We have developed standardized procedures and practices for infection of SCID-hu Thy/Liv mice with human immunodeficiency virus type 1 for the prophylactic administration of antiviral compounds and for evaluation of the antiviral effect in vivo. Endpoint analyses included quantitation of viral load by intracellular p24 enzyme-linked immunosorbent assay, DNA PCR for the presence of proviral genomes, flow cytometry to measure the representation of CD4 ؉ and CD8 ؉ cells, and cocultivation for the isolation of virus. Efficacy tests in this model are demonstrated with the nucleoside analogs zidovudine and dideoxyinosine and with the nonnucleoside reverse transcriptase inhibitor nevirapine. This small-animal model should be particularly useful in the preclinical prioritization of lead compounds within a common chemical class, in the evaluation of alternative in vivo dosing regimens, and in the determination of appropriate combination therapy in vivo.

Human herpesvirus 6 (HHV-6) is a potentially immunosuppressive agent that may act as a cofactor in the progression of AIDS. Here, we describe the first small animal model of HHV-6 infection. HHV-6 subgroup A, strain GS, efficiently infected the human thymic tissue implanted in SCID-hu Thy/Liv mice, leading to the destruction of the graft. Viral DNA was detected in Thy/Liv implants by quantitative polymerase chain reaction (PCR) as early as 4 d after inoculation and peaked at day 14. The productive nature of the infection was confirmed by electron microscopy and immunohistochemical staining. Atypical thymocytes with prominent nuclear inclusions were detected by histopathology. HHV-6 replication was associated with severe, progressive thymocyte depletion involving all major cellular subsets. However, intrathymic T progenitor cells (ITTPs) appeared to be more severely depleted than the other subpopulations, and a preferred tropism of HHV-6 for ITTPs was demonstrated by quantitative PCR on purified thymocyte subsets. These findings suggest that thymocyte depletion by HHV-6 may be due to infection and destruction of these immature T cell precursors. Similar results were obtained with strain PL-1, a primary isolate belonging to subgroup B. The severity of the lesions observed in this animal model underscores the possibility that HHV-6 may indeed be immunosuppressive in humans.

It is still uncertain whether multilineage hematopoietic progenitor cells are affected by human immunodeficiency virus-1 (HIV-1) infection in vivo. The SCID-hu Thy/Liv model is permissive of long-term multilineage human hematopoiesis, including T lymphopoiesis. This model was used to investigate the effects of HIV-1 infection on early hematopoietic progenitor function. We found that both lineage-restricted and multilineage hematopoietic progenitors were depleted from grafts infected with either a molecular clone or a primary isolate of HIV-1. Depletion of hematopoietic progenitors (including CD34+ cells, colony-forming units in methylcellulose, and long-term culture-initiating cells) occurred several days before the onset of thymocyte depletion, indicating that the subsequent rapid decline in thymocyte numbers was due at least in part to loss of thymocyte progenitors. HIV-1 proviral genomes were not detected at high frequency in hematopoietic cells earlier than the intrathymic T-progenitor cell stage, despite the depletion of such cells in infected grafts. Proviral genomes were also not detected in colonies derived from progenitor cells from infected grafts. These data demonstrate that HIV-1 infection interrupts both lineage-restricted and multilineage hematopoiesis in vivo and suggest that depletion of early hematopoietic progenitor cells occurs in the absence of direct viral infection.