Homeostatic cytokines reciprocally modulate the emergence of prenatal effector PLZF+CD4+ T cells in humans

The development of human prenatal adaptive immunity progresses faster than previously appreciated, with the emergence of memory CD4+ T cells alongside regulatory T cells by midgestation. We previously identified a prenatal specific population of promyelocytic leukemia zinc finger-positive (PLZF+) CD4+ T cells with heightened effector potential that were enriched in the developing intestine and accumulated in the cord blood of infants exposed to prenatal inflammation. However, the signals that drive their tissue distribution and effector maturation are unknown. Here, we define the transcriptional and functional heterogeneity of human prenatal PLZF+CD4+ T cells and identify the compartmentalization of T helper-like (Th-like) effector function across the small intestine (SI) and mesenteric lymph nodes (MLNs). IL-7 was more abundant in the SI relative to the MLNs and drove the preferential expansion of naive PLZF+CD4+ T cells via enhanced STAT5 and MEK/ERK signaling. Exposure to IL-7 was sufficient to induce the acquisition of CD45RO expression and rapid effector function in a subset of PLZF+CD4+ T cells, identifying a human analog of memory phenotype CD4+ T cells. Further, IL-7 modulated the differentiation of Th1- and Th17-like PLZF+CD4+ T cells and thus likely contributes to the anatomic compartmentalization of human prenatal CD4+ T cell effector function.

Heritable vaginal bacteria influence immune tolerance and relate to early-life markers of allergic sensitization in infancy

Maternal asthma status, prenatal exposures, and infant gut microbiota perturbation are associated with heightened risk of atopy and asthma risk in childhood, observations hypothetically linked by intergenerational microbial transmission. Using maternal vaginal (n = 184) and paired infant stool (n = 172) samples, we identify four compositionally and functionally distinct Lactobacillus-dominated vaginal microbiota clusters (VCs) that relate to prenatal maternal health and exposures and infant serum immunoglobulin E (IgE) status at 1 year. Variance in bacteria shared between mother and infant pairs relate to VCs, maternal allergy/asthma status, and infant IgE levels. Heritable bacterial gene pathways associated with infant IgE include fatty acid synthesis and histamine and tryptophan degradation. In vitro, vertically transmitted Lactobacillus jensenii strains induce immunosuppressive phenotypes on human antigen-presenting cells. Murine supplementation with L. jensenii reduces lung eosinophils, neutrophilic expansion, and the proportion of interleukin-4 (IL-4)⁺ CD4⁺ T cells. Thus, bacterial and atopy heritability are intimately linked, suggesting a microbial component of intergenerational disease transmission.

Homeostatic cytokines regulate the emergence of human prenatal effector T cells

In a healthy pregnancy, prenatal immunity promotes tolerance in utero and transitions to provide protective immunity after birth. In pathologic pregnancies, in utero immune activation and the ensuing fetal inflammatory response are predictive of severe neonatal morbidity and mortality. We previously demonstrated that the majority of prenatal intestinal memory CD4 T cells express the transcription factor PLZF and possess a heightened Th1 effector potential. These PLZF+ CD4+ T cells specifically accumulate in the intestine, are absent from the adult, and are enriched in the cord blood of infants exposed to in utero inflammatory pathology. However, the signals that drive their intestinal accumulation and maturation are not well understood. We show that PLZF+ CD4+ T cells are MHC Class II restricted and display tissue specific functional heterogeneity, including Th2-, Th22- and Th17-like capacities. We provide evidence that differences in homeostatic cytokines between the small intestine and mesenteric lymph node support the preferential accumulation and functional compartmentalization of prenatal PLZF+ CD4+ T cells. Specifically, we show that IL-7 and TGFβ reciprocally modulate the preferential expansion and functional maturation of PLZF+ CD4+ T cells. We provide evidence for the generation of both virtual memory T cells as well as classic antigen driven differentiation of Th1- and Th17-like effector PLZF+ CD4+ T cells. This work identifies critical mechanisms of human prenatal immune maturation and regulation and could inform the development of novel therapeutic strategies to address perinatal inflammatory pathologies.

Supported by K08 Mentored Clinical Scientist Development Award (AI128007) to Joanna Halkias and the Burroughs Wellcome Fund Preterm Birth Initiative (1019828)

Viable bacterial colonization is limited in the human intestine in utero

Mucosal immunity develops in the human fetal intestine by 11–14 weeks gestation, yet whether viable microbes exist in utero and interact with intestinal immunity is unknown. Structures consistent with coccoid bacterial morphology, embedded in fetal meconium were evident before mid-gestation by high-resolution scanning electron microscopy (n=4). Molecular methods indicated extremely low bacterial burden and simple profiles in fetal meconium (n=40 of 50) compared to controls (n=87). A subset of Micrococcaceae-dominated (n=9) meconium associated with proportions of lamina propria PLZF+ CD161+ CD4+ T cells and divergent intestinal epithelial transcriptomes. Fetal Micrococcus luteus was isolated only in the presence of a monocyte feeder cell line. This strain grew on placental hormones, remained viable within fetal antigen presenting cells, exhibited species-specific immunomodulatory capacity and genomic features indicating fetal adaptation. Thus, viable bacteria are highly limited and inconsistently detectable in human fetal meconium at mid-gestation.

Mechanisms of Fetal T Cell Tolerance and Immune Regulation

The developing human fetus generates both tolerogenic and protective immune responses in response to the unique requirements of gestation. Thus, a successful human pregnancy depends on a fine balance between two opposing immunological forces: the semi-allogeneic fetus learns to tolerate both self- and maternal- antigens and, in parallel, develops protective immunity in preparation for birth. This critical window of immune development bridges prenatal immune tolerance with the need for postnatal environmental protection, resulting in a vulnerable neonatal period with heightened risk of infection. The fetal immune system is highly specialized to mediate this transition and thus serves a different function from that of the adult. Adaptive immune memory is already evident in the fetal intestine. Fetal T cells with pro-inflammatory potential are born in a tolerogenic environment and are tightly controlled by both cell-intrinsic and -extrinsic mechanisms, suggesting that compartmentalization and specialization, rather than immaturity, define the fetal immune system. Dysregulation of fetal tolerance generates an inflammatory response with deleterious effects to the pregnancy. This review aims to discuss the recent advances in our understanding of the cellular and molecular composition of fetal adaptive immunity and the mechanisms that govern T cell development and function. We also discuss the tolerance promoting environment that impacts fetal immunity and the consequences of its breakdown. A greater understanding of fetal mechanisms of immune activation and regulation has the potential to uncover novel paradigms of immune balance which may be leveraged to develop therapies for transplantation, autoimmune disease, and birth-associated inflammatory pathologies.

Viable bacterial colonization is highly limited in the human intestine in utero

Mucosal immunity develops in the human fetal intestine by 11-14 weeks of gestation, yet whether viable microbes exist in utero and interact with the intestinal immune system is unknown. Bacteria-like morphology was identified in pockets of human fetal meconium at mid-gestation by scanning electron microscopy (n = 4), and a sparse bacterial signal was detected by 16S rRNA sequencing (n = 40 of 50) compared to environmental controls (n = 87). Eighteen taxa were enriched in fetal meconium, with Micrococcaceae (n = 9) and Lactobacillus (n = 6) the most abundant. Fetal intestines dominated by Micrococcaceae exhibited distinct patterns of T cell composition and epithelial transcription. Fetal Micrococcus luteus, isolated only in the presence of monocytes, grew on placental hormones, remained viable within antigen presenting cells, limited inflammation ex vivo and possessed genomic features linked with survival in the fetus. Thus, viable bacteria are highly limited in the fetal intestine at mid-gestation, although strains with immunomodulatory capacity are detected in subsets of specimens.

CD161 contributes to prenatal immune suppression of IFNγ-producing PLZF+ T cells

BACKGROUND

While the human fetal immune system defaults to a program of tolerance, there is concurrent need for protective immunity to meet the antigenic challenges encountered after birth. Activation of T cells in utero is associated with the fetal inflammatory response with broad implications for the health of the fetus and of the pregnancy. However, the characteristics of the fetal effector T cells that contribute to this process are largely unknown.

METHODS

We analyzed primary human fetal lymphoid and mucosal tissues and performed phenotypic, functional, and transcriptional analysis to identify T cells with pro-inflammatory potential. The frequency and function of fetal-specific effector T cells was assessed in the cord blood of infants with localized and systemic inflammatory pathologies and compared to healthy term controls.

RESULTS

We identified a transcriptionally distinct population of CD4+ T cells characterized by expression of the transcription factor Promyelocytic Leukemia Zinc Finger (PLZF). PLZF+ CD4+ T cells were specifically enriched in the fetal intestine, possessed an effector memory phenotype, and rapidly produced pro-inflammatory cytokines. Engagement of the C-type lectin CD161 on these cells inhibited TCR-dependent production of IFNγ in a fetal-specific manner. IFNγ-producing PLZF+ CD4+ T cells were enriched in the cord blood of infants with gastroschisis, a natural model of chronic inflammation originating from the intestine, as well as in preterm birth, suggesting these cells contribute to fetal systemic immune activation.

CONCLUSION

Our work reveals a fetal-specific program of protective immunity whose dysregulation is associated with fetal and neonatal inflammatory pathologies.

Bacteria in human intestine contribute to mucosal immune function in utero

Mucosal immunity influences host-microbial interactions and is evident in the human fetal intestine by 11–14 weeks of gestation; the developing intestine is populated by lymphoid aggregates, memory T cells and dendritic cells capable of responding to microbial stimuli. However, whether intestinal encounters with viable microbes occur in utero and shape immune maturation has not been investigated. Here, we identified subsets of fetal meconium relatively enriched in Lactobacillus or Micrococcaceae, using culture-independent methods, which related to divergent epithelial cell layer transcriptomes and proportions of lamina propria PLZF+ CD161+ CD4+ T cells in paired intestinal samples. Mimicking conditions in the fetal intestine permitted isolation of viable Lactobacillus and Micrococcus strains from fetal meconium. In contrast with phylogenetically related reference strains, fetal isolates utilized placental hormones for growth, remained viable within antigen presenting cells, and exhibited species-specific capacity to promote immune homeostasis. Fetal Lactobacillus isolates reduced activation of antigen presenting cells, while the Micrococcus isolate induced IL-10 production in antigen presenting cells and inhibited IFNγ production by memory PLZF+T cells. Fetal isolates were identified as strains of Lactobacillus jensenii or Micrococcus luteus by whole genome sequencing, with the latter encoding strain-specific genes for intracellular survival, immune modulation, and steroid uptake. These data suggest that pre-natal immunity is influenced by bacterial exposure in the fetal intestine, identifying a previously unknown component of human mucosal immune development.

Studying T Cell Development in Thymic Slices

Recently, tissue slices have been adapted to study both mouse and human T cell development. Thymic slices combine and complement the strengths of existing organotypic culture systems to study thymocyte differentiation. Specifically, the thymic slice system allows for high throughput experiments and the ability to introduce homogenous developmental intermediate populations into an environment with a well-established cortex and medulla. These qualities make thymic slices a highly versatile and technically accessible model to study thymocyte development. Here we describe methods to prepare, embed, and slice thymic lobes to study T cell development in situ.

Opposing chemokine gradients control human thymocyte migration in situ

The ordered migration of thymocytes from the cortex to the medulla is critical for the appropriate selection of the mature T cell repertoire. Most studies of thymocyte migration rely on mouse models, but we know relatively little about how human thymocytes find their appropriate anatomical niches within the thymus. Moreover, the signals that retain CD4+CD8+ double-positive (DP) thymocytes in the cortex and prevent them from entering the medulla prior to positive selection have not been identified in mice or humans. Here, we examined the intrathymic migration of human thymocytes in both mouse and human thymic stroma and found that human thymocyte subsets localized appropriately to the cortex on mouse thymic stroma and that MHC-dependent interactions between human thymocytes and mouse stroma could maintain the activation and motility of DP cells. We also showed that CXCR4 was required to retain human DP thymocytes in the cortex, whereas CCR7 promoted migration of mature human thymocytes to the medulla. Thus, 2 opposing chemokine gradients control the migration of thymocytes from the cortex to the medulla. These findings point to significant interspecies conservation in thymocyte-stroma interactions and provide the first evidence that chemokines not only attract mature thymocytes to the medulla, but also play an active role in retaining DP thymocytes in the cortex prior to positive selection.

Characterization of human thymocyte behavior in situ (111.46)

Thymocyte development occurs through the coordinated movement of developmental intermediates to defined anatomical locations within the thymus during which thymocytes sample surrounding cells and the microenvironment for signals to promote differentiation. The development of an experimental model in which purified, labeled cell populations migrate into a 3-dimensional living thymic slice gives us the first opportunity to visualize and monitor the behavior of human thymic subsets in situ. Double negative (DN) progenitors and CD4+CD8+ double positive (DP) human thymic subsets primarily localize to the cortex, whereas CD4+ and CD8+ single positive (SP) subsets localize appropriately to the medulla. In many respects, the migration of human thymic subsets mirror that of their murine counterparts: immature DP and DN cells move randomly with slow motility rates, DP cells migrate slower than DN, and both SP populations are fast and exhibit some directional movement presumably toward the medulla. These similarities between human and mouse thymocyte motility occur despite significant differences in the development and microenvironment of the thymus between the species. We are also examining cross-talk between mouse and human factors as well as the sensitivity of human thymic subsets to chemokine signals. The suitability of the thymic slice model to study human thymocyte development is a powerful system to study and manipulate processes that are otherwise inaccessible to researchers.

T cells in cryptopatch aggregates share TCR gamma variable region junctional sequences with gamma delta T cells in the small intestinal epithelium of mice

The role of cryptopatch aggregates in the development of intestinal intraepithelial lymphocytes (IEL) is a matter of controversy. Therefore, an important question is whether T cells in cryptopatch aggregates are lineally related to IEL. We hypothesized that if gammadelta+ IEL derive from T cells in cryptopatch aggregates, then a clonal relationship would exist between the two populations. To test this hypothesis, we compared the sequence of rearranged TCR gamma variable region 5 genes in gammadelta+ IEL and cryptopatch cells. We purified IEL by FACS and cryptopatch cells were isolated from frozen sections of the intestine by laser-assisted microdissection. PCR showed that TCR gamma variable region 5 was rearranged in gammadelta+ IEL and in CD3+ cryptopatch cells, but not in CD3- cryptopatch cells. DNA sequence analysis showed that the frequency of in-frame junctions in cryptopatch aggregates was at a level consistent with positive selection in both wild-type and athymic nude mice. In addition, the predicted amino acid sequences of V-J junctions present in gammadelta+ IEL and cryptopatch cells were encoded by identical nucleotide sequences. By contrast, the frequency of in-frame joints was significantly reduced in cryptopatch cells isolated from TCR delta-deficient mice, indicating that the enrichment of in-frame joints in cryptopatch cells must normally depend on expression of surface gammadelta TCR. Our results are consistent with the hypothesis that a subset of gammadelta+ IEL are related to T cells in cryptopatch aggregates. The precise role of cryptopatch aggregates in intestinal gammadelta+ T cell homeostasis still needs to be determined.

ApoA-II maintains HDL levels in part by inhibition of hepatic lipase. Studies In apoA-II and hepatic lipase double knockout mice

High density lipoprotein (HDL) cholesterol levels are inversely related to the risk of developing coronary heart disease. Apolipoprotein (apo) A-II is the second most abundant HDL apolipoprotein and apoA-II knockout mice show a 70% reduction in HDL cholesterol levels. There is also evidence, using human apoA-II transgenic mice, that apoA-II can prevent hepatic lipase-mediated HDL triglyceride hydrolysis and reduction in HDL size. These observations suggest the hypothesis that apoA-II maintains HDL levels, at least in part, by inhibiting hepatic lipase. To evaluate this, apoA-II knockout mice were crossbred with hepatic lipase knockout mice. Compared to apoA-II-deficient mice, in double knockout mice there were increased HDL cholesterol levels (57% in males and 60% in females), increased HDL size, and decreased HDL cholesteryl ester fractional catabolic rate. In vitro incubation studies of plasma from apoA-II knockout mice, which contains largely apoA-I HDL particles, showed active lipolysis of HDL triglyceride, whereas similar studies of plasma from apoA-I knockout mice, which contains largely apoA-II particles, did not. In summary, these results strongly suggest that apoA-II is a physiological inhibitor of hepatic lipase and that this is at least part of the mechanism whereby apoA-II maintains HDL cholesterol levels.