The study measured the natural antimicrobial peptide LL‑37 in blood and found it’s much higher in people with lung infections—especially tuberculosis—compared to healthy folks.

The antimicrobial peptide cathelicidin LL-37 plays a role in the immune response in the course of lung infections; however, the exact role of LL-37 in defense mechanisms against bacteria within the respiratory tract is has not been precisely described. The aim of our study was to evaluate LL-37 concentrations in the serum of pulmonary tuberculosis (TB) patients, patients with pneumonia caused by Gram-positive and Gram-negative bacteria and to compare them with those of healthy subjects. Thirty TB patients, 30 patients with pneumonia caused by Gram-positive bacteria, 30 patients with pneumonia caused by Gram-negative bacteria, and 30 healthy control subjects were enrolled in the study. Serum LL-37 concentration was measured using an enzyme-linked immunosorbent assay (ELISA). The mean (± SEM) LL-37 concentration in patients with TB (13.94±5.13 ng/mL) was significantly higher than that in patients with Gram-positive bacteria-induced pneumonia (7.87±4.58 ng/mL, P=0.00077), in patients with Gram-negative bacteria-induced pneumonia (10.27±3.60 ng/mL, P=0.00730), and in control healthy subjects (1.75±0.71 ng/mL, P=0.00004). Our data suggest that cathelicidin LL-37 is an important element of host defense in the course of bacterial diseases within the respiratory tract, particularly when the infection is caused by an intracellular pathogen.