HTLV-1 Infection of Human Hematopoietic Stem Cells Induces a Novel T Cell Lymphoma in Humanized SCID Mice

Human T-lymphotropic virus type-1 (HTLV-1) is a human retrovirus linked to cancer and is the etiologic agent of Adult T-cell leukemia/lymphoma (ATLL), an aggressive CD4⁺/CD25⁺ T cell malignancy. The early molecular events induced by HTLV-1 infection as well as the role of various viral genes in the induction of leukemia remain unclear, predominantly due to the lack of an animal model that recapitulates ATLL development. HTLV-1 infection of humanized NOD/SCID mice (HTLV-1- HU-SCID) was achieved by inoculation of NOD/SCID mice with CD34⁺ hematopoietic progenitor cells and stem cells (CD34⁺ HP/HSCs) infected ex vivo with HTLV-1. HTLV-1-HU-NOD/SCIDmice consistently developed CD4⁺CD25⁺ T cell lymphomas with clinical characteristics associated with ATLL and infected mice showed hyperproliferation of infected human stem cells (CD34⁺CD38⁻) in the bone marrow. Inoculation of NOD/SCID mice withCD34⁺ HP/HSCs transduced with a lentivirus vector (LV) expressing the HTLV-1oncoprotein (Tax1) also developed CD4⁺CD25⁺ lymphomas. The HTLV-1 bZIP protein(HBZ), encoded by the minus strand of the HTLV-1 genome, is expressed in all ATLL cells and has been implicated in the maintenance of leukemogenesis. HBZ has previously been previously shown to interact with numerous cellular factors and can modulate Tax1 activity in vitro. To establish the role of HBZ in HTLV-1 replication and leukemogenesis in vivo, HU-SCID mice were infected with an infectious proviral clone lacking functional HBZ (HTLV-1ΔHBZ). HTLV-1ΔHBZ-infected HU-SCID mice developed lymphoproliferations with an immature preleukemic CD4⁻CD8⁻CD90⁺ phenotype starting at ~10 weeks post-reconstitution. In contrast wild type HTLV-1 infection reproducibly induces a mature CD4⁺CD25⁺ CD90⁻ lymphoma. Lymphoma cells successfully engrafted naïve NOD/SCID mice when injected into the peritoneal cavity and these cells maintain the expression of viral proteins, gp46^env and p19^gag. HTLV-1 infection of CD34⁺ HP/HSCs and the recapitulation of a lymphoma similar to ATLL in HU-NOD/SCID mice suggest that hematopoietic stem cells provide a relevant cellular target and viral reservoir in vivo and that infection of these cells contribute to viral lymphomagenesis in humans. The HTLV-1-HU-SCID mouse model presents a compelling in vivo model to characterize molecular initiation and progression of events in the generation of ATL and to establish the role of HTLV-1 auxiliary proteins in viral pathogenesis.