ROLE OF CD4 ENDOCYTOSIS IN HUMAN-IMMUNODEFICIENCY-VIRUS INFECTION

We have analyzed the role of CD4 endocytosis in human immunodeficiency virus (HN) entry by measuring the infection of HeLa cells expressing various CD4 constructs with endocytosis rates of between 0.2 and 30%/m in in a quantitative infectious focus assay. For a number of laborator y-adapted HIV-I and HIV-2 strains, the highest levels of infection wer e found on cells with very limited CD4 endocytosis, while cells with e fficient CD4 uptake were only poorly infectable, suggesting that CD4 i nternalization is not required for HIV entry. This was confirmed in a modified assay involving prebinding of HIV-1(LAI), to HeLa CD4 cells a t CC, synchronized virus entry during warming to 37 degrees C, and neu tralization of virions remaining at the cell surface with anti-V3 loop antibodies. Warming cells in hypertonic medium inhibited CD4 endocyto sis but did not affect the rate or the extent of infection. These stud ies confirm that HIV infection does not require endocytosis and that l aboratory-adapted virus strains can enter HeLa-CD4 cells by fusion at the plasma membrane.