Murine neonatal CD4+ lymph node cells are highly deficient in the development of antigen-specific Th1 function in adoptive adult hosts

Intrathymic Regulation of Dendritic Cell Subsets and Their Contributions to Central Tolerance

Thymic dendritic cells (DCs) are critical mediators of central tolerance, cooperating with medullary thymic epithelial cells (mTECs) and B cells to establish T-cell self-tolerance to the proteome. The DC compartment is highly heterogeneous and is comprised of three major subsets, plasmacytoid dendritic cells (pDCs) and two conventional dendritic cell (cDC) subsets, cDC1 and cDC2. Thymic cDC1 and cDC2 arise from distinct progenitors and access the thymus at different stages of their differentiation, but both become activated by cellular and secreted cues received within the sterile thymus environment. Activated cDC1s and cDC2s have been implicated in presenting distinct types of self-antigens to induce central tolerance. Thus, understanding how the distinct cDC subsets are regulated within the thymus environment will provide important insights into mechanisms governing self-tolerance. Furthermore, the thymic DC compartment undergoes age-associated compositional and transcriptional changes that likely impact the efficiency and quality of central tolerance established over the lifespan. Here, we review recent findings from our lab and others on mechanisms regulating thymic DC activation, the distinct roles of thymic DC subsets in central tolerance, and age-associated changes in thymic DCs that could impact T-cell selection.

The Altered Neonatal CD8+ T Cell Immunodominance Hierarchy during Influenza Virus Infection Impacts Peptide Vaccination

Neonates are more susceptible to influenza virus infection than adults, resulting in increased morbidity and mortality and delayed clearance of the virus. Generating effective CD8+ T cell responses may be important for improving vaccination outcomes in vulnerable populations, but neonatal T cells frequently respond differently than adult cells. We sought to understand CD8+ T cell specificity and immunodominance during neonatal influenza infection and how any differences from the adult hierarchy might impact peptide vaccine effectiveness. Neonatal C57BL/6 mice displayed an altered CD8+ T cell immunodominance hierarchy during influenza infection, preferentially responding to an epitope in the influenza protein PA rather than the co-dominant adult response to NP and PA. Heterosubtypic infections in mice first infected as pups also displayed altered immunodominance and reduced protection compared to mice first infected as adults. Adoptive transfer of influenza-infected bone-marrow-derived dendritic cells promoted an NP-specific CD8+ T cell response in influenza-virus-infected pups and increased viral clearance. Finally, pups responded to PA (224-233), but not NP (366-374) during peptide vaccination. PA (224-233)-vaccinated mice were not protected during viral challenge. Epitope usage should be considered when designing vaccines that target T cells when the intended patient population includes infants and adults.

Age-Related Changes in Thymic Central Tolerance

Thymic epithelial cells (TECs) and hematopoietic antigen presenting cells (HAPCs) in the thymus microenvironment provide essential signals to self-reactive thymocytes that induce either negative selection or generation of regulatory T cells (Treg), both of which are required to establish and maintain central tolerance throughout life. HAPCs and TECs are comprised of multiple subsets that play distinct and overlapping roles in central tolerance. Changes that occur in the composition and function of TEC and HAPC subsets across the lifespan have potential consequences for central tolerance. In keeping with this possibility, there are age-associated changes in the cellular composition and function of T cells and Treg. This review summarizes changes in T cell and Treg function during the perinatal to adult transition and in the course of normal aging, and relates these changes to age-associated alterations in thymic HAPC and TEC subsets.

Alterations in the Thymic Selection Threshold Skew the Self-Reactivity of the TCR Repertoire in Neonates

Neonatal and adult T cells differ in their effector functions. Although it is known that cell-intrinsic differences in mature T cells contribute to this phenomenon, the factors involved remain unclear. Given emerging evidence that the binding strength of a TCR for self-peptide presented by MHC (self-pMHC) impacts T cell function, we sought to determine whether altered thymic selection influences the self-reactivity of the TCR repertoire during ontogeny. We found that conventional and regulatory T cell subsets in the thymus of neonates and young mice expressed higher levels of cell surface CD5, a surrogate marker for TCR avidity for self-pMHC, as compared with their adult counterparts, and this difference in self-reactivity was independent of the germline bias of the neonatal TCR repertoire. The increased binding strength of the TCR repertoire for self-pMHC in neonates was not solely due to reported defects in clonal deletion. Rather, our data suggest that thymic selection is altered in young mice such that thymocytes bearing TCRs with low affinity for self-peptide are not efficiently selected into the neonatal repertoire, and stronger TCR signals accompany both conventional and regulatory T cell selection. Importantly, the distinct levels of T cell self-reactivity reflect physiologically relevant differences based on the preferential expansion of T cells from young mice to fill a lymphopenic environment. Therefore, differences in thymic selection in young versus adult mice skew the TCR repertoire, and the relatively higher self-reactivity of the T cell pool may contribute to the distinct immune responses observed in neonates.

Prostaglandin E2 production during neonatal respiratory infection with mouse adenovirus type 1

Neonatal mice are more susceptible than adults to mouse adenovirus type 1 (MAV1) respiratory infection. In adult mice, MAV-1 respiratory infection induces production of prostaglandin E2 (PGE2), a lipid mediator that exerts suppressive effects on a variety of host immune functions. We tested the hypothesis that exaggerated PGE2 production in neonatal mice contributes to increased susceptibility to MAV-1. PGE2 concentrations were lower in lungs of uninfected neonatal mice than in adults. PGE2 production was induced by both MAV-1 and a nonspecific stimulus to a greater degree in neonatal mice than in adults, but only in adults was PGE2 induced in a virus-specific manner. Lung viral loads were equivalent in PGE2-deficient neonatal mice and wild type controls, as was virus-induced expression of IFN-γ, IL-17A, and CCL5 in the lungs. PGE2 deficiency had minimal effect on production of virus-specific IgG or establishment of protective immunity in neonatal mice. Collectively, our data indicate that lung PGE2 production is exaggerated early in life, but this effect does not mediate increased susceptibility to MAV-1 infection.

The Neonatal CD4+ T Cell Response to a Single Epitope Varies in Genetically Identical Mice

Neonatal infection is a major cause of morbidity and mortality worldwide. Increased susceptibility to infection in the neonate is attributed in part to defects in T cell-mediated immunity. A peptide:MHC class II tetramer-based cell enrichment method was used to test this hypothesis at the level of a single epitope. We found that naive T cells with TCRs specific for the 2W:I-A(b) epitope were present in the thymuses of 1-d-old CD57BL/6 mice but were barely detectable in the spleen, likely because each mouse contained very few total splenic CD4(+) T cells. By day 7 of life, however, the total number of splenic CD4(+) T cells increased dramatically and the frequency of 2W:I-A(b)-specific naive T cells reached that of adult mice. Injection of 2W peptide in CFA into 1-d-old mice generated a 2W:I-A(b)-specific effector cell population that peaked later than in adult mice and showed more animal-to-animal variation. Similarly, 2W:I-A(b)-specific naive T cells in different neonatal mice varied significantly in generation of Th1, Th2, and follicular Th cells compared with adult mice. These results suggest that delayed effector cell expansion and stochastic variability in effector cell generation due to an initially small naive repertoire contribute to defective peptide:MHC class II-specific immunity in neonates.

Enhanced survival during experimental Listeria monocytogenes sepsis in neonatal mice prophylactically treated with Th1 and macrophage immunoregulatory cytokines and mediators

BACKGROUND

Impairments in T-cell and macrophage-mediated host defense lead to increased infection-related morbidity and mortality in neonates, partly because of immaturity in T helper (Th)1 function. Listeria monocytogenes (Lm) is an intracellular pathogen disproportionately causing severe disease among neonates and the immunocompromised. Intact macrophage and Th1-mediated immune responses are critical for Lm clearance. We and others have previously demonstrated downregulation of Th1 and macrophage immunoregulatory cytokines in cord blood versus adult peripheral blood. We sought to determine whether therapeutic or prophylactic single agent or combination recombinant murine interleukin (rmIL)-12, rmIL-18, recombinant murine macrophage-colony stimulating factor (rmM-CSF), recombinant murine granulocyte macrophage-colony stimulating factor (rmGM-CSF) and recombinant murine interferon (rmIFN)-γ would enhance survival during experimental neonatal Lm sepsis.

METHODS

A 90% lethal dose (LD90) of Lm was established in C57/BL/6 neonatal mice. rmIL-12, rmIL-18, rmM-CSF, rmGM-CSF and rmIFN-γ were administered singly or sequentially, before or after LD90 Lm inoculation; ampicillin was administered 24 hours after inoculation.

RESULTS

Therapeutic doses of rmIL-12, rmIL-18, rmM-CSF, rmGM-CSF and rmIFN-γ as single agents and sequential therapy with rmM-CSF + (rmIL-12 and/or rmIL-18) + rmIFN-γ in addition to ampicillin were not associated with increased survival. However, prophylactic single doses of rmIL-12, rmIL-18, rmM-CSF and rmIFN-γ and prophylactic sequential doses of rmM-CSF + (rmIL-12 and/or rmIL-18) + rmIFN-γ in addition to ampicillin were associated with significantly enhanced survival compared with ampicillin alone.

CONCLUSIONS

These data suggest prophylactic administration of macrophage and Th1 immunoregulatory cytokines can potentially overcome deficits in neonatal immunity to protect against Lm.

The equine immune responses to infectious and allergic disease: a model for humans?

The modern horse, Equus caballus has historically made important contributions to the field of immunology, dating back to Emil von Behring's description of curative antibodies in equine serum over a century ago. While the horse continues to play an important role in human serotherapy, the mouse has replaced the horse as the predominant experimental animal in immunology research. Nevertheless, continuing efforts have led to an improved understanding of the equine immune response in a variety of infectious and non-infectious diseases. Based on this information, we can begin to identify specific situations where the horse may provide a unique immunological model for certain human diseases.

Proinflammatory effects of interferon gamma in mouse adenovirus 1 myocarditis

Adenoviruses are frequent causes of pediatric myocarditis. Little is known about the pathogenesis of adenovirus myocarditis, and the species specificity of human adenoviruses has limited the development of animal models, which is a significant barrier to strategies for prevention or treatment. We have developed a mouse model of myocarditis following mouse adenovirus 1 (MAV-1) infection to study the pathogenic mechanisms of this important cause of pediatric myocarditis. Following intranasal infection of neonatal C57BL/6 mice, we detected viral replication and induction of interferon gamma (IFN-γ) in the hearts of infected mice. MAV-1 caused myocyte necrosis and induced substantial cellular inflammation that was composed predominantly of CD3(+) T lymphocytes. Depletion of IFN-γ during acute infection reduced cardiac inflammation in MAV-1-infected mice without affecting viral replication. We observed decreased contractility during acute infection of neonatal mice, and persistent viral infection in the heart was associated with cardiac remodeling and hypertrophy in adulthood. IFN-γ is a proinflammatory mediator during adenovirus-induced myocarditis, and persistent adenovirus infection may contribute to ongoing cardiac dysfunction.

IMPORTANCE

Studying the pathogenesis of myocarditis caused by different viruses is essential in order to characterize both virus-specific and generalized factors that contribute to disease. Very little is known about the pathogenesis of adenovirus myocarditis, which is a significant impediment to the development of treatment or prevention strategies. We used MAV-1 to establish a mouse model of human adenovirus myocarditis, providing the means to study host and pathogen factors contributing to adenovirus-induced cardiac disease during acute and persistent infection. The MAV-1 model will enable fundamental studies of viral myocarditis, including IFN-γ modulation as a therapeutic strategy.

An alphavirus-based adjuvant enhances serum and mucosal antibodies, T cells, and protective immunity to influenza virus in neonatal mice

Neonatal immune responses to infection and vaccination are biased toward TH2 at the cost of proinflammatory TH1 responses needed to combat intracellular pathogens. However, upon appropriate stimulation, the neonatal immune system can induce adult-like TH1 responses. Here we report that a new class of vaccine adjuvant is especially well suited to enhance early life immunity. The GVI3000 adjuvant is a safe, nonpropagating, truncated derivative of Venezuelan equine encephalitis virus that targets dendritic cells (DCs) in the draining lymph node (DLN) and produces intracellular viral RNA without propagating to other cells. RNA synthesis strongly activates the innate immune response so that in adult animals, codelivery of soluble protein antigens induces robust humoral, cellular, and mucosal responses. The adjuvant properties of GVI3000 were tested in a neonatal BALB/c mouse model using inactivated influenza virus (iFlu). After a single immunization, mice immunized with iFlu with the GVI3000 adjuvant (GVI3000-adjuvanted iFlu) had significantly higher and sustained influenza virus-specific IgG antibodies, mainly IgG2a (TH1), compared to the mice immunized with antigen only. GVI3000 significantly increased antigen-specific CD4(+) and CD8(+) T cells, primed mucosal immune responses, and enhanced protection from lethal challenge. As seen in adult mice, the GVI3000 adjuvant increased the DC population in the DLNs, caused activation and maturation of DCs, and induced proinflammatory cytokines and chemokines in the DLNs soon after immunization, including gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), granulocyte colony-stimulating factor (G-CSF), and interleukin 6 (IL-6). In summary, the GVI3000 adjuvant induced an adult-like adjuvant effect with an influenza vaccine and has the potential to improve the immunogenicity and protective efficacy of new and existing neonatal vaccines.

IMPORTANCE

The suboptimal immune responses in early life constitute a significant challenge for vaccine design. Here we report that a new class of adjuvant is safe and effective for early life immunization and demonstrate its ability to significantly improve the protective efficacy of an inactivated influenza virus vaccine in a neonatal mouse model. The GVI3000 adjuvant delivers a truncated, self-replicating viral RNA into dendritic cells in the draining lymph node. Intracellular RNA replication activates a strong innate immune response that significantly enhances adaptive antibody and cellular immune responses to codelivered antigens. A significant increase in protection results from a single immunization. Importantly, this adjuvant also primed a mucosal IgA response, which is likely to be critical for protection during many early life infections.

A tetravalent alphavirus-vector based dengue vaccine provides effective immunity in an early life mouse model

Dengue viruses (DENV1-4) cause 390 million clinical infections every year, several hundred thousand of which progress to severe hemorrhagic and shock syndromes. Preexisting immunity resulting from a previous DENV infection is the major risk factor for severe dengue during secondary heterologous infections. During primary infections in infants, maternal antibodies pose an analogous risk. At the same time, maternal antibodies are likely to prevent induction of endogenous anti-DENV antibodies in response to current live, attenuated virus (LAV) vaccine candidates. Any effective early life dengue vaccine has to overcome maternal antibody interference (leading to ineffective vaccination) and poor induction of antibody responses (increasing the risk of severe dengue disease upon primary infection). In a previous study, we demonstrated that a non-propagating Venezuelan equine encephalitis virus replicon expression vector (VRP), expressing the ectodomain of DENV E protein (E85), overcomes maternal interference in a BALB/c mouse model. We report here that a single immunization with a tetravalent VRP vaccine induced NAb and T-cell responses to each serotype at a level equivalent to the monovalent vaccine components, suggesting that this vaccine modality can overcome serotype interference. Furthermore, neonatal immunization was durable and could be boosted later in life to further increase NAb and T-cell responses. Although the neonatal immune response was lower in magnitude than responses in adult BALB/c mice, we demonstrate that VRP vaccines generated protective immunity from a lethal challenge after a single neonatal immunization. In summary, VRP vaccines expressing DENV antigens were immunogenic and protective in neonates, and hence are promising candidates for safe and effective vaccination in early life.

Role of innate immunity in neonatal infection

Newborns are at increased risk of infection due to genetic, epigenetic, and environmental factors. Herein we examine the roles of the neonatal innate immune system in host defense against bacterial and viral infections. Full-term newborns express a distinct innate immune system biased toward T(H)2-/T(H)17-polarizing and anti-inflammatory cytokine production with relative impairment in T(H)1-polarizing cytokine production that leaves them particularly vulnerable to infection with intracellular pathogens. In addition to these distinct features, preterm newborns also have fragile skin, impaired T(H)17-polarizing cytokine production, and deficient expression of complement and of antimicrobial proteins and peptides (APPs) that likely contribute to susceptibility to pyogenic bacteria. Ongoing research is identifying APPs, including bacterial/permeability-increasing protein and lactoferrin, as well as pattern recognition receptor agonists that may serve to enhance protective newborn and infant immune responses as stand-alone immune response modifiers or vaccine adjuvants.

Susceptibility to acute mouse adenovirus type 1 respiratory infection and establishment of protective immunity in neonatal mice

There is an incomplete understanding of the differences between neonatal immune responses that contribute to the increased susceptibility of neonates to some viral infections. We tested the hypothesis that neonates are more susceptible than adults to mouse adenovirus type 1 (MAV-1) respiratory infection and are impaired in the ability to generate a protective immune response against a second infection. Following intranasal infection, lung viral loads were greater in neonates than in adults during the acute phase but the virus was cleared from the lungs of neonates as efficiently as it was from adult lungs. Lung gamma interferon (IFN-γ) responses were blunted and delayed in neonates, and lung viral loads were higher in adult IFN-γ(-/-) mice than in IFN-γ(+/+) controls. However, administration of recombinant IFN-γ to neonates had no effect on lung viral loads. Recruitment of inflammatory cells to the airways was impaired in neonates. CD4 and CD8 T cell responses were similar in the lungs of neonates and adults, although a transient increase in regulatory T cells occurred only in the lungs of infected neonates. Infection of neonates led to protection against reinfection later in life that was associated with increased effector memory CD8 T cells in the lungs. We conclude that neonates are more susceptible than adults to acute MAV-1 respiratory infection but are capable of generating protective immune responses.

Neonatal lung immune responses show a shift of cytokines and transcription factors toward Th2 and a deficit in conventional and plasmacytoid dendritic cells

The high incidence of lung-damaging life-threatening respiratory infections in infants may be related to the immaturity of their immune systems. To determine whether lung immune features differ in early life compared with those in adulthood, whole lung as well as lung T lymphocyte and DC responses were investigated in BALB/c neonates versus adults. Higher expression of GATA-3 and rapid and sustained production of type 2 cytokines by lung explants after in vitro exposure to anti-CD3 was the hallmark of the neonatal period, suggestive of a Th2 bias. Neonatal lung GATA-3-producing cells were identified as CD3(+), CD4 and CD8 double-negative T lymphocytes, a subset found at a higher frequency in neonatal than adult lung. The neonatal lungs contained fewer conventional DCs, with a lower ratio of CD103(+) to CD11b(+) DCs, and a much lower number of plasmacytoid DCs in comparison with adult lungs. Yet, when stimulated in vivo by BCG, neonatal lung DCs matured and primed adult naïve CD4(+) T cells toward Th1 as efficiently as adult BCG-primed lung DCs. Conversely, both adult and neonatal BCG-primed lung DCs induced a Th2 cytokine response from neonatal naïve lymph node T cells, suggestive of an intrinsic feature of neonatal T lymphocytes.

A single immunization near birth elicits immediate and lifelong protective immunity

Most existing vaccines do not induce protective immunity immediately following birth, nor do they retain protective efficacy in the latter years of life without booster doses. Using a mouse model, we present evidence that a live-replicating vaccine administered only once shortly after birth was able to induce both immediate and lifelong protection. Newborn mice immunized with a safe, highly attenuated strain of Listeria monocytogenes (Lm) were already protected by day 7 post-vaccination when challenged with a virulent strain of Lm. Furthermore, all mice remained fully protected for 2 years after only a single immunization. Vaccine-specific T cell immune responses were still detectable 2 years later, indicating long-lived immune memory even in neonatal vaccine recipients. Analysis of memory precursor subsets, specific for antigens homologous to Lm or a model vaccine (Ova), demonstrated remarkable similarity between adult and neonatal vaccine recipient effector and central memory CD8 T cell development. The magnitude of expansion of antigen specific memory T cells post-infectious challenge correlated with protection in both groups. This is the first direct evidence that vaccination--even in the absence of a booster dose--is capable of inducing immediate and lifelong protective immune memory regardless of age at the time of initial vaccination.

The migration of T cells in response to influenza virus is altered in neonatal mice

Influenza virus is a significant cause of mortality and morbidity in children; however, little is known about the T cell response in infant lungs. Neonatal mice are highly vulnerable to influenza and only control very low doses of virus. We compared the T cell response to influenza virus infection between mice infected as adults or at 2 d old and observed defective migration into the lungs of the neonatal mice. In the adult mice, the numbers of T cells in the lung interstitia peaked at 10 d postinfection, whereas neonatal T cell infiltration, activation, and expression of TNF-alpha was delayed until 2 wk postinfection. Although T cell numbers ultimately reached adult levels in the interstitia, they were not detected in the alveoli of neonatal lungs. Instead, the alveoli contained eosinophils and neutrophils. This altered infiltrate was consistent with reduced or delayed expression of type 1 cytokines in the neonatal lung and differential chemokine expression. In influenza-infected neonates, CXCL2, CCL5, and CCL3 were expressed at adult levels, whereas the chemokines CXCL1, CXCL9, and CCL2 remained at baseline levels, and CCL11 was highly elevated. Intranasal administration of CCL2, IFN-gamma, or CXCL9 was unable to draw the neonatal T cells into the airways. Together, these data suggest that the T cell response to influenza virus is qualitatively different in neonatal mice and may contribute to an increased morbidity.

Teaching tolerance: using the neonatal immune system to prevent allergic asthma

Evaluation of: Verhasselt V, Milcent V, Cazareth J et al. Breast milk-mediated transfer of an antigen induces tolerance and protection from allergic asthma. Nat. Med. 14(2), 170-175 (2008). Allergic asthma affects up to 20% of the population in developed countries, with increasing incidence. Lessons from epidemiologic studies suggest that increased emphasis on hygiene, coupled with decreasing frequency of childhood infections, may explain this rise in asthma. The present study demonstrates that maternal transfer of allergen plus immunoregulatory factors via breast milk can influence the developing immune response in the offspring, inducing the tolerant rather than the allergic phenotype. Taken with studies of pre- and postnatal early antigen exposure, these data suggest that learning to regulate responses to aeroallergens depends upon the maternal immune status, the composition of breast milk and the offspring's exposure to environmental stimuli. Extension of these findings may, ultimately, provide strategies for the prevention of asthma in children.

Yersinia enterocolitica promotes robust mucosal inflammatory T-cell immunity in murine neonates

Mucosal immunity to gastrointestinal pathogens in early life has been studied only slightly. Recently, we developed an infection model in murine neonates using the gastroenteric pathogen Yersinia enterocolitica. Here, we report that oral infection of neonatal mice with low doses of virulent Y. enterocolitica leads to vigorous intestinal and systemic adaptive immunity. Y. enterocolitica infection promoted the development of anti-LcrV memory serum IgG1 and IgG2a responses of comparable affinity and magnitude to adult responses. Strikingly, neonatal mesenteric lymph node CD4(+) T cells produced Yersinia-specific gamma interferon (IFN-gamma) and interleukin-17A (IL-17A), exceeding adult levels. The robust T- and B-cell responses elicited in neonates exposed to Y. enterocolitica were associated with long-term protection against mucosal challenge with this pathogen. Using genetically deficient mice, we found that IFN-gamma and CD4(+) cells, but not B cells, are critical for protection of neonates during primary Y. enterocolitica infection. In contrast, adults infected with low bacterial doses did not require either cell population for protection. CD4-deficient neonatal mice adoptively transferred with CD4(+) cells from wild-type, IFN-gamma-deficient, or IL-17AF-deficient mice were equally protected from infection. These data demonstrate that inflammatory CD4(+) T cells are required for protection of neonatal mice and that this protection may not require CD4-derived IFN-gamma, IL-17A, or IL-17F. Overall, these studies support the idea that Y. enterocolitica promotes the development of highly inflammatory mucosal responses in neonates and that intestinal T-cell function may be a key immune component in protection from gastrointestinal pathogens in early life.

Blue moon neurovirology: the merits of studying rare CNS diseases of viral origin

While measles virus (MV) continues to have a significant impact on human health, causing 150,000-200,000 deaths worldwide each year, the number of fatalities that can be attributed to MV-triggered central nervous system (CNS) diseases are on the order of a few hundred individuals annually (World Health Organization 2009). Despite this modest impact, substantial effort has been expended to understand the basis of measles-triggered neuropathogenesis. What can be gained by studying such a rare condition? Simply stated, the wealth of studies in this field have revealed core principles that are relevant to multiple neurotropic pathogens, and that inform the broader field of viral pathogenesis. In recent years, the emergence of powerful in vitro systems, novel animal models, and reverse genetics has enabled insights into the basis of MV persistence, the complexity of MV interactions with neurons and the immune system, and the role of immune and CNS development in virus-triggered disease. In this review, we highlight some key advances, link relevant measles-based studies to the broader disciplines of neurovirology and viral pathogenesis, and propose future areas of study for the field of measles-mediated neurological disease.

Maternal endotoxin exposure attenuates allergic airway disease in infant rats

Prenatal exposures to immunogenic stimuli, such as bacterial LPS, have shown to influence the neonatal immune system and lung function. However, no detailed analysis of the immunomodulatory effects of LPS on postnatal T helper cell differentiation has been performed. Using a rat model, we investigated the effect of prenatal LPS exposure on postnatal T cell differentiation and experimental allergic airway disease. Pregnant rats were injected with LPS on day 20 and 21 (term = 22 days). Some of the offspring were sensitized and challenged with ovalbumin. Positive control animals were placebo exposed to saline instead of LPS, whereas negative controls were sensitized with saline. Expression of T cell-related transcription factors and cytokines was quantified in the lung, and airway hyperresponsiveness was measured. Prenatal LPS exposure induced a T helper 1 (T(H)1) immune milieu in the offspring of rats [i.e., increased T-bet and T(H)1 cytokine expression while expression of T(H)2-associated transcription factors (GATA3 and STAT6) and cytokines was decreased]. Prenatal LPS exposure did not trigger T(H)17 cell differentiation in the offspring. Furthermore, prenatal LPS exposure reduced ovalbumin-induced (T(H)2-mediated) airway inflammation, eosinophilia, and airway responsiveness. Thus, in utero exposure to endotoxin promotes a T(H)1 immune environment, which suppresses the development of allergic airway disease later in life.

Trichloroethylene alters central and peripheral immune function in autoimmune-prone MRL(+/+) mice following continuous developmental and early life exposure

Trichloroethylene (TCE) is a widespread environmental toxicant known to promote CD4(+) T-lymphocyte activation, IFNgamma production, and autoimmunity in adult MRL(+/+) mice. Because developing tissues may be more sensitive to toxicant exposure, it was hypothesized that continuous TCE exposure beginning at conception might induce even more pronounced CD4(+) T-lymphocyte effects and exacerbate the development of autoimmunity in MRL(+/+) mice. In the current study, MRL(+/+) mice were exposed to occupationally-relevant doses of TCE from conception until adulthood (i.e., 7-8 wk-of-age). The CD4(+) T-lymphocyte effects in the thymus and periphery were evaluated, as well as serum antibody levels. TCE exposure altered the number of thymocyte subsets, and reduced the capacity of the most immature CD4-/CD8- thymocytes to undergo apoptosis in vitro. In the periphery, T-lymphocyte IFN(gamma) production was monitored in the blood prior to sacrifice by intracellular cytokine staining and flow cytometry. TCE induced a dose-dependent increase in T-lymphocyte IFN(gamma) as early as 4-5-week-of-age. However, these effects were transient, and not observed in splenic T-lymphocytes in 7-8-week-old mice. In contrast, the serum levels of anti-histone autoantibodies and total IgG(2a) were significantly elevated in the TCE-exposed offspring. The data illustrated that occupationally-relevant doses of TCE administered throughout development until adulthood affected central and peripheral immune function in association with early signs of autoimmunity. Future studies will address the possibility that early-life exposure to TCE may alter some aspect of self tolerance in the thymus, leading to autoimmune disease later in life.

Induction of protective immunity to Listeria monocytogenes in neonates

Neonates suffer unduly from infections and also respond suboptimally to most commonly used vaccines. However, a CD8 T cell response can be elicited in neonates if the Ag is introduced into the cytoplasm of APCs. Listeria monocytogenes (Lm) targets the cytoplasm of APC and is a strong CD8 and CD4 Th1-promoting vaccine vehicle in adult mice. We hypothesized that an attenuated strain of Lm would be safe and induce long-lasting protective immunity, even in neonates. We found that neonatal mice immunized only once with the attenuated strain DeltaactA-Lm developed robust primary and secondary CD8 and CD4 Th1 responses and were fully protected from lethal challenge with virulent wild-type Lm without the need for a booster immunization. Furthermore, DeltaactA-Lm expressing a heterologous recombinant Ag induced a strong CD8 and Th1 memory response to that Ag. Based on these data, we propose that DeltaactA-Lm or derivatives thereof might serve as a vaccine vehicle for neonatal immunization.

T-cell responses to allergens

The allergic response in human beings is engineered by CD4(+) T lymphocytes, which secrete T(H)2 cytokines in response to activation by allergen-derived peptides. Although T(H)2 cells have been well characterized, defining the properties of allergen-specific T cells has proved challenging in human beings because of their low frequency within the T-cell repertoire. However, recent studies have provided insight into the molecular signature of long-lived human memory T(H)2 cells, which are allergen-specific. T-cell responses directed against allergens develop in early life and are heavily influenced by the type and dose of allergen, and possibly coexposure to microbial products. These responses are susceptible to suppression by regulatory T cells. This article highlights recent advances in the characterization of allergen-specific memory T(H)2 cells and discusses the heterogeneous nature of regulatory T cells and possible mechanisms of action. The relevance of T-cell epitope mapping studies to understanding the unique nature of T-cell responses to different allergens, as well as to peptide vaccine development, is reviewed. Experimental techniques and approaches for analyzing allergen-specific T cells and identifying novel T-cell epitopes are described that may lead to new T-cell-based therapies.

Murine neonatal CD4+ cells are poised for rapid Th2 effector-like function

Murine neonates typically mount Th2-biased immune responses. This entails a cell-intrinsic component whose molecular basis is unknown. We found that neonatal CD4(+) T cells are uniquely poised for rapid Th2 function. Within 24 h of activation, neonatal CD4(+) cells made high levels of IL-4 and IL-13 mRNA and protein. The rapid high-level IL-4 production arose from a small subpopulation of cells, did not require cell cycle entry, and was unaffected by pharmacologic DNA demethylation. CpG methylation analyses in resting neonatal cells revealed pre-existing hypomethylation at a key Th2 cytokine regulatory region, termed conserved noncoding sequence 1 (CNS-1). Robust Th2 function and increased CNS-1 demethylation was a stable property that persisted in neonatal Th2 effectors. The transcription factor STAT6 was not required for CNS-1 demethylation and this state was already established in neonatal CD4 single-positive thymocytes. CNS-1 demethylation levels were much greater in IL-4-expressing CD4 single-positive thymocytes compared with unactivated cells. Together, these results indicate that neonatal CD4+ T cells possess distinct qualities that could predispose them toward rapid, effector-like Th2 function.

Immunopathology of RSV infection: prospects for developing vaccines without this complication

Respiratory syncytial virus is the most important cause of lower respiratory tract infection in infants and young children. RSV clinical disease varies from rhinitis and otitis media to bronchiolitis and pneumonia. An increased incidence of asthma later in life has been associated with the more severe lower respiratory tract infections. Despite its importance as a pathogen, there is no licensed vaccine against RSV. This is due to a number of factors complicating the development of an effective and safe vaccine. The immunity to natural RSV infection is incomplete as re-infections occur in all age groups, which makes it challenging to design a protective vaccine. Second, the primary target population is the newborn infant, which has a relatively immature immune system and maternal antibodies that can interfere with vaccination. Finally, some vaccines have resulted in a predisposition for exacerbated pulmonary disease in infants, which was attributed to an imbalanced Th2-biased immune response, although the exact cause has not been elucidated. This makes it difficult to proceed with vaccine testing in infants. It is likely that an effective and safe vaccine needs to elicit a balanced immune response, including RSV-specific neutralising antibodies, CD8 T-cells, Th1/Th2 CD4 T-cells and preferably secretory IgA. Subunit vaccines formulated with appropriate adjuvants may be adequate for previously exposed individuals. However, intranasally delivered genetically engineered attenuated or vectored vaccines are currently most promising for newborns, as they are expected to induce a balanced immune response similar to that elicited to natural infection and not be subject to interference from maternal antibodies. Maternal vaccination may be the optimal strategy to protect the very young infants.

The key regulators of adult T helper cell responses, STAT6 and T-bet, are established in early life in mice

Murine neonatal immunity is typically Th2 biased. This is characterized by high-level IL-4 production at all phases of the immune response and poor IFN-gamma memory responses. The differential expression of Th1/Th2 cytokines by neonates and adults could arise if the critical regulators of Th differentiation and function, STAT6 and T-bet, operate differently during the neonatal period. To test this idea, the Th cell responses of wild-type, T-bet-deficient, or STAT6-deficient mice were compared in vitro and in vivo. The absence of these factors had similar qualitative effects on the development of effector function in neonates and adults, i.e., if a Th lineage was inhibited or enhanced in adult animals, a similar phenomenon was observed in neonates. However, there was a striking difference observed in the in vivo Th1 memory responses of STAT6-deficient mice initially immunized as neonates. Antigen-specific IFN-gamma production was increased 50-100-fold in STAT6-deficient neonates, achieving levels similar to those of STAT6-deficient adults. These findings demonstrate that STAT6 and T-bet signals are central in shaping Th responses in wild-type neonates, as in adult mice, and that the master regulators of Th cell development and function are already firmly established in early life.

Dendritic cell immaturity during infancy restricts the capacity to express vaccine-specific T-cell memory

The capacity of the immune system in infants to develop stable T-cell memory in response to vaccination is attenuated, and the mechanism(s) underlying this developmental deficiency in humans is poorly understood. The present study focuses on the capacity for expression of in vitro recall responses to tetanus and diphtheria antigens in lymphocytes from 12-month-old infants vaccinated during the first 6 months of life. We demonstrate that supplementation of infant lymphocytes with "matured" dendritic cells (DC) cultured from autologous CD14+ precursors unmasks previously covert cellular immunity in the form of Th2-skewed cytokine production. Supplementation of adult lymphocytes with comparable prematured autologous DC also boosted vaccine-specific T-cell memory expression, but in contrast to the case for the infants, these cytokine responses were heavily Th1 skewed. Compared to adults, infants had significantly fewer circulating myeloid DC (P < 0.0001) and plasmacytoid DC (P < 0.0001) as a proportion of peripheral blood mononuclear cells. These findings suggest that deficiencies in the numbers of antigen-presenting cells and their functional competence at 12 months of age limit the capacity to express effector memory responses and are potentially a key factor in reduced vaccine responsiveness in infants.

Early life T cell responses to pneumococcal conjugates increase with age and determine the polysaccharide-specific antibody response and protective efficacy

Immunization with a tetanus-protein (TT) pneumococcal polysaccharide (PPS) conjugate vaccine (Pnc1-TT) induces protective immunity against lethal pneumococcal infections in neonatal and infant mice, but anti-PPS IgG response and protective efficacy is lower than in adult mice. Here, we show that reduced antibody (Ab) response and protection against infections is directly related to impaired T cell response to the carrier. Whereas spleen cells from adult mice immunized with Pnc1-TT responded with proliferation and IFN-gamma secretion to in vitro stimulation with TT, spleen cells from neonatal and infant mice did not. However, significant, but age dependent, Th2-cytokine responses were observed in mice immunized with Pnc1-TT. Impaired IFN-gamma production upon TT-stimulation in vitro was also reflected in reduced IFN-gamma/IL-5 ratio. The IL--5 response correlated with IgG anti-PPS titers, and the lack of PPS Ab in the majority of neonatal mice was clearly associated with absence of carrier-specific IL-5 production. These results show that immunization with Pnc1-TT induces carrier-specific T cell responses that increase with age and determine the levels of PPS-specific Ab elicited. Whereas a weak and Th2-biased response was observed in neonatal mice, infant mice showed a mixed Th1-Th2 response as observed in adults.

Choroid plexus selectively accumulates T-lymphocytes in normal controls and after peripheral immune activation

We determined T-lymphocyte migration into brain and choroid plexus (CPx) after enterotoxin-induced systemic immune activation. CPx T-lymphocytes/mm2 in control mice were > 3 logs more numerous than brain and increased by as much as 150-fold by post-enterotoxin Day 3 (p < 0.01). Flow cytometry of pooled CPx confirmed post-enterotoxin increases. Brain T-lymphocytes increased up to 17-fold after SEB and accumulated in subependymal and periventricular brain. T cell apoptosis was absent. These results show preferential T-lymphocyte migration to CPx over brain and suggest that brain T cells may be derived from the CPx by direct migration or by cerebrospinal fluid dissemination.

Upon TLR9 signaling, CD5+ B cells control the IL-12-dependent Th1-priming capacity of neonatal DCs

The susceptibility to infections and the strong Th2 bias observed in neonates are thought to be due to the immaturity of the dendritic cell (DC) compartment. We show that neonatal DCs, like their adult counterparts, elicit Th1 responses. We also demonstrate that during potentially harmful systemic inflammation, after Toll-like receptor (TLR) 9 triggering, neonatal B cells produce high concentrations of IL-10, preventing optimal IL-12 secretion by neonatal DCs and, thus, Th1 priming. Although both CD5+ and CD5- B cell subsets respond to CpG ODN stimulation, we found that only CD5+ B cells produce IL-10. Therefore, these results show the regulatory role of CD5+ B cells on DC activation in vivo for Th1/Th2 polarization and highlight the paradoxical effects of TLR triggering in vivo.

T cell homeostasis in tolerance and immunity

The size of the peripheral T cell pool is remarkably stable throughout life, reflecting precise regulation of cellular survival, proliferation, and apoptosis. Homeostatic proliferation refers to the process by which T cells spontaneously proliferate in a lymphopenic host. The critical signals driving this expansion are "space," contact with self-major histocompatibility complex (MHC)/peptide complexes, and cytokine stimulation. A number of studies have delineated an association between T cell lymphopenia, compensatory homeostatic expansion, and the development of diverse autoimmune syndromes. In the nonobese diabetic mouse model of type 1 diabetes, lymphopenia-induced homeostatic expansion fuels the generation of islet-specific T cells. Excess interleukin-21 facilitates T cell cycling but limited survival, resulting in recurrent stimulation of T cells specific for self-peptide/MHC complexes. Indeed, data from several experimental models of autoimmunity indicate that a full T cell compartment restrains homeostatic expansion of self-reactive cells that could otherwise dominate the repertoire. This review describes the mechanisms that govern T cell homeostatic expansion and outlines the evidence that lymphopenia presents a risk for development of autoimmune disease.

Development of an adult-like cell-mediated immune response in calves after early vaccination with Mycobacterium bovis bacillus Calmette-Guérin

Effects of neonatal vaccination on antigen-specific cellular and humoral immune responses of dairy calves have not been well described. The purpose of this study was to characterize the ontogeny of the adaptive immune response in calves sensitized to the attenuated strain of Mycobacterium bovis, bacillus Calmette-Guerín. Holstein bull calves were nonvaccinated (n = 6, vaccination controls) or vaccinated subcutaneously (n = 6) with bacillus Calmette-Guerín at 1 and 7 wk of age. Composition and functional capacities of blood mononuclear cell populations from calves were evaluated at 1 (prevaccination), 3, 6, 7, 8, 9, and 12 wk of age. Young adults (nulliparous heifers, n = 4) vaccinated in an identical manner were sampled concurrently to evaluate effects of animal maturity on the development of the adaptive immune response. Responses of nonvaccinated calves to recall antigen (Mycobacterium bovis purified protein derivative) ex vivo and in vivo (i.e., cutaneous delayed-type hypersensitivity) were minimal or nonexistent. Responses of cells from vaccinated calves and young adults to recall antigen, however, were evident as early as wk 2 after primary vaccination. Antigen-induced T cell subset proliferation, and secretion of interferon-gamma, nitric oxide, and tumor necrosis factor-alpha by cells from vaccinated calves were comparable to or greater than responses of vaccinated adults during the 11-wk study. Eleven weeks after primary vaccination, cutaneous responses of vaccinated calves and young adults to intradermal administration of antigen were pronounced and comparable, demonstrating the capacity of the bovine neonate to develop a vigorous cell-mediated immune response in vivo. Antibody responses (i.e., antibody concentrations in sera and in supernatants from antigen-stimulated cultures of blood mononuclear cells) of vaccinated calves, in contrast, were markedly lower than parallel responses of vaccinated adults. In conclusion, these results suggest that the bovine neonate can mount a vigorous, adult-like cell-mediated immune response when vaccinated at an early age.

Modulation of proinflammatory responses to Pneumocystis carinii f. sp. muris in neonatal mice by granulocyte-macrophage colony-stimulating factor and IL-4: role of APCs

Clearance of Pneumocystis carinii f. sp. muris (PC) organisms from the lungs of neonatal mice is delayed due to failure of initiation of inflammation over the first 3 wk after infection. The ability of neonatal lung CD11c(+) dendritic cells (DCs) to induce Ag-specific T cell proliferative responses was significantly reduced compared with adult lung DCs. However, neonatal bone marrow-derived DCs were as competent at presenting PC Ag as were adult bone marrow-derived DCs. Because GM-CSF mRNA expression and activity were significantly reduced in neonatal lungs compared with adults, we treated neonates with exogenous GM-CSF and IL-4 and found enhanced clearance of PC compared with untreated neonates. This was associated with increased lung TNF-alpha, IL-12p35, and IL-18 mRNA expression, indicating enhanced innate immune responses. Cytokine-treated mice had marked expansion of CD11c(+) DCs with up-regulated MHC-II in the lungs. Moreover, increased numbers of activated CD4(+)CD44(high)CD62L(low) cells in the lungs and draining lymph nodes suggested improved Ag presentation by the APCs. Together these data indicate that neonatal lungs lack maturation factors for efficient cellular functioning, including APC maturation.

Use of mouse models of allergic rhinitis to study the upper and lower airway link

PURPOSE OF REVIEW

Allergic rhinitis and asthma are examples of a continuum of airway diseases with diverse clinical manifestations. This review examines the most recent work in mouse models studying upper and lower airway links and interactions.

RECENT FINDINGS

The concept of united airways has been supported by investigative and epidemiological studies. Studies using mouse models of asthma and models of allergic rhinitis have demonstrated that analogous pathways lead to inflammation and airway hyperresponsiveness. Th2-type T cells and IL-13 play important immunopathologic roles. Recent studies have examined upper airway mucosal immune responses and development of both allergic and tolerant phenotypes. In a model of allergic airways disease, there is evidence of lower airway inflammation and airways hyperresponsiveness following application of allergen only to the nares, suggesting local stimulation can activate distal allergic responses. Immunomodulatory properties of the airway mucosa have also been explored. Allergen-specific tolerance can be induced by appropriate stimulation of airway mucosa and is associated with activation of IL-10-producing T cells. This effect is mediated by antigen presenting cells, especially dendritic cells.

SUMMARY

Immune stimulation of the airway mucosa, both in the upper and lower airways, results in active T-cell-mediated immune responses leading toward tolerance or asthma and allergic rhinitis. Regulation of these T-cell responses is currently under investigation. It is clear from these studies that antigenic stimulation of any part of the respiratory mucosa can have ripple effects along the entire airway and supports the concept of united airways.

Peripheral CD4+ lymphocytes derived from fetal versus adult thymic precursors differ phenotypically and functionally

There is growing evidence that the differentiation processes in the fetal and adult thymus are not identical. However, there is little information on whether these developmental differences influence the properties of mature cells that exit the thymus and seed peripheral lymphoid organs. We have addressed this issue by comparing the development of Ag-specific Th1/Th2 function by fetal vs adult thymic derived CD4(+) cells in the same adoptive adult hosts. Host mice were irradiated and transplanted with 14- to 15-day fetal thymic lobes from Thy-1 congenic mice. Ag (keyhole limpet hemocyanin)-specific Th1/Th2 responses of fetal-derived (donor) or adult-derived (host) CD4(+) cells were analyzed by ELISA following primary or secondary immunization. Fetal-derived cells produced up to 10-fold more of both Th1 (IFN-gamma) and Th2 (IL-4) cytokines than did adult-derived cells. Comparisons of the IL-4:IFN-gamma ratios showed that the responses of fetal-derived cells were Th2-skewed in an Ag dose-dependent manner. At low doses of Ag, the fetal-derived ratio was approximately 5 times higher than the adult-derived ratio. As the Ag dose was increased, the differences between the ratios of the fetal- and adult-derived responses were minimized. These relative responses were established initially during the primary effector phase but were maintained for weeks, into the memory phase of the immune response. Importantly, fetal-derived CD4(+) cells showed these properties whether the fetal thymic precursors matured within the fetal or adult thymic microenvironment. These results demonstrate that cells arising from fetal thymic precursors are functionally different both qualitatively and quantitatively from adult-derived cells.

TNF-alpha-dependent ICAM-1- and VCAM-1-mediated inflammatory responses are delayed in neonatal mice infected with Pneumocystis carinii

Neonatal mice have a delayed CD4-mediated inflammatory response to Pneumocystis carinii (PC) infection in the lungs that corresponds to a delayed TNF-alpha response and a delayed clearance of the organisms compared with adult mice. Since TNF-alpha is known to drive the up-regulation of adhesion molecules, we examined the expression and function of adhesion molecules in the lungs of neonatal mice. The expression of both ICAM-1 and VCAM-1 was significantly lower in the lungs of PC-infected neonatal mice compared with adults. Additionally, migration of neonatal T cells across endothelial cells expressing VCAM-1 and monocyte chemotactic protein-1 was aberrant compared with that in adult T cells, although alpha(4)beta(1) integrin-mediated adhesion of neonatal lymphocytes was comparable to that of adult lymphocytes. Treatment of neonatal mice with exogenous TNF-alpha resulted in increased expression of ICAM-1 and VCAM-1 as well as increased expression of chemokines, resulting in infiltration of CD4(+) cells into the lungs. Treatment with exogenous TNF-alpha resulted in a trend (although not statistically significant) toward a reduction of PC organisms from the lungs. These data indicate that neonatal lung endothelial cells do not up-regulate ICAM-1 and VCAM-1 in response to PC infection, probably due to depressed TNF-alpha production. Additionally, neonatal T cells are defective in the ability to migrate across endothelial cells.

CD8+ T Lymphocytes Regulating Th2 Pathology Escape Neonatal Tolerization

Transplantation tolerance induced by neonatal injection of semiallogeneic spleen cells is associated in several strain combinations with a pathological syndrome caused by Th2 differentiation of donor-specific CD4(+) T lymphocytes. We investigated the role of host CD8(+) T cells in the regulation of this Th2 pathology. IgE serum levels and eosinophilia significantly increased in BALB/c mice neonatally injected with (A/J x BALB/c)F(1) spleen cells when CD8(+) T cells were depleted by administration of anti-CD8 mAb or when beta(2)-microglobulin-deficient mice were used as recipients. In parallel, increased serum levels of IL-5 and IL-13 were measured in blood of tolerant CD8(+) T cell-deficient mice. Whereas neonatally injected mice were unable to generate anti-donor cytotoxic effectors, their CD8(+) T cells were as efficient as control CD8(+) T cells in reducing the severity of Th2 pathology and in restoring donor-specific cytotoxicity in vitro after in vivo transfer in beta(2)-microglobulin-deficient mice. Likewise, CD8(+) T cells from control and tolerant mice equally down-regulated the production of Th2 cytokines by donor-specific CD4(+) T cells in vitro. The regulatory activity of CD8(+) T cells depended on their secretion of IFN-gamma for the control of IL-5 production but not for IL-4 or IL-13. Finally, we found that CD8(+) T cells from 3-day-old mice were already able to down-regulate IL-4, IL-5, and IL-13 production by CD4(+) T cells. We conclude that regulatory CD8(+) T cells controlling Th2 responses are functional in early life and escape neonatal tolerization.

Impaired CD40-signalling in Langerhans' cells from murine neonatal draining lymph nodes: implications for neonatally induced cutaneous tolerance

Cutaneous tolerance to antigens may be induced in mice through application of antigen during the first few days following birth. The mechanism governing this neonatally induced tolerance remains uncertain. We employed a contact hypersensitivity model to analyse dendritic cell (DC) function and the expression of classical and non-classical lymphocyte populations within the neonate. Examination of draining lymph node DC after antigenic challenge of the skin revealed these DC to be significantly deficient in their ability to stimulate antigen-specific T cell proliferation. Co-stimulatory molecule (CD40, CD80 and CD86) expression of these cells was deficient in comparison to adult DC, and functional tests revealed these cells to possess a critical absence of CD40 signalling. A numerical analysis of classical and non-classical lymphocyte expression demonstrated that while the neonatal spleen is devoid of T cells, the lymph nodes have a normal repertoire of T, B, gammadelta and CD4+CD25+ lymphocytes but an increased expression of natural killer (NK) cells. This study indicates that functionally deficient DC are likely contributors to neonatally induced cutaneous tolerance.

Flt3 ligand-treated neonatal mice have increased innate immunity against intracellular pathogens and efficiently control virus infections

Flt-3 ligand (FL), a hematopoetic growth factor, increases the number of dendritic cells (DCs), B cells, and natural killer cells in adult mice but the effect in neonates was unknown. We show that FL treatment of newborn mice induced a >100-fold increase in the innate resistance against infection with herpes simplex virus type 1 and Listeria monocytogenes. This resistance required interferon (IFN)-alpha/beta for viral and interleukin (IL)-12 for bacterial infections. Long-term survival after viral but not bacterial infection was increased approximately 100-fold by FL treatment. After treatment, CD11c(+)/major histocompatibility complex type II(+) and CD11c(+)/B220(+) DC lineage cells were the only cell populations increased in the spleen, liver, peritoneum, and skin. DC induction was independent of IFNs, IL-2, -4, -7, -9, -15, and mature T and B cells. The data suggest that FL increases the number of DCs in neonates and possibly in other immune-compromised individuals, which in turn improves IFN-alpha/beta- and IL-12-associated immune responses.

Neonates support lymphopenia-induced proliferation

T cells expand without intentional antigen stimulation when transferred into adult lymphopenic environments. In this study, we show that the physiologic lymphopenic environment existing in neonatal mice also supports CD4 T cell proliferation. Strikingly, naive CD4 T cells that proliferate within neonates acquire the phenotypic and functional characteristics of memory cells. Such proliferation is inhibited by the presence of both memory and naive CD4 T cells, is enhanced by 3-day thymectomy, is independent of IL-7, and requires a class II MHC-TCR interaction and a CD28-mediated signal. CD44(bright) CD4 T cells in neonates have a wide repertoire as judged by the distribution of Vbeta expression. Thus, lymphopenia-induced T cell proliferation is a physiologic process that occurs during the early postnatal period.