The human cationic antimicrobial protein (hCAP-18) is expressed in the epithelium of human epididymis, is present in seminal plasma at high concentrations, and is attached to spermatozoa

Innate immunity is important for the integrity of the host against potentia lly invasive pathogenic microorganisms in the environment. Antibiotic pepti des with broad antimicrobial activity are part of the innate immune system. We investigated the presence of the cathelicidin, human cationic antimicro bial protein (hCAP-18), in the male reproductive system. We found strong ex pression of the hCAP-18 gene by in situ hybridization and hCAP-18 protein, as detected by immunohistochemistry, in the epithelium of the epididymis, b ut not in the testis. The highest expression in the epididymis was in the c audal part. Western blotting showed a doubler band, the upper part correspo nding to the size of hCAP-18 in plasma and neutrophils. Using a specific en zyme-linked immunosorbent assay (ELISA), levels of 86.5 +/- 37.8 mu g/ml (m ean +/- standard deviation; range, 41.8 to 142.8 mu g/ml; n = 10) were dete cted in seminal plasma from healthy donors, which is 70-fold higher than th e level in blood plasma. Flow cytometry and immunocytochemistry revealed th e presence of hCAP-18 on spermatozoa. ELISA measurement showed levels of 19 6 ng/10(6) spermatozoa, corresponding to 6.6 x 10(6) molecules of hCAP-18 p er spermatozoon. Our results suggest a key role for hCAP-18 in the antibact erial Integrity of the male reproductive system, The attachment of hCAP-18 to spermatozoa may implicate a role for hCAP-18 in conception.