Expression of an endogenous retroviral gene product in human placenta

To investigate the presence and potential pathophysiological role of endogenous retroviruses in humans, we prepared a recombinant protein using clone 4-I, a proviral sequence. DNA fragments containing the env region of clone 4-I were subcloned into a prokaryotic expression vector (pET3), and 2 fusion proteins, SU413 and SU415, were then expressed in Escherichia coli after treatment with isopropyl-beta-thiogalactopyranoside (IPTG). By sonicating lysates of the transformed E. coli, the recombinant protein SU413 was successfully separated from the native bacterial components, and was used to raise an antiserum in rabbits. In immunoblot analysis, this antiserum specifically recognized the recombinant protein, but did not react with other components of E. coli. This antiserum was then used for an immunofluorescence study of human placenta, in which the env gene transcript has been reported. As a result, the anti-SU413 serum detected substances in syncytiotrophoblasts and vascular endothelia from a human placenta. No such reactivity was detected in human kidney or human liver. Immunoblot analysis revealed that this antiserum reacted to a single molecule of 38-kDa in placenta, and its reactivity was reduced by the antiserum absorbed with SU413 antigen. These findings suggest that human placental syncytiotrophoblasts and vascular endothelia preferentially express a molecule encoded by human endogenous retrovirus clone 4-I.