All the human retroviruses belong to a single viral family, the Retroviridae. HTLV-I and HTLV-II belong to a subfamily known as the oncornaviruses, and HIV-1 and HIV-2 belong to the lentivirus subfamily. One other subfamily, the spumaviruses, includes human foamy virus, an agent with no known pathogenicity for humans.

The various retrovirus subfamilies have distinct in vitro and in vivo characteristics. Oncornaviruses transform cells in culture and can produce tumors in animals. Lentiviruses produce syncytia and cytopathic effects in cell culture and cause slowly progressive infections in the host. The spumaviruses induce vacuolization of cells in culture and do not cause apparent adverse effects in the host.

Human retroviruses have a peculiar tropism for lymphocytes. In the case of HIV, this tropism results from the high affinity of the viral envelope protein for the CD4 molecule on the surface of helper-inducer T lymphocytes. In addition to T lymphocytes, a variety of cells, including monocytes, macrophages, and microglial cells of the central nervous system (CNS), express the CD4 molecule. The hallmark of HIV infection is progressive loss of CD4⁺ T lymphocytes with inexorable diminution of immune function. The frequent occurrence of nervous system disease in individuals infected with HIV may be attributable in part to the introduction of virus into the CNS via infected monocytes and macrophages.

HTLV-I induces expression of interleukin-2 receptors on CD4⁺ T lymphocytes. The presence of excessive receptors for interleukin-2, a recognized T-cell growth factor, may help to explain the lymphoproliferation observed in some individuals infected with HTLV-I. Both HTLV-I and HTLV-II can cause in vitro proliferation of normal lymphocytes in the absence of exogenous antigens.

This chapter focuses on the epidemiology and clinical features of infection caused by HTLV-I and HTLV-II. For a thorough discussion of pediatric HIV infection and acquired immune deficiency syndrome, the reader is referred to Chapter 139.