Epithelial-Immune Cell Crosstalk Determines the Activation of Immune Cells In Vitro by the Human Cathelicidin LL-37 at Low Physiological Concentrations

The only human cathelicidin, LL-37, is a host defense antimicrobial peptide with antimicrobial activities against protozoans, fungi, Gram(+) and Gram(−) bacteria, and enveloped viruses. It has been shown in experiments in vitro that LL-37 is able to induce the production of various inflammatory and anti-inflammatory cytokines and chemokines by different human cell types. However, it remains an open question whether such cytokine induction is physiologically relevant, as LL-37 exhibited its immunomodulatory properties at concentrations that are much higher (>20 μg/mL) than those observed in non-inflamed tissues (1–5 μg/mL). In the current study, we assessed the permeability of LL-37 across the Caco-2 polarized monolayer and showed that this peptide could pass through the Caco-2 monolayer with low efficiency, which predetermined its low absorption in the gut. We showed that LL-37 at low physiological concentrations (<5 μg/mL) was not able to directly activate monocytes. However, in the presence of polarized epithelial monolayers, LL-37 is able to activate monocytes through the MAPK/ERK signaling pathway and induce the production of cytokines, as assessed by a multiplex assay at the protein level. We have demonstrated that LL-37 is able to fulfill its immunomodulatory action in vivo in non-inflamed tissues at low physiological concentrations. In the present work, we revealed a key role of epithelial-immune cell crosstalk in the implementation of immunomodulatory functions of the human cathelicidin LL-37, which might shed light on its physiological action in vivo.