Mechanism of HIV-1 Neutralization by Antibodies Targeting a Membrane-Proximal Region of gp41

Abstract

c Induction of broadly neutralizing antibodies (bNAbs) is an important goal for HIV-1 vaccine development. Two autoreactive bNAbs, 2F5 and 4E10, recognize a conserved region on the HIV-1 envelope glycoprotein gp41 adjacent to the viral membrane known as the membrane-proximal external region (MPER). They block viral infection by targeting a fusion-intermediate conformation of gp41, assisted by an additional interaction with the viral membrane. Another MPER-specific antibody, 10E8, has recently been reported to neutralize HIV-1 with potency and breadth much greater than those of 2F5 or 4E10, but it appeared not to bind phospholipids and might target the untriggered envelope spikes, raising the hope that the MPER could be harnessed for vaccine design without major immunological concerns. Here, we show by three independent approaches that 10E8 indeed binds lipid bilayers through two hydrophobic residues in its CDR H3 (third heavy-chain complementarity-determining region). Its weak affinity for membranes in general and preference for cholesterol-rich membranes may account for its great neutralization potency, as it is less likely than other MPER-specific antibodies to bind cellular membranes nonspecifically. 10E8 binds with high affinity to a construct mimicking the fusion intermediate of gp41 but fails to recognize the envelope trimers representing the untriggered conformation. Moreover, we can improve the potency of 4E10 without affecting its binding to gp41 by a modification of its lipid-interacting CDR H3. These results reveal a general mechanism of HIV-1 neutralization by MPER-specific antibodies that involves interactions with viral lipids.

Reactivation of Neutralized HIV-1 by Dendritic Cells Is Dependent on the Epitope Bound by the Antibody

“…In the Env CD4-unliganded prefusion state, both 2F5 and 4E10 first anchor the viral membrane, leading to binding to the MPER, with stronger affinity for the epitope in the CD4-postfusion conformational stage (34,35). However, the extremely potent 10E8 MPER bNAb is less dependent on binding the viral membrane prior to targeting its epitope (36), but like the other MPER bNAbs, affinity for Env in its prefusion state is weak (37). Thus, it is possible that Ab binding to the MPER in the prefusion state is too weak to withstand HIV-1 processing by DCs.…”

Antigenicity-defined conformations of an extremely neutralization-resistant HIV-1 envelope spike

“…Upon CD4 binding, the nonneutralizing epitopes in V3 became fully accessible, but the cluster I epitopes and those in the V2 loop did not. The MPER-directed bnAbs 10E8 and 4E10 did not bind the native Env trimer, because they target a fusion-intermediate conformation of gp41 (25,26). Another bnAb, PGT151, which binds at the gp120-gp41 interface, neutralized only very weakly with a maximum percent inhibition (MPI) of 45%; it bound at detectable levels to the CH120.6 Env trimer with or without CD4 (Fig.…”

Anti-MPER Antibodies with Heterogeneous Neutralization Capacity Are Detectable in Most Untreated HIV Infected Individuals

“…This is the case of the different bnAbs isolated [7,15]. Furthermore, the hydrophobic CDRH3 regions recognize lipids [7,15] and at least 2F5 and 4E10 bnAbs are also cross reactive with human proteins, thus suggesting that tolerance may limit anti-MPER responses [25,26]. All these limitations seem to favor the diversion of humoral immune responses towards other gp41 regions, in particular the external loop, which has been described as an immunodominant nonneutralizing region [27].…”

“…[21]. This is the case of the different bnAbs isolated [7,15]. Furthermore, the hydrophobic CDRH3 regions recognize lipids [7,15] and at least 2F5 and 4E10 bnAbs are also cross reactive with human proteins, thus suggesting that tolerance may limit anti-MPER responses [25,26].…”

“…The development of the B-cell cloning technology led to the recent isolation of the monoclonal antibody 10E8 [4], which is among the broadest and most potent neutralizing antibodies identified to date. Although it was shown initially to lack the limiting features presented by the previous anti-MPER bnAbs [4,14], it has been shown that 10E8 does bind membrane lipids by two hydrophobic residues in the CDRH3 loop, suggesting that anti-MPER bnAbs could mediate neutralization by similar mechanisms where the binding to the viral membrane plays a role [15]. Despite this controversy, it seems that the presentation of MPER epitopes in a lipid environment or in a soluble form may modify its recognition by anti-MPER antibodies [16,17].…”