The cellular tropism of the feline immunodeficiency virus (FIV) is affected by changes in variable region 3 (V3) of the surface (SU) envelope glycoprotein (Verschoor, E. J., et al., J. Virol. 69:4752-4757, 1995). By using high-dose DNA transfection, an FIV molecular clone with a non-CRFK-tropic V3 acquired the ability to replicate in CRFK cells. A single point mutation from a methionine to a threonine in the ectodomain of its transmembrane (TM) envelope glycoprotein was responsible for this change in viral tropism. This substitution is located in the putative SU interactive region, between the fusion peptide and the membrane-spanning region. Our results show that this region of the TM envelope glycoprotein constitutes an additional determinant for cell tropism.
The envelope is of cardinal importance for the entry of feline immunodeficiency virus (FIV) into its host cells, which consist of cells of the immune system including macrophages. To characterize the envelope glycoprotein determinants involved in macrophage tropism, chimeric infectious molecular clones were constructed containing envelope gene sequences from isolates that had been propagated in peripheral blood mononuclear cells (PBMC). The progeny virus was examined for growth in PBMC and bone marrow-derived macrophages and viruses with different replication kinetics in macrophages were selected. Envelope-chimeric viruses revealed that nucleotide sequences encoding variable regions 3 and 4 of the surface glycoprotein, SU, are involved in macrophage tropism of FIV. To assess the biological importance of this finding, the phenotypes of envelope proteins of viruses derived from bone marrow, brain, lymph node and PBMC of an experimentally FIV-infected, healthy cat were examined. Since selection during propagation had to be avoided, provirus envelope gene sequences were amplified directly and cloned into an infectious molecular clone of FIV strain Petaluma. The viruses obtained were examined for their replication properties. Of 15 clones tested, 13 clones replicated both in PBMC and macrophages, two (brain-derived clones) replicated in PBMC only and none replicated in Crandell feline kidney cells or astrocytes. These results indicate that dual tropism for PBMC and macrophages is a common feature of FIV variants present in vivo.
Hybridomas secreting monoclonal antibodies (MAbs) specific for the surface protein (SU) of feline immunodeficiency virus were generated. Four MAbs were obtained which could be assigned to two groups based on their neutralization and competition behaviour. Using SU protein fragments expressed in Escherichia coli the antigenic site recognized by one of the MAbs (2H11) could be mapped to the c terminus. The neutralizing MAb 1El did not bind to any of the SU protein fragments and was directed to a conformational epitope. Binding of the MAb 1El to native SU protein could be blocked with a rabbit serum raised against the SU3 fragment (amino acids 361 to 445). These data indicate that at least part of the epitope is located on this SU3 domain. In competition experiments most sera of naturally infected cats were able to inhibit binding of the MAbs. This shows the conserved and immunodominant nature of the epitopes involved.
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