HIV-1 and HIV-2 isolates differ in their ability to activate the complement system on the surface of infected cells

To analyse the ability of different HIV-1 and HIV-2 isolates to activate the complement system.

H9 cells chronically infected with various HIV isolates and the corresponding purified viruses were tested for complement activation. To identify the molecules responsible for complement activation on the surface of infected cells, the expression of complement inhibitors/regulators and viral proteins on the cell surface was analysed.

C3 deposition on the cell surface and the expression of viral and cellular antigens were determined by flow cytometry analysis. Complement activation by purified viruses was measured using a complement consumption assay and a C1 activation assay.

H9 cells infected with different HIV-1 and HIV-2 isolates showed varying degrees of complement activation on the cell surface, ranging from strong activation and deposition of large amounts of C3 to no increased C3 deposition compared to uninfected cells. The C3 deposition was eliminated by EDTA and reduced in the presence of EGTA. In contrast, all purified viral isolates tested activated the complement system in a comparable manner. While the expression of MCP, DAF and CD59 was not modified after infection with different viral isolates, the reaction of the infected cells with a monoclonal antibody (3D6) directed against a gp41 epitope (amino acids 601-620) was found to correlate with the complement activation on the cell surface.

Some HIV-1 as well as HIV-2 isolates activate the complement system on the surface of infected cells independent of anti-HIV antibodies, while other isolates fail to do so. Complement activation on the cell surface is mediated by the alternative and, to a lesser extent, the classical pathway. The differences in complement activation on the cell surface are not caused by a modified expression of membrane-bound complement inhibitors/regulators. C3 deposition on the cell surface correlates with the expression of an epitope lying within the major complement activating domain of gp41 (amino acids 591-620). These results suggest a role of gp41 for complement activation on HIV-infected cells as has been described previously for purified HIV.