Epididymis is a principal site of retrovirus expression in the mouse.

High levels of retrovirus particles are present in the reproductive tract of male mice. In this report epithelial cells that line the lumen of the epididymis are shown to be a principal site of virus synthesis. Aggregates of free virus were evident in the epididymal lumen in addition to the sperm-associated virus previously reported. Large intraluminal cells with characteristics of macrophages and engorged with virus particles were also seen. Virus particles were not detected in testis, liver, brain, or spleen. Thus, the epididymal epithelium is a principal reservoir for retrovirus expression. The virus would be ejaculated as free, cell-associated, and sperm-bound particles. The high level of expression and the relative isolation of epididymal virus from the immune system may relate to venereal transmission of retrovirus infections in mice and humans.