Mouse mammary tumor virus and the immune system.

Mouse mammary tumor virus (MMTV) is a nonacute transforming retrovirus that causes mammary tumors in susceptible strains of mice. Upon milk-borne transmission, B cells in the gut become infected and subsequently present a virus-encoded superantigen to cognate T cells. These T cells become activated and, in turn, stimulate neighboring lymphocytes, thereby establishing an infection-competent reservoir of lymphoid cells. During puberty and pregnancy, mammary epithelial cells actively divide, and viral transmission occurs from the lymphocytes that migrate to the mammary gland. Thus, MMTV utilizes the immune system to establish infection while simultaneously avoiding immune responses.