Essential Role of Human T Cell Leukemia Virus Type 1 orf-I in Lethal Proliferation of CD4 + Cells in Humanized Mice.

Human T cell leukemia virus type 1 (HTLV-1) is the ethological agent of adult T cell leukemia/lymphoma (ATLL) and a number of lymphocyte-mediated inflammatory conditions, including HTLV-1-associated myelopathy/tropical spastic paraparesis. HTLV-1 orf-I encodes two proteins, p8 and p12, whose functions in humans are to counteract innate and adaptive responses and to support viral transmission. However, the in vivo requirements for orf-I expression vary in different animal models. In macaques, the ablation of orf-I expression by mutation of its ATG initiation codon abolishes the infectivity of the molecular clone HTLV-1(p12KO). In rabbits, HTLV-1(p12KO) is infective and persists efficiently. We used humanized mouse models to assess the infectivity of both wild-type HTLV-1 (HTLV-1(WT)) and HTLV-1(p12KO). We found that NOD/SCID/gamma(-/-)(C) c-kit(+). mice engrafted with human tissues 1 day after birth (designated NSG-1d mice) were highly susceptible to infection by HTLV-1(WT), with a syndrome characterized by the rapid polyclonal proliferation and infiltration of CD4(+) CD25(+) T cells into vital organs, weight loss, and death. HTLV-1 clonality studies revealed the presence of multiple clones of low abundance, confirming the polyclonal expansion of HTLV-1-infected cells in vivo. HTLV-1(p12KO) infection in a bone marrow-liver-thymus (BLT) mouse model prone to graft-versus-host disease occurred only following reversion of the orf-I initiation codon mutation within weeks after exposure and was associated with high levels of HTLV-1 DNA in blood and the expansion of CD4(+) CD25(+) T cells. Thus, the incomplete reconstitution of the human immune system in BLT mice may provide a window of opportunity for HTLV-1 replication and the selection of viral variants with greater fitness. IMPORTANCE Humanized mice constitute a useful model for studying the HTLV-1-associated polyclonal proliferation of CD4(+) T cells and viral integration sites in the human genome. The rapid death of infected animals, however, appears to preclude the clonal selection typically observed in human ATLL, which normally develops in 2 to 5% of individuals infected with HTLV-1. Nevertheless, the expansion of multiple clones of low abundance in these humanized mice mirrors the early phase of HTLV-1 infection in humans, providing a useful model to investigate approaches to inhibit virus-induced CD4(+) T cell proliferation.