London: Researchers have found one of the human herpesvirus (HHV) that potentially infects the lining of the uterus and is behind the unexplained infertility in women. Approximately 25 per cent of female infertility cases are unexplained, leaving women with few options other than expensive fertility treatments, the study said. The findings showed that HHV-6A virus infects the lining of the uterus in 43 per cent of women with unexplained infertility. However, it was not found in the uterine lining of fertile women. Further, the response of the immune system to the HHV-6A virus may contribute to making the uterus less hospitable to a fertilised egg. HHV-6A can productively infect CD8+ T cells, natural killer cells, said the paper published in the journal PLOS ONE. The virus seems to activate immune cells called natural killer (NK) cells in the uterus and lead those cells to produce chemicals called cytokines -- tools the immune system uses to orchestrate an attack on a foreign invader, like a virus. However, the activated immune system cells and abnormal levels of certain cytokines may make it harder for a fertilised egg to lodge in the uterus and grow into a baby, the researchers said. Our study indicates, for the first time, that HHV-6A infection might be an important factor in female unexplained infertility development, with a possible role in modifying endometrial NK cells immune profile and ability to sustain a successful pregnancy, said Roberto Marci from University of Ferrara. HHV-6A, which was discovered in 1986 is one of nine human herpesviruses, can be diagnosed through a biopsy of the uterine lining. "This is a surprising and potentially important discovery. If confirmed, the finding may lead to treatments that improve the outcome for a large subset of infertile women," noted Anthony Komaroff, Professor at Harvard Medical School in Boston, US.