These results suggest that exposures to A. fumigatus prior to and during pregnancy were associated with diminished IgE production and airway eosinophilia. The latter occurred following prenatal exposure to both A. fumigatus and diesel. The parallel increases in IgG levels suggest that the antibody responses were specific to IgE. These findings indicate that prenatal exposure to A. fumigatus, may be associated with protection from systemic and airway allergic immune responses in adult offspring.
While these results may appear contradictory to several studies that show prenatal sensitization (i.e. to ovalbumin) is associated with greater allergic immune responses in the offspring , they are consistent with a few studies that suggest prenatal environmental exposures can suppress the subsequent risk for an asthma-related phenotype or induce tolerance. For example, transfer of antigens from mother to mouse pup via breast milk has induced oral tolerance and antigen-specific protection from allergic airway disease . In addition, prenatal sensitization to D. pteroynissinus was associated with lower total and D. pteroynissinus-IgE levels in the offspring. Similar to our model, exposure to allergen prior to mating reduced allergen sensitization in the offspring at the humoral level . In more recent work by the same group, the induction of allergic sensitization versus tolerance following prenatal exposure to ovalbumin was determined to be dependent on the dose and timing of exposure. Specifically, prenatal oral exposure to high dose ovalbumin was associated with lower ovalbumin-IgE in the pups at age 3 days following immunization. The effect was transient, and subsequent increases in ovalbumin-IgE levels were detected at age 25 days. Also, the effect was best observed if the ovalbumin treatments occurred during the first week of pregnancy. However, pups born to mothers who received prenatal oral administration of low dose ovalbumin showed similar decreases in ovalbumin IgE levels and antigen-specific T cell proliferation, but this tolerogenic effect was more sustained. Prenatal LPS exposure also was associated with suppression of IgG1 and IgE and reduction of interleukin (IL)-5 and IL-13 in splenic mononuclear cells . Besides endotoxin, prenatal oral exposure to the chemical bisphenol A has been associated with preferential T helper (Th) 1 immune responses in sensitized adult offspring mice . Hence, in the model described here, prenatal exposure to A. fumigatus and diesel may have timed or dosed so as to favor establishment of tolerance instead of allergic sensitization.
While postnatal exposure to mold has been associated with greater asthma severity or emergency room visits for asthma [24–27], recent studies suggest that exposure to mold allergen after birth may be protective. These include two cross sectional cohort studies that found higher levels of fungal β(1,3)-glucans, fungal extracellular polysaccharides and endotoxin in dust collected from mattresses used by asymptomatic children age 5-13 compared with those used by atopic children who wheezed [28, 29]. It has been hypothesized that fungal products besides the associated allergens, such as dust endotoxin, extracellular polysaccharides (EPS) and glucans may induce immunologic protection from the development of atopic disease [28, 29]. As another example, inner-city children, aged 2 to 6 years old, living in homes with either comparatively low ( < 2 μg/g Mus m 1) or high (> 29.9 μg/g Mus m 1) dust levels of mouse allergen developed attenuated humoral responses in comparison to those who lived in homes with a medium level of measured allergen in their dust (2-7.9 μg/g Mus m 1). This work also suggests that the development of protection from allergic sensitization occurs and may be related to the dose of allergen exposure. However, the extent of allergic sensitization, rather than the measured level of allergen detected in dust or delivered via aerosol, tends to be more strongly associated with allergy symptoms in an inner-city cohort study .
EPA has estimated occupational DEP exposures to range from 39 - 191 μ g/m3 for railroad workers, 4 - 748 μ g/m3 for firefighters, and 7 - 98 μ g/m3 for public transit workers and airport crews . So while the chronic administration of inhaled diesel exhaust particles may have mimicked some natural physiological conditions in this model, the levels employed are higher than most urban environments and some occupational ones. Diesel exposure has been associated with upregulation of the allergic immune responses and airway remodeling in both animal and human studies [32, 33]. However, the independent and synergistic effects of prenatal diesel exposure administered in this manner and reported here seem small. Adult offspring from mothers who received DEP alone, or A. fumigatus and DEP together, developed lower levels of total IgE, and greater levels of IgG1, when assessed after the fifth and sixth dose of allergen. Paradoxically, after the third dose of A. fumigatus, a reduction in IgG2a was detected among offspring from mice exposed to DEP compared with those treated with saline. Also, adult offspring of mothers that received both A. fumigatus and DEP developed significantly less airway eosinophilia compared to offspring of mothers that had received A. fumigatus alone. Combined, these results suggest that prenatal DEP exposure independently may have conferred some protection against allergic immune responses in the adult offspring in this model. These findings were unexpected, especially given previous research using the engine byproduct residual oil fly ash as the air pollutant that induced greater airway eosinophilia and hyperreactivity in the OVA-sensitized offspring . It is unclear whether the disparate phenotypes are related to the antigens administered (ovalbumin vs. A. fumigatus), components and dose of the air pollutants, strain of mouse, or age of the offspring following allergen sensitization (less than 5 weeks vs. 9-10 weeks).
Associations between prenatal exposure to DEP and/or A. fumigatus and airway arterial remodeling in adult offspring were not statistically significant, with only mild changes detected during histological examination. Exposure to A. fumigatus has been shown to exacerbate an asthma phenotype in rats by aggravating Th2 inflammation, increasing AHR, and inducing airway remodeling . Previously, a few mouse models have induced airway remodeling following repeated and chronic OVA exposure and the recruitment of eosinophils, IL-13 and profibrotic cytokines have been implicated [35–37]. Our group previously showed that adult C57BL/6 mice treated intermittently with A. fumigatus for a prolonged period of time developed remodeling of small to medium sized pulmonary arteries . In another mouse model, maternal exposure to cigarette smoke during pregnancy was found to be associated with airway remodeling in the offspring at ten weeks of age, as demonstrated by increases in airway smooth muscle thickness, collagen deposition and house dust mite induced increases in neutrophils, mast cells and goblet cell hyperplasia . From a cohort study, offspring of mothers who smoked during pregnancy developed permanent vascular damage that was not apparent in offspring of non-smoking mothers . This current study, to our knowledge, represents the first examination of the effects of these environmental exposures on airway remodeling across generations of mice.
Several plausible mechanisms may explain how prenatal exposures may help modulate the development of allergic and/or airway immune response in the offspring. It has been reported that antigen-specific T cell and B cell immune responses in the fetus can occur distinctly from those of the mother, as demonstrated by our group in response to vaccination against influenza . In addition, previous reports also suggested that the transfer of allergy across the placenta may be regulated by the transfer of cytokines that may influence the development of allergic sensitization. Supportive data include a murine model that demonstrates that administration of anti-IL-4 can inhibit allergic immune responses from sensitization to OVA in the offspring . In addition, combined inhaled diesel exhaust and A. fumigatus exposure has been shown to induce hypermethylation of multiple CpG sites of the interferon-gamma (IFNg) promoter and hypomethylation of one CpG site of the IL-4 promoter with associated changes in IgE levels, suggestive of the contribution of epigenetic regulation following environmental exposures . These mechanisms seem plausible in light of recent associations between prenatal exposure to polycyclic aromatic hydrocarbons or high methyl diet and DNA methylation of asthma candidate genes [6, 42]. However, these reports do not directly explain mechanistically how prenatal exposure to A. fumigatus, or diesel, may induce protection from allergy in the offspring, especially in light of past data that suggest A. fumigatus induces greater, not repressed, Th2 cytokine production . In one study, offspring of Balb/c mice whose mothers were tolerized with ovalbumin by means of oral application of antigen also were protected from the development of an asthma-like phenotype as late as 8 month after birth. This protection was blocked by inhibition of IFNγ . Transfer of IgG antibodies from suckling or from the placenta has also been shown to suppress IgE following prenatal exposures to egg albumin . Rats whose mothers were immunized with egg albumin during pregnancy experienced a diminished capacity to develop IgE and enhanced IgG responses during early adulthood, and these results were replicated when separate offspring were administered small quantities of immune serum 3 weeks after birth.
Some limitations of this study merit discussion. First, we used only one strain of mice to obtain the above findings even though it has been shown that when comparing acute injury responses, such as airway remodeling, patterns are unique to different strains. Also, the use of a mouse model does not give us a comprehensive representation of what occurs after prenatal sensitization in humans because we are not able to accurately replicate some human behaviors such as smoking and diet. Relatively higher inflammation scores among retired mothers and their adult offspring are difficult to explain, but could be a result of accumulated lung injury due to dust from dirty bedding in breeder cages or stress (personal observation).