Maternal exposure to ultrafine particles enhances influenza infection during pregnancy

Abstract

Background: Interactions between air pollution and infectious agents are increasingly recognized and critical to identify, especially to protect vulnerable populations. Pregnancy represents a vulnerable period for influenza infection and air pollution exposure, yet interactions during pregnancy remain unclear. Maternal exposure to ultrafine particles (UFPs, [Formula: see text] 100 nm diameter), a class of particulate matter ubiquitous in urban environments, elicits unique pulmonary immune responses. We hypothesized that UFP exposure during pregnancy would lead to aberrant immune responses to influenza enhancing infection severity.

Results: Building from our well-characterized C57Bl/6N mouse model employing daily gestational UFP exposure from gestational day (GD) 0.5-13.5, we carried out a pilot study wherein pregnant dams were subsequently infected with Influenza A/Puerto Rico/8/1934 (PR8) on GD14.5. Findings indicate that PR8 infection caused decreased weight gain in filtered air (FA) and UFP-exposed groups. Co-exposure to UFPs and viral infection led to pronounced elevation in PR8 viral titer and reduced pulmonary inflammation, signifying potential suppression of innate and adaptive immune defenses. Pulmonary expression of the pro-viral factor sphingosine kinase 1 (Sphk1) and pro-inflammatory cytokine interleukin-1β (IL-1 [Formula: see text]) was significantly increased in pregnant mice exposed to UFPs and infected with PR8; expression correlated with higher viral titer.

Conclusions: Results from our model provide initial insight into how maternal UFP exposure during pregnancy enhances respiratory viral infection risk. This model is an important first step in establishing future regulatory and clinical strategies for protecting pregnant women exposed to UFPs.

Keywords: Air pollution; Infection; Influenza; Mouse model; Particulate matter; Pregnancy; Ultrafine particles.

Fig. 1

Experimental design and phenotypic outcomes. A Experimental timeline indicating the acclimation and mating strategy, exposure duration to FA or UFPs, and viral infection window. B Percent weight gain between FA (gray) and UFP (blue) exposure groups post-inoculation. C Viral titer assessment via qPCR of the Influenza A segment 7 3’ splice site. D Viral titer assessment via TCID50. Data were analyzed using two-way ANOVA with Tukey’s multiple comparison test (*p < 0.05; **p < 0.002; ***p < 0.0002)

Fig. 2

Pulmonary histopathology. Two histological slides from a longitudinal section from the lungs including the mainstem bronchi and both right and left side lung lobes were evaluated for each animal. Representative area of average scores is shown (photographed at 20X magnification). A Dam exposed to FA and inoculation with HI, showing no histological lesions (avg. score = 0). B Dam exposed to PM and inoculated with HI, showing no histological lesions (avg. score = 0.33). C Dam exposed to FA and inoculated with PR8, showing moderate perivascular and peribronchiolar lymphoplasmacytic, histolytic and neutrophilic inflammation denoted by arrows (avg. score = 1.66). D Dam exposed to PM and inoculated with live PR8, showing minimal numbers of peribronchial lymphocytes (avg. score = 0). E Pathological scoring of lungs in each exposure group

Fig. 3

Pulmonary T cell profiles from dissociated lung tissue. A CD8 + T cell counts, showing noticeable reduction in the UFP-PR8 group compared to all other groups. B CD4 + T cell counts. C Count of CD4 + T cells belonging to the Th1 lineage (IFN- +). D Count of CD4 + T cells belonging to the Th2 lineage (IL-4 +). E Th1/Th2 cell ratio in the lung, used as an indicator of immune system bias. F Count of CD4 + T cells belonging to the Th17 lineage (IL-17 +), showing a slight reduction in the UFP-PR8 group compared to the FA-PR8 group

Fig. 4

Regulation of pro-viral and pro-inflammatory genes A Expression of Sphk1 between exposure groups, with the highest expression UFP-PR8 group; correlation analysis showed a strong positive correlation with viral titer (R² = 0.8366). B Expression of Il-1β between exposure groups, with the highest expression in the UFP-PR8 group; correlation analysis showed a moderate positive correlation with viral titer (R² = 0.6287). C Expression of Irf7 between exposure groups, with no significant changes or correlations with viral titer (R² = 0.02653). D Expression of Tgf-β between exposure groups with no significant changes or correlations (R² = 0.1602). E Expression of Il-33 between groups with no significant changes or correlations with viral titer (R² = 0.1052)

Fig. 5

Schematic depicting the sources of UFPs and the effects on influenza infection severity observed in our model. In urban environments, high number concentrations of UFPs are directly emitted into the atmosphere from traffic and industrial sources and/or produced from new particle formation relevant to photochemical oxidation, which is initiated by the hydroxyl radical (OH) or ozone (O₃), involving volatile organic compounds (VOCs) and sulfur dioxide (SO₂) in the presence of nitrogen oxides (NO_x = NO + NO₂) and ammonia (NH₃)^4,20–22. Our mouse model illustrates four major adverse health effects for aggravated respiratory infection from UFP exposure for pregnant women: (1) reduced weight gain, (2) reduced pulmonary immune responses, (3) elevated viral titer, and (4) enhanced pro-viral and pro-inflammatory gene expression