Nasal Exposure to Microcystin Can Trigger Immune Responses in the Lung and Gut Microbiome Dysbiosis via the Lung–Gut Axis: A Pilot Acute Mouse Study

Cyanotoxin events are among the most serious consequences of cyanobacterial harmful algal blooms. Microcystins (MCs), among the most prevalent cyanotoxins, are known to adversely affect human health. Recently, respiratory exposure has emerged as an important exposure route for MCs, with potential downstream effects on the gut microbiome through the lung–gut axis. In this pilot study, we investigated acute respiratory immune responses and gut microbiome alterations following MC inhalation using female C57BL/6J mice. Mice were assigned to three dose groups: control (0 µg/kg body weight), medium (25 µg/kg), and high (50 µg/kg). MC-LR was administered intranasally once daily for 3 days. Fecal samples were collected daily for 16S rRNA sequencing, and bronchoalveolar lavage (BAL) fluids were collected following sacrifice for immune cell analysis. MC exposure resulted in significantly increased monocyte counts (p < 0.1), while neutrophil, macrophage, and total cell counts did not significantly change (p > 0.1), suggesting selective lower respiratory tract inflammation. Functional prediction analysis of gut microbiota revealed significant increases (p < 0.1) in pathways associated with host health, including heme biosynthesis, sulfur oxidation, carbon metabolism, and antibiotic resistance. These findings suggest that inhaled MCs may induce respiratory inflammation and contribute to gut microbiome dysbiosis via the lung–gut axis.