Background:  We used infection of rhesus macaques with isogenic SHIV molecular clones to address the impact of co-receptor usage in target-cell selectivity and the induction of immunodeficiency.

Methods:  The V3 loop of the pathogenic CXCR4-tropic SHIV_SF33A2 clone was replaced with the corresponding region of the pathogenic CCR5-tropic SHIV_SF162P3 isolate to generate a clone, designated SHIV_SF33A(V3).

Results:  With the exception of a 35 amino acid V3 region that shares 71% homology, the genetic sequences of the SHIV_SF33A2 and SHIV_SF33A(V3) clones are identical. Functional assays demonstrate the substitution of the V3 loop of SHIV_SF33A2 Env gp120 resulted in a switch of co-receptor usage from CXCR4 to CCR5 for entry into HOS.CD4 cell lines. Further, infection of PBMC mediated by SHIV_SF33A(V3), but not  SHIV_SF33A2 env, was blocked by RANTES, confirming their co-receptor specificities. SHIV_SF33A(V3) is replication-competent in vivo, infecting 2 of 2 rhesus macaques by intravenous inoculation, with peak viremia >10⁶ RNA copies/mL plasma that is comparable to that seen in monkeys infected with the pathogenic X4 clone SHIV_SF33A2. But while acute infection with X4- SHIV_SF33A2 is accompanied by rapid and profound loss of peripheral and lymphoid CD4⁺ T lymphocytes, infection with the R5-tropic clone SHIV_SF33A(V3) resulted only in a modest and transient loss. However, substantial depletion of intestinal and lung lavage CD4⁺ T cells was observed in SHIV_SF33A(V3)-infected macaques, similar to that seen in monkeys infected with the pathogenic CCR5-tropic SHIV_SF162P3 isolate. Examination of the subset composition of circulating and tissue CD4⁺ T cells revealed rapid depletion of naïve T cells followed by memory T cells in X4-SHIV_SF33A2 infected macaques while R5-SHIV_SF33A(V3) infection mainly affected memory T cells of the mucosa.

Conclusions:  Our findings firmly establish co-receptor usage as the sole determinant of target-cell and tissue specificity of the virus in vivo, and dictates the dynamics of CD4⁺ T-cell depletion during HIV infection. Since progressive loss of CD4⁺ T lymphocytes underlies the development of AIDS in HIV-1- infected individuals, the two isogenic R5- and X4- SHIV clones described here will provide valuable tools to further advance our understanding of the impact of co-receptor usage on AIDS pathogenesis.

Keywords: Pathogenesis; Co-receptor usage; Animal disease model