Low levels of co-receptor CCR5 are sufficient to permit HIV envelope-mediated fusion with resting CD4 T cells.

The susceptibility of phenotypically CCR5-negative resting CD4 T cells for membrane fusion with a CCR5-specific HIV-1 envelope was analysed using a novel sensitive fusion assay. A very low overall density of CCR5 on T cells expressing high levels of CD4 was shown to be sufficient for HIV envelope-mediated membrane fusion. These findings are relevant to the understanding of how HIV-1 R5 strains enter and replicate in resting CD4 T cells in vivo.

Mutations in the env gene of human immunodeficiency virus type 1 NDK isolates and the use of African green monkey CXCR4 as a co-receptor in COS-7 cells.

A previous report from this laboratory described the isolation of the first CD4-independent human immunodeficiency virus type 1 isolate, m7NDK. This independence of CD4 is due to seven mutations located in the C2, V3 and C3 regions of the gp120 protein. The present report describes the entry features of the m5NDK virus, which contains five of the seven m7NDK mutations, located in the V3 loop and C3 region. The entry of this virus is strictly CD4-dependent but it can fuse with African green monkey (agm) COS-7 cells bearing human CD4 (h-CD4). This fusion is directly due to the five mutations in the envgene. It has also been shown that entry of m7NDK is CD4-independent in COS-7 cells. Since the wild-type NDK and m7NDK viruses use the human CXCR4 protein as co-receptor, agm-CXCR4 was cloned and used in transfection and fusion inhibition experiments to show that this receptor can be used by the m5 and m7NDK viruses. The wild-type NDK virus, which does not enter COS-7 cells, can use agm-CXCR4, but only when the receptor is transfected into target cells. Although co-receptor nature and expression levels are still major determinants of virus entry, this is the first case where a few mutations in the env gene can overcome this restriction.