Background:  Highly pathogenic SIV/HIV chimeric viruses (SHIVs) cause rapid, irreversible, and systematic depletion of CD4⁺ T lymphocytes in rhesus monkeys.  In the absence of this T cell subset, virus production is sustained for several months by tissue macrophage.  We recently reported that viral variants present in plasma during the late macrophage phase of SHIVDH12R infection carried changes that specifically targeted the V2 region of gp120; some of these SHIV variants were macrophage tropic (M-tropic).  In the present study, the gp120 structure, cell tropism, and coreceptor utilization properties of macrophage phase of SHIV, isolated directly from lymphoid tissues, have been examined.

Methods:  Five independent SHIVs were recovered directly from lymph nodes of late stage animals.  M-tropism was assessed by infecting alveolar macrophage (AM) recovered from rhesus macaques.  The gp160 regions of Env were cloned and sequenced.  Coreceptor usage was examined using small molecule coreceptor-targeted inhibitors, specific for CCR5 or CXCR4.

Results:  The highly pathogenic SHIVDH12R failed to produce detectable levels of progeny virus in AM.  In contrast, all 5 SHIV, isolated directly from lymph nodes of late stage monkeys, readily infected AM and each exhibited faster replication kinetics than the prototypic M-tropic SIV_mac316.  The tissue-derived M-tropic SHIVs had extensive mutations/deletions/loss of N-glycosylation sites in the V1 and V2 loops of gp120, notably 2-amino acid deletions affecting residues 164-165 and 186-187 within the V2 region and a large 6 amino acid deletion (residues 138-143) or loss of the glycosylation site at residue 141 or 197 in the V1 loop.  Infection of rh-PBMC by the starting highly pathogenic, T-cell depleting SHIV and tissue-derived M-tropic SHIV variants was potently suppressed by an inhibitor specific for CXCR4.  Moreover, infection of AM by M-tropic SHIVs was also suppressed by CXCR4, not CCR5 specific inhibitors.

Conclusions:  These results indicate that the tropism of the highly pathogenic SHIVDH12R changes during the transition from the acute T-cell to the macrophage phase of infection.  This change in the tropism is accompanied by specific alterations affecting the V1 and/or V2 regions of gp120.  The acquisition of M-tropism does not involve a switch in coreceptor usage.  Indeed, the initial T-tropic SHIV inoculum and tissue-derived M-tropic SHIVs both use CXCR4 for infections of rh-PBMC and AM.

Keywords: SHIV; macrophage; tropism