Antibody-Dependent, FcγRI-Mediated Neutralization of HIV-1 in TZM-bl Cells Occurs Independently of Phagocytosis

Abstract

We previously showed that expression of human Fc␥RI on TZM-bl cells potentiates neutralization by gp41 membrane-proximal external region (MPER)-specific antibodies. Here we show that lysosomotropic reagents known to block phagocytosis do not diminish this effect. We also show that Fc␥RI occasionally potentiates neutralization by antibodies against the V3 loop of gp120 and cluster I of gp41. We conclude that Fc␥RI provides a kinetic advantage for neutralizing antibodies against partially cryptic epitopes independent of phagocytosis. Many cells of the immune system express Fc receptors that bind immune complexes through the constant region of immunoglobulins to mediate a variety of innate and acquired immune defense mechanisms (1, 2). In the case of cell-free HIV-1, Fc␥Rs have been implicated in both infection enhancement (3) and virus destruction by phagocytosis (4,5,6). To study the impact of the four major Fc␥ receptors on cell-free HIV-1 in greater detail, we stably expressed each of the cDNAs for the human Fc␥Rs on TZM-bl cells. Using these new cell lines, we assayed a panel of HIV-1-positive sera and monoclonal antibodies (MAbs) for neutralization potency against several strains of HIV-1. We showed that expression of Fc␥RI/CD64 had a profound effect on HIV-1 neutralization by the gp41 membrane-proximal external region (MPER)-specific MAbs 2F5 and 4E10 (7). In the presence of Fc␥RI expression, neutralization titers for 4E10 and 2F5 were increased as much as 5,000-fold depending on the virus used. Given that both 2F5 and 4E10 have been shown to prevent viral infection in a macaque model (8) and that both MAbs are broadly neutralizing, an understanding of their neutralizing mechanism may be beneficial for vaccine development.As we proposed previously, at least two mechanisms might account for the observed positive effect of Fc␥RI on HIV-1 neutralization. One would involve the pre-positioning of Abs at the cell surface, thereby giving them a kinetic advantage in accessing epitopes that are more-readily exposed after virus attachment. This mechanism is supported by the finding that cell-free HIV-1 immune complex formation was not required prior to Ab-Fc␥RI engagement in order for Fc␥RI to augment the neutralization activities of 2F5 and 4E10 (7).A second mechanism that may potentially facilitate HIV-1 neutralization is phagocytosis. HeLa cells, from which the TZM-bl cell line was developed, are known to exhibit properties of nonprofessional phagocytes (9, 10, 11). Several mammalian cells have been shown to acquire phagocytic properties upon expression of Fc␥Rs (12,13,14). Thus, surface expression of Fc␥Rs may, in principle, turn TZM-bl cells into professional phagocytes.Macrophages, dendritic cells, and neutrophils use phagocytosis to internalize particles larger than 0.5 m. This process can be triggered by diverse membrane components, such as scavenger receptors, complement receptors, and Fc receptors (13,14). During phagocytosis, internalized particles are trafficked into a complex of membrane-bound structures ...

Broadly Neutralizing Antibodies Display Potential for Prevention of HIV-1 Infection of Mucosal Tissue Superior to That of Nonneutralizing Antibodies

“…Interestingly, 2F5 dIgA was ineffective in the majority of assays, with the notable exception of colorectal tissue. These observations confirm previous studies showing reduced activity for polymeric dIgA and pIgM isotypes (46, 47) and likely reflect the known influence of FcγR engagement on the neutralizing activity of 2F5 IgG (48). In contrast, MPER-specific 4E10, despite demonstrating good levels of inhibition across cellular models, was only able to inhibit viral infection in colorectal tissue and was inactive against onward transmission by migratory cells.…”

Deletion of Nlrp3 Augments Survival during Polymicrobial Sepsis by Decreasing Autophagy and Enhancing Phagocytosis

“…The phagocytic activity of peritoneal neutrophils was measured by the uptake of red fluorescent pHrodo Escherichia coli bioparticles (35, 36) as described. Briefly, 1× 10 6 neutrophils were suspended in 100 uL of HBSS containing 20 mM HEPES, pH 7.4, and mixed with 5uL of pHrodo E. coli bioparticles.…”

Fc Receptor-Mediated Immune Responses: New Tools But Increased Complexity in HIV Prevention

“…Moreover, controllers and untreated progressors exhibited greater phagocytic activity, accounted for by the skewed FcγRIIa recognition. FcγR1 can also mediate phagocytosis, but has exhibited an alternative function in enhancing the neutralization potency of MPER antibodies, attributed to a pre-positioning effect [63]. Expression of Fc RI on TZM-bl cells greatly increased the neutralization titers of monoclonal antibodies 4E10 and 2F5.…”