Rat strains differ in susceptibility to maternal and fetal infection with Mycoplasma pulmonis

Nicotine induces maternal and fetal inflammatory responses which predispose intrauterine infection risk in a rat model

Introduction

Both smoking and infection adversely impact pregnancy. Previously, our group identified in a rodent model that 6 mg/kg/d nicotine increased the risk of fetal infection at gestation day (GD) 18. Here, we investigate lower nicotine doses.

Methods

Pregnant Sprague-Dawley rats received nicotine infusion at 0, 1, or 3 mg/kg/d (no, low-, and mid-dose nicotine, respectively) from GD 6, with intravenous inoculation with Mycoplasma pulmonis (MP) at 10⁷ CFU (N = 20) or sterile broth (sham) (N = 11) on GD 14. Uterus and fetuses were retrieved on GD 18 for MP culture and histopathologic evaluation of maternal and fetal inflammatory responses (MIR and FIR).

Results

At 1 mg/kg/d nicotine, MP colonization rates were decreased, from 100% (9 of 9) to 40% (2 of 5) of MP-inoculated dams (p = .03), and 59% (66 of 111) to 39% (24 of 62) of fetuses (p = .01), versus no nicotine. Low-dose nicotine resulted in increased MIR and FIR in the sham-inoculated group; in the MP-inoculated group, this resulted in reduced relative risk (RR) for placental colonization (RR, 95% CI with high MIR = 0.14, 0.02 to 0.65; FIR = 0.38, 0.12 to 0.93). In contrast, 3 mg/kg/d nicotine treatment did not alter colonization rates; furthermore, FIR was completely suppressed, even in the face of placental or amniotic fluid colonization.

Conclusion

The 1 mg/kg/d nicotine dose decreased risk of intrauterine infection, with increased MIR and FIR. The 3 mg/kg/d nicotine dose inhibited FIR, and increased risk for intrauterine infection. Nicotine alterations of the intrauterine environment were markedly dose-dependent.

Implications

Nicotine exposure alters intrauterine infection and inflammation in a dose-dependent manner, potentially impacting fetal development and programming. Previous work in a rodent model showed that high-dose nicotine (6 mg/kg/d) exposure exacerbated intrauterine infection during pregnancy. The current study found that low-dose nicotine (1 mg/kg/d) exposure reduced colonization of placenta and amniotic fluid; this decrease was associated with increased intrauterine inflammation. Exposure to mid-dose nicotine (3 mg/kg/d) suppressed fetal inflammation. Elucidation of underlying mechanisms of these phenomena will inform public health and clinical care decisions, particularly in the context of risk assessment of nicotine replacement therapy during pregnancy for smoking cessation.

Prenatal infection with Mycoplasma pulmonis in rats exaggerates the angiotensin II pressor response in adult offspring

Exposure to different stressors in utero is linked to adult diseases such as obesity and hypertension. In this study, the impact of prenatal infection (PNI) on adult body weight and cardiovascular function was evaluated using a naturally occurring rodent pathogen, Mycoplasma pulmonis (MP). Pregnant Sprague-Dawley rats were infected with MP on gestational day 14 and gave birth naturally. Adult PNI offspring weighed more than controls, but resting mean arterial pressure (MAP) was unchanged. Subcutaneous injection of angiotensin II (10 μg/kg) elicited a rise in MAP that was greater in both male and female PNI offspring compared with controls (P < 0.03). The accompanying reflex bradycardia was similar to the controls, suggesting that PNI induced baroreflex dysfunction. Subcutaneous nicotine administration, a potent cardiorespiratory stimulus, also elicited a transient rise in MAP that was generally greater in the PNI group, but the change in MAP from baseline was only significant in the PNI females compared with controls (P < 0.03). Elevated body weight and cardiovascular reactivity in the PNI offspring was associated with an increase in the ratio of hypothalamic corticotrophin-releasing hormone receptors type 1 to type 2 gene expression in both sexes compared with controls. These findings support previous studies demonstrating that PNI induces alterations in cardiovascular function and body weight. Yet, unlike previous studies utilizing other models of PNI (e.g., endotoxin), MP PNI did not induce resting hypertension. Thus, our study provides a foundation for future studies evaluating the cardiovascular risks of offspring exposed to microbial challenges in utero.

Impact of gestational nicotine exposure on intrauterine and fetal infection in a rodent model

We investigated the interaction between prenatal nicotine exposure and intrauterine infection using established rat models. Beginning at gestation day (GD) 6, dams were continuously infused with either saline or 6 mg/kg/day nicotine (Nic). At GD 14, dams received either sterile broth or 10⁵ colony-forming units Mycoplasma pulmonis (MP), resulting in four treatment groups: control (4 dams, 33 fetal units); MP only (5 dams, 55 fetal units); Nic only (5 dams, 61 fetal units), and Nic + MP (7 dams, 82 fetal units). At GD 18, nicotine exposure significantly increased (P ≤ 0.02) the percentage of amniotic fluids and fetuses infected by MP but did not impact colonization rates of maternal sites. Nicotine exposure significantly reduced the numbers of MP in the placenta required for high microbial loads (≥10⁴ color-changing units) in the amniotic fluid (P < 0.01). Fetal inflammatory response lesions were most extensive in the Nic only and Nic + MP groups (P < 0.0001). Control and MP only placentas were interleukin (IL)10-dominant, consistent with an M2/Th2 environment. Placentas exposed to nicotine shifted to a neutral environment, with equivalent levels of interferon gamma (IFNG) and IL10. Both IL6 and tumor necrosis factor (TNF) levels in amniotic fluid were highly elevated when both nicotine and infection were present. Our study suggests that prenatal exposure to nicotine increases the risk for intrauterine infection, lowers the infectious dose required to breach the placental barrier and infect the amniotic fluid and fetus, and alters the pathology and inflammatory profile associated with maternal and fetal sites.

Impact of the Genetics and Source of Preclinical Safety Animal Models on Study Design, Results, and Interpretation

It has been long established that not only the species but also the strain and supplier of rodents used in preclinical safety studies can have a significant impact on the outcome of studies due to variability in their genetic background and thus spontaneous pathologic findings. In addition, local husbandry, housing, and other environmental conditions may have effects on the development and expression of comorbidities, particularly in longer-term or chronic studies. More recently, similar effects related to the source, including genetic and environmental variability, have been recognized in cynomolgus macaques ( Macaca fascicularis). The increased use of cynomolgus macaques from various sources of captive-bred animals (including nonnative, U.S./European Union-based breeding facilities or colonies) can affect study design and study results and outcome. It is important to acknowledge and understand the impact of this variability on the results and interpretation of research studies. This review includes recent examples where variability of preclinical animal models (rats and monkeys) affected the postmortem observations highlighting its relevance to study design or interpretation in safety studies.

Microbiological Quality Control for Laboratory Rodents and Lagomorphs

Mice (Mus musculus), rats (Rattus norvegicus), other rodent species, and domestic rabbits (Oryctolagus cuniculus) have been used in research for over 100 years. During the first half of the 20th century, microbiological quality control of lab animals was at best rudimentary as colonies were conventionally housed and little or no diagnostic testing was done. Hence, animal studies were often curtailed and confounded by infectious disease (Mobraaten and Sharp, 1999; Morse, 2007; Weisbroth, 1999). By the 1950s, it became apparent to veterinarians in the nascent field of comparative medicine that disease-free animals suitable for research could not be produced by standard veterinary disease control measures (e.g., improved sanitation and nutrition, antimicrobial treatments) in conventional facilities. Henry Foster, the veterinarian who founded Charles River Breeding Laboratories in 1948 and a pioneer in the large-scale production of laboratory rodents, stated in a seminar presented at the 30th anniversary of AALAS, “After a variety of frustrating health-related problems, it was decided that a major change in the company’s philosophy was required and an entirely different approach was essential”. Consequently, he and others developed innovative biosecurity systems to eliminate and exclude pathogens (Allen, 1999). In 1958, Foster reported on the Cesarean-originated barrier-sustained (COBS) process for the large-scale production of specific pathogen-free (SPF) laboratory rodents (Foster, 1958). To eliminate horizontally transmitted pathogens, a hysterectomy was performed on a near-term dam from a contaminated or conventionally housed colony. The gravid uterus was pulled through a disinfectant solution into a sterile flexible film isolator where the pups were removed from the uterus and suckled on axenic (i.e., germ-free) foster dams. After being mated to expand their number and associated with a cocktail of nonpathogenic bacteria to normalize their physiology and prime their immune system, rederived rodents were transferred to so-called barrier rooms for large-scale production. The room-level barrier to adventitious infection entailed disinfection of the room, equipment, and supplies, limiting access to trained and properly gowned personnel, and the application of new technologies such as high-efficiency particulate air-filtration of incoming air (Dubos and Schaedler, 1960; Foster, 1980; Schaedler and Orcutt, 1983; Trexler and Orcutt, 1999). The axenic and associated rodents mentioned in the COBS process are collectively classified as gnotobiotic to indicate that they have a completely known microflora. By contrast, barrier-reared rodent colonies are not gnotobiotic because they are housed in uncovered cages and thus acquire a complex microflora from the environment, supplies, personnel, and other sources. Instead, they are described as SPF to indicate that according to laboratory testing, they are free from infection with a defined list of infectious agents, commonly known as an ‘exclusion’ list.

Development of an ELISA using a recombinant P46-like lipoprotein for diagnosis of Mycoplasma pulmonis infection in rodents

Mycoplasma pulmonis is one of the most prevalent bacterial pathogens that infects laboratory mice and rats. To develop an M. pulmonis-specific antigen for serological diagnosis, we cloned the cDNA of P46-like lipoprotein (P46L), an M. pulmonis putative periplasmic protein. P46L is a homolog of P46, an M. hyopneumoniae antigen. We produced recombinant P46L fused to glutathione S-transferase (GST) in Escherichia coli. Immunoblot analysis revealed that sera from Mycoplasma-infected mice and rats contained anti-P46L antibodies. We developed an ELISA using the recombinant P46L-GST protein as an antigen. Thirteen of the 14 samples from rats naturally infected with M. pulmonis were determined to be positive according to the commercial ELISA (MONILISA Myco) and positive by our ELISA. Furthermore, 18/19 samples from mice experimentally infected with M. pulmonis were positive using our P46L-GST ELISA. In contrast, only 8/19 samples from infected mice were positive by the commercial ELISA. Our results indicate that P46L-GST was an appropriate antigen for developing a serological test to determine M. pulmonis infection in laboratory mice and rats.

Host genetic background impacts disease outcome during intrauterine infection with Ureaplasma parvum

Ureaplasma parvum, an opportunistic pathogen of the human urogenital tract, has been implicated in contributing to chorioamnionitis, fetal morbidity, and fetal mortality. It has been proposed that the host genetic background is a critical factor in adverse pregnancy outcome as sequela to U. parvum intra-amniotic infection. To test this hypothesis we assessed the impact of intrauterine U. parvum infection in the prototypical TH1/M1 C57BL/6 and TH2/M2 BALB/c mouse strain. Sterile medium or U. parvum was inoculated into each uterine horn and animals were evaluated for intra-amniotic infection, fetal infection, chorioamnionitis and fetal pathology at 72 hours post-inoculation. Disease outcome was assessed by microbial culture, in situ detection of U. parvum in fetal and utero-placental tissues, grading of chorioamnionitis, and placental gene expression of IL-1α, IL-1β, IL-6, TNF-α, S100A8, and S100A9. Placental infection and colonization rates were equivalent in both strains. The in situ distribution of U. parvum in placental tissues was also similar. However, a significantly greater proportion of BALB/c fetuses were infected (P<0.02). C57BL/6 infected animals predominantly exhibited mild to moderate chorioamnionitis (P<0.0001), and a significant reduction in placental expression of IL-1α, IL-1β, IL-6, TNF-α, S100A8, and S100A9 compared to sham controls (P<0.02). Conversely, severe protracted chorioamnionitis with cellular necrosis was the predominant lesion phenotype in BALB/c mice, which also exhibited a significant increase in placental expression of IL-1α, IL-1β, IL-6, TNF-α, S100A8, and S100A9 (P<0.01). Fetal pathology in BALB/c was multi-organ and included brain, lung, heart, liver, and intestine, whereas fetal pathology in C57BL/6 was only detected in the liver and intestines. These results confirm that the host genetic background is a major determinant in ureaplasmal induced chorioamnionitis with fetal infection and fetal inflammatory response.

Effect of experimental genital mycoplasmosis on gene expression in the fetal brain

Neurodevelopmental disorders may have their origins during intrauterine development. We used a well-defined animal model to test whether hematogenous infection with genital mycoplasma would alter the expression of genes associated with autism spectrum disorders (ASD). In a preliminary experiment, rats were exposed at 14 days gestation (GD14) to Mycoplasma pulmonis or sterile broth and sacrificed at GD18. Infection and inflammation status of the pups was ascertained by culture and cytokine ELISA. Intra-cardiac injection of 10(6)CFU M. pulmonis resulted in amniotic infection of 100% of the pups and was accompanied by higher levels of IL-1β in amniotic fluids. In a second experiment, animals were infected in a similar manner but dams and their litters were sacrificed at GD18, GD21 or postpartum day 3 (PPD3). Expression of proinflammatory cytokines and neurodevelopmental genes in the fetal brains was evaluated. M. pulmonis infection significantly increased the expression of IL-1β, TNF-α and COX-2 in fetal and neonatal brains. Expression of GFAP and CD11b, markers for activation on astrocytes and microglial cells, respectively, was also increased for infected animals. M. pulmonis significantly increased SHANK-3 gene expression at GD21 and PPD3 and PCP-2 expression at GD21. No effect of M. pulmonis infection on Reelin, PTEN, BDNF or HGF was detected. These data suggest that M. pulmonis infection at GD14 increases the expression of proinflammatory genes in the perinatal brain. Further studies with earlier time-points of infection and ones that use behavioral outcomes are needed to better understand the potential role of genital mycoplasmosis on pychopathology.

Colonization of maternal and fetal tissues by Porphyromonas gingivalis is strain-dependent in a rodent animal model

OBJECTIVE

The objective of this study was to develop a rodent model of Porphyromonas gingivalis infection during pregnancy.

STUDY DESIGN

Sprague Dawley rats were infected intravenously with 10(5), 10(7), or 10(9) CFU per dam of P gingivalis strain W83, ATCC 33277, or A7436 at gestational day 14 and necropsied at gestational day 18. Maternal organs were cultured to assess the spread of the infection. Six fetal units (placenta, amniotic fluid, membranes, and fetus) per dam were cultured; additional fetal units were examined by histopathology. Polymerase chain reaction was performed on placentas.

RESULTS

Colonization rates were dependent on the strain of P gingivalis used and the infection dose. At an infection dose of 10(9) CFU/dam, P gingivalis W83, ATCC 33277, or A7436 was detected in 33%, 83%, or 100% of placentas, respectively. Epithelial hyperplasia, cellular necrosis, and inflammatory infiltrate were observed in infected placental tissues.

CONCLUSION

This study demonstrated that P gingivalis can invade both maternal and fetal tissues, resulting in chorioamnionitis and placentitis.

Experimental genital mycoplasmosis causes increased levels of mRNA for IL-6 and TNF-alpha in the placenta

PROBLEM

Previous studies with animal models have shown that injection of lipopolysaccharide (LPS) results in fetal loss and increases production of proinflammatory cytokines at the maternal-fetal interface. Most intrauterine infections, however, are associated with Ureaplasma urealyticum, a microorganism that lacks a cell wall and therefore does not contain LPS. Previous work in our laboratory with an animal model for genital infection with a similar organism, Mycoplasma pulmonis, revealed that widespread infection in maternal and fetal tissues can be experimentally induced with minimal manipulation of the animal. For this project, we tested the hypothesis that administration of the organism by a hematogenous route at gestational day (gd) 14 would result in increased tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 production by the placenta.

STUDY DESIGN

Timed-pregnant, Sprague-Dawley rats were anesthetized on gd 14 and 10(7) CFU of M. pulmonis strain X1048 or an equivalent volume of sterile medium was injected into the heart. Rats were necropsied on gd 18 or 21, and ex vivo production of TNF-alpha and IL-6 was evaluated from six randomly selected placentas from each litter. The remaining placentas were harvested and either snap-frozen or placed in formalin. Frozen placentas were processed for real-time reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of TNF-alpha and IL-6 mRNA. Formalin-fixed placentas were sectioned and stained with hematoxylin and eosin for lesion analysis.

RESULTS

Concentrations of TNF-alpha but not IL-6 were significantly higher in conditioned medium from placentas harvested from infected dams at gd 21. Levels of mRNA for IL-6 and TNF-alpha, however, were increased by M. pulmonis at gd 18 and 21. Analysis of gd 21 placentas by light microscopy revealed that significant histological chorioamnionitis was present in infected animals with accumulations of neutrophils in the capsular decidua.

CONCLUSION

These data indicate that experimental infection with M. pulmonis causes histological chorioamnionitis, elevated mRNA levels of TNF-alpha and IL-6 in placental tissues, and the secretion of TNF-alpha by the placenta during late gestation.