Intracellular restriction factors in mammalian cells--An ancient defense system finds a modern foe

Cross-species transmission of retroviruses poses a threat to mammalian species. Zoonoses have given rise to devastating diseases because the host organism is not prepared to resist a new pathogen. Mammals have developed several layers of defense against viruses, including an intracellular antiretroviral defense, a part of innate immunity. Retroviral restrictions had been studied for decades using murine leukemia virus in mice, however it has become clear that primates too have intrinsic mechanisms to ward off infections by retroviruses. Several of these antiretroviral restriction mechanisms have recently been identified, with two particularly well described factors being members of the tripartite motif (Trim) and APOBEC families. Both systems provide a strong barrier against lentiviral infections. The viruses have developed countermeasures that allow them to replicate despite the host factors. This review discusses our current knowledge of this ancient battle between mammalian hosts and their retroviral opponents.