Human immunodeficiency virus, restriction factors, and interferon.

Recent discoveries have revealed previously unappreciated complexity with which retroviruses interact with their hosts. In particular, we have become aware that many mammals, including humans, are equipped with genes encoding so-called "restriction factors," that provide considerable resistance to retroviral infection. Such antiretroviral genes are sometimes constitutively expressed, and sometimes interferon-induced. Thus they can be viewed as comprising an intrinsic immune system that provides a pre-mobilized defense against retroviral infection or, alternatively, as a specialized extension of conventional innate immunity. Antiretroviral restriction factors have evolved at an unusually rapid pace, particularly in primates, and some startling examples of evolutionary change are present in genes encoding restriction factors. Our understanding of the mechanisms by which restriction factors interfere with retroviral replication, and how their effects are avoided by certain retroviruses, is accruing, but far from complete. Such knowledge could allow for novel forms of therapeutic intervention in pathogenic retroviral infections, as well as the development of animal models of human disease.