Molecular Basis for Cell Tropism of CXCR4-Dependent Human Immunodeficiency Virus Type 1 Isolates

Laboratory isolates of human immunodeficiency virus type 1 (HIV-1) that utilize CXCR4 as a coreceptor infect primary human macrophages inefficiently even though these express a low but detectable level of cell surface CXCR4. In contrast, infection of primary macrophages by primary CXCR4-tropic HIV-1 isolates is readily detectable. Here, we provide evidence suggesting that this difference in cell tropism results from a higher requirement for cell surface CXCR4 for infection by laboratory HIV-1 isolates. Transfected COS7 cells that express a high level of CD4 but a low level of CXCR4 were infected significantly more efficiently by two primary CXCR4-tropic HIV-1 isolates compared to the prototypic laboratory HIV-1 isolate IIIB. More importantly, overexpression of either wild-type or signaling-defective CXCR4 on primary macrophages dramatically enhanced the efficiency of infection by the laboratory HIV-1 isolate yet only modestly enhanced infection by either primary CXCR4-tropic virus. Overexpression of CD4 had, in contrast, only a limited effect on macrophage infection by the laboratory HIV-1, although infection by the primary isolates was markedly enhanced. We therefore conclude that the laboratory CXCR4-tropic HIV-1 isolate exhibits a significantly higher CXCR4 requirement for efficient infection than do the primary CXCR4-tropic isolates and that this difference can explain the poor ability of the laboratory HIV-1 isolate to replicate in primary macrophages. More generally, we propose that the cell tropisms displayed by different strains of HIV-1 in culture can largely be explained on the basis of differential requirements for cell surface CD4 and/or coreceptor expression levels.     

5. SOME NOTES RESULTING FROM A HOLIDAY TRIP THROUGH EAST GRIQUALAND

The study investigated the bird species diversity associated with vegetation communities found on a single mountain slope in the Usuthu Gorge Community Conservation Area, northern KwaZulu-Natal. Thirteen sample sites were surveyed on a monthly basis for 12 months. Over this period, 279 birds and 55 species were recorded, of which the Dark-capped Bulbul Pycnonotus tricolor was the most abundant. The Rattling Cisticola Cisticola chiniana was the indicator species in the highest community, Open Bushveld, which is characterised by grassland and bush patches. The White-bellied Sunbird Cinnyris talatala was an indicator in the second-lowest community, Dense Bushveld–Woodland, which is characterised by steep slopes and thickets. In Wooded Grassland, located between the above two communities, the highest beta diversity (species turnover) was recorded and this outcome was probably caused by large areas covered by rocks that compelled woodland birds to move through this community. Contrary to expectation, the gamma diversity per vegetation community increased with elevation from 18 to 33 species in Dense Bushveld–Woodland, Wooded Grassland and Open Bushveld. A future study should measure bird species diversity on larger elevational gradients, i.e. slopes from 500 to 3 000 m above sea level, which also include more distinct vegetation communities.