Delayed inflammatory response to Pneumocystis carinii infection in neonatal mice is due to an inadequate lung environment

Pneumocystis Pneumonia: Pitfalls and Hindrances to Establishing a Reliable Animal Model

Pneumocystis pneumonia is a severe lung infection that occurs primarily in largely immunocompromised patients. Few treatment options exist, and the mortality rate remains substantial. To develop new strategies in the fields of diagnosis and treatment, it appears to be critical to improve the scientific knowledge about the biology of the Pneumocystis agent and the course of the disease. In the absence of in vitro continuous culture system, in vivo animal studies represent a crucial cornerstone for addressing Pneumocystis pneumonia in laboratories. Here, we provide an overview of the animal models of Pneumocystis pneumonia that were reported in the literature over the last 60 years. Overall, this review highlights the great heterogeneity of the variables studied: the choice of the host species and its genetics, the different immunosuppressive regimens to render an animal susceptible, the experimental challenge, and the different validation methods of the model. With this work, the investigator will have the keys to choose pivotal experimental parameters and major technical features that are assumed to likely influence the results according to the question asked. As an example, we propose an animal model to explore the immune response during Pneumocystis pneumonia.

Bias of the Immune Response to Pneumocystis murina Does Not Alter the Ability of Neonatal Mice to Clear the Infection

Newborn mice are unable to clear Pneumocystis (PC) infection with the same efficiency as adults due, in part, to their inability to develop a robust immune response to infection until three weeks of age. It is known that infants tend develop a Th2 skewed response to antigen so we sought to determine whether a biased cytokine response altered the clearance of PC infection in neonatal mice. P. murina infection in neonatal mice resulted in increased IL-4 expression by CD4 T cells and myeloid cells, augmented IL-13 secretion within the airways and increased arginase activity in the airways, indicative of Th2-type responses. P. murina-infected IL-4Rα^-/- neonates had a shift towards Th1 cytokine production and increased numbers of CD4 and CD8 T cells within the lung as well as elevated levels of P. murina-specific IgG. IFNγ^-/- and IL-23 p19^-/- mice had altered CD4-T cell-dependent cytokine and cell responses. Though we could alter the T helper cell environment in neonatal knockout mice, there was no loss in the ability of these pups to clear infection. It is possible that the Th2 phenotype normally seen in neonatal mice protects the developing lung from pro-inflammatory immune responses without compromising host defense against P. murina.

RGS4 promotes allergen- and aspirin-associated airway hyperresponsiveness by inhibiting PGE2 biosynthesis

BACKGROUND

Allergens elicit host production of mediators acting on G-protein-coupled receptors to regulate airway tone. Among these is prostaglandin E2 (PGE2), which, in addition to its role as a bronchodilator, has anti-inflammatory actions. Some patients with asthma develop bronchospasm after the ingestion of aspirin and other nonsteroidal anti-inflammatory drugs, a disorder termed aspirin-exacerbated respiratory disease. This condition may result in part from abnormal dependence on the bronchoprotective actions of PGE2.

OBJECTIVE

We sought to understand the functions of regulator of G protein signaling 4 (RGS4), a cytoplasmic protein expressed in airway smooth muscle and bronchial epithelium that regulates the activity of G-protein-coupled receptors, in asthma.

METHODS

We examined RGS4 expression in human lung biopsies by immunohistochemistry. We assessed airways hyperresponsiveness (AHR) and lung inflammation in germline and airway smooth muscle-specific Rgs4-/- mice and in mice treated with an RGS4 antagonist after challenge with Aspergillus fumigatus. We examined the role of RGS4 in nonsteroidal anti-inflammatory drug-associated bronchoconstriction by challenging aspirin-exacerbated respiratory disease-like (ptges1-/-) mice with aspirin.

RESULTS

RGS4 expression in respiratory epithelium is increased in subjects with severe asthma. Allergen-induced AHR was unexpectedly diminished in Rgs4-/- mice, a finding associated with increased airway PGE2 levels. RGS4 modulated allergen-induced PGE2 secretion in human bronchial epithelial cells and prostanoid-dependent bronchodilation. The RGS4 antagonist CCG203769 attenuated AHR induced by allergen or aspirin challenge of wild-type or ptges1-/- mice, respectively, in association with increased airway PGE2 levels.

CONCLUSIONS

RGS4 may contribute to the development of AHR by reducing airway PGE2 biosynthesis in allergen- and aspirin-induced asthma.

Identification of Predictive Markers and Outcomes of Late-onset Pneumocystis jirovecii Pneumonia in Kidney Transplant Recipients

Background

In the era of prophylaxis, Pneumocystis pneumonia (PCP) has become a late-onset opportunistic infection requiring indications for prolonged prophylaxis to be defined. The primary objective of our study was therefore to evaluate risk factors associated with late-onset PCP. The secondary objective was to assess the impact of this infection on graft and patient survival.

Methods

We conducted a French case-control study in Bordeaux and Toulouse center by matching 1 case to 1–2 controls from the same center based on the transplant date and the type of induction treatment.

Results

Seventy cases and 134 controls were included. PCP occurred at a median of 3 years after transplantation. The total lymphocyte count and CD4+ and CD8+ T-lymphocyte values were lower in the cases than in their matched controls on the day of infection and annually up to 4 years earlier. The covariables independently associated with PCP were the total lymphocyte count 1 year before Pneumocystis, mTOR inhibitors used as maintenance immunosuppressive drugs, and the administration of corticosteroid boluses used in acute rejection. A total lymphocyte count threshold <1000/µL offered the best predictive value for infection occurrence. PCP was associated with high incidence of graft loss and patient death (30% and 17% respectively, 3 years after PCP).

Conclusions

Pneumocystis pneumonia has dramatic consequences in kidney transplant recipients; a targeted prophylaxis based on simple criteria, such as chronic lymphopenia and/or history of corticosteroid boluses, could be useful to avoid life-threatening complications.

Innate Immunity to Respiratory Infection in Early Life

Early life is a period of particular susceptibility to respiratory infections and symptoms are frequently more severe in infants than in adults. The neonatal immune system is generally held to be deficient in most compartments; responses to innate stimuli are weak, antigen-presenting cells have poor immunostimulatory activity and adaptive lymphocyte responses are limited, leading to poor immune memory and ineffective vaccine responses. For mucosal surfaces such as the lung, which is continuously exposed to airborne antigen and to potential pathogenic invasion, the ability to discriminate between harmless and potentially dangerous antigens is essential, to prevent inflammation that could lead to loss of gaseous exchange and damage to the developing lung tissue. We have only recently begun to define the differences in respiratory immunity in early life and its environmental and developmental influences. The innate immune system may be of relatively greater importance than the adaptive immune system in the neonatal and infant period than later in life, as it does not require specific antigenic experience. A better understanding of what constitutes protective innate immunity in the respiratory tract in this age group and the factors that influence its development should allow us to predict why certain infants are vulnerable to severe respiratory infections, design treatments to accelerate the development of protective immunity, and design age specific adjuvants to better boost immunity to infection in the lung.

The life cycle stages of Pneumocystis murina have opposing effects on the immune response to this opportunistic, fungal pathogen

The cyst cell wall β-glucans of Pneumocystis have been shown to stimulate immune responses in lung epithelial cells, dendritic cells, and alveolar macrophages. Little is known about how the trophic life forms, which do not have a fungal cell wall, interact with these innate immune cells. Here, we report differences in the responses of both neonatal and adult mice to the trophic and cystic life cycle stages of Pneumocystis murina The adult and neonatal immune responses to infection with Pneumocystis murina trophic forms were less robust than the response to infection with a physiologically normal mixture of cysts and trophic forms. Cysts promoted the recruitment of nonresident innate immune cells and T and B cells into the lungs. Cysts, but not trophic forms, stimulated increased IFN-γ cytokine concentrations in the alveolar spaces, and an increase in IFN-γ-producing CD4⁺ T cells. In vitro, bone marrow-derived dendritic cells (BMDCs) stimulated with cysts produced the proinflammatory cytokines IL-1β and IL-6. In contrast, trophic forms suppressed β-glucan-, LTA-, and LPS-induced IL-1β, IL-6, and TNFα production by BMDCs and antigen presentation to CD4⁺ T cells. The negative effects of trophic forms were not due to ligation of mannose receptor. Our results indicate that optimal innate and adaptive immune responses to Pneumocystis species are dependent on stimulation with the cyst life cycle stage. Conversely, trophic forms suppress β-glucan-induced proinflammatory responses in vitro, suggesting that the trophic forms dampen cyst-induced inflammation in vivo.

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.

Neonatal immunology: responses to pathogenic microorganisms and epigenetics reveal an "immunodiverse" developmental state

Neonatal animals have heightened susceptibility to infectious agents and are at increased risk for the development of allergic diseases, such as asthma. Experimental studies using animal models have been quite useful for beginning to identify the cellular and molecular mechanisms underlying these sensitivities. In particular, results from murine neonatal models indicate that developmental regulation of multiple immune cell types contributes to the typically poor responses of neonates to pathogenic microorganisms. Surprisingly, however, animal studies have also revealed that responses at mucosal surfaces in early life may be protective against primary or secondary disease. Our understanding of the molecular events underlying these processes is less well developed. Emerging evidence indicates that the functional properties of neonatal immune cells and the subsequent maturation of the immune system in ontogeny may be regulated by epigenetic phenomena. Here, we review recent findings from our group and others describing cellular responses to infection and developmentally regulated epigenetic processes in the newborn.

Nodular inflammatory foci are sites of T cell priming and control of murine cytomegalovirus infection in the neonatal lung

Neonates, including mice and humans, are highly susceptible to cytomegalovirus (CMV) infection. However, many aspects of neonatal CMV infections such as viral cell tropism, spatio-temporal distribution of the pathogen as well as genesis of antiviral immunity are unknown. With the use of reporter mutants of the murine cytomegalovirus (MCMV) we identified the lung as a primary target of mucosal infection in neonatal mice. Comparative analysis of neonatal and adult mice revealed a delayed control of virus replication in the neonatal lung mucosa explaining the pronounced systemic infection and disease in neonates. This phenomenon was supplemented by a delayed expansion of CD8⁺ T cell clones recognizing the viral protein M45 in neonates. We detected viral infection at the single-cell level and observed myeloid cells forming “nodular inflammatory foci” (NIF) in the neonatal lung. Co-localization of infected cells within NIFs was associated with their disruption and clearance of the infection. By 2-photon microscopy, we characterized how neonatal antigen-presenting cells (APC) interacted with T cells and induced mature adaptive immune responses within such NIFs. We thus define NIFs of the neonatal lung as niches for prolonged MCMV replication and T cell priming but also as sites of infection control.

Neonates are highly susceptible to a number of infections that usually cause disease only in immunocompromised individuals, most likely because of their incompletely developed immune system. Although this phenomenon has been frequently observed, immune responses of neonates remain largely undefined upon infections with viruses. There is lack of knowledge about the spatio-temporal dynamics of host-virus interaction, especially in comparative infection models of neonates and adults. In this study, with the use of virus reporter mutants, we provide elaborate insight into these aspects in the mouse model of CMV infection. We define hallmarks of virus tropism, early cellular immune responses and general infection dynamics, findings that are fundamental to understand neonatal antiviral immunity. Furthermore, we found that neonatal APCs induce T cell responses in nodular inflammatory foci of the lung, a process which was supposed to be restricted to lymphoid organs. However, the MCMV-specific T cell response was qualitatively different in neonates from that in adults, possibly explaining - in part - the higher susceptibility of newborns. These observations expand our understanding of where adaptive immunity can be initiated, highlights the importance of early local cellular immune responses and sheds more light on neonatal antiviral immunity.

Mucus clearance, MyD88-dependent and MyD88-independent immunity modulate lung susceptibility to spontaneous bacterial infection and inflammation

It has been postulated that mucus stasis is central to the pathogenesis of obstructive lung diseases. In Scnn1b-transgenic (Scnn1b-Tg⁺ mice, airway-targeted overexpression of the epithelial Na⁺ channel β subunit causes airway surface dehydration, which results in mucus stasis and inflammation. Bronchoalveolar lavage from neonatal Scnn1b-Tg⁺ mice, but not wild-type littermates, contained increased mucus, bacteria, and neutrophils, which declined with age. Scnn1b-Tg⁺ mice lung bacterial flora included environmental and oropharyngeal species, suggesting inhalation and/or aspiration as routes of entry. Genetic deletion of the Toll-interleukin-1 receptor adapter molecule MyD88 in Scnn1b-Tg⁺ mice did not modify airway mucus obstruction, but caused defective neutrophil recruitment and increased bacterial infection, which persisted into adulthood. Scnn1b-Tg⁺ mice derived into germ-free conditions exhibited mucus obstruction similar to conventional Scnn1b-Tg⁺ mice and sterile inflammation. Collectively, these data suggest that dehydration-induced mucus stasis promotes infection, compounds defects in other immune mechanisms, and alone is sufficient to trigger airway inflammation.

Alveolar macrophages in neonatal mice are inherently unresponsive to Pneumocystis murina infection

Pneumocystis pneumonia was first diagnosed in malnourished children and has more recently been found in children with upper respiratory symptoms. We previously reported that there is a significant delay in the immune response in newborn mice infected with Pneumocystis compared to adults (Garvy BA, Harmsen AG, Infect. Immun. 64:3987-3992, 1996, and Garvy BA, Qureshi M, J. Immunol. 165:6480-6486, 2000). This delay is characterized by the failure of neonatal lungs to upregulate proinflammatory cytokines and attract T cells into the alveoli. Here, we report that regardless of the age at which we infected the mice, they failed to mount an inflammatory response in the alveolar spaces until they were 21 days of age or older. Anti-inflammatory cytokines had some role in dampening inflammation, since interleukin-10 (IL-10)-deficient pups cleared Pneumocystis faster than wild-type pups and the neutralization of transforming growth factor beta (TGF-β) with specific antibody enhanced T cell migration into the lungs at later time points. However, the clearance kinetics were similar to those of control pups, suggesting that there is an intrinsic deficiency in the ability of innate immunity to control Pneumocystis. We found, using an adoptive transfer strategy, that the lung environment contributes to association of Pneumocystis organisms with alveolar macrophages, implying no intrinsic deficiency in the binding of Pneumocystis by neonatal macrophages. Using both in vivo and in vitro assays, we found that Pneumocystis organisms were less able to stimulate translocation of NF-κB to the nucleus of alveolar macrophages from neonatal mice. These data indicate that there is an early unresponsiveness of neonatal alveolar macrophages to Pneumocystis infection that is both intrinsic and related to the immunosuppressive environment found in neonatal lungs.

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.

Reducing or increasing β-cell apoptosis without inflammation does not affect diabetes initiation in neonatal NOD mice

The presentation of islet antigens in the pancreatic LNs (PLNs) of mice is a developmentally regulated process. It has been hypothesized that, during physiological tissue remodeling, a wave of neonatal β-cell apoptosis may initiate diabetes in autoimmune-prone strains of mice. If true, increasing or decreasing physiological β-cell apoptosis in neonatal NOD mice should alter the time-course of antigen presentation in the PLNs. We used transgenic over-expression of either an anti-apoptotic protein (Bcl-2) or a toxic transgene (rat insulin promoter-Kb) in mouse β cells to reduce or increase neonatal β-cell apoptosis, respectively. Neither intervention affected the timing of antigen presentation in the PLNs or the initiation of islet infiltration. This suggests that under physiological conditions and in the absence of inflammation, neonatal β-cell apoptosis in NOD mice is not the trigger for antigen presentation in the draining LNs.

In vivo activation of toll-like receptor-9 induces an age-dependent abortive lytic cycle reactivation of murine gammaherpesvirus-68

Infection of mice with murine gammaherpesvirus-68 (γHV-68) serves as a model to understand the pathogenesis of persistent viral infections, including the potential for co-infections to modulate viral latency. We have previously found that infection of neonates (8-day-old mice) with γHV-68 resulted in a high level of persistence of the virus in the lungs as well as the spleen, in contrast to infection of adult mice, for which long-term latency was only readily detected in the spleen. In this study we investigated whether stimulation of toll-like receptor (TLR)9 would modulate viral latency in mice infected with γHV-68 in an age-dependent manner. Pups and adult mice were injected with the synthetic TLR9 ligand CpG ODN at 30 dpi, at which time long-term latency has been established. Three days after CpG injection, the lungs and spleens were removed, and a limiting dilution assay was done to determine the frequency of latently infected cells. RNA was extracted to measure viral transcripts using a ribonuclease protection assay. We observed that CpG injection resulted in an increase in the frequency of latently-infected cells in both the lungs and spleens of infected pups, but only in the spleens of infected adult mice. No preformed virus was detected, suggesting that TLR9 stimulation did not trigger complete viral reactivation. When we examined viral gene expression in these same tissues, we observed expression only of the immediate early lytic genes, rta and K3, but not the early DNA polymerase gene or late gB transcript indicative of an abortive reactivation in the spleen. Additionally, mice infected as pups had greater numbers of germinal center B cells in the spleen following CpG injection, whereas CpG stimulated the expansion of follicular zone B cells in adult mice. These data suggest that stimulation of TLR9 differentially modulates gammaherpesvirus latency via an age-dependent mechanism.

Lactobacillus-mediated priming of the respiratory mucosa protects against lethal pneumovirus infection

The inflammatory response to respiratory virus infection can be complex and refractory to standard therapy. Lactobacilli, when targeted to the respiratory epithelium, are highly effective at suppressing virus-induced inflammation and protecting against lethal disease. Specifically, wild-type mice primed via intranasal inoculation with live or heat-inactivated Lactobacillus plantarum or Lactobacillus reuteri were completely protected against lethal infection with the virulent rodent pathogen, pneumonia virus of mice; significant protection (60% survival) persisted for at least 13 wk. Protection was not unique to Lactobacillus species, and it was also observed in response to priming with nonpathogenic Gram-positive Listeria innocua. Priming with live lactobacilli resulted in diminished granulocyte recruitment, diminished expression of multiple proinflammatory cytokines (CXCL10, CXCL1, CCL2, and TNF), and reduced virus recovery, although we have demonstrated clearly that absolute virus titer does not predict clinical outcome. Lactobacillus priming also resulted in prolonged survival and protection against the lethal sequelae of pneumonia virus of mice infection in MyD88 gene-deleted (MyD88(-/-)) mice, suggesting that the protective mechanisms may be TLR-independent. Most intriguing, virus recovery and cytokine expression patterns in Lactobacillus-primed MyD88(-/-) mice were indistinguishable from those observed in control-primed MyD88(-/-) counterparts. In summary, we have identified and characterized an effective Lactobacillus-mediated innate immune shield, which may ultimately serve as critical and long-term protection against infection in the absence of specific antiviral vaccines.

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.

Pneumocystis jirovecii Pneumonia

Pneumocystis jirovecii has gained attention during the last decade in the context of the AIDS epidemic and the increasing use of cytotoxic and immunosuppressive therapies. This article summarizes current knowledge on biology, pathophysiology, epidemiology, diagnosis, prevention, and treatment of pulmonary P jirovecii infection, with a particular focus on the evolving pathophysiology and epidemiology. Pneumocystis pneumonia still remains a severe opportunistic infection, associated with a high mortality rate.

Current understanding of Pneumocystis immunology

Pneumocystis jirovecii is the opportunistic fungal organism that causes Pneumocystis pneumonia (PCP) in humans. Similar to other opportunistic pathogens, Pneumocystis causes disease in individuals who are immunocompromised, particularly those infected with HIV. PCP remains the most common opportunistic infection in patients with AIDS. Incidence has decreased greatly with the advent of HAART. However, an increase in the non-HIV immunocompromised population, noncompliance with current treatments, emergence of drug-resistant strains and rise in HIV(+) cases in developing countries makes Pneumocystis a pathogen of continued interest and a public health threat. A great deal of research interest has addressed therapeutic interventions to boost waning immunity in the host to prevent or treat PCP. This article focuses on research conducted during the previous 5 years regarding the host immune response to Pneumocystis, including innate, cell-mediated and humoral immunity, and associated immunotherapies tested against PCP.

Pneumocystis pneumonia in children

Pneumocystis pneumonia (PCP) is a life-threatening infection in immunocompromised children with quantitative and qualitative defects in T lymphocytes. At risk are children with lymphoid malignancies, HIV infection, corticosteroid therapy, transplantation and primary immunodeficiency states. Diagnosis is established through direct examination or polymerase chain reaction (PCR) from respiratory secretions. Trimethoprim-sulphamethoxazole is used for initial therapy in most patients, while pentamidine, atovaquone, clindamycin plus primaquine, and dapsone plus trimethoprim are alternatives. Prophylaxis of high-risk patients reduces but does not eliminate the risk of PCP. Improved understanding of the pathogenesis of PCP is important for future advances against this life-threatening infection.

Infection of neonates with murine gammaherpesvirus 68 results in enhanced viral persistence in lungs and absence of infectious mononucleosis syndrome

We used the murine gammaherpesvirus 68 (γHV-68), which serves as a model for human gammaherpesvirus infection, to determine whether age at infection altered the pattern of gammaherpesvirus pathogenesis. We infected mice intranasally at 8 days old (pups) and 6 weeks old (adults) to investigate differences in γHV-68 pathogenesis. There was no difference between adults or pups in acute infection in the lungs at 6 days post-infection (p.i.). However, mice infected as pups exhibited a more disseminated viral infection with viral DNA detected in the spleen, liver and heart as measured by quantitative PCR (Q-PCR). In addition, viral DNA was detected in the lungs of mice infected as pups until 60 days p.i. Three viral transcripts (M2, M3 and M9) were expressed at both 30 and 60 days p.i. In contrast, no viral DNA or mRNA expression was detected in lungs of mice infected as adults at 30 or 60 days p.i. Mice infected as adults experienced a peak in latent infection in the spleen at 16 days p.i., corresponding with an increase in splenic weight and expansion of the Vβ4+ CD8+ T-cell population, similar to infectious mononucleosis observed following infection of young adults with Epstein–Barr virus. However, the increase in splenic weight of infected pups was not as pronounced and no significant increase in Vβ4+ CD8+ T-cell expansion was observed in infected pups. Together, these data suggest that the pathogenesis of murine gammaherpesvirus γHV-68 is age-dependent.

Innate immunity of the newborn: basic mechanisms and clinical correlates

The fetus and newborn face a complex set of immunological demands, including protection against infection, avoidance of harmful inflammatory immune responses that can lead to pre-term delivery, and balancing the transition from a sterile intra-uterine environment to a world that is rich in foreign antigens. These demands shape a distinct neonatal innate immune system that is biased against the production of pro-inflammatory cytokines. This bias renders newborns at risk of infection and impairs responses to many vaccines. This Review describes innate immunity in newborns and discusses how this knowledge might be used to prevent and treat infection in this vulnerable population.

Scavenger receptor A dampens induction of inflammation in response to the fungal pathogen Pneumocystis carinii

Alveolar macrophages are the effector cells largely responsible for clearance of Pneumocystis carinii from the lungs. Binding of organisms to beta-glucan and mannose receptors has been shown to stimulate phagocytosis of the organisms. To further define the mechanisms used by alveolar macrophages for clearance of P. carinii, mice deficient in the expression of scavenger receptor A (SRA) were infected with P. carinii, and clearance of organisms was monitored over time. SRA-deficient (SRAKO) mice consistently cleared P. carinii faster than did wild-type control mice. Expedited clearance corresponded to elevated numbers of activated CD4(+) T cells in the alveolar spaces of SRAKO mice compared to wild-type mice. Alveolar macrophages from SRAKO mice had increased expression of CD11b on their surfaces, consistent with an activated phenotype. However, they were not more phagocytic than macrophages expressing SRA, as measured by an in vivo phagocytosis assay. SRAKO alveolar macrophages produced significantly more tumor necrosis factor alpha (TNF-alpha) than wild-type macrophages when stimulated with lipopolysaccharide in vitro but less TNF-alpha in response to P. carinii in vitro. However, upon in vivo stimulation, SRAKO mice produced significantly more TNF-alpha, interleukin 12 (IL-12), and IL-18 in response to P. carinii infection than did wild-type mice. Together, these data indicate that SRA controls inflammatory cytokines produced by alveolar macrophages in the context of P. carinii infection.

Exogenous heat-killed Escherichia coli improves alveolar macrophage activity and reduces Pneumocystis carinii lung burden in infant mice

Pneumocystis carinii is an opportunistic fungal pathogen that causes life-threatening pneumonia in immunocompromised individuals. Infants appear to be particularly susceptible to Pneumocystis pulmonary infections. We have previously demonstrated that there is approximately a 3-week delay in the clearance of Pneumocystis organisms from pup mouse lungs compared to that in adults. We have further shown that there is approximately a 1-week delay in alveolar macrophage activation in pups versus adult mice. Alveolar macrophages are the primary effector cells responsible for the killing and clearance of Pneumocystis, suggesting that pup alveolar macrophages may be involved in the delayed clearance of this organism. Alveolar macrophages cultured in vitro with Pneumocystis alone demonstrate little to no activation, as indicated by a lack of cytokine production. However, when cultured with lipopolysaccharide (LPS) or zymosan, cytokine production was markedly increased, suggesting that pup alveolar macrophages are specifically unresponsive to Pneumocystis organisms rather than being intrinsically unable to become activated. Furthermore, pup mice treated with aerosolized, heat-killed Escherichia coli in vivo were able to clear Pneumocystis more efficiently than were control mice. Together, these data suggest that while pup alveolar macrophages are unresponsive to P. carinii f. sp. muris organisms, they are capable of activation by heat-killed E. coli in vivo, as well as LPS and zymosan in vitro. The lack of response of pup mice to P. carinii f. sp. muris may reflect protective mechanisms specific to the developing pup lung, but ultimately it results in insufficient clearance of Pneumocystis organisms.

A murine model of dual infection with cytomegalovirus and Pneumocystis carinii: Effects of virus-induced immunomodulation on disease progression

Despite the use of antimicrobial prophylaxis, cytomegalovirus (CMV) and Pneumocystis carinii (PC) pneumonia (PCP) are both leading causes of morbidity and mortality in immunocompromised patients. It has previously been reported that CMV infection modulates host immune responses with a variety of mechanisms which include the suppression of helper T cell functions and antigen presenting cell (APC) functions, both of which are critical for PCP resolution. However, the mechanisms of these interactions and other possible immune regulatory effects are not clearly understood. In this study, we investigated the impact of murine CMV (MCMV) induced immunomodulation on the progression of PCP in a co-infection model. Initial results show that dually infected mice had evidence of more severe PC disease, which include a greater loss of body weight, an excess lung PC burden and delayed clearance of PC from lungs, compared to mice with PC infection alone. At day 7 post-infection, dually infected mice had reduced numbers of MHC-II expressing cells in the lung interstitium and lymph nodes and reduced migration of CD11c+ cells to both the tracheobronchial lymph nodes and alveolar spaces. Dual infected mice showed elevated numbers of specific CD8 responses concomitant with a decrease in activated CD4+ T cells in both the lymph nodes and in alveolar spaces when compared to mice infected with MCMV alone. These data suggest that MCMV infection inhibits the immune responses generated against PC which contribute to the delayed clearance of the organism.

Immunity against the opportunistic fungal pathogen Pneumocystis

Species of the genus Pneumocystis exist as opportunistic fungal pathogens and are associated with severe pneumonia and pulmonary complications in immunocompromised individuals. Although prophylactic therapy for Pneumocystis has significantly decreased the overall incidence of infection, more than 80% of cases in current patient populations are considered breakthrough cases. In the HIV-infected population, in the years following the initiation of highly active antiretroviral therapy (HAART), significant reductions in the incidence of Pneumocystis infection were observed, although trends over the last several years suggest that the incidence of Pneumocystis has plateaued rather than decreased. Furthermore, with the more prominent usage of immunosuppressive therapies, the frequency of Pneumocystis infection in the HIV-negative population, such as those with hematologic malignancies and those who have undergone transplantation, has risen significantly. Investigating host defense mechanisms against P. carinii has historically been problematic due to the difficulty in achieving continuous in vitro propagation of proliferating Pneumocytis organisms. Nevertheless, clinical and experimental studies have documented that host defense against Pneumocystis involves a concerted effort between innate, cell-mediated (T lymphocyte) and humoral (B lymphocyte) responses. However, the pulmonary environment is a tissue site where heightened inflammatory responses can often lead to inflammation-mediated injury, thereby contributing to the pathogenesis of Pneumocystis infection. Accordingly, clearance of Pneumocystis from the pulmonary environment is dependent on a delicate equilibrium between the inflammatory response and immune-mediated clearance of the organism. Furthermore, innate and adaptive responses against Pneumocystis are strikingly similar to those against other medically-important fungi, thus providing additional evidence that Pneumocystis exists as a fungal organism.

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.

Passive immunization of neonatal mice against Pneumocystis carinii f. sp. muris enhances control of infection without stimulating inflammation

Pneumocystis carinii is an opportunistic fungal pathogen that causes life-threatening pneumonia in immunocompromised individuals. Infants appear to be particularly susceptible to infection with Pneumocystis. We have previously shown that there is a significant delay in clearance of the organisms from the lungs of neonatal mice compared to adults. Since alveolar macrophages are the effector cells responsible for killing and clearance of Pneumocystis, we have examined alveolar macrophage activity in neonatal mice. We found that alveolar macrophage activation is delayed about 1 week in Pneumocystis-infected neonates compared to adults. Opsonization of the organism by Pneumocystis-specific antibody resulted in increased clearance of the organism in neonatal mice; however, there was decreased expression of activation markers on neonatal alveolar macrophages and reduced levels of cytokines associated with macrophage activation. Mice born to immunized dams had significant amounts of Pneumocystis-specific immunoglobulin G in their lungs and serum at day 7 postinfection, whereas mice born to naive dams had merely detectable levels. This difference correlated with enhanced Pneumocystis clearance in mice born to immunized dams. The increase in specific antibody, however, did not result in significant inflammation in the lungs, as no differences in numbers of activated CD4+ cells were observed. Furthermore, there was no difference in cytokine or chemokine concentrations in the lungs of pups born to immune compared to naive dams. These findings indicate that specific antibody plays an important role in Pneumocystis clearance from lungs of infected neonates; moreover, this process occurs without inducing inflammation in the lungs.

Defective macrophage function in neonates and its impact on unresponsiveness of neonates to polysaccharide antigens

Neonates do not respond to thymus-independent (TI) antigens (Ag), making them vulnerable to infection with encapsulated bacteria. The antibody (Ab) response of adult and neonatal B cells to TI Ag requires certain cytokines, which are provided by T cells or macrophages (MPhi). Lipopolysaccharide (LPS) failed to induce neonatal MPhi to produce interleukin (IL)-1beta and tumor necrosis factor alpha (TNF-alpha) mRNA and to secrete IL-1beta, IL-12, and TNF-alpha. However, LPS induced neonates to secrete some IL-6 and three- to fivefold more IL-10 than adults. Accordingly, adding adult but not neonatal MPhi could restore the response of purified adult B cells to trinitrophenol (TNP)-LPS, a TI Ag. Increased IL-10 is causally related to decreased IL-1beta and IL-6 production, as IL-10(-/-) neonatal MPhi responded to LPS by secreting more IL-1beta and IL-6 than wild-type (WT) neonatal MPhi. When cultures were supplemented with a neutralizing Ab to IL-10, WT neonatal MPhi secreted increased amounts of IL-6 and allowed neonatal MPhi to promote adult B cells to mount an Ab response against TNP-LPS. Thus, neonates do not respond to TI Ag as a result of the inability of neonatal MPhi to secrete cytokines, such as IL-1beta and IL-6, probably as a result of an excess production of IL-10. This dysregulated cytokine secretion by neonatal MPhi may be a result of a reduction in expression of Toll-like receptor-2 (TLR-2) and TLR-4 and CD14.

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.

Experimental models of pulmonary infection

Experimental models of pulmonary infection are being discussed, focused on various aspects of good experimental design, such as choice of animal species and infecting strain, and route of infection/inoculation techniques (intranasal inoculation, aerosol inoculation, and direct instillation into the lower respiratory tract). In addition, parameters to monitor pulmonary infection are being reviewed such as general clinical signs, pulmonary-associated signs, complication of the pulmonary infection, mortality rate, and parameters after dissection of animals. Examples of pulmonary infection models caused by bacteria, fungi, viruses or parasites in experimental animals with intact or impaired host defense mechanisms are shortly summarized including key-references.

IL-10 modulates host responses and lung damage induced by Pneumocystis carinii infection

Host responses to Pneumocystis carinii infection mediate impairment of pulmonary function and contribute to the pathogenesis of pneumonia. IL-10 is known to inhibit inflammation and reduce the severity of pathology caused by a number of infectious organisms. In the present studies, IL-10-deficient (IL-10 knockout (KO)) mice were infected with P. carinii to determine whether the severity of pathogenesis and the efficiency of clearance of the organisms could be altered in the absence of IL-10. The clearance kinetics of P. carinii from IL-10 KO mice was significantly enhanced compared with that of wild-type (WT) mice. This corresponded to a more intense CD4(+) and CD8(+) T cell response as well as an earlier neutrophil response in the lungs of IL-10 KO mice. Furthermore, IL-12, IL-18, and IFN-gamma were found in the bronchoalveolar lavage fluids at earlier time points in IL-10 KO mice suggesting that alveolar macrophages were activated earlier than in WT mice. However, when CD4(+) cells were depleted from P. carinii-infected IL-10 KO mice, the ability to enhance clearance was lost. Furthermore, CD4-depleted IL-10 KO mice had significantly more lung injury than CD4-depleted WT mice even though the intensity of the inflammatory responses was similar. This was characterized by increased vascular leakage, decreased oxygenation, and decreased arterial pH. These data indicate that IL-10 down-regulates the immune response to P. carinii in WT mice; however, in the absence of CD4(+) T cells, IL-10 plays a critical role in controlling lung damage independent of modulating the inflammatory response.

Neonatal T cells in an adult lung environment are competent to resolve Pneumocystis carinii pneumonia

Initiation of the pulmonary inflammatory response to Pneumocystis carinii is delayed by 3 wk in mice infected as neonates compared with adults. There was no difference in the proliferative response of draining lymph node T cells from mice infected as neonates compared with adults when stimulated in vitro with either Con A or anti-CD3 mAB: However, TNF-alpha and IFN-gamma mRNA expression in the lungs of P. carinii-infected neonates was significantly lower than in adults indicating a lack of appropriate activation signaling in the local environment. This may have been due to active suppression because TGF-beta mRNA expression was significantly elevated in neonatal lungs compared with adults. To determine whether T cells from 10-day-old mice would effect resolution of P. carinii if harbored in an adult lung environment, cells were adoptively transferred to SCID mice with established P. carinii infections. There was no difference in the kinetics of T cell migration into the lungs or of clearance of P. carinii organisms when SCID mice were reconstituted with splenocytes from young mice as compared with adult mice. Furthermore, splenocytes from young mice stimulated both TNF-alpha and IFN-gamma mRNA expression to levels that were similar to that in the lungs of SCID mice reconstituted with adult cells. These data indicate that neonatal lymphocytes are competent to resolve P. carinii infection when harbored in an adult lung environment, suggesting that the neonatal lung environment, and not the T cells, is ineffective at responding to P. carinii infection.