Mucosal immunity develops in the human fetal intestine by 11–14 weeks gestation, yet whether viable microbes exist in utero and interact with the intestinal immune system is unknown. Bacterial-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, though strains with immunomodulatory capacity are detected in subsets of specimens.

Toxoplasma gondii infection occurs through the oral route, but we lack important information about how the parasite interacts with the host immune system in the intestine. We used two-photon laserscanning microscopy in conjunction with a mouse model of oral T. gondii infection to address this issue. T. gondii established discrete foci of infection in the small intestine, eliciting the recruitment and transepithelial migration of neutrophils and inflammatory monocytes. Neutrophils accounted for a high proportion of actively invaded cells, and we provide evidence for a role for transmigrating neutrophils and other immune cells in the spread of T. gondii infection through the lumen of the intestine. Our data identify neutrophils as motile reservoirs of T. gondii infection and suggest a surprising retrograde pathway for parasite spread in the intestine.neutrophil motility | dynamic imaging | gut | mucosal immunology T oxoplasma gondii infects around a third of humans worldwide and is widely dispersed in other warm-blooded hosts. Although clinical manifestations in the brain, eye, and developing fetus receive the most attention, T. gondii is an oral pathogen and first enters the body and establishes infection in the small intestine. Infection follows consumption of cyst-containing meat or oocyst-contaminated water and produce and is associated with the development of small intestinal pathology in a variety of nonhuman hosts (1). Most notably, experimental infection of C57BL/6 mice by the oral route results in an inflammation of the small intestine that shares immunological features with inflammatory bowel disease (2). This model is useful to further our understanding of host-pathogen interactions in the intestine and of common mechanisms underpinning the development of inflammatory bowel disease (3). Nevertheless, we have limited understanding of how and in which cells infection is established in the intestine, the extent to which the parasite replicates and spreads within the intestine, and how these factors contribute to the development of pathology (2, 4-9). The ability to label living parasites fluorescently and track them in the tissues of infected hosts provides an important tool for investigating these questions (10)(11)(12)(13)(14).Starting in the small intestine, T. gondii must travel long distances and surmount a variety of biological barriers to establish chronic infection in the brain. These barriers include the mucus, the intestinal epithelium, and the blood-brain barrier (7,15). Cells of the immune system are often highly motile and represent attractive transport vessels for pathogens seeking to reach and enter tissues while being protected from the external environment. Consequently, recent studies have focused on the role of immune cells in transporting parasites between tissues (4, 16-23). For example, cluster of differentiation 11b-positive (CD11b + ) cells have been implicated in the dissemination of T. gondii through the blood and across the blood-brain barrier (4, 19). Following oral infection, i...

BACKGROUND. While the human fetal immune system defaults to a program of tolerance, there is a 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 proinflammatory 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 with that of 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 proinflammatory cytokines. Engagement of the C-type lectin CD161 on these cells inhibited TCR-dependent production of IFN-γ in a fetal-specific manner. IFN-γ-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. cells in the development of protective immunity and their contribution to perinatal immune dysregulation is not known.Here, we performed a detailed analysis of human CD4 + T cell phenotype and function in fetal lymphoid and mucosal tissues. We show that Vα7.2 -PLZF + TCR-αβ + CD4 + T cells (herein referred to as PLZF + CD4 + T cells) specifically accumulated in the fetal intestine and were absent from the adult. Fetal PLZF + CD4 + T cells represent a transcriptionally unique subset of CD4 + T cells that are distinct from either innate-like, semi-invariant Va7.2 + T cells or PLZF -CD4 + T cells. Consistent with a primarily T effector memo-memory originates in utero. Innate-like T cells with rapid effector functions, such as γδ T cells, mucosa-associated invariant T (MAIT) cells, and innate-like NKT cells, are also present in fetal tissues (23)(24)(25). Promyelocytic leukemia zinc finger (PLZF), a transcriptional regulator that directs the differentiation of innate-like T cells (26,27), is widely expressed in human immune cells and is commonly associated with expression of CD161, a C-type lectin receptor (28). The human fetal thymus uniquely produces a subset of CD4 + T cells, distinct from NKT cells and MAIT cells, that expresses the transcription factor PLZF (29). However, the role of fetal PLZF ...

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.

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.