Y Takeuchi

We have isolated a variant of human immunodeficiency virus type 1 (HIV-1) which is highly infectious to fibroblastlike cells (BT cells) derived from human brain as well as CD4-positive T cells. This variant HIV-1, named HIV[GUN-1V], was obtained by infecting BT cells with a prototype HIV-1 isolate, named HIV[GUN-1WT], which is highly infectious to T cells but barely infectious to BT cells. HIV[GUN-1V] infects BT cells productively and this infection appeared to be mediated by CD4. To elucidate the viral gene responsible for the host range difference between the variant and prototype HIV-1s, we cloned and analyzed the provirus genomes of the two viruses. Examination of the infectivities of BT cells by various recombinant viruses and analyses of the nucleotide sequences of HIV[GUN-1V] and HIV[GUN-1WT] showed that a single nucleotide exchange was responsible for their difference in infectivity of BT cells: HIV[GUN-1V] contains a thymine residue instead of the cytosine residue in HIV[GUN-1WT] at position 931 of the env coding sequence. Replacement of cytosine by thymine at this position of the env coding sequence of the HIV[GUN-1WT] genome induced the ability to infect BT cells. The base exchange at this position was expected to change amino acid 311 of the envelope glycoprotein, gp120, from proline to serine, which is located in a variable region containing type-specific immunodominant epitopes. Thus, HIV[GUN-1V] acquired a wider host range than HIV[GUN-1WT] by a single point mutation in the env gene.

The fidelity of DNA synthesis by reverse transcriptases from human immunodeficiency virus and other retroviruses was compared by measuring the rates of misincorporation of dCMP in the place of TMP in cell-free DNA synthesis with polyadenylic acid as the template. The fidelity of human immunodeficiency virus reverse transcriptase was found to be about one-third of that of the reverse transcriptases of other retroviruses.

T-cell-line-tropic human immunodeficiency virus type 1 cannot infect CD4-positive, brain-derived cells. We isolated several new variants that readily infected brain-derived cells. Mutation of proline to serine, to alanine, or to threonine in the well-conserved GPGR sequence in the V3 region of the envelope glycoprotein was found in all these variants. This indicates the importance of amino acid sequences at the tip of the V3 region for brain cell tropism of human immunodeficiency virus type 1.