Human Complement Proteins C3b, C4b, Factor H and Properdin React with Specific Sites in gpl20 and gp4l, the Envelope Proteins of HIV1

Abstract

Recently we reported the basic phenomenon of an interaction between the envelope glycoproteins of HIV-1 gp120 and gp41 and components of the human complement system, i.e. activated C4 (C4b) and activated C3 (C3b) and the complement regulator proteins factor H and properdin. In this study we analyze these interactions in detail. Using 46 overlapping peptides of gp120 attached to microtiter plates, binding of activated human C3 to 6 regions in gp120 was found (aa 100-129, 161-190, 231-250, 301-328, 410-449, 470-499). In competition assays with soluble peptides, representatives of four of these regions were capable to partially inhibit Cab binding to immobilized gp 120. Activated human C4 interacted only with peptides covering as 410-449, but both in direct binding assays and fluid phase inhibition studies. The multi-reactivity of gp120 with C3b was also supported by the fact that gp120 agglutinated erythrocytes coated with Cab. Guided by partial as sequence homology of gp120 and human C4b binding protein (C4bp) as well as human properdin we detected binding of anti-properdin to as 100-129 in gp120 and of anti-C4bp to as 410-449 in gp120. This cross-reactivity was also confirmed by a monoclonal antibody directed against as 416-443 of gp120, which could be shown to bind C4bp. Interestingly, as 310-328, part of the V3-loop, were found to show an as sequence similarity to human complement receptor type 3 (cc-chain). Consequently, of the 4 (or possibly 6) interaction sites of gp120 with activated human C3, 3 may bind due to imitation of either properdin, CR3 or C4bp. In addition to C4b and C3b, we detected interaction of factor H with gp120; it selectively bound to as 102-129. Using 14 overlapping peptides of gp41 attached to plates, we identified 4 areas in gp41 (aa 561-585, 587-605, 615-635, 651-675) which bound human factor H. All of them except the first region partially inhibited factor H binding to gp41 in competition assays with soluble peptides. Properdin bound only to 2 regions (aa 584-614, 651475). The first 3 sites in gp41 were already shown by us to share homology to sites in human C3. The region around as 651-675 now also turned out to be similar to human C3. These data demonstrate that the interaction of both, gp120 and gp41, with the complement system is polyvalent and complex. It raises the possibility that this reaction potential of gp120 and gp41 might have relevance for HIV during its attachment to and intrusion into cells. Moreover, binding of factor H to both envelope proteins of HIV-1 could take part in the protection of HIV-1 from complement-mediated lysis.