Human T-cell lymphotropic virus type 1 p12^I enhances interleukin-2 production during T-cell activation

Human T-cell lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATLL) and a variety of lymphoproliferative disorders. The early virus-cell interactions that determine a productive infection remain unclear. However, it is well recognized that T-cell activation is required for effective retroviral integration into the host cell genome and subsequent viral replication. The HTLV-1 pX open reading frame I encoding protein, p12 ^I, is critical for the virus to establish persistent infection in vivo and for infection in quiescent primary lymphocytes in vitro, p12 ^I localizes in the endoplasmic reticulum (ER) and cis-Golgi apparatus, increases intracellular calcium and activates nuclear factor of activated T cells (NFAT)-mediated transcription. To clarify the function of p12^I, we tested the production of IL-2 from Jurkat T celis cells and peripheral blood mononuclear cells (PBMC) expressing p12^I. Lentiviral vector expressed p12^I in Jurkat T cells enhanced interleukin-2 (IL-2) production in a calcium pathway-dependent manner during T-cell receptor (TCR) stimulation. Expression of p12^I also induced higher NFAT-mediated reporter gene activities during TCR stimulation in Jurkat T cells. In contrast, p12 expression in PBMC elicited increased IL-2 production in the presence of phorbal ester stimulation, but not during TCR stimulation. Finally, the requirement of ER localization for p12^I-mediated NFAT activation was demonstrated and two positive regions and two negative regions in p12^I were identified for the activation of this transcription factor by using p12^I truncation mutants. These results are the first to indicate that HTLV-1, an etiologic agent associated with lymphoproliferative diseases, uses a conserved accessory protein to induce T-cell activation, an antecedent to efficient viral infection.