Cross-Generational Reproductive Fitness Enforced by Microchimeric Maternal Cells.

Exposure to maternal tissue during in utero development imprints tolerance to immunologically foreign non-inherited maternal antigens (NIMA) that persists into adulthood. The biological advantage of this tolerance, conserved across mammalian species, remains unclear. Here, we show maternal cells that establish microchimerism in female offspring during development promote systemic accumulation of immune suppressive regulatory T cells (Tregs) with NIMA specificity. NIMA-specific Tregs expand during pregnancies sired by males expressing alloantigens with overlapping NIMA specificity, thereby averting fetal wastage triggered by prenatal infection and non-infectious disruptions of fetal tolerance. Therefore, exposure to NIMA selectively enhances reproductive success in second-generation females carrying embryos with overlapping paternally inherited antigens. These findings demonstrate that genetic fitness, canonically thought to be restricted to Mendelian inheritance, is enhanced in female placental mammals through vertically transferred maternal cells that promote conservation of NIMA and enforce cross-generational reproductive benefits.

In situ mapping identifies distinct vascular niches for myelopoiesis.

In contrast to nearly all other tissues, the anatomy of cell differentiation in the bone marrow remains unknown. This is owing to a lack of strategies for examining myelopoiesis-the differentiation of myeloid progenitors into a large variety of innate immune cells-in situ in the bone marrow. Such strategies are required to understand differentiation and lineage-commitment decisions, and to define how spatial organizing cues inform tissue function. Here we develop approaches for imaging myelopoiesis in mice, and generate atlases showing the differentiation of granulocytes, monocytes and dendritic cells. The generation of granulocytes and dendritic cells-monocytes localizes to different blood-vessel structures known as sinusoids, and displays lineage-specific spatial and clonal architectures. Acute systemic infection with Listeria monocytogenes induces lineage-specific progenitor clusters to undergo increased self-renewal of progenitors, but the different lineages remain spatially separated. Monocyte-dendritic cell progenitors (MDPs) map with nonclassical monocytes and conventional dendritic cells; these localize to a subset of blood vessels expressing a major regulator of myelopoiesis, colony-stimulating factor 1 (CSF1, also known as M-CSF)1. Specific deletion of Csf1 in endothelium disrupts the architecture around MDPs and their localization to sinusoids. Subsequently, there are fewer MDPs and their ability to differentiate is reduced, leading to a loss of nonclassical monocytes and dendritic cells during both homeostasis and infection. These data indicate that local cues produced by distinct blood vessels are responsible for the spatial organization of definitive blood cell differentiation.

Offspring's Tolerance of Mother Goes Viral.

Pregnancy uniquely allows genetically discordant tissues of the mother and child to intimately coexist in harmony. In this issue of Immunity, Ou and colleagues show that hepatitis B virus exploits these naturally occurring immune tolerance pathways to establish persistent postnatal infection in offspring.

Persistent Zika Virus Clinical Susceptibility despite Reduced Viral Burden in Mice with Expanded Virus-Specific CD8+ T Cells Primed by Recombinant Listeria monocytogenes.

Vaccines against Zika virus (ZIKV) infection that target CD8+ T cells are of considerable interest because Abs may enhance infection susceptibility. However, whether CD8+ T cells are protective or promote susceptibility to clinical infection symptoms remains uncertain. To more precisely investigate ZIKV-specific CD8+ T cells in isolation, we engineered a Listeria monocytogenes-based vector to express a single MHC class I-restricted immune dominant peptide, E294-302, from ZIKV envelope protein. We show accumulation of activated ZIKV-specific CD8+ T cells primed by recombinant L. monocytogenes is associated with reductions in circulating virus levels after ZIKV challenge in type I IFN receptor-deficient mice and wildtype mice administered neutralizing Abs against type I IFN receptor. Interestingly, susceptibility to ZIKV clinical infection including weight loss and mortality each persists and is neither significantly improved nor worsened compared with isogenic L. monocytogenes-primed control mice. These data demonstrating persistent ZIKV clinical susceptibility despite reduced viral burden in mice with expanded virus-specific CD8+ T cells highlights the need for targeting other adaptive immune components in developing vaccines against ZIKV infection.

Preconceptual Zika virus asymptomatic infection protects against secondary prenatal infection.

Pregnant women, and their fetal offspring, are uniquely susceptible to Zika virus and other microbial pathogens capable of congenital fetal infection. Unavoidable exposure to Zika virus in endemic areas underscores the need for identifying at-risk individuals, and protecting expecting mothers and their fetal offspring against prenatal infection. Here we show that primary Zika virus asymptomatic infection in mice confers protection against re-infection, and that these protective benefits are maintained during pregnancy. Zika virus recovery was sharply reduced in maternal tissues and amongst fetal concepti after prenatal challenge in mothers with resolved subclinical infection prior to pregnancy compared with mice undergoing primary prenatal infection. These benefits coincide with expanded accumulation of viral-specific antibodies in maternal serum and fetal tissues that protect against infection by the identical or heterologous Zika virus genotype strains. Thus, preconceptual infection primes Zika virus-specific antibodies that confer cross-genotype protection against re-infection during pregnancy.

Immunosuppressive CD71+ erythroid cells compromise neonatal host defence against infection.

Newborn infants are highly susceptible to infection. This defect in host defence has generally been ascribed to the immaturity of neonatal immune cells; however, the degree of hyporesponsiveness is highly variable and depends on the stimulation conditions. These discordant responses illustrate the need for a more unified explanation for why immunity is compromised in neonates. Here we show that physiologically enriched CD71(+) erythroid cells in neonatal mice and human cord blood have distinctive immunosuppressive properties. The production of innate immune protective cytokines by adult cells is diminished after transfer to neonatal mice or after co-culture with neonatal splenocytes. Neonatal CD71(+) cells express the enzyme arginase-2, and arginase activity is essential for the immunosuppressive properties of these cells because molecular inhibition of this enzyme or supplementation with L-arginine overrides immunosuppression. In addition, the ablation of CD71(+) cells in neonatal mice, or the decline in number of these cells as postnatal development progresses parallels the loss of suppression, and restored resistance to the perinatal pathogens Listeria monocytogenes and Escherichia coli. However, CD71(+) cell-mediated susceptibility to infection is counterbalanced by CD71(+) cell-mediated protection against aberrant immune cell activation in the intestine, where colonization with commensal microorganisms occurs swiftly after parturition. Conversely, circumventing such colonization by using antimicrobials or gnotobiotic germ-free mice overrides these protective benefits. Thus, CD71(+) cells quench the excessive inflammation induced by abrupt colonization with commensal microorganisms after parturition. This finding challenges the idea that the susceptibility of neonates to infection reflects immune-cell-intrinsic defects and instead highlights processes that are developmentally more essential and inadvertently mitigate innate immune protection. We anticipate that these results will spark renewed investigation into the need for immunosuppression in neonates, as well as improved strategies for augmenting host defence in this vulnerable population.

Commensal microbes drive intestinal inflammation by IL-17-producing CD4+ T cells through ICOSL and OX40L costimulation in the absence of B7-1 and B7-2.

The costimulatory B7-1 (CD80)/B7-2 (CD86) molecules, along with T-cell receptor stimulation, together facilitate T-cell activation. This explains why in vivo B7 costimulation neutralization efficiently silences a variety of human autoimmune disorders. Paradoxically, however, B7 blockade also potently moderates accumulation of immune-suppressive regulatory T cells (Tregs) essential for protection against multiorgan systemic autoimmunity. Here we show that B7 deprivation in mice overrides the necessity for Tregs in averting systemic autoimmunity and inflammation in extraintestinal tissues, whereas peripherally induced Tregs retained in the absence of B7 selectively mitigate intestinal inflammation caused by Th17 effector CD4(+) T cells. The need for additional immune suppression in the intestine reflects commensal microbe-driven T-cell activation through the accessory costimulation molecules ICOSL and OX40L. Eradication of commensal enteric bacteria mitigates intestinal inflammation and IL-17 production triggered by Treg depletion in B7-deficient mice, whereas re-establishing intestinal colonization with Candida albicans primes expansion of Th17 cells with commensal specificity. Thus, neutralizing B7 costimulation uncovers an essential role for Tregs in selectively averting intestinal inflammation by Th17 CD4(+) T cells with commensal microbe specificity.

Cutting edge: committed Th1 CD4+ T cell differentiation blocks pregnancy-induced Foxp3 expression with antigen-specific fetal loss.

Pregnancy stimulates induced Foxp3 expression among maternal CD4(+) T cells with fetal specificity. Although sustained maternal regulatory CD4(+) T cell (Treg) expansion is essential for maintaining fetal tolerance during pregnancy, the necessity for Foxp3(+) cells with fetal specificity remains undefined. In this study, we demonstrate that mitigating Treg differentiation among maternal CD4(+) T cells with a single surrogate fetal specificity elicits Ag-specific fetal loss. Using recombinant Listeria monocytogenes to prime stably differentiated Th1 CD4(+) T cells with fetal I-A(b):2W1S55-68 specificity refractory to pregnancy-induced Foxp3 expression, we show that Ag delivery by cytoplasmic L. monocytogenes causes selective loss of 2W1S(+) offspring through CD4 cell- and IFN-γ-dependent pathways. In contrast, CD4(+) T cells primed by L. monocytogenes restricted from the cell cytoplasm are markedly more plastic for induced Foxp3 expression, with normal pregnancy outcomes. Thus, committed Th1 polarization blocks pregnancy induced Treg differentiation among maternal CD4(+) T cells with fetal specificity and triggers Ag-specific fetal loss.

Regulatory T cells: new keys for further unlocking the enigma of fetal tolerance and pregnancy complications.

The immunological alterations required for successful pregnancy in eutherian placental mammals have remained a scientific enigma since the discovery of MHC haplotype diversity and unique immune signatures among individuals. Within the past 10 years, accumulating data suggest that immune-suppressive regulatory T cells (Tregs) confer essential protective benefits in sustaining tolerance to the semiallogeneic fetus during pregnancy, along with their more established roles in maintaining tolerance to self and "extended self" commensal Ags that averts autoimmunity. Reciprocally, many human pregnancy complications stemming from inadequacies in fetal tolerance have been associated with defects in maternal Tregs. Thus, further elucidating the immunological shifts during pregnancy not only have direct translational implications for improving perinatal health, they have enormous potential for unveiling new clues about how Tregs work in other biological contexts. In this article, epidemiological data in human pregnancy and complementary animal studies implicating a pivotal protective role for maternal Tregs are summarized.

Reproductive outcomes after pregnancy-induced displacement of preexisting microchimeric cells.

Pregnancy confers partner-specific protection against complications in future pregnancy that parallel persistence of fetal microchimeric cells (FMcs) in mothers after parturition. We show that preexisting FMcs become displaced by new FMcs during pregnancy and that FMc tonic stimulation is essential for expansion of protective fetal-specific forkhead box P3 (FOXP3)-positive regulatory T cells (Treg cells). Maternal microchimeric cells and accumulation of Treg cells with noninherited maternal antigen (NIMA) specificity are similarly overturned in daughters after pregnancy, highlighting a fixed microchimeric cell niche. Whereas NIMA-specific tolerance is functionally erased by pregnancy, partner-specific resiliency against pregnancy complications persists in mothers despite paternity changes in intervening pregnancy. Persistent fetal tolerance reflects FOXP3 expression plasticity, which allows mothers to more durably remember their babies, whereas daughters forget their mothers with new pregnancy-imprinted immunological memories.

Maternal-fetal conflict averted by progesterone- induced FOXP3+ regulatory T cells.

Pregnancy stimulates an intricately coordinated assortment of physiological changes to accommodate growth of the developing fetus, while simultaneously averting rejection of genetically foreign fetal cells and tissues. Despite increasing evidence that expansion of immune-suppressive maternal regulatory T cells enforces fetal tolerance and protects against pregnancy complications, the pregnancy-associated signals driving this essential adaptation remain poorly understood. Here we show that the female reproductive hormone, progesterone, coordinates immune tolerance by stimulating expansion of FOXP3+ regulatory T cells. Conditional loss of the canonical nuclear progesterone receptor in maternal FOXP3+ regulatory T cells blunts their proliferation and accumulation, which is associated with fetal wastage and decidual infiltration of activated CD8+ T cells. Reciprocally, the synthetic progestin 17α-hydroxyprogesterone caproate (17-OHPC) administered to pregnant mice reinforces fetal tolerance and protects against fetal wastage. These immune modulatory effects of progesterone that promote fetal tolerance establish a molecular link between immunological and other physiological adaptions during pregnancy.

Epidemiology of Pregnancy Complications Through the Lens of Immunological Memory.

In the fifteen minutes it takes to read this short commentary, more than 400 babies will have been born too early, another 300 expecting mothers will develop preeclampsia, and 75 unborn third trimester fetuses will have died in utero (stillbirth). Given the lack of meaningful progress in understanding the physiological changes that occur to allow a healthy, full term pregnancy, it is perhaps not surprising that effective therapies against these great obstetrical syndromes that include prematurity, preeclampsia, and stillbirth remain elusive. Meanwhile, pregnancy complications remain the leading cause of infant and childhood mortality under age five. Does it have to be this way? What more can we collectively, as a biomedical community, or individually, as clinicians who care for women and newborn babies at high risk for pregnancy complications, do to protect individuals in these extremely vulnerable developmental windows? The problem of pregnancy complications and neonatal mortality is extraordinarily complex, with multiple unique, but complementary perspectives from scientific, epidemiological and public health viewpoints. Herein, we discuss the epidemiology of pregnancy complications, focusing on how the outcome of prior pregnancy impacts the risk of complication in the next pregnancy - and how the fundamental immunological principle of memory may promote this adaptive response.

Preconceptual Priming Overrides Susceptibility to Escherichia coli Systemic Infection during Pregnancy.

Maternal sepsis is a leading cause of morbidity and mortality during pregnancy. Escherichia coli is a primary cause of bacteremia in women and occurs more frequently during pregnancy. Several key outstanding questions remain regarding how to identify women at highest infection risk and how to boost immunity against E. coli infection during pregnancy. Here, we show that pregnancy-induced susceptibility to E. coli systemic infection extends to rodents as a model of human infection. Mice infected during pregnancy contain >100-fold-more recoverable bacteria in target tissues than nonpregnant controls. Infection leads to near complete fetal wastage that parallels placental plus congenital fetal invasion. Susceptibility in maternal tissues positively correlates with the number of concepti, suggesting important contributions by expanded placental-fetal target tissue. Remarkably, these pregnancy-induced susceptibility phenotypes are also efficiently overturned in mice with resolved sublethal infection prior to pregnancy. Preconceptual infection primes the accumulation of E. coli-specific IgG and IgM antibodies, and adoptive transfer of serum containing these antibodies to naive recipient mice protects against fetal wastage. Together, these results suggest that the lack of E. coli immunity may help discriminate individuals at risk during pregnancy, and that overriding susceptibility to E. coli prenatal infection by preconceptual priming is a potential strategy for boosting immunity in this physiological window of vulnerability.IMPORTANCE Pregnancy makes women especially vulnerable to infection. The most common cause of bloodstream infection during pregnancy is by a bacterium called Escherichia coli This bacterium is a very common cause of bloodstream infection, not just during pregnancy but in all individuals, from newborn babies to the elderly, probably because it is always present in our intestine and can intermittently invade through this mucosal barrier. We first show that pregnancy in animals also makes them more susceptible to E. coli bloodstream infection. This is important because many of the dominant factors likely to control differences in human infection susceptibility can be property controlled for only in animals. Despite this vulnerability induced by pregnancy, we also show that animals with resolved E. coli infection are protected against reinfection during pregnancy, including having resistance to most infection-induced pregnancy complications. Protection against reinfection is mediated by antibodies that can be measured in the blood. This information may help to explain why most women do not develop E. coli infection during pregnancy, enabling new approaches for identifying those at especially high risk of infection and strategies for preventing infection during pregnancy.

CD8+ T Cell Functional Exhaustion Overrides Pregnancy-Induced Fetal Antigen Alloimmunization.

Pregnancy necessitates physiological exposure, and often re-exposure, to foreign fetal alloantigens. The consequences after pregnancy are highly varied, with evidence of both alloimmunization and expanded tolerance phenotypes. We show that pregnancy primes the accumulation of fetal-specific maternal CD8+ T cells and their persistence as an activated memory pool after parturition. Cytolysis and the potential for robust secondary expansion occurs with antigen re-encounter in non-reproductive contexts. Comparatively, CD8+ T cell functional exhaustion associated with increased PD-1 and LAG-3 expression occurs with fetal antigen re-stimulation during subsequent pregnancy. PD-L1/LAG-3 neutralization unleashes the activation of fetal-specific CD8+ T cells, causing fetal wastage selectively during secondary but not primary pregnancy. Thus, CD8+ T cells with fetal alloantigen specificity persist in mothers after pregnancy, and protection against fetal wastage in subsequent pregnancies is maintained by their unique susceptibility to functional exhaustion. Together, distinct mechanisms whereby fetal tolerance is maintained during primary compared with subsequent pregnancies are demonstrated.

Commensal Candida albicans Positively Calibrates Systemic Th17 Immunological Responses.

Mucosal barriers are densely colonized by pathobiont microbes such as Candida albicans, capable of invasive disseminated infection. However, systemic infections occur infrequently in healthy individuals, suggesting that pathobiont commensalism may elicit host benefits. We show that intestinal colonization with C. albicans drives systemic expansion of fungal-specific Th17 CD4+ T cells and IL-17 responsiveness by circulating neutrophils, which synergistically protect against C. albicans invasive infection. Protection conferred by commensal C. albicans requires persistent fungal colonization and extends to other extracellular invasive pathogens such as Staphylococcus aureus. However, commensal C. albicans does not protect against intracellular influenza virus infection and exacerbates allergic airway inflammation susceptibility, indicating that positively calibrating systemic Th17 responses is not uniformly beneficial. Thus, systemic Th17 inflammation driven by CD4+ T cells responsive to tonic stimulation by commensal C. albicans improves host defense against extracellular pathogens, but with potentially harmful immunological consequences.

Enhanced survival following oral and systemic Salmonella enterica serovar Typhimurium infection in polymeric immunoglobulin receptor knockout mice.

BACKGROUND: Polymeric immunoglobulin receptor (pIgR) transport of secretory immunoglobulin A (SIgA) to mucosal surfaces is thought to promote gut integrity and immunity to Salmonella enterica serovar Typhimurium (S. Typhimurium), an invasive pathogen in mice. To elucidate potential mechanisms, we assessed intestinal barrier function and both oral and systemic S. Typhimurium virulence in pIgR knockout (KO) and wildtype (WT) mice. METHODS: In uninfected animals, we harvested jejunal segments for Ussing chamber analyses of transepithelial resistance (TER); mesenteric lymph nodes (mLN) for bacterial culture; and serum and stool for IgA. Separately, we infected mice either orally or intravenously (IV) with S. Typhimurium to compare colonization, tissue dynamics, and inflammation between KOs and WTs. RESULTS: Uninfected KOs displayed decreased TER and dramatically increased serum IgA and decreased fecal IgA vs. WT; however, KO mLNs yielded fewer bacterial counts. Remarkably, WTs challenged orally with S. Typhimurium exhibited increased splenomegaly, tissue colonization, and pro-inflammatory cytokines vs. pIgR KOs, which showed increased survival following either oral or IV infection. CONCLUSIONS: Absence of pIgR compromises gut integrity but does not exacerbate bacterial translocation nor S. Typhimurium infection. These findings raise the possibility that immune adaptation to increased gut permeability and elevated serum IgA in the setting of SIgA deficiency provides compensatory protection against invasive gut pathogens.

Immunological implications of pregnancy-induced microchimerism.

Immunological identity is traditionally defined by genetically encoded antigens, with equal maternal and paternal contributions as a result of Mendelian inheritance. However, vertically transferred maternal cells also persist in individuals at very low levels throughout postnatal development. Reciprocally, mothers are seeded during pregnancy with genetically foreign fetal cells that persist long after parturition. Recent findings suggest that these microchimeric cells expressing non-inherited, familially relevant antigenic traits are not accidental 'souvenirs' of pregnancy, but are purposefully retained within mothers and their offspring to promote genetic fitness by improving the outcome of future pregnancies. In this Review, we discuss the immunological implications, benefits and potential consequences of individuals being constitutively chimeric with a biologically active 'microchiome' of genetically foreign cells.

Commensal Fungi Recapitulate the Protective Benefits of Intestinal Bacteria.

Commensal intestinal microbes are collectively beneficial in preventing local tissue injury and augmenting systemic antimicrobial immunity. However, given the near-exclusive focus on bacterial species in establishing these protective benefits, the contributions of other types of commensal microbes remain poorly defined. Here, we show that commensal fungi can functionally replace intestinal bacteria by conferring protection against injury to mucosal tissues and positively calibrating the responsiveness of circulating immune cells. Susceptibility to colitis and influenza A virus infection occurring upon commensal bacteria eradication is efficiently overturned by mono-colonization with either Candida albicans or Saccharomyces cerevisiae. The protective benefits of commensal fungi are mediated by mannans, a highly conserved component of fungal cell walls, since intestinal stimulation with this moiety alone overrides disease susceptibility in mice depleted of commensal bacteria. Thus, commensal enteric fungi safeguard local and systemic immunity by providing tonic microbial stimulation that can functionally replace intestinal bacteria.

l-Citrulline Metabolism in Mice Augments CD4+ T Cell Proliferation and Cytokine Production In Vitro, and Accumulation in the Mycobacteria-Infected Lung.

Activation, recruitment, and effector function of T lymphocytes are essential for control of mycobacterial infection. These processes are tightly regulated in T cells by the availability of l-arginine within the microenvironment. In turn, mycobacterial infection dampens T cell responsiveness through arginase induction in myeloid cells, promoting sequestration of l-arginine within the local milieu. Here, we show T cells can replenish intracellular l-arginine through metabolism of l-citrulline to mediate inflammatory function, allowing anti-mycobacterial T cells to overcome arginase-mediated suppression. Furthermore, T cell l-citrulline metabolism is necessary for accumulation of CD4+ T cells at the site of infection, suggesting this metabolic pathway is involved during anti-mycobacterial T cell immunity in vivo. Together, these findings establish a contribution for l-arginine synthesis by T cells during mycobacterial infection, and implicate l-citrulline as a potential immuno-nutrient to modulate host immunity.

Programmed Death-1 Culls Peripheral Accumulation of High-Affinity Autoreactive CD4 T Cells to Protect against Autoimmunity.

Self-reactive CD4 T cells are incompletely deleted during thymic development, and their peripheral seeding highlights the need for additional safeguards to avert autoimmunity. Here, we show an essential role for the coinhibitory molecule programmed death-1 (PD-1) in silencing the activation of high-affinity autoreactive CD4 T cells. Each wave of self-reactive CD4 T cells that escapes thymic deletion autonomously upregulates PD-1 to maintain self-tolerance. By tracking the progeny derived from individual autoreactive CD4 T cell clones, we demonstrate that self-reactive cells with the greatest autoimmune threat and highest self-antigen affinity express the most PD-1. Reciprocally, PD-1 deprivation unleashes high-affinity self-reactive CD4 T cells in target tissues to exacerbate neuronal inflammation and autoimmune diabetes. Reliance on PD-1 to actively maintain self-tolerance may explain why exploiting this pathway by cancerous cells and invasive microbes efficiently subverts protective immunity, and why autoimmune side effects can develop after PD-1-neutralizing checkpoint therapies.