A block to human immunodeficiency virus type 1 assembly in murine cells

Human immunodeficiency virus type 1 (HIV-1) does not replicate in murine ce lls, We investigated the basis of this block by infecting a murine NIH 3T3 reporter cell line that stably expressed human CD4, CCR5, and cyclin T1 and contained a transactivatable HIV-1 long terminal repeat (LTR)-green fluore scent protein (GFP) cassette, Although: the virus entered efficiently, form ed provirus, and was expressed at a level close to that in a highly permiss ive human cell line, the murine cells did not support M-tropic HIV-1 replic ation. To determine why the virus failed to replicate, the efficiency of ea ch postentry step in the virus replication cycle was analyzed using vesicul ar stomatitis virus G pseudotypes. The murine cells supported reverse trans cription and integration at levels comparable to those in the human osteosa rcoma-derived cell line GHOST,RS, and human cyclin T1 restored provirus exp ression, consistent with earlier findings of others, The infected murine ce lls contained nearly as much virion protein as did the human cells but rele ased less than 1/500 the amount of p24(gag) into the culture medium. A smal l amount of p24(gag) was released and was in the form of fully infectious v irus. Electron microscopy suggested that aberrantly assembled virion protei n had accumulated in cytoplasmic vesicular structures. Virions assembling a t the cell membrane were observed but were rare. The entry of M-tropic JR.F L-pseudotyped reporter virus was moderately reduced in the murine cells, su ggesting a minor reduction in coreceptor function. A small reduction in the abundance of full-length viral mRNA transcripts was also noted; however, t he major block was at virion assembly. This could have been due to a failur e of Gag to target to the cell membrane. This block must be overcome before a murine model for HIV-1 replication can be developed.