Rapid-high, syncytium-inducing isolates of human immunodeficiency virus type 1 induce cytopathicity in the human thymus of the SCID-hu mouse.

Clinical deterioration in human immunodeficiency virus type 1 (HIV-1) disease is associated with an increased viral burden in the peripheral blood and a loss of circulating CD4+ T cells. HIV-1 isolates obtained prior to this stage of disease often have a "slow-low," non-syncytium-inducing (NSI) phenotype, whereas those obtained afterwards are often characterized as "rapid-high" and syncytium inducing (SI). Paired NSI and SI isolates from two different patients were inoculated into the human thymus implants of SCID-hu mice. The two slow-low, NSI isolates replicated to minimal levels in the grafts and did not induce thymocyte depletion. In contrast, the two SI isolates from the same patients showed high levels of viral replication and induced a marked degree of thymocyte depletion, accompanied by evidence of programmed cell death. These observations reveal a correlation between the replicative and cytopathic patterns of HIV-1 isolates in vitro and in the SCID-hu mouse in vivo and provide direct evidence that the biological phenotype of HIV-1 switch may be a causal and not a derivative correlate of HIV-1 disease progression.