Expression and detection of macrophage-tropic HIV-1 gp120 in the brain using conformation-dependent antibodies

HIV-1 envelope proteins gp120 and gp41 are likely to play a role in the pat hogenesis of HIV-associated neurocognitive disorders. While detection of gp 120 in HIV-infected cell cultures is easy, it has not yet been possible to identify gp120 in human or animal brains in situ. The difficulty in detecti ng gp120 could be due to low expression levels of the protein, to the shedd ing of gp120 from infected macrophages/microglia, or to the use of inapprop riate gp-specific antibodies. We addressed these questions by analyzing the subcellular localization, oligomeric structure, and shedding behavior of g p120 from a macrophage-tropic, CCR5-dependent primary isolate, BX08, expres sed by a Semliki Forest virus replicon (SFVenvBX08) in vitro. We used the s ame SFV system injected in vivo into the rat brain in an attempt to detect gp120 in situ, Our results show that gp120/41 is expressed as monomers, dim ers, and trimers in cell culture. Immunocytochemical analysis revealed that intracytoplasmic gp120 can be recognized by an anti-V3 antibody, whereas g p120 at the plasma membrane is detected exclusively by a conformation-depen dent antibody. In the rat brain, the SFV vector allows gene expression in n eurons from day 3 to day 9 after injection without any apparent brain damag e nor reactive astrogliosis. In SFVenvBX08-infected neurons only conformati on-dependent antibodies allowed gp120 labeling. These results suggest that previous difficulties in detecting gp120 in brain tissues may be due to the use of antibodies which were unable to recognize gp120 at the plasma membr ane. (C) 1999 Academic Press.