Chapter 18: Retroviruses: Human T-Lymphotropic Virus, Human Immunodeficiency Virus, and Acquired Immunodeficiency Syndrome

INTRODUCTION

Retroviruses are enveloped, single-stranded, positive-sense RNA viruses. These viruses are known as retroviruses because they encode an enzyme called reverse transcriptase, which converts the RNA genome into a double-stranded DNA copy that subsequently becomes integrated into the host chromosome. The discovery of reverse transcriptase in 1970 by two American virologists, David Baltimore and Howard Temin, earned them a Nobel Prize in Medicine. There are two major groups of retroviruses that infect humans: the oncoretroviruses (onco-, “related to a tumor”) and the lentiviruses (lenti-, “slow”). There are several other groups of retroviruses that infect animals. Endogenous retrovirus sequences are found throughout the human genome. Like most enveloped viruses, all retroviruses are highly susceptible to factors that affect surface tension and are thus not transmissible through air, dust, or fomites under normal conditions, but instead require intimate contact with the infecting sources, such as bodily fluids, blood, and blood-derived products.

Members of the oncoretrovirus, a subgroup of retroviruses, have long been associated with a variety of cancers in animals, including leukemias, lymphomas, and sarcomas. However, an oncoretrovirus was discovered in the late 1970s that infects humans known as human T-cell lymphotropic virus type I (HTLV-I). It causes adult T-cell leukemia and lymphoma (ATLL), a rare malignancy found only in Japan, Africa, and the Caribbean, although serologic evidence shows that it also occurs in the United States and has raised the possibility of an association with some chronic neurologic conditions. A relative of HTLV-I, HTLV-II has been associated with a few rare cases of T-cell malignancies, including hairy cell leukemia, but its precise role in these diseases remains unclear.

The most important disease resulting from a human retrovirus infection is called acquired immunodeficiency syndrome (AIDS), which is caused by a lentivirus known as human immunodeficiency virus (HIV). There are two types: HIV-1 and HIV-2, which cause AIDS. A devastating disease worldwide, for which there is no permanent cure or preventive vaccine for protection, AIDS has spurred unprecedented research efforts to determine the nature and immunopathogenic mechanisms of the virus in the hope of finding more and new effective drugs and a preventive AIDS vaccine. Most of our present knowledge of HIV is derived from studies on HIV-1, which is the major cause of AIDS worldwide. In 2008, two French virologists, Françoise Barré-Sinoussi and Luc Montagnier, shared the Nobel Prize in Medicine for their work on the discovery of HIV-1, the virus that causes AIDS.

Oncoretroviruses are not cytolytic in the sense that they do not kill the cells that they infect, ...