X-I AND X-II OPEN READING FRAMES OF HTLV-I ARE NOT REQUIRED FOR VIRUS-REPLICATION OR FOR IMMORTALIZATION OF PRIMARY T-CELLS IN-VITRO

In contrast to other retroviruses of the oncovirinae subgroup, the pri mate and bovine leukemia viruses (HTLV, STLV, and BLV) encode genes in the X-region of the genome, between the env gene and the 3' long term inal repeat. In HTLV-I, two overlapping open reading frames (ORFs) in the distal half of the X-region encode tax and rex genes, while two OR Fs (X-I and X-II) in the proximal half of this region potentially enco de proteins designated p12(XI) (or roil and p30(XII) (or tof). The bio logical functions and mechanisms of tax and rex have been studied exte nsively whereas the roles of the other ORFs have not yet been establis hed. To identify possible functions for ORFs X-I and X-II, an infectio us molecular clone of HTLV-I and a mutant provirus lacking these ORFs were compared with respect to virus replication, gene expression, and ability to immortalize primary T-cells. When transiently transfected i nto 293 cells, both intact and deleted proviruses directed the synthes is of virus mRNAs and proteins that were quantitatively and qualitativ ely identical. These viruses were also indistinguishable in their abil ities to infect and replicate in DBS-FRhL cells, which are permissive for HTLV-I propagation. Immortalized T-cell lines were established aft er cell-free or coculture methods for infection of activated, human pe ripheral blood or cord blood lymphocytes with each of the cloned virus es. The growth kinetics, cytokine dependence, and cell surface markers of the infected T-cell cultures were similar for each provirus clone. Thus, ORFs X-I and X-II are not essential for virus infectivity, repl ication, gene expression, or T-cell immortalization in vitro.