BACKGROUND: Two major cell targets of HIV-1 in infected patients are CD4 T lymphocytes and monocytes/macrophages. Although most primary isolates of HIV-1 can infect both cell types to some extent, there is a clear preference of certain H1V strains towards one or another cell type. While most authors agree that the major determinant of tropism is located within the envelope protein and controls binding and fusion events, other mechanisms involved in the regulation of HIV infectiviiy remain undefined. MATERIALS AND METHODS: Entry into cells, reverse transcription, nuclear translocation, and integration of HIV-1 strains with different cell tropisms were determined in primary human monocytes by a quantitative PCR-bascd technique. RESULTS- Upon infection with different HIV-1 strains, only minimal differences in virus entry were observed. These differences increased at each subsequent step of the viral life cycle, with the largest quantitative difference occurring at the nuclear translocation step. Blocking of nuclear translocation by an NLS-targeting compound H0294 reduced the amount of viral DNA and inhibited replication of all HIV-1 strains, demonstrating a critical role of tins event, (ntercstingly, the greaicst difference between the monocyte-iropic and T cdl-tropic HIV-1 strains was in the amount of extrachromosomal circular DNA. CONCLUSIONS: Our results indicate that differences in nuclear translocation account significantly for the variations in cell tropism between HIV-1 strains. They also suggest that extrachromosomal copies of viral genome may be responsible for a high-level replication of monocy te-tropic strains in primary monocytes.
