Elevated CO₂suppresses specific Drosophila innate immune responses and resistance to bacterial infection

Abstract

Elevated CO2 levels (hypercapnia) frequently occur in patients with obstructive pulmonary diseases and are associated with increased mortality. However, the effects of hypercapnia on non-neuronal tissues and the mechanisms that mediate these effects are largely unknown. Here, we develop Drosophila as a genetically tractable model for defining non-neuronal CO 2 responses and response pathways. We show that hypercapnia significantly impairs embryonic morphogenesis, egg laying, and egg hatching even in mutants lacking the Gr63a neuronal CO 2 sensor. Consistent with previous reports that hypercapnic acidosis can suppress mammalian NF-B-regulated innate immune genes, we find that in adult flies and the phagocytic immune-responsive S2* cell line, hypercapnia suppresses induction of specific antimicrobial peptides that are regulated by Relish, a conserved Rel/NF-B family member. Correspondingly, modest hypercapnia (7-13%) increases mortality of flies inoculated with E. faecalis, A. tumefaciens, or S. aureus. During E. faecalis and A. tumefaciens infection, increased bacterial loads were observed, indicating that hypercapnia can decrease host resistance. Hypercapnic immune suppression is not mediated by acidosis, the olfactory CO 2 receptor Gr63a, or by nitric oxide signaling. Further, hypercapnia does not induce responses characteristic of hypoxia, oxidative stress, or heat shock. Finally, proteolysis of the Relish IB-like domain is unaffected by hypercapnia, indicating that immunosuppression acts downstream of, or in parallel to, Relish proteolytic activation. Our results suggest that hypercapnic immune suppression is mediated by a conserved response pathway, and illustrate a mechanism by which hypercapnia could contribute to worse outcomes of patients with advanced lung disease, who frequently suffer from both hypercapnia and respiratory infections.COPD ͉ hypercapnia ͉ Relish ͉ NF-B ͉ Gr63a

NF-κB Links CO2 Sensing to Innate Immunity and Inflammation in Mammalian Cells

“…3). Our findings are consistent with recent studies in Drosophila that provide evidence for hypercapniamediated immunosuppression independent of acidosis (9).…”

“…NF-kB is a key transcriptional regulator of inflammation and innate immunity (11). Recent studies have implicated altered NF-kB signaling in the attenuation of inflammatory gene expression by hypercapnic acidosis and in the regulation of the Drosophila NF-kB homolog Relish by CO 2 (9,12). In this study, we describe a CO 2 -dependent nuclear localization of the NF-kB signaling protein IKKa.…”

“…Similarly, hypercapnic acidosis attenuates endotoxin-induced acute lung injury, supporting a generally anti-inflammatory effect of CO 2 (8). Furthermore, elevated CO 2 increases mortality in Drosophila exposed to pathogens and augments infection-induced injury in rats, indicating a role for CO 2 -sensing in suppression of innate immunity (9,10). NF-kB is a key transcriptional regulator of inflammation and innate immunity (11).…”

The physiological and molecular effects of elevated CO2levels

“…Unlike in C. elegans, CO 2 levels up to 13% do not affect lifespan of fruit flies, suggesting that fly health is not globally affected. 40 However, at 13% CO 2 adult females lay fewer eggs and embryonic development significantly slows down, with some embryos showing morphological abnormalities. At 20% CO 2 , females lay almost no eggs and embryonic development is severely disrupted.…”

“…Since patients with lung diseases suffer from frequent infections, these data suggest that hypercapnic immune suppression is mediated by a conserved response pathway by which hypercapnia could contribute to worse outcomes of these patients. 40 Similar to C. elegans, increased CO 2 levels cause an avoidance behavior in D. melanogaster adults. 43 This behavior is dose-dependent, with flies avoiding CO 2 concentrations of 0.1% and above.…”