Antimicrobial peptide LL-37 increases rhinovirus-induced interferon β expression in human airway epithelial cells through a Ca2+-dependent mechanism

The human cathelicidin LL-37 is active against both bacteria and viruses, but it also shows immunomodulatory properties. Here, we assess the impact of LL-37 on viral signaling in human airway epithelial BEAS-2B cells infected with the respiratory pathogen rhinovirus (RV). We show that LL-37 (4 μM) enhances RV-induced expression of interferon β (IFNβ) transcript and reduces viral-load. LL-37-evoked potentiation of RV-stimulated IFNβ does not involve up-regulation of the classical viral TLR3, MDA5 and RIG-I receptors. Moreover, the LL-37-induced stimulation of IFNβ expression in the presence of RV is abolished by chloroquine, an inhibitor of endosomal acidification. Interestingly, RV + LL-37-induced stimulation of IFNβ is observed in the absence but not in the presence of the Ca²⁺ chelating agent EGTA, indicating that Ca²⁺ is critical for this effect. Indeed, we demonstrate that LL-37 increases intracellular [Ca²⁺] in cells loaded with the fluorescent Ca²⁺ indicator Fluo-4 AM. Furthermore, we reveal that treatment with RV in combination with the Ca²⁺ ionophore A23187 promotes IFNβ expression, showing the importance of Ca²⁺. In conclusion, we demonstrate that LL-37 acts in synergy with RV to enhance IFNβ expression and that this effect involves LL-37-induced increase in intracellular [Ca²⁺].