Researchers found that the LL-37 precursor protein hCAP-18 is present in human semen, both on its own and attached to tiny vesicles called prostasomes. This suggests semen can store the antimicrobial peptide, possibly helping protect sperm and the reproductive tract from infections.

Cathelicidins are a group of antibiotic peptides with broad antimicrobial activity. They are considered to be an essential part of the innate immune system. The only known human cathelicidin is the human cationic antimicrobial protein (hCAP-18), from which the antimicrobial peptide LL-37 is released. In the present study, we purified hCAP-18 from seminal plasma and confirmed its identity by N-terminal amino acid sequencing. Gel filtration of seminal plasma showed the presence of hCAP-18 in both a low and a high molecular weight peak. Fractions corresponding to the high molecular form of hCAP-18 also contained dipeptidyl peptidase IV (CD26), a prostasome marker. This finding suggested that hCAP-18 found in fractions corresponding to high molecular weight molecules, is prostasome-associated. Flow cytometry confirmed the association of hCAP-18 with prostasomes and indicated that the molecule is surface bound. Western blot showed the presence of intact hCAP-18 in sperm, prostasomes and ultracentrifuged seminal plasma. These findings suggest that hCAP-18 may have an important role in antimicrobial defence during human reproduction. The binding of hCAP-18 to prostasomes indicates that protasomes can serve as a reservoir of this precursor of the antibiotic peptide LL-37.