Infant T cells are developmentally adapted for robust lung immune responses through enhanced T cell receptor signaling

Regional specialization within the mammalian respiratory immune system

The respiratory tract is exposed to infection from inhaled pathogens, including viruses, bacteria, and fungi. So far, a comprehensive assessment that integrates common and distinct aspects of the immune response along different areas of the respiratory tract has been lacking. Here, we discuss key recent findings regarding anatomical, functional, and microbial factors driving regional immune adaptation in the mammalian respiratory system, how they differ between mice and humans, and the similarities and differences with the gastrointestinal tract. We demonstrate that, under evolutionary pressure, mammals evolved spatially organized immune defenses that vary between the upper and lower respiratory tract. Overall, we propose that the functional specialization of the immune response along the respiratory tract has fundamental implications for the management of infectious or inflammatory diseases.

LINE1 modulate human T cell function by regulating protein synthesis during the life span

The molecular mechanisms responsible for the heightened reactivity of quiescent T cells in human early life remain largely elusive. Our previous research identified that quiescent adult naïve CD4+ T cells express LINE1 (long interspersed nuclear elements 1) spliced in previously unknown isoforms, and their down-regulation marks the transition to activation. Here, we unveil that neonatal naïve T cell quiescence is characterized by enhanced energy production and protein synthesis. This phenotype is associated with the absence of LINE1 expression attributed to tonic T cell receptor/mTOR complex 1 (mTORC1) signaling and (polypyrimidine tract-binding protein 1 (PTBP1)-mediated LINE1 splicing suppression. The absence of LINE1 expression primes these cells for rapid execution of the activation program by directly regulating protein synthesis. LINE1 expression progressively increases in childhood and adults, peaking in elderly individuals, and, by decreasing protein synthesis, contributes to immune senescence in aging. Our study proposes LINE1 as a critical player of human T cell function across the human life span.

Preschool-age children maintain a distinct memory CD4+ T cell and memory B cell response after SARS-CoV-2 infection

The development of the human immune system lasts for several years after birth. The impact of this maturation phase on the quality of adaptive immunity and the acquisition of immunological memory after infection at a young age remains incompletely defined. Here, using an antigen-reactive T cell (ARTE) assay and multidimensional flow cytometry, we profiled circulating severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-reactive CD3+CD4+CD154+ T cells in children and adults before infection, during infection, and 11 months after infection, stratifying children into separate age groups and adults according to disease severity. During SARS-CoV-2 infection, children younger than 5 years old displayed a lower antiviral CD4+ T cell response, whereas children older than 5 years and adults with mild disease had, quantitatively and phenotypically, comparable virus-reactive CD4+ T cell responses. Adults with severe disease mounted a response characterized by higher frequencies of virus-reactive proinflammatory and cytotoxic T cells. After SARS-CoV-2 infection, preschool-age children not only maintained neutralizing SARS-CoV-2-reactive antibodies postinfection comparable to adults but also had phenotypically distinct memory T cells displaying high inflammatory features and properties associated with migration toward inflamed sites. Moreover, preschool-age children had markedly fewer circulating virus-reactive memory B cells compared with the other cohorts. Collectively, our results reveal unique facets of antiviral immunity in humans at a young age and indicate that the maturation of adaptive responses against SARS-CoV-2 toward an adult-like profile occurs in a progressive manner.

Dietary iron deficiency impairs effector function of memory T cells following influenza infection

The establishment of memory T cell responses is critical to protection against pathogens and is influenced by the conditions under which memory formation occurs. Iron is an essential micronutrient for multiple immunologic processes and nutritional deficiency is a common problem worldwide. Despite its prevalence, the impact of nutritional iron deficiency on the establishment of memory T cell responses is not fully understood. In this study we investigate the impact of nutritional iron deficiency on the generation, phenotype, and function of memory T cell responses using a murine model of dietary iron modulation in the context of influenza infection. Iron deficient mice have decreased systemic iron levels and develop significant anemia. Increased T cell expression of the transferrin receptor (CD71) is seen in iron deficient mice at baseline. During primary influenza infection, iron deficient mice experience increased weight loss and phenotypic evidence of impairments in T cell activation. Following recovery from infection, iron deficient mice generate increased influenza specific memory T cells which exhibit impaired ability to produce IFNγ, most notably within the lung. Importantly, the ability to produce IFNγ and TNFα is not recovered by co-culture with iron replete dendritic cells, suggesting a T cell intrinsic alteration in functional memory formation. Altogether, these results isolate a critical effect of nutritional iron deficiency on T cell memory development and function.

Update on Early-Life T Cells: Impact on Oral Rotavirus Vaccines

Rotavirus infection continues to be a significant public health problem in developing countries, despite the availability of several vaccines. The efficacy of oral rotavirus vaccines in young children may be affected by significant immunological differences between individuals in early life and adults. Therefore, understanding the dynamics of early-life systemic and mucosal immune responses and the factors that affect them is essential to improve the current rotavirus vaccines and develop the next generation of mucosal vaccines. This review focuses on the advances in T-cell development during early life in mice and humans, discussing how immune homeostasis and response to pathogens is established in this period compared to adults. Finally, the review explores how this knowledge of early-life T-cell immunity could be utilized to enhance current and novel rotavirus vaccines.

Early life imprinting of intestinal immune tolerance and tissue homeostasis

Besides its canonical role in protecting the host from pathogens, the immune system plays an arguably equally important role in maintaining tissue homeostasis. Within barrier tissues that interface with the external microenvironment, induction of immune tolerance to innocuous antigens, such as commensal, dietary, and environmental antigens, is key to establishing immune homeostasis. The early postnatal period represents a critical window of opportunity in which parallel development of the tissue, immune cells, and microbiota allows for reciprocal regulation that shapes the long-term immunological tone of the tissue and subsequent risk of immune-mediated diseases. During early infancy, the immune system appears to sacrifice pro-inflammatory functions, prioritizing the establishment of tissue tolerance. In this review, we discuss mechanisms underlying early life windows for intestinal tolerance with a focus on newly identified RORγt+ antigen-presenting cells-Thetis cells-and highlight the role of the intestinal microenvironment in shaping intestinal immune system development and tolerance.

Larval T Cells Are Functionally Distinct from Adult T Cells in Xenopus laevis

The amphibian Xenopus laevis tadpole provides a unique comparative experimental organism for investigating the roles of innate-like T (iT) cells in tolerogenic immunity during early development. Unlike mammals and adult frogs, where conventional T cells are dominant, tadpoles rely mostly on several prominent distinct subsets of iT cells interacting with cognate nonpolymorphic MHC class I-like molecules. In the present study, to investigate whole T cell responsiveness ontogenesis in X. laevis, we determined in tadpoles and adult frogs the capacity of splenic T cells to proliferate in vivo upon infection with two different pathogens, ranavirus FV3 and Mycobacterium marinum, as well as in vitro upon PHA stimulation using the thymidine analogous 5-ethynyl-2'-deoxyuridine and flow cytometry. We also analyzed by RT-quantitative PCR T cell responsiveness upon PHA stimulation. In vivo tadpole splenic T cells showed limited capacity to proliferate, whereas the in vitro proliferation rate was higher than adult T cells. Gene markers for T cell activation and immediate-early genes induced upon TCR activation were upregulated with similar kinetics in tadpole and adult splenocytes. However, the tadpole T cell signature included a lower amplitude in the TCR signaling, which is a hallmark of mammalian memory-like T cells and iT or "preset" T cells. This study suggests that reminiscent of mammalian neonatal T cells, tadpole T cells are functionally different from their adult counterpart.

HIVEP3 inhibits fate decision of CD8+ invariant NKT cells after positive selection

CD8+ invariant natural killer T (iNKT) cells are functionally different from other iNKT cells and are enriched in human but not in mouse. To date, their developmental pathway and molecular basis for fate decision remain unclear. Here, we report enrichment of CD8+ iNKT cells in neonatal mice due to their more rapid maturation kinetics than CD8- iNKT cells. Along developmental trajectories, CD8+ and CD8- iNKT cells separate at stage 0, following stage 0 double-positive iNKT cells, and differ in HIVEP3 expression. HIVEP3 is lowly expressed in stage 0 CD8+ iNKT cells and negatively controls their development, whereas it is highly expressed in stage 0 CD8- iNKT cells and positively controls their development. Despite no effect on IFN-γ, HIVEP3 inhibits granzyme B but promotes interleukin-4 production in CD8+ iNKT cells. Together, we reveal that, as a negative regulator for CD8+ iNKT fate decision, low expression of HIVEP3 in stage 0 CD8+ iNKT cells favors their development and T helper 1-biased cytokine responses as well as high cytotoxicity.

Site-specific development and progressive maturation of human tissue-resident memory T cells over infancy and childhood

Infancy and childhood are critical life stages for generating immune memory to protect against pathogens; however, the timing, location, and pathways for memory development in humans remain elusive. Here, we investigated T cells in mucosal sites, lymphoid tissues, and blood from 96 pediatric donors aged 0-10 years using phenotypic, functional, and transcriptomic profiling. Our results revealed that memory T cells preferentially localized in the intestines and lungs during infancy and accumulated more rapidly in mucosal sites compared with blood and lymphoid organs, consistent with site-specific antigen exposure. Early life mucosal memory T cells exhibit distinct functional capacities and stem-like transcriptional profiles. In later childhood, they progressively adopt proinflammatory functions and tissue-resident signatures, coincident with increased T cell receptor (TCR) clonal expansion in mucosal and lymphoid sites. Together, our findings identify staged development of memory T cells targeted to tissues during the formative years, informing how we might promote and monitor immunity in children.

M cell maturation and cDC activation determine the onset of adaptive immune priming in the neonatal Peyer's patch

Early-life immune development is critical to long-term host health. However, the mechanisms that determine the pace of postnatal immune maturation are not fully resolved. Here, we analyzed mononuclear phagocytes (MNPs) in small intestinal Peyer's patches (PPs), the primary inductive site of intestinal immunity. Conventional type 1 and 2 dendritic cells (cDC1 and cDC2) and RORgt+ antigen-presenting cells (RORgt+ APC) exhibited significant age-dependent changes in subset composition, tissue distribution, and reduced cell maturation, subsequently resulting in a lack in CD4+ T cell priming during the postnatal period. Microbial cues contributed but could not fully explain the discrepancies in MNP maturation. Type I interferon (IFN) accelerated MNP maturation but IFN signaling did not represent the physiological stimulus. Instead, follicle-associated epithelium (FAE) M cell differentiation was required and sufficient to drive postweaning PP MNP maturation. Together, our results highlight the role of FAE M cell differentiation and MNP maturation in postnatal immune development.

Programmed and environmental determinants driving neonatal mucosal immune development

The mucosal immune system of neonates goes through successive, non-redundant phases that support the developmental needs of the infant and ultimately establish immune homeostasis. These phases are informed by environmental cues, including dietary and microbial stimuli, but also evolutionary developmental programming that functions independently of external stimuli. The immune response to exogenous stimuli is tightly regulated during early life; thresholds are set within this neonatal "window of opportunity" that govern how the immune system will respond to diet, the microbiota, and pathogenic microorganisms in the future. Thus, changes in early-life exposure, such as breastfeeding or environmental and microbial stimuli, influence immunological and metabolic homeostasis and the risk of developing diseases such as asthma/allergy and obesity.

Novel murine model reveals an early role for pertussis toxin in disrupting neonatal immunity to Bordetella pertussis

The increased susceptibility of neonates to specific pathogens has previously been attributed to an underdeveloped immune system. More recent data suggest neonates have effective protection against most pathogens but are particularly susceptible to those that target immune functions specific to neonates. Bordetella pertussis (Bp), the causative agent of "whooping cough", causes more serious disease in infants attributed to its production of pertussis toxin (PTx), although the neonate-specific immune functions it targets remain unknown. Problematically, the rapid development of adult immunity in mice has confounded our ability to study interactions of the neonatal immune system and its components, such as virtual memory T cells which are prominent prior to the maturation of the thymus. Here, we examine the rapid change in susceptibility of young mice and define a period from five- to eight-days-old during which mice are much more susceptible to Bp than mice even a couple days older. These more narrowly defined "neonatal" mice display significantly increased susceptibility to wild type Bp but very rapidly and effectively respond to and control Bp lacking PTx, more rapidly even than adult mice. Thus, PTx efficiently blocks some very effective form(s) of neonatal protective immunity, potentially providing a tool to better understand the neonatal immune system. The rapid clearance of the PTx mutant correlates with the early accumulation of neutrophils and T cells and suggests a role for PTx in disrupting their accumulation. These results demonstrate a striking age-dependent response to Bp, define an early age of extreme susceptibility to Bp, and demonstrate that the neonatal response can be more efficient than the adult response in eliminating bacteria from the lungs, but these neonatal functions are substantially blocked by PTx. This refined definition of "neonatal" mice may be useful in the study of other pathogens that primarily infect neonates, and PTx may prove a particularly valuable tool for probing the poorly understood neonatal immune system.