Sequences involved in the dimerisation of human T cell leukaemia virus type-1 RNA.

The formation of a genomic RNA dimer appears to be a critical step in the life cycle of all retroviruses. To investigate the site and nucleotide interactions involved in this process, a 531 bp DNA fragment encompassing sequences up- and downstream of the splice donor in human T cell leukaemia virus type 1 (HTLV-1) was inserted into a plasmid vector under the control of the SP6 promoter. RNA transcripts generated in vitro from this template formed dimers which could be dissociated by heating at 60-80 degrees C for 3 min. The physical properties of the dimeric RNA were not consistent with either Watson-Crick base pairing or guanine tetrad formation as being solely responsible for the interaction. Deletion mutagenesis identified a 32 nt sequence required for dimerisation. Computer modelling was carried out in order to identify putative RNA secondary structures within this essential region. A stem-loop structure was identified, the stem of which was conserved among different sequenced isolates of HTLV-1. This sequence also contains a 15 nt palindrome. We sought by disruptive and compensatory mutagenesis to define the possible roles of these two structures in dimer linkage.

Low expression of lymphocyte function-associated antigen (LFA)-1 and LFA-3 adhesion molecules is a common trait in Burkitt's lymphoma associated with and not associated with Epstein-Barr virus.

Lymphocyte function-associated antigens 1 and 3 (LFA-1, LFA-3) and intercellular adhesion molecule 1 (ICAM-1) are cell surface adhesion molecules necessary for immune processes requiring intercellular contact. It was recently proposed that malignant Burkitt's lymphoma cells (BL) may escape from immunosurveillance through the downregulation of LFA-1 (CD11a/CD18) or LFA-3 (CD58) and ICAM-1 (CD54) molecules. Expression of these three adhesion antigens was investigated in 19 BL lines. LFA-1 or LFA-3 expression was found to be absent or low in 8 of 11 Epstein-Barr virus (EBV) genome positive BL, but strongly expressed on all nonmalignant EBV genome positive lymphoblastoid cell lines (LCL). Negative or weak expression of LFA-1 and LFA-3 was also observed in 7 of 8 EBV genome negative BL. ICAM-1 was found to be expressed on the cell surface of the majority of BL lines. BL lines growing as individual cells did not express LFA-1, whereas clump-forming BL lines expressed this marker involved in B-cell homotypic aggregation. Expression of LFA-1 and LFA-3 was induced on in vitro infection of EBV-negative BL cells with the immortalizing EBV strain B95-8, and weakly with the nonimmortalizing EBV strain P3HR1. EBNA2 and LMP, two EBV encoded proteins expressed in LCL and in BL infected with B95-8 (BL/B95-8), are not expressed in P3HR1 infected BL cells (BL/P3HR1). Stable expression of EBNA2 after gene transfer in a BL/P3HR1 cell line did not restore the level of LFA-1 and LFA-3 found on BL/B95-8 cells. In EBV-positive BL cells expressing LFA-1 and LFA-3, LMP was found coexpressed, supporting the recent finding of the role of LMP in B-cell adhesion receptor activation. Consequently, diminished LFA-1 and LFA-3 expression appears to be a common characteristic of numerous EBV-positive BL as well as EBV-negative BL. These findings are discussed in the framework of BL pathogenesis.