Defective human T-lymphotrophic virus type I provirus in T-cell prolymphocytic leukaemia.

T-cell prolymphocytic leukaemia (T-PLL) is a rare form of post-thymic T-cell neoplasm, the aetiology of which remains unknown. We examined human T-lymphotropic virus (HTLV) provirus in five HTLV-I/II seronegative patients with T-PLL. Southern blotting did not show monoclonal integration of the HTLV-I genome in any of the DNA samples. However, two of the five DNA samples contained an HTLV-I tax sequence. Other sets of oligonucleotide primers for HTLV-I gag, pol, env and LTR regions were all negative. HTLV-I tax gene expression and p40tax antibody were not detected in samples from cases with HTLV-I tax sequence. Our findings suggest that there may be alternative mechanisms involved in HTLV-associated leukaemogenesis, in which HTLV-I genome insertion triggers T-PLL but the deletion of various regions of the integrated provirus subsequently prevents active replication and the expression of the virus.