Host–microbe interactions in the lung epithelium: Lacticaseibacillus rhamnosus modulates MAPK-associated inflammatory programs via soluble factors

Abstract

Microbial communities colonize nearly all mammalian body surfaces and contribute to organ homeostasis and immune regulation. Although the lower respiratory tract was once considered sterile, culture-independent studies have shown that bacterial communities can be detected in the lung, where their composition is shaped by microbial immigration, elimination, and local growth conditions and may influence pulmonary immunity. However, the genetic basis of lung-associated host–microbe interactions remains unclear. Recent studies linked lung-detected Lactobacillaceae abundance to host genomic regions containing Mk2 and Il10, key regulators of inflammatory signaling. Here, we investigated whether selected Lactobacillaceae taxa modulate epithelial inflammatory responses through MK2-associated pathways under basal and inflammatory conditions. Human lung epithelial A549 cells were co-cultured with Lacticaseibacillus rhamnosus, Limosilactobacillus reuteri, Lactiplantibacillus plantarum, and Ligilactobacillus murinus, with lipopolysaccharide (LPS) as an inflammatory challenge. None of the tested taxa induced cytotoxicity, reduced viability, or increased oxidative stress. RT-qPCR analysis revealed species-specific effects, with Lactobacillus rhamnosus associated with reduced MK2 and IL1B expression under basal conditions. Under LPS challenge, L. rhamnosus pre-treatment was associated with lower MK2, IL1B, IL6, and TNF expression relative to LPS alone, although not all were statistically significant. RNA-seq demonstrated that live L. rhamnosus and its heat-killed supernatant induced similar transcriptional responses, including downregulation of MAPK-associated signaling components (MK3, JNK1) and upregulation of stress-response genes (HSPA1A, HSPA1B). These findings identify L. rhamnosus as a modulator of lung epithelial inflammatory signaling and indicate that its effects do not require bacterial viability, suggesting that soluble bacterial mediators contribute to immune modulation through MAPK-associated pathways.