Understanding the basis of CD4⁺ T-cell depletion in macaques infected by a simian-human immunodeficiency virus

The efficacy of candidate AIDS vaccines to mediate protection against viral infection and pathogenesis is evaluated, at a preclinical stage, in animal models. One model that is favored because the infecting virus is closely related to HIV-1 and because of the rapidity of pathogenic outcomes is the infection of Old World monkeys by simian-human immunodeficiency virus (SHIV) chimerae. We investigated the basis for the depletion of CD4⁺ T lymphocytes in a SHIV-macaque model. Molecularly cloned SHIVs, SHIV-89.6 and SHIV-KB9, differ in the ability to cause CD4⁺ T-cell loss at a given level of virus replication in monkeys. The envelope glycoproteins of the pathogenic SHIV-KB9 mediate membrane-fusion in cultured T lymphocytes more efficiently than the envelope glycoproteins of the non-pathogenic SHIV-89.6. The minimal envelope glycoprotein region that specifies this increase in membrane-fusing capacity was sufficient to convert SHIV-89.6 into a virus that causes profound CD4⁺ T-cell depletion in monkeys. Conversely, two single amino acid changes that decrease the membrane-fusing ability of the SHIV-KB9 envelope glycoproteins also attenuated the CD4⁺ T-cell destruction that accompanied a given level of virus replication in SHIV-infected monkeys. Thus, the ability of the HIV-1 envelope glycoproteins to fuse membranes, which has been implicated in the induction of viral cytopathic effects in vitro, contributes to the capacity of the pathogenic SHIV to deplete CD4⁺ T lymphocytes in vivo.