Identification of novel antimicrobial peptides in the human respiratory tract

The respiratory tract is a major entry site for pathogens into the human body. To combat bacterial infections the immune system has a variety of mechanisms of host defence at his disposal. Part of the first line of defence are the so-called antimicrobial peptides (AMPs). To search for novel AMPs from the human respiratory tract, peptide libraries established from bronchoalveolar lavage fluid as well as lung tissue were screened for antimicrobial activity against Gram-positive and Gram-negative bacterial pathogens. After several testing-cycles and subsequent purification of active fractions we identified β-2 microglobulin (B2M) from the BAL library. B2M is a subunit of the major histocompatibility complex class I receptor complex that is present at the surface of every nucleated cell. It is known to express antibacterial activity against Listeria monocytogenes as well as Escherichia coli and to be a precursor of an antimicrobial agent acting against Staphylococcus aureus. Using commercially available B2M we could confirm a dose-dependent inhibition of Pseudomonas aeruginosa as well as Listeria monocytogenes. During investigation of the variables determining the biological effect of B2M a pH-dependent activity of B2M was observed. The strongest biological effect against the two tested bacterial strains was present under acidic conditions and decreased with environmental pH reaching neutral values. Congo Red assays indicate a pH-dependent fibril formation of B2M at acidic conditions suggesting that fibrillogenesis could mediate the observed pH-dependent antibacterial activity. Based on the results of SYTOX Green assays as well as transmission electron microscopy imaging, the inhibition of L. monocytogenes by B2M may be caused by membrane permeabilization. In conclusion B2M as part of a ubiquitous cell surface complex may be a potent inhibitor of bacterial invasion, especially in acidic environments as present in inflamed tissues.