To the Editor:
Environmental exposures, including pollutants, allergens, and microbes
may trigger asthma exacerbations.1 Inhaled exposures
interact with airway epithelial cells, which then activate the innate
immune system resulting in the release of DAMPs (danger-associated
molecular patterns) and cytokines via pattern recognition receptor
(PRRs), including TLRs. The activated immune system further orchestrates
inflammation and Th2 immune responses in the
airway.2,3 Exposure combinations may potentiate
exacerbations. In patients with asthma, eosinophilic lung inflammation
in response to house dust mite (HDM) allergen may be further amplified
by concomitant bacterial lipopolysaccharide (LPS)
exposure.4 Transcriptomic responses to environmental
stimuli (both individually and in combination), may uncover gene targets
relevant for exposures, with implications for asthma pathogenesis and
gene by environment interaction studies.
We performed transcriptomic analyses of well-differentiated primary
human bronchial epithelial cells (HBECs) from 3 donors with fatal asthma
and 3 without asthma (Supplemental Table 1 ). We stimulated
HBECs with HDM, LPS, or their combination. To identify differential
expression (stimulus vs. vehicle), we performed RNA sequencing (RNA-seq)
(Supplemental Methods ) and used DESseq2 (R statistical
software) for our statistical analyses.
In all subjects, the combination of HDM and LPS exhibited the greatest
number of differentially expressed (DE) genes (adjusted p
<0.05) (2,103 genes), followed by LPS (854 genes) and then HDM
(46 genes) (Figure 1A, 1K ). The majority (63%) of DE genes
with the combined exposure were not observed by either HDM or LPS alone
(Figure 1A) . For HDM alone, the top-ranked DE genes included
NOD-like receptor pathway (IL-8 ), ribosomal (RPS5, RPL37 ),
and histone genes (HIST1H4E , HIST1H4C)(Supplemental Table 2) . For LPS alone, the top-ranked DE genes
included IL-17 signaling (IL17C) , NFKB (BCL2A1 ), nitric
oxide synthesis (NOS2 ), and NOD-like receptor signaling genes
(CCL2 ) (Supplemental Table 3). For the combination of
HDM and LPS, the top-ranked DE genes overlapped with the results for
each stimulus and then extended to ubiquitin oxidoreductases and
mitochondrial encoded cytochromes pathways. (Supplemental Table
4 , Figure 1F-1J )
Next, to identify networks, we mapped DE genes onto protein-protein
interactions using the String Interactome feature in Network Analyst
(www.Networkanalyst.ca)
(Figure 1B-1I ). Compared to the HDM-specific network
(Figure 1B ), the LPS-specific network (Figure 1C )
contained a greater number of nodes and included hubs for NFKB1,
SRC, RAC1 , and CTNNB1 . By far, the most complicated network was
for the combined exposure (Figure 1D-1E ).
When stimulated, HBECs from donors with asthma (vs. non-asthma)
demonstrated a much larger number of DE genes, and much denser, more
highly connected networks with additional gene hubs (Figure
2A-2D) . For asthmatic HBECs (vs. non-asthmatic HBECs), HDM produced 315
DE genes (vs. 59), LPS 277 genes (vs. 119), and the combination of HDM +
LPS 1,709 genes (vs. 335). The HDM-specific network in asthmatic cells
showed immune regulatory genes such as TNFAIP3, NFKBIA , andFAT1 , none which were nodes in the HDM-specific network for
non-asthmatic cells (Fig 2A ). In the LPS-specific networks,
both asthmatic and non-asthmatic cells had nodes for NFKB1,
NFKB1A and TNFAIP3 represented; however, the hub JAK2 is
only observed for asthmatic cells (Fig 2B ).
We performed two validation studies. First, qPCR confirmed differential
expression of select top genes (Supplemental Figure 1 ). Second,
we performed another set of exposure experiments in HBECs from
additional donors with non-fatal asthma and without asthma and performed
qPCR analysis, which also produced similar results (Supplemental
Figure 2 ). These validations confirm generalizability of our results
across asthma phenotypes (i.e. HBECs from fatal asthma and non-fatal
asthma responded similarly).
Our findings provide evidence that the combined exposure to
environmental triggers of asthma exacerbation increases the number of DE
genes and the complexity of transcriptomic responses in HBECs as
compared to individual stimuli. Moreover, cells from donors with asthma
are more sensitive to environmental asthma triggers, eliciting a greater
number of DE genes. While our data should be confirmed in a larger
cohort, they serve as a solid basis for future studies of the effect of
multiple exposures (i.e. the exposome) on molecular signaling in the
bronchial epithelium during asthma exacerbations.
References
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- Berger M, de Boer JD, Bresser P, van der Poll T, Lutter R, Sterk PJ.
Lipopolysaccharide amplifies eosinophilic inflammation after segmental
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