Properties of the surface envelope glycoprotein associated with virulence of simian-human immunodeficiency virus SHIV(SF33A) molecular clones.

Simian-human immunodeficiency viruses (SHIVs) are chimeric viruses engineered to test the function or immunogenicity of human immunodeficiency virus (HIV) gene products in simian models. Frequently used constructs contain an HIV type 1 (HIV-1) DNA fragment containing the tat, rev, vpu, and env genes inserted in the proviral genome of SIVmac239, a molecular clone that causes simian AIDS in rhesus macaques (3, 11, 22, 28, 35, 37, 47, 49). SHIVs play a key role in the development of vaccines that are directed against the HIV-1 envelope glycoprotein and are also valuable tools in the understanding of AIDS pathogenesis (41, 42). Comparative studies of SHIVs that use CXCR4 and the CCR5 coreceptor (X4 and R5 SHIVs, respectively) have revealed the role of tropism in the rate and localization of CD4+ T-cell depletion (19). Pathogenic X4 SHIVs typically cause a severe depletion of CD4+ T lymphocytes in blood and peripheral lymph nodes within 1 month of infection (14, 21, 27, 36). Pathogenic isolates of dualtropic R5X4 SHIVs also cause rapid CD4+ T-cell depletion (29, 46, 50), while pathogenic R5 SHIVs cause a more protracted decrease of peripheral CD4+ T cells similar to that seen in SIVmac infection (18). Rhesus macaques chronically infected with pathogenic X4 or R5X4 SHIVs often maintain a high viral load in spite of an almost complete depletion of CD4+ T lymphocytes, a finding that may be explained by the productive infection of activated macrophages (25). SHIV constructs do not readily induce disease in rhesus macaques and require in vivo adaptation and often serial in vivo passages to acquire the capacity to replicate at high levels, deplete CD4+ T cells, and induce simian AIDS. Most pathogenic SHIV isolates, such as SHIVKU-1, SHIV-89.6P, and SHIV162P3, have been recovered after serial transfer of bone marrow or of blood from infected animals (19, 28, 46). Administration of an anti-CD8 monoclonal antibody (MAb) during primary infection with clone SHIVDH-12 resulted in CD4+ T-cell depletion in one rhesus macaque, indicating that immunosuppression could favor the emergence of pathogenic SHIVs (26). SHIVSF33 differs from other isolates in that it evolved into a pathogenic virus without serial in vivo passages or immunosuppressive treatment. The original virus replicated to intermediate levels in rhesus macaques and did not cause CD4+ T-cell depletion, except in one juvenile animal which developed signs of simian AIDS 2 years after inoculation. The isolate recovered from this animal, designated SHIVSF33A (A, adapted), replicated to high titers, induced a rapid and profound depletion of peripheral CD4+ T cells, and caused an AIDS-like syndrome when passaged intravenously in rhesus macaques (36, 37). In addition, SHIVSF33A could be transmitted by the vaginal route even though it used exclusively the CXCR4 coreceptor for entry (21). Comparison of original SHIV constructs with their in vivo-adapted counterparts provides a system to map the viral determinants of SHIV virulence (29, 56). Genetic analyses of pathogenic SHIV molecular clones have demonstrated that virulence is primarily modulated by adaptive changes in the two subunits of the envelope glycoprotein (2, 15, 16, 30, 33). We showed previously that, in the context of the SHIVSF33 genome, replacing the V1-to-V5 region of the env gene with corresponding sequences amplified from SHIVSF33A-infected animals was sufficient to confer pathogenicity (20). The extent of CD4+ T-cell depletion and the disease course in animals infected with recombinant molecular clone SHIVSF33A2 were similar to those caused by the SHIVSF33A isolate, demonstrating that pathogenic determinants mapped to the V1-to-V5 region of gp120. The aim of the present study was to characterize the properties conferred by the envelope glycoprotein of pathogenic SHIVSF33A molecular clones. We found that changes in gp120 were responsible for increases in fusogenicity, cytopathicity, replication capacity, and neutralization resistance of SHIVSF33A and thus conferred a set of properties that could account for the pathogenic phenotype observed in vivo. Furthermore, a conserved CD4-induced epitope that overlaps with the coreceptor binding site was spontaneously exposed at the surfaces of SHIVSF33A virions, even in the absence of CD4 binding. This finding provided a structural basis for the enhanced fusogenicity and replication capacity of pathogenic SHIVSF33A.