Scientists found that the protein, called human beta-defensin 1 (DEFB1), creates a shield around the sperm to fend off viruses, bacteria and fungi as sperm travels through the reproductive tract.
The research, by the Chinese University of Hong Kong and Shenzhen Second People's Hospital, may provide a new approach to treat male infertility associated with poor sperm motility and genital tract infection.
"DEFB1 is found in many parts of the body, but its role in regulating male fertility has not been investigated," said senior author Chen Xiaozhang, professor at the Chinese University of Hong Kong.
"Our previous studies have found a type of defensin from rat can affect sperm motility, so we want to explore how human defensins work," Chen was quoted as saying by the state run Xinhua news agency.
Infertility affects 10 per cent to 15 per cent of couples worldwide, and male infertility contributes about 50 per cent of these infertile cases.
Reduced sperm motility, also known as asthenozoospermia, is a common cause of infertility and accounts for about 18 percent of the male infertility cases.
Leukocytospermia, which is related to seminal tract infection, is another common cause of infertility. The problem is observed in about 11 per cent of infertile male patients.
In the study, Chen and his colleagues studied 190 normal individuals and 325 infertile men with either asthenozoospermia or leukocytospermia.
The researchers found that DEFB1 can create a shield against viruses, bacteria and fungi around human sperms as they travel through the reproductive tract.
In addition, the protein can also help promote sperm motility.
However, the amount of DEFB1 in sperm from infertile men with either leukocytospermia or asthenozoospermia is much lower compared to that in normal fertile sperm.
Further in-vitro fertilisation study showed that sperm treated with recombinant DEFB1, a chunk of the normal DEFB1 protein, has improved motility, working antimicrobial protection and increased egg-penetrating capacity.
"Our study suggests a feasible therapeutic approach for male infertility. But further investigation is still needed to determine its safety in humans," Chen said.
The study was published in the journal Science Translational Medicine.