Mouse-human heterokaryons support efficient human immunodeficiency virus type 1 assembly

Murine cells do not support human immunodeficiency virus type 1 (HIV-1) rep lication because of blocks to virus entry, proviral expression, and virion assembly. In murine 3T3 fibroblasts, the block to HIV-1 entry is relieved b y the introduction of human CD4 and CCR5 or CXCR4, and proviral expression is increased by the introduction of the Tat cofactor, human cyclin T1; howe ver, because of the assembly block virus fails to spread. A panel of rodent cell lines expressing human CD4, CCR5, and cyclin T1 was established and s tudied for the ability to support virus replication. Mus musculus lymphoid cell lines EU and L1-2 and Mus dunni fibroblasts supported only low levels of virus assembly and released small amounts of infectious virus. CHO and R at2 cell lines produced more infectious virus, but this production was stil l 40-fold lower than production in human cells. Only CHO cells expressing t he three human cofactors were partially permissive for HIV-1 replication. T o investigate the basis of the block to HIV-1 assembly, mouse-human heterok aryons were tested for ability to assemble and release virus. Fusion of hum an cells to HIV-1-infected mouse cells expressing CD4, CCR5, and cyclin T1 caused a 12-fold increase in virion release and a 700-fold increase in infe ctious virus production. Fusion of HIV-1-infected M, dunni tail fibroblasts to uninfected human cells caused a similar increase in virus release. More efficient virus release was not caused by increased proviral transcription or increased synthesis of virion components. Analysis of reciprocal hetero karyons suggested the absence of an inhibitor of virus assembly. Taken toge ther, the results suggested that murine fibroblasts lack a cofactor that is required for efficient virus assembly and release.