Respiratory Epithelial Cells Respond to Lactobacillus plantarum but Provide No Cross-Protection against Virus-Induced Inflammation

Virus-induced inflammation plays a critical role in determining the clinical outcome of an acute respiratory virus infection. We have shown previously that the administration of immunobiotic Lactobacillus plantarum (Lp) directly to the respiratory tract prevents lethal inflammatory responses to subsequent infection with a mouse respiratory virus pathogen. While Lp-mediated protective responses involve non-redundant contributions of both Toll-like receptor 2 (TLR2) and NOD2, the cellular basis of these findings remains unclear. Here, we address the impact of Lp and its capacity to suppress inflammation in virus-infected respiratory epithelial cells in two cell culture models. We found that both MLE-12 cells and polarized mouse tracheal epithelial cells (mTECs) were susceptible to infection with Influenza A and released proinflammatory cytokines, including CCL2, CCL5, CXCL1, and CXCL10, in response to replicating virus. MLE-12 cells express NOD2 (81 ± 6.3%) and TLR2 (19 ± 4%), respond to Lp, and are TLR2-specific, but not NOD2-specific, biochemical agonists. By contrast, we found that mTECs express NOD2 (81 ± 17%) but minimal TLR2 (0.93 ± 0.58%); nonetheless, mTECs respond to Lp and the TLR2 agonist, Pam2CSK4, but not NOD2 agonists or the bifunctional TLR2-NOD2 agonist, CL-429. Although MLE-12 cells and mTECS were both activated by Lp, little to no cytokine suppression was observed in response to Lp followed by virus infection via a protocol that replicated experimental conditions that were effective in vivo. Further study and a more complex approach may be required to reveal critical factors that suppress virus-induced inflammatory responses.