Scientists showed that putting mRNA that codes for the antimicrobial proteins LL‑37 and calprotectin into mouth‑lining cells makes those cells better at fighting off harmful bacteria for about two days, but it also slightly harms the cells themselves.

To protect against invading bacteria, oral epithelial cells appear to use two effector antimicrobial peptides (AMPs): calprotectin (S100A8-S100A9 heterodimer [S100A8/A9]) in the cytosol and cathelicidin antimicrobial protein (CAMP) in endosomes. We sought to learn whether innate immunity might be augmented benignly to increase resistance against invasive bacteria. Epithelial cells were transiently transfected with mRNA constructs containing either the CAMP, S100A8, and S100A9 open reading frames, A8-IRES-A9 (fusion sequence), or A8-nIRES-A9 (fusion with native internal ribosome entry site [IRES] sequence). CAMP, S100A8, and S100A9 protein levels generally peaked between 16 and 44 h after mRNA transfection, depending on the construct; CAMP was processed to LL-37 over time. Following transfection with the respective mRNAs, CAMP and S100A8/A9 each independently increased resistance of epithelial cells to invasion by Listeria and Salmonella for up to 48 h; tandem S100A8/A9 constructs were also effective. Cotransfection to express S100A8/A9 and CAMP together augmented resistance, but synergy was not seen. Independent of the new proteins produced, transfection reduced cell viability after 48 h by 20%, with only 2% attributable to apoptosis. Taken together, these results suggest that epithelial cell resistance to invasive pathogens can be augmented by transient transfection of antimicrobial mRNAs into epithelial cells.