Kaposi's sarcoma–associated herpesvirus (KSHV), also called human herpesvirus (HHV)-8, is a novel member of the lymphotropic human herpesvirus family. KSHV sequences were identified initially in Kaposi's sarcoma (KS) lesions of AIDS patients 1
. The virus has also been detected in a variety of AIDS-associated lymphoproliferative disorders, namely body cavity or primary effusion lymphoma (PEL; reference 2
) and multicentric Castleman's disease 3
. KS itself is a hyperproliferation of poorly differentiated endothelial cells and is associated with extreme neovascularization. To date, a wealth of epidemiological evidence points to a central role for KSHV in the development of KS (for review see references 4567
). For instance, KSHV DNA 8
and seroreactivity 9
directed against latent or lytic antigens can be detected in virtually all cases of KS. Seropositivity for KSHV precedes disease and is a major independent risk factor for KS in AIDS patients 1011121314
. Viral DNA can also be isolated from KS lesions of HIV-1–seronegative patients 8
, suggesting that KSHV is the principal viral factor involved in all epidemiological forms of this disease.
Like all herpesviruses, KSHV has two modes of infection: lytic replication, which generates infectious progeny and destroys the host cell, and latent replication, in which the viral genome persists in its host cell but with dramatically restricted gene expression and without cell destruction 15
. Transcripts of latent viral genes (e.g., K12/kaposin, orf73/LANA, orf72/v-cyclin) are found in most KS tumor cells 16171819
, and a subset of these transcripts (e.g., kaposin and v-cyclin) may confer a cell-autonomous proliferative advantage to the cells expressing them 2021
. By contrast, <3% of infected cells in KS lesions or in KSHV+
lymphomas display evidence of lytic KSHV gene expression 162223
. Certain lytic genes (K1, vGCR, vIRF) also transform cultures of NIH3T3 cells 242526
; however, the relevance of these observations to tumorigenesis in vivo remains to be established.
A major impediment to the study of KSHV biology has been the absence of a suitable animal or culture model for de novo infection. PEL cell lines harbor latent KSHV genomes and can be induced to produce virions by treatment with phorbol-12-myristate-13-acetate (TPA) or sodium butyrate 22
. However, most established cell lines or primary explants support only limited replication or maintenance of KSHV 2728
. Unlike its cousin, Epstein-Barr virus 29
, KSHV could not be propagated in SCID mice reconstituted with human peripheral blood cells 30
, and transmission of KSHV to other conventional laboratory animals has not been reported. This suggests that the cell type most susceptible to KSHV replication and latency might be highly specialized or not readily propagated ex vivo. To alleviate the problem of inadequate in vitro systems to study natural infection, we decided to test whether KSHV could infect cells in human Thy/Liv organs grafted onto SCID mice.
The SCID-hu Thy/Liv mouse model uses C.B-17 scid/scid
mice as recipients of human fetal liver and fetal thymus implants. Upon coimplantation of both tissues under the murine kidney capsule, human hematopoietic and lymphoid precursor cells reconstitute an organ that faithfully reproduces human multilineage hematopoiesis, including thymopoiesis 3132
. T lymphocytes in various stages of development comprise the bulk (>90%) of cells in the implant, but cells of all hematopoietic lineages (including monocytes and B cells) and stromal endothelial cells are present. After direct injection of virus into the implant, replication of a number of human viruses is observed in the lymphoid (e.g., HIV-1 33
, HTLV-1 , HHV-6 35
, and varicella-zoster virus 36
) and stromal (e.g., HIV-1 37
, CMV 38
, and measles virus 39
) compartments of the graft. Depending on the biology of the particular virus, the resulting infection may be noncytopathic, (e.g., CMV 38
) or may induce severe target cell depletion (e.g., HIV-1 40
and HHV-6 35
This paper shows that KSHV derived from cultured PEL cells can initiate a productive infection in SCID-hu Thy/Liv mice. Accumulation of viral DNA depends on intact virions and can be inhibited by ganciclovir. Latent and lytic transcripts can be detected principally in CD19+ lymphocytes, but the morphology of the implant remains intact and no cytopathic effects are seen during acute infection. SCID-hu Thy/Liv mice represent the first animal model in which KSHV replication and persistence can be observed.