The ability of human immunodeficiency virus (HIV-1) to persist and cause AIDS is dependent on its avoidance of antibody-mediated neutralization. The virus elicits abundant, envelope-directed antibodies that have little neutralization capacity. This lack of neutralization is paradoxical, given the functional conservation and exposure of receptor-binding sites on the gp120 envelope glycoprotein, which are larger than the typical antibody footprint and should therefore be accessible for antibody binding. Because gp120-receptor interactions involve conformational reorganization, we measured the entropies of binding for 20 gp120-reactive antibodies. Here we show that recognition by receptor-binding-site antibodies induces conformational change. Correlation with neutralization potency and analysis of receptor-antibody thermodynamic cycles suggested a receptor-binding-site 'conformational masking' mechanism of neutralization escape. To understand how such an escape mechanism would be compatible with virus-receptor interactions, we tested a soluble dodecameric receptor molecule and found that it neutralized primary HIV-1 isolates with great potency, showing that simultaneous binding of viral envelope glycoproteins by multiple receptors creates sufficient avidity to compensate for such masking. Because this solution is available for cell-surface receptors but not for most antibodies, conformational masking enables HIV-1 to maintain receptor binding and simultaneously to resist neutralization.

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SUMMARYComplement activation with formation of biologically potent mediators like C5a and the terminal C5b-9 complex (TCC) contributes essentially to development of inflammation and tissue damage in a number of autoimmune and inflammatory conditions. A particular role for complement in the ischaemia/ reperfusion injury of the heart, skeletal muscle, central nervous system, intestine and kidney has been suggested from animal studies. Previous experiments in C3 and C4 knockout mice suggested an important role of the classical or lectin pathway in initiation of complement activation during intestinal ischaemia/reperfusion injury while later use of factor D knockout mice showed the alternative pathway to be critically involved. We hypothesized that alternative pathway amplification might play a more critical role in classical pathway-induced C5 activation than previously recognized and used pathwayselective inhibitory mAbs to further elucidate the role of the alternative pathway. Here we demonstrate that selective blockade of the alternative pathway by neutralizing factor D in human serum diluted 1 : 2 with mAb 166-32 inhibited more than 80% of C5a and TCC formation induced by solid phase IgM and solid-and fluid-phase human aggregated IgG via the classical pathway. The findings emphasize the influence of alternative pathway amplification on the effect of initial classical pathway activation and the therapeutic potential of inhibiting the alternative pathway in clinical conditions with excessive and uncontrolled complement activation.

We have analyzed a panel of eight murine monoclonal antibodies (MAbs) that depend on the V2 domain for binding to human immunodeficiency virus type 1 (HlV-1) gpl20. Each MAb is sensitive to amino acid changes within V2, and some are affected by substitutions elsewhere. With one exception, the MAbs were not reactive with peptides from the V2 region, or only poorly so. Hence their ability to bind recombinant strain IIIB gpl20 depended on the preservation of native structure. Three MAbs cross-reacted with strain RF gpl20, but only one cross-reacted with MN gpl20, and none bound SF-2 gp120. Four MAbs neutralized HIV-1 IIIB with various potencies, and the one able to bind MN gpl20 neutralized that virus. Peptide serology indicated that antibodies cross-reactive with the HxB2 Vi and V2 regions are rarely present in HIV-1-positive sera, but the relatively conserved segment between the Vi and V2 loops was recognized by antibodies in a significant fraction of sera. Antibodies able to block the binding of V2 MAbs to IIIB or MN gpl20 rarely exist in sera from HIV-1-infected humans; more common in these sera are antibodies that enhance the binding of V2 MAbs to gpl20. This enhancement effect of HIV-1-positive sera can be mimicked by several human MAbs to different discontinuous gpl20 epitopes. Soluble CD4 enhanced binding of one V2 MAb to oligomeric gpl20 but not to monomeric gpl20, perhaps by inducing conformational changes in the oligomer.