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Demonstration of two distinct cytopathic effects with syncytium formation-defective human immunodeficiency virus type
1Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110.
Insights
Mutations in the HIV-1 fusion domain reduce syncytium formation, leading to lower infectivity but preserving single-cell lysis. This finding offers insights into HIV-1 pathogenesis and disease progression.
Area of Science:
- Virology
- Molecular Biology
- Immunology
Background:
- The cytopathic mechanisms of human immunodeficiency virus type 1 (HIV-1) are not fully understood, with potential roles for syncytia formation and single-cell lysis.
- The env gene's fusion domain is critical for HIV-1's interaction with host cells and viral pathogenesis.
Purpose of the Study:
- To investigate the role of the HIV-1 fusion domain in syncytium formation, single-cell lysis, and viral infectivity.
- To elucidate the molecular mechanisms underlying HIV-1-induced cell damage.
Main Methods:
- Insertion mutations were introduced into the env gene's fusion domain of the HIV-1 HXB2 proviral clone.
- Viruses produced from mutant clones (F3, F6) were analyzed for syncytium formation, cell viability, virus yield, and infectivity in T-cell lines.
- Envelope proteins were expressed in vaccinia virus to assess their fusion and binding properties.
Main Results:
- Mutant viruses F3 and F6 showed reduced or absent syncytium-forming ability.
- F6 virus infection led to a loss of syncytial cytopathic effect, increased cell viability, and higher virus yield in early infection.
- Infectivity was reduced in F3 and F6 viruses, while single-cell lysis remained comparable to the parental HXB2 virus.
- Mutant envelope proteins were defective in syncytia formation but showed no differences in cleavage, expression, or CD4 binding.
Conclusions:
- Loss of syncytium formation attenuates HIV-1 infectivity and syncytial cytopathic effect without compromising single-cell lysis.
- These findings suggest that syncytium production is a key factor in HIV-1 cytopathicity and may correlate with disease progression observed in vivo.
Abstract:
The mechanism of human immunodeficiency virus type 1 (HIV-1) cytopathicity is poorly understood and might involve formation of multinucleated giant cells (syncytia), single-cell lysis, or both. In order to determine the contributions of the fusion domain to syncytium formation, single-cell lysis, and viral infectivity and to clarify the molecular details of these events, insertion mutations were made in the portion of env encoding this sequence in the functional HIV-1 proviral clone HXB2. Viruses produced from these mutant clones were found to have a partial (F3) or complete (F6) loss of syncytium-forming ability in acutely infected CEM, Sup T1, and MT4 T-cell lines. During the early stage of acute infection by F6 virus, there was a loss of the syncytial cytopathic effect, which resulted in increased cell viability, and a 1.9- to 2.6-fold increase in virus yield in the cell lines tested. In the late stage of acute infection, the single-cell cytopathic effect of F6 virus was similar to that of the parental HXB2 virus. The F3 and F6 viruses were also found to have a 1.7- to 43-fold reduction in infectivity compared with the HXB2 virus. The mutant F3 and F6 and parental HXB2 envelope proteins were expressed in vaccinia virus, and the mutant envelope proteins were observed to be defective in their ability to form syncytia. BSC-40 cells infected with vaccinia virus recombinants revealed no differences in kinetics of cleavage, cell surface expression, or CD4 binding capacity of the mutant and parental envelope proteins. These results demonstrate that a loss of syncytium formation results in an attenuation of infectivity and a loss of the syncytial cytopathic effect without a loss of single-cell lysis. These mutants may reflect in tissue culture the changes observed in the HIV isolates in vivo during disease progression, which exhibit marked differences in syncytium production.