The study found that the human antimicrobial peptide LL‑37 can quickly make colon cells more leaky by pulling apart key tight‑junction proteins, even at levels that don’t kill bacteria. This effect was seen in human colon cell lines and also in mouse cells, and it worsened damage caused by a gut pathogen.

The intestinal barrier, held together by epithelial cells and intercellular tight junction (TJ) proteins, prevents the penetration of microbial pathogens. Concurrently, intestinal epithelial cells secrete antimicrobial peptides, including cathelicidin. Cathelicidin has direct antibacterial and immunomodulatory functions, although its role in intestinal integrity remains elusive. In this study, we demonstrate that direct stimulation of human colonic epithelial (T84) cells with human cathelicidin, LL-37, resulted in a rapid and transient increase in epithelial cell permeability. This increased permeability was associated with the TJ proteins occludin and claudin-2 degradation, mediated by these specific proteins' endocytosis and lysosomal degradation. While murine cathelicidin (CRAMP) failed to modify T84 cell permeability, LL-37 degraded TJ proteins in murine rectal epithelial (CMT-93) cells. The stimulus of (CMT-93) cells with LL-37 aggravated the cell permeability and furthered TJ degradation provoked by the intestinal pathogen, attaching/effacing (A/E) Citrobacter rodentium (C. rodentium). The number of C. rodentium that colonized CMT-93 cells was not severely impacted by the presence of LL-37. While a temporary disruption of tight junctions by LL-37 may lead to a 'leaky gut,' this study demonstrates that LL-37 increases epithelial cell permeability by degrading TJ proteins occludin and claudin-2 through endocytosis and lysosomal degradation. These immunomodulatory actions occurring at concentrations lower than those microbicidal uncover a new guise for cathelicidin modulating the epithelial barrier against A/E pathogens. Recognizing native cathelicidin's functions in a specified disease setting (e.g., colitis) will help establish it as an anti-infectious immunomodulator.