Selective accumulation of Th2-skewing immature erythroid cells in developing neonatal mouse spleen

The Distinct Immune Nature of the Fetal Inflammatory Response Syndrome Type I and Type II

Fetal inflammatory response syndrome (FIRS) is strongly associated with neonatal morbidity and mortality and can be classified as type I or type II. Clinically, FIRS type I and type II are considered as distinct syndromes, yet the molecular underpinnings of these fetal inflammatory responses are not well understood because of their low prevalence and the difficulty of postdelivery diagnosis. In this study, we performed RNA sequencing of human cord blood samples from preterm neonates diagnosed with FIRS type I or FIRS type II. We found that FIRS type I was characterized by an upregulation of host immune responses, including neutrophil and monocyte functions, together with a proinflammatory cytokine storm and a downregulation of T cell processes. In contrast, FIRS type II comprised a mild chronic inflammatory response involving perturbation of HLA transcripts, suggestive of fetal semiallograft rejection. Integrating single-cell RNA sequencing-derived signatures with bulk transcriptomic data confirmed that FIRS type I immune responses were mainly driven by monocytes, macrophages, and neutrophils. Last, tissue- and cell-specific signatures derived from the BioGPS Gene Atlas further corroborated the role of myeloid cells originating from the bone marrow in FIRS type I. Collectively, these data provide evidence that FIRS type I and FIRS type II are driven by distinct immune mechanisms; whereas the former involves the innate limb of immunity consistent with host defense, the latter resembles a process of semiallograft rejection. These findings shed light on the fetal immune responses caused by infection or alloreactivity that can lead to deleterious consequences in neonatal life.

The role of CD71+ erythroid cells in the regulation of the immune response

Complex regulation of the immune response is necessary to support effective defense of an organism against hostile invaders and to maintain tolerance to harmless microorganisms and autoantigens. Recent studies revealed previously unappreciated roles of CD71⁺ erythroid cells (CECs) in regulation of the immune response. CECs physiologically reside in the bone marrow where erythropoiesis takes place. Under stress conditions, CECs are enriched in some organs outside of the bone marrow as a result of extramedullary erythropoiesis. However, the role of CECs goes well beyond the production of erythrocytes. In neonates, increased numbers of CECs contribute to their vulnerability to infectious diseases. On the other side, neonatal CECs suppress activation of immune cells in response to abrupt colonization with commensal microorganisms after delivery. CECs are also enriched in the peripheral blood of pregnant women as well as in the placenta and are responsible for the regulation of feto-maternal tolerance. In patients with cancer, anemia leads to increased frequency of CECs in the peripheral blood contributing to diminished antiviral and antibacterial immunity, as well as to accelerated cancer progression. Moreover, recent studies revealed the role of CECs in HIV and SARS-CoV-2 infections. CECs use a full arsenal of mechanisms to regulate immune response. These cells suppress proinflammatory responses of myeloid cells and T-cell proliferation by the depletion of ʟ-arginine by arginase. Moreover, CECs produce reactive oxygen species to decrease T-cell proliferation. CECs also secrete cytokines, including transforming growth factor β (TGF-β), which promotes T-cell differentiation into regulatory T-cells. Here, we comprehensively describe the role of CECs in orchestrating immune response and indicate some therapeutic approaches that might be used to regulate their effector functions in the treatment of human conditions.

Erythroid Lineage Cells in the Liver: Novel Immune Regulators and Beyond

The lineage of the erythroid cell has been revisited in recent years. Instead of being classified as simply inert oxygen carriers, emerging evidence has shown that they are a tightly regulated in immune potent population with potential developmental plasticity for lineage crossing. Erythroid cells have been reported to exert immune regulatory function through secreted cytokines, or cell-cell contact, depending on the conditions of the microenvironment and disease models. In this review, we explain the natural history of erythroid cells in the liver through a developmental lens, as it offers perspectives into newly recognized roles of this lineage in liver biology. Here, we review the known immune roles of erythroid cells and discuss the mechanisms in the context of disease models and stages. Then, we explore the capability of erythroid lineage as a cell source for regenerative medicine. We propose that the versatile lineage of erythroid cells provides an underappreciated and potentially promising area for basic and translational research in the field of liver disease.

Regulation of bile duct epithelial injury by hepatic CD71+ erythroid cells

Extramedullary hematopoietic cells are present in the liver of normal neonates in the first few days of life and persist in infants with biliary atresia. Based on a previous report that liver genes are enriched by erythroid pathways, we examined the liver gene expression pattern at diagnosis and found the top 5 enriched pathways are related to erythrocyte pathobiology in children who survived with the native liver beyond 2 years of age. Using immunostaining, anti-CD71 antibodies identified CD71+ erythroid cells among extramedullary hematopoietic cells in the livers at the time of diagnosis. In mechanistic experiments, the preemptive antibody depletion of hepatic CD71+ erythroid cells in neonatal mice rendered them resistant to rhesus rotavirus-induced (RRV-induced) biliary atresia. The depletion of CD71+ erythroid cells increased the number of effector lymphocytes and delayed the RRV infection of livers and extrahepatic bile ducts. In coculture experiments, CD71+ erythroid cells suppressed the activation of hepatic mononuclear cells. These data uncover an immunoregulatory role for CD71+ erythroid cells in the neonatal liver.

The effects of advanced maternal age on T-cell subsets at the maternal-fetal interface prior to term labor and in the offspring: a mouse study

Women who conceive at 35 years of age or older, commonly known as advanced maternal age, have a higher risk of facing parturition complications and their children have an increased risk of developing diseases later in life. However, the immunological mechanisms underlying these pathological processes have yet to be established. To fill this gap in knowledge, using a murine model and immunophenotyping, we determined the effect of advanced maternal age on the main cellular branch of adaptive immunity, T cells, at the maternal-fetal interface and in the offspring. We report that advanced maternal age impaired the process of labor at term, inducing dystocia and delaying the timing of delivery. Advanced maternal age diminished the number of specific proinflammatory T-cell subsets [T helper type 1 (Th1): CD4⁺ IFN-γ⁺ , CD8⁺ IFN-γ⁺ and Th9: CD4⁺ IL-9⁺ ], as well as CD4⁺ regulatory T cells (CD4⁺ CD25⁺ FoxP3⁺ T cells), at the maternal-fetal interface prior to term labor. Advanced maternal age also altered fetal growth and survival of the offspring in early life. In addition, infants born to advanced-age mothers had alterations in the T-cell repertoire but not in CD71⁺ erythroid cells (CD3^- CD71⁺ TER119⁺ cells). This study provides insight into the immune alterations observed at the maternal-fetal interface of advanced-age mothers and their offspring.

Prenatal Maternal Stress Causes Preterm Birth and Affects Neonatal Adaptive Immunity in Mice

Maternal stress is a well-established risk factor for preterm birth and has been associated with adverse neonatal outcomes in the first and subsequent generations, including increased susceptibility to disease and lasting immunological changes. However, a causal link between prenatal maternal stress and preterm birth, as well as compromised neonatal immunity, has yet to be established. To fill this gap in knowledge, we used a murine model of prenatal maternal stress across three generations and high-dimensional flow cytometry to evaluate neonatal adaptive immunity. We report that recurrent prenatal maternal stress induced preterm birth in the first and second filial generations and negatively impacted early neonatal growth. Strikingly, prenatal maternal stress induced a systematic reduction in T cells and B cells, the former including regulatory CD4+ T cells as well as IL-4- and IL-17A-producing T cells, in the second generation. Yet, neonatal adaptive immunity gained resilience against prenatal maternal stress by the third generation. We also show that the rate of prenatal maternal stress-induced preterm birth can be reduced upon cessation of stress, though neonatal growth impairments persisted. These findings provide evidence that prenatal maternal stress causes preterm birth and affects neonatal immunity across generations, adverse effects that can be ameliorated upon cessation.

Nonproliferative and Proliferative Lesions of the Rat and Mouse Hematolymphoid System

The INHAND Project (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP), and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative changes in rats and mice. The purpose of this publication is to provide a standardized nomenclature for classifying changes observed in the hematolymphoid organs, including the bone marrow, thymus, spleen, lymph nodes, mucosa-associated lymphoid tissues, and other lymphoid tissues (serosa-associated lymphoid clusters and tertiary lymphoid structures) with color photomicrographs illustrating examples of the lesions. Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous lesions as well as lesions induced by exposure to test materials. The nomenclature for these organs is divided into 3 terminologies: descriptive, conventional, and enhanced. Three terms are listed for each diagnosis. The rationale for this approach and guidance for its application to toxicologic pathology are described in detail below.

Cytokine-Producing B Cells Promote Immune-Mediated Bile Duct Injury in Murine Biliary Atresia

Biliary atresia (BA) is a neonatal T cell-mediated, inflammatory, sclerosing cholangiopathy. In the rhesus rotavirus (RRV)-induced neonatal mouse model of BA (murine BA), mice lacking B cells do not develop BA, and the lack of B cells is associated with loss of T-cell and macrophage activation. The aim of this study was to determine the mechanism of B cell-mediated immune activation (antigen presentation versus cytokine production) in murine BA. Normal neonatal B cells in the liver are predominantly at pro-B and pre-B cellular development. However, BA mice exhibit a significant increase in the number and activation status of mature liver B cells. Adoptively transferred B cells into RRV-infected, B cell-deficient mice were able to reinstate T-cell and macrophage infiltration and biliary injury. Nonetheless, neonatal liver B cells were incompetent at antigen presentation to T cells. Moreover, 3-83 immunoglobulin transgenic mice, in which B cells only present an irrelevant antigen, developed BA, indicating a B-cell antigen-independent mechanism. B cells from BA mice produced a variety of innate and adaptive immune cytokines associated with immune activation. In vitro trans-well studies revealed that BA B cells secreted cytokines that activated T cells based on increased expression of T-cell activation marker cluster of differentiation 69. Conclusion: Neonatal liver B cells are highly activated in murine BA and contribute to immune activation through production of numerous cytokines involved in innate and adaptive immunity; this work provides increased knowledge on the capacity of neonatal B cells to contribute to an inflammatory disease through cytokine-mediated mechanisms, and future studies should focus on targeting B cells as a therapeutic intervention in human BA.

CD71+ Erythroid Suppressor Cells Promote Fetomaternal Tolerance through Arginase-2 and PDL-1

Survival of the allogeneic pregnancy depends on the maintenance of immune tolerance to paternal alloantigens at the fetomaternal interface. Multiple localized mechanisms contribute to the fetal evasion from the mother's immune rejection as the fetus is exposed to a wide range of stimulatory substances such as maternal alloantigens, microbes and amniotic fluids. In this article, we demonstrate that CD71⁺ erythroid cells are expanded at the fetomaternal interface and in the periphery during pregnancy in both humans and mice. These cells exhibit immunosuppressive properties, and their abundance is associated with a Th2 skewed immune response, as their depletion results in a proinflammatory immune response at the fetomaternal interface. In addition to their function in suppressing proinflammatory responses in vitro, maternal CD71⁺ erythroid cells inhibit an aggressive allogeneic response directed against the fetus such as reduction in TNF-α and IFN-γ production through arginase-2 activity and PD-1/programmed death ligand-1 (PDL-1) interactions. Their depletion leads to the failure of gestation due to the immunological rejection of the fetus. Similarly, fetal liver CD71⁺ erythroid cells exhibit immunosuppressive activity. Therefore, immunosuppression mediated by CD71⁺ erythroid cells on both sides (mother/fetus) is crucial for fetomaternal tolerance. Thus, our results reveal a previously unappreciated role for CD71⁺ erythroid cells in pregnancy and indicate that these cells mediate homeostatic immunosuppressive/immunoregulatory responses during pregnancy.

CD71+ erythroid cells from neonates born to women with preterm labor regulate cytokine and cellular responses

Neonatal CD71+ erythroid cells are thought to have immunosuppressive functions. Recently, we demonstrated that CD71+ erythroid cells from neonates born to women who underwent spontaneous preterm labor (PTL) are reduced to levels similar to those of term neonates; yet, their functional properties are unknown. Herein, we investigated the functionality of CD71+ erythroid cells from neonates born to women who underwent spontaneous preterm or term labor. CD71+ erythroid cells from neonates born to women who underwent PTL displayed a similar mRNA profile to that of those from term neonates. The direct contact between preterm or term neonatal CD71+ erythroid cells and maternal mononuclear immune cells, but not soluble products from these cells, induced the release of proinflammatory cytokines and a reduction in the release of TGF-β. Moreover, PTL-derived neonatal CD71+ erythroid cells (1) modestly altered CD8+ T cell activation; (2) inhibited conventional CD4+ and CD8+ T-cell expansion; (3) suppressed the expansion of CD8+ regulatory T cells; (4) regulated cytokine responses mounted by myeloid cells in the presence of a microbial product; and (5) indirectly modulated T-cell cytokine responses. In conclusion, neonatal CD71+ erythroid cells regulate neonatal T-cell and myeloid responses and their direct contact with maternal mononuclear cells induces a proinflammatory response. These findings provide insight into the biology of neonatal CD71+ erythroid cells during the physiologic and pathologic processes of labor.

Umbilical cord CD71+ erythroid cells are reduced in neonates born to women in spontaneous preterm labor

PROBLEM

Preterm neonates are highly susceptible to infection. Neonatal host defense against infection seems to be maintained by the temporal presence of immunosuppressive CD71+ erythroid cells. The aim of this study was to investigate whether umbilical cord CD71+ erythroid cells are reduced in neonates born to women who undergo spontaneous preterm labor/birth.

METHOD OF STUDY

Umbilical cord blood samples (n=155) were collected from neonates born to women who delivered preterm with (n=39) and without (n=12) spontaneous labor or at term with (n=82) and without (n=22) spontaneous labor. Time-matched maternal peripheral blood samples were also included (n=111). Mononuclear cells were isolated from these samples, and CD71+ erythroid cells were identified and quantified as CD3-CD235a+CD71+ cells by flow cytometry.

RESULTS

(i) The proportion of CD71+ erythroid cells was 50-fold higher in cord blood than in maternal blood; (ii) a reduced number and frequency of umbilical cord CD71+ erythroid cells were found in neonates born to women who underwent spontaneous preterm labor compared to those born to women who delivered preterm without labor; (iii) umbilical cord CD71+ erythroid cells were fewer in neonates born to term pregnancies, regardless of the process of labor, than in those born to women who delivered preterm without labor; and (iv) no differences were seen in umbilical cord CD71+ erythroid cells between neonates born to women who underwent spontaneous preterm labor and those born to women who delivered at term with labor.

CONCLUSION

Umbilical cord CD71+ erythroid cells are reduced in neonates born to women who had undergone spontaneous preterm labor.

Immune responses in perinatal brain injury

The perinatal period has often been described as immune deficient. However, it has become clear that immune responses in the neonate following exposure to microbes or as a result of tissue injury may be substantial and play a role in perinatal brain injury. In this article we will review the immune cell composition under normal physiological conditions in the perinatal period, both in the human and rodent. We will summarize evidence of the inflammatory responses to stimuli and discuss how neonatal immune activation, both in the central nervous system and in the periphery, may contribute to perinatal hypoxic-ischemic brain injury.

α-Galactosylceramide stimulates splenic lymphocyte proliferation in vitro and increases antibody production in vivo in late neonatal-age mice

The neonatal stage is characterized by weak responses to various infections and vaccines, thus the development of efficient formulas to improve vaccine effectiveness is of high priority. The glycolipid alpha galactosylceramide (αGalCer) is known as a potent immune modulator due mainly to natural killer (NK) T cell activation. Using a mouse tetanus toxoid (TT) immunization model, we observed that neonatal mice given αGalCer at the time of primary immunization on postnatal day (pnd) 17 had a significantly higher TT-specific immunoglobulin (Ig)M response as well as a memory IgG response, while αGalCer given on pnd 7 resulted in only marginal boosting. Consistently, immunostaining of the spleen sections from αGalCer-treated pnd 17 immunized neonates showed a higher number of Ki67(+) cells in the splenic germinal centre area, suggesting a stronger response after immunization. In-vitro kinetic studies revealed that spleen cells from newborn to pnd 7 neonates did not respond to αGalCer stimulation, whereas cell proliferation was increased markedly by αGalCer after pnd 7, and became dramatic around neonatal pnd 17-18, which was accompanied by increased B, T and NK T cell populations in the spleen. In addition, in pnd 17 spleen cells, αGalCer significantly stimulated the production of NK T cytokines, interleukin (IL)-4 and interferon (IFN)-γ, and promoted the proliferation of CD23(+) B cells, a subset of B cells enriched in germinal centres. These data suggest that αGalCer is an effective immune stimulus in the late neonatal stage, and thus may be useful in translational studies to test as a potential adjuvant to achieve a more efficient response to immunization.

New insight into an old concept: role of immature erythroid cells in immune pathogenesis of neonatal infection

Newborns are exceedingly susceptible to infection. However, very little is known about what governs the immunological differences seen in early life that result in extreme vulnerability to infection, nor how this changes during infancy. Herein, I provide evidence that the reduced ability to mount a protective immune response to pathogens is not due to an inherent immaturity of neonatal immune cells but instead the functions of these immune cells are actively suppressed by CD71⁺ erythroid cells. Furthermore, the role of CD71⁺ erythroid cells in host defense against infection is examined. CD71⁺ erythroid cells are enriched in newborns and have distinctive immunosuppressive properties that leave them vulnerable to infection. Moreover, immature erythroid cells possess exclusive immunomodulatory properties and may play a role in immune ontogeny. In addition to these distinct features, CD71⁺ erythroid cells impact digestive health by preventing excessive inflammation following the sudden transition from a sterile in utero setting to excessive colonization with commensals in the external environment. Ongoing research in identifying the beneficial and/or detrimental effects of immature erythrocytes on immune responses may serve to enhance protective newborn immune responses to infection and enable better vaccination strategies for the young to be designed.

Immune responses in neonates

Neonates have little immunological memory and a developing immune system, which increases their vulnerability to infectious agents. Recent advances in the understanding of neonatal immunity indicate that both innate and adaptive responses are dependent on precursor frequency of lymphocytes, antigenic dose and mode of exposure. Studies in neonatal mouse models and human umbilical cord blood cells demonstrate the capability of neonatal immune cells to produce immune responses similar to adults in some aspects but not others. This review focuses mainly on the developmental and functional mechanisms of the human neonatal immune system. In particular, the mechanism of innate and adaptive immunity and the role of neutrophils, antigen presenting cells, differences in subclasses of T lymphocytes (Th1, Th2, Tregs) and B cells are discussed. In addition, we have included the recent developments in the neonatal mouse immune system. Understanding neonatal immunity is essential to development of therapeutic vaccines to combat newly emerging infectious agents.