD patients. Moreover, potential advantages of performing GVHD-focused TRM research with "dirty mice" rather than laboratory mice were proposed. Understanding TRMs in allo-HSCT is a rapidly growing field requiring future studies to address unresolved questions regarding TRM heterogeneity, plasticity, longevity, alloreactivity, and roles in GVHD and tolerance after allo-HSCT.

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) Tcells. Here, we examined the contribution of ICOS in Trm cell differentiation. Upon transfer into WT mice, Icos-/- CD8+ Tcells 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+ Tcell priming did not determine Trm induction; rather, effector CD8+ Tcells showed reduced Trm differentiation after seeding into Icosl-/- mice. IcosYF/YF CD8+ Tcells 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+ Tcell 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.

Expression of the purinergic receptor P2RX7 marks early effector CD8+ T cells with increased potential for both long-lived circulating and resident memory cell generation

Abstract Memory CD8+ T cells protect against secondary viral infections. They develop and maintain exclusively in circulation (e.g. central memory - Tcm) or are excluded from re-circulation (resident memory - Trm). Circulating memory CD8+ T cells arises from memory precursors (MPs) which express memory, pro-survival, and pluripotency molecules. Trm generation, in contrast, is still not fully understood. Previous reports suggest that Trm and Tcm have a common precursor present at the early effector phase; yet, the nature of this precursor is elusive. We have found that the extracellular ATP receptor P2RX7 promotes both Tcm and Trm generation. Moreover, P2RX7 is quickly upregulated in some (but not all) early effectors and early P2RX7 ablation hinders both Tcm and Trm generation. Thus, we postulated that P2RX7 in early effectors may mark a Tcm/Trm common precursor. Comparison between high (P2RX7hi) and low (P2RX7lo) P2RX7-expressing CD8+ T cells shows a correlation between memory markers and P2RX7, with CD62L+ Tcm precursors exclusively found in P2RX7hi cells. RNA-seq of P2RX7hi and P2RX7lo early effectors show survival, memory and pluripotency genes and pathways are upregulated in P2RX7hi cells. Conversely, many terminal effector and apoptosis genes are upregulated in P2RX7lo cells. P2RX7lo early effectors and MPs have increased apoptosis. Upon transfer into infection-matched mice, P2RX7hi MPs preferentially survive and form Tcm compared to P2RX7lo MPs. This is even more apparent when P2RX7hi and P2RX7lo early effectors are compared: approximately 100x more P2RX7hi cells form memory when compared to P2RX7lo cells, both Tcm and Trm. Our data suggest that P2RX7 in early effector CD8+ T cells promotes the generation of a common Tcm/Trm precursor.

Engagement of ICOS in tissues promotes establishment of CD8+ tissue-resident memory T cells

Abstract Resident memory T cells (TRM) provide localized protective immunity in non-lymphoid tissues, but the signals and interactions within non-lymphoid tissues involved in generating TRM are incompletely defined. Inducible T-cell costimulator (ICOS) is a costimulatory molecule with well elaborated roles in regulating differentiation of CD4+ T cell populations, its function in CD8+ T cells is largely unexplored. With acute infection models, we found that ICOS-deficient CD8+ T cells showed minimal defects in the initial effector expansion and the generation of recirculating memory subsets. However, consistent with its high expression in TRM, ICOS deficiency significantly compromised the establishment of CD8+ TRM cells in diverse non-lymphoid tissues. We further confirmed that rather than pre-programming the TRM fate during initial T cell priming, the interaction between ICOS ligand (ICOSL) and ICOS in the non-lymphoid tissues during CD8+ T cell recruitment is critical for efficient tissue retention and acquisition of tissue residency signatures. ICOS signaling through PI3K pathway is key for TRM generation and ICOS stimulation promotes downregulation of KLF2, a transcription factor that regulates T cell trafficking, in CD8+ T cells after entering non-lymphoid tissues. Altogether, our data illustrate that engagement of ICOS is one of the critical local costimulatory cues that promote the establishment of tissue-resident populations, with potential implication for therapeutic manipulation.

Ptpn2 and KLRG1 regulate the generation and function of tissue-resident memory CD8+ T cells in skin.

Tissue-resident memory T cells (TRM cells) are key elements of tissue immunity. Here, we investigated the role of the regulator of T cell receptor and cytokine signaling, Ptpn2, in the formation and function of TRM cells in skin. Ptpn2-deficient CD8+ T cells displayed a marked defect in generating CD69+ CD103+ TRM cells in response to herpes simplex virus type 1 (HSV-1) skin infection. This was accompanied by a reduction in the proportion of KLRG1- memory precursor cells and a transcriptional bias toward terminal differentiation. Of note, forced expression of KLRG1 was sufficient to impede TRM cell formation. Normalizing memory precursor frequencies by transferring equal numbers of KLRG1- cells restored TRM generation, demonstrating that Ptpn2 impacted skin seeding with precursors rather than downstream TRM cell differentiation. Importantly, Ptpn2-deficient TRM cells augmented skin autoimmunity but also afforded superior protection from HSV-1 infection. Our results emphasize that KLRG1 repression is required for optimal TRM cell formation in skin and reveal an important role of Ptpn2 in regulating TRM cell functionality.

Discipline in Stages: Regulating CD8+ Resident Memory T Cells.

Resident memory CD8+ T (TRM) cells are a lymphocyte lineage distinct from circulating memory CD8+ T cells. TRM lodge within peripheral tissues and secondary lymphoid organs where they provide rapid, local protection from pathogens and control tumor growth. However, dysregulation of CD8+ TRM formation and/or activation may contribute to the pathogenesis of autoimmune diseases. Intrinsic mechanisms, including transcriptional networks and inhibitory checkpoint receptors control TRM differentiation and response. Additionally, extrinsic stimuli such as cytokines, cognate antigen, fatty acids, and damage signals regulate TRM formation, maintenance, and expansion. In this review, we will summarize knowledge of CD8+ TRM generation and highlight mechanisms that regulate the persistence and responses of heterogeneous TRM populations in different tissues and distinct microenvironments.

Exploiting albumin as a mucosal vaccine chaperone for robust generation of lung-resident memory T cells.

Tissue-resident memory T cells (TRMs) can profoundly enhance mucosal immunity, but parameters governing TRM induction by vaccination remain poorly understood. Here, we describe an approach exploiting natural albumin transport across the airway epithelium to enhance mucosal TRM generation by vaccination. Pulmonary immunization with albumin-binding amphiphile conjugates of peptide antigens and CpG adjuvant (amph-vaccines) increased vaccine accumulation in the lung and mediastinal lymph nodes (MLNs). Amph-vaccines prolonged antigen presentation in MLNs over 2 weeks, leading to 25-fold increased lung-resident T cell responses over traditional immunization and enhanced protection from viral or tumor challenge. Mimicking such prolonged exposure through repeated administration of soluble vaccine revealed that persistence of both antigen and adjuvant was critical for optimal TRM induction, mediated through T cell priming in MLNs after prime, and directly in the lung tissue after boost. Thus, vaccine persistence strongly promotes TRM induction, and amph-conjugates may provide a practical approach to achieve such kinetics in mucosal vaccines.

Generation of protective pneumococcal-specific nasal resident memory CD4+ T cells via parenteral immunization.

The generation of tissue-resident memory T cells (TRM) is an essential aspect of immunity at mucosal surfaces, and it has been suggested that preferential generation of TRM is one of the principal advantages of mucosally administered vaccines. We have previously shown that antigen-specific, IL-17-producing CD4+ T cells can provide capsular antibody-independent protection against nasal carriage of Streptococcus pneumoniae; but whether pneumococcus-responsive TRM are localized within the nasal mucosa and are sufficient for protection from carriage has not been determined. Here, we show that intranasal administration of live or killed pneumococci to mice generates pneumococcus-responsive IL-17A-producing CD4+ mucosal TRM. Furthermore, we show that these cells are sufficient to mediate long-lived, neutrophil-dependent protection against subsequent pneumococcal nasal challenge. Unexpectedly, and in contrast with the prevailing paradigm, we found that parenteral administration of killed pneumococci also generates protective IL-17A+CD4+ TRM in the nasal mucosa. These results demonstrate a critical and sufficient role of TRM in prevention of pneumococcal colonization, and further that these cells can be generated by parenteral immunization. Our findings therefore have important implications regarding the generation of immune protection at mucosal surfaces by vaccination.

The Functional Requirement for CD69 in Establishment of Resident Memory CD8+ T Cells Varies with Tissue Location.

Recent studies have characterized populations of memory CD8+ T cells that do not recirculate through the blood but are, instead, retained in nonlymphoid tissues. Such CD8+ tissue resident memory T cells (TRM) are critical for pathogen control at barrier sites. Identifying TRM and defining the basis for their tissue residency is therefore of considerable importance for understanding protective immunity and improved vaccine design. Expression of the molecule CD69 is widely used as a definitive marker for TRM, yet it is unclear whether CD69 is universally required for producing or retaining TRM Using multiple mouse models of acute immunization, we found that the functional requirement for CD69 was highly variable, depending on the tissue examined, playing no detectable role in generation of TRM at some sites (such as the small intestine), whereas CD69 was critical for establishing resident cells in the kidney. Likewise, forced expression of CD69 (but not expression of a CD69 mutant unable to bind the egress factor S1PR1) promoted CD8+ TRM generation in the kidney but not in other tissues. Our findings indicate that the functional relevance of CD69 in generation and maintenance of CD8+ TRM varies considerably, chiefly dependent on the specific nonlymphoid tissue studied. Together with previous reports that suggest uncoupling of CD69 expression and tissue residency, these findings prompt caution in reliance on CD69 expression as a consistent marker of CD8+ TRM.

Monocytes Acquire the Ability to Prime Tissue-Resident T Cells via IL-10-Mediated TGF-β Release

SUMMARYUsing non-human primates (NHPs), mice, and human primary cells, we found a role for interleukin-10 (IL-10) in the upregulation of the tissue-resident memory T cell (TRM) marker CD103. In NHPs, intravenous, but not subcutaneous, immunization with peptide antigen and an adjuvant combining an agonistic anti-CD40 antibody plus poly(IC:LC) induced high levels of CD103+ TRMs in the lung, which correlated with early plasma IL-10 levels. Blocking IL-10 reduced CD103 expression on human T cells stimulated in vitro with the adjuvant combination as well as diminished CD103 on lung-resident T cells in vivo in mice. Monocyte-produced IL-10 induced the release of surface-bound transforming growth factor β (TGF-β), which in turn upregulated CD103 on T cells. Early TGF-β imprinted increased sensitivity to TGF-β restimulation, indicating an early commitment of the T cell lineage toward TRMs during the priming stage of activation. IL-10-mediated TGF-β signaling may therefore have a critical role in the generation of TRM following vaccination.

The role of mTOR, TRAF1, and local antigen in 4-1BB-dependent establishment of Trm in the lung

Abstract The tumour necrosis factor receptor (TNFR) superfamily receptor 4-1BB is important in the establishment of a tissue-resident memory (Trm) population in the lung tissue following influenza infection. Moreover, supraphysiological boosting of 4-1BB during the boost phase of a prime-boost immunization regimen can greatly enhance the establishment of a long-lived Trm population that protects against lethal heterotypic challenge (Zhou et al. Mucosal Immunology 2017). However, little is known about how 4-1BB contributes to the establishment of the lung Trm population. Here, we investigated the mechanism by which 4-1BB induces lung Trm cells. Using competitive mixed bone marrow chimeras, we found that the effect of 4-1BB on lung resident influenza-specific T cells does not substantially change between day 9 and 45 post-infection, suggesting that the main effect of 4-1BB is to allow the persistence of CD8 T effector cells as they transition to Trm. The signaling adaptor TRAF1, downstream of 4-1BB was also shown to be important in enhancing the numbers of Teffectors and Trm in the lung. Using supraphysiological stimulation of 4-1BB in the boost phase of a prime boost immunization, we show 4-1BB-mediated Trm generation is dependent on local delivery of both antigen and costimulation and that this process is inhibited by rapamycin, suggesting a role for mTOR in this process. Also, using this prime-boost model we demonstrate that TRAF1 contributes to the effect of 4-1BB on Trm generation. Taken together, these data point to an important role for 4-1BB, TRAF1 and mTORC1 in allowing the lung effector T cells in the lung parenchyma to survive through the transition to tissue resident memory T cells. This work was funded by CIHR grant FDN-143250 to THW

Infant T cells exhibit increased TCR signaling and proliferation to respiratory infection

Abstract Infants are highly susceptible to respiratory infections and frequently experience recurrent episodes. Our previous study demonstrated that tissue resident memory T cells (TRM), which are critical in mediating long-term protection against pathogens, are reduced in infant mice compared to adult mice after influenza infection. However, the underlying mechanisms for reduced TRM generation in infants is still unknown. Our preliminary data show that infant T cells have enhanced TCR sensitivity leading to increased activation compared to adult T cells and thus are biased towards effector differentiation at the expense of memory formation. We show that infant OVA-specific (OTII) T cells exhibit lower T cell activation threshold for proliferation and enhanced sensitivity for low does of antigen. Infant T cells express higher levels of CD69 and CD25 with a greater induction of Nur77, a direct indicator of TCR signal strength, at peptide doses that fail to fully activate adult T cells. Co-transfer of infant and adult OTII T cells into a congenic host resulted in infant T cells dominating the effector response in the lungs (at a ratio of 3:1) after primary influenza challenge. At the peak of infection, infant effector T cells also expressed lower levels of TCF1 and Bcl-2, indicating transcriptional bias for terminal effector differentiation with reduced survival. Moreover, infant derived effector and memory T cells express lower levels of CD5, suggesting enhanced survival of lower affinity T cells after influenza infection. Taken together, these results show that enhanced sensitivity for antigenic stimulation promotes cell intrinsic mechanisms that govern decreased potential of infants to form durable and functional lung TRM.

4-1BB Regulates Effector CD8 T Cell Accumulation in the Lung Tissue through a TRAF1-, mTOR-, and Antigen-Dependent Mechanism to Enhance Tissue-Resident Memory T Cell Formation during Respiratory Influenza Infection.

The TNFR superfamily member 4-1BB is important in the establishment of tissue-resident memory T cells (Trm) in the lung tissue following influenza infection. Moreover, supraphysiological boosting of 4-1BB in the airways during the boost phase of a prime-boost immunization regimen increases the long-lived Trm population, correlating with increased protection against heterotypic challenge. However, little is known about how 4-1BB contributes to the establishment of the lung Trm population. In this study, we show that effects of 4-1BB on lung Trm accumulation are already apparent at the effector stage, suggesting that the major role of 4-1BB in Trm formation is to allow persistence of CD8 T effector cells in the lung as they transition to Trm. Using supraphysiological stimulation of 4-1BB in the boost phase of a prime-boost immunization, we show that the effect of 4-1BB on Trm generation requires local delivery of both Ag and costimulation, is inhibited by rapamycin treatment during secondary CD8 effector T cell expansion, and is dependent on the signaling adaptor TRAF1. The decrease in lung Trm following early rapamycin treatment is accompanied by increased circulating memory T cells, as well as fewer effectors, suggesting a role for mammalian target of rapamycin (mTOR) in the formation of Trm through effects on the accumulation of effector precursors. Taken together, these data point to an important role for 4-1BB, TRAF1, and mTOR in the persistence of CD8 effector T cells in the lung parenchyma, thereby allowing the transition to Trm.

Brain-Resident T Cells Following Viral Infection.

Activated CD8+ lymphocytes infiltrate the brain in response to many viral infections; where some remain stationed long term as memory T cells. Brain-resident memory T cells (bTRM) are positioned to impart immediate defense against recurrent or reactivated infection. The cytokine and chemokine milieu present within a tissue is critical for TRM generation and retention; and reciprocal interactions exist between brain-resident glia and bTRM. High concentrations of TGF-β are found within brain and this cytokine has been shown to induce CD103 (integrin αeβ7) expression. The majority of T cells persisting within brain express CD103, which aids in retention through interaction with E-cadherin. Likewise, cytokines produced by T cells also modulate microglia. The anti-inflammatory cytokine IL-4 has been shown to preferentially polarize microglial cells toward an M2 phenotype, with a corresponding increase in E-cadherin expression. These findings demonstrate that the brain microenvironment, both during and following inflammation, prominently contributes to the role of CD103 in T cell persistence. Further evidence shows that microglia, and astrocytes, upregulate programmed death (PD) ligand 1 during neuroinflammation, likely to limit neuropathology, and the PD-1: PD-L1 pathway also aids in bTRM generation and retention. Upon reactivation of quiescent neurotropic viruses, bTRM may respond to small amounts of de novo-produced viral antigen by rapidly releasing IFN-γ, resulting in interferon-stimulated gene expression in surrounding glia, thereby amplifying activation of a small number of adaptive immune cells into an organ-wide innate antiviral response. While advantageous from an antiviral perspective; over time, recall response-driven, organ-wide innate immune activation likely has cumulative neurotoxic and neurocognitive consequences.

Tumor Resident Memory T Cells: New Players in Immune Surveillance and Therapy.

Tissue resident memory T cells (Trm) are a subset of memory T cells mainly described in inflammation and infection settings. Their location in peripheral tissues, such as lungs, gut, or skin, makes them the earliest T cell population to respond upon antigen recognition or under inflammatory conditions. The study of Trm cells in the field of cancer, and particularly in cancer immunotherapy, has recently gained considerable momentum. Different reports have shown that the vaccination route is critical to promote Trm generation in preclinical cancer models. Cancer vaccines administered directly at the mucosa, frequently result in enhanced Trm formation in mucosal cancers compared to vaccinations via intramuscular or subcutaneous routes. Moreover, the intratumoral presence of T cells expressing the integrin CD103 has been reported to strongly correlate with a favorable prognosis for cancer patients. In spite of recent progress, the full spectrum of Trm anti-tumoral functions still needs to be fully established, particularly in cancer patients, in different clinical contexts. In this mini-review we focus on the recent vaccination strategies aimed at generating Trm cells, as well as evidence supporting their association with patient survival in different cancer types. We believe that collectively, this information provides a strong rationale to target Trm for cancer immunotherapy.

Armed and Ready: Transcriptional Regulation of Tissue-Resident Memory CD8 T Cells.

A fundamental benefit of immunological memory is the ability to respond in an enhanced manner upon secondary encounter with the same pathogen. Tissue-resident memory CD8 T (TRM) cells contribute to improved protection against reinfection through the generation of immediate effector responses at the site of pathogen entry. Key to the potential of TRM cells to develop rapid recall responses is their location within the epithelia of the skin, lungs, and intestines at prime entry sites of pathogens. TRM cells are among the first immune cells to respond to pathogens that have been previously encountered in an antigen-specific manner. Upon recognition of invading pathogens, TRM cells release IFN-γ and other pro-inflammatory cytokines and chemokines. These effector molecules activate the surrounding epithelial tissue and recruit other immune cells including natural killer (NK) cells, B cells, and circulating memory CD8 T cells to the site of infection. The repertoire of TRM effector functions also includes the direct lysis of infected cells through the release of cytotoxic molecules such as perforin and granzymes. The mechanisms enabling TRM cells to respond in such a rapid manner are gradually being uncovered. In this review, we will address the signals that instruct TRM generation and maintenance as well as the underlying transcriptional network that keeps TRM cells in a deployment-ready modus. Furthermore, we will discuss how TRM cells respond to reinfection of the tissue and how transcription factors may control immediate and proliferative TRM responses.

Developmental Regulation of Effector and Resident Memory T Cell Generation during Pediatric Viral Respiratory Tract Infection.

Viral respiratory tract infections (VRTI) remain a leading cause of morbidity and mortality among infants and young children. In mice, optimal protection to VRTI is mediated by recruitment of effector T cells to the lungs and respiratory tract, and subsequent establishment of tissue resident memory T cells (Trm), which provide long-term protection. These critical processes of T cell recruitment to the respiratory tract, their role in disease pathogenesis, and establishment of local protective immunity remain undefined in pediatric VRTI. In this study, we investigated T cell responses in the upper respiratory tract (URT) and lower respiratory tract (LRT) of infants and young children with VRTI, revealing developmental regulation of T cell differentiation and Trm generation in situ. We show a direct concurrence between T cell responses in the URT and LRT, including a preponderance of effector CD8+ T cells that was associated with disease severity. During infant VRTI, there was an accumulation of terminally differentiated effector cells (effector memory RA+ T cells) in the URT and LRT with reduced Trm in the early neonatal period, and decreased effector memory RA+ T cell and increased Trm formation with age during the early years of childhood. Moreover, human infant T cells exhibit increased expression of the transcription factor T-bet compared with adult T cells, suggesting a mechanism for preferential generation of effector over Trm. The developmental regulation of respiratory T cell responses as revealed in the present study is important for diagnosing, monitoring, and treating VRTI in the critical early life stages.