Human and simian immunodeficiency virus (HIV and SIV) replicate optimally in activated memory CD4(+) T cells, a cell type that is abundant in the intestine. SIV infection of rhesus monkeys resulted in profound and selective depletion of CD4+ T cells in the intestine within days of infection, before any such changes in peripheral lymphoid tissues. The loss of CD4+ T cells in the intestine occurred coincident with productive infection of large numbers of mononuclear cells at this site. The intestine appears to be a major target for SIV replication and the major site of CD4+ T cell loss in early SIV infection.

The T-cell tropic retrovirus of macaque monkeys STLV-III has morphologic, growth, and antigenic properties indicating that it is related to HTLV-III/LAV, the etiologic agent of the acquired immune deficiency syndrome (AIDS) in humans. Four of six rhesus monkeys died within 160 days of STLV-III inoculation with a wasting syndrome, opportunistic infections, a primary retroviral encephalitis, and immunologic abnormalities including a decrease in T4+ peripheral blood lymphocytes. These data show that an immunodeficiency syndrome can be produced experimentally in a nonhuman primate by an agent from the HTLV-III/LAV group of retroviruses. The STLV-III-macaque system will thus provide a useful model for the study of antiviral agents and vaccine development for human AIDS.

It has recently been shown that rapid and profound CD4؉ T-cell depletion occurs almost exclusively within the intestinal tract of simian immunodeficiency virus (SIV)-infected macaques within days of infection. Here we demonstrate (by three-and four-color flow cytometry) that this depletion is specific to a definable subset of CD4 ؉ T cells, namely, those having both a highly and/or acutely activated (CD69 ؉ CD38 ؉ HLA-DR ؉ ) and memory (CD45RA ؊ Leu8 ؊ ) phenotype. Moreover, we demonstrate that this subset of helper T cells is found primarily within the intestinal lamina propria. Viral tropism for this particular cell type (which has been previously suggested by various studies in vitro) could explain why profound CD4 ؉ T-cell depletion occurs in the intestine and not in peripheral lymphoid tissues in early SIV infection. Furthermore, we demonstrate that an acute loss of this specific subset of activated memory CD4 ؉ T cells may also be detected in peripheral blood and lymph nodes in early SIV infection. However, since this particular cell type is present in such small numbers in circulation, its loss does not significantly affect total CD4 ؉ T cell counts. This finding suggests that SIV and, presumably, human immunodeficiency virus specifically infect, replicate in, and eliminate definable subsets of CD4 ؉ T cells in vivo.