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Many AIDS vaccine candidates under development may elicit immune responses similar to those observed in and used to screen human immunodeficiency virus type 1 (HIV-1)-infected individuals. Therefore, it is important to develop vaccine candidates that incorporate antigenic markers and allow vaccinees to be distinguished from HIV-1 infectees. To this end, we introduced a series of mutations into and in the vicinity of the major immunodominant region (MIR) of gp41 (residues 598-609), a domain recognized by almost all HIV-1 infectees, and evaluated whether HIV-1-like particles incorporating such mutant glycoproteins could be expressed in mammalian cells. Results indicated that although up to three consecutive amino acids could be replaced within MIR without significantly affecting particle formation or gp160 processing, deletions within MIR impaired envelope processing. Replacement of HIV-1 MIR by part or most of the corresponding domain from other lentiviruses markedly decreased or abolished gp160 processing. Synthetic peptides corresponding to a mutated MIR incorporating three amino acid replacements were not recognized by a panel of sera from HIV-1 infectees, suggesting that HIV-1-like particles with this type of mutation represent potential candidate vaccines that could allow vaccinees to be distinguished from HIV-1 infectees., (Copyright 1999 Academic Press.)

The mechanism by which HIV-1 mediates cell fusion and penetrates target cells, subsequent to receptor (CD4) binding, is not well understood. However, neutralizing antibodies, which recognize the principal neutralizing determinants of the gp120 envelope protein (the V3 loop region, residues 296 to 331), have been shown to effectively block cell fusion and virus infectivity independent of the initial gp120-CD4 binding. To investigate the role of the V3 loop in an HIV infection, a series of site-specific mutations were introduced into the HIV-1 envelope gene. Specifically, each residue (312 to 315) in the strongly conserved tetrapeptide sequence, GPGR, which is positioned in the center of the V3 loop domain was individually altered. The processing, transport, and CD4 binding properties of the mutant envelope proteins were comparable to those of the wild-type protein, however, none of the mutants were able to form syncytia in the HeLa-T4 assay. Molecular HIV-1 clones containing mutations altering the G312, G314, or R315 residues produced noninfectious virions, whereas a clone with a P313A mutation was found to be infectious. These results demonstrate that certain V3 loop mutations can be lethal and clearly indicate that this region of the HIV-1 gp120 protein is essential for virus infectivity.

Administration of high-titered goat anti-FLV gp71 IgG to AKR mice during a narrow neonatal therapy "window" suppresses the early development of MuLV infectious cell centers (ICC) in spleen, thymus, and bone marrow. By 4-5 weeks of age ICC appear in spleen and marrow cell populations, rapidly increasing to plateau levels within 2 weeks in the absence of viremia. The thymus of treated animals remains devoid of detectable ICC throughout the 4-month period of testing. This pattern of ICC suppression occurs independently of virus neutralization as shown by the inability of F(ab')2 preparations, which retained neutralizing activity, to prevent early establishment of ICC. Immune IgG significantly decreases, but does not eliminate gp71 expression in all tissues tested. In control animals, ICC reside within a minor subpopulation of cortical, thymic T cells, whereas peripheral (i.e., splenic) ICC are totally devoid of conventional T cell, B cell, and macrophage phenotypic markers. Although thymocytes appear to be a major target of this antibody therapy, T-cell reactivity is not compromised.