Complement-Mediated Enhancement of HIV-1 Infection Reverses the Anti-HIV-1 Activity of Castanospermine

Mechanism for complement-mediated, antibody-dependent enhancement of human immunodeficiency virus type 1 infection in MT2 cells is enhanced entry through CD4, CD21, and CXCR4 chemokine receptors

Some antibodies neutralize Human Immunodeficiency Virus (HIV). However, antibody to HIV and complement can enhance HIV replication if cells express both complement receptors and CD4, a phenomenon described as complement-mediated, antibody-dependent enhancement (C'ADE). Although increased binding of opsonized virions has been reported, the mechanism by which C'ADE enhances HIV replication remains unproven. In this study, real-time polymerase chain reaction to detect HIV cDNA indicates that complement and anti-HIV antibodies enhance HIV entry 8- to 30- fold with similar increases in integrated provirus. Thus, complement increases HIV replication through a mechanism of enhanced entry. To further refine the mechanism of C'ADE, chemokine receptor antagonists were employed. JM2987, a CXCR4 chemokine receptor antagonist, blocked HIV infection and C'ADE; thus CD4, complement receptors, and CXCR4 chemokine receptors are required for enhanced entry of HIV into MT2 cells. Finally, anti-HIV immunoglobulin enhanced replication of not only group M clade B HIV but also group M clade D and group O isolates. These data demonstrate that antibodies mediating C'ADE of HIV infection are broadly reactive.

Complement-mediated binding of naturally glycosylated and glycosylation-modified human immunodeficiency virus type 1 to human CR2 (CD21)

Particulate glycoproteins lacking sialic acid, such as desialylated enveloped viruses, readily activate complement through the alternative pathway. Human immunodeficiency virus type 1 (HIV-1) contains two heavily glycosylated and partially sialylated envelope glycoproteins: a surface gp120 and a transmembrane gp41. The abilities of naturally glycosylated HIV-1 and glycosylation-modified HIV-1 to interact with the complement system were examined with a biological assay which measured the binding of whole virus particles to cells expressing CR2 (CD21), the complement receptor found naturally in abundance on follicular dendritic cells and immature B cells. HIV-1 IIIB was synthesized in the presence or absence of the mannosidase II inhibitor, swainsonine, to give rise to high-mannose-type, nonsialylated, nonfucosylated carbohydrate moieties. The virus also was treated with neuraminidase or endo-beta-galactosidase to remove terminal sialic acids. An enzyme immunoassay specific for HIV-1 p24 core protein was used to quantitate the amount of virus bound to cell surfaces. Virus particles incubated with 1:3-diluted, fresh HIV-1-negative human serum as a source of complement readily bound to MT-2 (CD4+ CR2+) and Raji-3 (CD4- CR2+) cells but not to CEM (CD4+ CR2-) cells, suggesting that the virus bound to CR2 independently of CD4. Compared with heat-inactivated or C3-deficient sera, fresh complement increased binding by as much as 62 times for naturally glycosylated virus, and 5 times more than this for glycosylation-modified virus. Similar observations were made with freshly isolated, non-mitogen-stimulated peripheral blood mononuclear cells. Additional evidence that HIV-1 bound to CR2 independently of CD4 was provided by the fact that binding was blocked by monoclonal antibody OKB7 (anti-CR2) but not by OKT4a (anti-CD4). Also, the virus bound to transfected K562 cells (CD4-) which expressed recombinant human CR2 but did not bind to untransfected K562 cells. Results obtained with complement component-deficient sera indicated that binding required the alternative complement pathway. Raji-3 and transfected K562 cells could not be infected with HIV-1 in the presence of complement, suggesting that utilization of CR2 as a receptor in the absence of CD4 does not allow virus entry. The demonstration of CR2 as a receptor for HIV-1 in the presence of complement, together with the ability to enhance binding by desialylation, provides new insights into mechanisms of HIV-1-induced immunity and immunopathogenesis.