Profound differences exist in the replicative capacities of various human immunodeficiency virus 1 isolates in primary human monocytes. To investigate the molecular basis for these differences, recombinant full-length clones were constructed by reciprocal DNA fragment exchange between a molecular clone derived from a monocyte-tropic isolate (ADA) and portions of two full- length clones incapable of infection or replication in primary monocyte cultures (HXB2 and NL4-3). Virions derived from proviral clones that contained ADA sequences encoding vpu and the N and C termini of the surface envelope glycoprotein (gp120) were incapable of replication in monocytes. However, a 283-base-pair ADA sequence encoding amino acids 240-333 of the mature gp120 protein conferred the capacity for high-level virus replication in primary monocytes. The predicted amino acid sequence of this ADA clone differed from NL4-3 and HXB2 at 22 of 94 residues in this portion of gp120, which includes the entire third variable domain. Only 2 of 11 residues implicated in CD4 binding are located in this region of gp120 and are identical in HXB2, NL4-3, and ADA. Alignment of the ADA sequence with published amino acid sequences of three additional monocyte-replicative and three monocyte-nonreplicative clones indicates 6 discrete residues with potential involvement in conferring productive human immunodeficiency virus 1 infection of primary monocytes.

Original languageEnglish
Pages (from-to)3097-3101
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number8
StatePublished - 1991


  • CD4
  • macrophage
  • third variable domain of gp120
  • tropism


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