Host Defense Peptide LL-37, in Synergy with Inflammatory Mediator IL-1β, Augments Immune Responses by Multiple Pathways

The human cathelicidin LL-37 is a cationic host defense peptide and serves as an important component of innate immunity. It has been demonstrated to be a multifunctional modulator of innate immune responses, although the mechanism(s) underlying this have not been well characterized. In this study, it was demonstrated that LL-37 synergistically enhanced the IL-1beta-induced production of cytokines (IL-6, IL-10) and chemokines such as macrophage chemoattractant proteins (MCP-1, MCP-3) in human PBMC, indicating a role in enhancing certain innate immune responses. Similarly, LL-37 synergistically enhanced chemokine production in the presence of GM-CSF, but IFN-gamma, IL-4, or IL-12 addition led to antagonism, indicating that the role of LL-37 in reinforcing specific immune responses is selective and restricted to particular endogenous immune mediators. The inhibition of G protein-coupled receptors and PI3K substantially suppressed the ability of IL-1beta and LL-37 to synergistically enhance the production of chemokine MCP-3. Consistent with this, the combination of IL-1beta and LL-37 enhanced the activation/phosphorylation of kinase Akt and the transcription factor CREB. The role of transcription factor NF-kappaB was revealed through the demonstration of enhanced phosphorylation of IkappaBalpha and the consequent nuclear translocation of NF-kappaB subunits p50 and p65, as well as the antagonistic effects of an inhibitor of IkappaBalpha phosphorylation. These results together indicate that the human host defense peptide LL-37 can work in synergy with the endogenous inflammatory mediator IL-1beta to enhance the induction of specific inflammatory effectors by a complex mechanism involving multiple pathways, thus reinforcing certain innate immune responses.