Functional role of human T-lymphotropic virus type 1 p12I in viral infectivity and host cell activation

Human T-lymphotropic virus type 1 (HTLV-1) causes adult T cell lymphoma/leukemia (ATL). Although many of the events of HTLV-1-mediated lymphocyte transformation have been elucidated, the early mechanisms allowing persistent infection remain unclear. The accessory protein p12I is critical for viral infectivity in vivo. HTLV-1 p12I localizes to the endoplasmic reticulum (ER) and binds to the calcium-binding proteins calreticulin and calnexin, as well as the IL-2 receptor β and γ_C chains.; To compare proviral loads between rabbits infected with a wild-type infectious clone of HTLV-1, ACH, and a p12I mutant clone, ACH.p12, we designed a quantitative competitive polymerase chain reaction (qcPCR). ACH.p12-infected animals did not reveal detectable levels of proviral DNA indicating that HTLV-1 p12I is necessary for viral infectivity in vivo. Rabbits infected with a mutant clone deleted in the accessory proteins p13 II and p30II showed a dramatic reduction in viral load demonstrating a role for p13II and p30II in maintenance of high viral loads in vivo. Furthermore, qcPCR was used to monitor HTLV-1-infected patients undergoing experimental antiviral therapy. To test, whether p12I was necessary for viral infectivity in quiescent primary lymphocytes in vitro, we developed three co-culture assays using HTLV-1 wild-type- and p12I-mutant-producing cells with quiescent and activated peripheral blood mononuclear cells (PBMC). While ACH and ACH.p12 had equal infectivity in activated target cells, the infectivity of ACH.p12 was dramatically reduced in quiescent primary lymphocytes. These data suggest a role for HTLV-1 p12I in activation of quiescent T lymphocytes.; We hypothesized that p12I modulates T cell signaling pathways leading to cellular activation. Therefore, we tested the influence of p12 I on the level of NFAT-, NF-κB- and AP-1-activity in Jurkat cells. NFAT-mediated transcription was increased 20-fold in the presence of p12I and phorbol ester. This induction was mediated by NFATc1 and was sensitive to cyclosporin A, BAPTA-AM and inhibition of the Ras/MAPK pathway. Inhibition of PLC-γ and LAT or PI₃K did not abolish p12_I-mediated NFAT activation. Furthermore, p12I colocalized with the ER-resident protein calreticulin.; Taken together, our findings suggest that HTLV-1 p12I increases viral infectivity by inducing lymphocyte activation via calcium- and NFAT-mediated events.